Biological Filtration

During biological filtration, bacteria are used to convert ammonium to nitrite and nitrate. The process is known as nitrification and involves two steps:

• Transfer of NH4(ammonium ion) to NO2 (nitrite)

• Transfer of NO2 (nitrite) to NO3 (nitrate)

The nitrification process is performed by bacteria which oxidize ammonia. These bacteria are autotrophic and use O2 as oxidizing agent and CO2 or HCO3 as a carbon source for growth. NH4+ is transformed to NO2 (nitrite) by Nitrosomonas bacteria and then to NO3 (nitrate) by Nitrobacter bacteria. Nitrate is typically non-toxic, unless it reaches high concentrations, which may become an issue is zero or very low-exchange recirculating aquaculture systems (RAS).

The bacteria grow in the biofilm on the filter medium. Nitrification takes place in this film so the biofilm must be established for the nitrification filter to function. The process creates more biofilm as the bacteria grow and divide, and the cell mass increases. The effectiveness of a biological (nitrification) filter can be described by the nitrification rate, defined as the amount of ammonium oxidized per unit biofilm surface area and unit time (mg NH4+/(m2 min)).

The efficiency of the nitrification process and the establishment of the biofilm process depend on several factors. It is important that the bacteria grow as optimally as possible.

The following important factors regulate the growth of the bacterial culture:

• Concentration of ammonia

• Temperature

• Oxygen concentration

• pH

• Salinity

• Organic substances

• Toxic substances

Why is this important and why should we have a general understanding of the processes taking place in a biological filter?

Because the biological filter contains living organisms, we must ensure that a suitable environment is provided and maintained, in order to allow the nitrifying bacteria to grow and serve its purpose removing ammonia.

A biological filter requires several weeks time to establish the biofilm of nitrifying bacteria. Once established, suitable conditions must be maintained in order to keep the biological filter “alive”. If conditions in the filter become anaerobic, or the nitrifying bacteria are starved, the bacteria may partially or completely die off and the filter will require another conditioning period to re-establish the bacterial biofilm. Understanding these biofilter requirements and factors that may inhibit or affect the nitrification process is important to ensure the filter becomes established and continues to function properly.

Biological filters come in many shapes and sizes. The most suitable type and required size of biological filter is chosen according to the specific application, expected Total Ammonia Nitrogen (TAN) production rate, and the nitrification rate or efficiency of the biofilter.

Contact Us or visit the Biological Filtration section of our shop for more information.

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