|Ph.D Student||Gendel Youri|
|Subject||Indirect Ammonia Electro-oxidation: Reaction Mechanism|
Investigation and Implementation on Intensive
|Department||Department of Civil and Environmental Engineering||Supervisor||Professor Ori Lahav|
|Full Thesis text|
The work was dedicated to investigating the mechanism of the indirect ammonia electro-oxidation process and its implementation for water treatment of intensive fish producing facilities.
The mechanism of indirect ammonia electrooxidation has been often described similarly to breakpoint chlorination. However, comparison of the chloramines concentrations which develop in batch indirect ammonia electrolysis and chloramination experiments suggests that reactions are different. Based on the results of chloramination and electrolysis experiments a new mechanism for indirect ammonia electrooxidation was proposed, according to which trichloramine forms from a reaction between NH4 and Cl2(aq), which occurs in the near anode area where pH is <2 and the Cl- concentration is high. NCl3, formed in the near anode area, decomposes to N2, NH2Cl and NHCl2 in the bulk solution or/and close to the cathode surface area, where pH>12. Both NH2Cl and NHCl2 that form in the bulk electrolyte are converted to NCl3 by Cl2(aq) upon return to the near-anode zone. The indirect ammonia electro-oxidation process was applied for NH4 conversion to N2 within a newly developed physico-chemical process aimed at ammonia removal from fresh-water recirculated aquaculture systems (RAS). The method is based on separating NH4 from RAS water through an ion-exchange resin, which is regenerated by chemical desorption and electrochemical ammonia oxidation. Approach advantages include (2) insensitivity to low temperatures, bacterial predators and chemical toxins; (2) no startup period is required and system can be switched on and off at will; and (3) the fish are grown in low bacterial concentration environment, making the potential for both disease and off-flavor, lower. A small pilot scale RAS was operated for 51 d for proving the process concept. The system was stocked by 105 tilapia fish (initial weight 35.8 g). The fish, which were maintained at high TAN concentrations (10 to 23 mgN L-1) and fish density of up to 20 kg m-3, grew at a rate identical to their established growth potential. NH3(aq) concentrations in the fish tank were maintained lower than the assumed toxicity threshold (0.1 mgN L-1) by operating the pond water at low pH (6.5-6.7). The low pH resulted in efficient CO2 air stripping, and low CO2(aq) concentrations (<7 mg L-1). The system was operated first at 10% and then at 5% daily makeup water exchange rate. The results show the process to be highly feasible from both the operational and economical standpoints.