Design of sewage treatment plant 2

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DESIGN OF SEWAGE TREATMENT PLANT
UNIT-I
1) Introduction 3-6
1.1 Definitions
1.2 Treatment of sewage
1.3 Sewage compositions
2) Origin of Sewage 7-8
2.1 Sewage origin
2.2 Sewage mixing with rain water
2.3 Industrial effluent
3) Types of Sewage 9
3.1 Domestic sewage
3.2 Industrial sewage
3.3 Storm sewage
4) Types of Sewerage Systems 10
4.1 Combined System
4.2 Separate system
4.3 Partially combined system
UNIT-II
5) Advantages and disadvantages 12
Of the Sewerage System
6) Objectives of the Study 13-14
6.1 Study of the sewage treatment plant
6.2 Physical characterization of the domestic waste water
6.3 Chemical characterization of the domestic waste water
6.4 Biological characterization of the domestic waste water

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UNIT -III
7) Treatment of Waste Water 18-27
7.1 Pretreatment
7.2 Sewage treatment
7.3 Grit removal
7.4 Flow equalization
7.5 Fat & grease removal
7.6 Primary sedimentation
7.7 Secondary sedimentation
7.8 Tertiary sedimentation
7.9 Filtration
7.10 Lagoons or ponds
7.11 Biological nutrient removal
7.12 Nitrogen removal
7.13 Phosphorus removal
7.14 Disinfection
7.15 Fourth treatment stage
UNIT-IV
8) Design of Sewage Treatment Plant 29-31
8.1 Plant Capacity
8.2 Sizing calculation for collection pit
8.3 Sizing calculation of bar screen
8.4 Sizing calculation of aeration tanks
8.5 Check for aeration period/hydraulic retention time
8.6 Sizing calculation for sludge drying beds
8.7 Term used in the model full form
Drawings 32-33
Conclusion 34
References 35

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UNIT-I
-1 Introduction
-2 Origin of Sewage
-3 Types of Sewage
-4 Types of Sewerage Systems

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Introduction -1
1.1 Definitions:
Sewage treatment is the process of removing contaminants from
wastewater primarily from household sewage. It includes physical,
chemical and biological processes to remove these contaminants and
produce environmentally safe treated wastewater (or treated effluent). A by-
product of sewage treatment is usually a semi-solid waste or slurry,
called sludge, which has to undergo further treatment before being suitable
for disposal or land application.
1.2 Treatment of Sewage:
Sewage treatment may also be referred to as wastewater treatment,
although the latter is a broader term which can also be applied to purely
industrial wastewater. For most cities, the sewer system will also carry a
proportion of industrial to the sewage treatment plant which has usually
received pretreatment at the factories themselves to reduce the pollutant
load. If the sewer system is a combined sewer then it will also carry urban
runoff (storm water) to the sewage treatment plant.
The term sewage treatment plant or sewage treatment works in some
countries is nowadays often replaced with the term wastewater
treatment plant.
Sewage can be treated close to where the sewage is created, which may
be called a decentralized system or even an on-site system. Alternatively,
sewage can be collected and transported by a network of pipes and pump
stations to a municipal treatment plant. This is called a centralized system
although the borders between decentralized and centralized can be variable.
For this reason, the terms semi-decentralized and semi-centralized are also
being used.
1.3 Sewage Compositions:
Pollution in its broadest sense includes all changes that curtail natural
utility and exert deleterious effect on life. The crisis triggered by the rapidly

