What impact do low dissolved oxygen levels (0.2-0.5 mg/L) have on wastewater treatment processes?

Low dissolved oxygen levels, particularly in the range of 0.2 to 0.5 mg/L, can lead to septic conditions in wastewater treatment processes. Dissolved oxygen is essential for aerobic bacteria, which play a crucial role in the breakdown of organic matter. When oxygen levels drop significantly, the aerobic processes become inhibited, leading to a shift toward anaerobic conditions where less desirable bacteria thrive.

These anaerobic conditions result in the production of gases such as hydrogen sulfide, which contributes to unpleasant odors and can create toxic environments that further hinder the treatment process. Moreover, septic conditions may cause operational issues, including the potential for increased foul odors and the formation of scum layers, which can disrupt the overall functioning of the treatment plant.

In contrast, the other options involve scenarios that do not accurately describe the direct impact of low dissolved oxygen levels. For instance, low oxygen does not improve bacterial activity, as aerobic bacteria rely on sufficient oxygen to effectively decompose waste. Similarly, low oxygen levels do not stabilize foam formation—often, excessive foam can occur in anaerobic conditions, and filtration efficiency is generally impeded rather than enhanced when dissolved oxygen is too low. Thus, the answer highlights a critical relationship between dissolved oxygen levels and the functionality of wastewater treatment