Ammonium Nitrogen Recovery From Wastewater By Magnesium Ammonium Phosphate Crystallization

One of the major environmental concerns associated with surface water environment is related to the pollution of ammonium nitrogen. Now, ammonium nitrogen is used as an additional controlled indicator for energy saving and emission reduction strategy of "12Five Year Plan". As a result, the technology for ammonium nitrogen removal will show great market potential and industrial applications prospects. Ammonium nitrogen removal by adding magnesium salt and phosphate to form magnesium ammonium phosphate hexahydrate (MAP) crystallization is a useful method. Recently, MAP precipitation of ammonium has been studied widely.However, the problem, parameter controlled, magnesium ammonium phosphate disposal, and crystallized-settled reactor designed, that hamper the wide application of MAP precipitation. In the present research of "Ammonium Nitrogen Recovery from Wastewater by Magnesium Ammonium Phosphate Crystallization", the problem, parameter controlled, magnesium ammonium phosphate disposal, and crystallized-settled reactor designed, that was investigated by the method of extract precipitation reaction control, PHREEQC thermodynamic modeling, response surface methodology evaluation, magnesium ammonium phosphate pyrogenation, chemical precipitation pyrogenation recycle, U-shape flow course design, and continuous stirred crystallized-settled reactor control. The research of MAP precipitation parameter control technology, thermodynamic modeling assessment technology, chemical precipitation recycle technology, and continuous stirred crystallized-settled reactor function technology, were examined. The main results are as follows:(1) Ammonium nitrogen recovery from wastewater by MAP precipitation Taking into account the problem of parameter controlled, ammonium nitrogen recovery from wastewater by MAP precipitation was investigated and the following conclusions could be obtained. When MgCl2·6H2O and Na2HPO4·12H2O were employed, the optimum pH was9.0-10.5and the molar ratio of Mg2+:NH4+:PO43-was controlled at1.1-1.2:1:1in order to recovery ammonium effectively and avoid higher concentration of PO43-in the effluent. MgCl2·6H2O plus Na2HPO4·12H2O was the most efficient for ammonium removal. The interfering ions of Ca+, CO32-, and K+could obviously affect ammonium nitrogen recovery efficiency. The ionic strength, and temperature could also affect ammonium nitrogen recovery efficiency. The kinetics experiment showed that the rate of reaction was closer to the first order kinetic model. Fourier transform infrared spectroscopy and X-ray diffraction analysis indicated that MAP was the main composition of the precipitates. Scanning electron microscopy with energy dispersive X-ray analysis indicated that the unshaped crystal was coarse and its size was irregular, the surface composition of the precipitates contains a great deal of O, P, Mg.(2) Thermodynamic modeling assessment for ammonium nitrogen recovery by MAP precipitationThermodynamic modeling was developed for predicting ammonium nitrogen recovery from wastewater by MAP precipitation. Response surface methodology (RSM) was applied to assist in understanding the relative significance of reaction factors and the interactive effects of solution conditions. The following conclusions were drawn. The saturation index (SI) of MAP followed a polynomial function of pH. The SI of MAP increased and followed a logarithmic function of the concentration of magnesium, ammonium, and phosphate, respectively. The SI of MAP decreased with an increase in the concentration of calcium, carbonate, respectively. The SI of MAP decreased with an increase in ionic strength. Temperature could obviously affect the SI of MAP. The RSM analysis indicated that the factors pH, Mg/N, CN, Ca/N,(Mg/N) x (CO32-/N),(pH)2,(Mg/N), and (Cn)2were significant. Thermodynamic modeling was validated by comparing it with the case study.(3) Ammonium nitrogen removal by chemical precipitation recycle technologyTaking into account the problem of magnesium ammonium phosphate disposal, chemical precipitation recycle technology (CPRT) for ammonium nitrogen removal from wastewater was examined. The pyrolysate resulting from magnesium ammonium phosphate pyrogenation in sodium hydroxide solution was recycled for ammonium nitrogen removal from wastewater, and the following conclusions were drawn. The MAP pyrolysate could be produced at the optimal condition of a hydroxyl to ammonium molar ratio of2:1, a heating temperature of110℃, and a heating time of3h. The pyrolysate could be recycled as a magnesium and phosphate source at an optimum pH of9.5. When the recycle times were increased, the ammonium nitrogen removal ratio gradually decreased if the pyrolysate was used without supplementation. When the recycle times were increased, the ammonium nitrogen removal efficiency was not decreased if the added pyrolysate was supplemented with MgCl2-6H2O plus Na2HPO4·12H2O during treatment. A high ammonium nitrogen removal ratio was obtained by using pre-formed MAP as seeding material.(4) New continuous crystallization reactor designed for ammonium nitrogen recovery as MAP precipitationTaking into account the problem of crystallized-settled reactor, a new continuous crystallization reactor was designed for ammonium nitrogen recovery as MAP precipitation. The reactor practical operation was studied, and the following conclusions were drawn. A continuous stirred crystallized-settled reactor (CSCSR), adopted U-shaped flow course design, consists of column coagulation implement, filler baffle-wall, taper stabilization implement, and settlement tank. The ammonium nitrogen recovery efficiency was91.08%at the condition of pH11.0, Mg/N1:1, HRT1.5h, and Ca/N0:1. The RSM analysis indicated that the factors Mg/N, and Ca/N were significant.