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.