EVALUATION OF RECIRCULATING SAND FILTERS IN MINNESOTA

Approximately 30% of Minnesota's residents rely on onsite technologies for their wastewater treatment. There is a growing need for 'alternative' technologies to aid in treatment for difficult sites and sensitive environmental areas. Recirculating sand filters (RSFs) have been used since the 1970s for small communities with flows >20,000 L per day, but use for small flow application (<5000 L/d) has been growing due to its small land use requirement. A research site was developed in southern Minnesota in 1995 to test alternative technologies, including two RSFs. In addition, in 1998, two RSFs were added to existing residential soil treatment systems that were having problems because of inadequate separation and fill soil conditions. All RSFs in this study used 0.6 meters of coarse sand for treatment, were loaded at approximately 204 L per day per square meter (5 gallons per square foot per day) and a recirculation rate of 5:1. All RSFs have effectively reduced Biochemical Oxygen Demand (BOD5), Total Suspended Solids (TSS), Fecal Coliform (FC) and Nutrients (nitrogen and phosphorus). These systems are able to achieve secondary effluent treatment levels for BOD5 and TSS. The median FC reduction was 90% with a value of 5.7 E4 cfu/100mL, indicating additional soil treatment is necessary to protect health and the environment. The RSFs consistently removed 25% or more total phosphorous (TP) and 40% or more total nitrogen (TN). The RSFs did not show significantly decreased performance during the winter months. Two of the RSFs receiving rather high strength waste were able to reduce a greater percentage of total nitrogen, indicated that the addition of carbon from the high strength waste is a benefit resulting in greater TN removal.