Tennessee Wastewater Operator Practice Exam

What is the typical solids retention time (SRT) for a conventional mesophilic digester?

1. 10 days

2. 15 days

3. 20 days

4. 25 days

The correct answer is: 20 days

The typical solids retention time (SRT) for a conventional mesophilic digester is generally around 20 days. SRT is a crucial parameter in anaerobic digestion, as it represents the average time that solid material remains in the digester. It reflects the balance between the rate of solids entering and leaving the digester. In a conventional mesophilic digester, a 20-day SRT allows for an optimal retention time for the microorganisms to break down organic material efficiently, thus maximizing biogas production and reducing solids volume. The mesophilic temperature range (around 30 to 38 degrees Celsius) is conducive for the activity of bacteria involved in anaerobic digestion, and maintaining this SRT ensures that the bacteria have enough time to carry out their processes effectively. The selection of 20 days aligns well with operational practices and performance standards commonly observed in the field. This SRT allows for a balance between effective digestion and operational efficiency. A significantly lower or higher SRT may lead to inadequate digestion or excessive buildup of solids, respectively, impacting the overall treatment process.