ENVIRONMENT BIOTECHNOLOGY PART 2

Arsenic

Long-term exposure to arsenic in drinking water can cause cancer in the skin, lungs, bladder and kidney. It can also cause other skin changes such as thickening and pigmentation.

Exposure to arsenic in the workplace by inhalation can also cause lung cancer. The likelihood of cancer is related to the level and duration of exposure. Smoking and arsenic exposure combined increase the risk of lung cancer.

Noise Pollution

Sound is essential to our daily lives, but noise is not. Noise is generally used as an unwanted sound, or sound which produces unpleasant effects and discomfort on the ears.

What are the sources of noise pollution?

Household sources:
Gadgets like food mixer, grinder, vacuum cleaner, washing machine and dryer, cooler, air conditioners, can be very noisy and injurious to health. Others include loud speakers of  sound systems and TVs, ipods and ear phones.

Social events:
Places of worship, discos and gigs, parties and other social events also create a lot of noise for the people living in that area. In many market areas, people sell with loud speakers, others shout out offers and try to get customers to buy their goods.

Commercial and industrial activities:
Printing presses, manufacturing industries, construction sites, contribute to noise pollutions in large cities. People who work with lawn mowers, tractors and noisy equipment are also required to wear noise-proof gadgets.

Transportation:
Aero planes flying over houses close to busy, over ground and underground trains, vehicles on road—these are constantly making a lot of noise.

Effects of noise pollution

Generally, problems caused by noise pollution include stress related illnesses, speech interference, hearing loss, sleep disruption, and lost productivity.

Hearing
The immediate and acute effect of noise pollution to a person, over a period of time, is impairment of hearing. Prolonged exposure to impulsive noise to a person will damage their eardrum, which may result in a permanent hearing.

Effects on general health
Health effects of noise include anxiety and stress reaction and in extreme cases fright. The physiological manifestations are headaches, irritability and nervousness, feeling of fatigue and decreases work efficiency.

Noise pollution prevention and control

Construction of soundproof rooms for noisy machines in industrial and manufacturing installations must be encouraged. This is also important for residential building—noisy machines should be installed far from sleeping and living rooms, like in a basement or garage.

Use of horns with jarring sounds, motorbikes with damaged exhaust pipes, noisy trucks to be banned.

Noise producing industries, airports, bus and transport terminals and railway stations to sighted far from where living places.

Community law enforcers should check the misuse of loudspeakers, worshipers, outdoor parties and discos, as well as public announcements systems.

Community laws must silence zones near schools / colleges, hospitals etc.

Vegetation (trees) along roads and in residential areas is a good way to reduce noise pollution as they absorb sound.

 

Pollution monitoring and measurement

 

SO₂ concentration is measured by the ultraviolet fluorescence method, where the analysed sample is exposed to UV-lamp irradiation with energetic excitation of SO₂ molecule. With the backward conversion of the molecule into the basic energetic level, energy as fluorescing radiation is released. This radiation is proportional to the sulfur dioxide concentration and is detected by a photomultiplier. 

NO_x concentration is measured by a chemiluminescence analyzer for the NO, NO₂ and NO_x concentration measurement. The principle of this method stands on the nitrogen molecule excitation by ozone. With the conversion of the molecule into the basic energetic level, liberation of radiation as chemiluminescence occurs. This radiation is detected by a photomultiplier. The analyzer design makes possible the acquirement of information on nitrogen monoxide (NO), nitrogen dioxide (NO₂) and nitrogen oxides (NO_x) concentrations. 

For PM₁₀ concentration (suspended particulate matter fraction up to 10µm particle size) measurement the radiometric method is used. It stands on beta-ray absorption in a sample captured on filtering material. The difference between the beta-ray absorption of the exposed and non-exposed filtering material, which is proportional to the mass of the captured suspended particle matter, gives the information on its concentration. 

The automated stations installed by the State Health Institute and some stations of the Public Health Service use for the suspended particulate matter continual monitoring the tapered element oscillating microbalance(TEOM). It measures the mass of the sample captured on a replaceable filter according to the oscillating tapered element frequency variation. The air sample passes through a filter where the dust particles are captured and runs through a hollow tapered element to a vacuum pump with an electronic flow control. 

CO concentration is measured by the method of IR-correlation absorption spectrometry. The radiation from an infra-red source passes through two parallel cells, one of which contains a non-absorbing background gas, the other contains the analyzed flowing sample of ambient air. The difference in energy between the sample and the reference cell is proportional to the carbon monoxide concentration. 

The ozone concentration measurement is based on ultraviolet absorption photometry, resting upon absorption of radiation with the wavelenght of 254 nm by ozone in the analyzed sample. The radiation source is an UV-lamp and clean air (zero) and the sample itself are alternately measured in cells. The presented method with automatic pressure and temperature compensation meets the challenging requirements for O₃ measurement. 

During the year 1997 continual measurement of aromatic hydrocarbons (benzene, toluene and xylenes) by BTX analyzers and gas chromatography method was introduced at two AMS (Libuš, Most). It is a case of standard linkage to a sampling probe in a container.

 

Venturi scrubber

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A venturi scrubber is designed to effectively use the energy from the inlet gas stream to atomize the liquid being used to scrub the gas stream. This type of technology is a part of the group of air pollution controls collectively referred to as wet scrubbers.

Principle

This gas scrubber accelerates the scrubber liquid, together with the air or gas exhaust stream, to high velocities and turbulence. This happens in the bottleneck of the venturi. Behind this bottleneck, the pressure drops, reducing flow velocity back to normal. At this point, contaminant particles are collected and removed.

Construction and working

A venturi scrubber consists of three sections: a converging section, a throat section, and a diverging section. The inlet gas stream enters the converging section and, as the area decreases, gas velocity increases (in accordance with the Bernoulli equation). Liquid is introduced either at the throat or at the entrance to the converging section.

The inlet gas, forced to move at extremely high velocities in the small throat section, shears the liquid from its walls, producing an enormous number of very tiny droplets.

Particle and gas removal occur in the throat section as the inlet gas stream mixes with the fog of tiny liquid droplets. The inlet stream then exits through the diverging section, where it is forced to slow down.

Venturis can be used to collect both particulate and gaseous pollutants, but they are more effective in removing particles than gaseous pollutants.

Liquid can be injected at the converging section or at the throat. Thus, the liquid coats the venturi throat making it very effective for handling hot, dry inlet gas that contains dust. Otherwise, the dust would have a tendency to cake on or abrade a dry throat. These venturis are sometimes referred to as having a wetted approach.

Since it is sprayed at or just before the throat, it does not actually coat the throat surface. These throats are susceptible to solids buildup when the throat is dry. They are also susceptible to abrasion by dust particles. These venturis are best used when the inlet stream is cool and moist. These venturis are referred to as having a non-wetted approach.

Advantages and disadvantages

• Advantages
  —  Relatively low maintenance
  —  High  removal yield
  —  Simple and compact construction
  —  No mechanical components
  —  Gaseous components are absorbed
  —  Able to deal with fluctuating gas flows
  —  No ventilator required

• Disadvantages
  — Large pressure drops
  — Signs of erosion when scrubbing abrasive mediums

Applications

The venturi scrubber is also used as a cooler to “quench” hot flue gases (up to 1000 °C).

Venturi scrubbers are used in a variety of settings, including:

—  The chemical industry, to separate dust and aerosols;
—  The metal industry for various types of waste gases;
—  Waste incineration installations;
—  Gasification processes;
—  Potato-processing industry for the removal of starch;
—  Glass industry;
—  Melting processes in metallurgy;
—  Foundries;
—  Sintering processes;
—  Drying processes;
—  Fertiliser production;
—  Pharmaceutical industry;
—  Plastics industry

 

Mechanically aided scrubber

 

Mechanically aided scrubbers are a form of pollution control technology. This type of technology is a part of the group of air pollution controls collectively referred to as wet scrubbers.

In addition to using liquid sprays or the exhaust stream, scrubbing systems can use motors to supply energy. The motor drives a rotor or paddles which, in turn, generate water droplets for gas and particle collection.

Systems designed in this manner have the advantage of requiring less space than other scrubbers, but their overall power requirements tend to be higher than other scrubbers of equivalent efficiency. Significant power losses occur in driving the rotor. Therefore, not all the power used is expended for gas–liquid contact.

Principle of Operation

+ Multi-Cyclone Scrubbers use centrifugal force to effectively remove solid particles and liquids from a gas. There are no moving parts. The high efficiency, low pressure drop, centrifugal separation is attained by using Assure Solutions proprietary designed Cyclone Tubes.

+ The entrainment laden gas stream enters the distribution chamber of the scrubber, which contains multiplicity of small cyclone tube arranged in parallel. The gas enters each of the tubes through two tangential opening located near the top. The resulting centrifugal action moves the liquid droplets and/or solid particles to the outer periphery of the tube and downward, causing them to drop into the collection chamber at the bottom of the vessel. The cleanse gas then reverses direction at the vortex of the cyclone tube and moves upward through the riser and into the exit plenum.

Advantages

• High collection efficiency of fine particulate matter;
• No secondary dust sources;
• Minimum space requirements;
• Ideal for combined separation of solid particle and gaseous pollutants (particularly adhesive substances);
• Suitable for operation at high gas temperatures, pressure and humidity;
• Low investment and maintenance costs (if no additional wastewater treatment is required).

TYPICAL APPLICATIONS

Exhaust gas cleaning in:

• Food frying and drying operations
• Industrial incinerators.
• General industrial classifying, grinding and transfer operations.
• Phosphate fertilizer plants.

Electrostatic precipitator

An electrostatic precipitator (ESP) is a highly efficient filtration device that removes fine particles, like dust and smoke, from a flowing gas using the force of an induced electrostatic charge minimally impeding the flow of gases through the unit.

Electrostatic precipitators take advantage of the electrical principle that opposites attract. A high voltage electrode negatively charges airborne particles in the exhaust stream. As the exhaust gas passes through this electrified field, the particles are charged. Typically 20,000 to 70,000 volts are used. A large, grounded flat metal surface acts as a collection electrode. Microscopic particles are attracted to this surface where they build-up to form a dust cake. Periodically, a rapper strikes the plate to knock the dust cake into a collection hopper.

        

ESP use for:

• Power generation
• Petrochemical and oil refineries
• Sinter plants
• Cement plants
• Heating plants
• Metallurgical
• Paper mills
• Waste incineration
• Sludge incineration
• Flue gas desulfurization
• Industrial and waste heat boilers
• Coke production

What is Environmental Degradation?

Environmental degradation is the disintegration of the earth or deterioration of the environment through consumption of assets, for example, air, water and soil; the destruction of environments and the eradication of wildlife. It is characterized as any change or aggravation to nature’s turf seen to be pernicious or undesirable. Ecological effect or degradation is created by the consolidation of an effectively substantial and expanding human populace, constantly expanding monetary development or per capita fortune and the application of asset exhausting and polluting technology. It occurs when earth’s natural resources are depleted and environment is compromised in the form of extinction of species, pollution in air, water and soil, and rapid growth in population.

Causes of Environmental Degradation

1. Land Disturbance: A more basic cause of environmental degradation is land damage. Numerous weedy plant species, for example, garlic mustard, are both foreign and obtrusive. A rupture in the environmental surroundings provides for them a chance to start growing and spreading. These plants can assume control over nature, eliminating the local greenery. The result is territory with a solitary predominant plant which doesn’t give satisfactory food assets to all the environmental life. Whole environments can be destroyed because of these invasive species.

2. Pollution: Pollution, in whatever form, whether it is air, water, land or noise is harmful for the environment. Air pollution pollutes the air that we breathe which causes health issues. Water pollution degrades the quality of water that we use for drinking purposes. Land pollution results in degradation of earth’s surface as a result of human activities. Noise pollution can cause irreparable damage to our ears when exposed to continuous large sounds like honking of vehicles on a busy road or machines producing large noise in a factory or a mill.

3. Overpopulation: Rapid population growth puts strain on natural resources which results in degradation of our environment. Mortality rate has gone down due to better medical facilities which has resulted in increased lifespan. More population simple means more demand for food, clothes and shelter. You need more space to grow food and provide homes to millions of people. This results in deforestation which is another factor of environmental degradation.

4. Landfills: Landfills pollute the environment and destroy the beauty of the city. Landfills come within the city due the large amount of waste that gets generated by households, industries, factories and hospitals. Landfills pose a great risk to the health of the environment and the people who live there. Landfills produce foul smell when burned and cause huge environmental degradation.

5. Deforestation: Deforestation is the cutting down of trees to make way for more homes and industries. Rapid growth in population and urban sprawl are two of the major causes of deforestation. Apart from that, use of forest land for agriculture, animal grazing, harvest for fuel wood and logging are some of the other causes of deforestation. Deforestation contributes to global warming as decreased forest size puts carbon back into the environment.

6: Natural Causes: Things like avalanches, quakes, tidal waves, storms, and wildfires can totally crush nearby animal and plant groups to the point where they can no longer survive in those areas. This can either come to fruition through physical demolition as the result of a specific disaster, or by the long term degradation of assets by the presentation of an obtrusive foreign species to the environment. The latter frequently happens after tidal waves, when reptiles and bugs are washed ashore.

