Nitrogen And Phosphorous Recovery From Source-separated Urine With Spontaneous Electricity Production

In the past two decades,eutrophication caused by excessive nitrogen and phosphorus discharge has become one of the major environmental pollution problems in China.Both the traditional ammonia synthesis and phosphate mining process consume a large amount of energy,and phosphate is also a non-renewable resource.Whereas,urine contains a large amount of nitrogen and phosphorus resources,and urine with a purity of more than 85%can be separated from urinals based on the existing urine source separation system.Therefore,the recovery of nitrogen and phosphorus from urine not only has important environmental significance,but also helps to solve the problems of sustainable human development.The use of electrochemical methods to recover nitrogen and phosphorus from urine wastewater in the form of struvite has being attracted more and more attention.Due to the high energy consumption,however,the electrochemical process is limited in practical application.To this end,this study combined electrochemical precipitation and electrochemical oxidation by using Magnesium air fuel cell?MAFC?technology to recover nitrogen and phosphorus from urine in the form of struvite,so as to achieve simultaneous nitrogen and phosphorus recovery and electricity generation.In addition,to solve the problem of long period ureolysis,the law of urine hydrolysis was studied,and the influential mechanism of urease dosage on electrochemical precipitation was explored,so as to shorten the storage time of urine and maximize the utilization of nitrogen and phosphorus resources.The research content is mainly divided into two parts.?1?The recovery of nitrogen and phosphorus from hydrolyzed urine and the simultaneous generation of electricity were studied.Preparation method of porous oxygen reduction catalyst was obtained,and MAFC was designed and constructed.The kinetics of nitrogen and phosphorus removal and reaction process were studied when hydrolyzed urine was used as MAFC electrolyte.The influence of external resistance of MAFC on properties of recovered precipitation was analyzed,and the variation and mechanism of power generation performance of MAFC were studied.?2?The effect of dosage of urease in urine on phosphorus recovery was investigated.The pH changing rule and the optimal initial pH of ureolysis were obtained.The kinetics patterns of nitrogen and phosphorus removal and reaction process in urine with different urease dosage as MAFC electrolyte were revealed.The influence of urine with different urease dosage on the composition and properties of recovered precipitation was analyzed.The main results of this study are as follows.?1?MAFC was designed and constructed by integrated electrochemical precipitation and electrochemical oxidation.MAFC was successfully applied to recover nitrogen and phosphorus from hydrolyzed urine and synchronous power generation.After 25-min treatment,up to 99%of phosphorus was removed,and the phosphorus concentration in effluent was 0.28 mg L^–1 which met the requirement of level A standard of China's discharge standard of pollutants for municipal wastewater treatment plant.The maximum output power density of the MAFC was 552.32 mW m^–3,and the recovered power was 1035.06 KWh Kg_P^–1.The nitrogen and phosphorus resources were successfully recovered in the form of struvite,and the purity of struvite was as high as 98%,which was in comparison to that of commercial struvite.Magnesium ions and hydroxide were generated by electrode reaction in situ to participate in the struvite precipitation reaction without additional chemical reagent,which reduced the operation cost.A reaction mechanism on the delay of passivation of magnesium plate and improvement of the electrochemical properties of MAFC was proposed.?2?The optimal initial pH value of urease promoting ureolysis was determined to be 5,and urease inactivation was observed as the pH value below 2.When the initial pH of the system increased from 5 to 11,the activity of urase decreased gradually as the increase of pH value,resulting in the declined ureolysis rate.Meanwhile,the production rate of ammonia nitrogen decreased from 1.818 mg L^–1 min^–1 to 0.366 mg L^–1 min^–1.When the urease dosage increased from 1 mg L^–1 to 5 mg L^–1,the duration for hydrolyzing ca.5.5%urine was shortened from 3.5 h to 12 min.With the increase of urease dosage,the phosphorus content and struvite purity in recovered precipitate increased while the Ca²⁺content decreased.In this paper,MAFC was used to recover nitrogen and phosphorus from urine in the form of high purity struvite,achieving 99%of phosphorus recovery and synchronous electricity generation.Controlling the discharge of nitrogen and phosphorus wastewater from the source and carrying out effective resource recovery can not only reduce the operation cost of sewage treatment plant,and enhance the effluent quality,but also create great economic benefits.