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Facultative
Ponds - Oxidation pond, in which both aerobic and anaerobic activities are carried out in a single unit is called facultative pond. They are the most widely used oxidation ponds. Facultative ponds are mostly rectangular in shape with a length-breadth ratio of 3: 1. They are usually l-2m deep. Shallow areas near the edge of the tank should be avoided to prevent breeding of mosquitoes.
Cutting the weeds or grasses and spraying of insecticides can also help. Banks of the ponds may be simply made of earth or in case of large ponds to prevent wave action slabs can be paved. The normal retention time for these ponds is 5 to 30 days depending upon the temperature.
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In facultative ponds, as mentioned earlier, both aerobic and anaerobic activities take place. Near the surface of the ponds chlorophyll bearing microorganisms, phytoflagellates and algae dominate. Representative genera of algae include Chlorella, Chlamydomonas, Scenedesmus, Euglena and Oscillatoria.
These organisms perform photosynthesis utilizing the inorganic materials and carbon dioxide. Carbon dioxide is obtained either from bacterial decomposition of organic matters or from the atmosphere which gives rise to dissolved CO2.
The oxygen produced by photosynthesis causes an increase in the dissolved oxygen (DO) level which favours aerobic bacteriological activity. Dominant member of the aerobic heterotrophic bacterial population are Pseudomonas, Achromobacter and Flavobacterium DO level may decrease after aerobic activities and it may also reach zero if the pond is overloaded.
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In the bottom region, anaerobic condition is maintained and decomposition of detritus materials occurs. This anaerobic activity stabilizes the sludge to some extent and releases some of the organic matter in soluble form which are degraded in the aerobic zone.
Removal of bacteria occurs by natural death and also by settlement, because of the long retention time in the tank. Bacteria and phytoplankton can also be removed by the preying action of ciliates, rotifers and crustaceans.
However some bacteria may escape these and are found in the effluent. Considerable removal of viruses also occurs and is enhanced by the light intensity. Sometimes large algal growth may occur in the tanks and may lead to a high suspended solid level in the effluent if not removed.
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