pollut

ENVIRONMENTAL POLLUTION CONTROL METHODS (11 – OE310)

• Introduction to Environmental Pollution Control Methods

• Water Pollution• Wastewater treatment• Air Pollution• Solid waste• Noise Pollution

• Briefing on Competencies acquired

• Sources, Types and Effects of Water pollutants:• Sources• Domestic wastewater (or) Municipal Sewage

Residential Buildings, Hotels, Educational institutions, Functional Halls etc.,

Industrial wastewater (or) Effluent – Industries –Bread , Beer, Milk Products, Wood pulp , Cotton bleaching, Chemicals, Sulphuric acid, Gasoline and Steel

Types of Water Pollutants• 1. Oxygen demanding wastes• 2. Disease-causing agents• 3. Synthetic organic compounds• 4. Plant nutrients• 5. Inorganic chemicals and minerals• 6. Sediments• 7. Radioactive substances• 8. Thermal discharges• 9. Oil

Effects:• Oxygen demanding waste:Dissolved oxygen (DO) is essential for sustaining the plant and animal life in any aquatic system. For example, warm-water fish requires a minimum DO level of at least 5mg/l (5ppm). If the DO level drops below the level necessary to sustain normal life, then the aquatic system is classified as polluted. There are four processes which actually affect the DO content in the water : reaeration, photosynthesis, respiration and the oxidation of wastes. Biochemical Oxygen DemandThe biochemical oxygen demand (BOD) is a measure of the oxygen utilised by microorganisms during the oxidation of organic materials.It is the most widely known measure for assessing the water pollution potential of a given organic waste.

Disease-causing agents

• The pathogens are carried in to the water bodies by sewage and wastes from farms and various industries, specially tanning and meat packaging industries. Contact with the pathogens can be made by drinking the water or through other activities involving contact with water.• Some bacteria are water-borne, and these include those responsible for

causing cholera, typhoid, amoebic dysentery and gastroenteritis. Viruses are also found in water, including strains, which are responsible for polio, infectious hepatitis and coxsackie fever.• The coliform group of bacteria are considered reliable indicators of the

hygienic quality of water.• A large concentration of E.coli in the water indicates faecal contamination

and a presumptive evidence of the presence of pathogens

Synthetic organic compounds

• These include pesticides, synthetic organic chemicals and detergents.• These compounds, in contrast to the organic wastes, are not biodegradable and

may persist for long periods.• These are of great concern to environmentalists because most of the synthetic

organic compounds are accumulative toxic poisons and ultimately may reach objectionable levels in water or in aquatic life.• Analyses of polluted waters show the presence of a wide variety of these

compounds and many others are probably not being detected. Of these, two (pesticides and deteregents) have been subjected to intensive study.

Pesticides: DDT, dieldrin and aldrin are hazardous mainly due to their concentration in the food chain Detergents: The basic active ingredient in detergents is the surfactant

or surface-active agent.It lowers the surface tension and allows dirt particles to become linked to water.

Plant nutrients

• Nitrogen and Phosphorus are essential elements which are required by plants and animals for maintaining their growth and metabolism.• These compounds may enter the water bodies directly from the

manufacture and use of fertilisers, and from the processing of biological materials such as food and textiles, or via domestic sewage treatment plants.• Thus, when unusually large concentrations of nutrients are present in

water bodies, an excess growth of algae, known as an algal bloom appears. This provides an unsightly green slime layer reduces light penetration and restricts atmospheric reoxygenation of the water. The dense algal growth eventually dies and the subsequent biodegradation produces an oxygen deficit which can result in foul-smelling anaerobic conditions.

Inorganic chemicals and minerals

• This category of water pollutants includes inorganic salts, mineral acids, finely divided metals and metal compounds. These pollutants enter the water bodies from municipal and industrial wastewaters and mine runoff. Most of these substances are toxic and are capable of killing living organisms in the water bodies.• The toxic properties of numerous inorganic compounds, particularly

heavy metals have been known for years. The metals of particular concern in industrial wastewaters are cadmium, chromium, lead, mercury and silver.• The itai-itai disease in Japan was probably due to the transport of

cadmium-containing particulates in water to the irrigation fields.The permissible level for cadmium in drinking water supplies is 0.01 mg/l.• Many plant species are adversely affected by chromium concentrations

of 5mg/l.