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growing population and industrialization with the resultant degradation of
the environment causes a grave threat to the quality of life. Degradation of
water quality is the unfavorable alteration of the physical, chemical and
biological properties of water that prevents domestic, commercial,
industrial, agricultural, recreational and other beneficial uses of water.
Sewage and sewage effluents are the major sources of water pollution.
Sewage is mainly composed of human fecal material, domestic wastes
including wash-water and industrial wastes. The growing environmental
pollution needs for decontaminating waste water result in the study of
characterization of waste water, especially domestic sewage. In the past,
domestic waste water treatment was mainly confined to organic carbon
removal. Recently, increasing pollution in the waste water leads to
developing and implementing new treatment techniques to control nitrogen
and other priority pollutants. Sewage Treatment Plant is a facility designed
to receive the waste from domestic, commercial and industrial sources and
to remove materials that damage water quality and compromise public
health and safety when discharged into water receiving systems. It includes
physical, chemical, and biological processes to remove various contaminants
depending on its constituents. Using advanced technology it is now possible
to re-use .sewage effluent for drinking water.
The present study comprises the study on quality of domestic waste
water that is discharged from the city Chandigarh and Panchkula, through
the kitchen outlets and bathroom effluents. The study includes
characterization tests for pH value, acidity, alkalinity, chloride, residual
chlorine, turbidity & DO.
When designing a new sewage works or extending an existing one,
the design should be based on the flow rate and sewage strength. Estimation
of the flow rate is therefore a critical factor in design. If an existing works is
to be extended and the flow records and plant performance data of the plant
are available, then careful assessment of the rate of flow, daily and seasonal
fluctuations in flow rate and sewage strength and operational data for the
various process units can be used to determine the design parameters of the

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extended plant. However if no reliable data are available, as is often the case
in designing new works, then estimates have to be made. This chapter
provides guidelines in estimating flow for design purposes. This is achieved
by establishing the size of the population for the contributing area to the
works, thereby making it possible to obtain a reasonable estimate of likely
flows to the works.
It is necessary to have good estimates of existing or likely wastewater
flows and biological loads over the period encompassed by the design.
When the design is an extension or refurbishment of an existing plant one is
usually able to access some data regarding flows and Wastewater strengths
as a basis for process sizing, but for new plants, in areas which are still in the
process of planning or development, it is necessary to make a number of
design assumptions.
In the case of residential areas or developments where there is no
existing data, the best estimates of flow and load are usually based on
population projections. The developers of the residential areas or estates will
have an overall plan of the number of stands and for smaller developments
may even have knowledge of the size of houses to be built, or the number of
bedrooms in the flats or apartments planned. From this it should be possible
to obtain a reasonable estimate of the likely population and in turn from this,
an estimate of likely wastewater flows and biological loads.
Population and flow projections for areas served by a wastewater
treatment plant should be made before sizing of treatment processes and
piping. Where possible, designs should be based on a 10-year design period
for any one phase of construction. However shorter periods or staged
developments often need to be implemented to match predicted growth
patterns. In considering staged development the ultimate development of
the collection area should be assessed to determine how the layout for the
plant may appear if the area is fully developed.
The population per dwelling unit depends on a number of factors.
Obviously the size of the house is relevant and a four-bedroom house is
likely to have a higher occupancy than a one or two bedroom unit.

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The area is also relevant. A holiday area is likely to be much closer to
being fully occupied in season than a development which houses permanent
residents. Income level and family size are also important. Many lower
income areas have larger families than high income areas and populations
of 6 to 8 persons per household are common in poorer areas compared to 4
persons per unit in a high income area.

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Origin of Sewage -2
2.1 Sewage Origin:
Sewage is generated by residential, institutional, commercial and
industrial establishments. It includes household waste liquid
from toilets, baths, showers, kitchens, and sinks draining into sewers. In
many areas, sewage also includes liquid waste from industry and commerce.
The separation and draining of household waste into greywater and black
water is becoming more common in the developed world, with treated
greywater being permitted to be used for watering plants or recycled for
flushing toilets.
2.2 Sewage mixing with rainwater:
Sewage may include storm water runoff or urban
runoff. Sewerage systems capable of handling storm water are known
as combined sewer systems. This design was common when urban
sewerage systems were first developed, in the late 19th and early 20th
centuries. Combined sewers require much larger and more expensive
treatment facilities than sanitary sewers. Heavy volumes of storm runoff
may overwhelm the sewage treatment system, causing a spill or overflow.
Sanitary sewers are typically much smaller than combined sewers, and they
are not designed to transport storm water. Backups of raw sewage can occur
if excessive infiltration/inflow (dilution by storm water and groundwater)
is allowed into a sanitary sewer system. Communities that
have urbanized in the mid-20th century or later generally have built separate
systems for sewage (sanitary sewers) and storm water, because precipitation
causes widely varying flows, reducing sewage treatment plant efficiency.
As rainfall travels over roofs and the ground, it may pick up various
contaminants including soil particles and other sediment, heavy
metals, organic compounds, animal waste, and oil and grease.
Some jurisdictions require storm water to receive some level of treatment
before being discharged directly into waterways. Examples of treatment
processes used for storm water include retention basins, wetlands,
buried vaults with various kinds of media filters, and vortex separators (to
remove coarse solids).

