Phosphate Removal And Recovery By Magnesium Ammonia Phosphate Crystallization Based On Source Control

Phosphorus is the main cause of the eutrophication, and it is a non-renewable resource.The phosphorus pollution mainly comes from sewage disposal. Therefore, it is necessary toremove and recover phosphorus from wastewater treatment plant. Processes for biologicalnitrogen and phosphorus removal from urban wastewater sewage are often short of carbonsources. Therefore, to study the technology of source control, enhanced phosphorus removalto the wastewater from the septic tanks, is an effect way to reduce and control the phosphorusflowed into wastewater treatment plant. In the methods of phosphate removal and recovery,magnesium ammonia phosphate crystallization is an effective method that can remove andrecover phosphate. Therefore, this study was deeply investigated the phosphate removal andrecovery from the wastewater of septic tanks by the magnesium ammonium phosphatecrystallization to explore a technology of phosphorus removal from wastewater based onsource control.Different phosphate wastewater was the object of study. The influence rule of molar ratioof reactants, initial pH value, reaction time, reaction dynamic conditions, assembly time andinitial phosphate concentration on magnesium ammonia phosphate crystallization werestudied. Kinetics of crystallization was also discussed. When steel slag, mineral slag,modified steel slag, modified mineral slag and quartz sand were used as the seed crystal, theinfluence rule of the seed crystal on phosphate removal efficiency were studied. And themechanism of the seed crystal could remove the phospate was discussed. The influences ruleof COD and ammonia nitrogen on magnesium ammonia phosphate crystallization weredetermined. The treatment effection of seed crystal media filter and influence of operationalconditions were determined. The crystallization products were analysed by XRD. And therecycling value of crystallization products was discussed.The results indicated that it was helpful to improve the initial pH value to increasephosphate removal efficiency. Some degrees of increasing the mole ratio of reactants, reactiontime, dynamic conditions and assembly time could increase the phosphate removal efficiency.The phosphate removal efficiency was higher when the initial phosphate concentration washigh than that when the initial phosphate concentration was low. When reaction time≥30min,stirring speed≥80r/min and assembly time≥30min, the phosphate removal efficiency wouldreduce.When the seed crystal used steel slag or modified steel slag, the phosphate removal efficiency was higher than the phosphate removal efficiency when the seed crystal usedmodified mineral slag or mineral slag. When quartz sand was used as seed crystal, thephosphate removal efficiency was the worst of all. If the surface of the seed crystal containssimilar components as magnesium ammonia phosphate, it was in favor of improving thephosphate removal efficiency. When there were the components which could broden thewidth of metastable region and lengthen the induction period, the phosphate removalefficiency would reduce.The results showed that COD had no influence on magnesium ammonia phosphatecrystallization. When the initial phosphate concentration was low, the influence of NH4+-Nwas high. When NH4+-N was the restriction factor, it would influence the phosphate removalefficiency. When the initial phosphate concentration was too high, NH4+-N was not therestriction factor. There would be no influence on the phosphate removal efficiency.Through the orthogonal experiment, the results of experiments with simulationwastewater in seed crystal media filter showed that when the initial phosphate concentrationwas lower, the phosphate removal efficiency was higher. When the initial phosphateconcentration was high, the optimal and worse order to phosphate removal efficiency wasHRT>COD>N. When the initial phosphate concentration was low, the optimal and worseorder to phosphate removal efficiency was N>COD>HRT. On the same pH or the same HRTconditions, the phosphate removal efficiency was higher when initial phosphate concentrationwas low than the phosphate removal efficiency when the initial phosphate concentration washigh. The phosphate removal efficiency would increase with HRT increasing. The results ofXRD showed that the crystallization degrees of products were good. The experiments resultsof seed crystal media filter treating the actual septic tank wastewater showed that thephosphate removal efficiency could below1mg/L for a long time when phosphateconcentration was75mg/L, ammonia nitrogen concentration was186mg/L, COD was324mg/L, magnesium was20mg/L and SS was100NTU. The results of XRD showed that thecrystallization degrees of products were good. It had high recovery value.