A sample from an aerated lagoon at 2:15 PM on a sunny July day shows DO 20 mg/L, pH 8.8, lagoon bright green, and aerators not running. What is the MOST appropriate action?

• A. Keep aerators off
• B. Apply algaecide
• C. Apply sulfuric acid
• D. Turn aerators on

Answer: (A)

In daylight, algae in an aerated lagoon photosynthesize and produce a lot of oxygen. The measured dissolved oxygen is already very high (20 mg/L), and the lagoon’s bright green color indicates an active algal bloom that is generating oxygen rather than consuming it. There’s no oxygen deficit to correct, so running the aerators would waste energy without improving treatment. Keeping the aerators off preserves energy and avoids unnecessary disturbance of the surface while the DO remains sufficient. The other options aren’t appropriate here: there’s no reason to apply algaecide in a wastewater lagoon simply because of a bloom, acidifying the lagoon isn’t needed at this DO/pH level, and turning on aeration would not address any problem and would expend energy.