Effects of Environmental Degradation

1. Impact on Human Health: Human health might be at the receiving end as a result of the environmental degradation. Areas exposed to toxic air pollutants can cause respiratory problems like pneumonia and asthma. Millions of people are known to have died of due to indirect effects of air pollution.

2. Loss of Biodiversity: Biodiversity is important for maintaining balance of the ecosystem in the form of combating pollution, restoring nutrients, protecting water sources and stabilizing climate. Deforestation, global warming, overpopulation and pollution are few of the major causes for loss of biodiversity.

3. Ozone Layer Depletion: Ozone layer is responsible for protecting earth from harmful ultraviolet rays. The presence of chlorofluorocarbons, hydro chlorofluorocarbons in the atmosphere is causing the ozone layer to deplete. As it will deplete, it will emit harmful radiations back to the earth.

4. Loss for Tourism Industry: The deterioration of environment can be a huge setback for tourism industry that rely on tourists for their daily livelihood. Environmental damage in the form of loss of green cover, loss of biodiversity, huge landfills, increased air and water pollution can be a big turn off for most of the tourists.

5. Economic Impact: The huge cost that a country may have to borne due to environmental degradation can have big economic impact in terms of restoration of green cover, cleaning up of landfills and protection of endangered species. The economic impact can also be in terms of  loss of tourism industry.

Environmental laws

In the Constitution of India it is clearly stated that it is the duty of the state to ‘protect and improve the environment and to safeguard the forests and wildlife of the country’. It imposes a duty on every citizen ‘to protect and improve the natural environment including forests, lakes, rivers, and wildlife’. Reference to the environment has also been made in the Directive Principles of State Policy as well as the Fundamental Rights. The Department of Environment was established in India in 1980 to ensure a healthy environment for the country. This later became the Ministry of Environment and Forests in 1985.

The constitutional provisions are backed by a number of laws – acts, rules, and notifications. The EPA (Environment Protection Act), 1986 came into force soon after the Bhopal Gas Tragedy and is considered an umbrella legislation as it fills many gaps in the existing laws. Thereafter a large number of laws came into existence as the problems began arising, for example, Handling and Management of Hazardous Waste Rules in 1989.

Following is a list of the environmental legislations that have come into effect:
General
Forest and wildlife
Water
Air 

General

1986 - The Environment (Protection) Act authorizes the central government to protect and improve environmental quality, control and reduce pollution from all sources, and prohibit or restrict the setting and /or operation of any industrial facility on environmental grounds.

1986 - The Environment (Protection) Rules lay down procedures for setting standards of emission or discharge of environmental pollutants.

1989 - The objective of Hazardous Waste (Management and Handling) Rules is to control the generation, collection, treatment, import, storage, and handling of hazardous waste.

1998 - The Biomedical waste (Management and Handling) Rules is a legal binding on the health care institutions to streamline the process of proper handling of hospital waste such as segregation, disposal, collection, and treatment.

1999 - The Environment (Siting for Industrial Projects) Rules, 1999 lay down detailed provisions relating to areas to be avoided for siting of industries, precautionary measures to be taken for site selecting as also the aspects of environmental protection which should have been incorporated during the implementation of the industrial development projects.

2000 - The Municipal Solid Wastes (Management and Handling) Rules, 2000 apply to every municipal authority responsible for the collection, segregation, storage, transportation, processing, and disposal of municipal solid wastes.

2000 - The Ozone Depleting Substances (Regulation and Control) Rules have been laid down for the regulation of production and consumption of ozone depleting substances.

2002 - The Noise Pollution (Regulation and Control) (Amendment) Rules lay down
such terms and conditions as are necessary to reduce noise pollution, permit use of loud speakers or public address systems during night hours (between 10:00 p.m. to 12:00 midnight) on or during any cultural or religious festive occasion

2002 - The Biological Diversity Act is an act to provide for the conservation of biological diversity, sustainable use of its components, and fair and equitable sharing of the benefits arising out of the use of biological resources and knowledge associated with it

Forest and wildlife

1927 - The Indian Forest Act and Amendment, 1984, is one of the many surviving colonial statutes. It was enacted to ‘consolidate the law related to forest, the transit of forest produce, and the duty leviable on timber and other forest produce’.

1972 - The Wildlife Protection Act, Rules 1973 and Amendment 1991 provides for the protection of birds and animals and for all matters that are connected to it whether it be their habitat or the waterhole or the forests that sustain them.

1980 - The Forest (Conservation) Act and Rules, 1981, provides for the protection of and the conservation of the forests.

Water

1897 - The Indian Fisheries Act establishes two sets of penal offences whereby the government can sue any person who uses dynamite or other explosive substance in any way (whether coastal or inland) with intent to catch or destroy any fish or poisonous fish in order to kill.

1956 - The River Boards Act enables the states to enroll the central government in setting up an Advisory River Board to resolve issues in inter-state cooperation.

1970 - The Merchant Shipping Act aims to deal with waste arising from ships along the coastal areas within a specified radius.

1974 - The Water (Prevention and Control of Pollution) Act establishes an institutional structure for preventing and abating water pollution. It establishes standards for water quality and effluent. Polluting industries must seek permission to discharge waste into effluent bodies.
The CPCB (Central Pollution Control Board) was constituted under this act.

1991 - The Coastal Regulation Zone Notification puts regulations on various activities, including construction, are regulated. It gives some protection to the backwaters and estuaries.

 

 

Air

1948 – The Factories Act and Amendment in 1987 was the first to express concern for the working environment of the workers. The amendment of 1987 has sharpened its environmental focus and expanded its application to hazardous processes.

1981 - The Air (Prevention and Control of Pollution) Act provides for the control and abatement of air pollution. It entrusts the power of enforcing this act to the CPCB .

1982 - The Air (Prevention and Control of Pollution) Rules defines the procedures of the meetings of the Boards and the powers entrusted to them.

1982 - The Atomic Energy Act deals with the radioactive waste.

1987 - The Air (Prevention and Control of Pollution) Amendment Act empowers the central and state pollution control boards to meet with grave emergencies of air pollution.

1988 - The Motor Vehicles Act states that all hazardous waste is to be properly packaged, labelled, and transported.

The Air (Prevention and Control of Pollution ) Act, 1981

This is an Act to provide for prevention, control and abatement of air pollution in the country so as to preserve the quality of air. Central and State Boards constituted u/s 3 & 4 of Water [ Prevention and Control and Pollution ] Act, 1974 were deemed also as Central and State Boards for prevention and control of air pollution.

An Act to provide for the prevention, control and abatement of air pollution, for the establishment, with a view to carrying out the aforesaid purposes, of Boards, for conferring on and assigning to such Boards powers and functions relating thereto and for matters connected therewith.

WHEREAS decisions were taken at the United Nations Conference on the Human Environment held in Stockholm in June, 1972, in which India participated, to take appropriate steps for the prevention of the natural resources of the earth which, among other things, include the preservation of the quality of air and control of air pollution.

AND WHEREAS it is considered necessary to implement the decisions aforesaid in so far as they relate to the preservation of the quality of air and control of air pollution.

The Salient Points of the Act are :

• The Act is applicable to whole India.
• U/s 19 of the Act, Central / State Govt. in consultation with SPCB is vested with power to declare AIR POLLUTION CONTROL AREA in which the provisions of the Act shall be applicable.
• As per provisions in section 21 [2], no person can establish or operate any industrial plant without the consent of State Pollution Control Board shall complete the formalities to either grant or refuse consent. During the course of the processing consent application, Board may seek any information about the industry after giving notice in Form-II.
• Under section 22, 22(A) operating any industrial plant so as to cause emission of any air pollutant in excess of standard laid down by State Board, is liable for litigation by the Board.

POWERS OF STATE BOARD

1. POWER TO ENTRY & INSPECTION [ u/s 24 ]

2. POWER TO TAKE SAMPLES… [u/s 26 ]

3. POWER TO GIVE DIRECTION…..[ u/s 31 ]

Collection of Solid Waste

Waste collection is the collection of solid waste from

point of production (residential, industrial commercial,

institutional) to the point of treatment or disposal.

Types of Collection System

• Refuse Collection Systems - Household waste removed from the home
• Commercial Waste Collection - Commercial waste removed primarily using dumpsters
• Recyclable Material Collection - Collection of recyclable materials separated at the source of generation

Municipal solid waste is collected in several ways:

1. House-to-House: Waste collectors visit each individual house to collect garbage. The user generally pays a fee for this service.

2. Community Bins: Users bring their garbage to community bins that are placed at fixed points in a neighborhood or locality. MSW is picked up by the municipality, or its designate, according to a set schedule.

3. Curbside Pick-Up: Users leave their garbage directly outside their homes according to a garbage

pick-up schedule set with the local authorities (secondary house-tohouse collectors not typical).

4. Self Delivered: Generators deliver the waste directly to

disposal sites or transfer stations, or hire third-party operators (or the municipality).

5. Contracted or Delegated Service: Businesses hire firms

(or municipality with municipal facilities) who arrange collection schedules and charges with but rather is placed out for collection in separate containers without first being ‘mixed’ together.

Sludge treatment and disposal

The residue that accumulates in sewage treatment plants is called sludge (or biosolids). Treatment and disposal of sewage sludge are major factors in the design and operation of all wastewater treatment plants. Two basic goals of treating sludge before final disposal are to reduce its volume and to stabilize the organic materials. Stabilized sludge does not have an offensive odour and can be handled without causing a nuisance or health hazard. Smaller sludge volume reduces the costs of pumping and storage.

Treatment processes

Thickening

Thickening is often the first step in a sludge treatment process. Sludge from primary or secondary clarifiers may be stirred (often after addition of clarifying agents) to form larger, more rapidly settling aggregates.^[2] Primary sludge may be thickened to about 8 or 10 percent solids, while secondary sludge may be thickened to about 4 percent solids. Thickeners often resemble a clarifier with the addition of a stirring mechanism.^[3] Thickened sludge with less than ten percent solids may receive additional sludge treatment while liquid thickener overflow is returned to the sewage treatment process.

Dewatering

Sewage sludge is sandwiched between two belt press filter cloths (shown green and purple). Filtrate is extracted initially by gravity, then by squeezing the cloth through rollers and returned to the sewage treatment plant, while dewatered sludge is scraped off for composting or disposal.

Sidestream treatment technologies

Sludge treatment technologies that are used for thickening or dewatering of sludge have two products: the thickened or dewatered sludge, and a liquid fraction which is called sludge treatment liquids, sludge dewatering streams, liquors, centrate (if it stems from a centrifuge), filtrate (if it stems from a belt filter press) or similar. This liquid requires further treatment as it is high in nitrogen and phosphorus, particuarly if the sludge has been anaerobically digested. The treatment can take place in the sewage treatment plant itself (by recylyling the liquid to the start of the treatment process) or as a separate process.

Phosphorus recovery

One method for treating sludge dewatering streams is by using a process that is also used for phosphorus recovery. Another benefit for sewage treatment plant operators of treating sludge dewatering streams for phosphorus recovery is that it reduces the formation of obstructive struvite scale in pipes, pumps and valves. Such obstructions can be a maintenance headache particularly for biological nutrient removal plants where the phosphorus content in the sewage sludge is elevated. For example, the Canadian company Ostara is marketing a process based on controlled chemical precipitation of phosphorus in a fluidized bed reactor that recovers struvite in the form of crystalline pellets from sludge dewatering streams. The resulting crystalline product is sold to golf courses and the general public as fertiliser under the trade name of Crystal Green.^[5]

Digestion

Sludge digestion is a biological process in which organic solids are decomposed into stable substances. Digestion reduces the total mass of solids, destroys pathogens, and makes it easier to dewater or dry the sludge. Digested sludge is inoffensive, having the appearance and characteristics of a rich potting soil.



Anaerobic digestion is a bacterial process that is carried out in the absence of oxygen. The process can either be thermophilic digestion, in which sludge is fermented in tanks at a temperature of 55 °C, or mesophilic, at a temperature of around 36 °C. Though allowing shorter retention time (and thus smaller tanks), thermophilic digestion is more expensive in terms of energy consumption for heating the sludge.

Aerobic digestion is a bacterial process occurring in the presence of oxygen resembling a continuation of the activated sludge process. Under aerobic conditions, bacteria rapidly consume organic matter and convert it into carbon dioxide. Once there is a lack of organic matter, bacteria die and are used as food by other bacteria. This stage of the process is known as endogenous respiration. Solids reduction occurs in this phase. Because the aerobic digestion occurs much faster than anaerobic digestion, the capital costs of aerobic digestion are lower. However, the operating costs are characteristically much greater for aerobic digestion because of energy used by the blowers, pumps and motors needed to add oxygen to the process. However, recent technological advances include non-electric aerated filter systems that use natural air currents for the aeration instead of electrically operated machinery.

