Aquaculture: Biological Filters prevent Poisoning of Fish

inside the filter tank and vigorously stirred by means of aeration from below. Water enters the filter at a convenient point and exits elsewhere, the position of these two points typically being opposite each other to allow for maximum residence time.

Two groups of bacteria arrive and colonise the media; they will arrive on their own in association with the fish but can be added to the system in various ways, but this is not necessary. The first group convert the ammonia, excreted by the fish, to nitrite, and the second group convert the nitrite to nitrate. Nitrate is only mildly toxic to fish and as such can accumulate to much higher concentrations in the recirculating water before a water change or the addition of plants through aquaponics, is required to prevent it from increasing further. In this way the biofilter enables fish farmers using RAS technology to stock their fish at far higher densities and heat the water cost effectively, as water changes are only infrequently required. The biofilter bacteria are aerobic and oxygen levels above 4 mg/l are essential for this conversion to occur fully. In addition, the breakdown of ammonia via nitrite to nitrate utilises alkalinity, and as such the pH of the water needs to be managed to prevent it from falling. A final consideration is that the biofilter bacteria thrive at a temperature around 27°C, which is superb if you farm warm water fish but those farming cold water species require a relatively larger biofilter due to the slower rate of the ammonia conversion.

Biofilters make RAS farming possible. These incredible little critters require minimal attention or care whilst performing an incredibly valuable function that enables us to harness the many benefits associated with farming fish in a RAS.

By: Leslie Ter Morshuizen, AquacultureAfrica

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