The Role of Soil Aquifer Treatment (SAT) for Effective Removal of Organic Matter, Trace Organic Compounds and Microorganisms from Secondary Effluents Pre-Treated by Ozone

Soil aquifer treatment (SAT) is an effective natural and economically feasible tertiary treatment for wastewater reuse. An innovative hybrid process based on biofiltration, ozonation and short SAT (sSAT, with ~22 days retention time) was demonstrated in a 6 m³/hr pilot system to remove emerging trace organic compounds (TrOCs), organic matter and control Mn²⁺ dissolution in reclaimed water. The biofiltration stage was proposed for nitrification of ammonia as well as removal of dissolved and particulate organic matter (DOM and POM), to enable efficient ozonation of secondary effluents. The pilot system was operated in two modes, where samples were periodically taken from all pilot stages to observe changes in product water quality. At first (Mode 1), biofiltered effluents were infiltrated through sSAT (i.e., no ozonation prior infiltration). During this operation, ammonia, nitrite and phosphate were completely removed, and pathogens were highly reduced. In addition, all measured TrOCs were effectively removed after sSAT, besides the persistent TrOCs Carbamazepine (CBZ) and Iodine-organic contrast media Iopamidol (IPDL). In Mode 2, biofiltered and ozonated (1.0–1.2 mg ozone/mg DOC) effluents were infiltrated through sSAT. In the final reclaimed product, values of DOC, UVA and Mn²⁺ were reduced to 0.8 mg/L, 2.2 L/m, and 29–35 µg/L, respectively. Furthermore, ammonia and nitrite were not detected in the product, and good bacterial quality was obtained. Following 56–75 days of operation at Mode 2, all TrOCs were reduced down to <100 ng/L. The delay in the effect of the pretreatment stages on TrOCs removal by sSAT (>56 days instead of ~22 days) could be explained by their displacement retardation in the upper soil layers of the pilot SAT (0–25 cm). In-depth sampling in the observation well after 111 days at Mode 2 showed homogeneity along the overall perforated section of the well (from −14 to −26 m) with 0.7–0.9 mg/L DOC, 2.1–2.2 1/m UVA and <10 ng/L CBZ. This result proved that the ozonated water completely covered the area around the observation well and positively affected the quality of the reclaimed water.