Water supply and treatment Engineering notes 18 cv46

WATER SUPPLY AND TREATMENT ENGINEERING (18CV46)
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MODULE 1
INTRODUCTION
Need for the protected water supply:
The water when exposed to the atmosphere contains many impurities which are harmful to any
living organism. if untreated water is consumed by living organisms it is likely to cause serious
harm to their health hence in order to make water potable and free from various impurities
purification methods are found out.
The soul of purification process of present day water supply schemes is the filtration. It is
preceded by pre filtration purification methods and followed by post filtration purification
methods. The former methods make the water fit for filtration and the latter methods treat the
impurities which have not been removed with the help of the process of filtration. The line of
treatment to be recommended for a particular quantity of water will naturally depend upon its
quality.
The water covers about three quarters of our planet and yet it is said to note that about 70 % of
the world’s population survive without clean water. The un served population is largely existing
to the extent of about 50 % in India, Pakistan and Bangladesh. India alone contributes 30 % to 35
% of the un served population.
Types of water demands
Domestic demand:
Drinking human body contains about 70% of water consumption of water by a man is required
for various physiological processes blood formation food assimilation its the quantity of water
which a man would require for drinking depends on various factors but on the average and under
normal conditions it is about to liters birthday this amount as will be seen is very small as
compared to various other uses of water but it is most essential to supply water for drinking
purposes with a high degree of purity of water for drinking contains undesirable elements with
mildly to epidemic in fact the drinking water should be protected potable and palatable.
Cooking:
Some quantity of water will also be required for cooking the quantity of water required for this
purpose will depend upon the stage of advancement of the family in particular and Society in

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general however for the purpose of estimation the amount of water required for cooking may be
assumed as about 5 liters per head per day.
Bathing:
The quantity of water required for bearing purpose will mainly depend on the habits of people
and type of climate. For an Indian bath this quantity may be assumed as about 30 to 40 liters per
head per day and for tub bath it may be taken as about 50 to 80 liters per head per day.
Washing hands face etc.
The quantity of water required for this purpose will depend on the habits of people and may
roughly be taken as 5 to 10 liters per head per day.
Household sanitary purposes under this division the water is required for washing clothes floors
utensils and it may be assumed to be about 50 to 60 liters per head per day.
Private gardening and irrigation:
In case of developed cities there will be particularly no demand of water for this purpose in case
of an developed cities the private wells are generally used to provide water for private gardening
and irrigation it is therefore not essential to include the quantity of water required for this
purpose in case of public water supply project.
Domestic animals and private vehicles:
The amount of water required for the use of domestic animals and private vehicles is not of much
concern to a water supply engineer. With the growth and development of town, the cattle
disappear and commercial stables come into existence. For information, the quantity of water
required for various types of domestic animals is mentioned in the table below
Water required for domestic animals.
Name of the domestic animal Quantity of water required in
liters per number per day
Cow or buffalo 40
Dog 10
Horse 50
Mule or pony 30
Sheep 5

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Industrial purposes:
1. Factories: the quantity of water required for the processes involved in factories will
naturally depend on the nature of products, size of factory etc and it has no relation with
the density of population. It is quite likely that the demand of water for factories may
equal or even exceed the demand of water for domestic purposes. The possibility of
recycling of water in the plant will also have appreciable effects on the demand of water
for a particular product table shows the typical water demand per kg products in liters for
some of the factories.
2. Power stations: A huge quantity of water will be required for working of power stations
but generally the power stations are situated away from the cities and they do not
represent a serious problem to public water supply
3. Railways and airports: In most of the cases the Railways and airports make their own
arrangements regarding their water requirements and hence the quantity of water to be
consumed by railways is not ordinarily induced in any public water supply scheme. For
the purpose of estimate, the Railways provide 25 to 70 litres of water per head per day
depending upon the nature of station and facilities like bathing ect. The airport authorities
usually make the provision of water at about 70 litres of water per head per day.
Business or trade purpose:
Some traits such as Diaries hotels laundries motor carriages restaurants stables School hospitals
cinema halls theatres etc require a large quantity of water such trades are to be maintained in
hygienic conditions and sanitation of such places should be strictly insisted. The number of such
business centers will depend upon the population and for moderate city and average value of
about 15 to 25 liters of water per head per day may be taken as water requirement for this
purpose.
Fire demand:
Usually a fire occurs in factories and stores. The quantity of water required for fire fighting
purposes should be easily available and always kept stored in the storage reservoir. The the Fire
hydrants are located in the mains at distances of not more than 150 metres or so. When a fire
occurs, the pumps are installed on trucks are immediately rushed to the site of fire occurrence

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and these pumps when connected to the fire hydrants are capable of throwing water with high
pressure the fire is thus brought under control.
Requirement of water for fire demand can be work doubt in a logical way as follows
1. Minimum number of streams required
2. Discharge of each stream
3. Duration of a fire and
4. Number of simultaneous fires
For a moderate fire about three fire streams will be required one for the property affected by fire
and one each for adjacent property on either side of the burning property. The discharge of
stream should be about 1100 litters per minutes. If a fire is assumed to last for 3 hours and if
provision is to be made for 4 fires at a time, the quantity of water will be.
3 X 1100 X 60 X 3 X 4 = 2376000 litres
Public purposes:
1. Road washing: the rose with heavy amount of dust is to be sprinkled with water to avoid
inconvenience to the users. Even in case of dust proof roads the periodical washing is
necessary. On an average, the quantity of water required for this purpose may be taken as
about 5 litres per head per day.
2. Sanitation purposes: in this division, the water is required for cleaning public sanitary
blocks large markets, etc and for carrying liquid wastes from houses. The quantity of
water required for this purpose will depend on the growth of Civilization and may be
assumed to be about 2 to 3 litres per head per day.
3. Ornamental purposes: in order to add on the town with decorative features the
fountains or lakes or Ponds are sometimes provided. These objects require huge quantity
of of water for their performances. As far as Indian towns are concerned, the quantity of
water required for this purpose may be treated as quite negligible since in most of the
towns, the quantity of water available is not enough even to meet with the most urgent
needs of the society.
Factors affecting per capita demand
1. Climate conditions: the requirement of water in summer is more than that in winter. So
also is the case with hotter and cooler places. Extreme cold, the people making water taps
open to avoid freezing of pipes. This may result in increased rate of consumption.

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2. Cost of water: the rate at which water is supplied to the consumers may also affect the
rate of demand the higher the cost the lower will be the rate of demand and vice versa.
3. Distribution pressure: the consumption of water increases with the increase in the
distribution pressure. This is due to increase in loss and water of high pressure. For
instance, an increase of pressure from 2 to 3 kg per CM square main lead to an increase
in consumption to the extent of about 25 to 30%. The designer therefore should only
provide for distribution pressure which is necessary for rendering satisfactory service.
Habits of population for high value premises the consumption rate of water will be more
due to better standard of living of persons. For middle class premises the consumption
rate will be average while in case of slum areas it will be much lower. A single water tap
may be serving several families in low value areas.
4. Industries: the presence or absence of industries in a city may also affect its rate of
demand. As there is no direct relation between the water requirement for industries and
population, it is necessary to calculate carefully present and future requirements of
industries.
5. Policy of metering: the quantity of water supplied to a building is recorded buy a water
meter and the consumer is then charged accordingly the installation of meters reduces the
rate of consumption. But the fact of adopting policy of meter is a disputable one.
• Arguments for policy of meeting
1. It becomes very easy to locate the points of leakage when metres are installed.
2. The consumer is charged in proportion to the quantity of water which is uses.
3. The reduction in consumption of water results in decrease ine loads on purification
plants, pumps, sewers etc.
4. The wastage of water has decreased. The careful consumer pays less and the
careless consumer pays more.
• Arguments against policy of metering
1. There is loss of pressure due to installation of metres and it adds to the pumping
cost.
2. The use of water for Gardens, fountains is greatly diminished. This decreases the
beauty of the locality.
3. The Limited use of water may lead to unhygienic conditions and may cause
epidemic.

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4. The policy of metering is expensive in the sense that the cost is to be encouraged
to buy, to install, to read and to maintain the metres.
5. It is suggested that the amount spent after introducing the policy of metering main
will be spent in improvement of the water supply scheme itself.
Variation in demand of water
The average daily rate of demand per head is the ratio of total quantity of water supplied during
the year to the number of persons multiplied by the days of years. This average daily rate of
demand per head is likely to debate if period of observation is shortened. Thus if average daily
rate of demand is say hundred, then
1. Seasonal maximum demand may be 130
2. Monthly maximum demand may be 140 and
3. daily maximum demand maybe 180
These variations are due to many factors habits of people climate conditions type of industries et
cetera the above figures for deviation from the average for seasonal monthly and daily demand
are taken for illustration only.
In practice the maximum daily rate of consumption is very important this maximum daily
consumption is to be consumed in 24 hours. But demand during 24 hours will not be uniform and
it will vary according to hour of day.
Seasonal variations occur due to larger use of water in summer season lesser use in winter in
much less in rainy season these variations may also be caused by seasonal use of water in
industries such as processing of cash crops every time of harvesting etc.
Day-to-day variations definite household and industry activity for example consumption is
generally more on Sundays and holidays on this of dust Storms etc again there are variations in
hour to hour demand for example the consumption in the early hours of morning is generally
small increases sharply as the day advances reaching a peak value between 8:00 a.m. to 11:00
a.m. then decreases sharply up to 1 p.m. remains constant up to about 4:00 p.m. again increases
in the evening reaching a peak between 7 to 9 p.m. finally falling to a low value in the late hours
of night as shown in the figure

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Factors governing design period
1. Useful life of component structures and the chances of their becoming old and
obsolete. Design periods should not exceed those respective values.
2. And difficulty that is likely to be faced in expansions if undertaken at future dates.
For example more difficult expansions mean using a higher value of the design
period.
3. Amount and availability of additional investment likely to be incurred for additional
provisions, for example if the funds are not available one has to keep a smaller
design period.
4. The rate of interest on the borrowings and the additional money invested for
example if the interest rate is small higher value of the design period may be
economically justified and therefore adopted
5. Anticipated rate of population growth including possible shift in communities
industries and commercial establishments for example if the rate of increase of
population is less higher figure for the design period may be chosen.

