The document describes the chemical oxygen demand (COD) test, which is used to quantify the amount of chemically oxidizable organic material in wastewater. In the COD test, a strong chemical oxidizing agent is used instead of bacteria to oxidize organic compounds. The COD test measures total oxidizable organic material rather than just biodegradable material, so COD results are higher than biochemical oxygen demand (BOD) tests on the same samples. The document also provides an overview of primary, secondary, and advanced wastewater treatment processes and describes processes like activated sludge treatment and trickling filters used in secondary biological treatment.

1. 1
Chemical Oxygen Demand (COD)
In the COD test, the oxidizing bacteria of the BOD test are replaced by a strong oxidizing agent
under acidic condition. 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 two 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 advantage over other oxidants in oxidizing power and applicability to a wide
variety of samples.
The COD test does not distinguished between organic materials that are biodegradable and those
that are not, and, hence, gives a measure of the total oxidizable organic material in the sample. Due
to this, the COD test results are higher than those of BOD tests carried out on the same samples. If
inorganic substances such as chlorides and nitrates are present in the wastewater, they interfere with
the COD test since they are also oxidized by dichromate and create an inorganic COD that leads to
an error in the measurement. Chloride interference can be eliminated by adding mercuric sulphate to
the sample prior to the addition of other reagents, and nitrite interference can be overcome by
adding sulphamic acid to the dichromate solution.
The COD test is much more useful than the BOD test for estimating the oxygen requirements of
certain industrial wastewaters. It is valuable for wastes where BOD test is not applicable due to the
presence of toxic substances, low rate of oxidation, or other similar factors. Ratios of BOD to COD
can be employed to get an indicatio9n of the degrees of biotreatability of the waste. Ratio of 0.8 or
higher indicates wastes that are highly amenable to biochemical treatment, while lower ratios
indicate that the wastes are not favorable to biological treatment.
Wastewater Treatment
The wastewater treatment processes are generally grouped as the primary treatment, the secondary
treatment, and the tertiary or the advanced waste treatment. Primary treatment removes suspended
solids and floating matters. In the secondary treatment, also known as the biological treatment,
organic matter that is soluble or in the colloidal form is removed. Advanced waste treatment may
involve physical, chemical or biological processes or their various combinations depending on the
impurities to be removed. These processes are employed to remove residual soluble non-
biodegradable organic compounds, including surfactants, inorganic nutrients, and salts, trace
contaminants of various types, and dissolved inorganic salts. The advanced waste treatment
processes are expensive, and are used only when waste produced is required to be of higher quality
than that produced by conventional secondary treatment so that the treated waste can be reclaimed
and put to some form of direct reuse.
Primary treatment
It comprises a pretreatment step and a sedimentation step.

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Pretreatment
Wastewater is pretreated to remove large floating and suspended solids which could interfere with
the normal operation of subsequent treatment processes. The pretreatment operation may also
include flow measurement and sometimes prechlorination to prevent any odors that may emanate
during subsequent processes. Pretreatment consists of screening and grit removal. Screens of
different sizes and shapes are used, depending on the nature of solids to be removed, and cleaning is
done either manually or mechanically. Bar screens made up of parallel metal bars and have
apertures in the range 25 to 50 mm. The channel approached velocities are in the range of 0.3 to 0.6
m/s. A perforated drainage plate is provided at the top of the racks where the rakings may be stored
temporarily for drainage. Screening process produces objectionable screenings which must be
disposed of in a satisfactory manner.
Fig. 1. Manually raked bar screen.
Methods of disposal include burial, incineration, grinding and digestion. To avoid the disposal
problems, some treatment plants use a device known as a comminutor instead of screens. The
comminutor grinds large solids which can then be satisfactorily handled in the sedimentation tank.
After screening, the wastewater enters a grit chamber for the removal of inorganic grit, consisting of
sand, gravel, cinders and pebbles. Grit chambers are provided to protect pumps from abrasion and
to reduce the formation of heavy deposits in pipes and channels.
Sedimentation
In this step, the settleable 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. 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. The conveyor scraper
scrapes the floating material into a screen though while it also pushes the settled solids into a sludge
hopper.
Inlet Outlet
Trough
Parallel bars

3. 3
Fig. 2. Rectangular sedimentation tank
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 effected by means of
a rotary sludge scrapper which forces the settled sludge down a slopping bottom into a central
hopper, from which it is withdrawn. Scum is removed by a surface skimming board, which is
attached to the rotary mechanism. Vertical flow tanks are often used in small treatment where the
feed is applied at a point or points along the bottom, and clarified effluent is collected at the top.
Inflow
Overflow
Scum trough
Sludge scrapper
Sludge underflow

