Electrochemical Reinforced Removal Of A Mmonia Based On The Struvite Process

Struvite crystallization with electrochemical method has gained interest to rec over nitrogen and phosphate from wastewater.However,since wastewater usually have a large molar excess of NH4+-N relative to PO43-,removal efficiency of a mmonia nitrogen was usually low in struvite process.Hence,an electrochemical method based on struvite process,which coupled the electrochemical precipitation and electrochemical oxidation to reinforce ammonia removal,was developed to i mprove the removal rate of ammonia nitrogen in struvite process.The main resu lts of the study are as follows:(1)In the electrochemical reactor with Mg/modified graphite electrode,the el ectrochemical precipitation and cathode oxidation occurred simultaneously in solut ion,which could improve the recovery/removal effect of nitrogen and phosphorus in solution.Under the same electrochemical parameters,both of the Mg/modifie d graphite electrode and Mg/steel electrode reactor provided a more than 97% re moval rate of phosphorus,while the removal rate of ammonia nitrogen was 4.2%higher than that of the latter.(2)In the Mg/modified graphite electrode reactor,the electrochemical precipit ation reaction was the main removal route of ammonia nitrogen,and the compos ition of precipitation produced was mainly struvite.The modified graphite cathod e can produce a certain amount of H2O2,which could oxidize ammonia to N2 a nd promote the removal of ammonia.(3)In the electrochemical reactor with Mg/Pt electrode,the electrochemical p recipitation reaction and anodic oxidation reaction occurred alternately in the solu tion,which was shown to be an effective method to removal/recover ammonia a nd phosphorus.Under the same electrochemical parameters,the removal rate of a mmonia nitrogen in the pulse reverse electrolysis was 59.5% and 10%,which w as higher than that of pulsed oxidation and pulsed precipitation respectively.(4)Many factors have a significant effect on the removal rate of ammonia n itrogen and PO43-,Such as duty cycle,voltage,frequency and initial p H.With d uty cycle increasing,the proportion of the electrochemical precipitation reaction i ncreased,and the proportion of the electrochemical oxidation reaction reduced.W hen duty cycle was 35%,the removal rate of ammonia was more than 95% after 180 min.The voltage affected the rate of the reaction by changing the magnitu de of the current in the solution.The electrolysis frequency has strong influence on the current and electrode passivation.By the way of increasing the initial pH,the precipitation in the solution occured earlier,which affected the removal of p ollutants.While increasing the influent N: P ratio can improve purity of struvite,but the removal rate of ammonia nitrogen was reduced..(5)Ammonia removal path was significantly affected by duty cycle factor.A t 45% duty cycle,the precipitation reaction was the main removal route of amm onia nitrogen,and the precipitated product was mainly struvite and magnesium p hosphate.(6)The result of XRD,ICP-MS and elemental analysis showed that the pa ssivation of the plate was mainly Mg(OH)2 in the Mg electrode in the process o f electrolysis.When the Mg electrode was at low potential,the H2 produced on the surface of the electrode can destroy the passivation layer,which will inhibit the occurrence of passivation.What’s more,electrolytic voltage and frequency ha d a significant effect on electrode passivation.The plates were characterized by s uperficial depth of 3D microscopy and SEM.The results showed that the passiva tion of Mg electrode was slowed down with the voltage increasing and promoted with the frequency increasing.