The popular dietary shift to fish and the limits on natural fish populations have caused rapid growth in fish farming and aquaculture. Optimized aquaculture systems are attempting to use as complete recycle as possible to obtain maximum control over the fish populations, and limit water and energy requirements. Closed systems aquaculture is severely limited by water quality since pollutants accumulate in the system. This study examined the possibility of simultaneous BOD5 reduction, SS removal, nitrification and denitrification with an expanded bed in complete recycle growth systems with Atlantic salmon and brook trout. The fish rearing tank volume was 10 m3 and the expanded bed volume was 2.6 m3. The total duration of the study was 180 days.

The expanded bed developed a film capable of nitrification and denitrification within 40 days. After this time, the SS and BOD were maintained at less than 6 mg/l. Ammonia, nitrates, and nitrites were below detectable levels, thus confirming the simultaneous one-stage removal potentials. Maximum nitrogen removal rates of 40 mg per liter of AFEB per day are similar to the higher removal rates predicted for maximum removal efficiencies at bulk solution ammonia-nitrogen concentrations of 0.5 mg/l. Dissolved oxygen concentrations in the expanded bed appear to be the most sensitive control parameter for these simultaneous reactions.

This one-stage pollution control process for complete recycle aquaculture would be useful in both freshwater and seawater applications. The system is easily modularized for fish rearing tank capacities of 1,000 to more than 10,000 kg. The treatment system volume would be less than half the rearing volumes at low fish densities and low ammonia concentrations (less than 0.5 mg/l of ammonia nitrogen).

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