4. 4
Fig. 3. Circular radial flow sedimentation tank
Fig. 4 Vertical flow sedimentation tank
Inflow
Overflow
Scraper
Sludge underflow
Sludge underflow
under
Sludge blanket
under
Inflow
Over flow

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Flotation
Flotation may be used in place of sedimentation primarily for treating industrial wastewaters
containing finely divided suspended solids and oily materials. Flotation technique is used in paper
industry to recover fine fibers from the screened effluent and in the oil industry for the clarification
oil-bearing waste. 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 matter. This
has the effect of increasing the buoyancy of the particles; as a result, the particles float to the surface
where they can be readily removed. To aid in the flotation process, chemical coagulants such as
aluminum and ferric salts or polymers 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
flotation. In the 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
usually about 1 mm in diameter and they usually cause turbulence which breaks up fragile flock
particles. Due to this, dispersed-air flotation is not a favored 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.
Secondary (Biological) Treatment
Much of the organic matter in wastewater is colloidal and dissolved solids, the primary treatment
processes are largely ineffective in removing it.
Decomposition of organic wastes
There are two important methods by which the organic matter could be decomposed.
1. Aerobic process, in which oxygen is used by microorganisms for the decomposition.
2. 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 oxidized by the
microorganisms resulting in very stable end products. The end products include CO2, H2O and new
cells. 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 BOD is generally less than 500 ppm. For high strength wastewater (BOD>1000
ppm), 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 acid 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 to methane gas and

6. 6
carbon dioxide. Unlike in the aerobic process, cell production is relatively low resulting in low
sludge formation.
Aerobic Biological Treatment
In biological treatment, oxygen supplied to the bacteria is consumed under controlled conditions so
that most of the BOD is removed in the treatment plant rather than in the watercourse. Thus, the
principle requirements of a biological waste treatment process are an adequate amount of bacteria
that feed on the organic material present in wastewater, oxygen and some means of achieving
contact between the bacteria and the organics.
Two most commonly used systems for biological waste treatment are the activated sludge system
and biological film system. In the activated sludge system the wastewater is brought into contact
with a diverse group of microorganisms in the form of a flocculent suspension in an aerated tank.
Whereas in the biological-film system, also known as trickling filters, the wastewater is brought
into intimate contact with a mixed microbial population in the form of a film of slime attached to
the surface of a solid support medium.
Activated Sludge Process
The process flow diagram for a typical activated sludge plant is given below.
1: Pretreatment, 2: Primary clarifier, 3: Aeration tank, 4; Secondary clarifier (Settling tank)
5: Thickener, 6: Sludge digestion
Fig. 5. Flow diagram of an activated sludge treatment plant
Liquid flow Sludge flow
1
Wastewater input
2 3 4
Inert solids
5
1
6 Stabilized sludge
Air
Effluent
Returned sludge

7. 7
Wastewater after primary treatment enters aeration tank, where the organic matter is brought out
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 diffuser or by surface aeration. The microorganisms utilize the oxygen in
the air and convert the organic matter into stabilized, low energy compounds such as NO3, SO4, and
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 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.
Basic theory and design
In the activated sludge system the major design parameter is the loading or the amount of organic
matter (food) added relative to the microorganisms (activated sludge) available. This ratio is known
as the food to microorganisms’ ratio (F/M). Unfortunately, measurement of either F or M accurately
is difficult and, hence, the ratio is usually expressed as the amount of BOD utilized per unit mass of
active biological solids. The combination of the liquid and microorganisms in the aeration tank is
known as "mixed liquor", and the suspended solids are called "mixed liquor suspended solids
(MLSS).
Sludge volume index (SVI)
The success of the activated sludge system depends on many factors, among which the separation of
the solid-liquid phase in the secondary settler is one of the more important ones. A conventional
measure of the settling ability of the sludge is the sludge volume index (SVI), defined as the volume
in mL occupied by one gram of sludge after it has settled in one litre cylinder for 30 min. The SVI
has units of mL/g and it can be calculated as follows:
SVI = (V /M) x 1000
Where,
V = sludge volume after settling for 30 min, mL/L
M = MLSS concentration, mg/L
SVI varies from 40 to 100 for a good sludge, but may exceed 200 for a poor sludge having a
tendency towards bulking.
Trickling filters
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. Milk processing, paper mill and
pharmaceutical wastes are among those treated by trickling filters. Conventional trickling filters
normally consist of a rock bed, 1 to 3 m 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 organics matter diffuse into