Composting

Composting is an aerobic process of mixing sewage sludge with agricultural byproduct sources of carbon such as sawdust, straw or wood chips. In the presence of oxygen, bacteria digesting both the sewage sludge and the plant material generate heat to kill disease-causing microorganisms and parasites.^[7]:20 Maintenance of aerobic conditions with 10 to 15 percent oxygen requires bulking agents allowing air to circulate through the fine sludge solids. Stiff materials like corn cobs, nut shells, shredded tree-pruning waste or bark from lumber or paper mills better separate sludge for ventilation than softer leaves and lawn clippings.^[8] Light, biologically inert bulking agents like shredded tires may be used to provide structure where small, soft plant materials are the major source of carbon.^[9]

Incineration

Incineration of sludge is less common because of air emissions concerns and the supplemental fuel (typically natural gas or fuel oil) required to burn the low calorific value sludge and vaporize residual water. On a dry solids basis, the fuel value of sludge varies from about 9,500 British thermal units per pound (980 cal/g) of undigested biosolids to 2,500 British thermal units per pound (260 cal/g) of digested primary sludge.^[11] Stepped multiple hearth incinerators with high residence time and fluidized bed incinerators are the most common systems used to combust wastewater sludge. Co-firing in municipal waste-to-energy plants is occasionally done, this option being less expensive assuming the facilities already exist for solid waste and there is no need for auxiliary fuel.^[7]:20–21 Incineration tends to maximize heavy metal concentrations in the remaining solid ash requiring disposal; but the option of returning wet scrubber effluent to the sewage treatment process may reduce air emissions by increasing concentrations of dissolved salts in sewage treatment plant effluent.^[12]

Drying beds

This simple evaporative sludge drying bed near Damascus in Syria illustrates the initial consistency of primary sludge being discharged from the primary settling tank via the pipe in the foreground.

Disposal or use as fertilizer

When a liquid sludge is produced, further treatment may be required to make it suitable for final disposal. Sludges are typically thickened and/or dewatered to reduce the volumes transported off-site for disposal. Processes for reducing water content include lagooning in drying beds to produce a cake that can be applied to land or incinerated; pressing, where sludge is mechanically filtered, often through cloth screens to produce a firm cake; and centrifugation where the sludge is thickened by centrifugally separating the solid and liquid. Sludges can be disposed of by liquid injection to land or by disposal in a landfill.

Land Disposal of Sludges

This is one of the lower cost alternatives for final sludge disposal. It is practised for two reasons: the need to dispose of the sludge (itself also a residue) and the opportunity to use the contents (organic matter, nitrogen and phosphorus) as fertilizer. Sludge land application has proven to be a viable substitute for commercial fertilizer, provided pathogenic control is applied.

Site selection is an important factor; it should be relatively far from waterbodies and the soil should be moderately permeable and well-drained, with a maximum slope of 5-8% in order to minimize erosion problems during application. Neutral to alkaline soils are preferred to reduce heavy metal mobility, the concentration of which should also be periodically checked.

The application of the sludges on the soil can be carried out by spreading from tank trucks or by sprinkling. Trenching or ploughing mixes the soil and sludges adequately, reduces odour and run-off.

 

Sludge Disinfection

The disinfection of sludges is especially important before they are applied to the land because of possible existence of pathogenic micro-organisms.

One of the simplest forms of disinfection is the addition of chlorine to stabilize and disinfect the sludge.

Types of solid waste

Solid waste can be classified into different types depending on their source:
a) Household waste is generally classified as municipal waste,
b) Industrial waste as hazardous waste, and
c) Biomedical waste or hospital waste as infectious waste.

Municipal solid waste

Municipal solid waste consists of household waste, construction and demolition debris, sanitation residue, and waste from streets. This garbage is generated mainly from residential and commercial complexes. With rising urbanization and change in lifestyle and food habits, the amount of municipal solid waste has been increasing rapidly and its composition changing. More than 25% of the municipal solid waste is not collected at all; 70% of the Indian cities lack adequate capacity to transport it and there are no sanitary landfills to dispose of the waste.

 Hazardous waste

Industrial and hospital waste is considered hazardous as they may contain toxic substances. Certain types of household waste are also hazardous. Hazardous wastes could be highly toxic to humans, animals, and plants; are corrosive, highly inflammable, or explosive; and react when exposed to certain things e.g. gases. Household waste that can be categorized as hazardous waste include old batteries, shoe polish, paint tins, old medicines, and medicine bottles.

Hospital waste contaminated by chemicals used in hospitals is considered hazardous. These chemicals include formaldehyde and phenols, which are used as disinfectants, and mercury, which is used in thermometers or equipment that measure blood pressure.

In the industrial sector, the major generators of hazardous waste are the metal, chemical, paper, pesticide, dye, refining, and rubber goods industries.

Direct exposure to chemicals in hazardous waste such as mercury and cyanide can be fatal.

 

Hospital waste

Hospital waste is generated during the diagnosis, treatment, or immunization of human beings or animals or in research activities in these fields or in the production or testing of biologicals. It may include wastes like sharps, soiled waste, disposables, anatomical waste, cultures, discarded medicines, chemical wastes, etc. These are in the form of disposable syringes, swabs, bandages, body fluids, human excreta, etc. This waste is highly infectious and can be a serious threat to human health if not managed in a scientific and discriminate manner.

DISPOSAL METHODS

Waste management simply means the collection, transport, processing or disposal, managing and monitoring of waste materials to minimize its' consequences on humans and environment.

How is waste treated and disposed off?

Waste management simply means the collection, transport, processing or disposal, managing and monitoring of waste materials to minimize its' consequences on humans and environment.

Incineration method of waste management:
This simply means burning waste. This method is common in countries with limited landfill space. Incineration chambers can be small for domestic use, but ther are large ones for municipal use as well. It is great for treating waste with contamination (like those from hospitals) and hazardous waste from factories, but the method produces too much carbon dioxide.

Sanitary Landfills as waste disposal:
Generally, this term means a large piece of land away from living places where all the waste from a town is deposited. But there is more to landfills. Proper landfill management involves sorting out all the waste (waste separation), and sending only the waste that cannot be recycled and composted to the site.

Proper landfills, are also lined at the bottom to minimize the leakage of soil pollutants and other toxins from getting into the water table. This method is effective, but expensive and difficult.

 Composting:

Composting operations of solid wastes include preparing refuse and degrading organic matter by aerobic microorganisms. Refuse is presorted, to remove materials that might have salvage value or cannot be composted, and is ground up to improve the efficiency of the decomposition process.

Recycling

Today, recyclable materials are recovered from municipal refuse by a number of methods, including shredding, magnetic separation of metals, air classification that separates light and heavy fractions, screening, and washing. Another method of recovery is the wet pulping process: Incoming refuse is mixed with water and ground into a slurry in the wet pulper, which resembles a large kitchen disposal unit. Large pieces of metal and other nonpulpable materials are pulled out by a magnetic device before the slurry from the pulper is loaded into a centrifuge called a liquid cyclone. Increasingly, municipalities and private refuse-collection organizations are requiring those who generate solid waste to keep bottles, cans, newspapers, cardboard, and other recyclable items separate from other waste.

Hazardous waste

Hazardous waste is waste that poses substantial or potential threats to public health or the environment.^[1] In the United States, the treatment, storage, and disposal of hazardous waste is regulated under the Resource Conservation and Recovery Act (RCRA). Hazardous wastes are divided into two major categories: characteristic wastes and listed wastes.^[2]

• Characteristic hazardous wastes are materials that are known or tested to exhibit one or more of the following four hazardous traits:
• ignitability (i.e., flammable)
• reactivity
• corrosivity
• toxicity
• Listed hazardous wastes are materials specifically listed by regulatory authorities as a hazardous waste which are from non-specific sources, specific sources, or discarded chemical products.^[3]

The requirements of RCRA apply to all the companies that generate hazardous waste as well as those companies that store or dispose of hazardous waste in the United States. Many types of businesses generate hazardous waste. For example, dry cleaners, automobile repair shops, hospitals, exterminators, and photo processing centers may all generate hazardous waste. Some hazardous waste generators are larger companies such as chemical manufacturers, electroplating companies, and oil refineries.

These wastes may be found in different physical states such as gaseous, liquids, or solids. A hazardous waste is a special type of waste because it cannot be disposed of by common means like other by-products of our everyday lives. Depending on the physical state of the waste, treatment and solidification processes might be required.

 

Final disposal of hazardous waste

Recycling

Many hazardous wastes can be recycled into new products. Examples might include lead-acid batteries or electronic circuit boards where the heavy metals these types of ashes go though the proper treatment, they could bind to other pollutants and convert them into easier-to- dispose solids, or they could be used as pavement filling. Such treatments reduce the level of threat of harmful chemicals, like fly and bottom ash^{[citation needed]}, while also recycling the safe product.

Portland cement

Another commonly used treatment is cement based solidification and stabilization. Cement is used because it can treat a range of hazardous wastes by improving physical characteristics and decreasing the toxicity and transmission of contaminants. The cement produced is categorized into 5 different divisions, depending on its strength and components. This process of converting sludge into cement might include the addition of pH adjustment agents, phosphates, or sulfur reagents to reduce the settling or curing time, increase the compressive strength, or reduce the leach ability of contaminants.

Incineration, destruction and waste-to-energy

A HW may be "destroyed" for example by incinerating it at a high temperature. Flammable wastes can sometimes be burned as energy sources. For example many cement kilns burn HWs like used oils or solvents. Today incineration treatments not only reduce the amount of hazardous waste, but also they also generate energy throughout the gases released in the process. It is known that this particular waste treatment releases toxic gases produced by the combustion of byproduct or other materials and this can affect the environment. However, current technology has developed more efficient incinerator units that control these emissions to a point that this treatment is considered a more beneficial option. There are different types of incinerators and they vary depending on the characteristics of the waste. Starved air incineration is another method used to treat hazardous wastes. Just like in common incineration, burning occurs, however controlling the amount of oxygen allowed proves to be significant to reduce the amount of harmful byproducts produced. Starved Air Incineration is an improvement of the traditional incinerators in terms of air pollution. Using this technology it is possible to control the combustion rate of the waste and therefore reduce the air pollutants produced in the process.

Hazardous waste landfill (sequestering, isolation, etc.)

A HW may be sequestered in a HW landfill or permanent disposal facility. "In terms of hazardous waste, a landfill is defined as a disposal facility or part of a facility where hazardous waste is placed or on land and which is not a pile, a land treatment facility, a surface impoundment, an underground injection well, a salt dome formation, a salt bed formation, an underground mine, a cave, or a corrective action management unit (40 CFR 260.10)."^[6][7]

Pyrolysis

Some hazardous waste types may be eliminated using pyrolysis in an ultra high temperature electrical arc, in inert conditions to avoid combustion. This treatment method may be preferable to high temperature incineration in some circumstances such as in the destruction of concentrated organic waste types, including PCBs, pesticides and other persistent organic pollutants.

Sewage sludge treatment

Sewage sludge treatment describes the processes used to manage and dispose of sewage sludge produced during sewage treatment. Sludge is mostly water with lesser amounts of solid material removed from liquid sewage. Primary sludge includes settleable solids removed during primary treatment in primary clarifiers. Secondary sludge separated in secondary clarifiers includes biosolids grown in secondary treatment bioreactors.

Treatment processes

Thickening

Thickening is often the first step in a sludge treatment process. Sludge from primary or secondary clarifiers may be stirred (often after addition of clarifying agents) to form larger, more rapidly settling aggregates.^[2] Primary sludge may be thickened to about 8 or 10 percent solids, while secondary sludge may be thickened to about 4 percent solids. Thickeners often resemble a clarifier with the addition of a stirring mechanism.^[3] Thickened sludge with less than ten percent solids may receive additional sludge treatment while liquid thickener overflow is returned to the sewage treatment process.

Digestion

Many sludges are treated using a variety of digestion techniques, the purpose of which is to reduce the amount of organic matter and the number of disease-causing microorganisms present in the solids. The most common treatment options include anaerobic digestion, aerobic digestion, and composting.

Anaerobic digestion

Anaerobic digestion is a bacterial process that is carried out in the absence of oxygen. The process can either be thermophilic digestion, in which sludge is fermented in tanks at a temperature of 55 °C, or mesophilic, at a temperature of around 36 °C. Though allowing shorter retention time (and thus smaller tanks), thermophilic digestion is more expensive in terms of energy consumption for heating the sludge.

Aerobic digestion

Aerobic digestion is a bacterial process occurring in the presence of oxygen resembling a continuation of the activated sludge process. Under aerobic conditions, bacteria rapidly consume organic matter and convert it into carbon dioxide. Once there is a lack of organic matter, bacteria die and are used as food by other bacteria. This stage of the process is known as endogenous respiration. Solids reduction occurs in this phase. Because the aerobic digestion occurs much faster than anaerobic digestion, the capital costs of aerobic digestion are lower. However, the operating costs are characteristically much greater for aerobic digestion because of energy used by the blowers, pumps and motors needed to add oxygen to the process.

Dewatering

Sewage sludge is sandwiched between two belt press filter cloths (shown green and purple). Filtrate is extracted initially by gravity, then by squeezing the cloth through rollers and returned to the sewage treatment plant, while dewatered sludge is scraped off for composting or disposal.

Composting

Composting is an aerobic process of mixing sewage sludge with agricultural byproduct sources of carbon such as sawdust, straw or wood chips. In the presence of oxygen, bacteria digesting both the sewage sludge and the plant material generate heat to kill disease-causing microorganisms and parasites.

