Research On Membrane Treatment Of Low Concentration Ammonia Nitrogen Wastewate

In this paper,the wastewater discharged from the production of wet-process phosphoric acid in a chemical plant in Guizhou,in which the content of ammonia nitrogen is about 30 mg/L,which belongs to low concentration ammonia nitrogen wastewater containing phosphorus and fluorine.It is treated by chemical precipitation pretreatment method and membrane separation method,in order to establish a reasonable process route to remove phosphorus,fluorine,and ammonia nitrogen substances in the wastewater,make it meet the discharge requirements in the comprehensive wastewater discharge standard（GB 8978-1996）,and solve the problem of wastewater discharge and comprehensive utilization.Firstly,the double alkali pretreatment method is used to remove most of the phosphorus containing substances and fluoride in the water,and remove most of the easy scaling ions（phosphate,fluoride,and calcium salts）in the raw water,in order to reduce the membrane pollution in the subsequent treatment process.After calcium oxide sodium carbonate treatment,the phosphorus removal rate is 99.98%and the fluorine removal rate is 79.64%,in which the phosphorus and fluorine contents meet the sewage discharge standard,and the total hardness of the wastewater is reduced from256 mg/L to 8.45 mg/L,but the residual ammonia nitrogen changes little.In addition,XRD and EDS were used to analyze the components of precipitated sludge,mainly calcium carbonate,a small amount of calcium hydroxyphosphate,and magnesium hydroxide.Secondly,the combined process of sand filtration and ultrafiltration was selected to remove insoluble impurities such as floating matter and suspended matter in water,and the influence of operating conditions in the process of sand filtration and ultrafiltration on effluent turbidity was explored.The results show that the optimum technological conditions of sand filtration are as follows:the particle size range of quartz sand is 0.38-0.58 mm,and the filtration rate is 1800 L·m^-2·h^-1,the filling height of the sand filter column is 140-180 mm;The best process condition of ultrafiltration:the inlet flow is 40 L/h,and the differential pressure is selected as 0.025 MPa.After sand filtration ultrafiltration treatment,the average turbidity of effluent is 0.25 NTU,and the removal rate of turbidity is more than 99%.Then,vacuum membrane distillation was used to remove ammonia nitrogen from wastewater.Various factors affecting ammonia removal rate and permeation flux were studied experimentally.The experimental results show that the p H of the raw material has the greatest influence on the ammonia removal rate.In acidic environment,heating is not conducive to the removal of ammonia nitrogen.Increasing the vacuum degree of permeation side and increasing the inlet flow rate can effectively increase the ammonia removal rate,while the influence of membrane aperture on the ammonia removal rate is not obvious.Under the experimental conditions（the feed p H is 4,the temperature is75℃,the vacuum degree is 0.09 MPa and the feed flow is 40 L/h）,VMD system running for two hours,ammonia nitrogen allowance in water can be reduced to 2 mg/L,ammonia nitrogen removal rate can reach 93.33%,the maximum osmotic flux can reach51 kg·（m²·h）^-1.In addition,the membrane fouling under long-time operation and the change of flux before and after cleaning were also studied to determine the antifouling ability of the selected membrane.Finally,based on the vacuum membrane distillation experiment,the mass transfer model equation is established by using the Knudsen diffusion model.The permeate flux calculated by the model fits well with the experimental value,which can better reflect and predict the permeation flux under different operating conditions.The permeation flux can be increased by optimizing process parameters and adjusting membrane modules,so as to provide a theoretical basis for subsequent large-scale application.