Study And Application Of Ammonia Electrooxidation On Coking Residual Ammonia Wastewater

Coking wastewater is generated from coke plants in coal coking,coke-oven gases purification and by-product recovering processes.Coking residual ammonia wastewater is the main wastewater source of coke plant,containing high concentrations of hazardous organic and inorganic pollutants and ammonia nitrogen concentration is above 3000 mg/L.The degradation of high concentration of ammonia nitrogen by conventional methods was very poor.Among many treatment alternatives,ammonia removal by electrooxidation have attracted much attention due to its effectiveness and ease in operation.This paper investigated the ammonia removal by electrooxidation.The investigation of this paper should be of benefit in the subsequent treatment of actual wastewater.In this study,we first discussed direct and indirect electrooxidation for ammonia removal of coking residual ammonia wastewater.The results indicated that ammonia oxidation rates resulted from both direct oxidation at electrode-liquid interfaces of anode and from indirect oxidation from active chlorine by addition of chloride.A remarkable increase in the removal of ammonia was observed by the indirect oxidation.The three-dimensional electrode reactor showed a higher ammonia oxidation rate than two-dimensional electrode reactor whether chloride was present or not.The paper then studied the influence of operating conditions(i.e.,chloride concentration,applied current density and initial wastewater pH value)on ammonia oxidation rate,energy consumption and current efficiency in PAC packed three-dimensional electrode reactor.The increase in chloride concentration and applied current density enhanced the indirect oxidation effect of ammonium removal.However,a higher current density and chloride concentration would result in higher energy consumption and lower current efficiency.It was found that electrolysis was more efficient in neutral to moderate alkaline conditions.Taking both ammonium removal rate and energy consumption into consideration,optimum conditions were obtained at Cl-concentration of 0.2 mol/L,current density of 80 mA/cm2 and initial pH value of 9.Under the optimum conditions,ammonia nitrogen were oxidized from3369 mg/L to 207 mg/L and removal efficiency reached 93.9% after 60 minutes electrolysis.Total nitrogen was decreased from 3621 mg/L to 1036 mg/L and removal efficiency reached 71.4%,indicating that ammonia mainly transferred to N2 in athree-dimensional electrode reactor.78.34% of COD was removed under the given condition.Hence,the electrooxidation method provided a feasible approach for treatment of coking residual ammonia wastewater with high chloride concentration and a final effluent with substantially reduced pollution load could be attained.Finally,the paper investigated the pseudo-kinetics for removal of ammonia by indirect electrooxidation.The reaction rate fomula was built up,depending on chloride concentration,applied current density and initial wastewater pH value.The oxidation rates of ammonia followed pseudo zero-order kinetics in electrochemical oxidation with the addition of chloride.A mathematical model that was used to describe the oxidation kinetics of synthetic wastewaters has been used to predict the kinetics of COD removal and the degradation reaction was confirmed to the first-order kinetic.