# WASTE WATER Engineering Interview Questions

## 100+ TOP WASTE WATER Engineering Questions and Answers

Q No: 1
For a continuous flow type of sedimentation tanks
A. Width of the tank is normally kept about 6 m
B. Length of the tank is normally kept 4 to 5 times the width
C. Maximum horizontal flow velocity is limited to 0.3 m/minute
D. All the above
ANS: D

Q No: 2
The asbestos cement sewers are
A. Light in weight
B. Not structurally strong to bear large compressive stress
C. Susceptible to corrosion by sulphuric acid
D. All the above
ANS: D

Q No: 3
For efficient working of a sewer, it must be ensured that
A. Minimum velocity of 0.45 m/sec, is maintained at its minimum flow
B. A maximum velocity of 0.90 m/sec, is maintained at its maximum flow
C. Both A. and B.
D. Neither A. nor B.
ANS: C

Q No: 4
Pick up the correct statement from the following:
A. The sewer pipes of sizes less than 0.4 m diameter are designed as running full at maximum discharge
B. The sewer pipes of sizes greater than 0.4 m diameter are designed as running 2/3rd or 3/4th full at maximum discharge
C. The minimum design velocity of sewer pipes is taken as 0.8 m/sec
D. All the above
ANS: D

Q No: 5
Pick up the correct statement from the following:
A. Maximum daily flow = 2 times the average daily flow
B. Maximum daily flow = average daily flow
C. Sewers are designed for minimum permissible velocity at minimum flow
D. All the above
ANS: D

Q No: 6
Bottom openings 15 cm × 15 cm in the standing baffle wall are provided
A. 15 cm c/c
B. 22.5 cm c/c
C. 30 cm c/c
D. 50 cm c/c
ANS: B

Q No: 7
The pressure exerted by
A. The sewage when running full from inside, is called internal pressure
B. The internal pressure if any, causes tensile stress in the pipe material
C. Pressure sewers are designed to be safe in tension
D. All the above
ANS: D

Q No: 8
For drainage pipes in buildings the test applied before putting them to use, is
A. Water test
B. Smoke test
C. Straightness test
D. All the above
ANS: B

Q No: 9
No treatment of the sewage is given if dilution factor is
A. Less than 150
B. Between 150 to 200
C. Between 200 to 300
D. More than 500
ANS: D

Q No: 10
Which of the following pumps in used to pump sewage solids with liquid sewage without clogging the pump is?
A. Centrifugal pump
B. Pneumatic ejector
C. Reciprocating pump
D. None of these
ANS: A

Q No: 11
Acid regression stage of sludge digestion at a temperature 21°C extends over a period of
A. 15 days
B. 30 days
C. 60 days
D. 90 days ANS: D

Q No: 12
Pick up the correct statement from the following:
A. Small sewers are cleaned by flushing
B. Medium sewers are cleaned by cane rodding
C. Medium sewers may be cleaned by pills
D. All the above
ANS: D

Q No: 13
The coagulant which is generally not used for treating the sewage, is
A. Alum
B. Ferric chloride
C. Ferric sulphate
D. Chlorinated copperas
ANS: A

Q No: 14
The layers of vegetable wastes and night soil alternatively piled above the ground to form a mound, is called
A. A heap
B. Plateau
C. Windrow
D. None of these
ANS: D

Q No: 15
If the flame of a miner’s safety lamp in the upper layers of the sewer forms an explosive, the sewer certainly contains
A. Hydrogen sulphide
B. Carbon dioxide
C. Methane
D. Oxygen
ANS: C

Q No: 16
The gas which may cause explosion in sewers, is
A. Carbondioxide
B. Methane
C. Ammonia
D. Carbon monoxide
ANS: B

Q No: 17
Pick up the correct statement from the following:
A. The water supply pipes carry pure water free from solid particles
B. The water supply pipes get clogged if flow velocity is less than self cleansing velocity
C. The sewers may be carried up and down the hills and valleys
D. The sewer pipes are generally laid along level hills
ANS: A

Q No: 18
Pick up the incorrect statement from the following:
A. The process of decomposing the organic matter under controlled anaerobic conditions, is called sludge digestion
B. Sludge digestion is carried out in sludge tank
C. The gases produced in sludge digestion process, contain 75% carbon dioxide
D. The gases produced in sludge digestion process, contain 75% methane
ANS: C

Q No: 19
The sewer pipes
A. Carry sewage as gravity conduits
B. Are designed for generating self-cleansing velocities at different discharge
C. Should resist the wear and tear caused due to abrasion
D. All the above
ANS: D

