The performance of a deep primary waste stabilization pond

Abstract

This thesis describes an investigation into the performance of a deep primary waste stabilization pond under warm climatic conditions. Wastewater treatment designers in developing countries are constantly faced with the problem of selecting a system that is both efficient and economic. The main objective to this study was to determine safe loading rate and retention time in a pond deeper than the ponds ·normally used in warm climates. A secondary objective was to study the effect of using a deep pond in conjunction with a secondary shallow pond. A pilot-scale pond of 3.6m depth as primary pond and oi1 drum of 1991 capacity as secondary shallow pond were used under field conditions for the experimental study. The performance was evaluated by measuring influent and effluent BOD, COD and TC along with the measurements of DO, temperature, and pH profiles. Experimental results have shown that the high loaded deep pond is effective in the removal of bacteria and organic matter. A linear regression equation has been obtained for BOD and COD of raw domestic wastewater in order to predict BOD from COD measurements. The straight line regression equations have also been obtained to predict BOD and COD areal removal rates, covering ranges of 575 to .1406kg BOD/ha.d and 1137 to 2912kg COD/ha.d. Average climatological factors from records of a 25 year period were analysed and the permissible areal organic loading computed. From this analysis and the experimental results obtained, it was concluded that the maximum areal loading should be limited to 640kg BOD/ha.d (or about 1300 kg COD/ha.d) with a minimum hydraulic retention time of 11 days. Experimental study also revealed that the combination of a deep pond followed by .a secondary pond can produce a high quality effluent in terms of soluble organics and bacterial numbers. An empirical equation to predict BOD removal efficiency when volumetric ·organic loading is known was developed by analysing the experimental results and data collected from several publications. Using this equation, a chart to select suitable pond depths for different loading rates has also been developed. Beneficial and adverse effects of tropical climates on pond performance are identified and discussed. The areas which require further study and development are also identified. And recommendations made f6r future investigations.