DEEP WASTE-WATER RESERVOIRS IN ISRAEL - EMPIRICAL-DATA FOR MONITORING AND CONTROL

Selected physico-chemical and biological variables were investigated in twelve reservoirs storing wastewater for irrigation. Organic load on the various reservoirs ranged from 1 to 15 kg BOD dunam-1* day-1; BOD of the reservoirs effluent ranged from 1 to 120 mg l-1; TSS from 6 to 150 mg l-1; chlorophyll a from 3 to 2700 μg l-1: DO from 0.1 to 19 mg l-1 at midday; compensation levels from 0.2 to 3.5 m; and rates of respiration from 0.07 to 5 mg O2 l-1 h-1. Significant correlation coefficients obtained between the above variables prove that, despite their wide heterogeneity, these reservoirs constitute a well defined category of aquatic ecosystems, where a predictable behaviour is induced by increasing concentration of organic matter. The fact that water column transparency appeared significantly correlated with the concentration of BOD, TSS, chlorophyll and dissolved oxygen, predestinates underwater light measurements to serve as a useful parameter for a rapid assessment of reservoir water quality. Under the existing design, operation and local climatic conditions BOD levels of 40-50 mg l-1 appeared to be the upper limit for the safe performance of these reservoirs. In order to support such concentrations the organic load should not exceed 6 kg BOD dunam-1 day-1. Reservoirs operated within the above limits store wastewater without causing nuisances and reduce the entering BOD by some 80%, which seems to be the maximal efficiency of these systems. © 1990.