Incineration

• Incineration of sludge is less common because of air emissions concerns and the supplemental fuel (typically natural gas or fuel oil) required to burn the low calorific value sludge and vaporize residual water.

Disposal or use as fertilizer

When a liquid sludge is produced, further treatment may be required to make it suitable for final disposal. Sludges are typically thickened and/or dewatered to reduce the volumes transported off-site for disposal. Processes for reducing water content include lagooning in drying beds to produce a cake that can be applied to land or incinerated; pressing, where sludge is mechanically filtered, often through cloth screens to produce a firm cake; and centrifugation where the sludge is thickened by centrifugally separating the solid and liquid. Sludges can be disposed of by liquid injection to land or by disposal in a landfill.

There is no process which completely eliminates the need to dispose of biosolids.

Waste Incineration

Environment has great influence in the life of all the living things on this earth. When it comes to wastage and its treatment, one of the very oldest effective waste treatments is waste incineration. It is basically a process where the domestic and industry waste materials are burnt. In this process, the waste materials turn into ash, flue gas and heat. On the basis of the type of waste materials, the incineration can of various scales, such as: small scale, medium scale and large scale.

In waste incineration method, waste materials or organic substances are burnt which incorporate households, hazardous and also medical wastes equipments. As the method of incineration involves combustion, therefore it is also known as thermal treatment.

The Process of Incineration

In the process of incineration, incinerators reduce the waste by burning it after the incinerator is initially fired up with gas or other combustible material. The process is then sustained by the waste itself. Complete waste combustion requires a temperature of 850º C for at least two seconds but most plants raise it to higher temperatures to reduce organic substances containing chlorine. Flue gases are then sent to scrubbers which remove all dangerous chemicals from them. To reduce dioxin in the chimneys where they are normally formed, cooling systems are introduced in the chimneys. Chimneys are required to be at least 9 meters above existing structures.

The method of incineration has a lot of benefits over other types of waste treatment system. While treating the waste materials, such as clinical and hazardous materials, waste incineration has proved to be more effective in this regard. By using this waste treatment method, the harmful pollutant and pathogens can be burnt completely in high temperature. This method of waste treatment has become extremely popular in countries having scarcity of lands.

However, while gong for waste incineration, one should also keep in mind that this process can have some negative effectives on our health due to environmental pollution. Production of ashes, flue gases and other releases of incineration can also lead to some serious consequences on mankind as well as on our natural atmosphere. In incineration, the waste materials get reduced in its amount and also get transformed into ashes that consist of some of the most venomous substances like: dioxins and heavy metals. These substances are difficult to destroy.

Advantages:

• requires minimum land
• can be operated in any weather
• produces stable odor-free residue
• refuse volume is reduced by half

Disadvantages:

• expensive to build and operate
• high energy requirement
• requires skilled personnel and continuous maintenance
• unsightly - smell, waste, vermin

The Pros and Cons of Incinerating Waste

We'll first look at the pros of incinerating some or the majority of our waste, before then focusing on the cons. You'll make up your own mind about this topic, but as with many environmental problems, like those discussed here on Ecoants, there is no easy answer.

The Advantages

• Perhaps the primary advantage of using incinerators would have to be the significant impact they have on reducing the amount of waste going to landfill. Depending on what is being incinerated the mass of the waste can be reduced by up to 85%, which is clearly a big benefit. This results in less land pollution in the form of landfill, reduced space required for landfill, and less potential harmful leakage from landfill waste seeping into surrounding environments and causing the problems we have already highlighted elsewhere.
• Many incinerators reach temperatures which can destroy most harmful pathogens and chemicals, which is why this method is used for dealing with clinical waste.
• Incineration of domestic and industrial waste can be used to produce energy in the form of electricity and also heat which can be piped to municipal heating schemes. In this way some use is being made of the heat being created when burning the waste.
• Modern incinerator technology is able to filter out many of the potential harmful emisions in the hot rising flue gases so that they do not escape into the outside environment, including various dioxins, particulates and some environmentally dangerous acidic gases like hydrogen chloride.

The Disadvantages

• Incineration can lead to a huge waste of important and finite natural resources which literally go up in smoke or end up as part of the resultant ash. Unless important resources such as all metals, plastics, and glass are removed before incineration this process can result in loss of resources which causes negative knock on environmental effects in the form of increased mining to replace those lost resources, and increased energy use to process and produce new resources. Increased mining leads to further land pollution and degradation of the land environments which are being mined. Increased energy use means increased fossil fuel use and therefore further increases in greenhouse gas emissions.
• Incinerators destroy reources which could easily be recycled. The use of incineration to deal with waste can also be a disincentive to use and promote recycling schemes and to compost organic household waste at a municipal level. This issue can be further exacerbated by the fact that the high costs of building an incinerator need to be offset with long contracts with municipal authorities to burn local waste or waste from other regions. These long contracts can encourage more incineration of waste locally rather than recycling.
• The location of an incinerator is often very controversial and most residents do not want one situated near to them. The problems they can bring include increased traffic to an area, which is specifically carrying waste for processing at the incinerator plant, a problem made worse if waste is imported from other regions.
• Even though modern incinerators use filters and processes to remove many of the harmful particulates and toxins from the hot flue exhaust gases produced as a result of the incineration process, they still do not filter out the smallest particles,. These particles could cause health problems in areas that are exposed to their emission downwind, with ongoing research seeking to establish the exact effects.
• A further problem with incinerated waste is what to do with the resultant ash that is collected. Part of this ash, the filtered fly ash, contains a significant proportion of toxins such as heavy metals.

Landfill Advantages

• A specific location for disposal that can be monitored.
• When a landfill is complete, it can be reclaimed, built on or used as parks or farming land.
• Waste going to a properly designed landfills can be processed to removeall recyclable materials before tipping.
• Waste going to a properly designed landfills can be processed to remove organic material and use it for compost or natural gas (methane) production.
• Properly managed landfills can capture the natural gas (methane) produced by the decomposing material underground.
• Properly managed landfills can minimize and/orcapture the leachate produced by the decomposing material underground.

 Landfill Disadvantages and Problems A poorly designed or operated landfill shares many problems observed at uncontrolled dumping areas:

• Landfills and the surrounding areas are often heavily polluted.
• Landfill can pollute the water, the air, and also the soil.
• It is difficult to keep dangerous chemicals from leaching out into the surrounding land.
• Dangerous chemicals can spread into the water table or into waterways.
• Landfill can attract animals and insects to come such as raccoons, rats, mosquitoes, cockroaches, and seagulls.
• Landfill can also cause sicknesses, illnesses, and diseases which might spread in communities.
• Landfill can increase the chances of global warming by releasing methane, a dangerous greenhouse gas.
• Landfills are taking up lots of our land and that can also take away habitats for other animals.
• Landfills contain a lot of kitchen scraps and organic material. As the landfill is constantly being covered with new garbage the organic material decomposes anaerobically (that is, without air).
• Anaerobic decomposition produces methane, which is a 20 times more dangerous greenhouse gas than carbon dioxide.

SANITARY LANDFILL

Advantages:

• volume can increase with little addition of people/equipment
• filled land can be reused for other community purposes

Disadvantages:

• completed landfill areas can settle and requires maintenance
• requires proper planning, design, and operation

INCINERATION

Advantages:

• requires minimum land
• can be operated in any weather
• produces stable odor-free residue
• refuse volume is reduced by half

Disadvantages:

• expensive to build and operate
• high energy requirement
• requires skilled personnel and continuous maintenance
• unsightly - smell, waste, vermin

Bioremediation

Bioremediation is a waste management technique that involves the use of organisms to remove or neutralize pollutants from a contaminated site.^[1] According to the EPA, bioremediation is a “treatment that uses naturally occurring organisms to break down hazardous substances into less toxic or non toxic substances”. Technologies can be generally classified as in situ or ex situ. In situ bioremediation involves treating the contaminated material at the site, while ex situ involves the removal of the contaminated material to be treated elsewhere. Some examples of bioremediation related technologies are phytoremediation, bioventing, bioleaching, landfarming, bioreactor, composting, bioaugmentation, rhizofiltration, and biostimulation.

2. How does it work?

Microorganisms must be active and healthy in order for bioremediation to take place. Bioremediation technologies assist microorganisms' growth and increase microbial populations by creating optimum environmental conditions for them to detoxify the maximum amount of contaminants. The specific bioremediation technology used is determined by several factors, for instance, the type of microorganisms present, the site conditions, and the quantity and toxicity of contaminant chemicals. Different microorganisms degrade different types of compounds and survive under different conditions.

Indigenous microorganisms are those microorganisms that are found already living at a given site. To stimulate the growth of these indigenous microorganisms, the proper soil temperature, oxygen, and nutrient content may need to be provided.

If the biological activity needed to degrade a particular contaminant is not present in the soil at the site, microorganisms from other locations, whose effectiveness has been tested, can be added to the contaminated soil. These are called exogenous microorganisms. The soil conditions at the new site may need to be adjusted to ensure that the exogenous microorganisms will thrive.

Bioremediation can take place under aerobic and anaerobic conditions. In aerobic conditions, microorganisms use available atmospheric oxygen in order to function. With sufficient oxygen, microorganisms will convert many organic contaminants to carbon dioxide and water. Anaerobic conditions support biological activity in which no oxygen is present so the microorganisms break down chemical compounds in the soil to release the energy they need. Sometimes, during aerobic and anaerobic processes of breaking down the original contaminants, intermediate products that are less, equally, or more toxic than the original contaminants are created.

Advantages

There are a number of cost/efficiency advantages to bioremediation, which can be employed in areas that are inaccessible without excavation. For example, hydrocarbon spills (specifically, petrol spills) or certain chlorinated solvents may contaminate groundwater, and introducing the appropriate electron acceptor or electron donor amendment, as appropriate, may significantly reduce contaminant concentrations after a long time allowing for acclimation. This is typically much less expensive than excavation followed by disposal elsewhere, incineration or other ex situ treatment strategies, and reduces or eliminates the need for "pump and treat", a practice common at sites where hydrocarbons have contaminated clean groundwater.

Phytoremediation

Phytoremediation is a bioremediation process that uses various types of plants to remove, transfer, stabilize, and/or destroy contaminants in the soil and groundwater.

Limitations and Concerns

The toxicity and bioavailability of biodegradation products is not always known.

Disposal of harvested plants can be a problem if they contain high levels of heavy metals.

The depth of the contaminants limits treatment.

The success of phytoremediation may be seasonal, depending on location. Other climatic factors will also influence its effectiveness.

The success of remediation depends in establishing a selected plant community.

If contaminant concentrations are too high, plants may die.

Some phytoremediation transfers contamination across media, (e.g., from soil to air).

Phytoremediation is not effective for strongly sorbed contaminants such as polychlorinated biphenyls (PCBs).

Phytoremediation requires a large surface area of land for remediation.

 

Applicability

Phytoremediation is used for the remediation of metals, radionuclides, pesticides, explosives, fuels, volatile organic compounds (VOCs) and semi-volatile organic compounds (SVOCs). It may be used to cleanup contaminants found in soil and groundwater.

Wastewater Characteristics

I.  Physical characteristics                           

A. total solids content: solid residual matter that remains after evaporation.

By dividing this value into two sub-categories described below: suspended solids and filterable solids.

1. suspended solids: this includes solids that are basically larger than1 micron in diameter.

When we flush the toilet, the fecal wastes have to go somewhere! Such wastes are, of course, part of the suspended solids. These wastes break up fairly quickly due to the action of water during transport. While fecal wastes are certainly part of this group, there may be many other sources (e.g., industrial wastes) depending on the sources of wastewater.

settleable solids: removed by gravity (usually in about one hour).

This is a sub-category of suspended solids. It is useful for us to know how much of the suspended solids will settle out in an hour, because these wastes are the easiest to remove.

2. filterable solids: This includes solids that are basically less than 1 micron in diameter.

These solids are much more difficult to remove from wastewater, because smaller particulates can remain suspended in water for a much longer time. In fact, they can pass through wastewater treatment facilities relatively unchanged. They generally fall into two sub-categories: colloidal and dissolved solids.

colloidal solids: This generally includes solids between 1 - 100 millimicrons in diameter.

dissolved solids: This generally includes wastes less than 1 millimicron in diamter.

 II. Chemical characteristics                                       

This refers to the amount of oxygen it takes to break down these organic wastes. In other words, instead of measuring all of the individual organic compounds, we have a general indicator (oxygen) for the strength of these wastes.

A. BOD (Biochemical Oxygen Demand)      

a measure of dissolved oxygen used by micro-organisms in the biological oxidation of organic matter.                

BOD is perhaps the most relevant measure of organics, because most treatment methods for organic wastes involve the use of microbes to digest the wastes.

B. COD (Chemical Oxygen Demand)         

a measure of dissolved oxygen used by an chemical oxidizing agent  (potassium dichromate) in the chemical oxidation of organic matter.

COD is faster than BOD, because a powerful chemical oxidizing agent can oxidize organics faster than microbes. This makes it a more convenient measure, but less relevant to many treament methods.

COD is generally higher than BOD, because there are some organics that cannot be digested by microbes, but can be oxidized by powerful chemical agents.

C. TOD (Total Oxygen Demand)            

a measure of oxygen used in the incineration (physical oxidation) of organic matter.

