Intensive fish culture at high ammonium and low pH

Abstract

Fish excrete two principal toxic metabolites to the water: NH 3 and CO 2, the former being typically toxic to fish at low (< 0.1 mg N l − 1 ) concentrations. However, allowing the accumulation of metabolic CO 2(aq) results in pH reduction, thereby reducing the fraction of NH 3 from TAN. Such operation strategy may allow increasing the design criteria for TAN, which can result in reduced water flow requirements in flow-through systems. In this study, growth parameters of sea bream Sparus aurata grown in high TAN and low pH values, were monitored. TAN toxicity was first tested in 27-l aquariums, where sea bream fingerlings were grown in TAN values of up to 20 mg N l − 1 and pH 6.8, without showing any significant negative effects. Following that, two 100 m 3 marine fish-culture tanks were stocked with 84 g fish and supplied with the same daily feed for 250 days. Liquid oxygen enrichment was affected and paddlewheel aerators were used for CO 2 stripping. Seawater was supplied to the high TAN experimental system at an average rate of 5.25 m 3 (kg feed) − 1 and to a control tank at an average rate of 22.9 m 3 (kg feed) −1 (normal flow-through practice). The experimental system included a solids filter, but not a nitrification unit. TAN concentrations measured in the experimental system were much higher than those in the control system (5.44 ± 1.2 mg N l − 1 and 1.34 ± 0.6 mg N l − 1 on average, respectively), however fish growth and fish mortality rates in both systems were statistically identical. The inorganic carbon mass balance differed significantly between the two systems emphasizing the important role of the CO 2 stripping device. The choice of stripping device allows controlling the CO 2(aq) concentration, which in turn controls the pH value for a given alkalinity value. Such control over the CO 2(aq) concentration enables operating the system at relatively high TAN concentrations while maintaining NH 3(aq) below the threshold concentration. An aquatic-chemistry model was developed to predict pH value, and consequently CO 2(aq) and NH 3(aq) concentrations, assuming steady state conditions. Model results were used to determine the minimal makeup water flow-rate that would allow safe operation with regard to the threshold metabolite concentrations. Model results indicated that under the conditions tested, a flow-through system could be operated safely with a ratio as low as 4.4 m 3 seawater (kg feed) − 1 with no need for a nitrification biofilter. The implications of growing fish at high TAN concentrations are extensive, the most important being a significant reduction in water treatment costs.