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Treatment of Secondary Sewage Effluent with a Peat-Sand Filter Bed¹

ABSTRACT

A peat-sand filter bed was constructed to provide additional treatment of the secondary sewage effluent from a campground located on the shore of North Star Lake, within the Chippewa National Forest in north-central Minnesota. This effluent contained an average of 30.9 mg/liter total nitrogen, 8.63 mg/liter total phosphorus, and 122,000 fecal coliforms/100 ml. An average of 6.8 cm/week was applied from late May through early October for 8 years. The peat-sand filter bed accomplished almost complete removal of fecal coliform bacteria and P. About 90% of the waste-water N was removed during the 2nd and 3rd years of operation, but this declined to about 50% by the 5th year, due to oxidation of the peat and release of N. The high Fe, Al, and ash content of the peat contributed to the filter's highly efficient removal of P. Microbial immobilization in the peat contributed to N and P removal during the first 2 or 3 years of waste water application. Rough-stalked bluegrass (Poa trivialis L.) planted on the peat surface of the filter was very important in overall N and P removal. Nutrient uptake by the bluegrass increased each year and accounted for 45% of the P and virtually all of the N removed from the waste water in the 5th year of the study.

Key Words: nitrogen • phosphorus • tertiary treatment • water quality

¹ Contribution of the USDA Forest Service, North Central Forest Exp. Stn., St. Paul, MN 55108.

² Research Soil Scientist, USDA Forest Service, Forestry Sci. Lab., Grand Rapids, MN 55744; and Assistant Director, Pacific Northwest Station, Corvallis, OR 97331; respectively.

Received for publication July 21, 1980.