Tennessee Wastewater Operator Practice Exam

With an unchanged influent BOD load and increasing SRT, what must be true?

1. The growth rate is increasing

2. The growth rate is decreasing

3. The settling quality is improving

4. The biomass concentration is decreasing

The correct answer is: The growth rate is decreasing

When the influent Biochemical Oxygen Demand (BOD) load remains constant and the Solids Retention Time (SRT) increases, it indicates that the biomass in the system is retained for a longer period. This extended retention time allows for more time for the microorganisms to process the BOD, but at the same time, the growth of new biomass may begin to level off or decrease. This is due to several factors, including the potential for established microbial populations to reach a point of diminishing returns in their growth due to nutrient limitations or reduced availability of organic material as it gets consumed. An increased SRT can lead to a steady-state condition in which the growth rate of the biomass decreases because there may not be enough influent substrate (BOD) for the biomass to maintain an increasing growth rate. In essence, while the existing biomass stays viable longer, the overall growth and reproduction of new organisms may not keep pace, leading to a decline in the growth rate. This situation does not inherently imply improvements in settling quality or a decrease in biomass concentration; instead, it focuses on the relationship between retention time and microbial activity in response to a constant organic load. Thus, an increasing SRT with an unchanged influent BOD load typically results in a