Florida Wastewater Practice Test 2026 – Complete Exam Prep

The increase in dissolved oxygen (DO) residual is a strong indicator that a toxic compound has entered and progressed through the activated sludge aeration basin. When a toxic substance infiltrates the system, it can inhibit the activity of microorganisms responsible for the biological treatment process. Typically, these microorganisms would consume organic material during the aeration phase, resulting in a consumption of oxygen and thus a decrease in DO levels.

However, when toxicity affects the microorganisms' ability to metabolize and break down organic compounds, their activity diminishes, leading to less oxygen being consumed. This results in higher residual levels of DO in the basin. Observing an increase in DO suggests that the biological processes are not functioning effectively, often due to the presence of toxic substances interfering with the intended microbial activity.

In contrast, a decrease in pH levels might suggest acidification rather than explicitly indicating toxicity. A decrease in MLSS (Mixed Liquor Suspended Solids) concentration typically signals a reduction in biomass or microbial activity but does not directly link to toxicity in the way DO does. Additionally, an increase in the Food-to-Microbe (F/M) ratio could reflect a change in substrate availability or biomass levels, but it does not necessarily correlate with the presence of a toxic compound.