Performance of a Recirculating Sand Filter Wastewater Treatment System Servicing the Iron Range Resources and Rehabilitation Agency

Abstract

Approximately 470,000 Minnesota residences rely on the use of onsite wastewater treatment systems and 27% of these systems may be in noncompliance with state rules or are failing to the surface. As part of a demonstration project to test new technologies in the region, a recirculating sand filter with shallow dispersal trenches was constructed and monitored at the Iron Range Resources and Rehabilitation Agency (IRRRA) office building near Eveleth, Minnesota in 1998 for a flow of 600 g/d. During the study period, flow averaged 520 g/d in summer and 492 g/d in winter, or 82-87% of design flow. Approximately 750,000 gallons of septic tank effluent (STE) have passed through the system to date, with the following STE quality: 166-178 BODs, 30 mg/L TSS, 10 mg/L TP, 64-80 mg/L TN, and 1,600,000 cfu/l00mL fecal coliform bacteria (pathogen indicator organisms). After only 3 years of operation, the septic tank was nearly full (80-90% ), presumably due to a significant loading of toilet paper, since the wastewater is largely human/toilet waste. The recirculating sand filter (RSF) performed well over the nearly 3 years tested at reducing wastewater contaminants before dispersed back into the environment. Average seasonal removal rates for the RSF were 92% removal of BOD5 (15 mg/L BOD5), 84% removal of TSS (5 mg/L TSS), 82-88% removal of fecal coliform bacteria (50,000-290,000 cfu/l00mL), 13-28% removal of phosphorus (8 mg/L TP), and 20-34% removal of nitrogen (42-64 mg/L TN). Secondary treatment standards for BOD5 and TSS were routinely achieved by the RSF, although pathogen levels were elevated in the effluent. Nitrogen removal (TN) was better in summer than winter, and the effluent was also more highly nitrified during the warmer season. Nitrate levels in RSF effluent, summer and winter, were similar (20-23 mg/L NO3). Temperature of the effluents were 15°C in summer, while winter temperatures averaged 9.5°C for STE and 7.5°C for RSF effluent. During the first 3 years, a single trench (120 fl:2) was used to disperse ~518,000 gallons of RSF effluent, at a loading rate of 4g/fl:2/day, with minimal ponding in the trench. Average daily flow to the soil dispersal trench was 500 g/d, but it varied from 187 g/d to 975 g/d. During the winter 2002-2003, both the forcemain from the pump tank to the soil dispersal system and the pressure line in each trench froze. To overcome this problem, a temporary forcemain was used to route RSF effluent into the old drainfield, which had drained almost entirely after resting for 21 months.