Study And Application Of Electrochemical Oxidation Technology For Ammonia Removal

Based on the functions of electrochemical reaction with the oxidation and microbubble production, the research of ammonia oxidation and activated sludge separation by electrolysis was proposed. The factors that affected ammonia electro-oxidation, characterization of electrode and ammonia electro-oxidation, oxidation mechanism, oxidation products and dynamic kinetics, preparation of electrode materials, the feasibility of electro-flotation for the solid-liquid separation of activated sludge were investigated. A novel chemical enhanced biological fluidized reactor was developed and applied for wastewater treatment. The following results were achieved.Electrode reaction was the reaction rate control step for ammonia electro-oxidation. Current density, chloride concentration and electrode material were the main factors for ammonia electro-oxidation. The influence of initial water temperature and electrode gap was rather little. Under alkalescence condition, the chloride ion would transform in a circle, it was favourable for ammonia removal. Metal ion such as Mg²⁺, Ca²⁺, Sn²⁺, Fe²⁺, Al3+, and anion such as HCO3-, CO32- and NO3- had little effect on ammonia electro-oxidation removal, but SO42- and PO43- had significant effect. There were completive oxidation between NO₂^- and ammonia, NO₂^- was oxidized prior to ammonia oxidation. COD and ammonia can be removed simultaneously by electrochemical oxidation and the removal of ammonia is the dominant reaction in this method.In the electrochemical process, ammonia was removed by either direct or indirect electrochemical oxidation process. When the concentration of chloride was low ([Cl-]<500mg/L), the hydroxyl radicals produced during electrochemical process was about 10-15mol/L, the free chlorine produced during electrochemical process had significant effect on ammonia oxidation. The oxidation products of ammonia related with the content of electrolyte and pH. When chloride presented, the main product was N2, a little chloramine and NO3-. The ammonia electro-oxidation followed pseudo-first-order kinetics. The reaction activation energy was 19.81kJ/mol. The dynamic kinetic equation could be expressed as:The over-voltages of three types of Ti/RuO₂-based electrode of oxygen evolution and chlorine evolution were low, and the difference of over-voltage between oxygen evolution and chlorine evolution was very small, which resulted in a low current efficiency of ammonia electro-oxidation (lower than 30%). The voltammetry curves showed that the electro-catalytic of ammonia on these three type electrodes was small and the main reactions were oxygen evolution and chlorine evolution. SEM and accelerated life test showed that these three types of DSA anode with good electro -chemical stability. They were suitable for wastewater treatment.The factors that affected the preparation of tin and antimony oxide electrode by thermal decomposition were investigated. The Ti/SnO₂-Sb₂O₃ electrode showed good performances of direct electro-oxidation of ammonia. Ti/SnO₂-Sb₂O₃ (-CeO₂) electrode had the same effect as well as Ti/RuO₂-TiO₂ electrode on indirect electro-oxidation of ammonia, but the Ti/SnO₂-Sb₂O₃(-CeO₂) electrode was cost effective.Electro-flotation using inert anode is a high efficiency technology for the solid-liquid separation of activated sludge. Combined the inner circulation three phase biological fluidized bed reactor with electrochemical technology, a novel chemical enhanced biological fluidized reactor was developed. Applied the new reactor for synthetic municipal wastewater treatment, the ammonia removal ratio increased 24%.