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Oxidation Ponds (lagons): These are shallow ponds, about 5 feet deep, where aeration of wastewater for oxidation is accomplished by diffusion though wind action and photosynthetic activity of algae. Oxygenation is usually incomplete with consequent odour problems. Bacteria digest and oxidize the organic material In sewage, whereas algae utilize CO2 and other substances resulting from bacterial action and through photosynthesis they produce O2 needed for aerobic bacterial action. Because of this interaction oxidation ponds reduce 75-90% BOD. This is low cost but inefficient system requiring long retention times. This process is recommended for small communities in rural areas for warmer climates where suitable and sufficient land is available (Fig. 17.1).

Fig. 17.1 Oxidation pond (top) and sedimentation tank (bottom).

(i)                       Activated Sludge Process: It is an efficient and popular biological treatment process of sewage treatment, where, the sewage is aerated vigorously for 4-8 hours to promote the growth of aerobic microorganisms, where aeration is provided by mechanical stirring or air injection. The finely suspended and colloidal matter along with many microorganisms forms aggregates (flocs). After repetition of this process several times, the excess sedimented floc thus obtained is known as activated sludge and it is made up of microorganisms along with organic matter. The liquid is passed into a settling tank to settle out the sludge. A portion of this sludge is returned to the previous chamber to maintain a sufficient population of microorganisms for digestion of new batch of sewage (hence the name of the process) and rest is pumped to an anaerobic digestion chamber. In this anaerobic chamber, organic compounds get decomposed anaerobically to produce soluble substances and various gases like, about 70% CH4, 25% CO2, H2 and N2· This gas mixture can be used as fuel and the sludge obtained after digestion can be used as manure after drying it in sun. The liquid from the settling tank does not contain any organic matter (higher BOD removal) but contains NO3, SO42-,PO43- etc. and a few pathogens and can be discharged into natural water body. In this technology, energy is needed for aeration of the wastewater.] or incinerated or composted or disposed in landfills.

Fig, 17.2 Activated sludge system.