Study On Wastewater Treatment With High Concentration Of Nitrogen And Phosphate By The Process Of Magnesium Ammonium Phosphate Crystallization

Nowadays, anthropogenic nitrogen and phosphorus discharged into the receiving waters has a major contribution to eutrophication. Ammonia nitrogen is a new controlled indicator in the "12th Five-Year Plan" and the discharge amount of ammonia nitrogen should be reduced10%; phosphorus, a non-renewable resource, will be exhausted in about100years. The magnesium ammonium phosphate (MAP) crystallization process is a valid alternative for removal and recovery of ammonia nitrogen and phosphate in wastewater as struvite precipitation by the addition of Mg2+The application of MAP crystallization technique for N and P removal and recovery has received much attention.In this thesis, the problems existing in the treatment of wastewater containing high concentrations of N and P through resources recovery——MAP crystallization controlled, MAP crystallization reactor performance and anaerobic digestion of wastewater containing high concentrations of organic nitrogen, were investigated by reaction condition control, response surface methodology (RSM) evaluation, seeding technique and technological combination design. The researches of process optimization of MAP crystallization technological conditions, performance optimization of reactor operational conditions and development of "anaerobic digestion-chemical precipitation" combined process for N recovery and methanogenic activity enhancement, were examined. The main conclusions are as follows:(1) Parameters evaluation of MAP crystallization technological conditions on phosphate recoveryThe main research object was interference of co-existing ions and RSM evaluation of parameters on phosphate recovery in synthetic urine. The results indicated that the co-existing ions of Ca2+and K+obviously affected phosphate recovery efficiency, but CO32-had a little effect. According to ANOVA of the response surface quadratic model, the terms of pH value (X1), Mg/P (X2), Ca/P (X3), X1×X3, X2×X3, X12, X22and X32had significant effects on the phosphate recovery efficiency. The experimental results showed that phosphate recovery efficiency could exceed99%at the conditions of pH value8.5-9.5and Mg/P1.0-1.2. X-ray diffraction (XRD), Scanning electron microscope-Energy dispersive X-ray (SEM-EDX) and Fourier transform infrared spectroscopy (FTIR) analysis indicated that K+and Na+had a little effect on phosphate recovery efficiency, but Ca2+had a remarkable interference effect. When Ca/P was below0.25, the purity of MAP exceeded85%and the impurity increased as Ca/P was above0.25. When Ca/P was0.5, the purity of MAP was only about70%.(2) Performance optimization of continuous crystallized-settled baffled reactor (CCSBR) operational conditionsThe main research object was performance optimization of CCSBR operational conditions and promoting effect of seeding technique. CCSBR, made up of coagulation zone, stabilization zone and settlement zone, was examined and the results indicated that Mg/P, Ca/P, Mg/P×Ca/P and Mg/P×Mg/P had significant effects on phosphate recovery efficiency. When Ca/P was below0.5in synthetic urine, a satisfactory phosphate recovery efficiency could be obtained by controlling the conditions at the urinary initial pH value10.5-11.0, Mg/P1.0-1.2, hydraulic detention time (HRT)1h and MAP seeds dosage2g/L. XRD, SEM-EDX and FTIR analysis indicated that the main component of the precipitate was MAP. When Ca/P was above0.5, phosphate recovery by MAP precipitation was inadvisable. Economic evaluation showed that the total cost of synthetic urine wastewater treatment was about￥13.04/m3and the income of MAP crystallized was about￥12.28/m3.(3) Development of "anaerobic digestion-chemical precipitation" combined process"Anaerobic digestion-chemical precipitation" combined process was based on anaerobic ammonification and MAP crystallization technique. For avoiding adverse effects of NH4+-N on anaerobic digestion, high concentrations of N and P in anaerobic effluent were recovered and supernatant was reflowed to anaerobic sludge area. Taking into account the factors of reflux ratio (RR), HRT and organic load rate (OLR), the "anaerobic digestion-chemical precipitation" combined process was examined in this study. The synthetic urine was used as the feed in this experiment. The results indicated that optimum parameters of process were as follow:UASB reactor: HRT9h, OLR<13kgCOD/(m3·d), RR14:1, temperature35±2℃;②CSBR: initial pH value11.0±0.2, Mg2+: NH4+: PO43-=1.15:1:1.15, MAP seeds dosage2g/L, HRT1h;③pH adjusted tank: acetic acid as adjusting reagent, final pH value6.5±0.1. The results indicated that under the optimum condition,①UASB reactor: An average effluent COD concentration of120mg/L and effluent pH value7.0was observed, the NH4+-N concentration in the effluent reached>720mg/L, ammonification rate>90%, the CH4yield and content was about0.29mL CH4/mg COD reduced and70%. Compared to the optimum condition of UASB reactor start-up stage, the CH4yield and content enhanced about20.7%and10%;②CSBR: the pH value of effluent8.7-9.3, more than96%of NH4+-N and60%of PO43--P present in synthetic urine could be recovered in struvite.