8. 8
the film to be metabolized 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. As the microorganisms utilize
the organic matter, the thickness of the slime layer or biofilm increases. Typical film thickness
ranges from 100 μm to 2 mm. If the biofilm thickness is large enough, all of the oxygen may be
deposited at some point in the film before the solid surface is reached. This creates anaerobic
conditions at the base of the film. As a result microorganisms near the support media enter into an
endogenous decay and lose their ability to cling to the solid media, and the film gets detached from
the surface. This process is known as sloughing. A settling tank following the trickling filter
removes the detached bacterial film and some suspended matter. A portion of the clarified
wastewater is recirculated to the top of the trickling filter, usually to dilute the high-strength influent
waste water and to provide even distribution of wastewater over the packing material thereby
increasing the contact efficiency
Fig. 6. A conventional trickling filter
Synthetic plastic materials have been used in recent times as packing media in trickling filters.
These filters are known as super-rate filters. The packing material is high and can be stacked many
times higher than conventional rock bed. It has a much higher degree of treatment capability
because of the increase in the available surface area and is mostly used for treating high-strength
wastes.
Sludge treatment and disposal
Handling and disposal of sludge from biological wastewater treatment plants is an important
problem and represents about half of the total cost of most sewage treatment plants. The
concentration of solids in the primary sewage sludge is about 5%, the activated sludge contains less
than 1% solids; and the sludge from trickling filters had about 2% solids.
Rocks covered by microbial slime
Feed
Effluent
Rotatingspray

9. 9
The sequence of operations for sludge treatment is shown in Fig. 7.
Fig. 7. Sequence of operations for sludge treatment
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 of thickening are gravity settling and flotation. Gravity settling can result 5 to 9% solid
while air flotation will give about 4% solid content.
Digestion
After concentration, the sludge is stabilized by digesting it under aerobic or anaerobic conditions.
Anaerobic digestion is the common method in which the organic content of the sludge decomposes
to give mainly methane and CO2 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 polyelectrolyte. These chemicals bind the
sludge particles together and encourage the release of absorbed water. Physical conditioning
methods such as heat treatment are becoming popular. The sludge is heated under pressure and after
a period of time the gel structure of the sludge breaks down so that the water is released. Heat
treatment has the advantage of sterilizing the sludge; at the same time the sludge is partially
oxidized and completely stabilized.
Dewatering
The thickened sludge is dewatered for efficient handling and disposal. Dewatering is accomplished
by mechanical methods, the most common being centrifugation and filtration. In centrifugation,
conditioned sludge is added to a rotating bowl that separates the sludge into a cake and a dilute
stream. The solid cake is transported within the bowl and is removed by a screw conveyor at one
end of the bowl; the liquid is removed at the opposite end. Filtration, using plate-and-frame pressure
filter or rotating drum vacuum filter, is widely used for dewatering digested sludge.
Oxidation
Before the final disposal, some sludge may be oxidized to reduce the organic content, with the
consequent destruction of bacteria and a significant reduction in their volumes. Incineration and wet
oxidation are the two common methods employed for sludge oxidation.
Sludge Concentration Digestion Conditioning
Dewatering
Oxidation
Disposal

10. 10
Disposal
Several methods are employed for the ultimate disposal of sludge. Wet digested sludge may be
sprayed onto cropland where it functions as a fertilizer.
Advanced Wastewater Treatment
The effluent from a typical secondary (biological) treatment plant still contains 20 to 40 ppm
suspended solids and 20 to 40 ppm BOD. Suspended solids in addition to contributing to BOD, may
settle on the stream bed and inhibit certain forms of aquatic life. The BOD, if discharged into a
stream with low flow, can cause damage to aquatic life by reducing the DO content. In addition, the
secondary effluent contains significant amounts of plant nutrients and dissolved solids. If the
wastewater is of industrial origin, it may also contain traces of organic chemicals, heavy metals and
other contaminants. A wide variety of methods are used in advanced water treatment to satisfy any
of several specific goals, which include the removal of:
1. Suspended solids
2. BOD
3. Plant nutrient
4. Dissolved solids, and
5. Toxic substances
Removal of suspended solids
Removal suspended solids in the advanced treatment implies the removal of those materials that
have been carried over from a secondary settler. Of the several methods proposed, the two
methods most widely utilized in this application are microstraining and chemical coagulation
followed by filtration and mixed media filtration.
Microstraining utilizes a rotating drum type filter to screen suspended solids. The filtering
media consists of a finely woven stainless steel fabric with a mesh size of 23 to 35 μm. The
fabric is mounted on the periphery of the drum and water in allowed to pass from inside to the
outside. Back-washing is accomplished by high pressure water jets placed at the highest point of
the drum. The solid which are retained on the fabric are wasted into a trough, which recycles the
solids to the sedimentation tank.
Coagulation is the method in which certain chemicals are rapidly dispersed in wastewater to
change the characteristics of the suspended particles so that they coalesce and form flocs which
sink rapidly. Coagulation is employed to improve or make possible the removal of negatively
charged colloidal suspensions which do not normally settle out and cannot be removed by
conventional physical treatment. Coagulation is done by the addition of positive ions, e.g. Al+3
,
which reduces the electrostatic repulsion between the particles.
The most widely used coagulants for wastewater treatment are aluminum and iron salts such as
aluminum sulphate (alum), ferric sulphate and ferric chloride. At high pH prevalent in the water
these salts produce insoluble aluminum hydroxide or ferric hydroxide flocs. As they form and
grow, the aluminum hydroxide flocs entrap the solid particles. The precipitate is then
flocculated to produce large dense settleable solids.