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Population forecasting methods
1. Arithmetic increase method
2. Geometric increase method
3. Increment increase method
4. Decreasing rate of growth method
5. Graphical method
PROBLEMS:

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REFERENCES:
1. Water Supply And Sanitary Engineering – “Rangwala”
2. Water Supply Engineerng – “Santhosh Kumar Garg”
FAQ
1. What are the various types of water demand? Explain them in brief.
2. The census records of a small town is as follows: Calculate the probable population in 2020,
2030, 2040 by decreasing growth method
Year 1980 1990 2000 2010
Population 9000 13000 17500 23000
3. Define per capita demand and design period. Explain the factors governing design period.
4. The census records of a town show the population as follows as follows:
Present population = 50,300
Population before one decade = 46,500
Population before two decades = 43,100
Population before three decades= 40,500
Calculate the probable population after one , two and three decades by using
i) Geometrical increase Method
ii) Incremental increase method

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MODULE 2
WATER TREATMENT AND INTAKE STRUCTURE
Raw water
Alum polymer Chlorine
Screening: removes large solids like logs, branches, rags, fish. It is as imple process which
incorporates mechanized trash removal system.Protects pumps and pipes in water treatment
plant.
Coagulation: small particles are not removed efficiently by sedimentation because they settle
too slowly they may pass through filters. They are easier to remove when clumped together
.Coagulated to form larger particles, but they don’t because they have negative charge which
repels each other. While coagulation we add chemical such as alum which produces positive
charges to neutralize the negative charges on the particles. These particles stick together due to
their opposite charges forming larger particles. These larger size particles acan be easily
removed.
Aluminium sulphate, Ferrous sulphate, Ferric sulphate Ferric chloride and Lime are some of the
coagulants. Following are the factors to be considered while selecting a coagulant.
1. Easily available in dry and liquid form
2. Economical
3. Effective over range of pH
4. Produces less sludge
5. Less harful for plant
6. Fast
Flocculation: After coagulation particles acquire the capability to stick to each other, water is
sent o tank with paddles that provide mixing. This brings particles together to form larger
particles called flocs. Mixing is done quite slowly and gently during flocculation. If the
flocculation is too fast, the flocs will break apart into small particles that are difficult to remove by
sedimentation or filtration
Screening
Coagulation
Flocculation
Sedimentation
Filtration
Disinfection
Storage
Distribution

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Sedimentation:. The water next flows to a sedimentation tank , here the gravity make the flocs
settle to the bottom . larger particles settle more rapidly than the smaller particles. It would take
a very large time for all particles to settle out and that would mean we would need a large
sedimentation tank. The clarified water moves from sedimentation tank to the filtration tank
where the finer particles are removed.
Filtration: The filtration apparatus is a concrete blocks which contain sand , gravel and under
drain. After the filter is operated the sand becomes clogged iwth particles and must be
backwashed. Flow through the filter is reversed and the sand and particles are suspended. The
particles are lighter than the sand. So they rise up and are flushed from the system. When
backwashing is complete the sand settles down onto the gravel Flow is reversed and the
process begins again.
Disinfection: After all teh particles being removed the only process we are left is the
disinfection, so that no pathogens remain in the water. Protozoa pathogens are large in size and
have been removed with other particles. Bacteria and viruses ae now destroyed by addition of
disinfectant. Chlorination: enough chlorine is added so that some remains to go out in the
water distribution system protecting hte public once the water leaves the plant.
Softening: Areas where water comes in contact with limestone, there may be high levels of
calcium and magnesium present. These chemicals make the water hard. This hardness is
removed by a process called softening.Two chemicals lime and soda ash are added to water
causing calcium and magnesium precipitate. Solid substances is then removed with other
particles by sedimentation and filtration.
Surface sources of water supplies
The important surface sources are:
1. Natural Ponds and lakes
2. Streams and rivers
3. Impounding reservoir
These sources are discussed below:
Ponds and lakes and surface sources of supplies
A natural large size depression formed within the surface of the earth when gets filled up with
water is known as a pond or a lake the difference between a pond and a lake is only that of size
if the size of the depression is comparatively small it may be termed as a pond and when the
size is larger it may be termed as a lake the flow of water in a lake is just like the flow in a
stream channel. Generally the surface runoff from the catchment area contributing to the
particular Lake enters the lake through small all drains are streams. Sometimes the
underground water through spring also enters natural depressions Natural depressions and gets

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collected there forming Ponds and lakes. The quality of water in a lake is generally good and
does not need much purification. Larger and older lakes however provide comparatively pure
water then smaller and you are lakes. Self purification of water due to sedimentation of
suspended matter, bleaching of colour removal of bacteria makes the lakes water pure and
better. On the other hand in still water of lakes and ponds the algae, weed and vegetables
growth take place freely imparting bad smells and colours to their waters.
The quantity of water available from lakes is however generally small. It depends upon the
catchment area of the lake basin annual rainfall and geological formations. Due to the smaller
quality of water available from them lakes are usually not considered as principal source of
water supplies. They are therefore useful for only small towns and hilly areas however when no
other sources are available largest lakes may become the principal sources of supplies.
Streams and rivers are surface sources of supplies small stream channels feed their water
to the lakes or rivers. Small streams are therefore generally not suitable for water supply
schemes because the quantity of water available in them is generally very small and they may
even sometimes go dry. They are therefore useful as sources of water only for small villages,
especially in hilly regions. Rivers are the most important source of water for public water supply
scheme it is well known smart that most of the cities are settled near the river and it is generally
easy to find a river for supplying water to the city. Rivers may be perennial or non perennial.
Perennial rivers are those in which the water is available throughout the year such rivers are
generally felt by rains during rainy season and by snow during summer season. Perennial rivers
can be used as sources of public supplies directly whereas the non perennial rivers can be used
as sources of public supplies by providing storage on the upstream of the intake works highly
non perennial river and maybe adopted even on a perennial river when water is used for
multiple uses such as irrigation hydropower etc the head works such as a Barrage or a beer
may also be constructed on those perennial rivers where supplies are considerable e reduced
during dry weather periods. The quality of water obtained from rivers is generally not reliable, as
it contains large amount of silt sand and lot of suspended metre. The disposal of the entry of
untreated are treated sewage into the rivers is father liable to contaminate their waters. The
river water must therefore be properly analysed and well treated before supplying to the public.
Storage Reservoir as surface sources of supplies
A water supply scheme drawing water directly from a river or a stream main fail to satisfy the
consumer's demand during extremely low floors while during high flows it may again become
difficult to carry out its operations due to devastating flood a barrier in the form of a dam main
therefore sometimes be constructed across the river so as to form a pool of water on the

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upstream side of the Mariya. This pool or artificial lake formed on the upstream side of the Dam
is known as storage reservoir. The quality of this Reservoir water is not much different from the
top of a natural lake. The water stored in the reservoir can be used easily not only for water
supplies but also for other purposes. Generally multipurpose reservoir are planned these days
and operated so as to get optimum benefits.
SAMPLING
1. The process of collecting a representative portion of water, as from the natural
environment or from an industrial site, for the purpose of analyzing it for constituents
2. The process of taking a portion of water for analysis or other testing.
3. e.g. drinking water to check that it complies, or river water to check for pollutants, or
bathing water to check that it is safe.
OBJECTIVES OF WATER SAMPLING:
4. The primary goal of water sampling is to observe and measure how water quality
changes over time.
1. An important premise to water sampling work is that high acidity or high alkalinity (pH
levels) in the water might be normal for a given environment or ecological region
2. water samples must be taken and analyzed repeatedly over a period of weeks, months,
years, and decades to determine more precisely how water conditions change.
PROCEDURE
1. If sampling a body of running water, point the mouth of the bag upstream and your
hands downstream to avoid contamination.
2. If sampling from a water faucet, run the faucet for 1 minute before obtaining a sample.
3. Rinse the bag twice with the sample water prior to filling and closing.
4. Fill bag as full as possible. Half-filling the bottle leaves more room for oxygen which will
promote degradation of your sample.
5. Collect data such as temperature and pH which affect the solubility of many ions.