Q No: 20
The settling velocity of the particles larger than 0.06 mm in a settling tank of depth 2.4 is 0.33 m per sec. The detention period recommended for the tank, is
A. 30 minutes
B. 1 hour
C. 1 hour and 30 minutes
D. 2 hours
ANS: D

Q No: 21 depends upon
The Chezy’s constant C in the formula V = C
A. Size of the sewer
B. Shape of the sewer
C. Roughness of sewer surface
D. All the above
ANS: D

Q No: 22
The discharge per unit plan area of a sedimentation tank, is generally called
A. Over flow rate
C. Over flow velocity
D. All the above
ANS: D

Q No: 23
Pick up the incorrect statement from the following for allowing workers to enter sewers
A. The particular manhole and one manhole on upstream and one manhole on downstream should remain open for 30 minutes
B. Proper tests for the presence of poisonous gases must be carried out
C. The men entering the manhole should be advised to smoke in the sewer
D. Warning signals should be erected
ANS: C

Q No: 24
For the open drain (N = 0.025) shown in the below figure, the discharge is
A. 26.88 cumecs
B. 27.88 cumecs
C. 28.88 cumecs
D. 29.88 cumecs ANS: C

Q No: 25
To prevent settling down of sewage both at the bottom and on the sides of a large sewer, self-cleaning velocity recommended for Indian conditions, is
A. 0.50 m/sec
B. 0.60 m/sec
C. 0.70 m/sec
D. 0.75 m/sec
ANS: D

Q No: 26
Assertion A.: The determination of pH value of sewerage is important.
Reason (R): The efficiency of certain treatment methods depends upon the availability of pH value.
A. Both A and R are true and R is the correct explanation of A
B. Both A and R are true but R is not a correct explanation of A
C. A is true but R is false
D. A is false but R is true
ANS: A

Q No: 27
The normal values of over flow rates for sedimentation tanks using coagulant, ranges between
A. 25,000 to 35,000 litres/sqm/day
B. 40,000 to 50,000 litres/sqm/day
C. 50,000 to 60,000 litres/sqm/day
D. 80,000 to 100,000 litres/sqm/day
ANS: C

Q No: 28
The drainage area of a town is 12 hectares. Its 40% area is hard pavement ( K = 0.85), the 40% area is unpaved streets (K = 0.20) and the remaining is wooded areas (K = 0.15). Assuming the time of concentration for the areas as 30 minutes and using the formula Ps = 900/(t + 60) the maximum run off is
A. 0.10 cumec
B. 0.12 cumec
C. 0.15 cumec
D. 0.20 cumec
ANS: C

Q No: 29
Sewer pipes are designed for maximum discharge with 25% to 33% vacant cross-sectional area for
A. Unexpected large scale infiltration of stream water
B. Unexpected increase in the population
C. Under estimates of maximum and average flows
D. All of the above
ANS: D

Q No: 30
The rational formula for peak drainage discharge, was evolved by
A. Fruhling
B. Lloyd David
C. Kuichling
D. All of these
ANS: D

Q No: 31
The sewer that unloads the sewage at the point of treatment is called
A. Main sewer
B. Outfall sewer
C. Branch sewer
D. House sewer
ANS: B

Q No: 32
During preliminary treatment of a sewage
A. Oil and grease are removed from skimming tanks
B. Floating materials are removed by screening
C. Girt and sand are removed by girt chambers
D. All the above
ANS: D

Q No: 33
For a grit chamber, if the recommended velocity of flow is 0.2 m/sec and detention period is 2 minutes, the length of the tank, is
A. 16 m
B. 20 m
C. 24 m
D. 30 m
ANS: C

Q No: 34
Dry weather flow is:
A. Average daily rate of flow
B. Average monthly rate of flow
C. Average annual rate of flow
D. Water supply allowance per capita
ANS: D

Q No: 35
Pick up the incorrect statement from the following:
A. The mixture of water and waste products, is called sewage
B. The treated sewage effluents, are generally used for irrigating the crops
C. The process of collecting, treating and disposing off the sewage, is called sewerage
D. The old convergence system was definitely better than water carried sewerage system ANS: D

Q No: 36
Pick up the correct statement from the following:
A. The maximum rate of storm runoff is called peak drainage discharge
B. Rational method of estimating peak run off, may be used precisely for areas less than 50 hectares
C. The period after which the entire area starts contributing to the runoff, is called the time of concentration
D. All the above
ANS: D

Q No: 37
If q is the average sewage flow from a city of population P, the maximum sewage flow
A. Q = [(4 + )/(18 + )] q
B. Q = [(18 + P)/(4 + )] q
C. Q = [(18 + )/(4 + )] q
D. Q = [(5 + )/((15 + )] q
ANS: C

