Management of biological filters
A biofilter will always need a certain start-up time before it becomes functional (Fig. 9.6) because it takes some times to establish the culture of nitrifying bacteria on the filter medium (the biofilm). Of course there must be nitrificants in the water and the environment must be suitable, for instance with a water temperature above 7–8°C, dissolved oxygen content above 4 mg/l and biological oxygen demand at 7°C (BOD7) values below 10 mg/l.
If a biofilter is put into service before the bacterial culture has been established, there will be no or low efficiency. Normally, the start-up time for the filter is between 20 and 40 days, depending upon, among other factors, temperature and pH.28,35 The start-up time can be reduced by inoculation or placing old bacterial cultures in the filter. It is normal to only clean a part of the biofilter each time so that there will always be some biofilm left on a parts of the filter. Old filter surfaces will always have some film left, so the start-up time for a pre-viously used filter surface will be reduced compared to completely new ones.
In the start-up period, some nitrite may be sup-plied to the fish with the water that has passed through the biofilter because this is the end product of the first process that takes place in the filter. This is therefore a critical period, because small concen-trations of nitrite can be toxic to the fish and care must therefore be taken at this stage. The best course of action is to have as few fish as possible in the start-up period or to inoculate the filter with bacteria.