Secondary treatment of sewage is known to reduce pollutants considerably. The present study reports on the reduction in major pollutants-Nitrate (NO3), Phosphate (PO4), and Sulphate (SO4), Biochemical Oxygen Demand (BOD) and Chemical Oxygen Demand (COD)-leading to increase in Dissolved Oxygen (DO) in municipal Raw Sewage (RS) when treated by secondary treatment, a conventional extended aeration activated sludge process, at Kalpakkam located on the east coast of Tamil Nadu in India. The town generates 0.6 million gallons of sewage per day. These water quality parameters were monitored monthly during pre-monsoon-2005 to post-monsoon-2006. It was found that NO3, PO4, SO4, BOD and COD in the RS were 42, 1.6, 45, 279 and 399 mg/L respectively, which were reduced considerably at each phase of the treatment. At Aeration Tank (AT), these reduced by 31%, 32%, 13%, 62% and 36% and at Secondary Clarifier (SC) by 57%, 69%, 37%, 92% and 65%, respectively. The DO in RS was only 0.6 mg/L that increased at AT and SC by 81% and 87%, respectively. The reduction in these pollutants at AT and SC showed considerable temporal variation across different seasons.

Canals and rivers in most of the developing countries are polluted by indiscriminate disposal of untreated as well as treated municipal sewage and industrial effluents, affecting their water quality (Campos and von Sperling, 1996; and Girija et al., 2007). Thus, municipal sewage, when treated in a conventional Sewage Treatment Plant (STP) prior to its release, minimizes the adverse impact on the receiving water bodies, as primary and secondary treatments reduce the concentration of pollutants (notably NO₃ and PO₄) from the Raw Sewage (RS) (Van Haandel and Lettinga, 1994). High nitrate concentrations are frequently encountered in treated wastewater, as a result of ammonium nitrogen, which is prevalent in raw waste, being totally or partially oxidized to nitrite and nitrate by microbial action. High nitrate levels in waste effluents could also contribute to the nutrient load of the receiving water bodies and contribute to their eutrophication (Fried, 1991). Phosphates in sewage arise from human wastes and phosphate-based detergents, and are undesirable anions in receiving waters as these anions cause the eutrophication that results in a variety of adverse ecological effects (Pretoria et al., 2000). Sewage also contains sulphates. Due to bacterial action, these sulphates produce hydrogen sulphide, which gives the foul rotten egg smell found in sewage. Hydrogen sulphide in the form of sulphur is poisonous.

Dissolved Oxygen (DO) is an important water quality parameter. When the sewage is released into the receiving water bodies, the active degradation (oxidation) process of organic matter in the water consumes the DO leading to its rapid depletion and results in the Biochemical Oxygen Demand (BOD), which refers to the amount of DO needed by the water to completely oxidize its organic pollution load (Momba et al., 2006). Water with a high BOD, when released into the natural receiving water bodies, destroys these systems that may support little aquatic life, unless there is a means for rapid replenishment of DO. The suspended substances, nutrients and organic load also contribute to COD and pollute the canals, rivers and lakes. The dissolved organic matter in a given volume of water, oxidized chemically to CO₂ and H₂O by strong chemical oxidation, brings about the Chemical Oxygen Demand (COD) that may also be used as a measure of general pollution. The removal of both the BOD and COD in wastewater is one of the fundamental goals of wastewater treatment (Ahsan et al., 2001).

Monitoring the Removal of Pollutants in Municipal Sewage by Secondary Treatment at Kalpakkam Urban System, Municipal sewage, Activated sludge treatment, Major pollutants, Chemical Oxygen Demand (COD), Dissolved Oxygen (DO), active degradation, organic matter.