This is typically achieved by injecting wastes into a platinum catalyzed combustion chamber, where we measure the amount of oxygen present before and after incineration. The incineration techniques are the fastest of all. It's a useful measurement technique, but in terms of practical treatment methods, we would never actually be able to incinerate millions of gallons of wastewater every day.

D. TOC (Total Organic Carbon)

a measure of the carbon that remains after the incineration (physical oxidation) of organic matter.           

Instead of using oxygen as the unit of measure, carbon becomes the unit of measure for this method, mostly from carbon dioxide that remains after complete combustion. We can measure this by infrared techniques. TOC is often used in the various models that predict the fate of different wastes, because it represents a more direct measure of the organics (i.e., all organics are composed of carbon).

Biochemical Oxygen Demand (BOD)

Biochemical Oxygen Demand, BOD, as it is commonly abbreviated, is one of the most important and useful parameters (measured characteristics) indicating the organic strength of a wastewater.  BOD measurement permits an estimate of the waste strength in terms of the amount of dissolved oxygen required to break down the wastewater.  The BOD test is one of the most basic tests used in the wastewater field.  It is essentially a measure of the biological and the chemical component of the waste in terms of the dissolved oxygen needed by the natural aerobic biological systems in the wastewater to break down the waste under defined conditions.  Generally the BOD test is carried out by determining the dissolved oxygen on the wastewater or a diluted mixture at the beginning of the test period, incubating the wastewater mixture at 20°C, and determining the dissolved oxygen at the end of 5 days.  The difference in dissolved oxygen between the initial measurement and the fifth day measurement represents the biochemical oxygen demand. 

While BOD describes the biological oxidation capacity of a wastewater, it is not a measure of the total potential oxidation of the organic compounds present in the wastewater.  A number of chemical tests are used to measure this parameter, either in terms of the oxygen required for virtually complete oxidation, or in terms of the element carbon.  Probably the most common test for estimating industrial wastewater strength is the Chemical Oxygen Demand (COD) Test.  This test essentially measures the chemical oxidation of the wastewater by a strong oxidizing agent in an acid solution.  The value for the COD test is always greater than the BOD test and is not always a good indication of BOD values for the same waste. 

Chemical Oxygen Demand (COD)

 

The Chemical Oxygen Demand (COD) test measures the oxygen equivalent consumed by organic matter in a sample during strong chemical oxidation. Chemical oxygen demand is measured as a standardized laboratory assay in which a closed water sample is incubated with a strong chemical oxidant under specific conditions of temperature and for a particular period of time. A commonly used oxidant in COD assays is potassium dichromate (K₂Cr₂O₇) which is used in combination with boiling sulfuric acid (H₂SO₄). Because this chemical oxidant is not specific to oxygen-consuming chemicals that are organic or inorganic, both of these sources of oxygen demand are measured in a COD assay.

Materials required

• Water sample

• Potassium dichromate solution (0.1N)

• Sodium sulphate (0.1N)- 15.811g of sodium thiosulphate in 100litres of distilled water.

• Potassium iodide solution      

• Starch solution (1%)

• Water bath

• Titration apparatus

• 100ml conical flask

Procedure

1.         Water sample is collected from a pond.

2.         Poured 10ml of water sample in three 100 ml conical flask labeled as Test1, Test2, and Test3.

3.         Simultaneously the distilled water is taken in three 100ml conical flask labeled as Blank1, Blank2 and Blank3. 

4.         Added 5ml of potassium dichromate solution in each of the six conical flasks

5.         Keep the flask in water bath at 100°C (boiling temperature) for 1 hour

6.         Allowed the samples to cool for 10 minutes

7.         Added 5ml of potassium iodide in each flasks

8.         Added 10ml of sulphuric acid in each flask

9.         titrated the contents of each flask with 0.1N Sodium thiosulphate until the blue color disappear completely

The advantages of the COD test as compared to the BOD test are: 

1. COD results are available much sooner.
2. The COD test requires fewer manipulations of the sample. 
3. The COD test oxidizes a wider range of chemical compounds. 
4. It can be standardized more easily.  

The major disadvantage of the COD test is that the results are not directly applicable to the 5-day BOD results without correlation studies over a long period of time.  The samples used for the COD analysis may be grab or composite.  Preservation of the sample can be accomplished by adding sulfuric acid to depress the pH to 2 and the holding time with preservation is 7 days. 

Total organic carbon

Total Organic Carbon (TOC) is a sum measure of the concentration of all organic carbon atoms covalently bonded in the organic molecules of a given sample of water. TOC is typically measured in Parts Per Million (ppm or mg/L), although some industries require more refined measurements expressed in parts per Parts Per Billion (ppb or µg/L), or even Parts Per Trillion (ppt).

As a sum measurement, Total Organic Carbon does not identify specific organic contaminants. It will, however, detect the presence of all carbon-bearing molecules, thus identifying the presence of any organic contaminants, regardless of molecular make-up.

A typical analysis for TOC measures both the Total Carbon (TC) as well as Inorganic Carbon (IC, or carbonate). Subtracting the Inorganic Carbon from the Total Carbon yields TOC. (TC-IC=TOC). Another common variant of TOC analysis involves removing the inorganic carbon by purging the acidified sample with carbon-free air prior to measurement, then measuring the remaining carbon. This measurement is more accurately called non-purgeable organic carbon (NPOC).

Measurement

Whether the analysis of TOC is by TC-IC or NPOC methods, it may be broken into three main stages:

1. Acidification
2. Oxidation
3. Detection and Quantification

Acidification

Addition of acid and inert-gas sparging allows all bicarbonate and carbonate ions to be converted to carbon dioxide, and this IC product vented along with any POC that was present.

Oxidation

The second stage is the oxidation of the carbon in the remaining sample in the form of carbon dioxide (CO₂) and other gases. Modern TOC analyzers perform this oxidation step by several processes:

1. High Temperature Combustion
2. High temperature catalytic oxidation (HTCO)
3. Photo-oxidation alone
4. Thermo-chemical oxidation
5. Photo-chemical oxidation
6. Electrolytic Oxidation

High temperature combustion

Prepared samples are combusted at 1,350 °C in an oxygen-rich atmosphere. All carbon present converts to carbon dioxide, flows through scrubber tubes to remove interferences such as chlorine gas, and water vapor, and the carbon dioxide is measured either by absorption into a strong base then weighed, or using an Infrared Detector.^[8] Most modern analyzers use non-dispersive infrared (NDIR) for detection of the carbon dioxide.

High temperature catalytic oxidation

A manual or automated process injects the sample onto a platinum catalyst at 680 °C in an oxygen rich atmosphere. The concentration of carbon dioxide generated is measured with a non-dispersive infrared (NDIR) detector.^[9]

Photo-oxidation (ultraviolet light)

In this oxidation scheme, ultra-violet light alone oxidizes the carbon within the sample to produce CO₂. The UV oxidation method offers the most reliable, low maintenance method of analyzing TOC in ultra-pure waters.

Ultraviolet/persulfate oxidation

Like the photo-oxidation method, UV light is the oxidizer but the oxidation power of the reaction is magnified by the addition of a chemical oxidizer, which is usually a persulfate compound.

Thermochemical persulfate oxidation

Also known as heated persulfate, the method utilizes the same free radical formation as UV persulfate oxidation except uses heat to magnify the oxidizing power of persulfate. Chemical oxidation of carbon with a strong oxidizer, such as persulfate, is highly efficient, and unlike UV, is not susceptible to lower recoveries caused by turbidity in samples.

Detection and quantification

Accurate detection and quantification are the most vital components of the TOC analysis process. Conductivity and non-dispersive infrared (NDIR) are the two common detection methods used in modern TOC analyzers.

Conductivity

There are two types of conductivity detectors, direct and membrane. Direct conductivity provides an all-encompassing approach of measuring CO₂. This detection method uses no carrier gas, is good at the parts per billion (ppb) ranges, but has a very limited analytical range. Membrane conductivity relies upon the filtering of the CO₂ prior to measuring it with a conductivity cell. Both methods analyze sample conductivity before and after oxidization, attributing this differential measurement to the TOC of the sample. During the sample oxidization phase, CO₂ (directly related to the TOC in the sample) and other gases are formed. The dissolved CO₂ forms a weak acid, thereby changing the conductivity of the original sample proportionately to the TOC in the sample.

Non-dispersive infrared (NDIR)

The non-dispersive infrared analysis (NDIR) method offers the only practical interference-free method for detecting CO₂ in TOC analysis. The principal advantage of using NDIR is that it directly and specifically measures the CO₂ generated by oxidation of the organic carbon in the oxidation reactor, rather than relying on a measurement of a secondary, corrected effect, such as used in conductivity measurements.

Factors affecting dissolved oxygen

 

Dissolved oxygen is a molecule of O2 that is dissolved into the water. It is invisible to our naked eye. It is not the bubbles in water, nor the oxygen component of the water molecule H2O. Dissolved oxygen can get into the water two ways, through atmospheric oxygen mixing into a stream in turbulent areas or by the release of oxygen from aquatic plants during photosynthesis.

 

All animals need oxygen to survive. Dissolved oxygen is what makes aquatic life possible. Changes in oxygen concentration may affect species dependent on oxygen-rich water, like many macroinvertebrate species. Without sufficient oxygen they may die, disrupting the food chain.

 

    * Aquatic life-animals living in water use up dissolved oxygen. Bacteria take up oxygen as they decompose materials. Dissolved oxygen levels drop in a water body that contains a lot of dead, decomposing material.

    * Elevation-the amount of oxygen in the atmosphere decreases as elevation increases. Since streams get much of their oxygen from the atmosphere streams at higher elevations will generally have less oxygen.

    * Salinity (saltiness)-Salty water holds less oxygen than fresh water.

    * Temperature-cold water holds more dissolved oxygen than warmer water.

    * Turbulence-more turbulence creates more opportunities for oxygen to enter streams.

    * Vegetation-riparian vegetation directly affects dissolved oxygen by releasing oxygen into the water during photosynthesis.  It indirectly affects dissolved oxygen concentrations because vegetation shading a stream may decrease water temperatures and as temperature decreases dissolved oxygen increases.

Clearing land (e.g., construction, logging)-may send excess organic matter into streams. Organic matter is decomposed by microorganisms, which use up oxygen in this process. Therefore, if there is a lot of organic waste in the stream the microorganisms use more oxygen than can be replaced in the stream.

Destruction of riparian areas (e.g., development or overgrazing) decreases the amount of shade and increases the water temperature. Warmer water holds less DO than colder water.

Dissolved or suspended solids- Oxygen is more easily dissolved into water with low levels of dissolved or suspended solids. As the amount of salt in any body of water increases, the amount of dissolved oxygen decreases.

Amount of nutrients in the water -Nutrients are food for algae, and water with high amounts of nutrients can produce algae in large quantities. When these algae die, bacteria decompose them, and use up oxygen. This process is called eutrophication. DO concentrations can drop too low for fish to breathe, leading to fish kills. However, nutrients can also lead to increased plant growth. This can lead to high DO concentrations during the day as photosynthesis occurs, and low DO concentrations during the night when photosynthesis stops and plants and animals use the oxygen during respiration. Organic Wastes -Organic wastes are the remains of any living or once-living organism. Organic wastes that can enter a body of water include leaves, grass clippings, dead plants or animals, animal droppings, and sewage. Organic waste is decomposed by bacteria; these bacteria remove dissolved oxygen from the water when they breathe. If more food (organic waste) is available for the bacteria, more bacteria will grow and use oxygen, and the DO concentration will drop.

Groundwater Inflow -The amount of groundwater entering a river or stream can influence oxygen levels. Groundwater usually has low concentrations of DO, but it is also often colder than stream water. Therefore, groundwater may at first lower the DO concentration, but as groundwater cools the stream or river, the ability of the water to hold oxygen improves.

OXYGEN SAG CURVE

DO sag equation is used in the study of water pollution as a water quality modelling tool. The model describes how dissolved oxygen (DO) decreases in a river or stream along a certain distance by degradation of biochemical oxygen demand (BOD). The equation is also known as the Streeter–Phelps equation.

A graph of the measured concentrations of Dissolved Oxygen in water samples collected (1) upstream from a significant Point Source (PS) of readily degradable organic material (pollution), (2) from the area of the discharge, and (3) from some distance downstream from the discharge, plotted by sample location. The amount of dissolved oxygen is typically high upstream, diminishes at and just downstream from the discharge location (causing a sag in the line graph) and returns to the upstream levels at some distance downstream from the source of pollution or discharge.

DO sag equation determines the relation between the dissolved oxygen concentration and the biological oxygen demand over time and is a solution to the linear first order differential equation^[1]

This differential equation states that the total change in oxygen deficit (D) is equal to the difference between the two rates of deoxygenation and reaeration at any time.

DO sag equation, assuming a perfectly mixed stream at steady state is then

where

• is the saturation deficit, which can be derived from the dissolved oxygen concentration at saturation minus the actual dissolved oxygen concentration (). has the dimensions .
• is the deoxygenation rate, usually in .
• is the reaeration rate, usually in .
• is the initial oxygen demand of organic matter in the water, also called the ultimate BOD (BOD at time t=infinity). The unit of is .
• is the oxygen demand remaining at time t, .
• is the initial oxygen deficit .
• is the elapsed time, usually .

