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Contaminants Reason for Importance
Suspended solids Suspended solids can lead to the development of sludge deposits and
anaerobic conditions when untreated wastewater is discharged in the aquatic
Nutrients Both nitrogen and phosphate, along with carbon, are essential nutrients for
growth. When discharged to the aquatic environment, these nutrients can lead
to the growth of undesirable aquatic life. When discharged in excessive
amounts on land, they can also lead to the pollution of groundwater.
Priority pollutants Organic and inorganic compounds selected on the basis of their known or
suspected carcinogenicity, or high acute toxicity.
Refractory organics These organics tend to resist conventional methods of wastewater treatment.
Typical examples include surfactants, phenols, and agricultural pesticides.
Heavy metals Heavy metals are usually discharged to wastewater from commercial and
industrial activities and have to be removed if the wastewater is to be reused.
Dissolved inorganics Inorganic constituents such as calcium, sodium, and sulphate are added to the
original domestic water supply as a result of water use and may have to be
removed if the wastewater is to be reused.
Characteristics of industrial waste and effect on
• 1. Soluble organics causing depletion of dissolved oxygen—Since
most receiving waters require maintenance of minimum dissolved
oxygen, the quantity of soluble organics is correspondingly
restricted to the capacity of the receiving waters for assimilation or
by specified .
• 2. Suspended solids—Deposition of solids in quiescent stretches of
a stream will impair the normal aquatic life of the stream. Sludge
blankets containing organic solids will undergo progressive
decomposition resulting in oxygen depletion and the production of
• 3. Priority pollutants such as phenol and other organics dis-
charged in industrial wastes will cause tastes and odours in
the water and in some cases are carcinogenic. If these
contaminants are not removed before discharge, additional
water treatment will be required.
• 4. Heavy metals, cyanide, and toxic organics—
• 5. Colour and turbidity—
These present aesthetic problems even though they may not
be particularly deleterious for most water uses. In some
industries, such as pulp and paper, colour removal can be
difficult and expensive.
• 6. Nitrogen and phosphorus—
When effluents are discharged to lakes, ponds, and other
recreational areas, the presence of nitrogen and
phosphorus is particularly undesirable.Excessive
accumulation in water bodies causes ‘Eutrophication’.
• 7. Oil and Grease – If not removed from industrial
wastewater it creates a layer on surface of water bodies
thus affecting oxygen transfer for photosynthesis. Also
causes aesthetical problems.
Difference between Sewage and Industrial
Sewage Industrial wastewater
Domestic wastewater is
wastewater originating from
activities such as restroom usage,
washing, bathing, food
preparation, and laundry.
Industrial wastewater is process
wastewater originating from
manufacturing, commercial businesses,
mining, agricultural production and
processing, and wastewater from clean-
up of petroleum and chemical
contaminated sites, to name a few.
Characteristics are constant. Not
Characteristics may change. Depends
upon processes adopted.
Does not contain heavy metals May contain heavy metals such as Pb.
Sludge generated in WTP can
be reused in Farm fields
Sludge can be hazardous in nature.
Cant be used in farm fields.
Tertiary treatment is optional. Tertiary treatment is needed for
recycling of wastewater
Fixed flow pattern No fixed flow pattern
Low pollution strength Higher pollution strength
Industry Wastes Produced Type of Pollution
1. Caustic Soda Mercury, Chlorine gas Air, water and land
2. Cement dust, smoke Particulate matter –
3. Distillery Organic waste Land and water
4. Fertiliser Ammonia, cyanide,
oxides of nitrogen, Air and water
oxides of sulphur
5. Dye Inorganic waste pigment Land and water
6. Iron and steel Smoke, gases, coal dust,
fly ash, fluorine
Air, water and land
7. Pesticides Organic and inorganic Water and land
8. Oil Refineries Smoke, toxic gases,
Air and water
9. Paper and Pulp Smoke, organic waste Air and water
10. Sugar Organic waste,
Land and water
11. Textiles Smoke, particulate
Land and water
12. Tanneries Organic waste Water
13. Thermal power Fly ash, SO2 gas Air and water
14. Nuclear power
Radioactive wastes Water and land
15. Food processing Alkalies, phenols
Water and land
• Pollution prevention (P2) is any practice that reduces, eliminates,
or prevents pollution at its source, also known as "source
• Source reduction is fundamentally different and more desirable
than recycling, treatment and disposal.
• There are significant opportunities for industry to reduce or
prevent pollution at the source through cost-effective changes in
production, operation, and raw materials use.
