Lamella dissolved air flotation treatment of fish farming effluents as a part of an integrated farming and effluent treatment concept

摘要

Nutrient emissions from fish farming can be reduced by a bag pen, i.e., a floating circular basin which serves simultaneously both as a fish cultivation tank and a swirl separation tank. Solid matter (excreta and uneaten feed) is collected at the bottom of the bag pen and pumped as an underflow to a dissolved air flotation (DAF) unit for nutrient removal. DAF equipped with lamella elements was studied in real conditions. Altogether 3000 rainbow trout females (2.0 kg each) were cultivated. Solid-water mixture was pumped from the bottom of the bag pen to an equalizing basin using a sequence of 2-min pumping followed by a 4-min pause. In some tests the influent was pumped directly and continuously from the bag pen to DAF. The influent quality changed substantially: average suspended solids (SS) and phosphorus (P) concentrations were 290mgl~(-1) ± 110mgl~(-1) and 3.2 mg l~(-1) ± 1.2mgl~(-1), respectively. When the influent was fresh and P strongly associated with SS, DAF without precipitation chemicals produced up to 86% SS and 83% P removals. The influence of chemical doses was studied using 6.4-29.2 mg Fe l~(-1) with hydraulic loadings (HLs) of 11.0-11.7 m h~(-1). SS and P removal did not change substantially and the effluent concentration levelled at 30mgSSl~(-1) and 0.20-0.30 mg Pl~(-1), respectively. The lamella DAF, coupled with ferric precipitation, produced up to 90% P and 80% nitrogen reductions. HLs, excluding recycle water flow and lamella projection, up to 21 m h~(-1) could be used.