Cleaning the digester can have what effect on hydraulic retention time (HRT)?

Cleaning the digester can indeed increase the hydraulic retention time (HRT) and improve the volatile solids reduction (VSR). When the digester is cleaned, it eliminates accumulated sludge and debris that can cause issues with fluid flow and gas exchange. This maintenance not only improves the efficiency of the digestion process but also can enhance the capacity of the digester to treat incoming waste.

With a cleaner environment, the biological processes can function more effectively, allowing for an increased HRT. A longer HRT means that microorganisms have more time to break down the organic material, which leads to greater volatile solids reduction. Proper management of HRT is essential for optimizing the performance of wastewater treatment processes, as it directly influences the extent to which organic materials are decomposed.

The other options suggest effects like no change or a decrease in HRT, which doesn't align with the expected outcomes of maintaining a cleaner digestion process.