|M.Sc Student||Raz Ben Asher|
|Subject||Environmental and Operational Aspects of Electrochemical|
Ammonia Removal and CO2(aq) Control in High Head
Recirculated Aquaculture Systems
|Department||Department of Civil and Environmental Engineering||Supervisor||Full Professor Lahav Ori|
|Full Thesis text|
In this work two water quality improvements, pertinent to operational and environmental aspects, were developed, tested and evaluated. The first part of this work focused on the development of a method aimed at nitrogen removal from RAS effluents by applying a two-step biological and physico-chemical process. A continuous pilot scale activated sludge denitrification reactor was operated with RAS intrinsic organic matter as an electron donor for NO3- reduction. Its effluents, containing low NO3- and relatively high TAN concentrations, were used for testing and comparing between two TAN oxidation applications at the lab scale. The first (batch) approach was to recirculate the effluents through an electrolyzer where indirect TAN electro-oxidation was performed. The second approach was to utilize the high Cl- concentration in the brackish water fed to the RAS for generating a Cl2-rich solution, which was thereafter allowed to react with the denitrification effluents at different Cl2/N ratios applying the breakpoint chlorination approach to eliminate the TAN residual concentration.
The second part of the work focused on a development of a CO2 control method for high head RAS and defining the effects of various CO2 concentrations on the growth performance of gilthead seabream (Sparus aurata). The sub-lethal effect of CO2(aq) concentration was tested (for the first time) on gilthead seabream fingerlings (4 to 25 g) and adult fish (~300-400 g), both in fully controlled pilot tests and the latter also in a full-scale HESY RAS. In the pilot experiment the parameters that were monitored at the concentration range 5.2 to 56.3 mgCO2/L were (1) specific growth rate; (2) mortality rate; (3) general health condition; and (4) skeleton distortions. In the full scale experiment fish from a similar batch were exposed to controlled (by external NaOH dosage) and uncontrolled pH conditions, resulting in exposure of the fish to significantly different CO2(aq) concentrations. The results showed, as expected, that the fish grew faster at the lower CO2 concentrations and that the growth rate of both the fingerlings and the grownup fish was reasonable up to ~20 mg CO2/L (relative to a theoretical curve adopted from the literature). Mortality rate was exceptionally high only at the highest CO2 concentration applied (~56 mgCO2/L). Deformities were generally not observed, apart from lack of swim bladders in the high CO2(aq) treatment. The full scale RAS results showed a clear, statistically significant, advantage for growing gilthead seabream at constant and relatively low (<16 mg/L) CO2(aq) concentrations.