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2.3 Industrial effluent:
In highly regulated developed countries, industrial effluent usually
receives at least pretreatment if not full treatment at the factories themselves
to reduce the pollutant load, before discharge to the sewer. This process is
called industrial wastewater treatment. The same does not apply to many
developing countries where industrial effluent is more likely to enter the
sewer if it exists, or even the receiving water body, without pretreatment.
Industrial wastewater may contain pollutants which cannot be removed
by conventional sewage treatment. Also, variable flow of industrial waste
associated with production cycles may upset the population dynamics of
biological treatment units, such as the activated sludge process.

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Types of Sewage -3
This modern water carriage sewerage system not only helps in
removing the domestic and industrial waste water, but also helps in
removing storm water drainage. The runoff resulting from the storms is also
sometimes. Sewage system has been divided into three category.
3.1 Domestic Sewage
3.2 Industrial Sewage
3.3 Storm Sewage
3.1 Domestic Sewage:
The wastewater from residences and institutions, carrying body
wastes primarily feces and urine, washing water, food preparation wastes,
laundry wastes, and other waste products of normal living, are classed as
domestic or sanitary sewage.
3.2 Industrial Sewage:
Liquid-carried wastes from stores and service establishments serving the
immediate community, termed commercial wastes, are included in the
sanitary or domestic sewage category if their characteristics are similar to
household flows. Wastes that result from an industrial processes such as the
production or manufacture of goods are classed as industrial wastewater,
not as sewage.
3.3 Storm Sewage:
Surface runoff, also known as storm flow or overland flow, is that
portion of precipitation that runs rapidly over the ground surface to a
defined channel. Precipitation absorbs gases and particulates from
the atmosphere, dissolves and leaches materials from vegetation and soil,
suspends matter from the land, washes spills and debris from urban streets
and highways, and carries all these pollutants as wastes in its flow to
a collection point.

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Types of Sewerage Systems -4
Sewerage system has been divided into three categories, namely it has
been listed below
4.1 Combined System
4.2 Separate System
4.3 Partially Separate System
4.1 Combined System:
When the drainage is taken along with the sewage then the system is
called combined system. In the modern days a separate system is generally
preferred to a combined system although each individual case should be
decided separately.
4.2 Separate System:
When the drainage and sewage are taken independently of each other
through two different sets of conduits then the system is called separate
system.
4.3 Partially Combined System:
Sometimes a part of drainage system especially that originating from
the roofs or paved courtyards of buildings is allowed to be admitted into the
sewers and similarly sometimes the domestic sewage coming out from the
residence or institutions, etc. is allowed to be admitted into the drains, the
resulting system is called partially combined system.

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UNIT-II
-5 Advantages and Disadvantages of Sewerage
Systems
-6 Objectives of the Study

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Advantages and disadvantages of -5
Sewerage System
Advantages:
 Capable of removing 97% of suspended solids.
 Biological nitrification without adding chemicals.
 Oxidization and nitration achieved.
 Solids ad liquids separation.
 Removes organics.
 Cost effective.
 Easily maintained mechanical work.
 Self-sustaining system.
Disadvantages:
 Cleaning is hassle.
 Most plants need at least three tanks.
 Temperature changes affect the tank greatly.

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Objectives of the Study -6
The principal objective of waste water treatment is generally to allow
human and industrial effluents to be disposed of without danger to human
health or unacceptable damage to the natural environment. An
environmentally-safe fluid waste stream is produced. No danger to human
health or unacceptable damage to the natural environment is expected.
Sewage includes household waste liquid from toilets, baths, showers,
kitchens, sinks and so forth that is disposed of via sewers. Sewage also
includes liquid waste from industry and commerce. The objectives of the
study are:
 Design of the sewage treatment plant.
 Physical, chemical and biological characterization of the domestic waste
water from the city Panchkula.
Waste water samples from the kitchen effluent and the bathroom waste of
the city of Panchkula of the residence.
The following physical characteristics were studied:
 Odor
 Taste
 Color
 Floatables
 Turbidity
The following chemical characteristics were studied:
 Total Iron
 Copper
 Zinc
 Potassium
 Lead
 Aluminum

