They function similar to domestic open septic tanks.
The settable solids in the raw wastewater settle down as a sludge layer where they are attacked by the acidogenic bacteria first.
The bacteria in this waste stabilization pond break down the carbohydrates, proteins and fats into fatty acids.
When this happens, the organic load does not reduce.
Later, at a temperature above 15 Deg.
C, the methanogenic bacteria convert the fatty acids into CO2 (Carbon Dioxide) and CH4 (Methane), which go up into air.
BOD removal is from 40% to 60% depending upon ambient temperature.
A scum layer forms on the surface which should not be disturbed.
It maintains the anaerobic conditions below and also controls the pond temperature.
Fly breeding in summer may be controlled by clean water sprays or final pond effluent spray, but never use insecticides in a waste stabilization pond.
Anaerobic ponds may appear purple or pink, due to sulfide oxidizing by photosynthetic bacteria.
They convert hydrogen sulfide to sulfur and their growth is advantageous.
Odor release (mainly H2S) is usually a major disadvantage of anaerobic ponds.
If designed for a loading of less than 400 gm BOD/m^3/day, odor nuisance does not occur.
500 mg of SO4 (Sulfate)/l is the limit.
The depth of the anaerobic ponds can be 2 -- 5m.
1 day is the minimum detention time.
Maturation Ponds Maturation ponds receive effluents from facultative ponds.
The number and size of the maturation ponds depends on the degree of bacterial quality of the effluent desired.
These ponds show less vertical qualitative stratification (better churning effect) and are well oxygenated for the whole day.
Depths can go up to 3m, but are usually kept the same as facultative ponds.
The parameters that govern the removal of fecal bacteria are temperature, detention time and organic loading.
Removal efficiency improves with a) increasing temperature b) increasing detention time and c) decreasing organic load.
However, the design of the waste stabilization pond is based only on temperature and detention time.
The removal mechanism of virus in the ponds is not well documented.
It is believed that the settlement of solids leads to absorption of virus.
Sedimentation removes excreted protozoan cysts and helminth eggs.
A series of ponds with overall detention time of 11 days or more, normally removes all cysts and eggs.