• Lead is present in industrial effluents arising from battery manufacture, printing, painting and dyeing. Lead is toxic to aquatic organisms, but its effects are less pronounced than those of cadmium and mercury. The permissible limit of lead concentration in drinking water supplies is 0.05mg/l.• Mercury is discharged into natural waters from various industrial

effluents, such as those from the manufacture of paint and paper, chlorine and caustic soda, fertilisers and pesticides. Mercury has proved to be the most toxic aquatic pollutant because of its rapid methylation in the aquatic environment and in this form, it gets accumulated in the food chains.• The recommended drinking water standard for mercury is 0.002mg/l.

Sediments

• Sediments include soil, sand and mineral particles washed into the aquatic environment by storms and flood waters. In addition, large deposits of sewage sludge, pulverized coal ash, and various industrial solids are disposed off into rivers and marine waters.• In suspension, the solids may cause thickening of fish gills which may

lead to eventual asphyxiation of the fish.

Radioactive substances

• Very little is known about the threshold of radiation damage to aquatic environment from wastes of uranium and thorium mining and refining, from nuclear power plants, and from industrial use of radioactive materials.• Radioactive substances can enter humans with food and water, and

get accumulated in blood and certain vital organs like the thyroid gland, the liver and bone and muscular tissues.• At present some low and medium level wastes are sealed in

containers and dumped in to the ocean.

Thermal discharges

• Power plants and industry use large quantities of water for cooling purposes. Used coolant water is usually discharged directly into water bodies. This could result in increase in temperature of the water bodies with deleterious consequences for aquatic inhabitations.• The hot layer, which itself holds less oxygen than the cooler layer

below, prevents the replacement of oxygen in the cooler layer as it is denied contact with the atmosphere. The DO level falls rapidly due to normal biological functions in the lower layer and may lead to anaerobic conditions.

Oil

• Oil is an important commodity involved in some way or other in virtually every activity of contemporary life. Oil and oil wastes enter rivers and other water bodies from several sources like industrial effluents, oil refineries and storage tanks, automobile waste oil, and petrochemical plants.• An oil slick on the surface of water can prevent oxygen transfer from

the atmosphere and lead to very low DO levels in the water due to microbial oxidative attack on the hydrocarbon molecules.

Lecture 3 :Water quality standardsWater Quality Standards:

ICMR WHO USPHS A B A B A BPhysical: Turbidity 5 25 5 25 5 -Colour 5 25 5 50 15 -

OdourNothing disagreeable Unobjectionable TO = 3

Chemical: pH units 7-8.5 6.5 or 9.2 7-8.5 6.5 or 9.2 - -Total Solids - - 500 1500 500 -Calcium 75 200 75 200 - -Magnesium 50 150 50 150 - -Iron 0.3 1 0.3 1 0.3 -Manganese 0.1 0.5 0.1 0.5 0.05 -Copper 1 3 1 1.5 1 -Sulphate 200 400 200 400 250 -Phenols 0.001 0.002 0.001 0.002 0.001 -Fluorides 1 2 0.5 1.0-1.5 0.6-1.7 -Nitrates 20 50 - 50-100 45 -Toxic : Arsenic - 0.2 - 0.2 0.01 0.05Barium - - - - - 1Cadmium - - - - - 0.01Chromium - 0.05 - 0.05 - 0.05Cyanide - 0.01 - 0.01 0.01 0.2Lead - 0.1 - 0.1 - 0.05Selenium - 0.05 - 0.05 - 0.01Silver - - - - - 0.05

Bacteriological -

1 coliform per 100ml

-

1 coliform per 100 ml

-

1 coliform per 100 ml

ICMR Indian Council of Medical Research

WHO World Health Organisation

USPHS United States Public Health Services

Lecture 4:Measurement of pollution loads:Determination of Organic matter• Dissolved Oxygen (DO)• Bio Chemical Oxygen Demand (BOD)• Chemical Oxygen Demand (COD)• Total Organic Carbon (TOC)

Dissolved Oxygen (DO)

• Surface waters of good quality should be saturated with dissolved oxygen. A fall in DO level is one of the first indications that a water body is polluted by organic matter. The DO level in water depends on physical, chemical and biochemical activities prevailing in the water body and, thus, it is one of the important parameters for assessing the purity of the water body.• It is usually determined by Winkler’s method, which is based on the

reaction of dissolved oxygen with manganese ions to form a precipitate of manganese dioxide.• The interference problems can also be overcome by using DO

analyzers with membrane electrodes.

