Key Takeaways & Executive Findings
- •• • Optimal biochar dosage of 2.5 g/L increased cumulative methane yield by 39.47% (to 11.63 L) compared to control, and by 43.30% relative to thermally hydrolyzed sludge, demonstrating significant enhancement in energy recovery. • • Methanogenic lag phase was reduced from (5.65±0.11) d to (4.33±0.12) d, indicating faster start-up and improved kinetics, which is critical for reducing reactor idle time and operational costs. • • Average VFAs during stable operation decreased from 1708 mg/L to 1033 mg/L with biochar addition, mitigating acidification risk and enhancing process stability, essential for continuous operation. • • Microbial community analysis showed enrichment of Synergistetes and Syntrophomonas, key for DIET, and Methanosarcina dominance, suggesting biochar promotes syntrophic metabolism, leading to higher methane production rates.
Abstract
Kitchen waste (KW) and excess sludge (ES) are urban biowastes with resource recovery potential, commonly treated via anaerobic digestion (AD) for methane production. However, KW mono-digestion suffers from acidification, while ES yields low methane. This study employed semi-continuous reactors to simulate practical AD, co-digesting KW and ES at a 4:1 volatile solids ratio with biochar addition (0.5, 1.0, 2.5, 5.0, 10.0 g/L). The optimal biochar dosage was 2.5 g/L, achieving cumulative biogas and methane volumes of 17.53 L and 11.63 L, respectively, representing 42.10% and 39.47% increases over the biochar-free control, and 34.45% and 43.30% enhancements relative to thermally hydrolyzed sludge. The methanogenic lag phase decreased from (5.65±0.11) d to (4.33±0.12) d. Process stability improved, with average volatile fatty acids (VFAs) during stable operation dropping from 1708 mg/L to 1033 mg/L. Microbial analysis revealed enhanced diversity and enrichment of Synergistetes and Syntrophomonas, indicating direct interspecies electron transfer (DIET) promotion. Biochar at low concentrations enhances AD by immobilizing microbes and facilitating electron transfer, while high concentrations (10 g/L) may inhibit methanogenesis due to fatty acid degradation blockage, yet total methane production remained above control. These findings demonstrate that biochar addition at 2.5 g/L effectively enhances methane production and process stability in KW-ES co-digestion.
1. Introduction
Anaerobic digestion (AD) of kitchen waste (KW) and excess sludge (ES) offers a dual benefit of waste reduction and renewable energy production. However, KW's high organic content often leads to volatile fatty acid (VFA) accumulation and system acidification, while ES's low carbon-to-nitrogen ratio results in suboptimal methane yields. Co-digestion of these substrates can balance nutrient ratios, but process instability remains a bottleneck. Biochar, a carbonaceous material, has emerged as a conductive additive to enhance AD by promoting direct interspecies electron transfer (DIET) and buffering pH. Yet, the optimal dosage and mechanistic insights in semi-continuous systems are not fully understood.
This study addresses these gaps by evaluating biochar addition in semi-continuous co-digestion of KW and ES at a 4:1 ratio. The research systematically investigates the impact of biochar dosages (0.5–10.0 g/L) on methane production, process stability, and microbial community structure. The findings identify 2.5 g/L as the optimal dosage, achieving a 39.47% increase in cumulative methane yield and a 23% reduction in lag phase. These results provide practical guidance for enhancing AD performance in full-scale applications, offering a cost-effective strategy to improve renewable energy recovery from urban organic wastes.
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WU Bowen, ZHENG Yijiang, ZHANG Tong, FENG Li, ZHANG Liqiu (2026). Biochar-Enhanced Anaerobic Co-digestion of Kitchen Waste and Excess Sludge: Performance, Stability, and Microbial Community Dynamics. Journal of Environmental Engineering Technology. https://doi.org/10.13205/j.hjgc.202607020
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Frequently Asked Questions
What is the optimal biochar dosage for maximum methane yield in co-digestion of kitchen waste and excess sludge, and what are the corresponding performance metrics?
The optimal biochar dosage is 2.5 g/L, yielding cumulative biogas and methane volumes of 17.53 L and 11.63 L, respectively. This represents a 42.10% increase in biogas and 39.47% increase in methane compared to the control without biochar. The methanogenic lag phase was reduced from (5.65±0.11) d to (4.33±0.12) d, indicating faster start-up.
How does biochar addition affect process stability in terms of pH and volatile fatty acids (VFAs) accumulation?
Biochar addition at optimal dosage (2.5 g/L) improved process stability by maintaining pH within a favorable range and reducing average VFAs during stable operation from 1708 mg/L to 1033 mg/L. This mitigates the risk of acidification, which is critical for continuous operation. However, excessive biochar (10 g/L) may lead to VFA accumulation, potentially inhibiting methanogenic activity.
What is the role of biochar in enhancing microbial community function, particularly regarding direct interspecies electron transfer (DIET)?
Biochar addition enriched syntrophic microorganisms such as Synergistetes and Syntrophomonas, which are known to participate in DIET. Additionally, Methanosarcina, a methanogen capable of DIET, became the dominant archaea. This suggests that biochar facilitates electron transfer between syntrophic bacteria and methanogens, thereby enhancing methane production. The porous structure of biochar also provides a surface for microbial attachment, reducing metabolic distances and promoting efficient electron exchange.
What are the implications of high biochar dosages (e.g., 10 g/L) on methane production and system resilience?
High biochar dosage (10 g/L) resulted in lower methane production compared to the optimal 2.5 g/L, likely due to inhibited fatty acid degradation. However, the system still produced more methane than the control, indicating that biochar provides a buffering capacity and enhances resilience against process disturbances. This suggests that even at suboptimal dosages, biochar can offer some benefits, but careful optimization is required to avoid negative effects.
How does the semi-continuous reactor mode used in this study compare to batch systems in terms of practical applicability?
Semi-continuous reactors better simulate real-world AD operations, where feedstock is added periodically. This study found that biochar addition at 2.5 g/L improved methane yield by 39.47% compared to control, which is consistent with batch studies but provides more realistic performance data. The reduced lag phase and enhanced stability observed in semi-continuous mode indicate that biochar can be effectively integrated into continuous full-scale systems, offering a practical solution for enhancing methane recovery from organic wastes.
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