| In order to simplify the operation and cut down the costs which was caused by magnesium source addition and pH increase in traditional magnesium ammonium phosphate(MAP)treatment method,a two chambers electrolysis cell with a sacrificial magnesium anode was proposed to achieve simultaneously adding the magnesium source and increasing pH.At the same time,an ion exchange membrane was used to stabilize the pH during the electrolysis process.In this study,the feasibility of the sequential operation mode and continuous operation mode was verified,the key operation parameters of these two modes were optimized by single factor experiment and response surface methodology,and the removal efficiency of phosphate was analyzed in both modes.In addition,this paper also studied the characteristics of membrane fouling existing in the electrolysis process,analyzed the influence of membrane fouling on phosphate removal.Thus,this paper could provide a reference for the application of this two chambers electrolysis cell with sacrificial magnesium anode in practice.The main results of this dissertation are as follows:(1)Under the sequencing operation mode,the phosphate removal efficiency first increased and then decreased as anolyte pH increased from 8.0 to 9.5 in simulated biogas slurry experiments.With the increase of current density from 11.1 A m-2 to 44.4 A m-2,the phosphate removal efficiency gradually increased,however,the initial current efficiency decreased in simulated biogas slurry experiments.When taking the phosphate removal efficiency and costs into account,the optimal parameters for sequencing batch mode were determined as the anolyte pH 8.5 and current density 22.2 A m-2.Under the optimal conditions,the chemical reagent was replaced by the cathode-anode subsection process,elevating the pH of the actual biogas slurry from 7.4 to 8.5-8.6,and 93%phosphorus removal efficiency was achieved stably.The run costs were estimated as ?81.6 kg-1 P.(2)The response surface methodology was used to optimize the key parameters in the continuous operation mode,and to describe the effects of current density and hydraulic retention time(HRT)on the phosphate removal efficiency.The results showed that the phosphate removal efficiency was significantly affected by linear terms of current density and HRT,quadratic terms of current density and HRT,and interaction terms of current density and HRT.In order to achieve the highest phosphate removal efficiency in the shortest HRT,the optimum parameters were obtained by the quadratic model as current density of 78.2 A m-2 and HRT of 40 min.Under the optimal condition,the actual removal efficiency of phosphate was kept higher than 85%.The run costs were estimated as ?71.7 kg-1 P.(3)The comparison between sequencing operation mode and continuous operation mode shows that the phosphate removal efficiency under sequencing operation mode was around 8%higher than that under continuous operation mode,while the running costs of sequencing operation mode was also 14%higher than the costs of continuous operation mode.Thus,the operation mode should be chosen by the needs of swine farm in practice.(4)To illustrate the extent of the membrane fouling and the changes of membrane characters,the membrane surfaces were determined by Scanning Electron Microscopy(SEM)-X-ray Energy Dispersive Spectrometry(EDS)and electrochemical impedance spectroscopy(EIS).The effect of electrolysis on membrane fouling was studied by comparing SEM results after immersion,MAP crystallization and magnesium anode electrolysis treatment.The results showed that membrane surfaces treated by immersion and MAP crystallization treatment were scarcely polluted.However,the magnesium anode electrolysis treatment caused serious membrane fouling.In addition,the multiple electrolysis treatment enlarged the coverage area of membrane fouling and promoted the sizes of crystal particles.However,the structure of the fouling layer became loose,so that the phosphate removal efficiency was not affected. |
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