Tennessee Wastewater Operator Practice Exam

A digester receiving both primary and secondary sludge will likely:

1. Consume less oxygen than one receiving only secondary sludge

2. Consume more oxygen than one receiving only secondary sludge

3. Operate at a higher temperature

4. Produce less gas than expected

The correct answer is: Consume more oxygen than one receiving only secondary sludge

When a digester receives both primary and secondary sludge, it typically consumes more oxygen compared to when it only processes secondary sludge. This increase in oxygen demand can be attributed to the higher organic content and complexities found in primary sludge. Primary sludge is rich in biodegradable materials because it is processed from the initial steps of wastewater treatment, which captures a larger proportion of organic matter. When mixed with secondary sludge, which has already undergone biological treatment and has a higher concentration of microbial activity, the combined organic load can lead to intensified microbial activity in the digester. As microorganisms break down the organic matter, they require oxygen for respiration, particularly in aerobic conditions. The combination of the two types of sludge enhances the microbial population and their metabolic processes, thereby increasing the overall oxygen consumption in the digester, which aligns with the reasoning for the correct answer. Understanding this dynamic is crucial for wastewater operators to