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# Multiple choice question for engineering

## Set 1

1. Which of the following is the correct expression of the aeration period in hours?
a) V/Q
b) 24V/Q
c) V/24Q
d) Q/V

Answer: b [Reason:] Aeration period = 24V/Q where, V is the volume of the tank and Q is the quantity of sewage. It is measured in hours.

2. Which of the following activated sludge process has minimum food to microorganism ratio?
a) Extended aeration
b) Step aeration
c) Modified aeration
d) Conventional

Answer: a [Reason:] Extended aeration process has minimum food to microorganism ratio of 0.5 which means it has the maximum BOD removal efficiency.

3. The maximum aeration period for a conventional activated sludge system is
a) 2 hours
b) 3 hours
c) 4 hours
d) 6 hours

Answer: d [Reason:] The detention period for a conventional activated sludge system is between 4 to 6 hours.

4. The organic loading in an activated sludge plant is defined as
a) Mass of BOD per day/Volume of the aeration tank
b) Volume of aeration tank/Mass of BOD per day
c) The mass of BOD per day/Volume of secondary clarifier
d) Aeration period * Volume of aeration tank

Answer: a [Reason:] The organic loading in an activated sludge plant is defined as the mass of BOD to the aeration tank in grams per unit volume of aeration tank in m3.

5. _________ is defined as a volume in ml occupied by 1gram of solid in the mixed liquor after 30 minutes of settling.
b) Suspended solid volume
c) Sludge volume index

Answer: c [Reason:] Sludge volume index is used to indicate the settling behavior of sludge. It is measured in ml/gm.

6. The design value of food to microorganism ratio in the tapered aeration sludge process is
a) 0.1
b) 0.2
c) 0.4
d) 0.8

Answer: c [Reason:] The design value of food to microorganism ratio is 0.3-0.4 for a tapered aeration sludge process.

7. Which of the following sludge process has maximum aeration period?
a) Conventional process
b) Extended process
c) Step aeration process
d) Tapered aeration

Answer: b [Reason:] Extended process has the maximum aeration period of 12 to 24 hours.

8. The maximum BOD removal efficiency of the Step aeration process of the activated sludge system is
a) 92%
b) 80%
c) 99%
d) 78%

Answer: a [Reason:] The BOD removal efficiency of the step aeration process of the activated sludge system is between 85% to 92%.

9. State whether the following statement is true or false.
BOD of sewage is dependent on food to microorganism ratio.
a) True
b) False

Answer: a [Reason:] If the food to microorganism ratio increases, BOD removal from the sewage will decrease.

10. _________ is the time at which sewage is detained in the aeration tank.
a) Average time
b) Detention time
c) Aeration period
d) Reaeration period

Answer: c [Reason:] Aeration period decides the loading rate at which sewage is introduced in the aeration tank.

11. Which of the following has minimum hydraulic retention time?
a) Conventional process
b) Extended process
c) Modified aeration process
d) Complete mix process

Answer: c [Reason:] Modified aeration process has a minimum hydraulic retention time with an aeration period of 1.5 hours.

## Set 2

1. Which of the following cannot be the velocity of flow in the horizontal flow sedimentation tank?
a) 12cm/min
b) 39cm/min
c) 5cm/min
d) 20cm/min

Answer: b [Reason:] The maximum value of the velocity of flow is 30cm/min, so it cannot be 39cm/min.

2. The deviation from the ideal flow of wastewater in the sedimentation tank causes
a) Whirlpool
b) Increase in tank efficiency
c) Short circuiting
d) Coagulation

Answer: c [Reason:] The deviation from the ideal flow of wastewater in the sedimentation tank causes short circuiting. It results in variation in the detention period in the top and bottom layer of wastewater in the tank.

3. The overflow rate for a plain sedimentation tank is ______ litre/day/metre2.
a) 50000
b) 40000
c) 12000
d) 30000

Answer: c [Reason:] The overflow rate for a plain sedimentation tank lies between 12000-18000 litre/day/metre2.

