Biofilter Sizing and Aeration System Design - Tech Talk 95
Biofilters consist of surface-providing media for attachment of microorganisms that remove wastes from the water. The media can be commercial products like Bio Barrels, Bio Balls, Bio Strata, Bio-Fill™, Biofilm Carrier Elements and Siporax® among others. Sand, rock, shells and other natural material can be used as biofilter media. The relative amount of usable surface area (square feet surface area/cubic foot of media) and their weight are important.
In aquaculture, biofilters are used to convert ammonia to nitrite, and ultimately nitrate, through an oxidation process called nitrification. The bacteria attach to the media surfaces where they use ammonia and nitrite as energy sources and carbon dioxide as a carbon source. These bacteria are aerobic, requiring oxygen for the conversion process.
The biofilter sizing process can be roughly summarized as follows:
1. Determine the maximum expected ammonia loading rate and the allowable total ammonia-nitrogen (TAN) concentration. Ammonia loading is a function of feed loading, protein content and digestion efficiency. The TAN limit is primarily a function of culture water pH and fish tolerance for un-ionized ammonia (see "Water Quality Guide" Tech Talk). High water replacement rates dilute culture water ammonia while impacting pH and other water quality parameters. Mass balance analysis is essential for determining the biofilter load, if any, for these “partial recirculating” and “flow-through” systems.
2. Select the best kind of biofilter for the application. Many types of biofilters have been used in aquaculture, including rotating biological contactors (RBCs), trickling filters, submerged filters, fluidized beds, bead filters and low-space bioreactors (LSBs). One may be better than another for a given application.
3. Calibrate the biofilter standard nitrification rate to the field water quality conditions. These conditions include hydraulic loading rate, TAN concentration, oxygen availability and temperature. Poor solid waste pre-filtration will reduce biofilter performance and require a larger biofilter.
4. Calculate the biofilter size based on the biofilter surface area, projected field nitrification rate and the maximum expected ammonia load.
Biofilter surface area requirements can range from 3 to 30 square feet per pound of fish depending on the biofilter type and the factors described above. For this reason, we recommend that the AES/B Numbers (pounds of fish supported, see "Biofilter Performance Standards" Tech Talk) as well as the feed and TAN limits listed in this catalog be used for comparison purposes only and not for biofilter sizing.