Study On Treatment Of High Concentration Nitrate Wastewater By Solventing-out Crystallization

Nitrate does not harm the human,but it can produce nitrite inside the body,which is not conducive to human health.The methods for removing nitrate from water mainly include chemical,biological and physical chemical methods.Ion exchange and reverse osmosis,which belong to physical and chemical methods,produce high concentrated wastewater when nitrate is removed.At present,the biological method is mainly used to treat high concentrated wastewater,but organic carbon source is added in the process of biological treatment.If the dosage is excessive,the secondary pollution will be caused,if the dosage is small,the denitrification will be incomplete.Moreover,the amount of sludge generated during the reaction process is large,and the subsequent treatment is troublesome,which has become a difficult problem to be solved urgently in the field of high nitrate and high salt wastewater treatment at home and abroad.Compared with the above biological method,the solventing-out crystallization method is unique for recycling inorganic salts in wastewater because of its advantages of low operating temperature,low energy consumption,recycling agent can be recycled,and does not cause secondary pollution.In this paper,in view of the high concentration nitrate wastewater discharged by reverse osmosis or ion exchange process,the solventing-out crystallization technique was proposed for nitrate nitrogen(NO3--N)removal in wastewater.The main purpose of this paper is to explore the feasibility of the solventing-out crystallization process for the treatment of high concentration nitrate wastewater.This paper studies the solid-liquid equilibrium of sodium nitrate in anhydrous ethanol-water solution.In this paper,the effects of multiple factors in the solution(anhydrous ethanol dosage,initial concentration of solution,Ca2+ concentration,temperature difference,solution pH and stirring rate)on the removal rate of NO3--N in the high-concentration nitrate wastewater by solventing-out crystallization with anhydrous ethanol as the solvent were studied.In this paper,Plackett-Burman designs and Box-Behnken experiments were carried out to determine the optimum technological conditions for NO3--N removal.And in this paper,the effect of multi-stage solventing-out crystallization on NO3--N removal rate was investigated.The main conclusions of the paper are as follows:1.According to the solubility data of sodium nitrate in anhydrous ethanol-water system with different mass ratios at different temperatures and using the solubility empirical model(C*=a/b+xc)to correlate the solubility data.This study laid a foundation for exploring the feasibility of using anhydrous ethanol as dissolving agent to treat high concentration nitrate wastewater2.By studying the effects of various factors in solution:the dosage of absolute ethanol,the initial concentration of solution,Ca2+concentration,temperature difference,solution pH and stirring rate on NO3--N removal rate.It was found that the addition of absolute ethanol,initial concentration,Ca2+ concentration and temperature difference all contribute to the removal of NO3--N.The removal of NO3--N can be facilitated by keeping proper alkalinity in the solution.The removal rate of NO3--N in the solution has no obvious relationship with the stirring speed in the dissolution process3.In order to improve the removal rate of NO3--N in solution,the Plackett-Burman designs and Box-Behnken designs were carried out for various factors in solution.According to the results of the Plackett-Burman designs,we can know that the order of influencing factors on the removal rate of NO3--N in solution was absolute ethanol dosage>initial concentration>Ca2+ concentration>temperature difference>pH>stirring rate.According to the results of the Box-Behnken designs,we can know that the optimum technological conditions.When the dosage of absolute ethanol is 80 mL,the initial concentration is 49.40 g·L-1,and the Ca2+ concentration is 0.84 g·L-1,the removal rate of NO3--N in the solution can reach 63.09%.4.The removal rate of NO3--N in residual solution can be improved by using multi-stage solventing-out crystallization process.