Biochemical Oxygen Demand (BOD)

The most widely used and accepted measure of biodegradable organic content of wastewater is the 5-day, 200C BOD value1. Two standard 300-ml BOD bottles are filled completely with the wastewater of which the BOD is to be measured and the bottles are sealed.2. Oxygen content of one bottle is determined immediately.3. The other bottle is incubated at 200c for 5days in total darkness, after which its oxygen content is measured.4. The difference between the two DO values is the amount of oxygen that is consumed by micro-organisms during the 5 days and is reported as the BOD5 value of the sample.

BOD5 (in mg/l) = D*[(DOt=0 – DOt=5)] – [(DOt=0 – DOt=5)]sample blank

where D* = dilution factor

Chemical Oxygen Demand (COD)• In the COD test, the oxidizing bacteria of the BOD test are replaced by a

strong oxidising agent under acidic conditions. • A sample of the wastewater containing organic material is mixed with

an excess of potassium dichromate and sulphuric acid and the mixture is heated under total reflux conditions for a period of 2 hours. • During digestion, the chemically oxidizable organic material reduces a

stoichiometrically equivalent amount of dichromate, the remaining dichromate is titrated with standard ferrous ammonium sulphate solution.• The amount of potassium dichromate reduced gives a measure of the

amount of oxidizable organic material. Dichromate has advantages over other oxidants in oxidising power and applicability to a wide variety of samples.

Total Organic Carbon (TOC)

• Total organic carbon test is based on the oxidation of the carbon of the organic matter to carbon dioxide, which is measured by a non-dispersive infrared analyser.

• Alternatively, the carbon dioxide can be reduced to methane, which is then measured by a flame ionisation detector.

Effluent discharge standardsTable: BIS (ISI) Standards for Discharge of Sewage and Industrial Effluents in Surface Water Sources and Public Sewers

Sl.No. Characteristic of the EffluentTolerance limit for Sewage Effluent

discharged into Surface Water sources, as per IS 4764-1973

Tolerance Limit for Industrial effluent discharged into

Inland surface waters, as per IS 2490-1974

Public sewers as per IS 3306-1974

1 BOD5 20 mg/l 30 mg/l 500 mg/l

2 COD - 250 mg/l -

3 pH value - 5.5 - 9.0 5.5 - 9.0

4 Total Suspended Solids (TSS) 30 mg/l 100 mg/l 600 mg/l

5 Temperature - 400C 450C

6 Oil and grease - 10 mg/l 100 mg/l

7 Phenolic compounds - 0.2 mg/l 2 mg/l

8 Arsenic - 0.2 mg/l -

9 Cadmium - 2 mg/l -

10 Copper - 3 mg/l 3 mg/l

11 Lead - 0.1 mg/l 1 mg/l

12 Chlorides - - 600 mg/l

13 Ammonical nitrogen (as N) - 50 mg/l 50 mg/l

14 insecticides - zero -

15 Total residual chlorine - 1 mg/l -

WASTEWATER TREATMENT

Pre-treatment : Screening, Grit removal Primary Treatment : Sedimentation tank, Floatation Secondary Treatment : Activated Sludge Process, Trickling filter and Secondary

Clarifier Tertiary Treatment : Sludge treatment

Typical Communiting – Wastewater treatment Plant

PRE TREATMENT PROCESS: SCREENINGThe pretreatment operation include to remove large floating and suspended solids which could interfere with the normal operation of subsequent treatment processes. The pre-treatment operation may also include flow measurement and sometimes prechlorination to prevent any odours that may emanate during subsequent processes.Pretreatment consists of screening and grit removal. Screens of various sizes and shapes are used, depending on the nature of solids to be removed, and cleaning is done either manually or mechanically. Fixed bar screens are the most common types of screens used in domestic wastewater treatment facilities.Bar screens are made up of parallel metal bars and have apertures in the range 25-50 mm for manually cleaned racks, and mechanically cleaned racks have openings ranging from 5 to 40mm. The channel approach velocities are designed to be in the range of 0.3 to 0.6m/s for hand-cleaned bar racks, and from 0.6 to 1.0 m/s for mechanically cleaned devices. A perforated drainage plate is provided at the top of the hand-cleaned racks where the rackings may be stored temporarily for drainage.The screening process produces objectionable screenings which must be disposed of in a satisfactory manner. Methods of disposal include burial, incineration, grinding and digestion.

