Study On Treatment Of Ammonia Nitrogen In Mariculture Wastewater By Electrochemical Technology

Ammonia nitrogen is the most common pollutant in aquaculture wastewater and very difficult to be removed. Biological filter method is most widely used because of its high efficiency, good mass transfer and low operation cost, however, biological filter method requires long startup procedures, and tends to be influenced by many factors. In comparison, electrochemical treatment is very easy to operate with excellent stability, which has drawn much attention in the removal of ammonia nitrogen from aquaculture saline water by electrochemical oxidation. Electrochemical treatment of seawater presents several advantages, as high salinity ensures an excellent electric conductivity that could reduce the energy consumption, and the high chloride concentration helps improve the indirect oxidation through the electro-generation of strong oxidants like free chlorine.In this paper, first, electrochemical oxidation of ammonia nitrogen in synthetic marine aquaculture wastewater was studied. Several factors affecting the removal of ammonia nitrogen by electrochemical oxidation were investigated, and the formation of free chlorine, hydroxyl radical and halogenated by-products during the treatment process was also evaluated. The electrodes were characterized to get the surface morphology and electrochemical properties. Secondly, removal of ammonia nitrogen from actual marine aquaculture wastewater using both static and dynamic experiment setup further proved the feasibility of ammonia nitrogen removal by electrochemical treatment. The main conclusions are as follows.(1) Anode material, current density and seawater salinity were the main factors that influenced the electrochemical oxidation of ammonia nitrogen. The effect of pH changing from 6 to 9 on ammonia nitrogen removal was not obvious. Both COD and ammonia nitrogen could be removed simultaneously by electrochemical oxidation, so there was a competition between them during the electrochemical oxidation process.(2) The removal of ammonia nitrogen by direct electrochemical oxidation was not obvious. There was almost no hydroxyl radical generated at electrode surface during the electrochemical oxidation process of synthetic marine aquaculture wastewater, and ammonia nitrogen was removed mainly by the oxidation reaction with free chlorine which was produced at anode.(3) The SEM and oxidation resistance test showed that Ti/TiO2-RuO2-IrO2 and BDD electrodes had good electrochemical stability. However, the BDD electrode was too expensive. The potential difference between oxygen evolution and chlorine evolution of graphite electrode was the minimum which resulted in more side reactions. Therefore, the Ti/TiO2-RuO2-IrO2 electrode was more suitable for the electrochemical treatment of actual marine aquaculture wastewater.(4) During the electrochemical oxidation process of actual marine aquaculture wastewater, complete ammonia nitrogen removal was all achieved after 30 min with static experiment setup when the current density was in the range of 3-10 mA/cm2. Under the condition of dynamic experiment setup, the highest ammonia nitrogen removal rate could reach up to 87.1% at J=10 mA/cm2 when the the hydraulic retention time was in the range of 2-10 minutes.(5) When actual marine aquaculture wastewater contained organic compounds, organics could react with free chlorine generated at anode during the electrochemical oxidation resulting in the formation of organic halogen compounds such as trihalomethanes, among which were mainly chlorinated halomethanes. If bromide ion exsisted in wastewater, it would result in the increase of proportion of brominated halomethanes in trihalomethanes. To solve this problem, further researches could combine with activated carbon to adsorb these free chlorine and halogenated compounds effectively.