Civil Engineering PE Water and Wastewater Treatment 4 — Questions and Answers
Question 1: Nitrification in biological wastewater treatment converts ammonia (NH₄⁺) to nitrate (NO₃⁻). Which bacteria perform the second step, converting nitrite (NO₂⁻) to nitrate?
- Nitrosomonas
- Nitrobacter (Correct answer)
- Pseudomonas
- Thiobacillus
Correct answer: Nitrobacter
Nitrobacter (and related genera) oxidize nitrite to nitrate in the second step of nitrification, requiring a long SRT due to slow growth.
Question 2: What is the primary purpose of a grit chamber at the headworks of a wastewater treatment plant?
- Remove floating grease and oils
- Remove heavy inorganic particles to protect downstream equipment (Correct answer)
- Reduce BOD by 30%
- Equalize flow variations during peak hours
Correct answer: Remove heavy inorganic particles to protect downstream equipment
Grit chambers remove sand, gravel, and heavy inorganic solids to prevent abrasion and clogging of pumps, pipes, and downstream treatment units.
Question 3: A water utility must achieve 3-log (99.9%) Giardia inactivation. If UV provides 2-log inactivation, how much additional log inactivation must chlorination provide?
- 0.5-log
- 1.0-log (Correct answer)
- 1.5-log
- 2.0-log
Correct answer: 1.0-log
Total required inactivation = 3-log; UV provides 2-log; therefore chlorination must provide 3 − 2 = 1-log inactivation.
Question 4: In anaerobic digestion of biosolids, what is the primary end product that is captured for energy recovery?
- Carbon dioxide only
- Hydrogen sulfide
- Methane (biogas) (Correct answer)
- Volatile fatty acids
Correct answer: Methane (biogas)
Anaerobic digestion produces biogas composed predominantly of methane (~65%) and CO₂ (~35%), which can be captured and used for energy.
Question 5: The Langelier Saturation Index (LSI) is used in water distribution systems to assess:
- Chlorine residual decay rate
- The tendency of water to precipitate or dissolve calcium carbonate (Correct answer)
- Microbial regrowth potential
- Turbidity removal efficiency
Correct answer: The tendency of water to precipitate or dissolve calcium carbonate
LSI = pH − pHs; a positive LSI indicates scale-forming water, a negative LSI indicates corrosive water, and zero indicates equilibrium with CaCO₃.
Question 6: A wastewater treatment plant achieves 90% BOD removal. If influent BOD = 250 mg/L, what is the effluent BOD?
- 25 mg/L (Correct answer)
- 225 mg/L
- 125 mg/L
- 10 mg/L
Correct answer: 25 mg/L
Effluent BOD = influent × (1 − removal efficiency) = 250 × (1 − 0.90) = 250 × 0.10 = 25 mg/L.
Question 7: Which membrane process is capable of removing dissolved salts and is commonly used for desalination or advanced water treatment?
- Microfiltration (MF)
- Ultrafiltration (UF)
- Nanofiltration (NF) and Reverse Osmosis (RO) (Correct answer)
- Bag filtration
Correct answer: Nanofiltration (NF) and Reverse Osmosis (RO)
NF and RO membranes have pore sizes small enough (< 1 nm) to reject dissolved ions and salts via solution-diffusion transport.
Civil Engineering PE Water and Wastewater Treatment 4
Nitrification in biological wastewater treatment converts ammonia (NH₄⁺) to nitrate (NO₃⁻).
Which bacteria perform the second step, converting nitrite (NO₂⁻) to nitrate?