CE 6605 EE-II

ANSWER KEY SET –A

PART – A
1. What are the types of treatment processes?
 Preliminary treatment
 Primary treatment
 Complete final treatment
 Secondary treatment
2. What are the various sources of wastewater generation?
 Industrial Wastes
 Domestic wastes
 Agricultural Wastes
3. Define Design period.
The future period for which the provisions are made in the designing capacities of the
various components of the sewerage scheme is known as design period.
4. What do you mean by time of concentration?
Time of concentration = Time of inlet + Time of flow
In the design of sewer, the storm whose duration is equal to the time of concentration
produces the maximum flow in sewer.
5. Distinguish between self cleansing velocity and non scouring velocity.
Self cleansing velocity flow, the velocity of sewage is minimum and the settling of
particles avoided. In non scouring velocity, the flow velocity of sewage is maximum
and avoids the scouring action of particles on the sewer pipes.

PART – B
6.(i) Describe the types of screens with need sketch.
Screening is the very first operation carried out at a sewage treatment plant, so as to
trap and remove the floating matter, such as pieces of cloth, paper, wood cork, hair, fiber,
kitchen refuge, fecal solids etc present in sewage. Thus, the main idea of providing screens it
to protect the pumps and other equipments from the possible damages due to the floating
matter of the sewage. Types of screens depending upon the size of the openings and screens
may be classified as
1) Coarse screen
2) Medium screens
3) Fine screens
Coarse Screen:
It is also known as racks and the spacing between the bars (i.e., opening size) is about 50 mm
or more. These screens do help in removing large floating objects
from sewage. The material separated by coarse screens, usually consists of rags, wood, paper
etc.
Medium Screen:
The spacing between bars a about 6 to 40 mm. These screens will ordinarily collect 30 to 90
lit of material per million liter of sewage. The screenings usually contain some quantity of
organic material may be dispersed of by incineration of burial.
Rectangular Shaped coarse and medium screens are now – a – days widely used at sewage
treatment plants. Now – a – days these screens are generally kept inclined at about 30 to 60%
to the direction of flow, so as to increase the opening area and to reduce the flow velocity and
there making the screening more effective.
Fine Screens:
Have perforations of 1.5mm to 3mm in size. The installation of there screens prove very
effective and they remove as much as 20% of the suspended solids from
sewage. These screens, however, get clogged very often, and need frequent cleaning. They
are, therefore, used only for treating the industrial waste waters or for
treating those municipal waste wasters, which are associated with heavy amounts of
industrial wastewaters. These screens will considerably reduce the load on
further treatment units. Brasses of Bronze plates or wire mesh are generally used for
constructing fine screens. The metal used to should be resistant to rest and Corrosion. Find
screens may be disc or drum type, and are operated continuously by electric motor.

6.(ii) Explain the method of laying sewer line for the designed /desired alignment and
gradient.
Laying the Sewer Pipes
Before laying the sewer pipes, the gradient of the bottom of the excavated trench
should be checked. The trench should heexcavated upto a level equal to the bottom of the
embedded concrete (wherever concrete bedding is required) After the bedding concrete has
been laid in the alignment and levels, the sewer pipes are lowered down into the trench.Sewer
of diameter less than 40cm with smaller length can be directly laid by masons and helper, and
larger pipe lengths
can be lowered by passing ropes around each end of the pipelength. For very larger pipes,
equipments are used to lower

Generally the sewer pipe lengths laid from the lowestpoint with their socket ends
facing upstream, as shown in.In this technique, the spigot of each pipe can be easilyinserted
in the socket of the already laid pipe.After thissuitable joining method is carried out. During
joining (orjointing), care should be taken to check that the pipe lengths remain in true level
and alignment.

7.(i) Explain classification of Treatment processes?

