Anaerobic digestion (AD) is one of the most widely used bioconversion technologies for renewable energy production from wet biowaste. However, such an AD system is so complicated that it is challenging to fully comprehend this process and design the operational conditions for a specific biowaste to achieve CH4-rich biogas. In this context, ensemble machine learning (ML) algorithms were employed to develop multitask models for jointly predicting the CH4 yield and content in biogas and understanding this complicated process. Based on the best ensemble model with the R 2 values of 0.82 and 0.86 for the multitask prediction of CH4 yield and content, the top three critical factors for CH4 yield/contents were identified and their interactions with process acid generation and microbial community in the AD process were comprehensively interpreted to unveil their importance on CH4 generation. Moreover, the well-developed ensemble model was integrated with an optimization algorithm to inversely design the AD process for a real-world food waste, in which the CH4 yield was as high as 468.7 mL/gVS and the calculation results were experimentally validated with relative errors of 9–16%. This work provides a creative approach to gain insights and inverse design for AD reactors, which is helpful to waste-to-energy technologists and practitioners.

Recoverable voltage loss was observed for a PEM fuel cell due to membrane chemical degradation under open circuit voltage (OCV) conditions. The anion analysis of the fuel cell effluent water, collected during both the OCV hold and voltage recovery stages, indicates that sulfate release rate is much higher during recovery than that during the OCV hold. The surge in sulfate anion release occurs simultaneously with the recovery of fuel cell voltage. It was found that the reversible voltage loss, and concurrent sulfate release rate during voltage recovery, is lower for Ce-mitigated NRE211 membrane compared to as-received membrane. The recoverable voltage loss is proposed to be mainly due to the sulfate anion generated by membrane chemical degradation adsorbing onto the platinum catalyst surface. IntroductionIt has been well known that operating at an open circuit voltage (OCV) condition can accelerate polymer electrolyte membrane chemical degradation in PEM fuel cells (1-3). This degradation leads to thinning of the membrane, eventually forming pin holes as characterized by a significant increase of hydrogen and oxygen gas crossover.Membrane decomposition generates various compounds including fluoride, sulfate, and other low-molecular-weight organic acids (3-5). Using direct gas mass spectroscopy, Teranishi et al reported the detection of a series of fragments of sulfuric acid in addition to HF, H 2 O 2 , and CO 2 in the cathode outlet gas produced during OCV operation (3). No additional high molecular weight products were observed. In the water extracts of severely degraded fuel cell MEAs, Healy et al found the existence of perfluoro(3-oxapentane)-1-sulfonic-4-carboxylic diacid through F19 NMR and mass spectroscopy analysis, which is believed to be a degradation product from the side chain of the PFSA membrane (5). Kabasawa et al investigated the impact of potential membrane chemical degradation products on PEM fuel cell performance using model compounds. Of the several compounds evaluated, including sulfuric acid, perflurocarboxylic acids and perfluorosufonic acids, sulfuric acid poses the most poisoning effect on the cathode catalyst, leading to a large mass activity reduction and performance drop (4). The impact of these chemical degradation products on fuel cell performance at practical operating conditions has rarely been systematically studied (4, 6).Kundu et al observed that the open circuit voltage loss is partially recoverable due to unintentional interruption of the OCV tests (7). The authors did not provide further mechanistic analysis for this reversible voltage decay phenomenon. However, they highlighted the necessity of understanding the difference between reversible and irreversible voltage decay modes. Sugawara et al conducted a detailed analysis on the performance decay under an open circuit voltage condition (6). Using exhaust water analysis, the loss in performance is tentatively attributed to the contamination of the catalyst surface by membrane chemical degradation products, such as sulfate anio...

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Magnetite-loaded biochar generated by gasification serves as a potential additive to facilitate electron transfer and boost methane production. In this study, the focus was placed on the mechanism of magnetite-loaded biochar-enhanced methanogens with live bacteria in real habitats, and the connection between live microorganisms and the methanogenic pathway was revealed. Magnetite-loaded biochar produced at a FeCl3-to-woodchip ratio (w/w) of 15/100 presented maximal methane production, where the daily methane yield was improved by 157% compared with the control. Magnetite accelerated the consumption of short-chain fatty acids through dissimilatory iron reduction, and it was converted into siderite and goethite after anaerobic digestion. The iron cycle coupling with the organic removal enhanced electron transfer efficiency and further transmitted electrons to promote methanogenesis. As indicated from the results, live high DNA (Atelge, M. R.; Atabani, A. E.; Banu, J. R.; Krisa, D.; Kaya, M.; Eskicioglu, C.; Kumar, G.; Lee, C.; Yildiz, Y. Ş.; Unalan, S.; et al. Fuel 2020, 270, 117494.) cells with high viability increased from 21 to 44%, which probably facilitated the electron transfer and direct interspecies electron transfer via membrane proteins. Moreover, the activities of the functional enzyme participating in the acetoclastic and hydrogenotrophic methanogenic pathways were enhanced. As revealed from the gene abundance from live microorganisms of the acetoclastic/hydrogenotrophic methanogenic pathway, the magnetite-loaded biochar facilitated the methanogenesis process, probably due to the DIET formed between the iron reduction bacteria Ruminococcaceae and methanogenesis Methanothrix and Methanosarcina by the magnetite-loaded biochar.