Effect of biogas recirculation strategy on biogas upgrading and process stability of anaerobic digestion of sewage sludge under slightly alkaline condition

Nitrogen-doped activated carbon-based steel slag composite material as an accelerant for enhancing the resilience of flexible biogas production process against shock loads: Performance, mechanism and modified ADM1 modeling

Anaerobic digestion for flexible biogas production can lead to digestion inhibition under high shock loads. While steel slag addition has shown promise in enhancing system buffering, its limitations necessitate innovation. This study synthesized the nitrogen-doped activated carbon composite from steel slag to mitigate intermediate product accumulation during flexible biogas production. Material characterization preceded experiments introducing the composite into anaerobic digestion systems, evaluating its impact on methane production efficiency under hydraulic and concentration sudden shocks. Mechanistic insights were derived from microbial community and metagenomic analyses, facilitating the construction of the modified Anaerobic Digestion Model No. 1 (ADM1) to quantitatively assess the material's effects. Results indicate superior resistance to concentration shocks with substantial increment of methane production rate up to 33.45% compared with control group, which is mediated by direct interspecies electron transfer, though diminishing with increasing shock intensity. This study contributes theoretical foundations for stable flexible biogas production and offers an effective predictive tool for conductor material reinforcement processes.

Effects of continuous low-speed biogas agitation on anaerobic digestion of high-solids pig manure: Performance and microbial community

This study aimed to investigate effects of continuous low-speed biogas agitation on the anaerobic digestion (AD) performance and microbial community of high-solids pig manure (total solids content of 10%). Our results reveal that at a biogas agitation intensity of 1.10 L/g feed VS/d, CH4 production increased by 16.67% compared to the non-agitated condition, the removal efficiency of H2S reached 63.18%, and the abundance of Methanosarcina was the highest. The presence of Hungateiclostridiaceae was associated with H2S concentrations. An increasing biogas agitation intensity led to an elevated pH and a decreased oxidation-reduction potential (ORP). Acetate concentrations, pH, and ORP values indicated changes in H2S concentrations. Sedimentibacter demonstrates the potential to indicate biogas agitation intensity and pH. We demonstrate that continuous low-speed biogas agitation effectively increases CH4 production and reduces H2S concentrations in AD of high-solids pig manure, offering a potential technical pathway for developing AD processes for high-solids pig manure, it also demonstrates that AD process can reduce the risk of pathogen and parasite transmission.

Up-flow anaerobic sludge blanket treatment of swine wastewater: Effect of heterologous and homologous inocula on anaerobic digestion performance and the microbial community

The effects of heterogenous (anaerobic sludge from treating distillery sewage, ASDS) and homologous (anaerobic sludge from treating swine wastewater, ASSW) inocula on anaerobic digestion and the microbial community in an up-flow anaerobic sludge blanket treating swine wastewater were compared. The highest chemical oxygen demand removal efficiencies with ASDS (84.8%) and ASSW (83.1%) were obtained with an organic loading rate of 15 kg COD/m3/d. For ASSW compared with ASDS, methane production efficiency was 15.3% higher and excess sludge production was 73.0% lower. The abundance of the cellulose hydrolyzing bacterium Clostridium sensu stricto_1 with ASDS (36.1%) was 1.5 times that with ASSW, while that of Methanosarcina with ASSW (22.9%) was > 100 times that with ASDS. ASDS reduced the content of pathogenic bacteria by 88.0%, while ASSW maintained a low level of pathogenic bacteria. ASSW greatly improved the methane production efficiency of wastewater and is more suitable for treating swine wastewater.

Biological nitrogen removal mechanisms during anaerobic digestion of swine manure: Effects of biogas circulation and activated carbon addition

This study investigated the biological nitrogen removal mechanisms during the anaerobic digestion of swine manure and the effects of biogas circulation and activated carbon (AC) addition. Biogas circulation, AC addition, and their combination increased the methane yield by 25.9%, 22.3%, and 44.1%, respectively, when compared to the control. Nitrogen species analysis and metagenomic results indicated that nitrification-denitrification was the dominant ammonia removal pathway in all digesters with little oxygen, while anammox did not occur. Biogas circulation could promote mass transfer and induce air infiltration to enrich nitrification- and denitrification-related bacteria and functional genes. And AC might act as an electron shuttle to facilitate ammonia removal. The combined strategies showed a synergetic effect on the enrichment of nitrification and denitrification bacteria and functional genes, significantly lowering the total ammonia nitrogen by 23.6%. A single digester with biogas circulation and AC addition could enhance methanogenesis and ammonia removal via nitrification and denitrification.