Sewage before being disposed of either in river streams or on land has generally to be
treated. So as to make it safe Sewage can be treated in difference ways treatment process are
often classified as
1) Preliminary treatment
2) Primary treatment
3) Secondary or (biological) treatment
4) Complete final treatment
Preliminary treatment:
Preliminary treatment consists solely in separating the floating materials (Like dead
animals, tree branches, papers, pieces of rags, wood etc) and also the heavy
settle able inorganic solids. It also helps in removing the oils and greases etc.
From the sewage this treatment reduces the BOD of the waste water, by about 15 to 30%.
The process used are screening for removing floating papers, rags, clothes etc. Grit chambers
or detritus tanks: For removing grit and sand
Slimming tanks: For removing oils and greases.
Primary Treatment
Primary treatment consists in removing large suspended organic solids. This is usually
this is usually accomplished by sedimentation on settling basins.
The liquid effluent from primary treatment often contains a large amount of suspended
organic material and has a high BOD about (60% of original).
The original solids which are separated out in the sedimentation tanks (in primary treatment)
are often stabilized by an anaerobic decomposition in a digestion
tanks or are incinerated. Sometimes the preliminary as well as primary
treatments are classified to gather under primary treatment.
Secondary treatment
Secondary treatment involves further treatment of the efficient, coming from the
primary sedimentation tank. This is generally accomplished through biological
decomposition of organic matter, which can be carried out either under aerobic or anaerobic
conditions. In these biological units, bacteria will decompose the fine organic matter, to
produce cleaner effluent.
The treatment reactors, in which the organic matter is decomposed (oxidized) by
aerobic bacteria are known as aerobic biological units; and may consists of
(i) Filters (intermittent sand filters as well as trick long filters).
(ii) Aeration tanks with the feed of recycled activated sludge (i.e., the sludge which I
settled in secondary sedimentation tank, receiving effluents from the aeration tank)
(iii) Exudation ponds and Aerated legions. Since the there aerobic units, generally
make use of primary settled sewage, they are early classified as secondary units.
The effluent from the secondary biological
treatment will usually contain a little BOD (5 to 10% of the original). The organic solids
sledge separated out in the primary as wells as in the secondary settling tank will be disposed
of by stabilizing them under anaerobic process
in a sludge digestion tank.
The final or advanced Treatment
Thus treatment is sometimes called tertiary treatment, and consists in removing the
organic local left after the secondary treatment, and particularly to kill the pathogence
bacteria. Thus treatment, which is normally carried out by chlorination

7.(ii)(a) A population of 30,000 is residing in a town having an area of 60 hectares. If the

average coefficient of runoff for this area is 0.60, and the time of concentration of the
design rain is 30 minutes, calculate the discharge for which the sewers of a proposed
combined system will be designed for a town.
Soln.
Assume average water supply per capita =120 litres/day/person
Quantity of sewage produced per day= (80/100] x 120x30, 000 litres
= 2880 cu.m
Quantity of sewage produced per second= 2880/ [24x60x60] =0.033 cumecs
Maximum sewage discharge = 3x 0.033 = 0.1 cumecs
The storm discharge Qp =1/36 K PcA
Using p=100/(T+20)
We have Pc=100/(30+20)
= 2cm/hr
Qp=(1/36) X 0.60 X 2 X 60 cumecs
= 2 cumecs
7.(ii)(b) The 5 day BOD 300C of sewage sample is 110 mg / l. Calculate its 5 days 200C
BOD. Assume the deoxgenation constant at 200C, K20 as 0.1
Soln:
K20 = 0.1
Using Equation,
KD(To) = KD(200) [1.047]T-20º
KD(30o) =( 0.1) [1.047]30-20º
KD(30o) = 0.158
Using Equation.
Yt= L[1-(10) –KD .t]
Y5= L[1-(10) –KD .5]
Y5 at 30 º = L[1-(10) –KD (30º ) x 5]
110 = L[1-(10) –0.158 x 5]
L = 131.3 mg/l
Y5 at 20 º = L[1-(10) –KD (20º ) x 5]
Y5 at 20 º = 131.3 [1-(10) – 0.1 x 5]
=89.88 mg/l
8.(i) What are the steps involved in laying of sewer pipes
All sewer pipes are laid starting from their outfall ends, towards their starting ends
Thecenter line of the sewer is marked by drawing a line on the ground parallel to it. Offset
line is offsetted at a distance of about half the trench width +0.6m Pegs- 15m interval
Temporary bench mark at 200 to300 m
Excavating trenches
Road pavements –removed Width of excavation depends on width of the trench Width of the
trench at the bottom is kept 15 cm more than dia of sewer Excavated material is deposited in
one end of the trench Offset line on the other side
Timbering or sheeting trenches
When trench is excavated in depths more than 2m-soil is supported by sheeting and bracing
the trenches The bracing will absorb the soil pressure.The sheeting are wooden planks and
placed in contact with trench sides .The braces or struts are the wooden pieces extending
from one side of trench to the other side Rangers or wales are timber blanks which transfer
the load from the sheeting boards to cross braces
Dewatering of trenches
Water table is higher than sewer pipes –ground water infiltrates in to the sewer The ground
water is removed through an open jointed drain constructed below sewer pipes which
discharges in to water course
Shaping the trench bottom to correct levels
Vertical post driven Horizontal board is fixed on the posts is called sight rail. Vertical pencil
line is marked with help of theodolite Horizontally projecting needle is fixed at a known
reduced level
The sting is stretched between the rails, This will establish a line which is parallel to sewer
pipes and with known height It used for measuring the exact depth of excavation, level of top
of foundation concretead the level of the sewer pipe
Laying of sewer pipes .Smaller than 40 cm dia can be handled by masons Larger pipes
lowered by passing ropes around Machines –used
8.(ii) Determine the diameter of a separate sewer section running 0.7 times full at
maximum discharge for a population of 50,000 persons. Water is supplied at a rate of
140 litres per capita per day. Assume 80% of water supplied turns as sewage. Take peak
factor as 2.25 and Mannings constant N= 0.013 at all depths of flow. The permissible
slope is 1 in 600.
Solution:
No. of persons=50,000
Water supplied=140 l/cap/day
Peak factor= 2.25
N= 0.013
S= 1/600
Maximum flow for total population q =2.25*140*(80/100)*50,000*(1/(24*60*60))
=0.1458 m3/sec
When d/D=0.7 then a =0.5903 D2 and p= 0.2981D
q = 1/n*a*r 2/3*s ½
0.1458 = 1/0.013* 0.5903 D2 *(0.2981D)2/3*(1/600)1/2
Diameter of the sewer = 0.5215 m
 ANSWER KEY SET –B
PART – A

