Fish farming wastewaters contain nutrients, phosphorus and nitrogen, which promote eutrophication in the typically shallow farming sites in Finland. Fish farming wastewater treatment is problematic because of large quantities of very dilute wastewater (200 – 600 m3/kg fish produced). In practice wastewater treatment is concentrated on suspended solids removal. Treatment can be done in two steps: concentration of the very dilute wastewater and subsequent treatment of the concentrated wastewater. Dissolved air flotation pilot trials were conducted using two types of concentrated wastewaters: settled solids from a sludge hopper of a cultivation basin and swirl separator concentrate. Two different pilot plants were used and performances compared. Both mechanical treatment and precipitation by ferric salts were applied. Depending on the influent quality, 70 to 90% phosphorus reductions were achieved without chemicals. Chemical precipitation and flotation produced 90% phosphorus reductions and effluent concentrations at the level of 0.05 mgP/l when 13 m3/(m2h) hydraulic loading was used.

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