Production and control of phosphorus
Phosphorus is an environmental pollutant associated with fish hatchery effluents because of its effect on the eutrophication of hatchery receiving waters (see Chapter 18). The obvious solution to reducing phosphorus in hatchery effluents is to reduce the amount of phosphorus in fish diets. Modern, low-phosphorus diets contain a minimum of 1% phosphorus. Green et al. (2002) reported that 31% of dietary phosphorus is retained in the fish, the remainder being released as waste. Thus, the phosphorus production from a fish hatchery can easily be estimated from the fish feeding rate.
Estimates of the total phosphorus associated with the solids fraction of fish wastes range from 30% to 84% (Cripps and Bergheim 2000) to 40% (True et al. 2004). Thus, efficient solids capture can substantially reduce hatchery effluent phosphorus levels. Summerfelt and Vinci (2008) stated that because phosphorus is primarily associated with solids, rapid solids removal from the system is the best way to remove phosphorus from recirculating aquaculture effluents. Removal of dissolved phosphorus from hatchery effluents is not practical, but chemical coagulation is a possibility for future efforts to control hatchery effluent phosphorus levels.
Rishel and Eberling (2006) investigated the use of alum and various synthetic polymers to treat the backwash water from an intensive aquaculture system. Total suspended solids, dissolved P, total P, and TN were reduced by 99%, 92%-99%, 98%, and 87%, respectively. Actual concentrations, after chemical treatment, were 4-20 mg/L TSS, 0.16 mg/L dissolved P, and
0.9-3.0 mg/L TP. The tests were conducted in jars so no recommendation on how the chemicals would be added to and mixed in a clarifier was given.
Primary sedimentation of municipal wastewaters removes only 10%-15% of P. If chemicals are added, effluent concentrations can be reduced to 0.8-1.5 mg P/L. If the effluent is filtered, the concentration can be reduced to 0.2 mg/L (Balmer and Hultman 1988).
Use of coagulants and flocculants for bulk-flow treatment of aquaculture effluents has not been documented in the scientific literature, but these reports indicate that chemical treatment of aquaculture effluents should be further investigated for situations with extreme P discharge limits. Sindilariu (2007) estimated that the chemical cost to treat 1000 gpm (3785 L/min) would be $59/day.