Tennessee Wastewater Operator Practice Exam

What happens when aeration is turned off for an extended period in a digester?

1. Settling occurs immediately

2. Supernatant layers are always formed

3. Sludge density increases

4. Solids may not settle properly

The correct answer is: Solids may not settle properly

When aeration is turned off for an extended period in a digester, the process can lead to the improper settling of solids. Aeration helps maintain the right conditions for microbial activity, which is essential for the breakdown of organic materials. When aeration ceases, the environment becomes anaerobic, and this can disrupt the balance of microbial populations. Without proper aeration, certain types of bacteria that thrive in aerobic conditions may die off or become less active. This can result in the formation of different microbial byproducts, which may alter the physical properties of the sludge. Consequently, the sludge may lose its ability to compact properly, leading to larger floc sizes that do not settle effectively. This condition can hinder the separation of solids from the liquid phase during the settling process, potentially causing issues with the quality of the supernatant and leading to difficulties in the subsequent treatment processes. In contrast, when aeration is active, it supports the growth of aerobic bacteria that help keep the solids well-distributed and facilitate effective settling. The absence of aeration hampers these processes, causing settling problems that can impact the overall efficiency and effectiveness of the digestion system.