[APD] Re: dead bio-balls, somewhere between myth & rule of

["Roeland Grommen" <roeland_grommen at UGent.be>]

It seems a lot of myths exist regarding the fate of nitrifying bacteria in
biofilters that are turned off. The greatest problem IMHO exists in wet/dry
filters where interrupting the flow can lead to drying of the biofilm, which
will kill off most bacteria. As long as the biofilms does not dry out,
things are fine.
In submerged filters and cannister filters there could be a lack of oxygen
after some time. Nitrifying bacteria can deal with this. It is true that
they are aerobic: they need oxygen to transform ammonia into nitrate. But
e.g. ammonia-oxidizers also have an anaerobic metabolism, which is very
slow, and which produces nitrogen gas from ammonia and nitrite.
Different strains of nitrifiers also have different affinities for oxygen
gas: the ones with the highest affinities can survive in the deeper parts of
the biofilm where oxygen concentrations are very low. But generally low
oxygen concentrations should not be a problem when doing maintenance to the
aquarium. Periods up to a day or 2 should not give rise to real problems.
Regarding the story that anaerobic bacteria will take over the biofilter.
Some anaerobic bacteria will always be present in the biofilter (even in
wet-dry filters). They have a role to play. The nutrient concentrations in
biofilters are just too low to allow drastic changes in the bacterial
populations (which requires fast growth).
The same stories goes for the absence of ammonia. When starting-up a
biofilter people tend to worry about the nitrifying bacteria to start dying
off as soon as there is no ammonia for them to grow on. It is true that
there can be no growth in the absence of ammonia, but these bacteria can
switch to a starvation metabolism, which helps them to overcome periods of
deprivation.
The take-home message is simple: don't worry! You can switch off the
biofilter without any problems for several hours.

Roeland

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