Activated Sludge Process

 

Activated sludge refers to a mass of microorganisms cultivated in the treatment process to break down organic matter into carbon dioxide, water, and other inorganic compounds. The activated sludge process has three basic components: 1) a reactor in which the microorganisms are kept in suspension, aerated, and in contact with the waste they are treating; 2) liquid-solid separation; and 3) a sludge recycling system for returning activated sludge back to the beginning of the process.

 

Primary effluent (or plant influent) is mixed with return activated sludge to form mixed liquor. The mixed liquor is aerated for a specified length of time. During the aeration the activated sludge organisms use the available organic matter as food producing stable solids and more organisms. The suspended solids produced by the process and the additional organisms become part of the activated sludge. The solids are then separated from the wastewater in the settling tank. The solids are returned to the influent of the aeration tank (return activated sludge). Periodically the excess solids and organisms are removed from the system (waste activated sludge). Failure to remove waste solids will result in poor performance and loss of solids out of the system over the settling tank effluent weir.

There are a number of factors that affect the performance of an activated sludge treatment system. These include:

• temperature
• return rates
• amount of oxygen available
• amount of organic matter available
• pH
• waste rates
• aeration time
• wastewater toxicity

To obtain desired level of performance in an activated sludge system, a proper balance must be maintained between the amount of food (organic matter), organisms (activated sludge) and oxygen (dissolved oxygen).

Activated Sludge Modifications

Many activated sludge process modifications exist. Each modification is designed to address specific conditions or problems. Such modifications are characterized by differences in mixing and flow patterns in the aeration basin, and in the manner in which the microorganisms are mixed with the incoming wastewater.

The major process modifications of the activated sludge process are:

1. conventional
2. tapered aeration
3. complete mix
4. step aeration
5. contact stabilization
6. extended aeration
7. pure oxygen systems

Advantages: Efficient removal of BOD, COD and nutrients when designed and professionally operated according to local requirements. The process itself has flexibility and numerous modifications can be tailored to meet specific requirements (e.g. for nitrogen removal). Activated sludge is the best documented and most widely used form of secondary wastewater treatment.

Disadvantages: Expensive in terms of both capital and O&M costs, requires a constant energy supply, needs trained operators who can monitor the system and react to changes immediately, and the availability of spare parts and chemicals may be an obstacle. The track record of activated sludge plants in the developing world is very poor, and few operate as designed or intended.

Sedimentation Tank designing

 

Sedimentation is a physical water treatment process using gravity to remove suspended solids from water.^[1] Solid particles entrained by the turbulence of moving water may be removed naturally by sedimentation in the still water of lakes and oceans. Settling basins are ponds constructed for the purpose of removing entrained solids by sedimentation.^[2] Clarifiers are tanks built with mechanical means for continuous removal of solids being deposited by sedimentation.

The limit sedimentation velocity of a particle is its theoretical descending speed in clear and still water. In settling process theory, a particle will settle only if:

1. In a vertical ascending flow, the ascending water velocity is lower than the limit sedimentation velocity.
2. In a longitudinal flow, the ratio of the length of the tank to the height of the tank is higher than the ratio of the water velocity to the limit sedimentation velocity.

Designs

Although sedimentation might occur in tanks of other shapes, removal of accumulated solids is easiest with conveyor belts in rectangular tanks or with scrapers rotating around the central axis of circular tanks.^[7] Settling basins and clarifiers should be designed based on the settling velocity of the smallest particle to be theoretically 100% removed. The overflow rate is defined as:

Overflow rate (V_o ) = Flow of water (Q (cubic metre per second)) /(Surface area of settling basin (A) )(m^2)

The unit of overflow rate is usually feet per second, a velocity. Any particle with settling velocity (Vs) greater than the overflow rate will settle out, while other particles will settle in the ratio Vs/Vo. There are recommendations on the overflow rates for each design that ideally take into account the change in particle size as the solids move through the operation:

• Quiescent zones: 0.031 ft/s
• Full-flow basins: 0.013 ft/s
• Off-line basins: 0.0015 ft/s^[8]

However, factors such as flow surges, wind shear, scour, and turbulence reduce the effectiveness of settling. To compensate for these less than ideal conditions, it is recommended doubling the area calculated by the previous equation.^[8] It is also important to equalize flow distribution at each point across the cross-section of the basin. Poor inlet and outlet designs can produce extremely poor flow characteristics for sedimentation.^{[citation needed]}

Settling basins and clarifiers can be designed as long rectangles (Figure 1.a), that are hydraulically more stable and easier to control for large volumes. Circular clarifiers (Fig. 1.b) work as a common thickener (without the usage of rakes), or as upflow tanks (Fig. 1.c).

Sedimentation efficiency does not depend on the tank depth. If the forward velocity is low enough so that the settled material does not re-suspend from the tank floor, the area is still the main parameter when designing a settling basin or clarifier, taking care that the depth is not too low.

Applications

Potable water treatment

Sedimentation in potable water treatment generally follows a step of chemical coagulation and flocculation, which allows grouping particles together into flocs of a bigger size. This increases the settling speed of suspended solids and allows settling colloids.

Waste water treatment

Sedimentation has been used to treat wastewater for millennia

Wastewater Treatment Ponds

Wastewater treatment using ponds can be an economical way of treatment which produces effluent that is highly purified. The number and the type of ponds used are the determining factors as to the degree of treatment that is provided.

Another name for wastewater treatment ponds is waste stabilization ponds. We use waste stabilization ponds because these ponds help to stabilize the wastewater before it is passed on to receiving water. They can also be referred to as oxidation ponds or sewage lagoons .

The waste stabilization pond is a biological treatment process , where bacteria use organic matter in the wastewater as food. The three types of bacteria at work in most ponds are the aerobic, anaerobic, and the facultative bacteria.

Facultative lagoons are a type of stabilization pond used for biological treatment of industrial and domestic wastewater. Sewage or organic waste from food or fiber processing may be catabolized in a system of constructed ponds where adequate space is available to provide an average waste retention time exceeding a month. A series of ponds prevents mixing of untreated waste with treated wastewater and allows better control of waste residence time for uniform treatment efficiency.

Facultative ponds are used the most to treat municipal wastewater. The ponds are usually 4 to 6 feet deep and the sludge at the bottom is anaerobic, while the 1 to 2 feet of the top of the pond is aerobic. In the middle, the amount of dissolved oxygen varies and either aerobic or anaerobic decomposition will take place, depending on how much dissolved oxygen is available.

Trickling Filter

A Trickling Filter is a fixed bed, biological filter that operates under (mostly) aerobic conditions. Pre-settled wastewater is ‘trickled’ or sprayed over the filter. As the water migrates through the pores of the filter, organics are degraded by the biomass covering the filter material.



The Trickling Filter is filled with a high specific surface-area material such as rocks, gravel, shredded PVC bottles, or special pre-formed filter-material. Pre-treatment is essential to prevent clogging and to ensure efficient treatment. The pre-treated wastewater is ‘trickled’ over the surface of the filter. Organisms that grow in a thin bio-film over the surface of the media oxidize the organic load in the wastewater to carbon dioxide and water while generating new biomass.

The incoming wastewater is sprayed over the filter with the use of a rotating sprinkler. In this way, the filter media goes through cycles of being dosed and exposed to air. However, oxygen is depleted within the biomass and the inner layers may be anoxic or anaerobic.

The filter is usually 1 to 3m deep but filters packed with lighter plastic filling can be up to 12m deep. The ideal filter material has a high surface to volume ratio, is light, durable and allows air to circulate. Whenever it is available, crushed rock or gravel is the cheapest option. The particles should be uniform such that 95% of the particles have a diameter between 7 and 10cm. Both ends of the filter are ventilated to allow oxygen to travel the length of the filter. A perforated slab that allows the effluent and excess sludge to be collected supports the bottom of the filter.

With time, the biomass will grow thick and the attached layer will be deprived of oxygen; it will enter an endogenous state, will lose its ability to stay attached and will slough off. High-rate loading conditions will also cause sloughing. The collected effluent should be clarified in a settling tank to remove any biomass that may have dislodged from the filter. The hydraulic and nutrient loading rate (i.e. how much wastewater can be applied to the filter) is determined based on the characteristics of the wastewater, the type of filter media, the ambient temperature, and the discharge requirements.

Types

• the treatment of small individual residential or rural sewage
• large centralized systems for treatment of municipal sewage
• systems applied to the treatment of industrial wastewater.

 

 

Advantages	Disadvantages/limitations
- Can be operated at a range of organic and hydraulic loading rates. - Small land area required compared to Constructed Wetlands.	- High capital costs and moderate operating costs - Requires expert design and construction. - Requires constant source of electricity and constant wastewater flow. - Flies and odours are often problematic. - Not all parts and materials may be available locally. - Pre-treatment is required to prevent clogging. - Dosing system requires more complex engineering.

 

Phosphorous removal from wastewater

 

Controlling phosphorous discharged from municipal and industrial wastewater treatment plants is a key factor in preventing eutrophication of surface waters. Phosphorous is one of the major nutrients contributing in the increased eutrophication of lakes and natural waters. Its presence causes many water quality problems including increased purification costs, decreased recreational and conservation value of an impoundments, loss of livestock and the possible lethal effect of algal toxins on drinking water.

Phosphate removal is currently achieved largely by chemical precipitation, which is expensive and causes an increase of sludge volume by up to 40%. An alternative is the biological phosphate removal (BPR).

 

Phosphorous removal processes

 

The removal of phosphorous from wastewater involves the incorporation of phosphate into TSS and the subsequent removal from these solids. Phosphorous can be incorporated into either biological solids (e.g. micro organisms) or chemical precipitates.

 

Phosphate precipitation

 

Chemical precipitation is used to remove the inorganic forms of phosphate by the addition of a coagulant and a mixing of wastewater and coagulant. The multivalent metal ions most commonly used are calcium, aluminium and iron.

 

Calcium:

 

it is usually added in the form of lime Ca(OH)2. It reacts with the natural alkalinity in the wastewater to produce calcium carbonate, which is primarily responsible for enhancing SS removal.

 

Ca(HCO3)2 + Ca(OH)2 à 2CaCO3 ↓+ 2H2O

 

As the pH value of the wastewater increases beyond about 10, excess calcium ions will then react with the phosphate, to precipitate in hydroxylapatite:

 

10 Ca2+ + 6 PO43- + 2 OH- ↔ Ca10(PO4)*6(OH)2 ↓

 

Because the reaction is between the lime and the alkalinity of the wastewater, the quantity required will be, in general, independent of the amount of phosphate present. It will depend primarily on the alkalinity of the wastewater. The lime dose required can be approximated at 1.5 times the alkalinity as CaCO3. Neutralisation may be required to reduce pH before subsequent treatment or disposal. Recarbonation with carbon dioxide (CO2) is used to lower the pH value.

 

Aluminium and Iron:

 

Alum or hydrated aluminium sulphate is widely used precipitating phosphates and aluminium phosphates (AlPO4). The basic reaction is:

 

Al3+ + HnPO43-n ↔ AlPO4 + nH+

 

This reaction is deceptively simple and must be considered in light of the many competing reactions and their associated equilibrium constants and the effects of alkalinity, pH, trace elements found in wastewater. The dosage rate required is a function of the phosphorous removal required. The efficiency of coagulation falls as the concentration of phosphorous decreases. In practice, an 80-90% removal rate is achieved at coagulant dosage rates between 50 and 200 mg/l. Dosages are generally established on the basis of bench-scale tests and occasionally by full-scale tests, especially if polymers are used. Aluminium coagulants can adversely affect the microbial population in activated sludge, especially protozoa and rotifers, at dosage rates higher than 150 mg/l. However this does not affect much either BOD or TSS removal, as the clarification function of protozoa and rotifers is largely compensated by the enhanced removal of SS by chemical precipitation.

 

Ferric chloride or sulphate and ferrous sulphate also know as copperas, are all widely used for phosphorous removal, although the actual reactions are not fully understood. The basic reaction is:

 

Fe3+ + HnPO43-n ↔ FePO4 + nH+

 

Ferric ions combine to form ferric phosphate. They react slowly with the natural alkalinity and so a coagulant aid, such as lime, is normally add to raise the pH in order to enhance the coagulation.

 

 

Aerobic Digestion

Aerobic digestion of waste is the natural biological degradation and purification process in which bacteria that thrive in oxygen-rich environments break down and digest the waste.

During oxidation process, pollutants are broken down into carbon dioxide (CO 2 ), water (H 2 O), nitrates, sulphates and biomass (microorganisms). By operating the oxygen supply with aerators, the process can be significantly accelerated. Of all the biological treatment methods, aerobic digestion is the most widespread process that is used throughout the world.

Biological and chemical oxygen demand

Aerobic bacteria demand oxygen to decompose dissolved pollutants. Large amounts of pollutants require large quantities of bacteria; therefore the demand for oxygen will be high.

The Biological Oxygen Demand (BOD) is a measure of the quantity of dissolved organic pollutants that can be removed in biological oxidation by the bacteria. It is expressed in mg/l.

The Chemical Oxygen Demand (COD) measures the quantity of dissolved organic pollutants than can be removed in chemical oxidation, by adding strong acids. It is expressed in mg/l.