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Sampling flow chart
Methods of sampling
1. Systematic sampling
2. Random sampling
3. Judgemental sampling
4. Stratified sampling
5. Haphazard
1. Systematic sampling: Here the points are selected at regular and even intervals , is
statistically unbiased- providing the co-ordinates of the first sampling point are
determined by random numbers. Systematic sampling does not generate clusters of
sampling points and is easier to use to survey sampling locations than random sampling.
A square grid is the commonest type systematic sampling pattern.
For example: The area to be analyzed may be divided by a grid , and a sample taken at each point of the grid.
2. Random sampling: with random sampling, sampling points are selected randomly but
not arbitrarily. A legitimate random number generator should be used to determine
sampling point coordinates. Most scientific calculators can generate numbers that are
sufficiently random for the intended purpose. The randomization process ensures any
location within the sampling area has an equal chance of being selected as a sampling
point, by chance , can cluster together. This makes them deficient for detecting hot spots
Sample
Planning
Water
sampling
preparation
Sampling at
the site
Transportation
Storage
Analyses

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and for giving an overall picture of the spatial distribution of the contamination. In
practice random sampling has limited use in contaminated site investigation.
3. Judgemental sampling : In this method, sampling points are selected on the basis of
investigator’s knowledge of the probable distribution of contaminants at the site. It is an
efficient sampling method which makes use of the site history and field observations but
has the disadvantage of being potentially biased. The quality of the sampling results
depends on the experience of the investigator and he available site history information,.
Judgemental sampling should not be used in validation sampling.
4. Stratified sampling: First divide the site into sub areas according to geological and
geographic features, nature of contamination, former usage pattern of the site, intended
future use of the subarea, and other relevant factors. Each sub area can then be treated
as individual site and different sampling patterns and sampling densities applied. A
stratified sampling pattern approach is best suited to investigations of large sites with
complex contaminant distributors. This sampling patters may require amore complex
statistical analysis.
5. Haphazard sampling: A sampling location or sampling time is chosen arbitrarily. This
type of sampling is reasonable for a homogenous system .since most environmental
systems have significant spatial or temporaral variability, haphazard sampling often
leads to biased results. However this method may be used as preliminary screening
technique to identify a possible problem before a full scale sampling done.
Types of Sampling:
1. Grab samples: A grab sample is a discrete sample which is collected at a specific
location at a certain point of time . if the environmental medium varies spatially or
temporarily , then a single grab sample is not representative and more samples need to
be collected.
2. Composite samples: A composite sample is made by thoroughly mixing severe grab
samples. The whole composites may be measured or random samples from the
composites may be withdrawn and measured.

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INTAKE STRUCUTRES
Whenever the water is withdrawn from source surface sources such as a lake or river or
reservoir and the entrance of the withdrawal conduit is not an integral part of a dam or any other
related structures then and intake structure must be constructed at the entrance of the conduit.
The basic function of the intake structure is to help in safely with drawing water from the sources
over a predetermined range of pool levels and then to discharge this water into the withdrawal
conduit through which it flows up to the water treatment plant.
An intake structure is constructed at the entrance of the conduits and thereby helping in
protecting the conduit from being damaged or clogged, it can vary from a simple concrete block
supporting the end of the conduit pipe to use concrete Towers housing intake Gates screens
pumps etc and even sometime is living quarters and shops for operating personnel.
Factors governing the location of an intake: the site for locating the Intake should be
selected carefully, keeping the following points in mind
1. As far as possible the site should be near the treatment plant so that the cost of
converting water to the city is less.
2. The intake must be located in the pura zone of the source so that the best possible
quality of water is withdrawn from the source thereby reducing the load on the treatment
plant.
3. The intake must never be located at the downstream or in the vicinity of the point of
disposal of waste water. When it becomes necessary to locate the intake in the close
proximity of the disposal of a drain etc it is advisable to construct a Weir or a Barrage
upstream of the disposal. And install the intake upstream of the Barrage.
4. The intake should never be located near the navigation channels as otherwise there are
chances of intake water getting polluted due to the discharge of refuse and waste from
ships and boats.
5. This site should be such as to permit greater withdrawal of water, if required at a future
date. Does there should be sufficient scope for future additions and expansions.
6. The intake must be located at a point from where it can draw water even during the
driest periods of the Year. Does the intake must be located in deep waters sufficiently
away from the Shore line.
7. The intake site should remain easily accessible during floods and should not get flooded
full stop over the flood water should not be concentrated in the vicinity of the intake.
8. In meandering rivers the intake should not be located on curves or at least on sharp
curves.

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Types of intakes
Simple submerged intakes: A simple submerged intake consists of a simple concrete block or
rock filled timber crib supporting the starting and of the withdrawal pipe. The withdrawal pipe is
generally taken up to the sump well at shore from where the water is lifted by pumps. The intake
opening is generally covered by screen so as to prevent the entry of debris into the withdrawal
conduit. In case of lakes where silts tends to settle down, the intake openings is generally kept
at about 2 to 2.5 m above the bottom of the lake and does to avoid the entry of large amount of
silt and sediment. These submersible intakes are cheaper and generally do not obstruct
navigation they are therefore widely used for small water supply projects drawing water from
streams are lakes having relative little change in water surface elevation throughout the year.
These intakes are not used on bigger projects on rivers and reservoirs, as their main
disadvantage is the fact that they are not easily accessible for cleaning repairing etc.
Wet intake Towers: A typical section of a wet intake tower is shown in the figure it may consist
of a concrete circular shell filled with water up to the Reservoir level and has a vertical inside
shaft which is connected to the withdrawal pipe. The withdrawal may be taken directly to the
treatment plant in case No lift is required or to the sump well in case a low lift is required. The
withdrawal conduits may lie over the bed of the rivers or may be in the form of tunnels below the
river bed openings are made into the outer concrete cell as well as into the inside shaft as
shown. Gates are usually placed on the shaft so as to control the flow of water into the shaft and
the withdrawal conduit. The water coming out of the withdrawal conduit may be taken to Pump
House for lift if the city is water treatment plant is located at higher elevation or may be taken
directly to the treatment plant if it is situated at lower elevation.

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Dry intake Tower: The essential difference between dry intake tower and await intake Tower is
that whereas innovate intake Tower the water enters from the entry ports into the tower and
then it enters into the conduit pipe through separate Gate controlled openings in a dry intake
Tower the water is directly drawn into the withdrawal conduits through the gate entry ports are
shown in the figure. A dry intake Tower will therefore have no water inside the tower if it Gates
are closed where as the fat intake Tower will be full of water even if its Gates are closed.
.

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When the entry ports are closed or dry intake Tower will be subject to additional buoyant forces
and hence must be heavier construction than the fat intake Towers. However the dry intake
towers are useful and beneficial in the sense that water can be withdrawn from many selected
level of the Reservoir by opening the port of the level. Intex towers are huge structures of
various designs standing in the reservoir of the river and hence should be located as not to
interfere with navigation they must be properly designed so as to with stand the worst possible
combination of various forces as hydrostatic pressure wind and earthquake forces and force is
caused by waves, ice, debris etc.
Canal intakes: figure shows the details of a Canal intake. And intake chamber is constructed in
the canal section full stop this results in the reduction of waterway which increases the velocity
of flow. It is therefore necessary to provide pitching on the downstream and upstream portions
of the canal near intake. The entry of water in the intake chamber takes through the course
screen and the top of outlet pipe is provided with fine screen. The inlet outlet pipe is of bell
mouth shape with perforations of fine screen on its surface. The outlet valve is operated from
the top and it controls the entry of water into the outlet pipe from where it is taken to the
treatment plant. As the water level in the canal section practically remains constant it is not
necessary to provide intake pipes at various levels. To reach up to the bottom of intake the
steps should be provided in zigzag manner starting from manhole. The flow velocity through the
outlet pipe is usually kept as about 1.5 m/s and it helps in determining the area and diameter of
the withdrawal conduit. For designing the area of course screen the velocity flow is limited to as
low as about 150 mm/s or so. The flow velocity to the bell mouth shaped inlet is limited to about
300 mm/s or so.

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Reservoir or lake intakes: figure shows the details of a Reservoir intake correlate intake. It
consists of an intake will which is placed near the dam. It is connected to the top of them by a
foot Bridge. The intake pipes are located at different levels with the common vertical pipe.
Peoples of intake pipes are operated from the top and they are installed in a valve room. Each
intake pipe is provided with Bell mouth entry with perforations of fine screen on its surface. The
outlet pipe is taken out through the body of dam. The outlet pipe should be suitably supported.
The location of intake pipes at different levels ensures supply of water from my level lower than
the surface level of water. When the value of an intake type is opened, the water is drawn off
from the reservoir to the outlet pipe through the common vertical pipe. To reach up to the
bottom of intake from the floor of valve room the steps should be provided in zigzag manner.
Sometimes the intakes are placed in the form of Wells inside these Lewis waves are openings
in the body of the Dam itself these are other Dam with Central impervious course for intake well
of or masonry Dam
River intakes: figure shows the details of a typical river intake an approach channel is
constructed to lead the water from the upstream side of the river to the Jack well. Depends talks
with screens are provided at different levels. The suction pipe is provided with strainer at its
lower end.The water from jackwell is pumped and sent to the treatment plant. To prevent the
back flow of water due to gravity a valve should also be provided on the rising main leading to
the treatment plant. To reach up to the bottom of intake from the floor of pump room, the ladder
in zigzag manner should be provided.

