A lagoon that forms the second treatment stage in waste stabilisation ponds (WSPs). It is a shallow pond (1 to 2m) consisting of an aerobic zone close to the surface and a deeper, anaerobic zone. AS algae grow on the surface they produce oxygen, which is consumed by aerobic bacteria in the middle of the pond degrading the BOD. In the lower zones of the pond, anaerobic digestion takes place.
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