• Suspension mixed lagoons, where there is less energy provided by the aeration equipment to keep the sludge in suspension. •
Facultative lagoons, where there is insufficient energy provided by the aeration equipment to keep the sludge in suspension and solids settle to the lagoon floor. The biodegradable solids in the settled sludge then degrade as in an
anaerobic lagoon.
Suspension mixed lagoons Suspension mixed lagoons flow through
activated sludge systems where the effluent has the same composition as the mixed liquor in the lagoon. Typically the sludge will have a residence time or
sludge age of 1 to 5 days. This means that the
chemical oxygen demand (COD) removed is relatively little and the effluent is therefore unacceptable for discharge into receiving waters. The objective of the lagoon is therefore to act as a biologically assisted flocculator which converts the soluble biodegradable organics in the influent to a biomass which is able to settle as a sludge. Usually the effluent is then put in a second pond where the sludge can settle. The effluent can then be removed from the top with a low chemical oxygen demand, while the sludge accumulates on the floor and undergoes anaerobic stabilisation. ==Methods of aerating lagoons or basins==