Ammonium Removal From Wastewater Via Struvite Pyrolysate Recycling

One of the major environmental concerns associated with water environment of china is related to the pollution of ammonium nitrogen, ammonium nitrogen is used as an additional controlled indicator for energy saving and emission reduction strategy of "12Five Year Plan". Many new and effective wastewater treatment processes have received great interest in water treatment, as an attractive method for the recovery of high concentrations of ammonium from wastewater, struvite (MgNH4PO4·6H2O, MAP) precipitation is a useful method by adding magnesium salt and phosphate to form magnesium ammonium phosphate hexahydrate crystallization. Recently, MAP precipitation has been studied widely.However, the problem, consumption of phosphate, phosphate concentration in the supernate, the high costs of magnesium and phosphate reagent, the utilization of struvite, that are urgent problem for the practical application of MAP chemical precipitation. In this study of "Ammonium removal from wastewater via struvite pyrolysate recycling", the problem, consumption of phosphate, phosphate concentration in the supernate, the high costs of magnesium and phosphate reagent, the utilization of struvite, that were investigated by the method of acidolysis regulated, effective split ammonia from struvite precipitate by dry pyrolysis, the dynamics modeling, ammonium removal via struvite pyrolysate recycling with NaOH/Mg(OH)2/mixed bases mediated pyrolysis. The study of parameter optimization of struvite with NaOH/Mg(OH)2/mixed bases mediated pyrolysis, ammonium removal via struvite pyrolysate, ammonium removal via struvite pyrolysate recycling and a thermo-kinetics study of struvite decompositon, were examined. The study can provide the scientific foundation for the extensive application of MAP precipitation. The main results are as follows.(1) The parameter optimization of struvite with NaOH/Mg(OH)2pyrolysisThe problem of parameter optimization of struvite with NaOH/Mg(OH)2pyrolysis, the formation and mechanism of the derivative were investigated, and the conclusions thus obtained were as follows. For struvite with NaOH mediated pyrolysis, the best parameters for the NaOH:NH4+molar ratio, pyrolysis temperature and reaction time were1:1,110℃and3h, respectively; While ammonia release ratio of MAP with Mg(OH)2mediated pyrolysis was less than that of with NaOH mediated pyrolysis, but which was more than that of direct pyrolysis, the optimum molar ratio of Mg(OH)2to NH4+was1:1, a temperature was110℃and a time was3h; Through detects the pyrolysate, shows that magnesium phosphate is created in the process, and the derivative, Mg2P2O7, is formed; The asymmetrical stretching vibration of P-O-P (Mg2P2O7) was increased with rising temperature.(2) The parameters optimization for improving ammonium removal from wastewater by struvite pyrolysateAmmonium removal from wastewater by struvite pyrolysate was studied, the effect of temperature, OH:NH4+molar ratio and reaction time on ammonium removal was investigated, and the following conclusions could be obtained. The optimum pH for ammonium removal from fertilizer wastewater by adding MAP pyrolysate was9.5; The optimal MAP pyrolysate dosage used in this study was Mg2+:NH4+molar ratios of1:1(the Mg2+concentrations were calculated based on MgHPO4); The best stirring time was60min; Ammonium removal ratio was the best,84%by the pyrolysate with direct pyrolysis at110℃, while which was87%with NaOH mediated pyrolysis at NaOH:NH4molar ratio1:1, pyrolysis temperature110℃, and reaction time3h, the phosphate concentration in the supernate was2.3mg/L; During the process of struvite with Mg(OH)2mediated pyrolysis, ammonium removal rate was about84%by the pyrolysate at the molar ratio of Mg(OH)2to NH4+1:1, a temperature110℃and a time3h, while the biggest advantage with Mg(OH)2mediated pyrolysis lies in the phosphate concentration in the supernate, which was0.02mg/L; When the molar ratio of NaOH:Mg(OH)2:NH^was0.5:0.5:1(with mixed-bases mediated pyrolysis), ammonium removal rate was above90%, and the phosphate concentration in the supernate was less than0.5mg/L.