RAS Filtration Systems

RAS Filtration Systems

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The biological filter, or biofilter, is a key component in the filtration portion of a recirculating aquaculture system (RAS). The biofilter houses the nitrifying bacteria and is the primary site where biological nitrification occurs. Nitrifying bacteria process dissolved nitrogenous waste products excreted by the aquatic organisms being cultured.

All cultured organisms, vertebrates or invertebrates, finfish or shellfish, produce waste as a result of the nutrition they receive. Finfish excrete ammonia (NH3), mostly from their gills, and it dissolves in the water in which the fish must live. This waste product is toxic to the fish and is an environmental stressor that causes reduced appetite, reduced growth rate, and death at high concentrations.

Fortunately, naturally occurring bacteria oxidize the ammonia, use it to grow, and convert it to nitrite (NO2-). This is an aerobic process that needs oxygen to occur. The bacteria that convert ammonia to nitrite are known collectively by their genus name Nitrosomonas. Like ammonia, the nitrite produced by the Nitrosomonas bacteria is toxic to aquatic organisms and must be oxidized further to a less toxic form of nitrogen. This is accomplished by another naturally occurring genus of bacteria referred to as Nitrobacter. These bacteria metabolize and oxidize the nitrite (NO2-) produced by Nitrosomonas and convert it to nitrate (NO3). Oxygen is also consumed in the creation of nitrate. Nitrate is the end product of the conversion of ammonia and nitrite and is non-toxic at the levels usually found in a RAS (>200 mg/L).

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