Serial particle size fractionation and characterisation of an aquacultural effluent

Abstract

The particulate phase of the effluent from a salmonid hatchery was serially divided into seven size fractions using 200, 100, 85, 65, 47, 25, and 5 μm pore size nylon membranes. The main aim was to determine the nutrient distribution associated with the particles and use this information to determine the screen pore size that would remove those particles containing the greatest nutrient load. Treatment effort could thus be targeted more efficiently on the particles likely to have the greatest environmental impact. It was considered likely that particles of a specific size would contain elevated nutrient concentrations, resulting from their origin in the diet. The total, dissolved and particulate phosphorus and nitrogen, suspended solids, and various particle size parameters, such as mean particle diameter and total particle volume, of each fraction were determined. Ranges in concentrations of the fractions were: total phosphorus, 131.7-67.9 μg/l; total nitrogen, 0.70-0.47 mg/l; suspended solids, 6.9-1.8 mg/l; total particle volume concentration, 26.17-2.15 μm 3/l · 10 9; and particle number concentration, 602-378/l · 10 3. No single separated particle size fraction contained a disproportionate concentration of total phosphorus, total nitrogen, total particle volume concentration, or particle number concentration. The phosphorus content, in both the suspended solids and the volume of the particles, increased significantly with decreased particle size. The concentrations of suspended solids and total particle volume in the small pore size fractions were, however, less than in larger fractions. Total phosphorus and total nitrogen concentrations were therefore unaffected by variations in the particulate phosphorus concentration associated with different size particles. The nutrients' and solids' removal efficiency data for each fraction could be used to estimate the minimum achievable removal efficiencies of a commercial effluent screening device with a specified pore size. Total particle volume concentration was significantly correlated with suspended solids concentration and so could be used in place of the latter term. Total particle volume concentration could be measured to lower concentrations than suspended solids concentration. Particle size analysis, used in conjunction with other techniques, such as fractionation and nutrient analysis, is a powerful tool for the characterisation of aquacultural wastes and for the improvement in wastewater treatment efficiency.