In primary facilitative ponds (those that receive raw wastewater), two mechanisms reduce BOD: Sedimentation – settled solids undergo anaerobic digestion.
Upper layers oxidize non-settleable organic solids and the solubilized products of anaerobic digestion. Part of the oxygen comes by surface aeration of wastewater, but mostly it is provided by the photo-synthetic activities of the micro-algae, which have profuse growth in the facilitative pond. The algae in return gets almost all of carbon dioxide from the end product of bacterial metabolism. A symbiotic relationship exists between the heterotrophic bacteria and the autotrophic algae.
In secondary facilitative pond (those that receive what comes from the anaerobic pond), the first mechanism (sedimentation) is very little.
BOD removal in both the types of facilitative ponds is in the range 70 – 80%. The depth of facilitative ponds is 1 m to 2 m. A depth of 1.5 m is usually preferred. Depth, less than 0.9 m encourage growth of rooted plants (a natural habitat for mosquito breeding).
Due to the photo-synthetic activities of the facilitative pond algae, a variation of DO concentration is noticed. After sunrise, the DO level gradually rises to a peak value. From mid-afternoon, it gradually drops down and reaches a minimum at night. Oxyphase and pH values change. When algae activity is at its peak, the bi-carbonate ions break to release more carbon dioxide to the algae, so excessive hydroxyl ions that are left over boost pH up to or above 10.
Wind and the churning of pond liquid takes place resulting in an even distribution of DO, BOD, bacteria and algae throughout the depth and hence a better stabilization. When this is absent, the algae population stratifies in narrow bonds (about 20 cm thick), during daylight hours. These bonds move up and down through the top 50 cm of the facilitative pond in response to the changes in sunlight intensity. Samples collected from these will show increased BOD, COD and suspended solids, which is not a true representation of everything.