基于膜蒸馏的沼液资源化处理研究进展

沼液可占湿法厌氧发酵后发酵剩余物总质量的80%以上,在农田土地承载量和运输成本的双重限制条件下,大型沼气工程的沼液很难通过还田利用的方式进行完全消纳。对沼液实行资源化处理既能减少沼液体积和降低对环境的潜在威胁,还可实现高附加值的资源回收,促进可持续的农业循环经济发展。作为膜分离技术中的重要分支,膜蒸馏在沼液处理过程中具有适应性强、膜污染程度低、避免发泡与快速脱氨等多方面的优势。在沼液处理与农业废弃物资源回收中具有广阔发展前景。为此,该研究从介绍膜蒸馏的基本原理出发,就膜蒸馏处理沼液过程中最核心的氨氮与水分回收部分进行详细的综述,并针对沼液处理过程中的营养物质回收与减量化处理进行了综合分析,最后对膜蒸馏用于沼气工程中的可行性进行简要计算。相比于其他沼液处理技术,膜蒸馏可在低成本与低碳足迹下实现沼液的资源回收与减量化处理,其处理沼液的成本与反渗透过程基本一致。在无外部能源供给的沼气工程中,膜蒸馏更适用于高有机负荷沼液处理,或对反渗透后剩余的高浓度沼液进行处理。

Research progress of biogas slurry resourceful treatment by membrane distillation

Abstract: Biogas slurry can account for more than 80% of the total mass of anaerobic digestates in biogas production. A large amount of biogas slurry has posed a great challenge on the carrying capacity of farmland and transportation cost. Particularly, returning to the farmland cannot completely consume such a great amount of incurred biogas slurry in a large-scale plant. The resourceful treatment is widely expected to reduce the volume of biogas slurry, and the potential threat to the agro-ecological environment for high value-added resource recovery in the sustainable development of the agricultural circular economy. For instance, membrane distillation serves as an important branch of membrane separation available for the resourceful treatment of biogas slurry in recent years. Excellent performance of membrane distillation has been achieved, including strong adaptability, rapid ammonia removal, as well as less membrane fouling and foaming. However, the high heat consumption and low flux have confined to the more efficient application of membrane distillation, compared with other technologies of membrane separation. In this study, a special process of membrane distillation was firstly introduced to systematically review the ammonia nitrogen and water recovery from biogas slurry. Water can normally be recovered from the acidified biogas slurry, while the nutrients were retained, including nitrogen, phosphorus, and potassium in the concentration phase. The water recovery can also be promoted, because the acidified biogas slurry can be utilized to suppress the ammonia volatilization, while relieving the membrane fouling. Typical reverse and forward osmosis concentrated the biogas slurry up to about 5 times than before, meaning that about 20% concentrated biogas slurry was left. The thermal-driven membrane distillation can even be used for the resourceful treatment of concentrated biogas slurry after reverse osmosis, where little biogas slurry was left. Nevertheless, membrane distillation presented a relatively low water flux for water recovery, compared with the typical reverse osmosis. Conversely, ammonia can be recovered from the biogas slurry, and then serve as ammonium fertilizer or aqueous ammonia solution for CO2 absorption. Consequently, the resulting biogas slurry was more suitable for agricultural utilization after ammonia removal. To date, membrane distillation behaved the highest ammonia recovery ratio of about 99%, compared with the reverse and forward osmosis. Meanwhile, the membrane used for ammonia recovery was a benefit to control the greenhouse gas emission. In addition, the multi-stage and multi-effect membrane distillation was introduced to reduce heat consumption. The reason is that the huge heat consumption can inevitably result in the high operation cost for the treatment of biogas slurry in a single membrane distillation. The heat consumption for water recovery was reduced from 2 000-3 500 kW·h/m3 to 100-200 kW·h/m3. Finally, the feasibility of membrane distillation was briefly evaluated for the biogas slurry treatment in a large-scale plant. The treatment cost of biogas slurry can even be much lower than that of a typical pressure-derived membrane process, where the heat and power were used from the Combined Heat and Power (CHP) in a biogas plant. Membrane distillation can efficiently realize resource recovery of biogas slurry in a facile, cost-saving, and environment-friendly way. Specifically, the cost of membrane distillation for biogas slurry was basically consistent with that of reverse osmosis. Consequently, membrane distillation was suitable for the treatment of high organic load or high residual concentration of biogas slurry after reverse osmosis, without any supplement of external heat source in a biogas plant.