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For determination of inorganic non-metallic constituents we
determined the:
 Alkalinity
 Acidity
 Chloride
 Residual Chlorine
 Sulphate
 Ph. of the sample
 Biochemical Oxygen Demand
 Dissolved Oxygen
6.1 Physical characteristic of waste water:
 Odour:
It depends on the substances which arouse human receptor cells on
coming in contact with them. Pure water doesn’t produce odour or taste
sensations. Thus waste water which contains toxic substances has pungent
smell which makes it easy to distinguish. Odor is recognized as a quality
factor affecting acceptability of drinking water. The organic and inorganic
substance contributes to taste or odor. The ultimate odor tasting device is the
human nose. The odor intensity is done by threshold odor test.
 Taste:
The sense of taste result mainly from chemical stimulation of sensory
nerve endings in tongue. Fundamental sensations of taste are, by convention
more than by research evidence, salt, sweet, bitter, and sour. The rating
involves the following steps:
a) Dilution series including random blanks is prepared.
b) Initial tasting of about half the sample by taking water into mouth and
holding it for several seconds and discharging it without swallowing.
c) Forming an initial judgment on the rating scale.
d) A final rating made for the sample.
e) Rinsing mouth with taste and odor free water.
f) Resting.

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 Color:
Color in water results from the presence of natural metallic ions such
as Fe or Mg, humus and peat materials, planktons and weeds. It is removed
to make water suitable for general and industrial applications. After
turbidity is removed the apparent color and that due to suspended matter is
found out. Tristimulus, Spectroscopic and Platinum cobalt method is used.
 Total solids:
It refers to matters suspended or dissolved in water and waste water.
Solids affect the water or effluent quality adversely in a number of ways.
Water with highly dissolved solids are not palatable and may cause
physiological reaction in transient consumer. A limit of 500 mg dissolved
solids/L is desirable for drinking waters.
Evaporation method is used to separate total solids and their weight is found
out.
 Floatables:
One important criterion for evaluating the possible effect of waste
disposal into surface water is the amount of floatable material in the waste.
It is important because it accumulates on the surface and may contain
pathogenic bacteria and viruses. Two general types of floating matters are
found.
a) Particulate matters like grease balls.
b) Liquid component capable of spreading as thin visible film over large
areas.
 Turbidity:
Clarity of water is important in producing products destined for
human consumption and in many manufacturing uses. It is caused by
suspended matter such as clay, silt and finely divided organic and inorganic
matter, soluble colored organic compounds. Turbidity is an expression of the
optical property that causes light to be scattered and absorbed rather than
transmitted in straight lines through the sample. The standard method for
determination of turbidity has been based on the Jackson candle
Turbiditimeter and Nephlometer.

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6.2 Chemical characteristic of waste water:
Chemical characteristics of water state the presence of metals their
treatment, the determination of inorganic non-metallic constituents and the
determination of organic constituents. Here goes a brief description of all the
experiments we have performed.
6.3 Biological characteristic of waste water:
Water quality has a key role in deciding the abundance, species
composition, stability, productivity and physiological condition of
indigenous populations of aquatic communities. Their existence is an
expression of the quality of the water. Biological methods used for
evaluating water quality include the collection, counting and identification
of aquatic organisms. Most microorganisms known to microbiologists can
be found in domestic wastewater like Bacteria, Protozoa, Viruses, and Algae.
Planktons, Periphyton, Macro-python, Macro-invertebrates, Fish,
Amphibians and Aquatic reptiles are the biotic group of interdependent
organism. Wastewater contains vast quantities of bacteria and other
organisms. Aerobic bacteria break down organic matter in the presence of
available oxygen. Anaerobic bacteria disintegrate organic matter which is
shut off from free oxygen, such as in the interior of a mass of feces or a dead
body. The products of anaerobic decomposition have an extremely
nauseating odor. Matter in which this condition exists is said to be septic. A
multitude of the bacteria in wastewater are coliform bacteria: those found in
the digestive tract of normal humans. It is these comparatively few
pathogenic organisms that pose the greatest public health hazard. Waste
water which is not properly treated may eventually find its way into a
community water source and spread waterborne diseases.

