Effects of soil texture, soil depth, and treatment on septic tank effluent renovation

This study compared the effects of soil texture, soil depth, and additional treatment, by constructed wetland (CW) and recirculating sand filter (RSF) systems, on wastewater renovation in a texture group 2 (loam) and a texture group 3 (clay loam) soil. The soils with depths of 15, 30, and 45 cm were placed in the columns with a Gidding's hydraulic soil probe in order to retain the soils structure in situ. The texture group 2 columns were dosed with septic tank eflluent (STE), constructed wetland effluent (eWE), and recirculating sand filter effluent (RSFE) at 685 cm³ per day and the texture group 3 columns were dosed at 670 cm³ per day with the same effluents. These dosing events were split into 2 separate (12 h) events. The leachate from both groups of soil columns were analyzed for fecal coliforms (FC), biochemical oxygen demand (BODs), ammonium (NH₄-N), nitrate (NO₃-N), phosphate (PO₄-P), pH, total dissolved solids (TDS), electrical conductivity (EC), and chloride.

In the texture group 2 soil the reduction in fecal coliform numbers was 88% and 99%, respectively, for the CW and the RSF and, in the texture group 3 soil was 91% and 99%, respectively, for the CW and the RSF. There was a reduction in BODs concentrations of 32% and 69% ,respectively, for the CW and the RSF. There was a 42% and 84% decrease in NH₄ concentrations, respectively, for the CW and the RSF. These reductions show that treatment of STE by CW or RSF can be substituted for soil depth in texture group 2 and 3 soils and will improve drainfield performance. Texture group 2 soils are less effective at STE renovation than texture group 3 soils at all soil depths. This was evidenced by higher FC counts and concentrations of BOD, N, and P in the leachates from these columns.