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根据第21届联合气候变化大会通过的《巴黎气候协定》,我国提出到2030年实现“碳达峰”、2060年实现“碳中和”的“双碳”目标。在污水处理过程中,由于大量药剂,以及曝气、污泥脱水设备、水泵等的电耗非常大,因此,污水处理行业在保护水环境的同时,实际为高耗能产业。同时,一些污水处理过程还伴有CH4、N2O等温室气体排放。据估算,全球污水处理行业的整体温室气体贡献率约为1%~3%[1-2]。因此,污水处理过程的碳排放问题不可小觑。
以实现碳中和 (Carbon neutrality) 或能量自给自足(Energy self-sufficiency)为目标,多个国家对污水处理碳中和运行制定了相关政策。荷兰提出NEWs概念,将未来污水处理厂描述为“营养物 (Nutrient) ”、“能源 (Energy) ”、“再生水 (Water) ”三厂 (Factories) 合一运行的模式;新加坡国家水务局推行“NEWater”计划,并制定水行业能源自给自足的三阶段目标,其远期目标为完全实现能源自给自足,甚至向外提供能量;美国以“Carbon-free Water”为目标,期望实现对水的取用、分配、处理、排放全过程以实现碳中和;日本发布“Sewerage Vision 2100”,宣布本世纪末将完全实现污水处理过程中能源的自给自足。
而已有国家通过不同手段已实现了污水处理厂的“能量中和”或“碳中和”运行[3-8]。奥地利Strass污水处理厂利用初沉池可截留进水悬浮物 (SS) 中近60%的COD,并以A/B工艺最大化富积剩余污泥,将初沉与剩余污泥共厌氧消化并热电联产 (CHP) 后可实现108%的能源自给率[3]。美国Sheboygan污水处理厂利用厂外高浓度食品废弃物与剩余污泥厌氧共消化并热电联产实现产电量与耗电量比值达90%~115%、产热量与耗热量比值达85%~90%[4]。德国Bochum-Ölbachtal污水处理厂通过节能降耗与热电联产实现能源中和率96.9%、碳中和率63.2%[9]。德国Köhlbrandhöft/Dradenau污水处理厂通过厌氧消化与污泥干化焚烧实现能源中和率>100%,并实现42.3%的碳中和率[10]。希腊Chania污水处理厂通过厌氧消化实现70%的能源中和率,碳中和率达到58.5%[11]。德国布伦瑞克市Steinhof污水处理厂通过剩余污泥单独厌氧消化并热电联产获得79%的能源中和率,再通过补充出水农灌、污泥回田等手段额外实现了35%的碳减排量,使碳中和率高达114%[2]。芬兰Kakolanmäki污水处理厂通过热电联产与余温热能回收最终实现高达640%能源中和率与332.7%碳中和率[12-13]。
以上案例表明,为实现碳中和目标,国外污水处理厂大都采取超量有机物厌氧消化并热电联产的方案。然而,我国市政污水处理厂普遍存在碳源低下的情况,故该思路可能无法实现。这就需要全方位分析污水自身潜能及利用方式来制定适宜于我国污水处理领域的碳中和规划。在国内,基于碳中和的污水处理运行机制研究才刚起步。在技术层面,各种节能降耗、能量回收方式直接或间接补偿污水处理碳排放量似乎是实现污水处理碳中和的重要方式[4-6,9,13]。基于此,本文从能量中和与碳中和基本概念入手,梳理污水处理行业的碳减排策略,同时探讨其能量潜力、技术路径及可操作性等,以期为我国污水处理领域选择适宜的碳中和路径提供参考。
污水处理行业实现碳中和的路径及其适用条件分析
Analysis on the path and applciable conditions of carbon neutrality in wastewater treatment industry
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摘要: 节能降耗、厌氧消化产甲烷、工艺相关的能源利用等策略可有助于碳减排,但这些常规方法的潜力距碳中和目标仍有相当距离。国外诸多案例表明,污水余温热能利用技术是污水处理领域实现碳中和运行的可行方案。在总结污水处理领域碳减排策略的基础上,评价分析其对碳中和的贡献。通过对国内案例计算分析余温热能潜力并与有机 (COD) 能转化率进行比较发现,污水中蕴含的余温热能潜力为有机能的9倍。余温热能利用可使污水处理厂达到碳中和目标,还可将剩余热能 (约75%~85%) 向外以供热/制冷形式输出,或用于原位低温干化污泥,实现污水处理厂向“能源工厂”的转型。Abstract: Strategies such as energy conservation, anaerobic digestion and methanogenesis, and process-related energy use can contribute to carbon emission, but the potential of these conventional approaches is still far from carbon neutrality. Many cases abroad indicated that waste water heat energy utilization technology was a feasible scheme to achieve carbon neutrality in wastewater treatment. Based on the summary of carbon reduction strategies in the field of wastewater treatment, the contribution to carbon neutrality was evaluated and analyzed. By calculating and analyzing the waste heat energy potential of domestic cases and comparing with the conversion rate of organic energy (COD), it was found that the waste heat energy potential contained in sewage was 9 times that of organic energy. The use of residual heat energy can make the waste water treatment plants (WWTP) achieve carbon neutrality, and also export the residual heat energy (about 75% ~85%) outward for outside heating/cooling, or for in situ low temperature desiccating sludge to realize the transformation of WWTP into “energy plants”.
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