Performance of Pre-engineered Modular Peat Filters for the Treatment of Domestic Wastewater at the Northeast Regional Correction Center

Abstract

Approximately 500,000 Minnesota residences rely on the use of onsite wastewater treatment systems and >50% of these systems may be in noncompliance with state rules or are failing to the surface. A research site at the Northeast Regional Correction Center near Duluth was established in 1995, involving~ 50 private and public sector partners, to design, construct, and monitor the performance of advanced onsite treatment systems. The systems were generally designed to treat 250 gal/day of septic tank effluent from single family homes to meet secondary treatment standards; 25 mg/L total suspended solids (TSS), 30 mg/L biochemical oxygen demand (BOD5), and 200 cfu/lOOmL fecal coliform bacteria. In 1998, pre-engineered peat filters (Puraflo®, Bord Na M6na, Inc.) were installed to compare the performance of a standard Irish peat to a Bord Na M6na specified Minnesota peat. Initially, the peat filters were operated in the recirculating mode for enhanced nitrogen removal, but were changed to single-pass mode after ponding occurred. Hydraulic failure likely resulted due to cold temperatures (since the modules were not insulated) coupled with high strength waste. With minor design modifications, including foam-insulated covers, the single-pass peat filters have operated without problems. In the recirculating mode, seasonal removal rates for both types of peat ranged 86-97% TSS, 94- 97% BOD5 96-99% fecal coliform bacteria, 6-20% total phosphorus (TP), and 31-45% total nitrogen (TN). As single-pass filters, removal rates were comparable at 89-96% TSS, 95-98% BOD5 >99% fecal coliform bacteria, 94-99% coliphages, 0-20% TP, and 24-37% TN for both peat types. The module Puraflo® peat filter systems, using both the standard Irish peat and a Minnesota peat, performed comparably and generally exceeded secondary treatment standards.