1. What are the factors affecting the quantity of storm water?

a) Population b) rate of water supply c) industrial and its type d) infiltrations &
exfiltration
2. Define BOD/COD ratio and state its significance.
The quantity of BOD5 to the COD is referred as BOD/COD ratio and it is always less
than 1. This ratio is very important for deciding the quantity and the type of treatment
given to waste water for the safe disposal.
3. Define sewerage system and list out the components of it.
A sewerage system consists of a net work of sewers for carrying the sewage from
individual units to the sewage treatment plant. This system consists of house sewers,
lateral sewers, branch sewers, main sewers, trunk sewers, man holes and catch basins.
4. What are the situations where the pumping of sewage becomes essential in
sewage management?
a) The place where the basement level of the houses are lower than the invert level of
the sewer.
b) To lift the sewage from low lying area
5. Where are manholes provided in the sewerage system?
Man holes are provided in sewerage system at every bend, junction, change of
gradient or change of sewer diameter.

PART – B
6.(i) Discuss briefly the various factors affecting the dry weather flow.
Factors influencing the dry weather flow are
1. Rate of water supply
2. Population growth
3. Type of area served
4. Infiltration of ground water
Rate of water supply
The quantity of waste water produced from a community would naturally
depend on the rate of water supplied. The quantity of waste water entering the sewer
would be less than the total quantity of water supplied, because water may be lost in
domestic consumption, evaporation, lawn sprinkling etc. In general, waste water
generated may be assumed to be 80% of the water supply.
Population growth
The sewerage system is designed not only for the present population, but also for
the population for the next few years. The population forecast maybe done by
1. Arithmetic increase method
2. Geometrical increase method
3. Decreased rate of growth method
4. Graphical comparison method
5. Zoning method
6. Master plan method
 Type of area served

The quantity of waste water produced depends upon whether the area is
commercial, residential or industrial.

Infiltration of sub soil

Ground water may infiltrate into the sewers through leaky joints

6.(ii) A town has a population of 1, 00,000 persons with a per capita water supply of 200
litres per day. Design a sewer running 0.7 times full at maximum flow condition. Take
N= 0.013 at all depths of flow, slope=1 in 500 and peak factor of 3

Solution:
Maximum water supplied = 1,00,000*200*80/100*3/(24*60*60*1000)

= 0.556 m3/sec
When d/D =0.7
then a= 0.5872 D2 and
p= 1.9823D as a/p =m, m =0.2962D
When q= 1/N * a*m 2/3*i1/2
By solving we get D= 0.835

7.(i) What are the physical, chemical and biological characteristics of sewage.
Characteristics of sewage can be classified as

1 Physical Characteristics

2 Chemical Characteristics

3 Biological Characteristics

Physical Characteristics

Colour: Fresh sewage has a light brown or yellowish orange colour. As the
decomposition starts with the time the colour of sewage begins to get black. Colour of
the industrial sewage depends on the chemical process in the industries.

Odour: Fresh sewage has slightly oily odour, but the stale sewage has offensive
odour of hydrogen sulphide and other sulphur compounds.

Temperature: Generally the temperature of the sewage is slightly higher than the
water supply.

