Polishing Treatment – Before the wastewater can be discharged safely into the natural environment, it must go through different stages of treatment to remove contaminants. These treatment categories include primary and secondary treatments involving aerobic (aeration wastewater treatment) and anaerobic processes. For example, sludge treatment is an integral part of wastewater treatment processes. Another integral part of these treatment processes is clarifier wastewater treatment system which involves separating the wastewater from suspended particles using gravitational influence.
Polishing is the tertiary and final effluent wastewater treatment stage before the wastewater can eventually be discharged into natural water bodies. Polishing process involves removal of remaining suspended solids and biological oxygen demand (BOD) that may be left after secondary effluent treatment. This helps in making the water more hygenic and environmentally safe before release.
The polishing process starts with filtration. This is done by passing tertiary wastewater over filters (which can be bed of sand or charcoal). This causes the particulate matter from water to attach to the filter media thus making the water free of such most of such matter.
The next step in polishing treatment process is called lagooning. During the polishing treatment, the water is kept in natural condition with full exposure to air in one or many, usually compartmentalized, open water bodies which are called polishing ponds. These ponds are usually from five to ten feet deep and the water is stored in these ponds for comparatively shorter duration which can vary from one to three days. During this time, sedimentation of non-degraded and degraded suspended particles at the bottom of the pond is facilitated in a natural way. Further, aquatic plants, invertebrates and weed eating fish are introduced in the polishing pond to absorb and consume any remaining particulate matter.
The water may still be very nutrient rich with especially high phosphorus and nitrogen content. This can promote growth of algal blooms. Bacteria consume these nutrients to produce products which can be removed from wastewater in much easier way. At the same time, fish helps in keeping algal growth in check by eating algae and unnecessary weeds. Eventually, chlorine is added to water, within safe limits allowed by local regulations, to kill off microbes thus disinfecting the water before eventual release.
Before eventually releasing the water into nature, it is recommended to test it for contamination. Apart from gauging the water quality, it also helps in measuring and improving the efficiency of polishing treatment. One test involves measuring concentration of eggs of parasitic worms which are commonly found in faeces and sewer waste. Another, more stringent, test involves measuring faecal coliforms in water. This is because these can be commonly found in faeces of animals and humans and take much longer to be removed from water as compared to eggs of parasitic worms.
Once the water meets the strict regulatory criteria of governmental and environmental agencies, it can be safely released into natural environment, for example, water bodies. The water, thus released, may not still be suitable for direct human consumption but it can be used, for example, for irrigating agricultural land, industrial use etc.