DESIGN, OPERATION AND PERFORMANCE OF STABILIZATION RESERVOIRS FOR WASTE-WATER IRRIGATION IN ISRAEL

Stabilization reservoirs are used for the seasonal storage and treatme nt of wastewater for irrigation. These reservoirs are not steady-state flow reactors, but rather accumulative batch ones with a relatively a brupt discharge. A new parameter, the percentage of fresh effluents wi thin the reservoir (PFE) is proposed as a tool to analyze the age dist ribution of effluents in this kind of reactor. The equations to calcul ate the mean residence time (MRT) and the surface organic loading are also provided. The relatively small fraction of fresh effluents repres ented by PFE and not the whole age distribution of the effluents (MRT) determines the removal percentages of COD and BOD in the reservoir an d the quality of the effluents released for irrigation. The effect of environmental parameters such as water temperature and solar radiation are overridden by the operational parameters (PFE). In steady-state f low reactors MRT can substitute PFE to evaluate the reactor performanc e because the ratio PFE/MRT is constant in this particular case. A val ue of 30-40 kg BOD/ha/day is recommended as the maximum allowable surf ace organic loading of stabilization reservoirs. The seasonal storage of wastewater in reservoirs reduces BOD, COD, TSS and detergents by 50 -80%, and total coliforms by 90%, in terms of annual means. The qualit y of the effluents is maximal at the beginning of the irrigation seaso n when the reservoir is full of old effluents, but sharply deteriorate s when water level drops and new wastewater is pumped into the reservo ir. Design and operational alternatives to overcome this problem can a ssure removals of at least 1 order of magnitude for chemical component s, and of at least 3-4 orders of magnitude for total coliforms. These alternatives are based on a change from seasonal to multi-seasonal sto rage of wastewater, and/or a change from single reservoir to two or mo re reservoirs supplying effluents of different quality.