Strategies for Optimizing Anaerobic Denitrification During the Cultivation of Penaeus vannamei in Biofloc Systems
Author: Maria de Fátima Gomes Silva (Currículo Lattes)
Advisor: Dr. Wilson Wasielesky
Abstract
Anaerobic denitrification consists of the microbial conversion of nitrate and nitrite into molecular nitrogen or nitrous oxide, promoting the removal of these compounds from aquatic environments. In aquaculture, this process improves water quality, prevents eutrophication, and reduces reliance on chemical inputs, thus enhancing sustainability. This study aimed to test different strategies to optimize anaerobic denitrification in biofloc systems during the cultivation of Penaeus vannamei, evaluating different denitrification volumes, suspended solids concentrations, and pH control methodologies. The study is divided into two chapters. In the first chapter, the efficiency of nitrate removal was evaluated based on different volumes of water allocated for denitrification during P. vannamei cultivation in biofloc systems and the influence of varying total suspended solids concentrations on the anaerobic denitrification process. The first experiment lasted 55 days and was conducted in 500 L tanks containing P. vannamei juveniles (400 shrimp/m³, average weight 0.94 ± 0.44 g). Each experimental tank included a reactor with a volume equivalent to each treatment (5%, 25%, and 50%). Denitrification was induced by adding regular sugar whenever nitrate concentrations exceeded 75 mg/L. The pH was maintained between 8 and 9 through NaOH addition, and shrimp were fed twice daily with balanced feed and underwent weekly biometrics. In the second experiment, lasting 4 days, the influence of suspended solids concentration on denitrification efficiency and rate was analyzed. For this, 60 L tanks containing 25 L of water were subjected to six treatments with different suspended solids concentrations (500, 550, 1000, 1500, 2000, and 2500 mg/L), using water from P. vannamei cultivation in biofloc systems.
The second chapter, consisting of two experiments, aimed to determine the optimal NaOH dosage to maintain pH above 8 in the initial stage of anaerobic denitrification and to evaluate different strategies for its addition. In the first 4-day experiment, the total NaOH required to maintain pH stability between 8 and 9 was quantified. In the second experiment, also lasting 4 days, six NaOH addition strategies were compared. The control (TC) involved the addition of 50 mL of sodium hydroxide solution whenever pH dropped below 8, until stabilization. Experimental treatments (TD0, TD, TD2, TD3, and TGOT) tested different NaOH dosing strategies. In TD0, the total pre-determined volume of NaOH was added only when dissolved oxygen reached approximately 0 mg/L. In TD, the full volume was added immediately after the addition of organic carbon, before oxygen depletion. In TD2, the volume was divided into two doses: one at the beginning and another when dissolved oxygen approached 0 mg/L. In TD3, three doses were applied: at the start, when dissolved oxygen dropped near 0 mg/L with a sharp pH decrease, and six hours after the second dose to compensate for further pH decline. In TGOT, NaOH was continuously dripped at 0.53 mL/min during the first 12 hours of the experiment. In both chapters, pH, alkalinity, temperature, salinity, dissolved oxygen, total ammonia nitrogen, nitrite, nitrate, total and settleable suspended solids were monitored daily. In the reactors, pH was measured hourly until stabilization, and other parameters were analyzed three times daily.
Results from the first chapter showed that the 50% volume treatment was the most effective in reducing nitrate concentrations and yielded the highest survival rate (85.2%). In the second experiment, suspended solids concentrations above 550 mg/L significantly reduced the nitrate removal time from 45 to 24 hours. In the second chapter, the ideal NaOH dosage was determined to be 14 mL/L to maintain pH between 8 and 9. All treatments resulted in final nitrate and nitrite concentrations close to 0 mg/L, confirming the efficiency of the denitrification process. However, significant differences were observed in dissolved oxygen, alkalinity, phosphate, and total suspended solids levels. The pH decreased in the initial hours due to hydroxide release during microbial reactions, stabilizing thereafter. These results confirm the relevance of biological denitrification for water quality maintenance and show that the implementation of optimized techniques can contribute to sustainability and healthy growth of P. vannamei in intensive aquaculture systems.