A facultative lagoon must be started up in the winter. To what depth should the lagoon be filled?

• A. 1 ft
• B. 3 ft
• C. 5 ft
• D. 7 ft

Answer: (C)

Starting up a facultative lagoon in winter requires a depth that supports both the upper, oxygen-producing layer created by algae and the deeper zone where solids settle and are digested. If the water is too shallow, the lagoon can freeze more easily and digestion may be incomplete. If it’s too deep, light won’t reach the algae well enough to provide oxygen, slowing startup and increasing odors. A moderate depth—roughly in the middle of the usual design range for facultative lagoons—gives enough water column for the anaerobic processes at the bottom while still allowing light to reach the surface to sustain algal activity during daylight. This balance helps establish a stable, self-sustaining system as temperatures change.