Diffused & Mechanical Aeration Systems Questions and Answers

This set of Waste Water Engineering Multiple Choice Questions & Answers (MCQs) focuses on “Diffused and Mechanical Aeration Systems”.

1. Aeration consumes less energy in activated sludge process compared to other processes.
a) True
b) False
View Answer

Answer: b
Explanation: Aeration occurs in many water and wastewater treatment processes, but in the activated sludge process and its variants, it consumes more energy than other processes, by far.

2. What does OTR stand for?
a) Oxygen transfer rate in standard conditions
b) Oxygen transfer rate in clean water
c) Oxygen transfer efficiency in clean water
d) Oxygen transfer efficiency in standard conditions in the clean water
View Answer

Answer: b
Explanation: OTR stands for oxygen transfer rate in clean water. SOTR stands for oxygen transfer rate in clean water. OTE stands for Oxygen transfer efficiency in the clean water. SOTE stands for oxygen transfer efficiency in standard conditions in clean water.

3. Which of the following is not a standard condition for aeration?
a) 40˚C
b) 1 atm
c) Zero salinity
d) Zero DO
View Answer

Answer: a
Explanation: Various standard conditions are mentioned for the aeration process. The standard temperature is considered to be 20˚C along with 1 atmospheric pressure, zero salinity and zero dissolved oxygen.

4. What is the symbol used to define the ratio of the process to clean water mass transfer?
a) Alpha
b) Beta
c) Gamma
d) Sigma
View Answer

Answer: a
Explanation: The ratio of the process to clean water mass transfer is called alpha factor and is symbolized by alpha.

5. _________ is the actual mass of oxygen transferred per unit time.
a) OTR
b) SOTR
c) OTE
d) SOTE
View Answer

Answer: a
Explanation: The OTR is the actual mass of oxygen transferred per unit time and it is the key process variable for design. The DO saturation concentration is the concentration of dissolved oxygen at saturation with no reactions in the liquid.

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6. High solubility of CO₂ ______ pH.
a) Maintains
b) Reduces
c) Increases
d) Nullifies
View Answer

Answer: b
Explanation: High solubility of CO₂ reduces the pH of water which causes excessive consumption of lime or other neutralizing agents in coagulation and softening process. The corrosiveness of water is also higher at lower pH values.

7. What is the range of pH above which aeration is required?
a) 2 mg/L
b) 5 mg/L
c) 8 mg/L
d) 10 mg/L
View Answer

Answer: d
Explanation: Exposure of water droplets to air for 2 seconds will lower the CO₂ by 70-90%. If CO₂ > 10mg/L aeration is recommended. Otherwise, the lime addition should be used to neutralize the CO₂.

8. H₂S is highly soluble in water.
a) Tue
b) False
View Answer

Answer: a
Explanation: H₂S is highly soluble in water. Hydrogen sulphide poisoning is one of the leading causes of accidents. If its concentration is above 5 ppm moderate odour is present. If its concentration increases above 300 ppm, it leads to unconsciousness and death.

9. Which of the following is not used for iron removal?
a) Oxidation
b) Reduction
c) Gasification
d) Absorption
View Answer

Answer: b
Explanation: Oxygenation of water is done by gasification and absorption. The removal of iron from water or wastewater can be done by various methods such as oxidation, gasification and absorption.

10. How many types of aerators are present?
a) 2
b) 3
c) 4
d) 5
View Answer

Answer: c
Explanation: There are four types of aerators. They are gravity aerators, spray aerators, diffused air aeration systems and mechanical aerators. Air stripping is done to remove toxic volatile organics and volatile compounds.

11. What is the value for Ɵ for mechanical and diffused aeration for the following equation?
K_LA(T) = K_{LA(20 Degree Celsius)}Ɵ^T-20
a) 1.015
b) 1.1
c) 1.024
d) 1.04
View Answer

Answer: c
Explanation: The value for Ɵ is usually between 1.015 to 1.04. In case of mechanical and diffused aeration this is 1.024. K_LA is Oxygen mass transfer coefficient.

12. What is the value for α for diffused aeration for the following equation.
α= K_{LA(wastewater)} /K_{LA(Tap water)}
a) 0.4-0.8
b) 0.2-0.3
c) 0.8-1
d) >1
View Answer

Answer: a
Explanation: Usually the value for α varies between 0.3-1.2. The value for α is usually between 0.4-0.8 for diffused aeration. K_LA is Oxygen mass transfer coefficient.α is the correction factor.

13. What is the value for α for mechanical aeration aeration for the following equation.
α= K_{LA(wastewater)} /K_{LA(Tap water)}
a) 0.6-1.2
b) 0.2-0.3
c) 0.3-0.5
d) >1.2
View Answer

Answer: a
Explanation: Usually the value for α varies between 0.3-1.2. The value for α is usually between 0.6-1.2 for mechanical aeration. K_LA is Oxygen mass transfer coefficient.α is the correction factor.α depends on the geometry of the basin, degree of mixing and wastewater characteristics.