12
Portable intakes: In case of emergencies such as festival war etc it sometimes becomes
necessary to draw water with the help of movable intake. It essentially consist of a truck fitted
with a pumping plant the truck is brought to the site and it is placed in such a position that it
becomes possible to immerse the suction pipe of the pump. The end of the suction pipe is kept
just above the bed level of water source and it is also provided with screen. Does the water
lifted by the portable intake is relative relief from suspended solids. The water is then conveyed
through the discharge pipe of the pumping plant.
Aeration:
Under the process of aeration water is brought in intimate contact with air so as to absorb
Oxygen and to remove carbon dioxide gas. It may also help in killing bacteria to a certain extent.
It also helps in removing H2S gas and iron and manganese to a certain extent from the treated
water the aeration of water can be carried out in the following way
By using spray nozzle: In this method water is sprinkled in air atmosphere through special
nozzles which breaks the water into droplets does permitting the escape of dissolved gases.
Carbon dioxide gas is does considerably removed in this method. However considerable hand
of water is required for the working of these nozzles which function efficiently at a pressure of 10
to 14 m head of water.
By permitting water Trickle over cascades: In this method the water is made to fall through a
certain height 2 to 3m over a series of steps with the fall of about. 0.15 to 0.3 m in each step.
The structure so formed is known as a Freefall aerator. The simplest type of the following
greater is known as a cascade aerator. Figure shows the design of a cascade aerator. Such
aerators are widely used in water features. We will take large quantities of water in a

13
comparatively small area at low head are simple to be kept clean should preferably be installed
in open air. However for protection against air pollution freezing and algal growth it can be
installed in a small house having plenty of air inlet. The cascade aerators are efficient in raising
dissolved oxygen content of water but not for Carbon dioxide removal which is removed only in
the range of 60% to 70%. In cascade aerators usually the rate of Flow may vary between 20 to
100 m3
/h per m length of weir. To allow entrained air to mix in the water receiving basin should
have a pool of water of depth 0.3 m to 0.5 m. If the water is allowed to cling to the steps,
especially at low discharge rates the efficiency is reduced.
By air diffusion: In this method compressed air is bubbled through the water so as to
thoroughly mix it with water. The compressed air is does bubbled up from the bottom of the
tank. During its upward movement through the water body it gets thoroughly mixed up with the
water contained in the tank there by completing the aeration process.
By using trickling beds: In this method the water is allowed to Trickle down the beds of Coke
supported over the perforated bottom trays, and arranged vertically in series. Generally
three beds are used, the depth of each being about 0.6 m with a clear distance of about 0.45 m
in between. The water is applied from the top through perforated distribution pipes and allowed
to trickle down, up to the bottom bed. During this downward, motion the water gets mixed up
with air, and aeration takes place. The size of coke to be used, usually ranges between 50 to 75

14
mm. This method gives better results than what can be obtained by cascades, but less effective
than the method of spray nozzles.
Aeration thought helpful in removing iron and manganese besides removing volatile gases, such
as carbon dioxide and hydrogen sulphide, yet cannot be relied upon to remove even reduce the
tastes and odours of all kinds. Moreover, it cannot completely remove the carbon dioxide, and its
residuals always remain in practice even after aeration. However, the oxidation made possible by
aeration, helps in removing oils and decomposing products of algae and other aquatic vegetation,
and thus helps in removing the odours, tastes and colours due to their presence.
Aeration should however be used only to a limited extent because too much of absorbed oxygen
will make the water corrosive and may necessitate the de aeration process.

15

16
REFERENCES:
1. Water Supply And Sanitary Engineering – “Rangwala”
2. Water Supply Engineerng – “Santhosh Kumar Garg”
FAQ
1. Explain the objectives of water treatment
2. List the physical water qualities
3. Discuss the complete sequence of water treatment plant with a flow diagram
4. List the chemical properties of water
5. Explain different types of intake structures
6. Discuss the factors affecting selection of sources of water for water supply scheme
7. Give the maximum permissible limits as per BIS for water quality parameters
8. What are the objectives of sampling
9. What is sampling? Explain the different types of sampling

WATER SUPPLY AND TREATMENT ENGINEERING
1
MODULE 3
SEDIMENTATION AND FILTERATION
Most of the suspended impurities present in water do have a specific gravity greater than that of
water in still water these impurities will there for 10 to settle down under gravity although in
normal draw supplies they remain in suspension because of the turbulence in water. Hence as
soon as the turbulence is retarded by offering storage to the water these impurities tend to settle
down at the bottom of the tank offering such storage. This is the principle behind sedimentation.
The basin in which flow of water is retarded is called the settling tank of sedimentation tank and
the theoretical average time for which the water is detained in the tank is called the detention
period.
Theory of sedimentation
The settlement of a particle in water brought to rest is opposed by the following factors:
1. The velocity of flow which carries the particle horizontally. The great father the floor
area the lesser is the velocity and hence more easily the particle will settle down.
2. The viscosity of water in which the particle is traveling. The viscosity varies inversely
with temperature. Warm water is less viscous and therefore offers less resistance to
settlement. However the temperature of water cannot be controlled by any appreciable
extent in water purification processes and hence this factor is generally ignored.
3. The size shape and specific gravity of a particle the greater is the specific gravity
more rapidly the particle will settle. The size and shape of the particle also affect the
settling great. For example the weight and volume of spherical shaped particles varies
with the cube of its diameter or its size and its area varies with the square of the
diameter. Hence very small size particle will settle very slowly it therefore clearly follows
that the shape and size of the particle do affect their settling velocities.
Types of sedimentation tank: sedimentation tank are basically divided into two types
1. Horizontal flow tank and
2. Vertical or upflow tanks
Horizontal flow tanks in the design of horizontal flow tank the aim is to achieve as nearly as
possible the ideal condition of equal velocity at all points lying on each vertical line in the settling
zone. The direction of flow in the tank is substantially horizontal among the horizontal flow tanks
we have different types of designs

2
 Rectangular tank with longitudinal flow such as the one shown in the figure they may be
provided with mechanical scrapping devices to scrap the sludge to the sludge pit located
usually towards the influent and from where it is continuously or periodically removed
without stopping the working of the tank. Such tanks are known as continuous flow type of
sedimentation tanks. In such a tank the flow velocity is only reduced and the water is not
brought to complete reset. In other types of such tanks mechanical scrappers may not be
provided and the tank may have to be cleaned by stopping the operation of the tank. In
such intermittent type of sedimentation tanks raw water is simply stored and kept at rest for
a certain period say about 24 hours. During this rest period the suspended particles settle
down to the bottom of the tank. The clear water from the tank is finally taken out and the
tank is cleaned of the settled silt. The tank is again filled with water to continue the next
operation. This type of tank does functions intermittently as a period of about 30 to 36
hours is usually required to put the tank again in working order. This necessitates the
commissioning of at least two tanks. Search intermittent thanks are usually not preferred in
modern treatment plants as they lead to wastage of time and Labour requiring more units
full stop continuous flow type of tanks are there for invariably used these days the working
of such a tank is simple as the water enters from one end and comes out from the other
and. The velocity is sufficiently reduced by providing sufficient length of travel. The velocity
is so designed that the time taken by the sand particle to travel from one end to another is
slightly more than the time required for settlement of the particle.

3
 Circular tank with radial flow with Central feed such as the one shown in figure in such
a tank the water enters at the center of the tank into a circular well provided with multiple
ports from which it emanates out to flow radially outward in all directions equally the water
that flows horizontally and radially from the center towards the Periphery of the circular tank
stop the am here is to provide uniform radial flow with decreasing horizontal velocity
towards the Periphery from where the water is withdrawn from the tank through the effluent
structure this large is scrapped to the central sump mechanically and continuously from
where it is withdrawn during operation. This removal mechanism consists of scrapper
blades mounted on two or four arms revolving slowly. Second type of circular tanks are
provided with peripheral feed these tanks differ from the central feed circular tank in that
the raw water here enters the tank from the Periphery are the rim full stop it has been
demonstrated that the average detention time is Greater in peripheral feat basins leading to
better performance the possible arrangements are shown in the figure.

4
 Vertical or upflow settling tanks vertical flow tanks usually combine sedimentation with
flocculation all the day may also be used for plain sedimentation they may be square or
circular in plan and may have copper bottoms the influent enters at the bottom of the unit the
up flow velocity decreases with the increase cross sectional area of the tank be clarified
water is withdrawn through the circumferential or central wear. When used with coagulants
deflocculating takes place in the bottom of the tank leading to formation of blanket of floc
through which the rising floc must pass because of this phenomenon these tanks are also
called the upload large blanket clarifiers.
Design concept in plain sedimentation tank in sedimentation tanks we use a term called
overflow velocity or over flow rate or surface over flow rate or surface loading which may be
represented by the symbol we not it is that flow velocity at which the tank is designed to
operate. The other term relating to sedimentation has already been defined and is called the
settling velocity vs of the particle the importance of these two terms can be understood easily by
considering an a floor sedimentation tank as shown in the figure in this design the particles fall

5
down word and the water rises vertically upwards. The rate at which the particle is settling is vs
and the velocity of the liquid rising is vo. Evidently if the particle is to be removed from the
bottom of the tank and not go out in the clarified water then the particle settling velocity must be
greater than the liquid rise velocity vo. In other words vs should be equal to or greater than vo of
an vo is kept at about 80% of vs for an upflow clarifier.
The liquid rise velocity vo is called the overflow rate because the water is flowing over the top of
the tank into the air system. This term is also referred to as surface loading rate because its
units are discharge per unit of plan area.
In the case of horizontal sedimentation tank we can similarly show that the particle remover is
likewise depend only upon the overflow rate. Such horizontal sedimentation tank is designed
based on the following three assumptions
1. Particles are their velocity vectors are evenly distributed across the tank cross section
this is the function of the inlet zone.
2. The liquid moves as an ideal slowing down the length of the tank.
3. Any particle hitting the bottom of the tank is removed.
Sedimentation with coagulation as pointed out earlier very fine suspended mud particles and
the colloidal matter present in water cannot settle down in plain sedimentation tank of ordinary
detention period. Such particles can however be removed easily by increasing their size by
changing them into flocculated particles. For this purpose certain chemical compounds called
coagulants are added to the water which on throw mixing former gelatinous precipitate called
floc. The very fine colloidal particles present in water get attracted and absorbed in these flocs