• The opportunities for source reduction are often not realized
because existing regulations focus upon treatment and disposal.
• Pollution prevention approaches include:
• increasing efficiency in energy use;
• use of environmentally friendly fuel sources.
• In the industrial sector, examples of P2 practices include:
1. Modifying a production process to produce less waste
2. Using non-toxic or less toxic chemicals as cleaners,
degreasers and other maintenance chemicals
3. Implementing water and energy conservation
4. Reusing materials such as drums and pallets rather
than disposing of them as waste
• Control of Industrial Pollution:
• Some important control measures are:
1. Control at Source:
It involves suitable alterations in the choice of raw materials and
process in treatment of exhaust gases before finally discharged and
increasing stock height up to min. 30 metres in order to ensure
proper mixing of the discharged pollutants.
2. Selection of Industry Site:
The industrial site should be properly examined considering the
climatic and topographical characteristics before setting of the
3. Treatment of Industrial Waste:
The industrial wastes should be subjected to proper treatment
before their discharge.
Intensive plantation in the region, considerably reduces the dust,
smoke and other pollutants.
5. Stringent Government Action:
Government should take stringent action against industries
which discharge higher amount of pollutants into the
environment than the level prescribed by Pollution Control
6. Assessment of the Environmental Impacts:
Environmental impact assessment should be carried out
regularly which intends to identify and evaluate the
potential and harmful impacts of the industries on natural
7. Strict Implementation of Environmental Protection Act:
Environment Protection Act should be strictly followed and
the destroyer of the environment should be strictly
• Use good housekeeping practices. Sweep, vacuum and
mop floors rather than hosing them down, and don’t
leave sweepings outside where rain can wash them into
storm drains. Clean up spills immediately. Sweep
parking lots, before the rains come. Be aware that rubber
from tires and other products from automobiles
contribute to water pollution.
• Store chemicals and liquids sensibly.
Store chemicals so they can be found and identified easily.
• Spill prevention and control.
• Use chemicals only in designated areas where spills can be contained.
Avoid moving chemicals long distances from storage to use. Properly
dispose of rags and absorbents.
• Train employees- All employees—whether or not they work with
chemicals—should receive training about the products in use, storage
requirements, spill procedures and potential hazards.
• Reduce chemical use whenever possible.
• Equalization is a method of retaining waste in a basin so that the
effluent discharged is fairly uniform in its water quality
characteristics (pH, colour, turbidity, alkalinity, biochemical
oxygen demand [BOD], and so forth).
• A secondary but significant effect is that of lowering the
concentration of effluent contaminants.
• This is accomplished not only by ironing out the slugs of a high
concentration of contaminants but also by physical, chemical,
and biological reactions that may occur during retention in
Objectives of Equalization
• Equalization improves sedimentation efficiency by improving
hydraulic retention time
• The efficiency of biological process can be increased because
of uniform characteristics and minimization of the impact of
shock loads and toxins during operation
• Manual and the automated control of flow- rate-dependent
operations, such as chemical feeding, disinfection, and sludge
pumping are simplified.
• Treatability of the waste water is improved and some BOD
reduction and odour removals provided if aeration is used for
mixing in the equalization basin
• A point of return for recycling concentrated waste stream is
provided, thereby mitigating shock loads to primary setters or
• Air is sometimes injected into these basins to provide:
(1) better mixing;
(2) chemical oxidation of reduced compounds;
(3) some degree of biological oxidation; and
(4) agitation to prevent suspended solids from settling.
• The size and shape of the basins vary with the quantity of waste and
the pattern of its discharge from the factory.
• Most basins are rectangular or square.
Capacity and Detention period of ET
• The capacity should be adequate to hold, and render
homogeneous, all the waste from the plant.
• Almost all industrial plants operate on a cycle basis;
• Thus, if the cycle of operations is repeated every 2 hours, an
equalization tank that can hold a 2-hour flow will usually be
• If the cycle is repeated only every 24 hours, the equalization
basin must be big enough to hold a 24-hour flow of waste.
• This mixing may be brought about in the following ways:
(1) proper distribution and baffling;
(2) mechanical agitation;
(3) aeration; and
(4) combinations of all three.
• Proper distribution and baffling is the most economical,
though usually the least efficient, method of mixing.
• Still, this method may suffice for many plants.
• Horizontal distribution of the waste is achieved by using
either several inlet pipes, spaced at regular intervals
across the width of the tank, or a perforated pipe across
the entire width.