Turbidity: It directly depends on the quantity of solid matter present in the suspended
state.

Solids: Sewage contains more than 99.9% of water and only 0.1% of solids. These
solids are present in sewage in suspended, dissolved and colloidal states.

Chemical Characteristics
 Sewage contains complex organic matters derived from urine, faeces, etc. and
inorganic chemicals. Normally fresh sewage is alkaline in nature but tends to acidity
as it becomes stale. The organic compounds can be divided as containing nitrogen and
free from nitrogen.

The sand, gravel, debris are the inorganic matters present in the sewage which
becomes street washings, kitchen and court yard washings.

Biological Characteristics

Sewage contains large quantity of bacteria which come from excreta matter. All the
bacteria present in the sewage are not harmful. More quantity of bacteria is harmless.
They help to treat sewage and reduce the cost of treatment plants. The main living
organisms in sewage are

1.Bacteria: Disease causing bacteria are known as pathogens. They may cause
typhoid fever, dysentery, cholera etc. Therefore it is necessary to kill the pathogens by
proper treatment.

2.Algae: Algae are unicellular plants containing chlorophyll. The physiological

process helps algae in absorbing carbon di oxide and release oxygen.

3. Protozoa: These include all unicellular animals. Most protozoa are bacteria eaters
and destroy the pathogens.

7.(ii) How to estimate the design sewage discharge . Explain time of concentration.

Addition due to unaccounted private water supplies Public uses Private wells and tube
wells in addition with public distribution
Addition due to infiltration
Sewer pipes are laid below ground water table-water seeps in to pipes through faulty
joints or cracks- quantity depends on height of water table and extends of faults
Ground water table is below sewer pipes- infiltration occurs it depends on permeability
of ground soil
Subtraction due to water losses
Water lost due to leakage in distribution system does not appear as sewage
Subtraction due to water not entering the sewerage system
Water used in boilers, streets, lawns, and gardens does not produce sewage
Net quantity of sewage produced =75 to 80% of the accounted water supplied from water
works
The time of concentration consist of two part
i. The inlet time or overland flow time or time of equilibrium
Ti= (0.885 L3/H) 0.385
Where Ti =inlet time in hours
L=length of overland flow in kilometres from the critical point to the mouth of the
drain
H=total fall of level from the critical point to the mouth of the drain in meters
ii .The channel flow time or gutter flow time Tf
 Tf = Length of drain / velocity in the drain
Total time of concentration Tc = Ti + Tf

8.(i) Explain various systems of sanitary plumbing for buildings.

One pipe system:

In this system, one main vertical pipe is provided for collecting the sullage and night
soil through the branch sewers. This main pipe is ventilated by providing cowl at its
top. A separate vent pipe is also provided. This system involves less number of pipes,
and hence this system is economical. This system has two pipes only.
Two pipe system:
In this system, two main vertical pipes are provided for collecting the sullage and
night soil through the branch sewers. The pipe used for carrying the night soil is
called soil pipes and the pipe used to carry the sullage is called sullage pipes or waste
pipes. The soil pipes and sullage pipes are separately ventilated by providing separate
vent pipes and anti siphonage pipes. This system involves a large number of pipes and
hence this system is a costly method. This system has four pipes

8.(ii) Design a sewer to serve a population of 36,000. The daily per capita water supply
allowance being 135 litres, of which 80 per cent finds its way into the sewer. The slope
available for the sewer to be laid is 1 in 625 and the sewer should be designed to carry
four times the dry weather flow when running full. What would be the velocity of flow
in the sewer when running full? Assume ‘n’ = 0.012 in Manning’s formula.

Given:
Population=36,000
Per capita water supply=135litres
Available slope for sewer=1 in 625
To Find:
Velocity of flow in Sewer when
 running full?
Solution:
Population = 36,000
Per Capita water supply = 135 litres/person/day
Avg water supplied daily = 36,000 x 135 litres/day
= 4860000 litres/day
Avg water supplied in cumecs = 4860000/(1000x24x60x60)
= 0.0562 cumecs.
Avg Sewage discharge = 80% of water supplied
= 0.8 x 0.0562 cumecs
= 0.045 cumecs
D.W.F = 0.045 cumecs
Maximum discharge for which sewer should be designed running full,
= 4 x 0.045 cumecs
= 0.18 cumecs
Now, using Manning’s formula,
q = 1/n*a*r 2/3*s ½
D = 0.51m Hence use 0.51m dia sewer pipe.
Velocity of flow when running full,
V=Q/A
= 0.18 / (π . 0.312 / 4)
V = 2.39 m/sec