(3) A thermo-kinetics modeling of struvite decompositionThe effect of NaOH and Mg(OH)2on struvite decomposition was investigated, a thermo-kinetics of struvite decomposition was studied, the results showed that one step could be observed in sturvite decomposition with direct pyrolysis, the temperature of the endothermic peak was130.3℃(5K/min); the mechanism of sturvite decomposition with direct pyrolysis is an autocatalytic set with a reduction in the reaction rate, the degradation has more than two pathways, the activation energy by direct pyrolysis were30KJ/mol to100KJ/mol, an n-th order model was obtained, the parameters of equation was f(a)=ko(1-α)n, the reaction order was2.51, and the coefficient of linear correlation was0.995; While two steps were found in sturvite decomposition with NaOH mediated pyrolysis, the temperature of the endothermic peak was113.4℃(5K/min); They lower the activation energy needed to initiate reaction by added NaOH during struvite pyrolysis, from about100KJ/mol to about70KJ/mol, and the degradation has only one pathways; the activation energy with NaOH mediated pyrolysis was about70KJ/mol (calculated by Friedman), which was about74KJ/mol by OFW method; The struvite thermal decomposition was bimolecular reaction, the kinetic function is f(a)=aα-(1-α)n, a Bna model, the coefficient of linear correlation was0.996and the reaction order was5.08. Degradation process of struvite with Mg(OH)2addition was three steps, the temperature of the endothermic peak was117.0℃(5K/min); The mechanism of sturvite decomposition with Mg(OH)2mediated pyrolysis is an autocatalytic set with a reduction in the reaction rate also, the degradation has more than two pathways, the activation energy of stripping ammonia from struvite was from34.4KJ/mol to112.9KJ/mol; The kinetics equation is f(a)=k0(1-α)n, the coefficient of linear correlation was0.994and the reaction order was6.12. Three steps were observed in sturvite decomposition with mixed bases mediated pyrolysis, the temperature of the endothermic peak was114.1℃(5K/min); The activation energy of initiate reaction with mixed bases mediated pyrolysis was71.6KJ/mol, when the extent of conversion was less than20%, NaOH mediated should play a leading role in struvite pyrolysis, a reaction pathway with lower energy intermediate state is choice; When the extent of conversion was more than20%but less than50%, NaOH and Mg(OH)2mediated should play a role in struvite pyrolysis, the degradation has more than two pathways; When the extent of conversion was more than50%, Mg(OH)2begins to break down, and the degradation has only one pathway; The activation energy of struvite with mixed bases mediated pyrolysis (the extent of conversion<50%) was62.1KJ/mol calculated by Friedman, and which was69.3KJ/mol by OFW method; The kinetics equation with mixed bases mediated pyrolysis is f(a)=aα-(1-a)n, the coefficient of linear correlation was0.995and the reaction order was5.92.(4) Ammonium removal from wastewater by struvite pyrolysate recyclingAmmonium removal from wastewater by struvite pyrolysate recycling was investigated by the method of struvite pyrolysis and reagent recycling, and ammonium removal rate with the increase of recycle was studied by added acidolysis, the results indicated that the optimum pH was1, temperature was120℃and time was2h; Pre-formed MAP was added to the solution at the beginning of the experiment as a seed crystal, the seed load in this experiment corresponded to a molar ratio of pre-formed struvite to NH4+in the wastewater of5:100; The NH4-N removal ratio from fertilizer wastewater was more than80%at pH9.5, and which was stable during struvite pyrolysate recycling; Ammonium removal rate was increased by added acidolysis, with the increase of number of recycle, ammonium removal rate was not decreased; however, the supernatant phosphate concentration with NaOH pyrolysis was198.2mg/L, and the existence of NaOH seriously affects the growing habit of the struvite crystals; Ammonium removal rate by struvite pyrolysate recycling with Mg(OH)2mediated pyrolysis was less than that of with NaOH mediated pyrolysis, but which was more than that of direct pyrolysis, about75%, and the phosphate concentration in the supernate was less than2mg/L; However, during struvite pyrolysate recycling with mixed bases mediated pyrolysis, ammonium removal rate was more than80%and the level keeps relatively stable during the cycle of struvite pyrolysate, the phosphate and magnesium ions concentration in the supernate for combined mixed-base technology were less than2mg/L and40mg/L, respectively; The particle size of MAP with mixed bases mediated pyrolysis recycling was d(0.5)=26.97μm, which was close to that of direct pyrolysis.