Q No: 38
¾th or ¼th extra space is left in sewer pipes at maximum discharge for
A. Low estimates of the average and maximum flows
B. Large scale infiltration of storm water
C. Unexpected increase in population
D. All the above
ANS: D

Q No: 39
If the flame of a miner’s safety lamp in a manhole extinguishes within 5 minutes, the sewer certainly contains
A. Hydrogen sulphide
B. Carbon dioxide
C. Methane
D. Oxygen
ANS: B

Q No: 40
B. Difference in heads between two points in circular pipes
C. Mean of radii in a pipe line of varying cross -sections
D. Cross-sectional area/wetted perimeter
ANS: D

Q No: 41
If the depth of flow in a circular sewer is 1/4th of its diameter D, the wetted perimeter is
A. /2
B. /4
C. /3
D. D
ANS: C

Q No: 42
Pick up the correct statement from the following:
A. Anaerobic bacteria flourish in the absence of oxygen
B. Aerobic bacteria flourish in the presence of oxygen
C. Facultative bacteria flourish with or without oxygen
D. All the above
ANS: D

Q No: 43
The Brake Horse power of the motor (efficiency 60%) required for a pump of capacity 0.075 cumec for a total lift of 12 m, is
A. 10
B. 15
C. 20
D. 25

Q No: 44
Aerobic bacterias
A. Flourish in the presence of free oxygen
B. Consume organic matter as their food
C. Oxidise organic matter in sewage
D. All the above
ANS: D

Q No: 45
At the junction of sewers
A. Top of smaller sewer is kept lower
B. Top of larger sewer is kept lower
C. Tops of both the sewers are at the same level
D. None of these
ANS: D

Q No: 46
Traps
A. Are water seals which prevent the entry of foul gases
B. Are used to trap the rats entering sewers
C. Dissolve the foul gases
D. Create symphonic action to increase the quick disposal of sewerage
ANS: A

Q No: 47
The sewage treatment units in which anaerobic decomposition of organic matter is used, are called
A. Imhoff tanks
B. Trickling filters
C. Sludge sedimentation tanks
D. None of these
ANS: B

Q No: 48
Pick up the correct statement from the following:
A. In treated sewage, 4 ppm of D.O. is essential
B. Only very fresh sewage contains some dissolved oxygen
C. The solubility of oxygen in sewage is 95% that is in distilled water
D. All the above
ANS: D

Q No: 49
If the diameter of a sewer is 150 mm, the gradient required for generating self cleansing velocity, is
A. 1 in 60
B. 1 in 100
C. 1 in 120
D. None of these
ANS: B

Q No: 50
The sewer which received discharge from two or more main sewers, is known as
A. A trunk sewer
B. An outfall sewer
C. A main sewer
D. An intercepting sewer
ANS: A

Q No: 51
The grit and silt of the grit chambers, may not be used for
A. Raising low lying areas by dumping
B. Concreting
C. Both A. and B.
D. Neither A. nor B.
ANS: B

Q No: 52
For treating the sewage of a large city, you will recommend
A. A sedimentation tank and an activated sludge treatment plant
B. A plant consisting of Imhoff tanks with low rate trickling filters
C. Sedimentation tanks with high rate trickling filters
D. None of these
ANS: A

Q No: 53
Imhoff cone is used to measure
A. Total organic solids
B. Total solids
C. Total in organic solids
D. Settleable solids
ANS: D

Q No: 54
Pick up the correct statement from the following:
A. The ratio of the quantity of the diluting water to that of the sewage, is known as dilution factor
B. The automatic purification of polluted water, is known self purification phenomenon
C. The photosynthesis is carried out in the presence of sun light
D. All the above
ANS: D

Q No: 55
1000 kg of sewage contains
A. 0.112 kg in suspension
B. 0.112 kg in solution
C. 0.225 kg in solution
D. Both A. and C. of above
ANS: D

Q No: 56
Dilution method of disposing of sewage, is not preferred to
A. When sewage is fresh
B. When diluting water has high dissolved oxygen content
C. When diluting water is used for water supply near the point of sewage disposed
D. When the diluting water is having flow currents
ANS: C

Q No: 57
If the peak discharge of a storm water drain (S.W. Drain) is expected to exceed 150 cumecs, the free board to be provided, is
A. 100 cm
B. 90 cm
C. 80 cm
D. 50 cm
ANS: A

Q No: 58
The reduced levels of the string at the consecutive sight rails A and B are 203.575 m, 203.475 m respectively. If the difference of their R.D.s is 10 m, the gradient of the sewer line is
A. 1 in 100 upward
B. 1 in 500 upward
C. 1 in 100 downward
D. 1 in 503 upward
ANS: C