6
forming the bigger size flocculated particles the colloidal particles do in fact causes surface
charges resulting from preferential adsorption are from ionization of chemical groups on the
surface. Most of the colloidal particles in water or wastewater are negatively charged. The
stationary charged layer on the surface is surrounded by a boundary layer of water as shown in
the figure. In this boundary layer called the Stern layer ions of opposite charge drawn from the
bulk solution producer rapid drop in potential called the stern potential drop called the potential
occurs between the surface of the groundwater layer and the point of Electro neutrality in the
solution as shown in the figure.
The surface charge on colloidal particles gives them long term stability and hence the particle
which might otherwise settle are mutually repelled by the like charges. Relation is a chemical
technique which is directed towards the destabilization of the charged colloidal particles.
Flocculation on the other hand is the slow mixing technique which promotes the agglomeration
of the destabilize particles. For all practical purposes however the entire process of addition of
chemicals and mixing is usually referred to as coagulation. The coagulated water is finally made
to pass through the sedimentation tank where the flocculated particles settle down and our does
removed.
The use of coagulants is generally necessary for clarifying raw containing turbidity is greater
than 30 to 50 mg/l. But in actual practice plane sedimentation is rarely used these days and the
coagulation before sedimentation is almost universally adopted in all the major water treatment
plants and is followed by rapid sand filtration.

7
Chemicals used for coagulation
Various Chemicals such as alum, iron salts ferrous sulphate ferric chloride ferric sulphate etc
are generally used as coagulants these Chemicals are most effective when water is slightly
alkaline in the absence of such and alkalinity in RO supplies external alkalis like sodium
carbonate online etc are added to the water so as to make it slightly alkaline and does to
increase the effectiveness of the arguments full stop the important coagulants and the chemical
reactions associated are as described below
1. Use of alum as coagulant the name given to the Aluminium Sulphate with its chemical
formula Al2 (So4)3 24 H2O. The alum when added to raw water reacts with the bicarbonate
alkaline it is which are generally present in Raw supplies so as to form a gelatinous
precipitate of aluminium Hydroxide. This floc attracts other fine particles and suspend
matter and does grows in size and finally settled down to the bottom of the tank.
The chemical equation is
Al2 (So4)3 24 H2O + 3Ca(HCO3)2  3CaSo4 +2Al(OH)3 +6CO2
From the above equation it becomes evident that the addition of alum to the water in parts
permanent hardness to it in the form of calcium sulfate. The carbon dioxide gas which is
evolved causes corrosiveness
From the equation it can also be seen that the alum reacts with Bicarbonate alkalinity all the
total alkalinity is a measure of bicarbonate carbonate and Hydroxide ions. Since the alkalinity in
water in the form of bicarbonate alkalinity is typically pH below 8.3 and above the pH of 4.5 or
so it fixes a limit in on maintaining and optimum pH value of about 6.5 to 8.3 which is required
for the formation of good sweet floc .
The amount of alum required for coagulation depends on the turbidity and colour of the raw
water. The use of optimum amount of coagulant is indicated by the formation of large feathery
flakes and can be approximately determined by laboratory testing the dose of alum may vary
from 5 mg/l for relatively clear waters to 85 mg/l for highly turbid water the average normal
dose is about 17 mg/l.
However if raw supplies are not sufficiently alkaline then the external alkalis like lime or soda
Ash are generally added and the following reactions takes place

8
Alum or filter alum has proved to be very effective coagulant and is now extensively used
throughout the world it is quite cheap forms an excellent stable floc and does not require any
skilled supervision for handling the water obtained is also quite clear as it helps in reducing
taste and colour of raw water in addition to removing its turbidity.
The main problem in using alum till recently was that it is difficult to di water this large formed
and is not easy to dispose it of acid is found and suitable for filling low lying lands. But the
recent research has however shown that it is possible to recover album from this sludge and it
can be reused for coagulation. The cost of recovery is also one fourth of the cost of the
recovered alum. The only drawback in the use of alum now is that the effective pH range for its
use a small that is 6.5 to 8.3 and may in many cases required the addition of external colour
salts thereby rendering it costlier
2. Use of copperas coagulant copperas is the name given to the ferrous sulphate with its
chemical formula as FeSo4.7H2O. Copperas is generally added to raw water in conjunction with
lime. Lime may be added either to Copperas or vice versa. When lime is added first the
following reactions takes place.
FeSo4.7H2O+Ca (OH)2  CaSo4 + Fe(OH)2 +7H2O
Copperas is extensively used as a coagulant for raw water that are not coloured. It is generally
cheaper than alum and functions effectively in the pH range of 8.5 and above. For coloured raw
water, it is however not used as it does not give satisfactory results full stop the quantity of
copper as required is almost the same as that of alum.
3. Use of chlorinated copperas as coagulant when chlorine is added to a solution of copper as
the to react chemically so as to form ferric sulphate and ferric chloride full stop the chemical
equation is as follows
6(FeSo4.7H2O)+3Cl2  2Fe2(SO4)3 +2FeCl3+42H2o

9
The resultant combination of ferric sulphate and ferric chloride is known as Chlorinated
copperas and is valuable coagulant for removing colours especially with the raw water has a low
pH value.
Both the constituent of the chlorinated copper as along with lime are effective Goa glands and
their combination is often quite effective the chemical reactions that take place are given below
2Fe2(SO4)3 + 3Ca(OH)2 3CaSO4 +2Fe (OH)3
Ferric sulphate is effective in the pH range of 4 to 7 and above 9 whereas ferric chloride is quite
effective in the pH range of 3.5 to 6.5 and above 8.5. The combination has therefore proved to
be very effective coagulant for treating low pH water.
4. Use of sodium aluminate as a coagulant beside alum and iron salts, sodium aluminate is
also sometimes used as a coagulant this chemical when dissolved in mixed with water reacts
with the salts of calcium and magnesium present in raw water resulting in the formation of
precipitate of calcium or magnesium illuminate full stop the chemical reactions that are involved
are
This coagulant is about one and half times costlier than alum and is therefore generally
avoided for treating ordinary public supplies but however it is very useful for treating water
which do not have the natural desired alkalinity and thus cannot be treated with pure alum.
As it is evident from the above equation is this chemical for the reduce the temporary as well as
the permanent hardness present in Raw supplies rather than increasing the same as is done
by this coagulant is therefore widely used for treating Boiler feed water which permit very low
values of hardness.
Comparison of alum and iron salt

10
The alum and the iron salts are having their own advantages and disadvantages are
summarized:
1. Iron salts produce heavy floc and can therefore remove much more suspended matter than
the alum
2. Iron salts, being good oxidizing agents, can remove hydrogen sulphide and its
corresponding tastes and odours from water.
3. Iron salts can be used over a wide range of pH values.
4. Iron salts cause staining and promote the growth of iron bacteria in the distribution system
5. Iron salts impart more corrosiveness to water than that which is imparted by alum.
6. The handling and storing of iron salts more skill and control, as they are corrosive and
deliquescent. Whereas, no such skilled supervision is required for handling alum.
Design and Dosage problem

11

12
For more problems refer class notes

13
Feeding Devices: The chemical coagulant may be fed into the raw water either in a powdered
from or in a solution form. The former is known as dry feeding, and the latter is known a wet
feeding. Wet feeding equipments are generally costlier than the dry feeding equipments, but
they have the advantage that they can be easily controlled and adjusted. The choice between
these two types of equipments depends upon the following factors
1. The characteristics of the coagulant and the convenience with which it can be applied:
chemicals which clog or which are non-uniform in composition cannot be fed by dry
feeding. For example, alum being fairly fine and uniform in size, can be fed easily by dry
feeding, but copperas may give trouble in dry feeding, because the water of crystallization
present in it, may change with temperature, thereby turning it into a solid or sticky mass.
Similarly hydrated lime cannot be fed by dry feeding, because it may bridge the orifices by
taking the atmospheric moisture.
2. The amount of the coagulant to be used: the amount of the coagulant to be used is an
important factor in choosing the type of feeding arrangement. For example if the dose of
the coagulant is very small, then for reasons of accuracy, it must be fed in a solution form.
3. The cost of the coagulant and the size of the plant: in a plant which uses a great deal of
coagulant, the chemicals should be purchased in its cheapest form and the plant should be
equipped to use the chemical in that form. The cost of the feeding machine is therefore less
important as compared to the cost of the coagulant in a large plant. Whereas, if the plant is
small, the cost of chemicals may be purchased in the dry form, because dry fed machines
are cheaper.
The wet feeding equipments in addition require making and preparing solutions of required
strength and to keep them available when needed. Sufficient watch and ward staff is
therefore required for this additional work. Hence large plants naturally use wet feeding and
small plants utilize dry feeding unless objected to by the characteristics of the coagulant.
Dry feeding device: The common devices which are used for dry feeding of the coagulants are
shown in the fig. They are in the form of a tank with hooper bottom. Agitating plates are placed
inside the tank, so as to prevent the arching of the coagulant. The coagulant in the powdered
form is filled in the tank and is allowed to fall in the mixing basin. Its dose is regulated by the
speed of a toothed wheel or helical screw. The speed of the toothed wheel or the helical screw
is in turn controlled by connecting it to a venture device installed in the raw water pipes bringing
water to the mixing basins. The quantity of the coagulant released is thus controlled in
proportion to the quantity of the raw water entering the mixing tank.