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UNIT-III
-6 Treatment of Waste Water

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Treatment of Waste Water -7
Sewage collection and treatment is typically subject to local, state and
federal regulations and standards. Treating wastewater has the aim to
produce an effluent that will do as little harm as possible when discharged
to the surrounding environment, thereby preventing pollution compared to
releasing untreated wastewater into the environment.
7.1 Pretreatment:
Pretreatment removes all materials that can be easily collected from the
raw sewage before they damage or clog the pumps and sewage lines of
primary treatment clarifiers. Objects commonly removed during
pretreatment include trash, tree limbs, leaves, branches, and other large
objects.
Fig: - Pretreatment

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The influent in sewage water passes through a bar screen to remove all
large objects like cans, rags, sticks, plastic packets etc. carried in the sewage
stream. This is most commonly done with an automated mechanically bar
screen in modern plants serving large populations, while in smaller or less
modern plants, a manually cleaned screen may be used. The raking action of
a mechanical bar screen is typically paced according to the accumulation on
the bar screens and flow rate. The solids are collected and later disposed in
a landfill, or incinerated. Bar screens or mesh screens of varying sizes may
be used to optimize solids removal. If gross solids are not removed, they
become entrained in pipes and moving parts of the treatment plant, and can
cause substantial damage and inefficiency in the process.
7.2 Sewage Treatment:
Sewage treatment generally involves three stages, called primary,
secondary and tertiary treatment.
Fig: - Sewage Treatment

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 Primary Treatment:
It consists of temporarily holding the sewage in a quiescent basin
where heavy solids can settle to the bottom while oil, grease and lighter
solids float to the surface. The settled and floating materials are removed
and the remaining liquid may be discharged or subjected to secondary
treatment. Some sewage treatment plants that are connected to a
combined sewer system have a bypass arrangement after the primary
treatment unit. This means that during very heavy rainfall events, the
secondary and tertiary treatment systems can be bypassed to protect
them from hydraulic overloading, and the mixture of sewage and storm
water only receives primary treatment.
 Secondary Treatment:
Secondary treatment removes dissolved and suspended biological
matter. Secondary treatment is typically performed by indigenous, water-
borne micro-organisms in a managed habitat. Secondary treatment may
require a separation process to remove the micro-organisms from the
treated water prior to discharge or tertiary treatment.
 Tertiary Treatment:
Tertiary treatments sometimes defined as anything more than
primary and secondary treatment in order to allow rejection into a highly
sensitive or fragile ecosystem. Treated water is sometimes disinfected
chemically or physically prior to discharge into a
stream, river, bay, lagoon or wetland, or it can be used for
the irrigation of a golf course, green way or park. If it is sufficiently clean,
it can also be used for groundwater recharge or agricultural purposes.
7.3 Grit removal:
Pretreatment may include a sand or grit channel or chamber, where
the velocity of the incoming sewage is adjusted to allow the settlement of
sand, grit, stones, and broken glass. These particles are removed because
they may damage pumps and other equipment. For small sanitary sewer
systems, the grit chambers may not be necessary, but grit removal is

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desirable at larger plants. Grit chambers come in 3 types: horizontal grit
chambers, aerated grit chambers and vortex grit chambers. The process is
called sedimentation.
Fig: - Great Removal
7.4 Flow equalization:
Clarifiers and mechanized secondary treatment are more efficient under
uniform flow conditions. Equalization basins may be used for temporary
storage of diurnal or wet-weather flow peaks. Basins provide a place to
temporarily hold incoming sewage during plant maintenance and a means
of diluting and distributing batch discharges of toxic or high-strength waste
which might otherwise inhibit biological secondary treatment including
portable toilet waste, vehicle holding tanks, and septic tank pumpers. Flow
equalization basins require variable discharge control, typically include

