Design, operation and performance of stabilization reservoirs for wastewater irrigation in Israel

Abstract

Stabilization reservoirs are used for the seasonal storage and treatment of wastewater for irrigation. These reservoirs are not steady-state flow reactors, but rather accumulative batch ones with a relatively abrupt discharge. A new parameter, the percentage of fresh effluents within the reservoir (PFE) is proposed as a tool to analyze the age distribution of effluents in this kind of reactor. The equations to calculate the mean residence time (MRT) and the surface organic loading are also provided. The relatively small fraction of fresh effluents represented by PFE and not the whole age distribution of the effluents (MRT) determines the removal percentages of COD and BOD in the reservoir and 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 flow reactors MRT can substitute PFE to evaluate the reactor performance because the ratio PFE/MRT is constant in this particular case. A value of 30–40 kg BOD/ha/day is recommended as the maximum allowable surface 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 quality of the effluents is maximal at the beginning of the irrigation season when the reservoir is full of old effluents, but sharply deteriorates when water level drops and new wastewater is pumped into the reservoir. Design and operational alternatives to overcome this problem can assure removals of at least I order of magnitude for chemical components, and of at least 3–4 orders of magnitude for total coliforms. These alternatives are based on a change from seasonal to multi-seasonal storage of wastewater, and/or a change from single reservoir to two or more reservoirs supplying effluents of different quality.