Recirculating systems for tilapia in tank

Recirculating systems generally recycle 90 to 99 percent of the culture water daily. The rearing tank is aerated as in flow- through systems with low exchange rates. Recirculating systems require a clarifier (settling tank) to remove solid waste (feces and uneaten feed) and a biofilter to remove toxic waste products (ammonia and nitrite) that are produced by the fish. A cylindrical clarifier with a conical bottom (60° slope) and center drain facilitates solids removal, but often rectangular tanks are used and the solids are pumped or siphoned off the bottom. Baffles are used near the inlet to slow the incoming water flow and near the outlet to retain floating sludge. If a few tilapia fingerlings (of one sex to prevent breeding) are placed in the clarifier, their movement will concentrate sludge in the lowest portion of the tank. They should not be fed, as they will obtain adequate nutrition from the sludge and wasted feed. For efficient solids removal, clarifiers have a water retention time of 25 to 30 minutes and a minimal depth of 4 feet. There are many effective biofilter designs, but they all operate on the same principle of providing a large surface area for the attachment of vitrifying bacteria that transform ammonia (NH3), excreted from the gills of fish, into nitrite (NO2), which in turn is converted to nitrate (NO3).

Nitrate is relatively non-toxic to fish, but an accumulation of ammonia and nitrite can cause mortality. Tilapia begin to die at ammonia concentrations around 2 mg/liter (expressed as NH3-N) and nitrite levels of 5 mg/liter (as NO2-N). Gravel biofilters, which once were common, are being replaced by plastic- media biofilters because they are lightweight and easy to clean. Biofilters now consist of self-supporting stacks of honeycombed modules, columns or tanks containing loosely packed rings, or a series of discs on an axle that floats at the water surface and rotates, alternately exposing the media to water and air.

Regardless of design, biofilters generally have the same requirements for efficient vitrification: 1) DO of not less than 2 mg/liter or 3 to 5 mg/liter for maximum efficiency; 2) pH 7 to 8; 3) a source of alkalinity for buffer since vitrification produces acid and destroys about 7 mg of alkalinity for every mg of NH3- N oxidized; 4) moderate levels of organic waste (less than 30 mg/liter measured as biochemical oxygen demand), thereby requiring good clarification; 5) water flow velocities that do not dislodge bacteria. Biofilters can be sized by balancing ammonia production rates with ammonia removal rates. Unfortunately, these rates are highly variable. In a growout study on tilapia in tanks, ammonia production averaged 10 grams/100 pounds of fish/day (range:4 to 21). Ammonia production depends on quality of feed, feeding rate, fish size and water temperature, among other factors.

Ammonia removal rates may range from 0.02 to 0.10 grams/ft2 of biofilter surface area/day depending on type of media, biofilter design, and the factors that affect vitrification. The required biofilter surface area can be obtained by dividing total ammonia production for the maximum standing crop by the ammonia removal rate. The filter volume can be determined by dividing the required biofilter surface area by the specific surface area (ft2/ft3) of the media. For example, assume that a biofilter containing l-inch pall rings is being designed to support 1,000 pounds of tilapia. The ammonia production rate is estimated to be 10 grams/100 pounds of fish/day. Therefore, total ammonia production would be 100 grams/day. The ammonia removal rate is estimated to be 0.05 grams/ft2/day. Dividing total ammonia production by the ammonia removal rate gives 2,000 ft2 as the required biofilter surface area. One-inch pall rings have a specific surface area of 66 ft2/ft3. Dividing the required biofilter surface area by the specific surface area gives 30 ft3 as the biofilter volume needed to remove ammonia.