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Efficiency of sand filter beds for the removal of bacteria from residential wastewater

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Title: Efficiency of sand filter beds for the removal of bacteria from residential wastewater
Author: Samimian Tehrani, Peyman
Department: School of Engineering
Advisor: Joy, D.M.
Abstract: On-site septic systems are potential source for environmental and ground water pollution. In Ontario, residential sewage systems with total daily design flow up to 10,000 L/day are regulated by Ontario Building Code (OBC). It is accepted in Ontario that wastewater return to environment through a soil absorption system (SAS) to receive adequate treatment before reaching groundwater. A minimum 900 mm of unsaturated soil is required for any SAS from the release point of wastewater vertically down to the limiting layer, 'i.e'. ground water table or bedrock to ensure the wastewater receives acceptable level of treatment. Although several researches have been conducted to measure the contamination removal in soil, data is not generally available on level of contamination below SAS under field conditions. This research is an attempt to provide long term data on level of bacterial contamination at different depths under surface of a filter bed. Liquid samples were collected from various depths of two filter beds and the respective septic tanks and analyzed for ' E. coli' concentration. Research was carried on for over a year. The septic tank effluent (STE) which was collected at the outlet of the septic tanks just before the effluent filter, had mean 'E. coli' concentrations of 8.5E+05 and 6.8E+05 CFU/100 mL at each site. The concentrations at different depths were generally in range of 1E+02 CFU/100 mL, with variations. The results suggest that the current depths of sand filter indicated in OBC provide sufficient treatment in terms of bacterial removal. Most of the removal occurs in the first 375 mm of sand filter. The addition of 150 mm of native soil (from 750 to 900 mm) does not increase treatment significantly. The weather data, which is well within the range of long term data for the region, did not appear to affect the performance of the filter beds, nor affected the concentrations in septic tank.
Date: 2009
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