14. Calculate the theoretical oxygen requirement for the following details.
Flow: 200 m³/h
BOD load: 800 mg/L
a) 45.5 Kg/h of O₂/hr
b) 23.05 Kg/h of O₂/hr
c) 25.05 Kg/h of O₂/hr
d) 50.5 Kg/h of O₂/hr
View Answer

Answer: b
Explanation: BOD load in Kg/day = Inlet BOD in mg/l x Flow in m³/day /1000 =800 x 200 / 1000=160 Kg/day. Theoretical oxygen requirement = BOD load x Kg of O₂/Kg of BOD= 160 x 1.8 = 288 Kg of O₂/day. Standard O₂ Requirement = Theoretical O₂ requirement/ Alpha x [Beta x Csw – D.O.] x Theta^(Tww-20°C)/ C_ss= 288/24/[0.7 x [0.95 x 8.64 – 2] x 1.024^(30-20)]/9.86=23.05 Kg of O₂/hr.

15. Calculate the amount of air required for the following details.
Flow: 300 m³/h
BOD load: 800 mg/L
a) 550 m³/hr
b) 518 m³/hr
c) 700m³/hr
d) 618 m³/hr
View Answer

Answer: c
Explanation: BOD load in Kg/day = Inlet BOD in mg/l x Flow in m³/day /1000 =800 x 300 / 1000=240 Kg/day. Theoretical oxygen requirement = BOD load x Kg of O₂/Kg of BOD= 240 x 1.8 = 432 Kg of O₂/day. Standard O₂ Requirement = Theoretical O₂ requirement/ Alpha x [Beta x Csw – D.O.] x Theta^(Tww-20°C)/ C_ss= 432/24/[0.7 x [0.95 x 8.64 – 2] x 1.024^(30-20)]/9.86=38.29 Kg of O₂/hr. Air requirement = Actual Oxygen requirement/ (Density of air x % of O₂ in air by weight x SOTE%) = 38.29 /( 1.201 x 0.232 x 0.1599)= 696 m³/hr. (Rounded off to 700 m³/hr). Factors assumed: SOTE=17.27%; Alpha=0.7; Beta=0.95; Theta=1.024; Surface saturation C=7.97 (Calculated); Site basin saturation C_sw=8.64 (Calculated); Standard basin saturation C_ss=9.86 (Calculated).

16. Calculate the number of diffusers for the following.
Blower Flow rate: 535 m³/hr
Coarse diffusers are used.
a) 54
b) 60
c) 65
d) 70
View Answer

Answer: a
Explanation: Number of diffusers equals =flow/ flow rate per diffuser. Flow rate considered for coarse diffusers are 10 m³/hr. Therefor number of diffusers= 535/10= 54.

17. Calculate the number of fine diffusers required for the following data.
Flow: 300 m³/h
BOD load: 1000 mg/L
a) 150
b) 146
c) 170
d) 186
View Answer

Answer: b
Explanation: BOD load in Kg/day = Inlet BOD in mg/l x Flow in m³/day /1000 =1000 x 300 / 1000=300 Kg/day. Theoretical oxygen requirement = BOD load x Kg of O₂/Kg of BOD= 300 x 1.8 = 540 Kg of O₂/day.
Standard O₂ Requirement = Theoretical O₂ requirement/ Alpha x [Beta x Csw – D.O.] x Theta^(Tww-20°C)/ C_ss= 540/24/[0.7 x [0.95 x 8.64 – 2] x 1.024^(30-20)]/9.86=48 Kg of O₂/hr. Air requirement = Actual Oxygen requirement/ (Density of air x % of O₂ in air by weight x SOTE%) = 48/( 1.201 x 0.232 x 0.1599)= 875 m³/hr. Factors assumed: SOTE=17.27%; Alpha=0.7; Beta=0.95; Theta=1.024; Surface saturation C=7.97 (Calculated); Site basin saturation C_sw=8.64 (Calculated); Standard basin saturation C_ss=9.86 (Calculated).
Number of fine diffusers required = 875/6=146. (flow rate considered for fine diffusers is 6 m³/hr).

18. Calculate the amount of air required for the equalization tank with the following details
Flow: 500 m³/h
BOD load: 800 mg/L
a) 950 m³/hr
b) 800 m³/hr
c) 2000 m³/hr
d) 785 m³/hr
View Answer

Answer: c
Explanation: Volume of tank = flow x retention time considered = 500 x 8 =4000 m³. The air requirement is 0.5 m³/hr/ m³ tank volume. Therefor the air requirement considered is 4000 x 0.5 = 2000 m³/hr.

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