Introduction
Over the last 20 years, Biofloc Technology Culture System (BFT system) has been developed with the aim of cultivating shrimp with reduced or zero water exchange. Nitrification processes are very well understood. However, the accumulation of certain substances still poses a problem for the BFT system. Nitrate, the final product of the nitrification process, accumulates in the culture water, potentially reaching lethal or sublethal concentrations for farmed shrimp. In addition, phosphate is another substance that can accumulate in the culture tanks. This comes from feeds that contain different forms of phosphate in their formulation. Despite the low toxicity of phosphorus, high concentrations can promote the growth of cyanobacteria. Therefore, experiments have been carried out to (1) reuse culture water, (2) apply denitrification processes to lower nitrate concentrations and release N2 into the atmosphere, (3) perform phosphate adsorption to extract it from the system and also, (4) use aquatic plants to reduce both nitrate and phosphate concentrations. This paper will present the main results obtained by researchers at the Federal University of Rio Grande (FURG), as a way to mitigate the problems of nitrate and phosphate accumulation in Penaeus vannamei crops in BFT systems.
Material and methods
Different experiments were carried out as part of master's dissertations and doctoral theses in the postgraduate program at FURG. In the first phase, tests were conducted to analyze which larger fractions of mature water could be used in a new culture. Subsequently, the minimum percentages (biofloc inoculum) of mature water to accelerate nitrification processes were verified. In the second phase, experiments were carried out to define the best conditions for carrying out denitrification processes in the culture effluents. Subsequently, tests were carried out to perform the denitrification processes simultaneously with the cultures. In the third phase, tests were and are being carried out for the adsorption of phosphate to clays and the reduction of the concentrations of this substance in the culture effluents. In parallel, experiments have been carried out with macroalgae to comparatively analyze the efficiency in removing nitrate and phosphate from shrimp cultures in a BFT system.
Results and discussion
The results obtained in experiments with different water reuse fractions confirmed that it is possible to reuse large quantities, from 75 to 100%, of mature water from a previous cultivation if the nitrate and phosphate concentrations do not exceed safety levels. On the other hand, the minimum concentrations of total solids (TSS) determined for an inoculum to accelerate the nitrification processes of a new cultivation were estimated at 5 mg/L of TSS.
In experiments conducted to define the best conditions for carrying out denitrification processes in cultivation effluents and reuse of this mature water, it was determined that anaerobic denitrification should use organic fertilizers with a C:N ratio of 2 to 4 carbons for each nitrogen in the form of nitrate present in the medium. In tests carried out to analyze denitrification simultaneously with cultivation, it was determined that it is possible to carry out denitrification parallel to cultivation with experimental units equivalent to 25% of the cultivation units. Tests are being conducted by decreasing the size of these units and increasing the frequency of denitrification. In ongoing work to control phosphates, the necessary clay concentrations of 3.5-7.0 g/L were verified over periods of 5 to 10 days. Tests to analyze the comparative efficiency of nitrate and phosphate removal by macroalgae in a BFT system demonstrated the great capacity of these organisms to remove these substances from the water used in shrimp farming in a BFT system. The results confirm that the different methods tested are efficient in improving the water quality of the BFT system, making the use of mature water feasible in shrimp farming under a BFT system. Further work should be carried out to define the best strategies and logistics for reusing mature water over several farming cycles.
Acknowledgments
This work was supported by the National Council for Scientific and Technological Development (CNPq - processes number: CNPq 403469/2023-6 and 307741/2022-2), the Research Support Foundation of Rio Grande do Sul State (FAPERGS - processes number: 24/2551-0002133-7 and 25/2551-0002791-8).
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