Removal of bacteria in subsurface flow wetlands

Removal of E. coli and total coliforms in subsurface flow constructed wetlands is investigated in field surveys and pilot experiments. Both systems use reed beds with 5-10 mm gravel medium receiving secondary effluents. A diurnal pattern of numbers was indicated in the survey of an operational tertiary reed bed at Leek Wootton. Removals of E. coli and total coliforms were compared in dry and wet periods in surveys on two successive years. Removals of about 1.5 to 2.1 log were found in dry weather. Removals fell in wet weather although no change was detected in removal of BOD5, TSS and amm N. The effect of different flow rates was compared using a pilot reed bed. A trend of increasing removal was seen between retention times of 12, 24, 48 and 120 hrs but variation between samples implied caution. All effluent samples from the pilot had less than 1000 cfu E. coli/100 ml at retention times of 24 hrs or more.