The inflow rates of the secondary effluent in the four cells were accordingly fixed at 300 L d−1, 150Ld−1, 100Ld−1 and 75 L d−1 respectively, for 1, 2, 3 and 4 days HRT. The applied hydraulic loads ranged between aminimum of 59.05mmd−1 in the cell with 4 days HRT and a maximum of 236.22mmd−1 in the cell with 1 day HRT