COLIFORM REMOVAL IN A STABILIZATION RESERVOIR FOR WASTE-WATER IRRIGATION IN ISRAEL

The removal of faecal and total coliforms is studied in a reservoir where wastewater is stored during the rainy winter to be used for irrigation during the dry summer. The operational regime of the reservoir is irregular, changing between non-steady-state flow and batch. The ranges of the reservoir's key parameters are: inflow faecal coliforms, 10(6)-10(7)/100cm(3); water level, 2.3-5.5 m; water temperature, 10-30 degrees C; pH, 6.9-8.6; mean hydraulic residence time, 50-180 days; surface organic loading, 0-340 kg COD/ha/day. Coliform removal is high in the epilimnion where high pH values occur due to algal activity, and low in the hypolimnion where pH values are much lower. When the reservoir is operated as a flow reactor, coliform removal is determined mainly by the percentage of fresh effluents within the reservoir (PFE) and not by the mean residence time of the effluents (MRT). The coliform removal percentage is one to two orders of magnitude when the reservoir is operated as a flow reactor, but it can reach more than five orders of magnitude when operated as a batch reactor. A proper combination of flow operation during winter and batch operation during summer can assure a coliform removal of at least five orders of magnitude during the whole irrigation season. Coliform removal can also be improved by vertical mixing of the water column, the use of a horizontal diffuser device in the outlet, and the design of elongated reservoirs.