Enhancement Mechanism Of Ammonia Nitrogen Recovery Performance From Biogas Slurry Induced By Biomass Ash

Wet anaerobic digestion biogas project as an effective treatment of waste technology,has been mature application at home and abroad,but the process will produce abundant biogas slurry rich in ammonia nitrogen.If the biogas slurry is not properly treated,it will cause serious secondary pollution to the environment.The recovery of nutrients such as ammonia nitrogen from biogas slurry is one of the proper treatment methods,which can realize the resource utilization of biogas slurry.Membrane distillation technology can effectively remove ammonia nitrogen and volatile organic compound in biogas slurry by only allowing volatile steam to pass through the membrane.As one of the membrane distillation technologies,vacuum membrane distillation has the advantages of stable treatment effect,flexible operation,and low energy consumption,and has realized good results in the recovery of ammonia nitrogen.However,the p H value of feed liquor is a crucial factor to determine the separation performance of ammonia in the process of recovering ammonia nitrogen from biogas slurry by vacuum membrane distillation.The poor recovery performance of ammonia nitrogen in biogas slurry is due to its weak alkalinity and low p H value,so it needs to be strengthened.Because biomass ash contains a large number of alkaline metal elements,in theory can enhance the p H value of biogas slurry.Based on this,the removal of ammonia nitrogen from biogas slurry by vacuum membrane distillation（VMD）was studied by raising the initial p H value of biogas slurry using biomass ash as exogenous additive,and to discuss the feasibility of biogas slurry improved by biomass ash for ammonia removal by vacuum membrane distillation.The main results are as follows:（1）The p H value enhancement performance and mechanism of biogas slurry were studied when different kinds of biomass ash were added to the biogas slurry.The results showed that the p H value of the biogas slurry improved from 8.03 to 8.81～12.24 with the increase of biomass ash addition level at normal temperature,and the p H value improved steadily after the addition level was increased to 50 g/L.The effect of adding cotton pole ash is particularly obvious.The main reason of raising p H value of biogas slurry by OH^-is released by hydration or hydrolysis reaction of alkaline components in biomass ash after being leached from the biogas slurry,improving the p H value of biogas slurry.With the increase of the initial concentration of ammonia nitrogen,the p H value of biogas slurry was not significantly improved by the addition of biomass ash.Compared with conventional leaching,the leaching efficiency of biomass ash is better when ultrasonic-assisted leaching is used,and the leaching time can be effectively shortened.（2）After improving the p H value of biogas slurry with different exogenous additives(Na OH,Ca（OH）_2,and biomass ash),the characteristics of ammonia nitrogen recovery from biogas slurry were studied by vacuum membrane distillation.The results showed that the ammonia nitrogen removal efficiency of biogas slurry enhanced by biomass ash could reach71.74%,the ammonia separation factor was 3.39 and the total mass transfer coefficient was1.14×10^-5 m/s,which is equivalent to the addition of Na OH,it can be used to strengthen biogas slurry instead of alkaline agents.Based on the utilization of biomass ash to enhance ammonia nitrogen recovery of biogas slurry,the effects of temperature,absolute pressure,initial ammonia-nitrogen concentration,and feed flow rate on ammonia removal efficiency,ammonia flux,and total mass transfer coefficient of biogas slurry were investigated,The results show that the ammonia nitrogen removal performance is better when the initial ammonia nitrogen concentration is 2 g-N/L,the feed temperature is 70°C,the flow rate is140 m L/min and the absolute pressure is 15 k Pa.The removal efficiency of ammonia nitrogen can reach 84.55%,the ammonia flux is 61.34 g/m²h,and the total mass transfer coefficient can reach 1.59×10^-5 m/s.Under these conditions,the fouling of the membrane during the long-term operation was analyzed concerning membrane flux and ammonia removal.The results showed that the ammonia flux and the efficiency of ammonia removal decreased slightly with the increase of the operating time in the Na OH and biomass ash strengthening treatment groups,but the change was not significant in the Ca（OH）₂treatment group.By analyzing the water contact angle and SEM-EDS of the membrane for experiments,the Ca（OH）₂ treated membrane has the least influence and the surface is clean,However,when using biomass ash and Na OH for p H increase,pollutants will be deposited on the surface of the membrane in the long-term operation,thereby reducing the hydrophobicity of the membrane surface.（3）The absorption of CO₂ and the physiological toxicity of plants in the residual biogas slurry after recovery of ammonia nitrogen were studied,The results showed that the net absorption of CO₂ was 0.0654 mol/L,the net absorption amounts of CO₂ in the biogas slurry with Na OH,Ca（OH）₂ and biomass ash enhanced vacuum membrane distillation were0.1227,0.0900 and 0.0695 mol/L respectively,so it could be used for CO₂ absorption in theory.On the other hand,the content of ammonia nitrogen in biogas liquid decreased greatly after the removal of ammonia nitrogen by vacuum membrane distillation,and the germination index of biogas liquid increased greatly after the removal of ammonia nitrogen,especially the germination index of biomass ash enhanced test group was 0.84,indicating that it had no physiological toxicity to the germination of lettuce seeds.