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provisions for bypass and cleaning, and may also include aerators. Cleaning
may be easier if the basin is downstream of screening and grit removal.
7.5 Fat and grease removal:
In some larger plants, fat and grease are removed by passing the
sewage through a small tank where skimmers collect the fat floating on the
surface. Air blowers in the base of the tank may also be used to help recover
the fat as a froth. Many plants, however, use primary clarifiers with
mechanical surface skimmers for fat and grease removal.
7.6 Primary Sedimentation:
In the primary sedimentation stage, sewage flows through large tanks,
commonly called pre-settling basins, primary sedimentation tanks or
primary clarifiers. The tanks are used to settle sludge while grease and oils
rise to the surface and are skimmed off. Primary settling tanks are usually
equipped with mechanically driven scrapers that continually drive the
collected sludge towards a hopper in the base of the tank where it is pumped
to sludge treatment facilities. Grease and oil from the floating material can
sometimes be recovered soap making.
7.7 Secondary Sedimentation:
Secondary sedimentation is designed to substantially degrade the
biological content of the sewage which are derived from human waste, food
waste, soaps and detergent. The majority of municipal plants treat the settled
sewage liquor using aerobic biological processes. To be effective,
the biota require both oxygen and food to live. The bacteria and
protozoa consume biodegradable soluble organic contaminants and bind
much of the less soluble fractions into floc.
Some secondary treatment methods include a secondary clarifier to
settle out and separate biological floc or filter material grown in the
secondary treatment bioreactor.

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7.8 Tertiary Sedimentation:
The purpose of tertiary sedimentation is to provide a final treatment
stage to further improve the effluent quality before it is discharged to the
receiving environment (sea, river, lake, wet lands, ground, etc.) More than
one tertiary sedimentation process may be used at any treatment plant. If
disinfection is practiced, it is always the final process. It is also called effluent
polishing.
7.9 Filtration:
Sand filtration removes much of the residual suspended matter.
Filtration over activated carbon, also called carbon adsorption, removes
residual.

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Fig: -Filtration
7.10 Lagoons or Ponds:
Lagoons or ponds provide settlement and further biological
improvement through storage in large man-made ponds or lagoons. These
lagoons are highly aerobic and colonization by native macrophytes,
especially reeds, is often encouraged. Small filter feeding invertebrates such
as Daphnia and species of rotifers greatly assist in treatment by removing
fine particulates.
7.11 Biological Nutrient Removal:
Biological nutrient removal (BNR) is regarded by some as a type of
secondary treatment process and by others as a tertiary treatment process.
Waste water may contain high levels of the nutrients nitrogen and
phosphorus. Excessive release to the environment can lead to a buildup of
nutrients, called eutrophication, which can in turn encourage the
overgrowth of weeds, algae, and blue-green algae. This may cause an algal
bloom, a rapid growth in the population of algae. The algae numbers are
unsustainable and eventually most of them die. The decomposition of the

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algae by bacteria uses up so much of the oxygen in the water that most or all
of the animals die, which creates more organic matter for the bacteria to
decompose. In addition to causing deoxygenation, some algal species
produce toxins that contaminate drinking water supplies. Different
treatment processes are required to remove nitrogen and phosphorus.
7.12 Nitrogen removal:
Nitrogen is removed through the biological oxidation of nitrogen
from ammonia to nitrate, followed by denitrification, the reduction of
nitrate to nitrogen gas.
Fig: Nitrogen Removal
Nitrogen gas is released to the atmosphere and thus removed from the
water. Since denitrification is the reduction of nitrate to dinitrogen gas,
an electron donor is needed. This can be, depending on the waste water,
organic matter or an added donor like methanol. The sludge in the
denitrification tanks must be mixed well. Over time, different treatment