14
Wet feeding device: In wet feeding, the solution of required strength of coagulant is prepared
and stored in a tank, from where it is allowed to trickle down into the mixing tank through an
outlet. The level of coagulant solution in the coagulant feeding tank is maintained constant by
means of afloat controlled valve, in order ensure a constant rate of discharge for a certain fixed
rate of raw water flow in the mixing basin. When the rate of inflow of raw water changes, the
rate of outflow of coagulant must also change. In order to make these two flows in proportion to
each other, a conical plug type arrangement as shown in the fig may be used.
The working of conical plug type arrangement is very simple. The mixing basin and the float
chamber are interconnected together, so that the water level remains the same in both of them.
As the flow of raw water increases, the depth of water and therefore its level in the float

15
chamber increases and thereby lifting the float of the float chamber. As the float rises, the pinion
and pulley rotates in the same direction, thereby lifting the conical plug and allowing more
quantity of coagulant solution to fall down into mixing basin. When the flow of water decreases,
the conical plug descends down and allows the feeding tank, thus automatically controls the
dose of coagulant.
Laboratory testing for the determination of optimum coagulant quantities (JAR TEST)
Jar test is carried out to determine the optimum quantity of coagulant in laboratory. The
apparatus which is used in this test is shown in the fig
The sample of raw water to be tested is placed in a number of jars each having a capacity of
about 1 liter. Normally, six jars are used. Different amounts of coagulant are then added to each
jar. The driving unit is started. The paddles placed inside the jars and connected with the driving
shaft through vertical stirring rods are thus made to rotate. The formation of the floc in each jar
is noted. The amount of coagulant in the jar which produces a good floc with the least amount of
coagulant indicates the optimum dosage. The speed of paddles and the time of mixing may also
be varied for different test during determining this least optimum dosage.
FILTRATION
Screening and sedimentation removes a large percentage of the suspended solids and organic
matter present in raw supplies. The percentage removal of the fine colloidal matter increases
when coagulants are also used before sedimentation. But, however the resultant water will not
be free of impurities, and may contain some very fine suspended particles and bacteria present
in it. To remove or to reduce the remaining impurities still further and to produce potable and
palatable water, the water filtered through the beds of fine granular materials such as sand etc.

16
The process of passing the water through the beds of such granular material is known as
filtration.
Theory of filtration
1. Mechanical straining: The suspended particles present in water, and which are of bigger
size than the size of the voids in the sand layers of the filter, cannot pass through these
voids and get arrested in them. The resultant water will, therefore be free from them. Most of
the particles are removed in the upper sand layers. The arrested particles including the
coagulated flocs forms a mat on the top of the bed, which further helps in straining out the
impurities
2. Flocculation and sedimentation: It has been found that the filters are able to remove even
particles of size smaller than the size of the voids present in the filter. This fact may be
explained be assuming that the void spaces act like tiny coagulation-sedimentation tanks.
The colloidal matter arrested in these voids is a gelatinous mass and therefore attract other
finer particles. These finer particles thus settle down in the voids and get removed.
3. Biological metabolism: Certain microorganisms and bacteria are generally present in the
voids of the filters. They may either reside initially as coatings over sand grains or they may
be caught during the initial process of filtration. Nevertheless, these organisms therefore
utilize such organic impurities and convert them into harmless compounds so formed,
generally form a layer on the top, which is called dirty skin. This layer further helps in
absorbing and straining out the impurities.
4. Electrolyte changes: The purifying action of filter can also be explained by the theory of
ionization. According to this theory, a filter helps in purifying water by changing the chemical
characteristics of water. This may be explained by the fact that the sand grains of the filter
media and the impurities in water carry electrical charges of opposite nature. When these
oppositely charged particles and the impurities comes in contact with each other, they
neutralize each other, thereby changing the character of the water and making it purer. After
a certain interval, the electrical charges of sand grains get exhausted and have to be
restored by cleaning the filter.

17
TYPES OF FILTERS AND THEIR CLASSIFICATION
1. Slow sand filters:
FILTERS
SLOW SAND FILTERS RAPID SAND FILTERS
RAPID GRAVITY
FILTERS
PRESSURE
FILTERS

18
1. Enclosure tank: It consists of open water tight rectangular tank, made of masonry or
concrete. The bed slope is kept at about 1 in 100 towards the central drain. The depth of the
tank may vary from 2.5 to 3.5 m. The plan area of the tank may vary from 100 to 2000 sq m
or more depending upon the quantity of water to be treated
2. Filter media: The filtering media consists of sand layers about 90 to 110 cm in depth and
placed over a gravel support. The effective size D10 of the sand varies from to mm and the
uniformity coefficient varies from 1.8 to 2.5 or 3 . The top 15 cm layer of this sand is
generally kept finer variety than that of the rest, which is generally kept uniform in grain size.
However, if different gradations of sand are used then coarsest layer should be placed near
the bottom and the finest towards the top. The finer sand used, the purer will be the
obtained water as more impurities and bacteria will be removed
3. Base material: The base material is gravel, and it supports the sand. It consists of 30 to 75
mcm thick of different sizes, placed in layers. Generally, three to four layers of each of 15 –
20 cm depth are used. The coarsest gravel is used in the bottom most layers, and the finest
gravel is used in the topmost layer. The size of gravel in the bottom most layer is generally
kept 40 to 65 mm, in the intermediate layers as varying between 20 to 40 mm and 6 to 20
mm and in the top most layer as 3 to 6 mm.
4. Under drainage system : The gravel support is laid on the top of an un der drainage
system, The underdrainage system consists of a central drain and lateral drain as shown in
the fig. The laterals are open jointed pipe drains or some other kind of porous drains placed
3 to 5m part on the bottom floor and sloping towards a main covered central drain. The
laterals collect the filtered water and discharge it into the main drain, which leads the water
to the filtered water well. Sometimes instead of placing it in the center, the main drain is
placed along one side of the tank and laterals slope towards it.
5. Inlet and outlet arrangements: An inlet chamber is constructed for admitting the effluent
from the plain sedimentation tank without disturbing the sand layers of the filter and to
distribute it uniformly over the filter bed.
Operation and cleaning of slow sand filters: The treated water from the sedimentation tank is
allowed to enter the inlet chamber of the filter unit and get distributed uniformly over the filter
bed. The water percolates through the filter media and gets purified during the process of
filtration. The water then enters the gravel layer and comes out as the filtered water. It gets
collected in the laterals through the open joints, which discharge into the main drain. The main

19
drain finally discharges into the filtered water well from where the filtered water is supplied to the
stored tank for supplies.
It may be noted that the water entering the slow and filter should not be treated by coagulants.
This is due to the fact that the dirty skin formed by the floc and carried to the filter considerably
affects the economical working of the filter.
The loss of head called filter head is generally limited to a maximum value of about 0.7 to 1.2m.
When this limiting value, which is roughly kept as 0.7 to 0.8 times the depth of the filter sand, is
reached. The filter unit must be put out of service and the filter is cleaned.
The cleaning of slow sand filter is not done by back washing as done for rapid gravity filters, but
is done by scrapping and removing the 1.5 to 3 cm of top sand layer. The top surface is finally
raked, roughened, cleaned and washed with good water. The amount of wash water required is
generally small, say of the order of 0.2 to 0.6 percent of the total filtered water. Cleaning is
repeated as often as necessary until the sand depth is reduced to about 40 cm or so. A lot of
manual labor is required in cleaning such filters , although very small quantities of wash waters
are needed.
The rate of filtration that can be obtained ranges between 100 to 200 liters per hour per sq.m of
filter areas. The extent of bacteria removal is up to 98 to 99% or more.
Uses of SSF: These are best suited for smaller plants and for purifying waters with low colors,
low turbidities and low bacterial contents. However because of their smaller rate of filtration they
require huge surface area and large volumes of filtering materials. This makes them costly and
uneconomical, especially for treating large scale supplies. They are therefore being replaced by
rapid gravity filters at all the major cities and towns . They may however be preferred for smaller
plants for village supplies or for individual industries supplies, especially in hotter and under
developed countries

20

21
Rapid sand filters: Rapid sand filters are divided into tow
1. Rapid sand gravity filters
2. Pressure filters
Rapid sand gravity filters: These are the one which utilize comparatively larger sized sand
particles, which allow greater rate of filtration as compared to that of slow sand filters. They are
called rapid gravity filters.
Construction of Rapid gravity filters:
1. Enclosure tank: It consists of an open water tight rectangular tank made of masonry or
concrete. The depth of the tank may vary from 2.5 to 3.5 m. In order to acieve uniform
distribution of water, the area of filter unit should not be kept larger, and is generally limited
to about 10 to 80 m2
for each unit. The number of unit at a filter plant may be roughly
estimated by the equation developed by Morrell and Wallace which states that
N = 1.22 √Q
Where N = number of filter units
Q = plant capacity in million liters per day
There should be at least two filter units in any plant .And for plant of more than 9 million
liters per day capacity , no single unit should have capacity greater than one fourth of the
capacity of that plant.

