Hydraulic and purification behaviors and their interactions during wastewater treatment in soil infiltration systems

Four three-dimensional lysimeters were established in a pilot laboratory wi th the same medium sand and either an aggregate-laden (AL) or aggregate-fre e (AF) infiltration surface and a 60- or 90-cm soil vadose zone depth to gr ound water. During 48 weeks of operation, each lysimeter was dosed 4 rimes daily with septic tank effluent (STE) at 5 cm/d (AL) or 8.4 cm/d (AF). Week ly monitoring was done to characterize the STE, percolate flow and composit ion, and water content distributions within the lysimeters. Bromide tracer tests were completed at weeks 0, 8, and 45 and during the latter two times, ice nucleating active (INA) bacteria and MS-2 and PRD-1 bacteriophages wer e used as bacterial and viral surrogates. After 48 weeks, soil cores were c ollected and analyzed for chemical and microbial properties. The observatio ns made during this study revealed a dynamic, interactive behavior for hydr aulic and purification processes that were similar for all four lysimeters. Media utilization and bromide retention times increased during the first t wo months of operation with the median bromide breakthrough exceeding one d ay at start-up and increasing to two days or more. Purification processes w ere gradually established over four months or longer, after which there wer e high removal efficiencies (>90%) for organic constituents, microorganisms , and virus, but only limited removal of nutrients. Soil core analyses reve aled high biogeochemical activity within the infiltrative zone from 0 to 15 cm depth. All four lysimeters exhibited comparable behavior and there were no significant differences in performance attributable to infiltrative sur face character or soil depth. It is speculated that the comparable performa nce is due to a similar and sufficient degree of soil clogging genesis coup led with bioprocesses that effectively purified the wastewater effluent giv en the adequate retention times and high volumetric utilization's of the sa nd media. (C) 2001 Elsevier Science Ltd. All rights reserved.