筑波大学University of Tsukuba博士（環境学）Doctor of Philosophy in Environmental Studies2021【要旨】thesi

Zhao, Jiamin

ザオ, ジャミン

赵, 佳敏

English

Tsukuba Repository  (Tulips-R)

    氏   名          赵 佳敏 (Zhao Jiamin)      学位の種類          博 士（環境学）     学位記番号          博 甲 第  10119  号     学位授与年月日      令和 3 年 9 月 24 日     学位授与の要件      学位規則第４条第１項該当     審査研究科      生命環境科学研究科      学位論文題目  Biogas Recirculation-based Anaerobic Digestion for Sustainable Management of Sewage Sludge    (持続可能な下水汚泥マネジメントのためのバイオガスリサキュレーション法を用いた下水汚泥の嫌気性消化)      主査             筑波大学教授      博士（農学）  張 振亜     副査       筑波大学准教授     工学博士    雷 中方     副査       筑波大学准教授     博士（農学）  清水 和哉     副査       筑波大学准教授     博士（学術）  水野谷 剛      論  文  の  要  旨 Abstract of thesis     Anaerobic digestion (AD) of sewage sludge can simultaneously achieve energy recovery, sludge stabilization and reduction, pathogens removal, and odor emissions reduction that can ease its final and safe disposal. However, such challenges as low AD efficiency, high sludge dewatering cost, and low heating values of the produced biogas are pending for more research works on this process. Therefore, the sustainable and cost-effective management of sewage sludge is still crucial to significantly improve its energy utilization efficiency and safe disposal of treated sludge. Besides, biogas recirculation as a mixing mode of the digester is an effective means for simultaneous biogas production and upgrading, and it is to some extent beneficial for sludge or microbial self-aggregation. The objective of this work was to develop the novel biogas recirculation-driven AD system for sewage sludge treatment, targeting biogas upgrading, sludge stabilization, and sludge aggregation simultaneously with resultant reduction in sludge management costs.     This dissertation is divided into 6 chapters. In Chapter 1, the author introduced the sewage sludge issue, AD process and its challenges. To address the enhancement on AD performance, some operation strategies were summarized. Specifically, the author proposed biogas recirculation as a promising means that can achieve not only well mixing but also biogas upgrading in addition to its application for wastewater treatment and organic solid waste treatment. At last, the author arrived at the research objectives and the framework of this research. In Chapter 2, the author investigated the effect of biogas recirculation strategy on the semi-continuous AD of sewage sludge. Results show that AD of sewage sludge under neutral pH (~7.5) condition yielded 6 - 7% higher CH4 content and 52 - 53% less H2S production, while slightly alkaline condition (pH ~8.0) achieved 14 - 15% higher CH4 content and 76 - 77% less H2S production under biogas recirculation in comparison to the control (no biogas recirculation). Endogenous free ammonia nitrogen-dissolved inorganic carbon-volatile fatty acids (FAN-DIC-VFAs) buffer system was found to regulate the pH and alkalinity in the bulk liquor, in which CO2 is a promising alternative pH buffer in this system and the carbonate buffer system showed the closed system characteristics. In Chapter 3, the author examined the effect of alkali addition strategies with or without Ca2+. Results show that high CH4 content and good sludge dewaterability could not be achieved simultaneously by adding a single NaOH in the biogas recirculation-driven AD system, but achieved by the combination of NaOH and Ca(OH)2 addition strategy. Owing to the attraction between negatively charged sludge particles and Ca2+ ions, the available Ca2+ in the former AD system may facilitate the re-flocculation and P immobilization in solid digestate, sequest partial CO2 with less CO2 emission, and bridge with some sludge flocs. In Chapter 4, the author established a novel sewage sludge treatment system, i.e., granular sludge system coupling with biogas recirculation, which is expected to maintain biological metabolisms through the dense anaerobic aggregates even under harsh conditions and improve the dewaterability of digested sludge simultaneously. Results show that biogas recirculation could enhance the CH4 production rate by 31 - 44% and shorten the lag-phase duration to 0.08 - 0.2 day with simultaneous increment of CH4 content in this granular sludge system. The reason is mainly associated with the stronger interspecies electron transfer under the biogas recirculation condition. In addition, 37 - 40% better dewaterability of the digested sludge was achieved, implying the occurrence of self-aggregation of microbial cells induced by biogas recirculation. Moreover, almost no obvious influence was noticed on the seed granules’ composition and properties. These findings suggest that the biogas recirculation-driven anaerobic granular sludge system could be a promising alternative for sewage sludge treatment, which can improve biogas quality and sludge dewaterability simultaneously. In Chapter 5, the author analyzed the underlying mechanisms, energy balance, and cost-effectiveness involved in the biogas recirculation-driven AD system. 12.6% lower net cost for sludge management was achieved by the biogas recirculation-driven AD system coupling with the combination alkali addition strategy, and 78 - 85% higher net energy recovery with no more addition of other chemicals by biogas recirculation was achieved in the novel granular sludge system, which is a promising integrated multi-purpose system for sludge treatment. Finally, in Chapter 6, the author summarized the major conclusions with future research being proposed.  審  査  の  要  旨 Abstract of assessment result     This research investigated the potentials of biogas recirculation-driven AD systems under different operation strategies for sewage sludge treatment. Biogas recirculation under slightly alkali condition could achieve biogas upgrading with higher CH4 content (up to 83 - 90%) and less H2S production during AD of sewage sludge, in which CO2 is a promising alternative pH buffer that reflects the characteristics of a closed carbonate buffer system. The comparison was also conducted on the AD performance and sludge properties among the different alkali addition strategies with or without Ca2+. Biogas upgrading and better sludge dewaterability were achieved simultaneously in this novel system in addition to 12% higher P immobilization in the solid digestate, lower CO2 emission potential from the liquid digestate, and 13% lower net cost for sludge management. Sludge self-aggregation with 37 - 40% better sludge dewaterability was also realized in the novel granular sludge system coupling with biogas recirculation, which is expected to achieve efficient post-treatment of digested sludge with no additional chemicals addition. Still, more attempts are necessary on the optimization of its operation strategies before being applied in wastewater treatment plants in practice.     The final examination committee conducted a meeting as a final examination on 12 July, 2021. The applicant provided an overview of the dissertation, addressed questions and comments raised during Q & A session. All of the committee members reached a final decision that the applicant has passed the final examination.      Therefore, the final examination committee approved that the applicant is qualified to be awarded the degree of Doctor of Philosophy in Environmental Studies. 

持続可能な下水汚泥マネジメントのためのバイオガスリサキュレーション法を用いた下水汚泥の嫌気性消化

https://tsukuba.repo.nii.ac.jp/record/2002049/files/A10119.pdf

持続可能な下水汚泥マネジメントのためのバイオガスリサキュレーション法を用いた下水汚泥の嫌気性消化

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