4. The detention period for a sedimentation tank where coagulation takes place is
a) 30 seconds
b) 1 minute
c) 1hour
d) 2hour

Answer: d [Reason:] The detention period for a sedimentation tank without coagulation is 4 hours, whereas with coagulation is 2 – 2.5 hours.

5. The test for finding the optimum coagulant dose in water is
a) Chromatography
b) Jar test
c) Kjeldahl test
d) Dilatancy test

Answer: b [Reason:] Jar test is used to find the optimum coagulant concentration that is required to cause coagulation in water so that the settling of suspended impurities takes place.

6. The average time required by water to pass through the settling tank is called _______
a) Detention time
b) Flowing through period
c) Time of flow
d) Mean time

Answer: b [Reason:] The average time required by water to pass through the settling tank is called ‘flowing through period’ which is always less than the detention period.

7. The quantity of water flowing per hour per unit horizontal area is called ____
a) Detention time
b) Flowing through period
c) Displacement time
d) Overflow rate

Answer: d [Reason:] Overflow rate = Q/A where Q is discharge rate in litre per day, A is the horizontal area in m2.

8. In a rectangular horizontal flow tank, the maximum permissible velocity is ____ m/Sec.
a) 3
b) 0.3
c) 0.03
d) 0.003

Answer: b [Reason:] In the rectangular tank, the ideal conditions are that the flow is horizontal, velocity is uniform in all directions and the maximum permissible limit of velocity is 0.03m/Sec.

## Set 3

1. Which of the following is called as air diffusion method?
a) Activated sludge aeration
b) Diffused air aeration
c) Mechanical aeration
d) Combined aeration

Answer: b [Reason:] Diffused air aeration method is also called as air diffusion method where diffused air aerators are used for diffusing the air in the aeration tank.

2. The maximum quantity of air required in diffused aerator is ______ m3 per million litres of sewage.
a) 4000
b) 5000
c) 6000
d) 8000

Answer: d [Reason:] The minimum and maximum quantity of air required in diffused aerator is 6000 and 8000 m3 respectively per million litres of sewage.

3. The effective area of the diffuser plate used in the aeration tank is
a) 400cm2
b) 780cm2
c) 1160cm2
d) 800cm2

Answer: b [Reason:] Diffuser plates are provided at the bottom of the aeration tank . It is square in shape with the dimension of 30cm x 30cm.

4. The maximum depth of aeration tank used in activated sludge process is
a) 1m
b) 2m
c) 4.5m
d) 5.5m

Answer: c [Reason:] The depth of aeration tank lies between 3 to 4.5m and is 4-6m wide.

5. Which device is used in the aeration tank for cleaning purposes?
a) Diffuser plate
c) Tube diffuser

Answer: c [Reason:] Tube diffuser is suspended in the aeration tank and can be used for cleaning without emptying the tank.

6. With how much pressure the air is filled in the aeration tank?
a) 70-90kN/m2
b) 34-56kN/m2
c) 10-20kN/m2
d) 35-70kN/m2

Answer: d [Reason:] The air is compressed in the diffused air aerator with the pressure of 35-70kN/m2 resulting in the formation of the bubbles, improving the efficiency of aeration.

7. The effective area of tube diffuser used in the aeration tank is
a) 400cm2
b) 780cm2
c) 1160cm2
d) 800cm2

Answer: c [Reason:] Tube diffuser is 60cm long tube having an internal diameter of 7.5cm and an effective area of 1160cm2.

8. With which device, the air is introduced in the aeration tank?
a) Diffuser plate
c) Cone with spiral vanes

Answer: a [Reason:] The air is introduced in the diffuser air aerators with the help of diffuser plates. It rotates and results in the formation of small bubbles causing aeration of sewage.

## Set 4

1. The unit of dynamic viscosity is
a) Metre2/Sec
b) Metre/Sec
c) Pa * Sec
d) Dimensionless

Answer: c [Reason:] The dynamic viscosity is the ratio of shear stress to velocity gradient. Its unit is Pa * Sec.

