Wastewater Operator Secondary Treatment and Microbiology Questions and Answers — Questions and Answers
Question 1: An operator observes a large population of stalked ciliates and rotifers during a microscopic examination of activated sludge. What does this MOST likely indicate about the treatment process?
- A recent toxic shock load has occurred.
- The system has a high food-to-microorganism (F/M) ratio.
- A stable, mature sludge with a good settling quality and low effluent turbidity. (Correct answer)
- The dissolved oxygen level in the aeration basin is critically low.
Correct answer: A stable, mature sludge with a good settling quality and low effluent turbidity.
Stalked ciliates and rotifers are higher life-form microorganisms that are indicators of a stable and mature activated sludge process. Their presence signifies a well-established food web, low BOD, and sufficient aging of the sludge (high Mean Cell Residence Time), which typically results in good floc formation, excellent settling, and a clear final effluent.
Question 2: Which of the following is a key difference between the activated sludge process and a trickling filter?
- Activated sludge is an anaerobic process, while trickling filters are aerobic.
- Trickling filters use a suspended growth of microorganisms, while activated sludge uses attached growth.
- Activated sludge processes typically require less operational control and skill.
- In a trickling filter, microorganisms grow on a media surface, whereas in activated sludge, they are suspended in the mixed liquor. (Correct answer)
Correct answer: In a trickling filter, microorganisms grow on a media surface, whereas in activated sludge, they are suspended in the mixed liquor.
The fundamental difference lies in how the biomass is maintained. Trickling filters are an "attached growth" system where microorganisms form a slime layer (biofilm) on a fixed media (like rocks or plastic). In contrast, the activated sludge process is a "suspended growth" system where microorganisms are kept in suspension in an aeration tank.
Question 3: A wastewater treatment plant is required to remove nitrogen from its effluent. This is typically achieved through the biological processes of nitrification and denitrification. Which sequence and environmental condition is correct for this removal process?
- Denitrification in an anoxic zone, followed by nitrification in an aerobic zone.
- Nitrification in an anoxic zone, followed by denitrification in an aerobic zone.
- Denitrification in an aerobic zone, followed by nitrification in an anaerobic zone.
- Nitrification in an aerobic zone, followed by denitrification in an anoxic zone. (Correct answer)
Correct answer: Nitrification in an aerobic zone, followed by denitrification in an anoxic zone.
Biological nitrogen removal is a two-step process. First, nitrification occurs in an aerobic (oxygen-rich) environment where bacteria like Nitrosomonas and Nitrobacter convert ammonia to nitrate. Second, denitrification occurs in an anoxic (low oxygen, but nitrate is present) environment where different bacteria convert the nitrate to nitrogen gas, which is then released into the atmosphere.
Question 4: An operator is troubleshooting an activated sludge system experiencing "bulking sludge," which is settling very poorly in the secondary clarifier. What is a common microbiological cause of this condition?
- Excessive growth of amoebas and flagellates.
- Over-predation by rotifers and nematodes.
- Proliferation of filamentous bacteria that interfere with floc compaction. (Correct answer)
- A lack of floc-forming bacteria in the mixed liquor.
Correct answer: Proliferation of filamentous bacteria that interfere with floc compaction.
Filamentous bulking is a common operational problem where the excessive growth of filamentous bacteria creates a web-like structure within the flocs. This structure prevents the flocs from compacting and settling properly in the clarifier, leading to a high sludge volume index (SVI) and potential solids washout.
Question 5: In the context of secondary treatment, what is the primary role of protozoa?
- To convert ammonia directly into nitrogen gas.
- To consume organic waste through anaerobic digestion.
- To act as the primary decomposers of complex organic molecules.
- To consume free-swimming bacteria, promoting floc formation and clarifying the effluent. (Correct answer)
Correct answer: To consume free-swimming bacteria, promoting floc formation and clarifying the effluent.
While bacteria are the primary decomposers, protozoa play a crucial role as predators. They consume dispersed, free-swimming bacteria, which helps in several ways: it clarifies the effluent by removing turbidity, encourages the remaining bacteria to form larger, settleable flocs, and helps control the overall bacterial population.
Question 6: The conversion of ammonia (NH4+) to nitrite (NO2-) is the first step in nitrification. Which group of bacteria is primarily responsible for this specific conversion under aerobic conditions?
- Nitrobacter
- Pseudomonas
- Nitrosomonas (Correct answer)
- Zoogloea
Correct answer: Nitrosomonas
Nitrification is a two-step process carried out by specific groups of autotrophic bacteria. The first step, the oxidation of ammonia to nitrite, is performed by ammonia-oxidizing bacteria (AOB), with Nitrosomonas being the most well-known genus. The second step, the oxidation of nitrite to nitrate, is then carried out by nitrite-oxidizing bacteria (NOB) like Nitrobacter.
