Tennessee Wastewater Operator Practice Exam

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What is the typical solids retention time for mesophilic anaerobic digesters?

10 days
20 days
30 days
40 days

The correct answer is: 20 days

The typical solids retention time (SRT) for mesophilic anaerobic digesters is generally around 20 days. This duration allows for the effective breakdown of organic matter by microorganisms that thrive in mesophilic conditions, which typically function optimally at temperatures between 30°C to 37°C (86°F to 98.6°F). This SRT balances the need for sufficient time for the microbial community to stabilize and effectively digest the organic solids while also ensuring that the digester operates efficiently. A retention time too short may lead to incomplete digestion of the solids, while a retention time that is too long can reduce the overall efficiency of the digestion process and increase operational costs. The specific selection of a 20-day retention time reflects operational practices demonstrated to optimize biogas production and minimize the accumulation of inert material, thus maintaining the overall health and productivity of the anaerobic digester system.