The BOD/COD gives an indication of the fraction of pollutants in the wastewater that is biodegradable.

Advantages of Aerobic Digestion

Aerobic bacteria are very efficient in breaking down waste products. The result of this is; aerobic treatment usually yields better effluent quality that that obtained in anaerobic processes. The aerobic pathway also releases a substantial amount of energy. A portion is used by the microorganisms for synthesis and growth of new microorganisms.

Path of Aerobic Digestion

Anaerobic Digestion

Anaerobic digestion is a complex biochemical reaction carried out in a number of steps by several types of microorganisms that require little or no oxygen to live. During this process, a gas that is mainly composed of methane and carbon dioxide, also referred to as biogas, is produced. The amount of gas produced varies with the amount of organic waste fed to the digester and temperature influences the rate of decomposition and gas production.  

Anaerobic digestion occurs in four steps:

•  Hydrolysis : Complex organic matter is decomposed into simple soluble organic molecules using water to split the chemical bonds between the substances.

•  Fermentation or Acidogenesis: The chemical decomposition of carbohydrates by enzymes, bacteria, yeasts, or molds in the absence of oxygen.

•  Acetogenesis: The fermentation products are converted into acetate, hydrogen and carbon dioxide by what are known as acetogenic bacteria.

•  Methanogenesis: Is formed from acetate and hydrogen/carbon dioxide by methanogenic bacteria.

The acetogenic bacteria grow in close association with the methanogenic bacteria during the fourth stage of the process. The reason for this is that the conversion of the fermentation products by the acetogens is thermodynamically only if the hydrogen concentration is kept sufficiently low. This requires a close relationship between both classes of bacteria.

The anaerobic process only takes place under strict anaerobic conditions. It requires specific adapted bio-solids and particular process conditions, which are considerably different from those needed for aerobic treatment.

Path of Anaerobic Digestion  

Advantages of Anaerobic Digestion

Wastewater pollutants are transformed into methane, carbon dioxide and smaller amount of bio-solids. The biomass growth is much lower compared to those in the aerobic processes. They are also much more compact than the aerobic bio-solids.

The Anaerobic Digestion

Anaerobic digestion is the most common process for dealing with wastewater sludge containing primary sludge.  Primary sludge is the solids which settle out of the wastewater in the sedimentation tanks just after the wastewater passes through the grit chambers. 
Secondary sludge from the clarifier is also sent to the digester.  Secondary sludge is generated when the over flow from the settling tanks goes into the aeration chambers and the aerobic bacteria convert the dissolved organics into carbon dioxide, water and solids.  The solids settle out in the clarifier.

Anaerobic digestion is preferred to reduce the high organic loading of primary sludge because of the rapid growth of the biomass that would ensue if the sludge were treated aerobically.  Anaerobic decomposition creates considerably less biomass than the aerobic process.  Anaerobic digestion converts as much of the sludge as possible to end products, such as, liquid and gases while producing as little residual biomass as possible.

An anaerobic sludge digester is designed to encourage the growth of anaerobic bacteria, particularly the methane producing bacteria that decreases organic solids by reducing them to soluble substances and gases, mostly carbon dioxide and methane.

There are three basic stages in anaerobic digestion.  The first stage is production of carbon dioxide and organic acids from fermentation.  The second stage is metabolizing of organic acids to hydrogen, carbon dioxide and other organic acids.  The third stage utilizes the products of the preceeding stages to produce methane from carbon dioxide, hydrogen and acetic acid.

Anaerobic Digestion Process

The purpose of the anaerobic process is to convert sludge to end products of liquid and gases while producing as little biomass as possible.  The process is much more economical than aerobic digestion.

It was originally thought that anaerobic digestion was accomplished in three stages:  (1)  hydrolysis of insoluble polymers, (2)  fermentation of monomeric breakdown products and (3)  generation of methane. 

The process has now been described by the following four steps:

1. Hydrolysis:  large polymers are broken down by enzymes.
2. Fermentation:  Acidogenic fermentations are most important, acetate is the main end product.  Volatile fatty acids are also produced along with carbon dioxide and hydrogen.
3. Acetogenesis:  Breakdown of volatile acids to acetate and hydrogen.
4. Methanogenesis:  Acetate, formaldehyde, hydrogen and carbon dioxide are converted to methane and water.  

When operating properly, the digester receives sludge, primary and secondary, from the other treatment processes.  The sludge is then held in the tank for 10 to 90 days depending on the system.  The sludge goes into the digester, methane, carbon dioxide and traces of hydrogen sulfide go out the gas outlet, supernatant, from the water generated by the process and the water in the sludge, is drawn off as necessary and sent back through the plant and stabilized sludge is pulled off the bottom to go to the drying beds.


               


Reactors for anaerobic digesters consist of closed tanks with air tight covers.  Treatment plants processing less than 4000 cubic meters/day of wastewater often use standard rate digestion for economic reasons or simplicity of operation.  Sludge separates in the reactor as shown, although some mixing occurs in the zone of active digestion and in the supernatant because of withdrawal and return of heated sludge.  Sludge is fed to the reactor on an intermittent basis and the supernatant is withdrawn and returned to the secondary treatment unit.  The digested sludge accumulates in the bottom to await removal to sludge disposal facilities.

 
     


    High rate digesters are more efficient and often require less volume than single stage digesters.  In the first stage the sludge is mechanically mixed to ensure better contact between the organics and the bacteria.  The unit is heated to increase the metabolic rate of the microorganisms, thus speeding up the digestion process.  In the second stage the sludge is allowed to stratify and separate into layers.  Little gas is generated in the second stage.  The second stage has a floating cap and is equipped for gas recovery.  The second stage is not heated because gas production doesn't occur in this stage.  The supernatant, scum and digested sludge are drawn out of this unit.

Comparison of anaerobic and aerobic digestion

The following article is a comparison of aerobic and anaerobic digestion. In both aerobic and anaerobic systems the growing and reproducing microorganisms within them require a source of elemental oxygen to survive.^[1]

In an anaerobic system there is an absence of gaseous oxygen. In an anaerobic digester, gaseous oxygen is prevented from entering the system through physical containment in sealed tanks. Anaerobes access oxygen from sources other than the surrounding air. The oxygen source for these microorganisms can be the organic material itself or alternatively may be supplied by inorganic oxides from within the input material. When the oxygen source in an anaerobic system is derived from the organic material itself, then the 'intermediate' end products are primarily alcohols, aldehydes, and organic acids plus carbon dioxide. In the presence of specialised methanogens, the intermediates are converted to the 'final' end products of methane, carbon dioxide with trace levels of hydrogen sulfide.^[2] In an anaerobic system the majority of the chemical energy contained within the starting material is released by methanogenic bacteria as methane.^[3]

Anaerobic compositing is an expensive process to complete. It requires continual introduction of large quantities of feedstock in order for the process to work efficiently. This is one of the reasons that it generates large quantities of methane gas as the food waste decomposes. That methane gas is not only highly combustible, methane gas is one of the most potent greenhouse gasses on the planet. Further, as this gas builds up within the system, the pressures within make it highly explosive and a safety hazard that must be monitored closely.

Additionally, as the compost is broken down by anaerobic digestion, it creates a sludge-like material that is even more difficult to break down. This requires time and considerable amounts of energy to accomplish. As a matter of fact, it can take up to a year before an anaerobic composter can fully break down the raw material into a viable compost.

In an aerobic system, such as composting, the microorganisms access free, gaseous oxygen directly from the surrounding atmosphere. The end products of an aerobic process are primarily carbon dioxide and water which are the stable, oxidised forms of carbon and hydrogen. If the biodegradable starting material contains nitrogen, phosphorus and sulfur, then the end products may also include their oxidised forms- nitrate, phosphate and sulfate.^[1] In an aerobic system the majority of the energy in the starting material is released as heat by their oxidisation into carbon dioxide and water.^[3]

The process of aerobic digestion that takes place within in-vessel aerobic composters is very similar to the process that occurs without any human assistance in nature. However, instead of taking place on the forest floor beneath the pitter patter of hooves and the like, the process takes place in a container that is easily monitored and maintained.

As aerobic digestion within the in-vessel composter takes place, the byproducts are simply heat, water, and carbon dioxide (CO₂). While CO₂ is a greenhouse gas, it is at least 1/20th as potent as methane. To minimize the impact on the environment, the CO₂ gas can be safely collected via a gas collection system that will prevent the gas from seeping out into the environment.

Naturally, one of the most important benefits of aerobic compositing is that the heat which is produced during the decomposition process is great enough that it kills harmful bacteria and pathogens within the pile. This is not the heat of Hades or Phoenix in July, but rather it ranges between 55F and 140F, and it usually lasts for just a few days or so. While this heat is killing the harmful bacteria, it is also facilitating the growth of beneficial bacteria species including psychrophilic, mesophilic, and thermophilic bacteria which thrive at the higher temperature levels. These bacteria facilitate a healthy biomass that plants feed on and thrive in.

Composting systems typically include organisms such as fungi that are able to break down lignin and celluloses to a greater extent than anaerobic bacteria.^[4] Due to this fact it is possible, following anaerobic digestion, to compost the anaerobic digestate allowing further volume reduction and stabilisation.^[5]

WHAT IS BIOFIFLTRATION?

Biofiltration is a relatively new pollution control technology. It is an attractive technique for the elimination of malodorous gas emissions and of low concentrations of volatile organic compounds (VOCs).

The most common style biofilter is just a big box. Some can be as big as a basketball court or as small as one cubic yard. A biofilter’s main function is to bring microorganisms into contact with pollutants contained in an air stream. The box that makes up this biofilter contains a filter material, which is the breeding ground for the microorganisms. The microorganisms live in a thin layer of moisture, the "biofilm", which surrounds the particles that make up the filter media. During the biofiltration process, the polluted air stream is slowly pumped through the biofilter and the pollutants are absorbed into the filter media. The contaminated gas is diffused in the biofilter and adsorbed onto the biofilm. This gives microorganisms the opportunity to degrade the pollutants and to produce energy and metabolic byproducts in the form of CO₂ and H₂O.

This biological degradation process occurs by oxidation, and can be written as follows:

Organic Pollutant + O₂ � CO₂ + H₂O + Heat + Biomass

Biofiltration process

A biofilter is a bed of media on which microorganisms attach and grow to form a biological layer called biofilm. Biofiltration is thus usually referred to as a fixed–film process. Generally, the biofilm is formed by a community of different microorganisms (bacteria, fungi, yeast, etc.), macro-organisms (protozoa, worms, insect’s larvae, etc.) and extracellular polymeric substances (EPS) (Flemming and Wingender, 2010). The aspect of the biofilm^[5] is usually slimy and muddy.

Water to be treated can be applied intermittently or continuously over the media, upflow or downflow. Typically, a biofilter has two or three phases, depending on the feeding strategy (percolating or submerged biofilter):

1.a solid phase (media);

2.a liquid phase (water);

3.a gaseous phase (air).

Organic matter and other water components diffuse into the biofilm where the treatment occurs, mostly by biodegradation. Biofiltration processes are usually aerobic, which means that microorganisms require oxygen for their metabolism. Oxygen can be supplied to the biofilm, either concurrently or counter currently with water flow. Aeration occurs passively by the natural flow of air through the process (three phases biofilter) or by forced air supplied by blowers.

Microorganisms' activity is a key-factor of the process performance. The main influencing factors are the water composition, the biofilter hydraulic loading, the type of media, the feeding strategy (percolation or submerged media), the age of the biofilm, temperature, aeration, etc.

Advantages of Biofiltration

   Biofiltration has many advantages over traditional VOC's and HAP destruction methods.

1. VOC's and HAP's are Oxidized at Ambient Temperatures - The low temperature oxidation eliminates the high costs associated with combustion.
2. Intrinsically Safe - The low temperature oxidation and high moisture content eliminate the fears associated with combustion.
3. Low Annual Operating Cost - There are only two major power consumers in a biofiltration system: a recirculation pump for humidification and a fan to pull the gas stream through the equipment.
4. Low Pressure Drop - Much lower pressure drop than catalytic or regenerative thermal oxidizers resulting in fan power consumption savings.
5. Proven Effective Technology - Biofiltration systems are in place with historical operating experience demonstrating regulatory compliance.
6. Environmentally Friendly - Zero NOx emissions, Zero SOx emissions and substantially lower Carbon Dioxide emissions.
7. Low Maintenance - Very few moving parts result in lower maintenance cost.

Disadvantages of Biofiltration

1. Biofiltration cannot successfully treat some organic compounds, which have low adsorption or degradation rates. This is especially true for chlorinated VOCs.
2. Contaminant sources with high chemical emissions would require large biofilter units or open areas to install a biofiltration system.
3. Sources with emissions that fluctuate severely or produce large spikes can be detrimental to the of a biofilter’s microbial population and overall performance.
4. Acclimation periods for the microbial population may take weeks or even months, especially for VOC treatment

COMMERCIAL APPLICATIONS

There have been over 50 commercial biofilters using compost-type material installed in Europe and the United States over the past 15 years.

