|M.Sc Student||Noa Efraim Rolider|
|Subject||Improvement and Quantitative Assessment of a Physical-|
Chemical Approach for Growing Striped-Bass in a
Freshwater and Brackish Water
|Department||Department of Civil and Environmental Engineering||Supervisors||Full Professor Lahav Ori|
|Dr. Birnhack Liat|
|Full Thesis text - in Hebrew|
RAS (Recirculated aquaculture systems) for growing fish have been in use for more than three decades. RAS are tank-based systems in which fish can be grown at a high density under controlled environmental conditions.
In this work we operated a RAS system that employs a nitrogen removal unit based on a physicochemical approach. Ammonium present in the fishpond is continuously separated from fresh water by an ion-exchange (IX) column. In order to maintain a high driving force for the IX separation step, High TAN (Total Ammonia Nitrogen) concentration and low pH (pH6.7 and below) is required. Once the ion-exchange capacity on the column is exhausted, it undergoes regeneration. The regeneration process includes passing water with a high counter cation concentration through the resin. In order to be able to reuse the regeneration solution for multiple regeneration cycles and to convert NH4 to N2(g) simultaneously, NH4 discharged from the IX resin is oxidized electrochemically. In the rearing water, for each mole of NH3 excreted from the fish gills, one mole of H is consumed. In order to compensate for the loss of protons and to maintain a pH level lower than 7 an exact amount of strong acid (e.g. H2SO4) should be added to the fishpond.
This main research objectives were to: a. Determine feasibility of growing hybrid striped bass at high TAN and low-pH water at fish density of up to 25 kg fish m3. b. Determine feasibility of operating a physico-chemical system for growing hybrid striped bass in freshwater and brackish water. c. Quantify phenomena occurring while operating the system . d. Develop an “acidification unit” that could lower the potential microbial concentration in the rearing water, by recirculating the rearing water through an acidic media (pH < 3) to reduce the activity of the bacterial population, including pathogens and nitrifying bacteria.
This system operated continuously for 24 days while growing hybrid striped bass as a test organism in freshwater and 28 days while growing bass in brackish water. In the freshwater growing period 177 fish grew from 72 to 93 grams. The actual growth rate correlated 85% with the expected growth rate. FCR was 2.08 g-feed/g-fish and weekly pathological turned out normal. During the brackish water growing period, 133 fish grew from 136 to 187 grams. The actual growing rate exceeded the expected growth rate and the FCR was 1.86 g-feed/g-fish. Pathological tests were normal. The results showed that growing hybrid striped bass in high TAN and low pH water at a fish density of 12-25 kg fish m-3 is possible. The daily water discharge volume was 8% for the freshwater growing period and 12% for the brackish water growing period. Repeated experiments showed that exposing the rearing water to pH 2.5-3 for 10-30 minutes can reduce the bacterial concentration (CFU/ml) by 3 orders of magnitude and furthermore, can hinder the activity of ammonia oxidizing bacteria. In addition, genomic tests that were performed on the treated samples showed that lowering the media’s pH caused significant changes in the microbial population.