Simultaneous Removal Of Ammonia Nitrogen And Phosphate By Electrolysis Coupled With Chemical Crystallization

Nitrogen and phosphorus are the basic elements of all organisms and play a vital role in the development of industry and agriculture.However,if the content of nitrogen and phosphorus in water exceeds the standard,it will lead to eutrophication.In addition,as phosphorus is a non-renewable resource,with the continuous development of modern industry and agriculture,the global phosphorus reserves are decreasing.Therefore,the research on phosphorus recovery has important theoretical and practical significance for the control of water eutrophication and the sustainable development of phosphorus resources.Firstly,the effect and mechanism of phosphorus removal by electrolysis and chemical precipitation were investigated.On this basis,the combined process of simultaneous nitrogen and phosphorus removal was further studied in order to develop efficient phosphorus recovery process and provide theoretical basis and technical support for the development of phosphorus recovery technology.The comparative study of phosphorus removal by electrolysis and chemical precipitation shows that using iron and aluminium salts as precipitators,the effect of solution pH value and amount of precipitator on phosphate recovery is significant,and the effect of reaction time on Fe²⁺as precipitator can not be neglected;using iron and aluminium as sacrificial electrodes,current density and pH value are the key control parameters in the process of electrolysis.Under the same conditions,the phosphate recovery efficiency of iron electrode is obviously higher than that of aluminium electrode.The phosphorus recovery efficiency is similar to that of chemical precipitation.However,adding aeration device in the process of iron electrode electrolysis can further improve the phosphate removal rate.The comparative analysis shows that the chemical precipitation method needs to increase the dosage of chemical reagents,while the electrolysis method does not need to add chemical reagents,and the operation is simple,so it is more feasible to remove phosphorus by electrolysis.In order to further investigate the effect of phosphorus removal by chemical precipitation,K-struvite crystallization process was used to treat urine wastewater from source separation.The results showed that the phosphorus and potassium removal efficiency of low-grade magnesium oxide and concentrated phosphoric acid were the best,with removal rates of 71%and 82%respectively.Because of the competitive relationship between Na⁺and K⁺in wastewater,when the molar ratio of Na:K increases to 10:1,the content of sodium magnesium phosphate in precipitated solid is higher than that of potassium magnesium phosphate.In order to eliminate the influence of Na⁺on the crystallization process of K-struvite,magnesium chloride,magnesium alloy electrode plate and magnesium hydroxide were used as magnesium sources,and the recovery effects of phosphorus and potassium were investigated.The results showed that with soluble magnesium chloride as magnesium source,Na⁺had little influence on the crystallization process of K-struvite,and the recovery effect of phosphorus and potassium was the best.The removal rate of phosphorus and potassium was 98%and 77%respectively by adding magnesium chloride step by step.On the basis of the above experiments,the effect of simultaneous nitrogen and phosphorus removal by the combined process was further investigated.The combined process of electrolysis and chemical precipitation was used to remove ammonia nitrogen and phosphate simultaneously.Dolomite was used as H⁺neutralizer and phosphate capture agent.Under the optimum conditions,ammonia nitrogen was basically completely removed,and the phosphate removal rate could reach more than 95%.In conclusion,compared with electrolysis and chemical precipitation,the combined process of electrolysis and chemical precipitation has more advantages in simultaneous nitrogen and phosphorus removal.It not only makes the phosphate in wastewater meet the discharge standard,but also solves the problem of unsatisfactory ammonia nitrogen removal effect in traditional electrolysis simultaneous nitrogen and phosphorus removal process.