Q No: 59
R.M.O. expenses include
A. Running expenses
B. Maintenance expenses
C. Operation expenses
D. All the above
ANS: D

Q No: 60
For the COD test of sewage, organic matter is oxidised by K2Cr2O7 in the presence of
A. H2SO4
B. HNO3
C. HCl
D. None of these
ANS: A

Q No: 61
The sludge does not contain waste water from
A. Bath rooms
B. Wash basins
C. Kitchen sinks
D. Toilets
ANS: D

Q No: 62
In sewers the velocity of flow should not be
A. More than the self-cleansing velocity
B. Less than the self-cleansing velocity
C. Less than 10 m/sec
D. More than 20 m/sec
ANS: B

Q No: 63
Pick up the correct statement from the following:
A. Hydrogen sulphide gas in excess, may cause corrosion of concrete sewers
B. 4 ppm of Dissolved Oxygen (D.O.) is ensured before discharging the treated sewage in river
C. Solubility of oxygen in sewage is 95% of that of distilled water
D. All the above
ANS: D

Q No: 64
In sewers the effect of scouring is more on
A. Top side
B. Bottom side
C. Horizontal side
D. All sides
ANS: B

Q No: 65
The rainfall at any place may be determined by
A. Its intensity
B. Its direction
C. Its frequency
D. All the above
ANS: D

Q No: 66
Pick up the correct statement from the following:
A. The circular section of sewers provides maximum hydraulic mean depth
B. The circular sewers are provided for separate sewerage system
C. The circular sewers work efficiently if the sections run at least half full
D. All the above
ANS: D

Q No: 67
Pick up the correct statement from the following:
A. Rate of digestion of sludge is more at higher temperature
B. Thermophilic organisms digest the sludge if the temperature ranges from 40° to 60°C
C. Mesophilic organisms digest the sludge if the temperature is between 25° and 40°C
D. All the above
ANS: D

Q No: 68
Sewer ventilating columns are generally placed at
A. Distances 150 m to 300 m
B. Upper ends of branch sewers
C. Every change in the size of sewers
D. All the above
ANS: D

Q No: 69
The sewer which resists sulphide corrosion, is
A. Brick sewer
B. Cast iron sewer
C. R.C.C. sewer
ANS: D

Q No: 70
In a sedimentation tank (length L, width B, depth D) the settling velocity of a particle for a discharge Q, is
A. Q/(B × D)
B. Q/(L × D)
C. Q/L
D. Q/(B × L)
ANS: D

Q No: 71
The formula which accepts the value of rugosity coefficient n = 0.012 to be used in Manning’s formula, is given by
A. Bazin
B. Crimp and Bruges
C. William-Hazen
D. Kutter
ANS: B

Q No: 72
The amount of oxygen consumed by the aerobic bacterias which cause the aerobic biological decomposition of sewage, is known
A. Bio-Chemical Oxygen Demand (B.O.D.)
B. Dissolved Oxygen (D.O.)
C. Chemical Oxygen Demand (C.O.D.)
D. None of these
ANS: B

Q No: 73
Water content of sewage is about
A. 90 %
B. 95 %
C. 99 %
D. 9.9 %
ANS: D

Q No: 74
Skimming tanks are
A. Used to remove the grease and oil
B. Those from which sludge is skimmed out
C. Tanks provided with self- cleansing screens
D. Improved version of grit chambers
ANS: A

Q No: 75
The most efficient cross-section of sewers in a combined sewerage system is
A. Parabolic
B. Circular
C. Rectangular
D. New egged
ANS: B

Q No: 76
The detention period for plain sedimentation water tanks, is usually
A. 4 to 8 hours
B. 8 to 16 hours
C. 16 to 24 hours
D. 24 to 36 hours
ANS: A

Q No: 77
The factor responsible for purification of sewage in river is
A. Hydrology
B. Dissolved oxygen in water
C. Temperature
D. All the above
ANS: D

Q No: 78
Hume steel pipes are
A. Steel pipes
B. Steel shell coated from inside with cement mortar
C. Steel shell coated from outside with cement mortar
D. Both B. and C.
ANS: D

Q No: 79
To maintain aerobic biological activity, the moisture content of the compost mass should be about
A. 45 %
B. 50 %
C. 55 %
D. 60 %
ANS: C

Q No: 80
Pick up the correct statement from the following:
A. The larger the sewer in size, more will be velocity
B. The smaller the sewer in size, less will be velocity
C. The larger the sewer in size, no deposition will take place
D. The larger the sewer in size, deposition will take place
ANS: D

