Tennessee Wastewater Operator Practice Exam

What parameter determines whether a digester is considered low or high rate?

1. Temperature

2. Hydraulic retention time

3. Volatile solids loading rate

4. pH level

The correct answer is: Volatile solids loading rate

The classification of a digester as low or high rate is primarily based on the volatile solids loading rate. This parameter measures the amount of biodegradable organic material (volatile solids) added to the digester per unit volume and time. In a high-rate digester, the volatile solids loading rate is significantly higher, enabling more efficient digestion and faster processing of organic material. These digesters can operate effectively with shorter hydraulic retention times due to the higher concentration of active microorganisms and enhanced digestion kinetics. Conversely, low-rate digesters operate with a lower volatile solids loading rate, leading to longer hydraulic retention times. This can be necessary for certain types of organic material that require more time for complete breakdown. The classification impacts not only the type of wastewater treatment technology employed but also the design, operation, and performance expectations of the digester. While temperature, hydraulic retention time, and pH level are important factors in digestion processes, they do not directly categorize digesters as low or high rate. Instead, they may reflect the conditions under which the digester operates and can influence performance, but the key determinant in this classification is the volatile solids loading rate.