Study On Removal Of NH₄⁺-N From Wastewater By MAP Method And The Recycling Of Pyrolysis Products

Ammonia nitrogen is not only one of the key factors that causing eutrophication of water body,but also an indispensable precious element of animals and plants.The ammonia nitrogen in water body has two forms:ammonium nitrogen?NH₄⁺-N?and free nitrogen?NH₃-N?.In this paper,Mg²⁺and PO₄^3-can be used to generate insoluble complex salts?MgNH₄PO₄·6H₂O,for short:MAP?with NH₄⁺-N,so as to achieve the purpose of removing ammonia nitrogen,which is called chemical precipitation method.This method has many advantages:for example,not only simple operation,rapid response,high efficiency of purification,not susceptible to impurities toxins and other factors in water,and the product can be used as fertilizer.However,the higher cost of pharmaceuticals,limited sales of fertilizers,limits the application of chemical precipitation.In this paper,based on the optimization of MAP precipitation process,the direct thermal decomposition of MAP;the thermal decomposition of MAP with Mg?OH?₂/NaOH and the effect of products on removal of NH₄⁺-N were investigated,Which includes the thermal decomposition temperature,time and pharmaceutical formulations,reaction conditions such as pH,reaction time,temperature,etc.The morphological structure and composition of the precipitated products and thermal decomposition products were analyzed by scanning electron microscopy,X-ray diffraction and infrared analysis.The removal rate of NH₄⁺-N was higher than 95%under the condition of 30 min reaction for NH₄⁺-N wastewater with initial concentration of 400 mg/L treated with n(Mg²⁺):n(PO₄^3-):n?NH₄⁺-N?=1.2:1:1,at room temperature,pH 9.3,MgCl₂ and Na₂HPO₄.Then,the thermal decomposition conditions of MAP with or without alkaline me dia were explored to obtain the best thermal decomposition conditions:direct thermal decomposition:105?,3 h;Mg?OH?₂involved:140?,2 h;NaOH Participation:100?,2 h.The results show that the main thermal decomposition products are magnesium hydrogen phosphate?MgHPO₄·3H₂O?,magnesium phosphate?Mg₃?PO₄?₂·10H₂O?,magnesium magnesium phosphate?MgNaPO₄?.Based on the influence factors of the removal of NH₄⁺-N of products under different thermal decomposition conditions,the advantages and disadvantages of three different thermal decomposition methods were compared.The removal rate of NH₄⁺-N:the third group>the second group>the first group;cycling performance:the third group>the first group>the second group;the concentration of effluent PO₄^3--P:the third group>the first group>the second group.Three groups of reactions can be used for the removal of NH₄⁺-N in waste water.From the perspective of NH₄⁺-N removal rate,the third and the second groups are better;from the perspective of recycling,the third and first are more appropriate;from the remaining PO₄^3--P concentration control,the second group is better.Choosing which solution needs to be combined the actual processing object.