Waste Water Engineering Multiple Choice Questions on “Biological Nitrification and Denitrification”.
1. Nitrification is performed by a small group of _______
a) Autotrophic bacteria
b) Eutrophic bacteria
c) Fungi
d) Viruses
Answer: a
Clarification: Nitrification is an aerobic process performed by small groups of autotrophic bacteria and archaea. This process was discovered by the Russian microbiologist Sergei Winogradsky.
2. In nitrification, ammonia is converted to _____
a) Nitrogen
b) Nitrate
c) Nitrite
d) Amide
Answer: b
Clarification: The first step is nitrification, which is the conversion of ammonia to nitrate through the action of nitrifying bacteria. The second step is the nitrate conversion (denitrification), which is carried out by facultative heterotrophic bacteria under anoxic conditions.
3. Nitrate conversion takes place only through assimilatory cellular functions.
a) True
b) False
Answer: b
Clarification: The first step is nitrification, which is the conversion of ammonia to nitrate through the action of nitrifying bacteria. The second step is the nitrate conversion (denitrification), which is carried out by facultative heterotrophic bacteria under anoxic conditions.
4. In dissimilatory denitrification, ______ serves as the electron acceptor in energy metabolism.
a) Nitrate
b) Nitrite
c) Nitrogen
d) Ammonia
Answer: a
Clarification: In dissimilatory denitrification, nitrate serves as the electron acceptor in energy metabolism and is converted to various gaseous end products but principally molecular nitrogen, N2, which is then stripped from the liquid stream.
5. Microbial yield under aerobic conditions is lower than anoxic conditions.
a) True
b) False
Answer: b
Clarification: The microbial yield under anoxic conditions is considerably lower than under aerobic conditions. Hence a relatively small fraction of the nitrogen is removed through assimilation.
6. Denitrification releases _______
a) Nitrogen
b) Oxygen and nitrogen
c) Carbon-dioxide
d) Nitrogen and carbon-monoxide
Answer: a
Clarification: Denitrification releases nitrogen which escapes as an inert gas to the atmosphere while oxygen released stays dissolved in the liquid and thus reduces the oxygen input needed into the system.
7. How many molecules of oxygen are required for one molecule of nitrogen?
a) 1
b) 2
c) 3
d) 4
Answer: d
Clarification: Four molecules of oxygen are required for one molecule of nitrogen during nitrification but releases back 2.5 molecules in denitrification. Thus, theoretically, 62.5% of the oxygen used is released back in denitrification.
8. Which of the below is used to denitrify nitrates?
a) Carbon
b) Aluminium
c) Iron
d) Copper
Answer: a
Clarification: The use of artificial carbon source should be avoided as far as possible since it adds to the cost of treatment and also some operating difficulties may arise from dosing rate of methanol.
9. What is the process for converting organic-nitrogen to ammonia?
a) Hydrolysis
b) Nitrification
c) Denitrification
d) Oxidation
Answer: a
Clarification: Maximum percentage of nitrogen in sewage is converted from nitrogen to ammonia through a process called hydrolysis.
10. What is the minimum amount of dissolved oxygen required for nitrification?
a) 0.5 mg/L
b) 1.0 mg/L
c) 1.5 mg/L
d) 2.0 mg/L
Answer: b
Clarification: Nitrification occurs only under aerobic conditions at dissolved oxygen levels of 1.0 mg/L or more. When DO concentrations are less than 0.5 mg/L, the growth rate is minimal.
11. What is the optimum pH for nitrosomonas?
a) 6.5-7.5
b) 7.5-8.5
c) 9.5-10.5
d) 13.5-14
Answer: b
Clarification: For Nitrosomonas optimum pH is between 7.5 and 8.5 and as well as for Nitrobacter. Treatment plants effectively nitrify with a pH of 6.5 to 7.0. Nitrification stops at a pH below 6.0.
12. What is the minimum temperature required for denitrification process?
a) 3˚C
b) 5˚C
c) 15˚C
d) 30˚C
Answer: b
Clarification: The minimum temperature required for denitrification process is 5˚C. The rates increase with temperature and type of organic source present. When methanol or acetic acid used, the highest growth rate can be found.
13. What is the denitrification rate for the following data?
Temperature: 10 degree Celsius
Dissolved oxygen: 10mg/L
a) 0.038 per day
b) 0.0038 per day
c) 0.38 per day
d) 3.8 per day
Answer: a
Clarification: Overall denitrification rate (ODR)= specific denitrification rate (SDNR)x 0.42 x(1-Dissolved oxygen). SDNR is assumed as 0.1 Kg NO3-N/kg MLVSS per day. ODR = 0.1x 0.42 x (1-0.1) = 0.038 per day.
14. Calculate the residence time for the following data.
Influent nitrate: 22 mg/L
Effluent nitrate: 3 mg/L
Overall denitrification rate: 0.038 per day.
MLVSS: 2000 mg/L
a) 1.5 hrs
b) 5 hrs
c) 6 hrs
d) 9 hrs
Answer: c
Clarification: Residence time: (Influent nitrate – Effluent nitrate)/ ODR x MLVSS. Residence time= (22-3)/(0.038 x 2000)= 19/(0.038 x2000). Residence time= 6 hrs.
15. Which of these chemicals is added in the post anoxic step?
a) Acetate
b) Propanol
c) Butanol
d) Chloroform
Answer: a
Clarification: Acetate is added in the post anoxic step. Sometimes even methanol is added. This is done in order to increase the rate of denitrification.
16. Ammonia is converted into nitrogen by which type of bacteria in anaerobic conditions?
a) Autotrophic
b) Heterotrophic
c) Organotrophs
d) Lithotrophs
Answer: a
Clarification: Ammonia is converted into nitrogen by autotrophic bacteria in anaerobic conditions. Under aerobic conditions ammonia is converted to nitrogen by aerobic bacteria. Organotrophs and lithotrophs are a type of heterotrophic bacteria.
17. What is the temperature at which the denitrification process takes place when it is carried out in a fluidised bed reactor?
a) 30-35 degree
b) 35 degree
c) 40 degree
d) 25 degree
Answer: a
Clarification: The temperature at which the denitrification process takes place is 30-35 degree. This can be done aerobically or anaerobically. Post anoxic tanks have a slower rate of denitrification when compared to post anoxic tanks.