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configurations have evolved as denitrification has become more
sophisticated.
7.13 Phosphorus removal:
Every adult human excretes between 200 and 1000 grams of
phosphorus annually. Studies of United States sewage in the late 1960s
estimated mean per capita contributions of 500 grams in urine and feces,
1000 grams in synthetic detergents, and lesser variable amounts used as
corrosion and scale control chemicals in water supplies.
Phosphorus can be removed biologically in a process called enhanced
biological phosphorus removal. In this process, specific bacteria,
called polyphosphate-accumulating organisms (PAOs), are selectively
enriched and accumulate large quantities of phosphorus within their cells.
When the biomass enriched in these bacteria is separated from the treated
water, these bio solids have a high fertilizer value. Phosphorus removal can
also be achieved by chemical precipitation, usually with salts of iron or
lime. This may lead to excessive sludge production as hydroxides
precipitates and the added chemicals can be expensive.
7.14 Disinfection:
The purpose of disinfection in the treatment of waste water is to
substantially reduce the number of microorganisms in the water to be
discharged back into the environment for the later use of drinking, bathing,
irrigation, etc. The effectiveness of disinfection depends on the quality of the
water being treated (e.g., cloudiness, pH, etc.), the type of disinfection being
used, the disinfectant dosage (concentration and time), and other
environmental variables. Cloudy water will be treated less successfully,
since solid matter can shield organisms, especially from ultraviolet light or
if contact times are low.
Chlorination remains the most common form of waste water
disinfection in North America due to its low cost and long-term history of
effectiveness. One disadvantage is that chlorination of residual organic
material can generate chlorinated-organic compounds that may
be carcinogenic or harmful to the environment. Residual chlorine or
chloramines may also be capable of chlorinating organic material in the

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natural aquatic environment. Further, because residual chlorine is toxic to
aquatic species, the treated effluent must also be chemically de-chlorinated,
adding to the complexity and cost of treatment.
Ultraviolet (UV) light can be used instead of chlorine, iodine, or other
chemicals. Because no chemicals are used, the treated water has no adverse
effect on organisms that later consume it, as may be the case with other
methods. UV radiation causes damage to the genetic structure of
bacteria, viruses, and other pathogens, making them incapable of
reproduction. The key disadvantages of UV disinfection are the need for
frequent lamp maintenance and replacement and the need for a highly
treated effluent to ensure that the target microorganisms are not shielded
from the UV radiation.
7.15 Fourth treatment stage
Micro pollutants such as pharmaceuticals, ingredients of household
chemicals, chemicals used in small businesses or industries, environmental
persistent pharmaceutical pollutant (EPPP) or pesticides may not be
eliminated in the conventional treatment process (primary, secondary and
tertiary treatment) and therefore lead to water pollution. Although
concentrations of those substances and their decomposition products are
quite low, there is still a chance to harm aquatic organisms. They mainly
belong to the group of environmental persistent pharmaceutical pollutants.
Techniques for elimination of micro pollutants via a fourth treatment stage
during sewage treatment are being tested in Germany, Switzerland and the
Netherlands. However, since those techniques are still costly, they are not
yet applied on a regular basis. Such process steps mainly consist of activated
carbon filters that adsorb the micro pollutants

28 | S . D . D . I . E . T
Unit-IV
-8 Design of Sewage Treatment Plant

29 | S . D . D . I . E . T
Design of Sewage Treatment Plant -8
8.1 Plant Capacity:
Average water supply per day = 423000 lit (Say)
= 0.423 mld*
Average sewage generated per day = 85% of supplied water
= 0.85*0.423=0.36 mld
= 360 kld**
Average sewage generated per hour=360/24=15 cum/hr
Peak factor = 3
Design flow capacity (maximum) = 13 x 3=45 cum/hr
*mld – Million liter per day
**kld – Kilo Liter per day
8.2 Sizing Calculation for Collection Pit:
Retention time required = 4 h
Average design flow = 15 m3/h
Capacity of collection sump = 4 x 15=60 m3
Assume liquid depth = 5 m
Area required for collection pits = 60/5 = 12 m2
Let it is a circular tank
 r = 1.93m
Volume of the pit provide = π/4 x 4 x 4 x5
= 62.8 m3
Thus Area of the pit provided = 12.6 m2
8.3 Sizing Calculation of Bar Screen:
Peak discharge = 45 m3/h
Average discharge = 15 m3/h
Average velocity @ average flow isn’t allowed to exceed 0.8 m/sec
Average spacing between bar 20 mm
The velocity = 0.3*60=18 m/h/ m2
Cross sectional area required = flow/velocity
= 45/18
= 2.5 m2