Grit removal chamber:

• Grit chambers are provided to protect pumps from abrasion and to reduce the formation of heavy deposits in pipes and channels. There are two basic types of grit chambers: horizontal flow and aerated. • In the horizontal flow type chamber, the velocity of flow is controlled

by the dimensions of the unit or by placing weirs at the effluent end of the channel. • A velocity close to 0.3 m/s is maintained and this velocity is sufficient

to permit the settling of the grit material and allows the organic particles to pass through the chamber.• The length of the channel is governed by the depth required by the

settling velocity and the cross-sectional area is governed by the rate of flow and by the number of channels.

Primary treatment – Sedimentation Tank

In this step, the settle able solids are removed by gravitational settling under quiescent conditions. The sludge formed at the bottom of the tank is removed as underflow either by vacuum suction or by raking it to a discharge point at the bottom of the tank for withdrawal.The clear liquid produced is known as the overflow and it should contain no readily settleable matter. If the sedimentation tank is poorly designed the overflow may contain solid particles or the underflow may be more dilute than desired.The sedimentation operation in waste treatment applications may be carried out in rectangular horizontal flow, circular radial flow, or vertical flow basins.Figure shows the three main types of arrangements. In rectangular tanks, feed is introduced at one end along the width of the tank and the overflow is collected at the surface, either across the other end or at different points along the length of the tank. An endless conveyor scrapes the floating material into a screen though while it also pushes the settled solids into a sludge hopper.

• In the circular radial flow tanks, the feed is introduced through a centre well and the clarified effluent is collected at weirs along the periphery of the tank. Sludge removal is collected by means of rotary sludge scraper which forces the settled sludge down a slopping bottom in to a central hopper, from which it is withdrawn.• Scum is removed by a surface skimming board, which is attached to the rotary

mechanism and positioned in such a manner that the scum can be collected in to a trough situated at the surface.• Vertical flow tanks are often used in small treatment plants where the feed is

applied at a point or points along the bottom, and clarified effluent is collected at the top. A sludge blanket is maintained in the lower part of the tank through which the suspension rises. It is important to control the sludge withdrawal and bleed carefully to avoid losing the blanket, which acts as a filter for small particles.

Basic Theory and Design

• Two types of particle behaviour can be distinguished in sedimentation. These are the discrete particle and the flocculent particle behaviour. In discrete settling behaviour particles do not interfere with one another;and their sizes, shapes and masses remain constant during settling, whereas in flocculent behaviour the particles agglomerate during settling and thus do not have constant characteristics. The settling of particles in a suspension depends upon their concentration and their flocculating properties. Generally, three types of settling are recognized:

1. Settling of dilute suspensions of discrete particles2. Settling of dilute suspensions of flocculent particles3. Zone settling which includes hindered settling and compressive settling

Lecture 11 : Floatation

• Flotation may be used in place of sedimentation, primarily for treating industrial wastewaters containing finely divided suspended solids and oily matter. Flotation technique is used for treating of industrial wastewater.• Particles of density very close to that of water are very difficult to

settle in normal sedimentation tanks and take a long time for separation. In such cases, the separation can be speeded up by aerating the effluent whereby air bubbles are attached to the suspended particles float to the surface where they can be readily removed.• To aid in the flotation process, chemical coagulants such as aluminium

and ferric salts or polymer coagulant-aids are often used. These chemicals increase the flocculent structure of the floated particles so that they can easily entrap the air bubbles.

• Two methods of flotation are currently available: (1) dispersed air flotation and (2) dissolved-air flotationIn dispersed-air flotation, air is introduced directly into the liquid through a revolving impeller or through diffusers. The air bubbles generated in dispersed air flotation systems are normally about 1mm in diameter and they usually cause turbulence which breaks up fragile floc particle. Due to this, dispersed-air flotation is not a favoured technique in the treatment of municipal wastewaters, although it finds a limited application in treating industrial wastes containing oil, grease and fine powders.

In dissolved-air flotation, air is intimately brought into contact with the wastewater at a pressure of several atmospheres when air is dissolved.

Lesson 12: Decomposition of Organic Waste :

There are two important methods by which the organic matter could be decomposed:1. Aerobic process, in which oxygen is used by the microorganisms for the

decomposition2. Anaerobic process, in which oxygen is not used by the microorganisms

for the decomposition.