Q No: 81
The pH value of sewage is determined with the help of
A. Imhoff Cone
B. Turbid meter
C. Potentiometer
D. None of these
ANS: C

Q No: 82
If the length of overland flow from the critical point to the mouth of drain is 13.58 km and difference in level between the critical point and drain mouth is 10 m, the inlet time is
A. 2 hours
B. 4 hours
C. 6 hours
D. 8 hours
ANS: D

Q No: 83
The normal value of over flow rates for plain primary sedimentation tanks, ranges between
A. 25,000 to 35,000 litres/sqm/day
B. 40,000 to 50,000 litres/sqm/day
C. 50,000 to 60,000 litres/sqm/day
D. 80,000 to 100,000 litres/sqm/day
ANS: B

Q No: 84
Pick up the incorrect statement from the following:
A. Septic tanks are horizontal continuous flow type of sedimentation tanks
B. Septic tanks are generally provided a detention period of 12 to 36 hours
C. Septic tanks are completely covered and high vent shafts are provided for the escape of foul gases
D. None of these
ANS: D

Q No: 85
Which one of the following statements regarding septic tanks is wrong?
A. A gap of 7.5 cm between the bottom of the covering slab and the top level of scum is provided
B. The outlet invert level is kept 5 to 7.5 cm below the inlet invert level
C. The minimum width of septic level is 90 cm
D. The depth of tank is kept equal to its width
ANS: D

Q No: 86
In a trickling filter
A. Filtration process is used
B. Biological action is used
C. Neither A. nor B.
D. Both A. and B.
ANS: B

Q No: 87
In areas where rainy season is limited to a few months, the type of sewerage system recommended is
A. Combined system
B. Partially separate system
C. Separate system
D. None of these
ANS: C

Q No: 88
The rate of accumulation of sludge in septic tanks is recommended as
A. 30 litres/person/year
B. 25 litres/person/year
C. 30 litres/person/month
D. 25 litres/person/month
ANS: A

Q No: 89
Dried sewage after treatment is used as
A. Fertilizer
B. Building material
C. Chemical for lowering B.O.D.
D. Base material for paints
ANS: A

Q No: 90
15 cumecs, the depth d and width are related by For drains up to
A. d = 0.2 B
B. d = 0.5 B
C. B = 0.2 d
D. B = 0.5 d
ANS: B

Q No: 91
In areas where light rains are uniformly distributed throughout the year, the type of sewerage system to be adopted is
A. Separate system
B. Combined system
C. Partially combined system
D. None of these
ANS: B

Q No: 92
Self-cleansing velocity is
A. Velocity at dry weather flow
B. Velocity of water at flushing
C. Velocity at which no accumulation remains in the drains
D. Velocity of water in a pressure filter
ANS: C

Q No: 93
5 days-biochemical oxygen demand (BOD5) is taken at a temperature of
A. 0°C
B. 15°C
C. 20°C
D. 25°C
ANS: C

Q No: 94
Pick up the correct statement from the following:
A. Inlets are provided on the road surface at the lowest point for draining rain water
B. Inlets are generally provided at an interval of 30 m to 60 m along straight roads
C. Inlets having horizontal openings, are called horizontal inlets
D. All the above
ANS: D

Q No: 95
If a 2% solution of sewage sample is incubated for 5 days at 20°C and the dissolved oxygen depletion was found to be 8 mg/l. The BOD of the sewage is
A. 100 mg/l
B. 200 mg/l
C. 300 mg/l
D. 400 mg/l
ANS: D

Q No: 96
Pick up the in-correct statement from the following:
A. Manholes are provided in sewer pipes at suitable intervals
B. Catch basins are generally provided in sewers for carrying drainage discharge
C. Inlets are generally provided in all sewers
D. None of these
ANS: C

Q No: 97
A well oxidized sewage contains nitrogen mainly as
A. Nitrates
B. Nitrites
C. Free ammonia
D. None of these
ANS: A

Q No: 98
The moisture content of sludge is reduced from 90% to 80% in a sludge digestion tank. The percentage decrease in the volume of sludge, is
A. 25 %
B. 50 %
C. 10 %
D. 5 %
ANS: B

Q No: 99
In a sludge tank, the gas mainly produced, is
A. Oxygen
B. Nitrogen
C. Hydrogen
D. Carbon dioxide
ANS: D

Q No: 100
Bio-chemical oxygen demand (BOD) for the first 20 days in generally referred to
A. Initial demand
B. First stage demand
C. Carbonaceous demand
D. All of these
ANS: D