22
2. Filter Media:
The filter media consists of sand layers, about 60 to 90 cm in depth and placed over a gravel
support. The effective size (D10) of the sand varies from 0.35 to 0.55 mm and the uniformity
coefficient Cu= D60/D10 generally varies as 1.3 to 1.7
3. Base Material: In slow as well as rapid gravity filters, the base materials are gravel and it
supports the sand. But, in rapid gravity filter, in addition supporting the sand it distributes
the wash water. It consists of 60 to 90 cm thick gravels of different sizes, placed in layers.
Generally, five to six layers each of 10 to 15 cm in depth are used. The coarsest gravel is
used in the top most layers. The size of the gravel in the bottom most layers is thus
generally kept between 20 to 40 mm, in the intermediate layers between 12 to 20 mm, and
6 to 12mm, and in the top most layer between 3 to 6 mm. In rapid gravity filter, the
distribution of the wash water is the critical function of the gravel layer and hence careful
grading and equally careful placing of the materials is important.
4. Underdrainage system: In slow sand filters, the under drainage system was provided only
to receive and deliver the filtered water. Whereas in rapid gravity filters, the under drainage

23
system serves two purposes 1. Receive and collect the filtered water 2. Allow the
backwashing for cleaning of filter. The underdrainage system should therefore be designed
in such a way that in addition to collecting he filtered water during its downward journey; it
should be capable of passing the wash water upward at sufficiently high velocity. Back
washing in fact consists of passing filtered water upward through the bed at such a velocity
that it causes the sand bed to expand until its thickness is 25 % to 40 % greater than during
filtering, depending upon the media. The grains move through the rising water, rub against
each other and are cleaned of deposits. A bed is usually washed when head loss through
the filter reaches of about 2 to 3m.
Operational trouble in Rapid gravity filters:
Formation of mud balls: The mud balls from the atmosphere usually accumulate on the sand
surface, so as to form a dense mat. During inadequate washing of the filter this mud may sink
down into the sand bed. This mud then sticks to the sand grains and other arrested impurities,
thereby forming mud balls. Mud balls slowly and steadily go on increasing in size and weight.
The may sink down into the gravel, thus interfering with the upward movement of wash water
during cleaning. They cause turbulence around them and thus hinder with uniform application of
wash water. The high velocities created around the edges of these balls, also displays the
gravel and thereby forming mounds.
Thus, when once the mud ball formation starts they go on increasing in number, until the entire
space in the filter box gets filled up with them.
The various measurements which may be adopted to control but ball formations are given
below:
1. Mud balls may be broken with some mechanical rakes and the broken mud particles are
washed off. .
2. Mud balls can also be broken by water stream by using a 10 mm diameter pipe having a
pointed closed end and to perforations a bit above the point. The pipe is connected to a
rubber hose furnishing water under pressure.
3. Compressed air score during backwashing for about 4 minutes supported with manual
surface raking may help in effective removal of mud balls.
4. The bed of filter is first washed then water is drawn of to 10 cm adept above the bed
and caustic soda is applied. After 12 hours of soaking the sand is thoroughly agitated
preferably, with air wash and eight hours later it is washed until the water runs clear.
5. When the filter gets badly clogged and damaged then it may have to be totally replaced.

24
Cracking of filters: The fine sand contained in the top layers of the filter beds, shrinks and
causes the development of shrinkage cracks in the sand bed. Cracks are more prominent near
the wall junctions. With the use of filter the loss of head and therefore the pressure on the sand
bed goes on increasing, with further goes on widening these cracks. The floc mud and other
impurities arrested in the filter penetrate deep into the filter through these cracks and does
empowering both the washing of the shelter and efficiency of the filtration.
Rate of filtration: the rate of filtration that can be obtained from Rapid gravity filters is very high
and is generally of the order of 3000 to 6000 lit/hour/sqm filter area. This high rate of filtration
leads to considerable saving of space as well as filter materials.
Efficiency and performance of rapid gravity filters: Rapid gravity filters compared
,to slow sand filters are less efficient in removing bacteria and turbidities. They can remove
about 80 to 90% of bacterial load present in water. The remaining bacteria are removed in
disinfection units. They can remove turbidity is to about 35 to 40 mg/L. But since the waters
entering these filters are given pretreatment in coagulation sedimentation tanks. They are
comparatively less turbid. Such turbidity can be easily removed by these filters and brought to
permissible limits.
Use of rapid gravity filters: Rapid gravity filters are best and most economical and therefore,
invariably used for treating public supplies, especially for large towns and cities. The treated
waters are however not so much safe as those obtained from slow sand filters and need for the
treatment before they can be supplied to the general public.

25

26
Pressure filters: These are just like small Rapid gravity filters placed in a closed vessel and
through which water to be treated is passed under pressure. Since water is forced through such
filters at a pressure greater than the atmospheric pressure, it is necessary that these filters are
located in airtight vessels. The raw water is pumped into the vessels by means of pumps, the

27
pressure so developed me normally very between 30 to 70 m head of water that is 300 to 700
kN/m2.
Construction of pressure filters: The filter vessel may be installed either in a horizontal or in a
vertical position, depending upon which they may be classified as horizontal pressure filters are
vertical pressure filters. Steel cylinders are used as pressure vessels and may be riveted or
welded. Their diameters generally vary between 1.5 to 3 m and their lengths our heights may
vary from 3.5 to 8 m. Inspection Windows are provided at top for inspection purposes. The
position of various valves is also clearly shown in the figure.
Working and operation of pressure filters: A pressure filter is operated like an ordinary Rapid
gravity filter except that the raw coagulated water is neither flocculated not segmented before it
enters the filter. The flocculation takes place inside the pressure filter itself. Under normal
working conditions the coagulated water under pressure enters the filter vessels to the inlet
valve 1, and the filtered water comes out of the outlet valve 2. Hence under this condition only
these two valves are kept open and all other valves are kept closed. The commonly used
coagulant is alum and is kept in a pressure container connected to the influent line to the filter.
Little time is available for this coagulant to get mixed properly or to form flocs outside the filter
vessel.
The cleaning of the filter may be carried out by backwashing as is done in the normal Rapid
gravity filter. The compressed air may also be used if designed in order to agitate the sand
grains. For cleaning the inlet and outlet valves are closed and the wash water valve 3 and wash
water gutter valve 4 are opened. After the completion of cleaning these valves may be closed
and raw supplies restored. However the filtered supplies should not be collected for a little time
and waste through valve for as is done in a rapid gravity filter.
These filters are cleaned when the loss of head due to clogging exceeds a certain fixed value.
Pressure filters require a slightly more frequent cleaning as compared to that required by Rapid
gravity filters, because the impurities which are removed in the sedimentation tank in case of
rapid gravity filters are also removed by filter in the case of pressure filters.
Rate of filtration of pressure filters: The pressure filters can yield filtered water at rates much
higher that is 2 to 5 times than what can be obtained from rapid gravity filters. There rate of
filtration normally ranges between 6000 to 15000l/h/sqm of filter area. The lower rates are used
for domestic supplies medium rates for industrial supplies and hire rates for recirculating
swimming pool supplies.
Efficiency and suitability of pressure filters: The pressure filters are less efficient than the
Rapid gravity filters in removing bacteria and turbidities. The quality of their effluent is poorer

28
and they are generally not used for public supplies. But since operational filter provides a
compact and easy handling machine they may be preferred for treating smaller quantities of
comparatively clearer water. Hence they may be installed for colonies of few houses individual
Industries Private estate swimming pool railway station etc.
Advantages and disadvantages of pressure filters
Advantages
1. A pressure filter is a compact machine and can be handled easily. Even completely
automatic units have been designed.
2. It requires lesser space and lesser filtering material for treating the same quantity of
water because the rate of filtration is higher.
3. Sedimentation and coagulation tanks are avoided.
4. They are more flexible as the rate of filtration can be changed by changing the pumping
pressure.
5. When installed on a large scale for treating turbid waters, pressure filters though prove
costlier yet may prove economical for treating smaller quantities of comparatively clearer
water. Hence they can be best adopted for supplying water to Cooperative housing
societies, individual industrial plants, private estate swimming pools etc. Since the water
coming out of the filter possesses sufficient residential head the pumping of the filtered
water is not required as in the case of rapid gravity filters.
Working and cleaning of rapid gravity filters
The working of rapid gravity filters explain with reference to the figure various valves have been
numbered as shown in the table below.
Valve Number Name of valve
1 Inlet valve
2 Waste water valve to drain water from inlet chamber
3 Waste water valve to drain water from main drain

