Fate of major pollutants in Pagla sewage treatment plant

Abstract

General complaints of shortcomings of the overall performance, health risk for marketing fish grown in the facultative lagoons and prospect of the effluent and sludge to be used in agriculture, have led to this concerted efforts in understanding, monitoring, and evaluating the overall performance of Pagla sewage treatment plant. The Pagla Sewage Treatment Plant was designed to treat a maximum flow of 120,000 m3/day of domestic sewage generated from about 18 per cent population of the metropolitan city, The final effluent from the treatment plant is discharged into the adjacent Buriganga River. The primary objective of the study was to assess the efficiency of the plant in removing pollutants and quality of effluent and sludge after treatment prior to disposal in the environment. Secondary aims include determining the efficacy of the plant to be used in beneficial purpose in addition to sewage treatment. For this reason the concentration of the pollutants in effluent, sludge and biofabric was tested, and the toxicity of sludge was also assessed to determine the mobility of the toxic elements from sludge to water. Initially water sample from five locations, inlet, after coarse screening, primary sedimentation tank at the end of preparatory treatment, fag end of both the lagoons and outlet prior to chlorination, were collected, Observing some aberration in the effluent quality after chlorination, few samples were collected after chlorination, Flow characteristics, pH at different locations, BODs, organic loading, COD, nutrients and heavy metals (chromium, cadmium and lead) including arsenic were tested, A wide variation of BODs was detected especially in the lagoons, Depending on the rate of reduction obtained from BODs determination, calculation for extended period shows that a detention period of 10 days instead of present 7days is required in the lagoons under present operating condition to meet the Bangladesh Standard of 40 mglL for effluent BODs prior discharging into inland surface waters, The Pagla STP was found quite effective in removmg suspended solids, The residual suspended solids in the effluent meet the requirement of Bangladesh standard for the discharge of sewage effluents into surface and inland waters, The number of coliforms found before and after chlorination plant indicates that proper chlorine dose (3,0 ppm as suggested) is not currently applied to reduce FC counts to acceptable level for discharge of the effluent in inland surface water. In assessing the suitability of the efflucnt from Pagla STP as irrigation watcr, the heavy mctal concentration in wastewater was also determined. It can be inferred from thc result that concentrations of As, Cr and Cd in effluent are quite low and well within acceptable level, but Pb content in the effluent is higher than the acceptable levels for irrigation and discharge in surface water. From the data it has also been revealed that unlike other three toxic substances, waste stabilization pond is not efficient in lead removal. Sludges from sludge lagoons were collected to determine the efficacy of the sludge for agricultural manuring and disposal in landfills. The concentrations of toxic heavy metals and their leaching characteristics meet the guideline values recommended by EEA (European Environment Agency) and USEPA (US Environmental Protection Agency) for agricultural usc and disposal in landfills. Hence the nutrients present in the sludge can be safely recycled in the agricultural lands. In view of very low potential for groundwater contamination, the sludge can also be disposed off in landfills. But before taking any steps, more comprehensive study over a long period is required, as leachate from sludge is a very long term phenomena. One of the targets of this study was to assess the suitability of the lagoons to be used for fish farming. Although the concentration of toxic metals in flesh is within acceptable limit, irrespective of size and species the accumulation of toxic metals is high in head, which is an edible organ. Therefore, fish farming should remain banned until entry of toxic substances from industrial wastewater can be prevented.