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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