29
4 Filtered water supply valves
5 Compressed air valve
6 Wash water supply valve
valve 1 is first of all opened which leads to the appliance of the correlation sedimentation basin
to enter the inlet Chamber of the filter. This water gets filtered through the filter and the filter
water can be taken out from the main drain by opening valve 4. Filtered water can be taken to
the disinfection unit. Thus when filter is in working condition only these two valves shall be kept
open and all other valves get closed.
Back washing: When sand becomes dirty as indicated by the excessive loss of head, the filter
must be cleaned and washed. For cleaning the raw water supplies as well as the filter supplies
have to be cut off bed drained down, and wash water sent back upward through the filter beds.
This force upward movement of water and compressed air will agitate the sand particles and
does removing the suspended impurities from it this can be accomplished as follows:
Valves 1 and 4 are closed and valves 5 and 6 are opened. The wash water and compressed are
thus forced upward from the under drainage through the gravel and sand beds. Valve 5 is
closed after supplying the required amount of air. The dirty water resulting from the washing
overflows into the wash water troughs, and is removed by opening valve 2, through the inlet
chamber into the wash water gutter. The process of washing the filters and removing the dirty
water is generally continued for a period of 3 to 5 minutes.
After the washing of the filter has been completed, valves 2 and 6 will be closed and valves 1
and 3 are opened. This restores the inlet supplies through valve 1, but the filtered water in the
beginning is not collected and washed for a few minutes through valve 3 to the gutter. This is
necessary because the remains of the wash water must be removed from the voids of the filter
and the surface mat must be allowed to be formed on sand. Ultimately
Valve 3 is closed and valve 4 is opened to get the filtered supplies again.
The entire process of backwashing the filters and Re maintaining filter supplies takes about 15
minutes, and the filter unit remains out of operation for this much of time. The amount of water
required for washing a rapid gravity filter may vary from 2 to 5% of the total amount of water
filtered. The Rapid gravity filters get locked very frequently and have to be washed every 24 to
48 hours.
Operational trouble in Rapid gravity filters:

30
Formation of mud balls: The mud balls from the atmosphere usually accumulates on the sand
surface, so as to form a dense mat. During inadequate washing of the filter this mud may sink
down into the sand bed. This mud then sticks to the sand grains and other arrested impurities,
thereby forming mud balls. Mud balls slowly and steadily go on increasing in size and weight.
The may sink down into the gravel, thus interfering with the upward movement of wash water
during cleaning. They cause turbulence around them and thus hinder with uniform application of
wash water. The high velocities created around the edges of these balls, also displays the
gravel and thereby forming mounds.
Thus, when once the mud ball formation starts they go on increasing in number, until the entire
space in the filter box gets filled up with them.
The various measurements which may be adopted to control but ball formations are given
below:
1. Mud balls may be broken with some mechanical rakes and the broken mud particles are
washed off. .
2. Mud balls can also be broken by water stream by using a 10 mm diameter pipe having a
pointed closed end and to perforations a bit above the point. The pipe is connected to a
rubber hose furnishing water under pressure.
3. Compressed air score during backwashing for about 4 minutes supported with manual
surface raking may help in effective removal of mud balls.
4. The bed of filter is first washed then water is drawn of to 10 centimetre adept above the
bed and caustic soda is applied. After 12 hours of soaking the sand is thoroughly
agitated preferably, with air wash and eight hours later it is washed until the water runs
clear.
5. When the filter gets badly clogged and damaged then it may have to be totally replaced.
Cracking of filters: The fine sand contained in the top layers of the filter beds, shrinks and
causes the development of shrinkage cracks in the sand bed. Cracks are more prominent near
the wall junctions. With the use of filter the loss of head and therefore the pressure on the sand
bed goes on increasing, with further goes on widening these cracks. The floc mud and other
impurities arrested in the filter penetrate deep into the filter through these cracks and does
empowering both the washing of the shelter and efficiency of the filtration.
Rate of filtration: the rate of filtration that can be obtained from Rapid gravity filters is very high
and is generally of the order of 3000 to 6000 lit/hour/sqm filter area. This high rate of filtration
leads to considerable saving of space as well as filter materials.

31
Efficiency and performance of rapid gravity filters: Rapid gravity filters compared to slow
sand filters are less efficient in removing bacteria and turbidities. They can remove about 80 to
90% of bacterial load present in water. The remaining bacteria are removed in disinfection units.
They can remove turbidity is to about 35 to 40 mg/L. But since the waters entering these filters
are given pretreatment in coagulation sedimentation tanks., they are comparatively less turbid.
Such turbidity can be easily removed by these filters and brought to permissible limits.
Use of rapid gravity filters: Rapid gravity filters are best and most economical and therefore,
invariably used for treating public supplies, especially for large towns and cities. The treated
waters are however not so much safe as those obtained from slow sand filters and need for the
treatment before they can be supplied to the general public.
Pressure filters: These are just like small Rapid gravity filters placed in a closed vessel and
through which water to be treated is passed under pressure. Since water is forced through such
filters at a pressure greater than the atmospheric pressure, it is necessary that these filters are
located in airtight vessels. The raw water is pumped into the vessels by means of pumps, the
pressure so developed me normally very between 30 to 70 mhead of water that is 300 to 700
kN/m2.
Construction of pressure filters: The filter vessel may be installed either in a horizontal or in a
vertical position, depending upon which they may be classified as horizontal pressure filters are
vertical pressure filters. Steel cylinders are used as pressure vessels and may be riveted or
welded. Their diameters generally vary between 1.5 to 3 m and their lengths our heights may
vary from 3.5 to 8 m. Inspection Windows are provided at top for inspection purposes. The
position of various valves is also clearly shown in the figure.

32
Working and operation of pressure filters: A pressure filter is operated like an ordinary Rapid
gravity filter except that the raw coagulated water is neither flocculated not segmented before it
enters the filter. The flocculation takes place inside the pressure filter itself. Under normal
working conditions the coagulated water under pressure enters the filter vessels to the inlet
valve 1, and the filtered water comes out of the outlet valve 2. Hence under this condition only
these two valves are kept open and all other valves are kept closed. The commonly used
coagulant is alum and is kept in a pressure container connected to the influent line to the filter.
Little time is available for this coagulant to get mixed properly or to form flocs outside the filter
vessel.
The cleaning of the filter may be carried out by backwashing as is done in the normal Rapid
gravity filter. The compressed air may also be used if designed in order to agitate the sand
grains. For cleaning the inlet and outlet valves are closed and the wash water valve 3 and wash

33
water gutter valve 4 are opened. After the completion of cleaning these valves may be closed
and raw supplies restored. However the the filtered supplies should not be collected for a little
time and waste through valve for as is done in a rapid gravity filter.
These filters are cleaned when the loss of head due to clogging exceeds a certain fixed value.
Pressure filters require a slightly more frequent cleaning as compared to that required by Rapid
gravity filters, because the impurities which are removed in the sedimentation tank in case of
rapid gravity filters are also removed by filter in the case of pressure filters.
Rate of filtration of pressure filters: The pressure filters can yield filtered water at rates much
higher that is 2 to 5 times than what can be obtained from rapid gravity filters. There rate of
filtration normally ranges between 6000 to 15000l/h/sqm of filter area. The lower rates are used
for domestic supplies medium rates for industrial supplies and hire rates for recirculating
swimming pool supplies.
Efficiency and suitability of pressure filters: The pressure filters are less efficient than the
Rapid gravity filters in removing bacteria and turbidities. The quality of their effluent are poorer
and they are generally not used for public supplies. But since operational filter provides a
compact and and easy handling machine they may be preferred for treating smaller quantities of
comparatively clearer water. Hence they may be installed for colonies of few houses individual
Industries Private estate swimming pool railway station etc.
Advantages and disadvantages of pressure filters
Advantages
1. A pressure filter is a compact machine and can be handled easily. Even completely
automatic units have been designed.
2. It requires lesser space and lesser filtering material for treating the same quantity of
water because the rate of filtration is higher.
3. Sedimentation and coagulation tanks are avoided.
4. They are more flexible as the rate of filtration can be changed by changing the pumping
pressure.
5. When installed on a large scale for treating turbid waters, pressure filters though prove
costlier yet may prove economical for treating smaller quantities of comparatively clearer
water. Hence they can be best adopted for supplying water to Cooperative housing
societies, individual industrial plants, private estate swimming pools etc. Since the water
coming out of the filter possesses sufficient residential head the pumping of the filtered
water is not required as in the case of rapid gravity filters.

34
Disadvantages:
1. Although the rate of filtration is high, the filter unit being smaller, the overall capacity of
the plant is small.
2. They are less efficient in removing bacteria and turbidities and hence the quality of the
filtered effluent is poorer.
3. They are costlier particularly for treating large scale municipal supplies.
4. Since the process of filtration as well as that of back washing takes place in a closed
tank proper inspection and quality control is not possible.
5. Inspection, cleaning and replacement of sand, gravel and underdrainage systems is
difficult.
6. Because of their inherent curved shapes properly designed wash water gutters which
can trap the washed impurities without flushing them back to other portions of the sand
beds cannot be provided easily.
7. Since these filters are operated under pressure, the normal tendency is to pump the
water at higher rates and thus obtaining still poorer quality of effluents.
Difference between SSF and RSF

35

36
FAQ
1. Define surface flow rate and detention time for a sedimentaion tank .
2. Describe briefly the various constituents of coagulation sedimentaion tank.
3. Explain with a neat sketch the working and backwashing of rapid gravity filters.
4. Explain in detail theory of filtration.
5. Discuss in detail the operational problems in filters.
6. Explain briefly the design elements of rectangular sedimentation tank.
7. What are the characteristics of good coagulant?
8. Explain the causes for fouling of membrane and how it can be controlled.
9. Differentiate between SSF and RSF
10. All problems
References:
1. “Water supply Engineering” by Santhosh Kumar Garg, Khanna publishers.
2. Previous year question papers

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Water supply and treatment Engineering notes 18 cv46

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