Long Path, Plug Flow, Submerged Biological Filter
Operators of recirculating aquaculture systems are blessed with a wide variety of choices when it comes to biological filters. Most of the filters used are either well mixed reactors with medium to long retention times or short path plug flow reactors with relatively short retention times. These systems are sufficiently efficient and effective when used for raising crops like tilapia that are tolerant of poor water quality. The problems start when farmers try to raise crops that are not tolerant of poor water quality. The biofilters commonly used today are very inefficient when very low levels of ammonia are required.
The tough part about maintaining the water quality necessary to raise delicate species is doing it while feeding the fish at the levels necessary to produce a crop in an economic time frame. It is not that difficult to maintain water quality while feeding at .5% of body weight in a system stocked at .1 lb of fish/gal of water. It is entirely different to maintain water quality while feeding at 3% of body weight in a system stocked at .75 lb/gal. It is the difference between a hobby aquarium and a profitable recirculating aquaculture system.
For the foreseeable future, the most intelligent solution for biofiltration and suspended solids control in recirculating systems will be to employ several different types of water quality control devices simultaneously. Each device should have one primary function. This has numerous advantages.
- Each individual device can be simpler.
- Simpler devices tend to be more reliable.
- In the event of a malfunction or failure of a single device, the overall effect on the system will not be as great. In some cases, other components of the system can compensate for the loss of one.
- Downtime for maintenance of one component in a multicomponent system has less of an effect than losing the 'do it all” type biofilter/solids filter.
- If a multi component strategy is adopted, it is easier to add more components as the need arises.
- Each type of biofilter or solids control device has its own characteristic advantages and weaknesses. By using several different devices simultaneously these devices can compliment each other and do a better overall job of maintaining water quality.
- Using numerous devices increases the flexibility of the overall system and enables the farmer to change crops as the market dictates the need.