PERFORMANCE OF PREFABRICATED PACKAGE PLANTS FOR ON-SITE WASTEWATER TREATMENT IN THE VANSJØ- AND HOBØL WATERSHED (MORSA), NORWAY Funksjonskontroll av minirenseanlegg i Vansjø- og Hobølvassdraget (Morsa), Norge

Prefabricated package wastewater treatment plants are increasingly used to treat on-site discharges in Norway. The performance of these plants, and their compliance with national and regional legislation, has been thoroughly evaluated. During 2006–2008 two separate studies investigated 91 on-site wastewater treatment plants, in the Vansjo- and Hobol Watershed. The removal of organic matter was satisfactory with an average effluent concentration of 17.7 mg BOD5/L. The removal of phosphorus did not satisfy discharge requirements with an average effluent concentration of 1.9 mg Tot-P/L. Insufficient phosphorus removal was caused by a variety of factors, such as; suboptimal pH for precipitation, suspended solids loss, insufficient sludge collection, low chemical dosage, equipment malfunction and insufficient maintenance. Many plants experienced unintentional nitrification. Chemical precipitation was effective for many plants even at very low pH values (pH 4 to 5).