Removal of E. coli during intermittent filtration of wastewater effluent as affected by dosing rate and media type

Wastewater effluent dosing rates of 25 and 50 mm/day were intermittently applied in eight daily doses of 3.125 or 6.25 mm each, to 15-cm diameter 80 cm high columns packed with two types of Light Weight Aggregates (LWA) and one type of activated carbon aggregates. After three months of wastewater effluent application at 25 mm/day to stabilize the filter systems, Escherichia coli was spiked once each day onto the surface of the columns and wastewater effluent was applied at 25 mm/day for the months. The same procedure was repeated for effluent application rate of 50 mm/day. During operation, hydraulic behavior was monitored by moisture tensiometers located 5, 10, 20 and 40 cm below the filter surface as well as by radiotracer studies. Removal behavior was assessed by sampling and analysis of the column percolate and media within the column. The removal of E. coli was decreased as a result of increasing the dosing rate for all three media. In all media, the highest removal rates were observed in the upper part of the columns. Sorption head measurements showed that each effluent dose rapidly penetrates through the upper part of the filters, until a steady state, unsaturated flow was established in the lower sections. Different flow patterns were observed for the two dosing rates. For the dosing rate of 50 mm/day, the flow was penetrating faster, and to a deeper level before establishing steady unsaturated flow. Fast flow through the upper part of the filter, where the bacterial removal is most effective, may explain the significantly lower removal for the dosing rate of 50 mm/day. The dynamic behavior of the filter columns showed that most of the water movement took place right after dose application, during intermittent dosing. This indicates that dose size may be just as important for bacterial removal as the daily dosing rate.