Effects of sand depth on domestic wastewater renovation in intermittently aerated leachfield mesocosms

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 0.21mol mol and containing 7.5, 15, or 30cm of sand that were dosed with septic tank effluent every 6h for 328 days (12cm/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 30cm than for 7.5 and 15cm during the first 70d 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 (≤7.5cm) below the infiltrative surface, with the medium below contributing little to renovation. © 2008 ASCE