PERFORMANCE OF PEAT FILTERS IN THE TREATMENT OF DOMESTIC WASTEWATER IN MINNESOTA

Abstract

Approximately 500,000 Minnesota residences rely on the use of onsite wastewater treatment systems, and50-70% of these are not in compliance with state codes or are failing hydraulically due to restrictive site andsoil conditions. Multi-interest research sites were established in northern and southern Minnesota in the fall of1995 to design, construct, and monitor the performance of advanced pretreatment systems. The pretreatmentsystems were designed to treat septic tank effluent from a single family home, ~950 liters/day (250 gal/day) tomeet secondary treatment levels of 25 mg/L total suspended solids (TSS), 30 mg/L biochemical oxygendemand (BOD5), and 200 cfu/100mL fecal coliform bacteria. Replicate in-ground (lined excavation)intermittent peat filters with gravity distribution, experienced hydraulic failure after 15 months at the northernsite, but were later modified with pressure distribution, and have operated successfully since. Removalefficiencies are 98% TSS, >99% BOD5, >99.99% fecal coliform bacteria, >42% TP, and >17% TN.Similarly constructed in-ground intermittent peat filters at the southern site also experienced hydraulic failure.However, a peat filter with pressure distribution is still in operation and functioning (since 1996) in a partialanaerobic condition. During the summer and winter of 1998, the in-ground intermittent peat filters at thenorthern site were spiked with Salmonella choleraesuis, and had an overall 9 log removal efficiency.Modular recirculating peat filters (Puraflo7) were installed at the northern site in the summer of 1998 tocompare a proprietary Irish peat with a Bord Na Mna specified proprietary Minnesota peat. Removalefficiencies for both have been >92% TSS, >96% BOD5, >99% fecal coliform bacteria, 3-20% TP, and 29-41%TN. Both the in-ground and the modular peat filters are performing well and consistently exceedingsecondary levels of treatment.