Nitrification-dominated bioflocs are a common challenge in aquaculture systems, where nitrate production becomes constant and accumulates in the system. The build-up of nitrate over several cycles can harm farmed animals, causing reduced growth and survival. In order to overcome this production bottleneck and treat the water, strategies such as the biological denitrification process must be adopted to make the water suitable for culture or disposal. This study aimed to evaluate the performance of Penaeus vannamei and the water quality after being subjected to a biological anaerobic denitrification process using water from a biofloc technology (BFT) system. By assessing the effects of the denitrified water on the growth and survival of the shrimp, as well as on water quality parameters, this study provides valuable insights into the potential benefits of using denitrified water in BFT systems, ultimately contributing to the sustainable development of aquaculture.
Material and Methods
A 63-day experiment was carried out at the Marine Station of Aquaculture of the Federal University of Rio Grande. P. vannamei juveniles (1.30 ± 0.48 g) were s tocked in 150-liter tanks at a density of 500 animals/m³. The experiment consisted of comparing denitrified water and natural seawater (never used for shrimp cultivation) under different biofloc formation strategies: with or without mature biofloc inoculum. Thus, the treatments were: DWI – denitrified water with inoculum; DWF – denitrified water without inoculum; SWI – sea water with inoculum; and SWF – seawater without inoculum , with three replicas each. In the treatments with biofloc inoculum, 15 L of water from a mature biofloc system was, added to the tanks (10% of the working volume) . In the treatments without inoculum, the bioflocs formation was stimulated from zero through the manipulation of the C/N ratio. Temperature, salinity, dissolved oxygen, pH, ammonia, nitrite, nitrate, alkalinity, and total suspended solids in the water were monitored. To assess growth and health conditions, 30 animals were randomly collected each week from each unit, weighed individually , and then returned to the tanks. At the end of the experiment, survival, final weight, weekly growth , and productivity were evaluated.
Animals exposed to denitrified water treatment without biofloc inoculum (DWF) died after one week of cultivation . The 100% mortality found in the treatment using denitrified water without biofloc inoculum could have been caused by various factors such as the colonization of opportunistic microorganisms in the environment, leading to diseases and ultimately death of the shrimps; the formation of nitrogenous disinfection by-products, which are known to be toxic to aquatic organisms; some unidentified metabolite or compound produced during the denitrification process, leading to adverse effects on the shrimps. Further investigation is necessary to identify the exact cause of the mortality and prevent future occurrences. Either way, no differences in survival (<89%) were found in the treatments using denitrified seawater with inoculum and natural seawater treatments, which indicates that the biofloc inoculum served as a biological treatment in denitrified seawater, readjusting the water for cultivation . Differences in the water quality parameters were found in ammonia, nitrite, nitrate concentrations , as well as in water exchange volume, organic carbon, and alkalizing agents between treatments. Total ammonia concentrations were highest in the treatment without biofloc inoculum, and nitrite concentrations were higher in the same treatment. Final nitrate concentrations were highest in the denitrified water treatment and lowest in the seawater treatment without inoculum. The treatment with denitrified water had higher pH values, and no significant difference was found in alkalinity concentrations between treatments. The treatments with inoculum had no water ex changes and organic carbon supplementation. The results indicate that it is possible to use denitrified seawater from a biofloc system for shrimp farming. However, alternatives for water treatment after the denitrification process need to be investigated.
The findings suggest that water from a biofloc system can be recycled by undergoing anaerobic biological denitrification. The denitrification process used is straightforward and can be performed in sedimentation basins or the same tanks used for cultivation once the animals are removed. This reduces the requirement for equipment and operational procedures. However, alternatives for further treatment of denitrified water are still necessary to ensure its suitability for cultivating P. vannamei, irrespective of the use of biofloc inoculum.