Treatment of septic effluent for fecal coliform and nitrogen in coarse-textured soils : Use of soil-only and sand filter systems

Groundwater effluent sample collectors (zero-tension lysimeters) were installed directly below the drainfields of three residential onsite treatment systems in the Clover/Chambers Creek aquifer region of Pierce County near Tacoma, WA. The use of a split effluent delivery system from the septic tank, where half the effluent was delivered under pressure to a normal native soil-only filter system and half was delivered to a sand filter system, allowed the direct comparison of the two commonly-utilized septic systems for treatment levels. Septic tank effluent (from the septic tank) and percolating water (between 0.3 and 0.9 m beneath the effluent distribution lines) was collected between May 1991 and April 1994 on 30 occasions. Samples were analyzed for fecal coliform bacteria, nitrate, nitrite, ammonium and total reduced (Kjeldahl) nitrogen. Results of this study indicate that the use of sand filters greatly increased removal of fecal coliform bacteria and total nitrogen. Soil-only filter systems had an average of 91% removal of fecal coliforms and 47% of total N; whereas sand filter systems had an average of 99.8% removal of fecal coliforms and 80% of total N.