Septic Tank Treatment of the Effluent from a Small‐Scale Commercial Recycle Aquaculture System

The efficiency of a conventional domestic septic tank for primary treatment of the wastewater effluent from a small‐scale commercial recycle aquaculture system (RAS) was evaluated. The aquaculture facility had five 39.2‐m 3 dual‐drain culture tanks and a total system volume of 249.9 m 3 . The mean standing stock of fish during the study was 4,837 kg (25.6 kg/m 3 ) consisting of 9.6% largemouth bass Micropterus salmoides , 32.1% walleye Sander vitreus , 17.8% hybrid striped bass (white bass Morone chrysops × striped bass M. saxatilis ), and 40.5% rainbow trout Oncorhynchus mykiss . The entire RAS volume was exchanged with makeup water only once every 62 d. Inflow to the septic tank contained 25.3% total nitrogen (TN) and 55.1% total phosphorus (TP) added to the culture system as fish feed. Relative to inflow, outflow from the septic tank reduced biochemical oxygen demand by 5.1%, TN by 59.5%, TP by 35.3%, settleable solids by 92.6%, total dissolved solids by 9.6%, and total suspended solids by 69.8%. Septic tank effluent was discharged to a subsurface drain tile that passed below a row crop field to a pond; the pond did not discharge from the operator's property during the study (i.e., zero discharge). Of the nutrients fed, 9.6% of TN and 34.5% of TP were present in the septic tank effluent. Septic tank sludge was pumped out monthly and dewatered to 14.9% moisture by air‐drying. It contained 3.6% TN and 2.6% TP (percent dry weight) and was test‐marketed as a soil amendment for greenhouse use.