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

The removal of faecal and total coliforms is studied in a reservoir wh ere wastewater is stored during the rainy winter to be used for irriga tion during the dry summer. The operational regime of the reservoir is irregular, changing between non-steady-state flow and batch. The rang es 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 de grees C; pH, 6.9-8.6; mean hydraulic residence time, 50-180 days; surf ace 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 l ow in the hypolimnion where pH values are much lower. When the reservo ir is operated as a flow reactor, coliform removal is determined mainl y by the percentage of fresh effluents within the reservoir (PFE) and not by the mean residence time of the effluents (MRT). The coliform re moval 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 as sure a coliform removal of at least five orders of magnitude during th e whole irrigation season. Coliform removal can also be improved by ve rtical mixing of the water column, the use of a horizontal diffuser de vice in the outlet, and the design of elongated reservoirs.