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RECIRCULATING GRAVEL FILTER FOR SECONDARY TREATMENT OF SEPTIC TANK EFFLUENT AND USE OF TIME-DOSED WETLANDS TO REMOVE NITRATE FROM GRAVEL FILTER EFFLUENT

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

Citation:  Paper number  701P0104,  . (doi: 10.13031/2013.15786)
Authors:   M.E. Byers, K.E. Zoeller, J.D. Fletcher
Keywords:   Septic systems, onsite, wastewater, gravel filters, wetlands

The use of recirculating media filters loaded with gravels such as pea gravel is increasing. These recirculating gravel filters (RGF) seem to be relatively robust. However, a paucity of published monitoring data exists for such filters using gravel media. These filters are being applied to single-family dwellings, multi-family clusters and some commercial installations. They seem well suited for clustering treatment. The tested RGF devices loaded with 3/8 pea gravel, achieved secondary treatment and advanced secondary treatment a high percentage of times tested (>90%). Due to the variable nature of the waste stream, perfection in treatment is impossible for onsite as well as centralized. Fourteen RGF systems were monitored. Although these recirculating filters are effective at some nitrogen removal (>50% removal), some locations require even better nitrogen removal to meet a tertiary standard. To accomplish this, a Z-Cell Wetland (anaerobic up-flow wastewater filter with emergent macrophytes) as a tertiary component was studied. Since the RGF largely produced nitrate-N in its effluent, time dosing a z-cell, which is largely anaerobic and fixes its own carbon has the potential to scavenge nitrate and further the denitrification process. A test system was set up at an industrial location. Septic tank effluent at this commercial test site was rich in ammonia (79 +/- 25 ppm). The RGF removed/converted most of the ammonia. The wetland cell further removed nitrate, but not to completion. Temperature affected nitrate removal. As temperature decreased, nitrate removal slowed. Also, the introduction of secondary treated effluent with residual oxygen seemed to affect treatment. Descriptions of filters, medias, and the pilot wetland tertiary filter are described.

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