In aerobic decomposition, a wide spectrum of organic matter could be oxidised by the microorganisms resulting in very stable end products. The end products include CO2, H2O and new cell tissue. Most aerobic organisms are capable of high growth rates resulting in the generation of large amounts of biological sludge.

• Aerobic decomposition is suitable for large quantities of dilute wastewater whose BOD5 is generally less than 500mg/l. For High strength wastewater, aerobic decomposition is not recommended and anaerobic decomposition may be the preferred method.• Anaerobic decomposition is basically a two-step process. In the first step,

complex organic compounds are broken down and converted to low molecular weight fatty acids, the most common of which are acetic and propionic acids. • The microorganisms responsible for this conversion are facultative in nature

and are identified as “acid formers”.• In the second step, methanogenic bacteria, which are strict anaerobes,

convert the organic acids formed in the first step to methane gas and carbon dioxide. Unlike in the aerobic process, the cell production is relatively low resulting in low sludge generation. Anaerobic method is usually used to stabilise the sludge produced during the aerobic decomposition.

Lesson 13 : Role of Microorganisms

• A variety of microorganisms, mainly bacteria, is used in the stabilisation of organic matter. The microorganisms convert the colloidal and dissolved carbonaceous organic matter into various gases and into cell tissue known as protoplasm.• Since the density of the protoplasm is slightly higher than that of water,

it can be removed from the treated liquid by gravity settling. It is necessary to remove the cell tissue from the solution, otherwise, the tissue, which itself is organic in nature, will be measured as BOD in the effluent.

Bacterial Population DynamicsFor effective design and control of biological treatment processes, an understanding of the population dynamics of bacteria, the microorganisms of primary interest is essential. In batch cultures, the bacterial growth pattern has four distinct phases:(a) The lag phase(b) The log-growth phase(c) The stationary phase(d) The death phaseIn the lag phase (A), the bacteria initially acclimatize to their new surroundings and start synthesizing new cells. In the log-growth phase (B), the bacterial cells divide exponentially at a rate depending on their ability to process food and their generation time.

As the food for the bacteria becomes exhausted the log-growth phase tapers off and at some point the population remains constant as a result of a balance between the growth of new cells and the death of old cells. This phase is called the stationary phase (C). Finally, as the limitations in the food supply increase, the bacterial death rate exceeds the production of new cells. This is shown as the death phase (D) in the figure.

Growth Kinetics

• When dealing with large number of mixed cultures of microorganisms found in wastewater treatment systems, it is convenient to deal with biomass rather than bacterial numbers. In the batch culture discussed previously, the rate of increase of cell biomass in the log growth phase is defined by the following relationship:

rg = uXWhere X = Concentration of microbial mass, mg/l

u = Specific growth rate, s-1

rg= rate of growth of cell biomass, mg/l.s

Lesson 14: Revision

Lesson 15 : Secondary treatment - Biological Treatment Systems - Activated Sludge Process

• The process flow diagram for a typical activated sludge plant is shown in above figure. The essential features of the process are : an aeration stage, solids-liquid separation following aeration, and a sludge recycle system.• Wastewater after primary treatment enters an aeration tank where the

organic matter is brought into intimate contact with the sludge from the secondary clarifier.• This sludge is heavily laden with microorganisms which are in an active

state of growth. Air is introduced into the tank, either in the form of bubbles through diffusers or by surface aerators. The microorganisms utilize the oxygen in the air and convert the organic matter into stabilized, low-energy compounds such as NO3,SO4,CO2 and synthesize new bacterial cells.• The effluent from the aeration tank containing the flocculent microbial

mass, known as the sludge, is separated in a settling tank, sometimes called a secondary settler or a clarifier.

• In the settling tank, the separated sludge exists without contact with the organic matter and becomes activated. A portion of the activated sludge is recycled to the aeration tank as a seed; the rest is wasted.• If all the activated sludge is recycled, then the bacterial mass would keep

increasing to the stage where the system gets clogged with solids. It is, therefore, necessary to “waste” some of the microorganisms, and this wasted sludge is the one which is processed and disposed.

