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 Effects of Applied Wastewater Quality on Soil Treatment of Effluent
Charlotte Dimick
Item Number: NRL2006AC-4
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Is there benefit to employing additional treatment beyond a septic tank before dispersing wastewater to the soil? In order to evaluate the effects of effluent quality and hydraulic loading rate on soil treatment of wastewater, a field study was conducted at the Colorado School of Mines (CSM) at the Mines Park Test Site. Eight in-situ test cells, installed in a sandy loam soil, received septic tank effluent (STE). A portion of the STE was treated additionally by a Membrane Bioreactor (MBR) and delivered to eight separate test cells, and yet another portion of the STE was further treated by a textile filter unit (TFU) and delivered to the remaining eight test cells. Two hydraulic loading rates were studied, 0.5 gpd/ft2 (2 cm/day) and 2 gpd/ft2 (8 cm/day), for each type of effluent during operation from start-up in April 2004 to November 2004 (6.5 months). To better understand the fate and transport of organic carbon, nitrogen, phosphorus and other constituents during wastewater effluent infiltration and percolation through the vadose zone, thirty stainless steel micro porous suction lysimeters were installed to collect soil solution at depths of two, four and eight feet below the infiltrative surface. Soil solution samples were collected on six occasions during an initial period of operation. Although unexpected, soil solution samples at the deeper depths of vadose zone (4 and 8 ft.) tended to have similar or slightly higher DOC concentrations than the 2 ft. samples, possibly due to a flushing downward of naturally occurring organic carbon in the soil. Nitrogen compounds from the already nitrified MBR and TFU effluents were detected earlier at various depths of the vadose zone. By the end of the 6.5 month monitoring period, however, the MBR and TFU test cells yielded reduced concentrations of total nitrogen at the 2 ft. depth, as compared to the cells receiving STE. All effluents (STE, TFU and MBR) demonstrated lower concentrations of total N at the 4 ft. depth versus the 2 ft. depth. Phosphorous removal in the soil was nearly complete in 2 ft. of percolation through the vadose zone during the 6.5 months of operation described in this paper.