Civil Engineering PE Water and Wastewater Treatment Questions and Answers — Questions and Answers
Question 1: An operator at an activated sludge wastewater treatment plant observes persistent sludge bulking, characterized by a high Sludge Volume Index (SVI) and poor settling in the secondary clarifier. Microscopic examination reveals a predominance of filamentous bacteria. Which of the following operational conditions is the MOST likely cause of this type of bulking?
- High dissolved oxygen (DO) concentration in the aeration basin
- A consistently low Food-to-Microorganism (F/M) ratio (Correct answer)
- A short Mean Cell Residence Time (MCRT)
- Excessive influent alkalinity
Correct answer: A consistently low Food-to-Microorganism (F/M) ratio
A low Food-to-Microorganism (F/M) ratio creates an environment where slow-growing filamentous bacteria can out-compete the desired floc-forming bacteria for the limited available substrate. This proliferation of filaments interferes with sludge settling and compaction, leading to bulking. While other factors like low DO can also cause bulking, a low F/M ratio is a classic and very common cause for specific types of filamentous organisms.
Question 2: In the process of water disinfection, what does the 'breakpoint' in the breakpoint chlorination curve represent?
- The point of maximum formation of combined chlorine residuals (chloramines).
- The initial dose of chlorine required to overcome immediate demand from inorganic reductants like iron.
- The point at which the chlorine demand has been satisfied, and any further chlorine added results in a proportional increase in free available chlorine residual. (Correct answer)
- The optimal pH for chlorine disinfection, which is achieved by the addition of chlorine gas.
Correct answer: The point at which the chlorine demand has been satisfied, and any further chlorine added results in a proportional increase in free available chlorine residual.
The breakpoint is the point on the chlorination curve where the chlorine demand from ammonia and other compounds has been fully met. Before the breakpoint, added chlorine reacts to form combined chlorine (chloramines), which are then destroyed by further chlorine addition. After the breakpoint, nearly all the ammonia has been oxidized, and any additional chlorine will create a stable free available chlorine residual that increases in direct proportion to the dose.
Question 3: A rectangular primary sedimentation basin at a water treatment plant is 100 ft long, 30 ft wide, and has a water depth of 12 ft. If the plant flow rate is 3.0 MGD (Million Gallons per Day), what is the surface overflow rate?
- 1,000 gpd/ft² (Correct answer)
- 83,333 gal/day
- 1,250 gpd/ft²
- 83 gpd/ft²
Correct answer: 1,000 gpd/ft²
The surface overflow rate (SOR) is calculated by dividing the flow rate by the surface area of the basin. First, calculate the surface area: Area = Length × Width = 100 ft × 30 ft = 3,000 ft². Next, convert the flow rate to gallons per day: Flow = 3.0 MGD = 3,000,000 gpd. Finally, calculate the SOR: SOR = Flow / Area = 3,000,000 gpd / 3,000 ft² = 1,000 gpd/ft². The depth of the basin is not used in this calculation.
Question 4: A wastewater treatment plant is being designed to achieve total nitrogen removal. Which of the following sets of conditions is essential for the biological process of denitrification?
- An aerobic environment (DO > 2.0 mg/L) and the presence of ammonia
- An anoxic environment (DO < 0.2 mg/L) and an available carbon source (Correct answer)
- A low pH (< 6.0) and a short solids retention time (SRT)
- A high concentration of phosphorus and an aerobic environment
Correct answer: An anoxic environment (DO < 0.2 mg/L) and an available carbon source
Denitrification is the biological conversion of nitrate (NO3-) to nitrogen gas (N2). This process is carried out by facultative heterotrophic bacteria. These bacteria require an anoxic environment, where free dissolved oxygen is absent, forcing them to use the oxygen on the nitrate molecule for respiration. They also require a source of organic carbon (BOD) as their food/energy source to drive the reaction.
Question 5: A public water system is struggling with high levels of disinfection byproducts (DBPs), specifically trihalomethanes (THMs) and haloacetic acids (HAAs), exceeding regulatory limits. Which of the following strategies is generally considered a highly effective method for reducing the formation of these DBPs?
- Increasing the chlorine dose to ensure complete oxidation of organic precursors.
- Lowering the water's pH prior to chlorination to increase the potency of hypochlorous acid.
- Lengthening the chlorine contact time to allow for DBP degradation.
- Switching the secondary disinfectant from free chlorine to chloramines for the distribution system. (Correct answer)
Correct answer: Switching the secondary disinfectant from free chlorine to chloramines for the distribution system.
DBPs like THMs and HAAs form when a disinfectant, like free chlorine, reacts with natural organic matter (NOM) in the water. Chloramines are a weaker oxidant than free chlorine and are significantly less reactive with NOM. Therefore, switching the secondary disinfectant (used to maintain a residual in the distribution system) to chloramines is a common and effective strategy to dramatically reduce the formation of regulated DBPs.
Question 6: What are the two primary objectives of utilizing anaerobic digestion for the stabilization of municipal wastewater sludge?
- To increase the sludge volume and sterilize the material with high pressure.
- To convert ammonia to nitrate and increase the dewaterability of the sludge.
- To reduce the mass of volatile solids and produce methane-rich biogas. (Correct answer)
- To disinfect the sludge and precipitate heavy metals.
Correct answer: To reduce the mass of volatile solids and produce methane-rich biogas.
Anaerobic digestion is a biological process that uses microorganisms in the absence of oxygen to break down organic matter in the sludge. This has two main benefits: 1) It converts a significant portion of the volatile solids into gas, which reduces the total volume and mass of sludge that requires final disposal. 2) It produces biogas, which is primarily composed of methane (CH4) and can be captured and used as a renewable energy source to generate heat or electricity.
Civil Engineering PE Water and Wastewater Treatment Questions and Answers
An operator at an activated sludge wastewater treatment plant observes persistent sludge bulking, characterized by a high Sludge Volume Index (SVI) and poor settling in the secondary clarifier.
Microscopic examination reveals a predominance of filamentous bacteria.
Which of the following operational conditions is the MOST likely cause of this type of bulking?