Tennessee Wastewater Operator Practice Exam

When does washout of methanogens occur in anaerobic digesters?

1. When the temperature is too high

2. When SRT is too short

3. When the feed sludge is overloaded

4. When the pH is too low

The correct answer is: When SRT is too short

Washout of methanogens in anaerobic digesters primarily occurs when the solids retention time (SRT) is too short. The SRT is a critical parameter in any anaerobic digestion process as it indicates the average time that the microbial population is retained in the digester. Methanogens, which are a group of microorganisms responsible for converting substrates into methane gas, typically grow slowly. Consequently, if the SRT is not adequately maintained, these organisms can be washed out before they have enough time to replicate and establish a stable population. When the SRT is too short, the removal rate of the biomass surpasses the growth rate, leading to insufficient numbers of methanogenic organisms to maintain effective digestion processes. This depletion can adversely affect the overall efficiency of the digester, leading to problems such as reduced biogas production and increased levels of volatile fatty acids. In this context, while other factors like temperature, feed sludge overload, and pH do have implications on the functioning of an anaerobic digester, the primary concern for the survival and viability of methanogens is their retention time within the system. A proper understanding of SRT management is essential for optimizing the conditions necessary for successful anaerobic digestion and ensuring the stability of methanogenic