Control of pH and total hardness in a biofloc system, evaluating water quality and performance of Nile Tilapia Oreochromis niloticus (L.)

Author: Gabriel Bernardes Martins (Currículo Lattes)
Supervisor: Dr Ricardo Berteaux Robaldo

Abstract

In a biofloc system (BFT), the high biomass of fish and organic matter, associated with the cycling of inorganic nitrogenates, cause intense consumption of alkalinity and pH reduction. Therefore, frequent additions of alkalizing compound are required. In this context, as a way to improve the BFT system for Nile tilapia Oreochromis niloticus, this study evaluates the use of alkalinizing sources and pH and hardness values, verifying the growth performance and water quality during the nursery phase. All tests were performed at the “Chasqueiro Fish Farming Laboratory”, at Universidade Federal de Pelotas. For the tests, boxes with 37.5 L of useful volume, continuous aeration (porous stone), heater (28 ° C) and three replicates were used for each treatment. The first chapter, carried out for 60 days,demonstrates that the best growth and productivity performance occurs when using sodium bicarbonate (NaHCO3) (44.1 ± 0.9 g and 23.5 ± 0.5 kg / m3), compared to calcium carbonate (CaCO3) (38, 3 ± 1.3 g and 20.8 ± 0.70 kg / m3), while calcium hydroxide (Ca (OH) 2) (40.6 ± 1.7 g and 21.6 ± 0.9 kg / m3) it was similar to the others. The reduced growth for the CaCO3 treatment was attributed to the excessive increase in total suspended solids (TSS) and a higher frequency of clarifications, which consequently reduce the total protein content of the bioflocs. In addition, when using Ca (OH) 2 or CaCO3, the water became extremely hard (> 1000 mg / L CaCO3), however it does not appear to have compromised growth. Survival (approx. 80%) and feed conversion (approx. 1.1) did not differ between treatments. To assess the physiological status of fish,at the end of the test, tissues (blood, liver and gills) were collected to assess hematological parameters and antioxidant capacity. Both parameters showed no change, indicating that the use of alkalinizers does not cause physiological disturbance. The second chapter, testing those with pH 6.5, 7.5 and 8.3 for 60 days, demonstrated superior growth and productivity for pH 7.5 (44.1 ± 0.9g and 23.5 ± 0.5kg / m3), compared to 8.3 (37.1 ± 3.9 g and 19.1 ± 2.0 kg / m3), while 6.5 (40.4 ± 4.1 g and 22.6 ± 2, 3 kg / m3) was similar to the others. Survival (approx. 80%) and feed conversion (approx. 1.1) did not differ between treatments. The results indicate that it is advantageous for nitrification to initiate the formation of bioflocs with a pH close to 8.0, and that maintenance up to pH 6.5 does not compromise the nitrogenous cycling. Further,maintaining the system at a lower pH reduces the production of solids, which favors an increase in the protein content of bioflocs, due to the lesser need to remove solids. The hematological and oxidative evaluations showed no difference between treatments, suggesting that the pH between 6.5 and 8.3 is not a challenge for tilapia. In the third chapter, when testing hardness concentrations (250.0, 700.0 mg / L CaCO3 and control (79.0 mg / L CaCO3) for 20 days, using CaCl2, the best growth and productivity performance occurred for 250 (7.6 ± 0.4 g and 4.8 ± 0.2 kg / m3) and 700 (7.4 ± 0.3 g and 4.6 ± 0.05 kg / m3), compared to the control (6.3 ± 0.1 g and 3.6 ± 0.4 kg / m3), while survival (approx. 80%) and feed conversion rate (approx. 1.0), did not differ between treatments.SST concentrations showed a positive increase with increasing hardness. This is due to the greater Ca adhesion in the biofloc structure, increasing the density, which makes the flake volume index (FVI, mL / g) in the treatments with hardness correction more stable throughout the test. Finally, as a way to optimize performance, the results obtained can be used in the production of Nile O. niloticus tilapia in a BFT system, indicating the formation of bioflocs using pH around 8.0, maintenance at pH up to 6.5, as well as the use of alkalizing NaHCO3 or Ca (OH) 2. Furthermore, the use of Ca (OH) 2 causing an excessive increase in hardness does not compromise growth when maintained up to 700 mg / L CaCO3.

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