Biofloc technology system is used for the purpose of preserving water quality in intensive aquaculture systems. In this system, the production of heterotrophic bacteria helps in the formation of bioflocs. The nitrifying bacteria transform ammonia into nitrite and nitrite into nitrate. Nitrate tends to accumulate reaching high concentrations (>100 mg/L), which can cause a decrease in growth and even cause mortalities. To solve this problem, several strategies are applied that involve biological processes such as the denitrification process. Therefore, the present work aimed to evaluate the effectiveness of denitrification in reducing nitrate levels concomitantly with the culture of Penaeus vannamei in a biofloc system in 8 weeks.
The experiment consisted of 1 control treatment, where the no denitrification process was carried out, and 3 experimental treatments which denitrifications were carried out in bioreactors of different volumes relative to the volumes of the culture tanks (5%, 25% and 50%). Each treatment was done with 3 replications. P. vannamei juveniles were stocked at a density of 400 shrimp/m3 in 500-liter tanks. Water quality parameters were evaluated daily, such as pH, alkalinity, temperature, salinity, dissolved oxygen, total ammonia nitrogen, nitrite, nitrate, total suspended solids, and settleable solids. Biometrics were performed weekly to estimate shrimp growth. The shrimp were fed twice a day with extruded commercial feed. To carry out the denitrification process, water from each experimental units was pumped to bioreactors without aeration, where organic carbon (sugar) was added in a carbon/nitrogen ratio of 3/1 to stimulate the process. Calcium hydroxide was used to maintain pH levels above 7.4. Denitrification was considered complete when nitrite and nitrate concentrations achieved around 0 mg/L, in denitrification tanks. At the end of the experiment, water quality and zootechnical parameters were submitted to statistical analysis (ANOVA).
Significant differences (p<0.05) were observed among the water quality parameters and zootechnical performance in the different treatments. It was confirmed that is possible to denitrify nitrate during trial and keep nitrate in target concentrations.