Effects of Sand Depth on Domestic Wastewater Renovation in Intermittently Aerated Leachfield Mesocosms

The depth of soil below the absorption trench of a septic system is considered an important factor in protection of groundwater. We examined the effects of depth on the ability of intermittently aerated sand-filled leachfield mesocosms to renovate domestic wastewater. Mesocosms (n=3) consisted of lysimeters with a headspace O2 concentration maintained at 0.21 mol∕mol and containing 7.5, 15, or 30 cm of sand that were dosed with septic tank effluent every 6 h for 328 days (12 cm∕d) . Sand depth had no effect on pH, dissolved O2 , PO4 , NH4 , or BOD5 levels in percolate water. Nitrate levels in percolate water were higher for 30 cm than for 7.5 and 15 cm during the first 70 d of the experiment, after which no differences were observed. Time-averaged removal rates of N, P, fecal coliform bacteria, and BOD5 were 22–28, 13–18, 81–92, and 81–99%, respectively, and were unaffected by depth. Wastewater renovation in intermittently aerated leachfield mesocosms appears to take place in a narrow zone (≤7.5 cm) below the infiltrative surface, with the medium below contributing little to renovation.