27 POC Filter

The POC filter is an active biofilter that removes nitrogen (nitrate) and phosphorus (phosphate) by assimilation. It is often sold under the name pellet filter. It is characterised by the use of particulate organic carbon (POC). It can also be used for denitrification (nitrate reduction) under certain operating conditions.

The organic material can be operated either as a solid bed filter (e.g. "deniballs") or in a fluidised bed process (e.g. material in pellet or spherical form).

1. assimilation:
Bacteria grow on the material and get their energy by biodegrading the carbon source (POC). Nitrate and phosphate are also taken from the water to build up the bacterial biomass. For maximum performance, the water flow rate should be high.

2. Heterotrophic denitrification:
Bacteria also grow on the organic growth bodies and cover their energy by decomposing the carbon source. Under oxygen deficiency conditions, the bacteria begin to breathe the nitrate present in the water = denitrification. Only a small amount of water may be fed into the filter (bypass system) in order to establish the low-oxygen environment in the filter. If very small biobodies are used (pellet or spherical form), the process should be operated in a reactor in which an internal circuit ensures that the biobodies are always in motion. It makes sense to control the water milieu by means of a redox measurement.