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Volume 18
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10.3390/en18174692
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Article

Enhanced Methane Production in the Anaerobic Digestion of Swine Manure: Effects of Substrate-to-Inoculum Ratio and Magnetite-Mediated Direct Interspecies Electron Transfer

by
Jung-Sup Lee
1,
Tae-Hoon Kim
1,
Byung-Kyu Ahn
1,
Yun-Ju Jeon
1,
Ji-Hye Ahn
1,
Waris Khan
1,
Seoktae Kang
2,
Junho Kim
2 and
Yeo-Myeong Yun
1,*
1
Department of Environmental Engineering, Chungbuk National University, 1 Chungdae-ro, Seowon-gu, Cheongju 28644, Republic of Korea
2
Department of Civil and Environmental Engineering, Korea Advanced Institute of Science and Technology, 291 Daehak-ro, Yuseong-gu, Daejeon 34141, Republic of Korea
*
Author to whom correspondence should be addressed.
Energies 2025, 18(17), 4692; https://doi.org/10.3390/en18174692
Submission received: 15 July 2025 / Revised: 18 August 2025 / Accepted: 2 September 2025 / Published: 4 September 2025
(This article belongs to the Special Issue Advanced Technologies for Energy and Resource Recovery from Water and Wastewater)
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Abstract

Improving the anaerobic digestion (AD) of swine manure is crucial for sustainable waste-to-energy systems, given its high organic load and process instability risks. This study examined the combined effects of substrate-to-inoculum ratio (SIR, 0.1–3.2) and magnetite-mediated direct interspecies electron transfer on biogas production, effluent quality, and microbial community dynamics. The highest methane yield (262 ± 10 mL CH4/g COD) was obtained at SIR 0.1, while efficiency declined at higher SIRs due to acid and ammonia accumulation. Magnetite supplementation significantly improved methane yield (up to a 54.1% increase at SIR 0.2) and reduced the lag phase, particularly under moderate SIRs. Effluent characterization revealed that low SIRs induced elevated soluble COD (SCOD) levels, attributed to microbial autolysis and extracellular polymeric substance release. Furthermore, magnetite addition mitigated SCOD accumulation and shifted molecular weight distributions toward higher fractions (>15 kDa), indicating enhanced microbial activity and structural polymer formation. Microbial analysis revealed that magnetite-enriched Syntrophobacterium and Methanothrix promoted syntrophic cooperation and acetoclastic methanogenesis. Diversity indices and PCoA further showed that both SIR and magnetite significantly shaped microbial structure and function. Overall, an optimal SIR range of 0.2–0.4 under magnetite addition provided a balanced strategy for enhancing methane recovery, effluent quality, and microbial stability in swine manure AD.
Keywords: anaerobic digestion; swine manure; substrate-to-inoculum ratio (SIR); magnetite; direct interspecies electron transfer (DIET); microbial community anaerobic digestion; swine manure; substrate-to-inoculum ratio (SIR); magnetite; direct interspecies electron transfer (DIET); microbial community
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MDPI and ACS Style

Lee, J.-S.; Kim, T.-H.; Ahn, B.-K.; Jeon, Y.-J.; Ahn, J.-H.; Khan, W.; Kang, S.; Kim, J.; Yun, Y.-M. Enhanced Methane Production in the Anaerobic Digestion of Swine Manure: Effects of Substrate-to-Inoculum Ratio and Magnetite-Mediated Direct Interspecies Electron Transfer. Energies 2025, 18, 4692. https://doi.org/10.3390/en18174692

AMA Style

Lee J-S, Kim T-H, Ahn B-K, Jeon Y-J, Ahn J-H, Khan W, Kang S, Kim J, Yun Y-M. Enhanced Methane Production in the Anaerobic Digestion of Swine Manure: Effects of Substrate-to-Inoculum Ratio and Magnetite-Mediated Direct Interspecies Electron Transfer. Energies. 2025; 18(17):4692. https://doi.org/10.3390/en18174692

Chicago/Turabian Style

Lee, Jung-Sup, Tae-Hoon Kim, Byung-Kyu Ahn, Yun-Ju Jeon, Ji-Hye Ahn, Waris Khan, Seoktae Kang, Junho Kim, and Yeo-Myeong Yun. 2025. "Enhanced Methane Production in the Anaerobic Digestion of Swine Manure: Effects of Substrate-to-Inoculum Ratio and Magnetite-Mediated Direct Interspecies Electron Transfer" Energies 18, no. 17: 4692. https://doi.org/10.3390/en18174692

APA Style

Lee, J.-S., Kim, T.-H., Ahn, B.-K., Jeon, Y.-J., Ahn, J.-H., Khan, W., Kang, S., Kim, J., & Yun, Y.-M. (2025). Enhanced Methane Production in the Anaerobic Digestion of Swine Manure: Effects of Substrate-to-Inoculum Ratio and Magnetite-Mediated Direct Interspecies Electron Transfer. Energies, 18(17), 4692. https://doi.org/10.3390/en18174692

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