EFFECT OF LONG-TERM EFFLUENT RECHARGE ON PHOSPHATE SORPTION BY SOILS IN A WASTEWATER RECLAMATION PLANT

摘要

The Soreq recharge basins, used for wastewater reclamation employing the Soil-Aquifer Treatment (SAT) system, have been recharged, on average, by about 1,800 m depth of secondary effluent during their operation period of ～25 years. An estimated amount of ～6 kg P m~(-2) was added to the soil/sediment column during this period. The objective of this study was to compare phosphorous sorption characteristics of representative pristine soils in the Soreq recharge site to those of the basin soils sampled after a long period of effluent recharge. Batch isotherm experiments were conducted: samples of one g of soil were equilibrated with 25 mL of 0.02 M NaCl solution containing 0-3.2 mM of phosphate for 7 days at 25 ± 1℃ and P sorption was measured. Long-term effluent recharge significantly decreased the maximum P sorption capacity of the top sandy soil (0.15-0.3 m) and only very slightly decreased maximum P isotherm capacity of the deep clayey-sand soil (10-10.5 m). The retention of P in the basin sandy soil primarily involved sorption and surface precipitation reactions on soil carbonates. In the basin clayey-sand soil, P was retained by its sorption on surfaces of Fe, Al, Mn oxide/hydroxides and clay minerals. Long-term effluent recharge increased EPC_0, (the equilibrium P concentration in solution at which there is no sorption or desorption to or from the soil under the given conditions), of the basin soils compared to the pristine soils. Due to loading of the top horizons with P by prolonged recharge and reduced P concentration in the effluent, EPC_0 of the basin sandy soil is now equal to the average P concentration of the recharged effluents. If effluent P concentration will decrease further, the top sandy soil will become a source of P to the reclaimed water, rather than a sink. The clayey-sand layers and lenses in the vadose zone of the SAT system of the Soreq site offer a large capacity for P adsorption. With gradual leaching of carbonate minerals and synthesis of secondary clay minerals, driven by long-term effluent recharge, P retention mechanisms in the basin soil may be changed, but this process would be extremely slow.