30 | S . D . D . I . E . T
Liquid depth required= 1 m
Velocity through screen at the peak flow = 1.6 m/sec
Clear area = 2.5/1.6 = 1.3
No. of clear spacing = 1.3/0.02 = 65
Width of channel = (65 x 20) + (67 x 6) = 1702 mm
Width of screen = 1700 mm
8.4 Sizing Calculation of Aeration Tank:
Bod in the feed sewage = 100 ppm
No. of aeration tank = 2
Average flow = 360/2 = 180 kld
Total bod load to the aeration tank = 15 x 24 x 100 = 36 kgs
Let mlss = 2000 mg/l, f/m=0.15
Volume of tank required = (Q x bod load) / (fm x mlss)
= (180 x 100)/0.15 x 2000
= 60 m3
Assume liquid depth = 3.5 m
Area = 60/3.5=17.143 m2
Tank size provided = 4.5 x 4.5 x 3.7
So, Volume of tank = 75 m3
8.5Check for Aeration Period/Hydraulic Retention Time:
Hydraulic retention time t = 75 x 24/180 = 10 h
So, the tank retention time is more than the required time.
8.6 Sizing Calculation for Sludge Drying Beds
Maximum design flow rate = 45 m3/h, 360 kld
Total feed suspended solid = 250 ppm
Total outlet suspended solid = 50 ppm
Load to the clarifier = 250-50 = 200 ppm
Sludge generated per day = 360x 200/1000
= 72 kg/day
Solid content in the feed= 3%
Specific gravity of the sludge= 1.015
Volume of sludge= ((72/0.03)/ (1000x1.015))
= 2.36 m3

31 | S . D . D . I . E . T
For Rourkela weather condition, the beds get dried out about 7 days
No. of cycles per year = 365/7
= 52 cycles
Period of each cycles = 7 days
Volume of sludge per cycle = 2.36 x 7
= 16.55 m3/cycle
Spreading a layer 1m per cycle,
Area of bed required = 16.55/1
= 16.55 m2
So area of 16.55 m2 for 2 drying beds is provided for the above system
Hence, Sludge Drying Bed has dimensions of 4.5 m x 4.5 m x 1 m of two
numbers.
8.7 Terms used in the Model Full Form
CW Collection Well
CBD Coarse Bubble Diffusion
AT Aeration Tank
PCT Primary Clarifier Tank
SCT Secondary Clarifier Tank
SDB Sludge Drying Bed

32 | S . D . D . I . E . T
Drawings
Fig: - Top view of Sewage Treatment Plant

33 | S . D . D . I . E . T
Fig: -Layout of Sewage Treatment Plant

34 | S . D . D . I . E . T
Conclusion
 The average ranges of physical, chemical and biological characteristics of
waste water quality are experimented and found out.
 The pH ranges from 7.8 to 8.01. The Turbidity ranged from 10 to 120.
 The value of Turbidity was found to be within the permissible limit.
 The Chloride and Alkalinity were in the range of 3.5 to 120 mg/l and 15
to 80 mg/l respectively.
 The Total Iron content was in the range of 0 to 3 mg/l.
 The Zinc content was in the limits of 0.1 to 2 mg/l.
 Copper content ranged from 0 to 0.2 mg/l.
 Potassium was present in the limits of 2 to 12 mg/l.
 The parameters studied resemble the waste water quality.

35 | S . D . D . I . E . T
References
 A STUDY ON THE WATER QUALITY OF THE CITY.
 IS: 3025 (PART 10) – 1984 METHODS OF SAMPLING AND TEST
(PHYSICAL AND CHEMICAL) FOR WATER AND WASTE WATER,
PART 10 - TURBIDITY.
 IS: 3025 (PART 15) – 1984, METHODS OF SAMPLING AND TEST
PART 15 - TOTAL RESIDUE (TOTAL SOLIDS — DISSOLVED AND
SUSPENDED).
 IS: 3025 (PART 16) – 1984, METHODS OF SAMPLING AND TEST
PART 16 - FILTERABLE RESIDUE (TOTAL DISSOLVED SOLIDS).
 IS: 3025 (PART 21) - 1983, METHODS OF SAMPLING AND TEST
PART 21 - TOTAL HARDNESS).
 IS: 3025 (Part 51) – 2001, METHODS OF SAMPLING AND TEST
PART 51 – CARBONATE AND BICARBONATE.
 IS: 3025 (Part 22) – 1986, METHODS OF SAMPLING AND TEST
PART 22 - ACIDITY.

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