In a biological filter ammonium (NH4+) is trans-formed into nitrate (NO3−). With a high degree of water re-use and large loads on the filter (high fish biomass and rapid growth) the amount of NO3− may exceed the tolerance of the fish, and result in mortality. To inhibit this, it may be necessary to add a new filter with a denitrification step. A denitrification filter transforms nitrate to nitrite (NO2−) and further on to nitrogen gas (N2), which is degassed from the water. Denitrification is for instance used in some eel farms that have a very high degree of water re-use.
Biofilm-hosting bacteria may also carry out den-itrification. This is an anaerobic two-step process. In the first step nitrate is transformed to nitrite and in the second step nitrite to nitrogen gas. The bacteria that carry out this process need a supply of organic carbon for growth, not oxygen. Normally there is not enough easily available organic carbon in the outlet water so carbon must be supplied. Carbon is commonly added as methanol, ethanol or sugar in liquid form before the denitrification filter. When using methanol the following equations apply:
CH3OH + 3NO−3→ 3NO−2+ CO2+ 2H2O
CH3OH + 2NO−2→ N2+ CO2+ H2O + 2OH−
If the created cell mass is described empirically by the formula C6H7O2N, the total equation, including steps 1 and 2, is as follows:
CH3OH + 0.92NO3−+ 0.92H+→ 0.06C6H7O2N + 0.43N2+ 0.7CO2+ 2.25H2O
Since denitrification is an anaerobic process it will not happen in the presence of oxygen. There-fore it is necessary to remove the oxygen from the water to get denitrifaction to start. Methanol and ethanol may also be used as a means of removing the oxygen; after addition of methanol or ethanol there will be no free oxygen left in the water (i.e. the concentration of dissolved oxygen will be close of 0 mg/l).
The design of the denitrification filters is the same as that of nitrification filters, requiring a large area where the bacteria can grow. An established biofilm is also necessary. However, the filter medium must be submerged to avoid ingress of oxygen. Trickling filters and drum filters are not therefore used.