What Does starting a Biofilter involve?
Starting a biofilter involves managing and controlling the seeding of nitrifying bacteria cells in a biological filter. A biological filter consists of non-corroding material such as plastic, fiberglass, ceramic or rock that has large amounts of surface area nitrifying bacteria cells can colonise. To make biofilters more compact, material that has a large surface area per unit volume is usually chosen. This unit of measure is usually referred to as the specific surface area (SSA) of the biofilter media. Simply stated, the more surface area available, the more bacteria cells can be grown and the greater the nitrification capacity, which means that higher feed rates can be achieved. A biofilter with a higher SSA will be more compact than one with a lower SSA.
Keep in mind, however, that some biofilter media with a higher SSA can become clogged with bacteria. Thus, there must be a balance between a high SSA and an operationally reliable biofilter media. Nitrifying bacteria cells grow on all surfaces of the biological filter media and, in fact, on all wet surfaces of the system, such as the insides of pipes, tank walls, etc.
Bacteria follow a continual cycle of growing and multiplying, maturing and dying, sloughing off of the media, and being replaced by new cells. A biofilter is started by adding bacteria to the system, which can be done in several ways. Nitrifying bacteria can be introduced with water or bits of biofilter media from an already operating system, with pond sediment or barnyard soil, or with small numbers of “starter” animals. These animals will have to survive the rigours of elevated ammonia and nitrite concentrations while bacteria cells reproduce and colonise the biofilter media.
Whatever method is used to add bacteria to a system, there is a danger of introducing pathogens as well. The choice of a method should be evaluated as part of the overall facility management and biosecurity plan.
One strategy for starting a biofilter, sometimes called the cold start method, involves stocking the cultured species without having an activated biofilter. Operators must be prepared to deal with the resulting rapid increase in ammonia and nitrite concentrations and to reduce them to tolerable levels through water exchanges. Feed rates must be reduced or feeding must be suspended until biofilter activation occurs. The cold start method has the advantage of using the bacteria that entered the system with the first animals introduced, bacteria that must have been well-suited to the conditions from which those animals came.
This passive biofilter activation, however, can be a slow and stressful process for the animals and the system operator. A more desirable method is to develop the nitrifying bacteria within the biofilter before stocking the species to be cultured. The advantages of “seeding” nitrifying bacteria are:
- Reducing stress on the newly introduced stock
- Shortening the growing cycle with higher feed rates from the first day of stocking (which is important to the economics of an RAS)
- Creating better water quality, which improves health, growth rates, and survival.