The mass balance for the microorganisms under steady-state conditions can be written as:[Rate of inflow of microorganisms in to the system] – [Rate of outflow of microorganisms from the system] + [Net rate of cell growth in the aeration tank] = 0 Equation may be written as:

QXo – [(Q-Qw)Xe + QwXr] + rg1V = 0

• Where Q= Volumetric flow rate of the influent, m3/d Qw = Volumetric flow rate of liquid containing the microorganisms

to be wasted, m3/d Xo = Concentration of microorganisms mass in the influent, mg/l

Xe = Concentration of microorganisms mass in the effluent, mg/l Xr = Concentration of microorganisms mass in waste sludge, mg/l V = Volume of the aeration tank, m3

Lesson 16 : Trickling Filters

• The second commonly used biological waste treatment process is the trickling filter method. It has good adaptability to handle peak shock loads and the ability to function satisfactorily after a short period of time. However, like all biological units, trickling filters are affected by temperature; therefore, cold weather slows down biological activity in the filter.• Conventional trickling filters normally consist of a rock bed, 1 to 3

meters in depth, with enough openings between rocks to allow air to circulate easily. The influent is sprinkled over the bed packing which is coated with a biological slime.• As the liquid trickles over the packing, oxygen and the dissolved

organic matter diffuse in to the film to be metabolised by the microorganisms in the slime layer.• End products such as CO2,NO3, etc., diffuse back, out of the film and

appear in the filter effluent.

• A portion of the clarified wastewater is recirculated to the top of the trickling filter; usually to dilute the high strength influent wastewater and to provide even distribution of wastewater over the packing material thereby increasing the contact efficiency.• Conventional trickling filters have been designed based on hydraulic and organic

loadings, and are classified as “standard or low-rate” or “high-rate” trickling filters.Eckenfleder proposed the following equation for predicting the performance of trickling filters:Se/Si = exp [ -KLAsm (A/Q)n]Where Se = concentration of settled effluent from the filter, mg/lSi = concentration of the influent to the filter, mg/lK = empirical rate constant, m/d ; Q = Waste water flow rate, m3/SAs = Specific surface area of the filter ; A = Cross sectional area of filter, m2

m,n = empirical constantsL = filter depth, m

Lesson 17 :Rotating - Biological Contactors

Lesson 18 : Secondary clarification

Lesson 19 : Tertiary treatment - Sludge treatment

• Handling and disposal of sludge from biological wastewater treatment plants is an important problem and represents about half the cost of most sewage treatment plants. • The concentration of solids in the primary sewage sludge is about 5% ; the

activated sludge contain less than 1% solids; and the sludge from trickling filters has about 2% solids. • This means that the sludge is composed almost entirely of water and

volume reduction is the key to economic disposal. In addition to reducing it high water content, the sludge must be stabilised so that its biological activity and tendency towards putrefaction are reduced drastically.• The common unit operations of sludge treatment and disposal involve

concentration or thickening, digestion, conditioning, dewatering, oxidation and safe disposal.

• Concentration: The purpose of concentration or thickening is to remove water from the sludge and reduce its volume as much as possible so that the sludge can be handled more efficiently. The common methods for thickening are gravity settling and flotation.• Digestion: After concentration, the sludge is stabilized by digesting it under

aerobic or anaerobic conditions. Anaerobic digestion is the most common method in which the organic content of the sludge decomposes to give mainly methane and carbon dioxide and at the same time the bound water is released from the sludge.• Conditioning: The sludge after stabilization may be conditioned to improve its

dewatering characteristics. This is done by adding chemicals like iron salts, alum, lime and polyelectrolytes. These chemicals bind the sludge particles together and encourage the release of absorbed water.• Dewatering: The thickened sludge is dewatered for efficient handling and

disposal. Dewatering is accomplished by mechanical methods, the most common being centrifugation and filtration, which includes pressure filtration and vacuum filtration.

Lesson 20: Low Cost Treatment Systems: Stabilization ponds

• This is the most frequently encountered type. These ponds have an aerobic upper zone and an anaerobic lower zone. Operation of a typical facultative pond is shown in above figure. The organic waste enters at one end of the pond where the suspended solids settle to the bottom.• At the bottom, an anaerobic layer develops and the settled sludge is

degraded by anaerobic organisms to produce CO2, NH3 and CH4. • In the upper zone aerobic bacterial degradation of the waste takes place.

A facultative zone exists between these two zones, which is generally variable. • It can be either aerobic or anaerobic at various times so that growth of

facultative organisms, which are able to adapt to either condition, is favoured.