Biological removal of ammonium ion
When using a biological filter, bacteria are used to oxidize ammonium to nitrite and nitrate, and perhaps further to molecular nitrogen. The bacteria are grown in a biofilm. Three processes are included in the biological removal of ammonium:
· Transfer of NH4+ (ammonium ion) to NO2− (nitrite)
· Transfer of NO−2 (nitrite) to NO−3 (nitrate)
· Transfer of NO−3 to N2 (molecular nitrogen)
The two first processes are carried out simultaneously and are known as nitrification; the process is performed in a nitrification filter. The third process is denitrification and is performed in a denitrification filter. The two first are aerobic, so air must be added. The last process is anaerobic so air must be removed from the water. Two different filters involving different bacteria are therefore used. In most cases only the nitrification process will be needed for aquaculture purposes, because fish have a higher tolerance for nitrate than for ammonia. Very high degrees of water re-use and high fish den-sities might require a denitrification filter, but knowledge of denitrifiaction filter function and optimization for fish farming is scant.
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