Performance Study Of Ammonia Nitrogen From Biogas Slurry Using Membrane Contactor Coupled With Air Stripping

After anaerobic fermentation of organic waste,a amount of biogas slurry is generated,rich in nutrients such as nitrogen and phosphorus.The recovery of ammonia nitrogen in the biogas slurry can not only reduce the impact on the environment but also increase the economic benefit.As a common ammonia nitrogen recovery technology,membrane distillation only allows vapor to pass through membrane pores and achieves the removal of volatile substances such as ammonia nitrogen in the feed liquid.Hollow fiber membrane contactor has the advantages of large gas-liquid contact area,compact structure,convenient in fluent management,which is suitable for ammonia nitrogen recovery from biogas slurry.However,biogas slurry has a strong buffer ability.Using direct contact membrane distillation will consume a lot of alkaline chemicals.When the p H value is not adjusted,the recovery efficiency of ammonia nitrogen by single membrane distillation process will be very low.Part of dissolved CO₂can be removed by air stripping,thus increasing the p H value of biogas slurry.However,the characteristics of sweeping gas membrane distillation for the removal of CO₂from biogas slurry are not clear,and how to couple air stripping with direct contact membrane distillation needs to be further studied.In addition,it still lacks complete equipment and automatic operation control module.Therefore,in this paper,a typical commercial hollow fiber membrane contactor was used to study the ammonia nitrogen recovery performance from biogas slurry.The influence of various factors on the ammonia nitrogen removal performance was studied.In addition,the long-term stability test of the membrane contactor system was carried out to analyze the membrane pollution.The complete equipment and automatic control system for recovering biogas slurry ammonia nitrogen were constructed.The main results are as follows:（1）Under isothermal conditions,direct contact membrane distillation technology was used to treat biogas slurry with initial ammonia nitrogen concentration of 4000 mg/L,with the aim of achieving no less than 95%ammonia nitrogen recovery.The influence of main operating parameters were studied.The treatment temperature of 65℃and flow rate of 200 m L/min were required for recovery with one-stage membrane contactor;the temperature can be reduced to 55℃with two-stage membrane contactor;the flow rate can be increased to 350 m L/min with three-stage membrane contactor;the temperature can be reduced to 35℃with four-stage membrane contactor.It was worth noting that when the flow rate of biogas slurry was as low as 100 m L/min,increasing temperature can not improve the recovery rate of ammonia nitrogen.Under the same conditions,the higher concentration of ammonia nitrogen of biogas slurry,the higher recovery rate of ammonia nitrogen.When p H of biogas slurry was increased to 10,the recovery of ammonia nitrogen was better.Compared with the biogas slurry without p H adjustment,the recovery of ammonia nitrogen was increased by about 60%after the one-stage membrane contactor treatment.There was no significant difference between monoammonium phosphate solution and sulfuric acid solution as absorbing acid solution in the treatment of ammonia nitrogen,and the difference of ammonia recovery between them was less than 2%.The selection of absorbing acid solution should be combined with the subsequent nitrogen fertilizer production.The results proved the feasibility of direct contact membrane distillation for rapid recovery of biogas slurry ammonia nitrogen,and provided the basis for the setting of complete equipment and automatic control program for biogas slurry ammonia nitrogen recovery.（2）The direct contact membrane distillation technology coupled with air stripping was adopted to recover ammonia nitrogen of biogas slurry.The shell path of the sweeping gas membrane contactor and continuous cycle processing of biogas slurry were required to achieve the ideal goal of ammonia nitrogen removal of biogas slurry.Compared with the condition without air stripping,when the air stripping speed was 1 L/min,the ammonia nitrogen removal rate increased by about 23%after cyclic treatment of 100 min,and the ammonia nitrogen loss rate of biogas slurry was less than 3%.Taking 35℃as the control group,the ammonia nitrogen removal rate of biogas slurry increased by about18%after the temperature was raised to 55℃and cyclic treatment of 100 min,and continued temperature increase had no significant effect.It indicated that the biogas slurry after moderate and high temperature fermentation can directly enter the membrane contactor without additional heating after pretreating.In particular,in the process of membrane contactor coupled air stripping treatment of biogas slurry,the change of p H value of biogas slurry could be controlled within 0.5,which was beneficial to the reuse of biogas slurry or subsequent biological treatment of biogas slurry.The coupling modes of9 types of membrane contactor and air stripping were also explored.The results showed that the coupling mode of air stripping by the one-stage membrane contactor and acid absorption by the two-stage membrane contactor had a good effect on ammonia nitrogen recovery in biogas slurry,which could increase the ammonia nitrogen removal rate by28%compared with the situation without stripping.（3）The continuous operation test of ammonia nitrogen recovery in biogas slurry for30 days was carried out to investigate the long-term stability of polytf（PTFE）hollow fiber membrane.The results showed that although pollutants accumulated gradually on the membrane surface after 10 days,the ammonia recovery performance was not significantly changed by pollutants.After SEM,EDS and ATR-FTIR analysis,it was found that the main pollutants on the membrane surface included Ca CO₃,polysaccharide and other substances,and the formation factor was mainly caused by the mixed accumulation of inorganic matter and organic matter.On the basis of process stability verification,a complete set of equipment for ammonia nitrogen recovery of biogas slurry was developed step by step.the development board was used as the control element and each component was used as the actuator.When the complete set of equipment was used to recover ammonia nitrogen from biogas slurry without adjusting p H value,the recovery rate of ammonia nitrogen can be more than 90%after 4h treatment at medium temperature（35℃）,while it only took 1h to recover ammonia nitrogen at high temperature（55℃）.This paper studies the removal characteristics of ammonia nitrogen in typical direct contact distillation process,analyses the influence of main operating parameters on the recovery performance of ammonia nitrogen and obtains the basic data.In view of the high alkali consumption of membrane contactor in recovering ammonia nitrogen from biogas slurry,the membrane contactor coupled air stripping process is proposed and proved.The long-term stability of hollow fiber membrane recovering biogas slurry ammonia nitrogen has been studied and completed equipment and process construction has been completed.The sudy results can provide reference for the process and equipment of low-cost recovery of ammonia nitrogen in biogas slurry.