DEVELOPMENT OF A BIOFLOC SYSTEM FOR THE PRODUCTION OF TILAPIA
James E. Rakocy*, Jason J. Danaher, Donald S. Bailey and R. Charlie Shultz
University of the Virgin Islands
Agricultural Experiment Station
Abstract
A 200-m3 circular tank was stocked with sex-reversed Nile tilapia (Oreochromis niloticus) and evaluated in four production trials. Water treatment methods consisted of aeration, mixing, solids removal, nitrification and denitrification. The fish were fed ad libitum twice a day with a complete (32% protein), floating pellet. Production results are shown in Table 1. Ammonia and nitrite concentrations were generally acceptable for tilapia growth. During the four trials, there were two non-toxic TAN spikes (~8 mg/L) and three nitrite-nitrogen spikes (14-18 mg/L) that were prevented from being lethal by adding chloride ions (~300 mg/L) at the outset of the trials. The nitrate-nitrogen concentration increased throughout the first two trials and reached 654 and 707 mg/L in Trials 1 and 2, respectively, which indicated a high rate of nitrification in the water column and the need for a denitrification treatment process. Two external denitrification channels (15.2 m x 1.2 m x 0.6 m) were established and used in Trials 3 and 4, resulting in lower peak nitrate-nitrogen concentrations (341 and 364 mg/L, respectively). Total suspended solids (TSS) increased throughout the first two trials and reached peaks values of 1,300 and 1,960 mg/L in Trials 1 and 2, respectively. Horizontal water velocity was too high for effective sedimentation of suspended solids for removal by a 45º cone situated in the center of the tank bottom. The addition of an external clarifier (1.8 m3, 60º slope) to the system for the last 3 weeks of Trial 2 removed 360 kg of dry weight solids, resulting in the reduction of TSS levels from to 1,700 to 600 mg/L. The reduction of TSS improved other water quality parameters and fish feeding response. The use of the central cone was discontinued, and the external clarifier was used throughout Trials 3 and 4, in which TSS reached peak values of 540 and 550 mg/L and averaged 317 and 368 mg/L, respectively. In Trial 4 the entire tank was covered with bird netting in lieu of less effective bird deterrence methods. As a result, survival increased from a high of 86% in Trial 3 to 99.7% in Trial 4. For optimal performance the UVI biofloc system requires an external clarifier, a denitrification unit and complete enclosure with bird netting.
Table 1. Results of four tilapia production trials in a 200-m3 biofloc system.
Trial / Stocking Rate (#/m3) / Initial Size(g) / Final Size
(g) / Culture Period
(d) / Growth Rate
(g/d) / Final
Biomass
(kg/m3) / FCR / Survival
(%) / Power
(kWh/
d)
1 / 20 / 214 / 912 / 175 / 4.0 / 14.4 / 2.2 / 78.9 / 52.8
2 / 25 / 73 / 678 / 201 / 3.0 / 13.7 / 1.9 / 81.0 / 52.8
3 / 25 / 70 / 707 / 182 / 3.5 / 15.3 / 1.8 / 86.0 / 58.9
4 / 25 / 154 / 745 / 183 / 3.2 / 18.6 / 2.0 / 99.7 / 71.7