SLUDGE REMOVAL FROM SALMONID TANK EFFLUENT USING ROTATING MICROSIEVES

Effluent from tanks stocked with adult Atlantic salmon was treated using a UNIK Rotary Filter (URF) with submersed microsieves with screen mesh sizes of 350 to 60 mum. The mean removal efficiency of suspended solids and total phosphorus was 63-68%. Using a standard backwashing system the URF normally produces 10-20 litres of sludge water per m3 treated tank effluent. By running the URF with vacuum suction equipment a reduction to about 0.3 litre of sludge water per m3 tank effluent is possible. By using a two-step mechanical dewatering system which includes microsieving and sedimentation (UNIK), it was possible to achieve a further concentration in the order of 200-5000 times. This system produced daily a sedimented sludge volume of 1-1.5 litre/kg feed supplied (FCR c. 1.2 kg feed/kg fish produced). The dry matter content of the sludge was as high as 50-100 g/litre.