Particle size distribution of wastes from freshwater fish farms

Waste water treatment on freshwater fish farms is problematic as waste material and water flows can vary greatly on a daily basis, and, in terms of effluent standards, fish farm effluent represents a dilute waste water output. A study was undertaken to investigate in detail the nature of the waste outputs under field conditions. Waste water samples were split by meshes into the following size ranges: >200μm, 200-100μm, 100-60μm, 60-30μm and <30μm. Waste water quality parameters, suspended solids (SS), biochemical oxygen demand (BOD₅) and total phosphorus (TP) were examined for each size range at two freshwater fish farm sites in Scotland, as part of a wider investigation into waste water quality of aquaculture operations. Results indicated that during periods of peak waste output i.e. tank cleaning, approximately 80% of BOD₅ and SS was present in a particle size range of 100-60μm, but only 66% of TP transport occurred in this size range. At other times, low levels (≤40%) of entrapment of wastes by the chosen meshes was observed, suggesting a reversion to predominantly dissolved material transport. Compared against a larger data set of outflow concentrations obtained from another section of the study, maximum removal rates of 46%, 48% and 30% for BOD₅, SS and TP respectively were determined. The implications for waste water treatment at fish farms are discussed.