For deep cell facultative lagoons, what is the typical depth distribution?

• A. 12-18 ft with top aerobic, middle facultative, bottom anaerobic
• B. 6 ft uniform across
• C. 25 ft with mixed thirds
• D. 3 ft shallow

Answer: (A)

In deep cell facultative lagoons, a vertical oxygen gradient forms because oxygen from the air diffuses downward but is quickly consumed by microbes near the surface. This creates three distinct microbial zones in a typical 12–18 ft depth: a top aerobic layer where oxygen is available and organic matter is oxidized, a middle facultative layer where organisms can use oxygen if present but can also operate without it, and a bottom anaerobic layer where oxygen is absent and anaerobic digestion occurs. This layered arrangement supports effective treatment, helps settle solids, and reduces odors.

Smaller depths, like about 3 ft, don’t allow a bottom anaerobic zone to develop, so you’d lose the three-layer structure. A uniform 6 ft or a deeper 25 ft with “mixed thirds” wouldn’t establish the stable, stratified gradient designed into deep cell lagoons. Hence, the 12–18 ft depth with top aerobic, middle facultative, and bottom anaerobic is the typical distribution.