A system for the treatment of sludge from land-based fish-farms

Abstract

This paper describes both an experimental and a commercial-scale system for sludge dewatering and stabilisation. In the experimental system, back-wash water from rotating disk microsieves was settled in a conical sedimentation tank. This tank functioned well, commonly removing more than 75−80 % of the solids, at an overflow rate of 1.0−2.7 m·h⁻¹. The hydraulic load was maintained low, so treatment efficiency was significantly positively influenced by inlet concentration and not inflow rate. Lime was added to the settled sludge. More than 99.9 % of the pathogenic viruses and bacteria studied were killed within 7 days at pH 12. In the commercial system, a newly developed combined effluent treatment and sludge processing system, was located in a large Norwegian salmon (Salmo salar) smolt farm. Four drum microsieves were used to separate particles from the primary effluent flow. The back-wash water, amounting to a maximum of 0.3 % of the 30−35 m³·min⁻¹ primary flow, was dewatered using another drum microsieve. Dewatered back-wash water from this sieve was pumped to a sedimentation tank with a top surface area of 3.3 m² and a volume of 5.5 m³. This system produced on average 0.7 L settled sludge containing ca. 10 % dry matter per kg of feed supplied. Sludge tapped from the bottom of the sedimentation tank was stabilised by mixing with lime. This system produced on average 0.7 L settled sludge containing ca. 10 % dry matter per kg of feed supplied. After stabilisation, the stored sludge was diluted with cattle manure and spread on agricultural land. The primary treated effluent was discharged into the receiving marine water body. The running costs of effluent and sludge treatment, including sieving, settling and stabilisation, amounted to US$ 0.056 per smolt produced, or about 5 % of the total production costs. In the recipient, no settled solids were detected on the seabed at the outlet point of the treated effluent.