2. Which of the following has minimum detention period?
a) Grit chamber
b) Sedimentation tank
c) Oxidation ditch
d) Oxidation pond

Answer: a [Reason:] Grit chamber has a minimum detention period with a value of 40-60 seconds.

3. The relation between dynamic and kinematic viscosity is given by
a) Dynamic viscosity = Kinematic viscosity * Density of water
b) Kinematic viscosity = Dynamic viscosity + Density of water
c) Dynamic viscosity = Kinematic viscosity * Density of water
d) Dynamic viscosity = Kinematic viscosity / Density of water

Answer: c [Reason:] μ = v * ρ, μ = dynamic viscosity, v = kinematic viscosity and ρ = density of water.

4. The dimension formula of dynamic viscosity is
a) ML-1T
b) ML-1T-1
c) ML-2T-2
d) ML-1T-3

Answer: b [Reason:] The dimension formula of dynamic viscosity is ML-1T-1 and it can be measured in kg m-1 s-1.

5. The dimension formula of kinematic viscosity is
a) L-1T
b) ML-1T-1
c) ML-2T-2
d) L2T-1

Answer: d [Reason:] The dimension formula of kinematic viscosity = ML-1T-1 / ML-3 = L2 T-1.

6. The coefficient of drag for a transition settling is
a) CD = 18.5/R0.6
b) CD = 24/R
c) CD = 8.5/R0.6
d) CD = R/24

Answer: a [Reason:] When Reynolds number ‘R’ lies between 1 and 1000, the coefficient of drag CD is given by CD = 18.5/R0.6.

7. In which of the following, large impurities are attached with air bubbles to remove it from the wastewater?
a) Sludge digestion tank
b) Primary sedimentation
c) Vacuum floatation
d) Oxidation pond

Answer: c [Reason:] In skimming tanks, large impurities are attached with air bubbles to decrease its density, they float in sewage and are finally removed by skimming troughs.

8. Which of the following has minimum detention period?
a) Sludge digestion tank
b) Primary sedimentation
c) Skimming tank
d) Oxidation pond

Answer: c [Reason:] The skimming tank has a minimum detention period with a value of 3 minutes.

9. The design velocity of flow of a detritus tank is
a) 1m/Sec
b) 1.2m/Sec
c) 0.9m/Sec
d) 0.5m/Sec

Answer: c [Reason:] The detritus tanks are able to remove grit and sand particles by having a design velocity of 0.9m/Sec.

10. Which of the following device is used for removal of oil and grease?
a) Skimming tank
b) Grit chambers
c) Tube settlers
d) Flocculator

Answer: a [Reason:] The skimming tank is used after grit chamber to remove oil and grease from the sewage.

11. The detention period of detritus tank is
a) 40 seconds
b) 1 minute
c) 3 minute
d) 1 hour

Answer: c [Reason:] The detention period of detritus tank is 3 minutes. It is used to remove the grit present in the wastewater.

12. A pressure difference of ___ is applied to vacuum floatation method to remove impurities from sewage.
a) 0.5 atm
b) 1 atm
c) 1.5 atm
d) 2 atm

Answer: b [Reason:] In the vacuum floatation method, pressure of 1 atm is applied in sewage followed by the vacuum that creates a pressure difference of 1 atm, resulting in the formation of bubbles that helps in the removal of large impurities in sewage.

## Set 5

1. How many stages are there in sludge digestion process?
a) 2
b) 3
c) 4
d) 5

Answer: b [Reason:] The sludge digestion process consists of acid fermentation, acid regression and alkaline fermentation.

2. The form of sludge which is free from pathogenic bacteria is
a) Undigested sludge
b) Digested sludge
c) Primary sludge
d) Supernatant liquor

Answer: b [Reason:] The digested sludge is free from pathogenic bacteria. It has the behavior of rich humus soil.

