29 Carbon Nitrate Filter

The carbon nitrate filter is an active biofilter that reduces the nitrogen concentration (nitrate) in the aquarium water by denitrification (nitrate reduction).

In contrast to autotrophic denitrification with a degradable sulfur source, the carbon-fiter works heterotrophically, i.e. the bacteria growing in the filter supply themselves with carbon from the added organic carbon. The required energy is also taken from the carbon source (organotrophic). The C source is added in dissolved form DOC (dissolved organic carbon) or as particles POC (particular organic carbon) in the form of degradable biobodies.

The nitrate filter must have only a small water inflow (bypass system), so that a low-oxygen environment is formed in the filter. For better reaction, internal circulation is useful.

The carbon nitrate filter can be operated either as a fixed bed, e.g. "Bioballs", or as a fluidized bed with e.g. quartz sand and a liquid C-source, alternatively so-called "pellets" i.e. degradable organic particles are possible. Redox monitoring is necessary. The operation is not very easy, because e.g. the redox electrode can overgrow with bacteria and show wrong results or important systems like pumps can clog.