• Over and under baffles are advisable when the tank is
wide because they provide more efficient horizontal and
• Mechanical agitation eliminates most of the need for
baffles and generally provides better mixing than baffles
• One typical arrangement, shown in Figure, uses three
wooden gate type agitators spaced equidistantly along the
centre line of the length of the tank.
• Agitators operated at a speed of 15 rotations/min (rpm) by
a 3-horsepower (hp) motor are usually adequate.
Types of equalization tank
In-Line Equalization tank
Volume of Equalization Tanks
Volume can be
determined by Mass
• Proportioning means the discharge of industrial wastes in
proportion to the flow of municipal sewage in the sewers or to the
stream flow in the receiving river.
• In most cases, it is possible to combine equalization and
proportioning in the same basin.
• The effluent from the equalization basin is metered into the sewer or
stream according to a predetermined schedule.
• The objective of proportioning in sewers is to keep constant the
percentage of industrial wastes to domestic sewage flow entering the
municipal sewage plant.
• To protect municipal sewage treatment using chemicals from
being impaired by a sudden overdose of chemicals contained
in the industrial waste
• To protect biological treatment devices from strong loads of
industrial wastes , which may inactivate the bacteria
• To minimize fluctuations of sanitary standards in the treated
• The rate of flow of industrial waste varies from instant to
instant, as does the flow of domestic sewage, and both
empty into the same sewage system.
• Therefore, the industrial waste must be equalized and
retained, and then proportioned to the sewer or stream
according to the volume of domestic sewage or stream
• To facilitate proportioning, a holding tank should be
constructed with a variable-speed pump to control the
• There are two general methods of discharging industrial waste
in proportion to the flow of domestic sewage at the municipal
1. manual control related to a well defined domestic sewage
flow pattern, and
2. automatic control by electronics.
• Manual control is lower in initial cost but less accurate.
• It involves determining the flow pattern of domestic sewage for
each day of the week over a period of months.
• Usually one does this by examining the flow records of the sewage
plant or by studying the hourly water-consumption figures for the
• It is better to spend time on a careful investigation of the actual
sewage flow than to make predictions based on miscellaneous non
• Actual investigative data should be used to support those records
that are applicable to the case.
• Automatic control of waste discharge according to sewage flow involves
placing a metering device that registers the amount of flow at the most
convenient main sewer connection.
• This device translates the rate of flow in the sewer to a recorder located
near the plant’s holding tank.
• The pen on the recorder actuates either a mechanical (gear) or a
pneumatic (air) control system for opening or closing the diaphragm of
• There are, of course, many variations of automatic flow-control systems.
Although their initial cost is higher than that of manual control, they will
usually return the investment many times by the savings in labour costs.
What is Neutralization?
• Neutralization involves adjusting the pH of a liquid to approach
the “neutral” pH of 7.0 (neither acid nor base).
• Generally, neutralization involves the use of an acid (pH less
than 7) to lower the pH of a tank of basic (or alkaline) liquid
(pH greater than 7),
• or the use of abase (or alkali) to raise the pH of a tank of acidic
• Excessive acid or alkaline wastes should not be discharged without
treatment into a receiving stream. There are many acceptable methods for
neutralizing over-acidity or over-alkalinity of wastewaters such as :
i) Mixing wastes ;
ii) Limestone treatment for acid waste ;
iii) Lime slurry treatment for acid waste ;
iv) Caustic Soda treatment for acid waste ;
v) Waste-boiler - Flue-Gas ;
vi) CO2 treatment for alkaline waste ;
vii) Producing CO2 in alkaline wastes ; and
viii) Sulfuric acid treatment for alkaline waste.
1. Mixing of wastes
• Mixing of wastes can be accomplished within a single plant operation
or between neighbouring industrial plants.
• Acid and alkaline wastes may be produced. Individually within one
plant and proper mixing of these waste at appropriate time. results in
• For example, if one plant produces an alkaline waste which can be
pumped conventionally to an area adjacent to a plant discharging an
acid waste, an economical and feasible system of neutralization
results for each plant.
Industry A Producing
Industry B Producing
A. Neutralization of Acidic Waste
2. Lime stone treatment for acid wastes
• In this method, acid wastes is passed through beds of
limestones (CaCO3) at a rate of about one gallon per minute
per square feet (1gpm/ft2).
• Neutralization takes place with following reaction :
CaCO3 + H2SO4 ———————> CaSO4 + H2CO3
• This reaction will continue as long as excess limestone is
available and in an active state.
• Disposing of the used limestone beds can be a serious
drawback to this method of neutralization.
• Since the used limestone must be replaced by fresh at periodic
interval, frequency of replacement depending on the quantity
and quality of acid wastes being passed through a bed.
