High-Flowrate Water Recirculation System Incorporating a Hydrocyclone Prefilter for Rearing Fish

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Abstract
Particulate filtration was achieved with hydrocyclones and biological nitrification occurred on a substrate of polypropylene biorings (40 mm in diameter) in a 175-L fiberglass tank. Recirculation and makeup water rates were 150 and 5.0 L/min, respectively. In a 1,700-L tank, 261 fish increased in average weight from 92.1 to 330.0 g over a 23-week test period, resulting in a compound weekly weight gain of 5.5%. At the end of the test, total weight of fish in the tank was 84.5 kg for a loading density of 49.7 kg/m³. Cumulative feed conversion ratio was 1.41 g of feed per gram of weight gain. Ammonium-nitrogen and nitrite-nitrogen, temperature, dissolved oxygen, and pH were satisfactorily controlled. Hydrocyclone prefliters proved highly effective in removing particulates, thereby overcoming a major complication of earlier systems by lengthening the biological filter backwash intervals to more than 3 weeks, and requiring only 300 L of water per backwash.