Discharge management in recirculating aquaculture systems (RAS) plays a crucial role for environmental friendliness, but also for economic viability. Low-cost solutions such as woodchip bioreactors and wetlands have been explored and adopted in RAS. A small-scale RAS accompanied by a passive water treatment field consisting of a woodchip bioreactor (50 m3 ), a vertical flow constructed wetland (45 m2), and a sand infiltration unit (650 m3 ) was constructed at the Laukaa research station (Natural Resources Institute Finland). The RAS included four 5 m3 raceway tanks, drum filters, fixed-bed bioreactors, air diffusers, and low-head oxygenators.
An intensive experiment was carried out where RAS water was pumped to the water treatment field and back to the RAS. Sludge from the RAS was thickened with polyalumini um chloride, and the supernatant was pumped to the water treatment field. Relative water renewal rate from the lake was 190 L kg-1 feed whereas 476 L kg-1 feed was pumped through the water treatment field. Sludge was removed at a rate of 106 L kg-1 feed, indicating a water loss of 84 L kg-1 feed either through evaporation or small leaks. Water quality remained good in the RAS, where mean nitrate, total ammonia nitrogen, and nitrite values were 49, 0.85, and 0.28 mg L-1 respectively. Rainbow trout (Oncorhynchuss mykiss ) were reared in four fish tanks and showed excellent growth and low mortality (Table 1). This experiment showed that RAS effluent can be treated so efficiently that it can be reused without any harmful effects on the fish. With improved sludge treatment, even small amounts of fresh water are required for efficient aquaculture.