VOC applications to date have included the following industries:

• Chemical and petrochemical industry
• Oil and gas industry
• Synthetic resins
• Paint and ink
• Pharmaceutical industry
• Waste and wastewater treatment
• Soil and Groundwater remediation

Odor abatement applications to date have included the following industries.

• Sewage treatment
• Slaughter houses
• Rendering
• Gelatin and glue plants
• Agricultural and meat processing
• Tobacco, cocoa and sugar industry
• Flavor and fragrance

Modes of biogas purification

Whatever the valorisation of biogas (heat or electricity), the presence of a very large variety of components in the biogas, of which some, like the hydrogen sulphide have poisonous and corrosive properties which impose the implementation of biogas purification.

General

Of the many processes traditionally and presently employed, that have been used for large-scale desulphurization of technical gases, only the so-called "dry" process is suitable on a smaller scale for biogas plants. They are acceptable from the point of view of technical complexity and maintenance and the degree of purification is satisfactory.

The desulphurization of biogas is based on a chemical reaction of H₂S with a suitable substance.

The lime process

The oldest process is the desulphurization of gases with quick lime, slaked lime in solid form or with slaked lime in liquid form. The process using quick or slaked lime has not been applied on a large scale for a long time. The large amounts of odourous residue that are produced cannot be satisfactorily disposed of. The handling of large amounts of dissolved or suspended slaked lime requires elaborate equipment.

Large concentrations of CO₂ which are present in biogas make the satisfactory removal of H₂S difficult. The CO₂ also reacts with the quick and slaked lime and uses it up quickly. The Ca(HCO₃)₂ formed reacts with Ca(SH)₂ which is formed by the reaction of H₂S with Ca(OH)₂ thus resulting in the reoccurance of H₂S. However, a large scale biogas plant in Germany with the cogeneration of heat and power has recently been constructed using a lime purifier. The results of long term tests are not yet available.
In as far as enough lump, quick lime is available in the countries concerned, this process could be considered for desulphurization. The apparatus for utilizing quick lime corresponds in construction and function to that used for the desulphurization with iron-containing substances.

Ferrous materials

Ferrous materials in the form of natural soils or certain iron ores are often employed to remove H₂S.

 

 

Principle

The ferrous material is placed in a closed, gas tight container (of steel, brickwork or concrete). The gas to be purified flows through the ferrous absorbing agent from the bottom and leaves the container at the top, freed from H₂S.

Chemistry

The absorbing material must contain iron in the form of oxides, hydrated oxides or hydroxides. These react as follows:

2 Fe(OH)₃ + 3 H₂S à Fe₂S₃ + 6 H₂O Fe(OH)₂ + H₂S à FeS + 2H₂O

This process terminates, of course, after some time. The greater part of the iron is then present as a sulphide.

Regeneration

However, by treating the sulphidized absorbent with atmospheric oxygen, the iron can be returned to the active oxide form required for the purification of the gas:

2 Fe₂S₃ + 3 O₂ + 6 H₂O à 4 Fe(OH)₃ +3 S₂
2 FeS + O₂ + 2 H₂O à 2 Fe(OH)₂ + S₂

The used absorbent can, therefore, be "regenerated". This regeneration cannot be repeated indefinitely. After a certain time the absorbent becomes coated with elementary sulphur and its pores become clogged. Purifying absorbents in gasworks (coke plants) acquire a sulphur content of up to 25% of their original weight, i.e. 40% sulphur by dry weight.

Process techniques

There are three different, dry desulphurizing processes available.

Without regeneration

The purification chamber consists of a box or drum. The absorbent is placed inside it on several, intermediate trays (sieve floors) to ensure that the depth of the absorbent is not more than 20-30 cm. Otherwise the absorbent would easily press together causing an increase in the resistance to the gas flow.

The biogas is fed in at the bottom of the box, flows through the absorbent and leaves the purification chamber at the top, freed from H₂S. When the absorbent becomes loaded with iron sulphides, the gas leaving the chamber contains increasingly more H₂S. The chamber is then opened at the top and the trays with the spent absorbent are removed. Then fresh absorbent is placed on the trays.

After the air in the purification chamber has again been displaced with biogas, the gas connection to the user is re-opened.
The spent absorbent is disposed of as described under the heading "Disposal of spent absorbent".

With regeneration

The spent, sulphide containing absorbent can also be regenerated by exposing it to oxygen. This can either be done by taking the used absorbent out of the chamber and exposing it to the air, or inside the purification chamber by simply sucking ambient air through it.
Since regeneration inside the chamber requires precautions against the formation of unwanted and dangerous air-gas mixtures and would require powerful fans, regeneration outside the chamber is usually preferred. The absorbent that is to be regenerated, is spread out on the ground in as thin a layer as possible. From time to time it is turned over with a shovel. After a few days it is ready for use again.
This regeneration process can be repeated up to ten times, after which the absorbent is finally spent.

Simultaneous regeneration and loading

Simultaneous regeneration and loading of the absorbent is a special case. Here a certain, small amount of air is added to the biogas. Then sulphide formation and regeneration occur at the same time and place. As such, the absorbent acts effectively as a catalyst.
Expensive gas-measuring and mixing equipment is required for this process, however, so that it is not suitable for small biogas plants.

Eutrophication

Eutrophication (Greek: eutrophia—healthy, adequate nutrition, development; German: Eutrophie) or more precisely hypertrophication, is the ecosystem response to the addition of artificial or natural substances, mainly phosphates, through detergents, fertilizers, or sewage, to an aquatic system.^[1] One example is the "bloom" or great increase of phytoplankton in a water body as a response to increased levels of nutrients. Negative environmental effects include hypoxia, the depletion of oxygen in the water, which may cause death to aquatic animals.

When algae die, they decompose and the nutrients contained in that organic matter are converted into inorganic form by microorganisms. This decomposition process consumes oxygen, which reduces the concentration of dissolved oxygen. The depleted oxygen levels in turn may lead to fish kills and a range of other effects reducing bio-diversity.

Eutrophication is one of the causes of the deterioration of water quality. In the North Sea and the English Channel, this is mainly due to human activities. Nutrients can have a natural or anthropogenic origin and come from:

• Domestic wastewater
• Industrial waste
• Agriculture (fertilizer use)Atmospheric deposition of nitrogen (livestock and gases)

The overload of nitrogen, phosphorus and other organic material can result in a series of 'side effects'. The main effects of eutrophication are:

• Increasing biomass of phytoplankton resulting in 'algal blooms'.
• Hypoxia (reduced dissolved oxygen content of a body of water).
• An increasing number of incidents of fish kills.
• The water can have a bad taste, color and odeur which has a negative impact on tourism. Governments have to invest more in waste water treatment.
• Decline or loss of species biodiversity (commercially important species may disappear).
• Some phytoplankton species produce toxins that cause severe symptoms such as diarrhea, memory loss, paralysis and in severe causes death.

Biomagnification

Biomagnification, also known as bioamplification or biological magnification, occurs when the concentration of a substance, such as DDT or mercury, in an organism exceeds the background concentration of the substance in its diet.^[1] This increase can occur as a result of:

• Persistence – where the substance can't be broken down by environmental processes
• Food chain energetics – where the substance concentration increases progressively as it moves up a food chain
• Low or non-existent rate of internal degradation or excretion of the substance – often due to water-insolubility

Biological magnification often refers to the process whereby certain substances such as pesticides or heavy metals move up the food chain, work their way into rivers or lakes, and are eaten by aquatic organisms such as fish, which in turn are eaten by large birds, animals or humans. The substances become concentrated in tissues or internal organs as they move up the chain. Bioaccumulants are substances that increase in concentration in living organisms as they take in contaminated air, water, or food because the substances are very slowly metabolized or excreted.

Classic example: DDT

DDT stands for dichloro, diphenyl trichloroethane.  It is a chlorinated hydrocarbon, a class of chemicals which often fit the characteristics necessary for biomagnification.

The figure shows how DDT becomes concentrated in the tissues of organisms representing four successive trophic levels in a food chain.

The concentration effect occurs because DDT is metabolized and excreted much more slowly than the nutrients that are passed from one trophic level to the next. So DDT accumulates in the bodies (especially in fat). Thus most of the DDT ingested as part of gross production is still present in the net production that remains at that trophic level.

This is why the hazard of DDT to nontarget animals is particularly acute for those species living at the top of food chains.

Lapse rate

The lapse rate is defined as the rate at which atmospheric temperature decreases with increase in altitude. The terminology arises from the word lapse in the sense of a decrease or decline. While most often applied to Earth's troposphere, the concept can be extended to any gravitationally supported ball of gas.

lapse rate, rate of change in temperature observed while moving upward through the Earth’s atmosphere. The lapse rate is considered positive when the temperature decreases with elevation, zero when the temperature is constant with elevation, and negative when the temperature increases with elevation (temperature inversion). The lapse rate of nonrising air—commonly referred to as the normal, or environmental, lapse rate—is highly variable, being affected by radiation, convection, and condensation; it averages about 6.5 °C per kilometre (18.8 °F per mile) in the lower atmosphere (troposphere). It differs from the adiabatic lapse rate, which involves temperature changes due to the rising or sinking of an air parcel. Adiabatic lapse rates are usually differentiated as dry or moist.

In general, a lapse rate is the negative of the rate of temperature change with altitude change, thus:

where is the lapse rate given in units of temperature divided by units of altitude, T = temperature, and z = altitude.

Types of lapse rates

There are two types of lapse rate:

• Environmental lapse rate – which refers to the actual change of temperature with altitude for the stationary atmosphere (i.e. the temperature gradient)
• The adiabatic lapse rates – which refer to the change in temperature of a parcel of air as it moves upwards (or downwards) without exchanging heat with its surroundings. There are two adiabatic rates:^[6]
• Dry adiabatic lapse rate
• Moist (or saturated) adiabatic lapse rate

Dry deposition

In aerosol physics, deposition is the process by which aerosol particles collect or deposit themselves on solid surfaces, decreasing the concentration of the particles in the air. It can be divided into two sub-processes: dry and wet deposition. Dry deposition is a continuous process, while wet removal can be realized only in the presence of precipitation. Therefore, despite of the dry process is slower than wet deposition, the accumulated removal quantity of a pollutant could be more important in case of dry deposition. Both the dry and wet depositions depend on the properties of the gases or particles like : near-surface concentration, physical and chemical properties of tracer, weather condition, soil, surface, vegetation properties, chemical reactions.

Dry deposition is caused by:

• Gravitational sedimentation – the settling of particles fall down due to gravity.
• Interception. This is when small particles follow the streamlines, but if they flow too close to an obstacle, they may collide (e.g. a branch of a tree).
• Impaction. This is when small particles interfacing a bigger obstacle are not able to follow the curved streamlines of the flow due to their inertia, so they hit or impact the droplet. The larger the masses of the small particles facing the big one, the greater the displacement from the flow streamline.
• Diffusion or Brownian motion. This is the process by which aerosol particles move randomly due to collisions with gas molecules. Such collisions may lead to further collisions with either obstacles or surfaces. There is a net flux towards lower concentrations.
• Turbulence. Turbulent eddies in the air transfer particles which can collide. Again, there is a net flux towards lower concentrations.
• Other processes, such as: thermophoresis, turbophoresis, diffusiophoresis and electrophoresis.

Inversion (meteorology)

In meteorology, an inversion is a deviation from the normal change of an atmospheric property with altitude. It almost always refers to a "temperature inversion," i.e., an increase in temperature with height, or to the layer ("inversion layer") within which such an increase occurs.^[1]

An inversion can lead to pollution such as smog being trapped close to the ground, with possible adverse effects on health. An inversion can also suppress convection by acting as a "cap." If this cap is broken for any of several reasons, convection of any moisture present can then erupt into violent thunderstorms. Temperature inversion can notoriously result in freezing rain in cold climates.

Causes of Temperature Inversions

Normally, air temperature decreases at a rate of 3.5°F for every 1000 feet (or roughly 6.4°C for every kilometer) you climb into the atmosphere. When this normal cycle is present, it is considered an unstable air mass and air constantly flows between the warm and cool areas. As such the air is better able to mix and spread around pollutants.

During an inversion episode, temperatures increase with increasing altitude. The warm inversion layer then acts as a cap and stops atmospheric mixing. This is why inversion layers are called stable air masses.

Temperature inversions are a result of other weather conditions in an area. They occur most often when a warm, less dense air mass moves over a dense, cold air mass. This can happen for example, when the air near the ground rapidly loses its heat on a clear night. In this situation, the ground becomes cooled quickly while the air above it retains the heat the ground was holding during the day. Additionally, temperature inversions occur in some coastal areas because upwelling of cold water can decrease surface air temperature and the cold air mass stays under warmer ones.

 

Consequences of Temperature Inversions

Some of the most significant consequences of temperature inversions are the extreme weather conditions they can sometimes create. One example of these is freezing rain. This phenomenon develops with a temperature inversion in a cold area because snow melts as it moves through the warm inversion layer. The precipitation then continues to fall and passes through the cold layer of air near the ground. When it moves through this final cold air mass it becomes "super-cooled" (cooled below freezing without becoming solid). The super-cooled drops then become ice when they land on items like cars and trees and the result is freezing rain or an ice storm.

Intense thunderstorms and tornadoes are also associated with inversions because of the intense energy that is released after an inversion blocks an area’s normal convection patterns.