3. The BOD of supernatant liquor in a sludge digestion process is ____ ppm.
a) 3000
b) 0
c) 1000
d) 250

Answer: a [Reason:] The range of BOD of supernatant liquor in a sludge digestion process is between 2000 and 3000 ppm. It is a form of sludge that is obtained in the sludge digestion process.

4. Which of the following is correct regarding pH in different stages of the sludge digestion process?
a) Acid fermentation > Acid regression > Alkaline fermentation
b) Acid regression > Acid fermentation > Alkaline fermentation
c) Acid fermentation < Acid regression < Alkaline fermentation
d) Acid fermentation < Acid regression > Alkaline fermentation

Answer: c [Reason:] The pH value in Alkaline fermentation stage is 7.5 whereas in Acid regression and Acid fermentation stage, the pH>7, having the least in acid fermentation stage.

5. Which gas evolves the most in the sludge digestion process?
a) SO2
b) CO2
c) N2
d) CH4

Answer: d [Reason:] Methane gas evolves the most during the sludge digestion process by approximately 70% of the total gas evolved.

6. Which of the following is incorrect about aerobic digestion in the sludge digestion process?
a) It has a low capital cost
b) It is easier to manage.
c) It has high residual energy
d) It decreases the BOD

Answer: c [Reason:] The residual energy in aerobic digestion is very less in comparison to anaerobic digestion.

7. The color of digestive sludge is
a) Blue
b) Black
c) Brown
d) Gray

Answer: b [Reason:] The color of digestive sludge is black. It has an inoffensive odor.

8. Which of the following is correct regarding anaerobic digestion in the sludge digestion process?
a) It contains high amount of biomass
b) It is a low energy process
c) It causes infertility in soil
d) Aerobic bacteria oxidize the organic matter

Answer: b [Reason:] Anaerobic digestion is a low energy process which contains low amount of biomass and where both facultative and anaerobic bacteria solubilize the organic matter.

9. The maximum temperature at which thermophilic microorganisms remain active in sludge digester is
a) 60oC
b) 40oC
c) 30oC
d) 20oC

Answer: a [Reason:] The maximum and minimum temperature at which thermophilic microorganisms remain active in sludge digester are 60oC and 40oC respectively.

10. Which of the following is obtained in the secondary tank in two stage sludge digester?
a) Partially digested sludge
b) Partially digested sludge and supernatant liquor
c) Digested sludge and supernatant liquor
d) supernatant liquor

Answer: c [Reason:] In two stage sludge digester, the partially digested sludge is obtained in primary tank, whereas digested sludge is obtained in the secondary tank along with supernatant liquor.

11. The high rate sludge digestion tanks are used when the capacity of the tank is ____ million liters per day.
a) 1
b) 3
c) 4
d) 10

Answer: c [Reason:] High rate digesters are used when the capacity of tank is 4 million litres per day. Complete mixing and uniform feeding of raw sludge takes place in high rate digesters.

12. In which of the following, both primary and secondary tanks are used?
a) Two stage sludge digesters
b) Single stage sludge digesters
c) Low rate digesters
d) High rate digesters

Answer: a [Reason:] In two stage sludge digesters, the sludge is passed in primary tank for 7 days and then it is passed in the secondary tank followed by the removal of residual matter.

13. The minimum size of sludge digestion tank is
a) 0.5m
b) 1m
c) 2m
d) 3m

Answer: d [Reason:] The minimum size of sludge digestion tank is 3m. They are cylindrical in shape with a funnel shaped at the bottom.

14. The range of temperature at which mesophilic microorganism becomes active in sludge digester is
a) 30-40oC
b) 10-40oC
c) 20-50oC
d) 20-40oC

Answer: d [Reason:] The mesophilic microorganism becomes active at 20-40oC in the sludge digester. The rate of digestion, increases with increase in temperature.

15. The sludge digestion process is independent of which of the following?
a) Temperature of sludge
b) pH of sludge
c) Shape of tank
d) Mixing of sludge