Particle size distribution of wastes from freshwater fish farms

Abstract

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.