Water recovery and changes in water quality in land application of domestic wastewater

Abstract

Secondary effluent of domestic wastewater was irrigated using sprinkler at sloping area in forest land, and changes in water quality during a penetration through soil layers in aerobic conditions or estimation on removed portion for each species was determined. The chemical composition of descending effluent was found to vary with modes of spraying. The concentrations of most cations, PO₄^3- and reducing substances (determined as COD) were decreasing, while those of Ca²⁺, SiO_2< and NO₃^- were increasing. An unfavorable influence of land application of secondary effluent on groundwater quality should be monitored by measuring continuously the concentrations of the species of the latter group in draining water. The removed portion for any species was defined as : (Total amount of the species contained in effluent-Total amount of the species remaining in drained water) /Total amount of the species contained in effluent. The calculation showed that the average removed portion (%) for some cations ranges from 94.3% (highest) for K⁺ to 56.7% (lowest) for Na⁺. The average values for Cl^- and SO₄^2- were 46.9% and 35.4%, respectively, demonstrating that the removal of anions during the penetration, therefore, is not effective. The removed portion as defined is a function of water recovery, or the volume ratio of drained water to effluent. The plot of removed portion against water recovery gave a linear relationship, and the slope of the linear equation is related to the retention of the species studied by soils. The retention was found to decrease in the order of K⁺, Fe³⁺, Mg²⁺ and Na⁺.