**3.4 Aerobic lagoons (AELs)**

Aerobic lagoons (AELs) are designed and operated to exclude algae. This is accomplished by two means. First, sufficient mixing is used to keep all biomass from the treatment system in suspension, thereby providing turbidity that restricts penetration of light into the water column. The mixing also has the effect of making the solid retention time (SRT) equal to the hydraulic retention time (HRT). Second, the HRT is controlled to values less than the minimum SRT for algal growth (about two days). Because algae are excluded, oxygen must be delivered by mechanical means [8].
