Production of energy (biodiesel) and recovery of materials (biochar) from pyrolysis of urban waste sludge

Abstract Safe disposal of sewage sludge is one of the most pressing issues in the wastewater treatment cycle: at the European Union level, sludge production is expected to reach 13 Mt by year 2020. Sludge disposal costs may constitute up to, and sometimes above, 50% of the total cost of operation of a WWTP, and contribute to over 40% of its GHGs emissions. The most common disposal options at the moment are landfilling, disposal in agriculture (about 40% EU-wide), incineration or co-incineration, and use in the industrial production of bricks, asphalts and concrete. Sewage sludge, however, still contains beneficial resources such as nutrients, that can be recovered through specific processes (e.g. precipitation as struvite) and energy, recoverable through a variety of approaches. Microwave-assisted pyrolysis of urban waste sludge was applied for the production of oil, (Syn)gas, and biochar that were afterwards characterized and compared to mainstream alternative fuels (biodiesels) and other material recovery options. Sustainability issues related to the production of biodiesel/biochars from urban wastewater treatment sludge are also discussed. The paper shows that waste urban sludge can indeed be a full component of the urban circular economy by allowing, if properly processed, recovery of energy resources at multiple levels: bio-oils (biodiesel), syngas and bio-char, all having definite advantages for final residues use and disposal. Biodiesel, in particular, allowing energy recovery as liquid fuel, offers a much more flexible and efficient utilization.

摘要 污泥的安全处置是污水处理循环领域最为紧迫的问题之一：在欧盟层面，截至2020年污泥产量预计将达13百万吨。污泥处置成本可占污水处理厂（Wastewater Treatment Plant，简称WWTP）总运营成本的50%，部分场景下甚至超出该比例，且其产生的温室气体（Greenhouse Gases，简称GHGs）排放占污水处理厂总排放量的40%以上。当前主流的污泥处置方案包括卫生填埋、农用处置（欧盟范围内占比约40%）、焚烧或协同焚烧，以及用于砖块、沥青与混凝土的工业生产环节。然而，污泥中仍留存有可通过特定工艺回收的营养物质等有益资源（例如以鸟粪石（struvite）形式沉淀回收），以及可通过多种途径回收利用的能源。本研究采用微波辅助热解工艺处理城市废弃污泥，制备得到油类、合成气（(Syn)gas）与生物炭，随后对上述产物进行表征分析，并与主流替代燃料（生物柴油）及其他物料回收方案开展对比。本文同时探讨了以城市污水处理污泥生产生物柴油/生物炭相关的可持续性问题。研究结果表明，若经过合理工艺处理，城市废弃污泥可成为城市循环经济的完整组成部分，实现多层级的能源资源回收：生物油（即生物柴油）、合成气与生物炭均在最终残余物的利用与处置方面具备明确优势。其中，生物柴油作为液体燃料实现能源回收，其利用方式更为灵活高效。