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Soil Treatment Unit Performance as Affected by Hydraulic Loading Rate and Applied Effluent Quality

Published by the American Society of Agricultural and Biological Engineers, St. Joseph, Michigan www.asabe.org

Citation:  Eleventh Individual and Small Community Sewage Systems Conference Proceedings, 20-24 October 2007, Warwick, Rhode Island  701P1107cd.(doi:10.13031/2013.24026)
Authors:   Kathryn S Lowe, Sheila M Van Cuyk, Robert L Siegrist
Keywords:   Onsite wastewater systems, treatment units, effluent quality, soil clogging

Treatment units, such as sand filters, textile media filters, and membrane bioreactors, are designed to enable higher or equivalent performance to a septic tank so that soil treatment can be accomplished at higher hydraulic loading rates and/or with less unsaturated soil depth. A field study was conducted to evaluate the performance of an overall treatment train with the soil treatment unit receiving effluent from three treatment units (septic tank [ST], ST with textile filter unit [TFU], and ST with membrane bioreactor [MBR]). The effluents from the three treatment units were applied to 18 in situ test cells at two hydraulic loading rates (2 or 8 cm/d). Each effluent was characterized and the effects of the effluent quality on the hydraulic and purification performance during soil treatment were studied. The treatment units achieved very different treatment efficiencies for organic matter, solids, nutrients, and bacteria (relative efficiency of ST < TFU < MBR). Addition of a treatment unit to produce effluent of higher quality than typical septic tank effluent can retard soil clogging development and enable application of higher hydraulic loading rates to soil, however, the increase in hydraulic loading rate is likely limited by the hydraulic properties of the natural soil. After 32 months of operation, the treatment and purification was >99% removal for total phosphorus and >87% removal of dissolved organic carbon after 60 cm of soil for each effluent quality. Nitrogen removal rates were ~60% in the TFU test cells and ~35% in the ST test cells. The treatment trains including a TFU or MBR, generally performed better with respect to purification and were less affected by HLR than the treatment train based on only a ST and soil treatment.

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