11. 11
Removal of dissolved solids
The dissolved solids are of both organic and inorganic types. The most common methods used
for the removal of soluble organics from wastewater is adsorption on activated carbon. Solvent
extraction is also used to recover certain organic chemicals like phenols and amines from
industrial wastewaters. A number of methods have been investigated for the removal of
inorganic constituents from wastewater. Three methods which are finding wide application in
advanced waste treatment are ion exchange, electrolysis and reverse osmosis.
Advanced Biological Systems
New biological methods are being investigated for wastewater treatment. The use of shallow
oxidation ponds or lagoons has proved very effective for the treatment of domestic wastewater.
The ponds are clarified into four main types:
1. Aerobic ponds
2. Facultative ponds
3. Aerated ponds, and
4. Anaerobic ponds
1. Aerobic ponds:
Wastewaters containing organic impurities are purified by the action of aerobic bacteria and algae.
Oxygen is supplied by natural diffusion across the pond surface, and by algal photosynthesis. These
are shallow ponds (depth of less than 0.3 m) designed to maximize the growth of algae.
2. Facultative 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 Fig. 8. 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 microorganisms 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 favored.

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Fig. 8. Basic reactions in a facultative pond system
3. Aerated ponds
These are similar to the activated sludge process units where oxygen is supplied by mechanical
aerators. The basic difference between the aerated ponds or lagoons and the activated sludge is that
recycling and wasting of the sludge is provided in the latter as a means of controlling the solids in
the aerator. In aerated ponds no recycling of the sludge is provided and the digested material leaves
the system.
4. Anaerobic ponds
These ponds are maintained in an anaerobic condition by applying a BOD load that exceeds oxygen
production from photosynthesis. Anaerobic ponds are usually employed as pre-treatment ponds for
the treatment of high-temperature, high-strength wastewaters where the reduction in waste strength
is more important than the effluent quality.
Aerobic
Oxidation
Organic waste
Soluble and
suspended solids CO2 +
New cells
Settleable
solids
Variable interface
Aerobic or anaerobic zone
Sludge zone
Bacteria Organic
acids
Bacteria CO2 + CH4 +
New cells
Photosynthesis: CO2 O2
Light
Aerobic
zone
Facultative
zone
Anaerobic
zone
Sunlight

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Chemical oxidation
In the field of wastewater treatment, chemical oxidants, such as chlorine, ozone and hydrogen
peroxide are widely used for disinfection, removing organic materials that are resistance to
biological or other treatment processes, and conversion f cyanides to innocuous products.
1. Chlorine: Initially when it is added to water; chlorine forms hypochlorous acid (HOCl):
Cl2 + H2O HOCl + H+
+ Cl-
Hypochlorous acid is the disinfecting agent is referred to as free residual or free available
chlorine. However, if any reducing agents such as ferrous ions or hydrogen sulphide are
present in water, chlorine reacts with them, and the concentration of chlorine available to
destroy pathogenic bacteria is reduced.
H2S + 4Cl2 + 4H2O H2SO4 + 8 HCl
Wastewater usually contains ammonia. In the presence of ammonia, HOCl reacts to form,
sequentially, monochloramine (NH2Cl), dichloramine (NHCl2) and trichloramine (NCl3)
according to the following reactions:
NH3 + HOCl H2O + NH2Cl
HOCl + NH2Cl H2O + NHCl2
HOCl + NHCl2 H2O + NCl3
Monochloramine and dichloramine are referred to as combined residuals and are more stable
than free residuals, but are less effective as disinfectants. Once all ammonia has been
reacted, further addition of chlorine converts the combined residuals into a free residual, the
conversion being proportional to the dose at the break point. This is the limit beyond which
all the residual chlorine is available as free chlorine.
Chlorine is used to oxidize cyanide in industrial wastewaters to harmless carbon and
nitrogen compounds. This is done in alkaline media at pH greater than 8.5 to prevent the
generation of poisonous hydrogen cyanide gas. The overall reaction may be represented as:
2CN-
+ 5Cl2 + 8OH-
10 Cl-
+ 2CO2 + N2 + 4 H2O
2. Ozone
Ozone is becoming more attractive as an oxidant, particularly as a substitute for chlorine. It
is a powerful oxidizing agent and found to be effective disinfectant. It is also useful for the
removal of color, taste and odor. It is effective in the oxidation of many complex organic
materials including pesticides surfactants, cyanides and phenols.

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