• When there are extremely high acid loads, foaming may occur,
especially when organic matter is also present in the waste.
3. Lime-slurry treatment for acid wastes
• Mixing acid wastes with lime slurries is an effective procedure
for neutralization. The reaction is similar to that obtained with
limestone beds But the advantages of using lime slurry than lime
stone beds are :
i) In this case, lime is used up continuously because it is converted
to CaSO4 and which is carried out in the waste ;
ii) Lime possesses a high neutralizing power and its action can be
hastened by heating or by oxygenating the mixture.
iii) It is relatively inexpensive, but in large quantities the cost can
be an important item.
4. Caustic-Soda treatment for acid wastes
• Concentrated solutions of caustic soda or sodium carbonate cause
neutralization of acid waste. These neutralizers are more powerful
• lime or limestone and some agents are required in small volume.
• Another advantage is that the reaction products are soluble and
thus they do not increase the hardness of receiving waters.
• When sodium hydroxide is used as a neutralizing agent for carbonic
and sulfuric acid wastes, the following reactions take place :
• Na2CO3 + CO2 + H2O →2NaHCO3
Carbonic Acid Waste
2NaOH + CO2 →NaHCO3 + H2O
NaOH + H2SO4 → NaHSO4 + H2O
Sulfuric Acid Waste
NaHSO4 + NaOH →Na2SO4 + H2O
• Both these neutralizations take place in two steps and the end
products depend on the final pH desired.
5. Neutralization of Alkaline Wastes : Using Waste-
• It is a most economical method for neutralization of alkaline waste.
• In this method, when waste boiler-flue gas (containing 14 percent CO2) is
passed through the alkaline waste CO2 dissolve in waste water and will
form carbonic acid (a weak acid) which in turn reacts with caustic wastes
to neutralize the excess alkalinity as follows :
i. CO2 + H2O→ H2CO3
Flue gas + Waste Water Carbonic acid
ii. H2CO3 + 2NaOH→ Na2CO3 + 2H2O
Carbonic acid Soda ash
iii. H2CO3 + Na2CO3 →2NaHCO3 + H2O
Excess Soda ash Sodium Carbonic Acid in waste bicarbonate in waste
6 Carbon - Dioxide Treatment for Alkaline Wastes
• Bottled CO2 is applied to wastewaters which neutralizes
alkaline wastes on the same principle as boiler-flue gas [i.e. it
forms a weak acid (carbonic acid) dissolved in water.
• When the quantity of alkaline waste is large, there is
operational difficulty as well as high cost.
7 Producing Carbon-dioxide in Alkaline Wastes
• CO2 produced by fermentation of an alkaline, organic waste
help in neutralization of alkaline waste by decreasing the pH.
• Similar by fermenting alkaline fat-sugar wastes with yeast, CO2
is produced which can be used for neutralization.
8 Sulfuric-Acid Treatment for Alkaline Wastes
• The addition of sulfuric acid to alkaline wastes causes
• In this method, storage and feeding equipment requirement
are low due to high acidity value of Na2S04, but it is difficult to
handle because of its corrosiveness.
• The neutralization reaction which occurs, when it is added to
wastewater is as follows :
• 2NaOH + H2SO4 → Na2SO4 + 2H2O
Find pH of mix if two equal volume wastes are mixed together with pH 6
and 4 respectively.
Q1. Differentiate between sewage and Industrial wastewater.
Q2. Discuss control of industrial pollution in detail.
Q3. Explain Equalization and proportioning.
Q4. Enlist methods used for neutralization of acidic and alkaline
wastes. Explain any one method in detail.
Q6. Determine pH of two industrial wastes having pH 6 and 8
respectively. Assume equal volumes of waste are mixed together.
1. Industrial waste water treatment is required for _____________.
(to protect receiving water, recycling of wastewater, recovery of
2. Organic content in industrial wastewater may deplete____ in
3. Oil and grease if discharged in wastewater interferes with ______.
4. Equalisation tank can be used for tackling fluctuations in
________ and _______.
5. ____________ and ______________ are types of equalization
6. Proportioning is method of discharging industrial waste in
proportion with discharge of ________ or ___________.
7. _______ method of proportioning id cheap but not accurate.
8. If proportioning of industrial wastewater is not done w.r.t. sewer
discharge, then it may affect ________processes and reduce its
9. pH +pOH = ____.
10. __ gas can be used for neutralization of alkaline waste.
11. ___________ process of acidic waste neutralization produces
more quantity of sludge.
12. Lime slurry process of neutralization is _____than lime stone