Question

In an anaerobic digester, if the hydraulic retention time is decreased below the critical doubling time of methanogenic bacteria, what happens to the methane production?

Accepted Answer

If the hydraulic retention time is decreased below the critical doubling time of methanogenic bacteria, methane production will cease, and the anaerobic digester will experience a process failure known as washout. The hydraulic retention time is the average amount of time that liquid material, such as wastewater, spends inside the digester. Methanogenic bacteria are the microorganisms responsible for converting organic acids into methane gas. The doubling time is the period required for a population of these bacteria to divide and grow in number. If the liquid moves through the digester faster than the methanogens can reproduce, the bacteria are physically flushed out of the system faster than they can replace themselves. As the population of these essential microbes declines, the conversion of organic matter into methane stops. Once the methanogens are depleted, the system can no longer produce methane, and the accumulation of intermediate organic acids will cause the pH level to drop, further inhibiting the environment for any remaining biological activity.

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In an anaerobic digester, if the hydraulic retention time is decreased below the critical doubling time of methanogenic bacteria, what happens to the methane production?