Waste Water Engineering Multiple Choice Questions on “Diffused and Mechanical Aeration Systems”.
1. Aeration consumes less energy in activated sludge process compared to other processes.
a) True
b) False
Answer: b
Clarification: 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
Answer: b
Clarification: 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
Answer: a
Clarification: 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
Answer: a
Clarification: 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
Answer: a
Clarification: 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.
6. High solubility of CO2 ______ pH.
a) Maintains
b) Reduces
c) Increases
d) Nullifies
Answer: b
Clarification: 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
Answer: d
Clarification: 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
Answer: a
Clarification: 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
Answer: b
Clarification: 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
Answer: c
Clarification: 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?
KLA(T) = KLA(20 Degree Celsius)ƟT-20
a) 1.015
b) 1.1
c) 1.024
d) 1.04
Answer: c
Clarification: The value for Ɵ is usually between 1.015 to 1.04. In case of mechanical and diffused aeration this is 1.024. KLA is Oxygen mass transfer coefficient.
12. What is the value for α for diffused aeration for the following equation.
α= KLA(wastewater) /KLA(Tap water)
a) 0.4-0.8
b) 0.2-0.3
c) 0.8-1
d) >1