Integration of two-stage anaerobic digestion process with in situ biogas upgrading

High impurity concentration of biogas limits its wide commercial utilization. Therefore, the integration of two-stage anaerobic digestion process with in situ biogas upgrading technologies is reviewed, with emphasis on their principles, main influencing factors, research success, and technical challenges. The crucial factors that influence these technologies are pH, alkalinity, and hydrogenotrophic methanogenesis. Hence, pH fluctuation and low gas-liquid mass transfer of H₂ are some major technical challenges limiting the full-scale application of in situ upgrading techniques. Two-stage anaerobic digestion integration with various in situ upgrading techniques to form a hybrid system is proposed to overcome the constraints and systematically guide future research design and advance the development and commercialization of these techniques. This review intends to provide the current state of in situ biogas upgrading technologies and identify knowledge gaps that warrant further investigation to advance their development and practical implementation.

Propionate-cultured sludge bioaugmentation to enhance methane production and micropollutant degradation in landfill leachate treatment

This research investigates the use of propionate-cultured sludge to enhance methane (CH₄) production and micropollutant biodegradation in biochemical methane potential (BMP) experiment treating landfill leachate. The experiments were carried out using non-acclimatized and acclimatized seed sludge with variable food to microorganism ratios of 1:1 and 1:2. Under the propionate-cultured sludge bioaugmentation, the concentrations of propionate-cultured sludge were varied between 10, 20, and 30 % (v/v). The acclimatized seed sludge exhibited high microbial abundance and diversity which promoted the CH₄ production and micropollutant biodegradation. The modified Gompertz model indicated that the optimal condition was the acclimatized seed sludge with 30% (v/v) propionate-cultured sludge, achieving the lag time (λ), maximum CH₄ production rate (R_max), and maximum CH₄ potential yield (P_max) of 0.57 day, 17.35 NmL/h, and 140.58 NmL/g COD. The research novelty lies in the use of propionate-cultured sludge bioaugmentation in landfill leachate treatment to enhance CH₄ production and micropollutant biodegradation.

Mutual effects of CO2 absorption and H2-mediated electromethanogenesis triggering efficient biogas upgrading

Anaerobic digestion coupled with bioelectrochemical system (BES) is a promising approach for biogas upgrading with low energy input. However, the alkalinity generation from electromethanogenesis is invariably ignored which could serve as a potential assistant for CO₂ removal through the transformation into dissolved inorganic carbon (DIC). Herein, a novel bioelectrochemical CO₂ conversion in the methanogenic BES was proposed based on active CO₂ capture and in-situ microbial utilization. It was found that the BES using a stainless steel/carbon felt hybrid biocathode (BES-SSCF reactor) achieved a CH₄ yield of 0.33 ± 0.03 LCH₄/gCOD_removal and increased CH₄ production rate by 28.3% of BES-CF reactor at 1.0 V applied voltage. As the experiment progressed, CH₄ content increased to 93.1% and CO₂ content in the upgraded biogas maintained at below 3%. The continuous proton consumption from H₂ evolution reaction in the hybrid biocathode was capable of creating a slightly alkaline condition in the BES-SSCF reactor and thereby the CO₂ capture as bicarbonate was enhanced through endogenous alkalinity absorption. Microbial community analysis revealed that significant enrichment of Methanobacterium and Methanosarcina at the BES-SSCF cathodic biofilm was favorable for bicarbonate reduction into CH₄ via establishment of H₂-mediated electron transfer. Consequently, the remained CO₂ and DIC only accounted for 12% of total carbon in the BES-SSCF reactor and the high conversion rate of CO₂ to CH₄ (82.3%) was achieved. These results unraveled an innovative CO₂ utilization mechanism integrating CO₂ absorption with H₂-mediated electromethanogenesis.

Performance and microbial community evaluation of full-scale two-phase anaerobic digestion of waste activated sludge

"Temperature Staging and Biological Phasing" (TSBP) is an improved two-phase anaerobic digestion (AD) technology. This technology hydrolyzes waste activated sludge (WAS) at 45 °C and converts methane at mesophilic temperature (35-38 °C), with hydraulic retention times of 3-5 d and 14-17 d, respectively. In this study, the performance and microbial community dynamics of full-scale TSBP-based sludge anaerobic digestion system were studied, and the technology was evaluated by energy balance and ecological benefit analysis. The stable operation for 390 d showed that the cumulative biogas yield was about 349,041 m³, the maximum biogas yield rate was 563.68 L/kg VS, and the VS degradation rate of organic matters in the sludge was 47.19%. Proteobacteria and Firmicutes were found to be the dominant bacteria in both thermophilic and mesophilic reactors. Methanobacterium and Methanosarcina were the two most abundant methanogenic genera in the AD samples. The aceticlastic methanogenesis was likely the predominant production pathway of methane in AD processes based on metagenomics. The TSBP system operated stably, and the recovered energy could achieve energy self-sufficiency, which provided technical reference for the anaerobic treatment of sludge.

Effects of temperature and HRT on biogas production in moving and fixed bed of a novel upflow anaerobic hybrid (UAHB) reactor

The upflow anaerobic hybrid (UAHB) reactor combines the advantages of a upflow anaerobic sludge blanket (UASB-type) reactor and an anaerobic filter in a single compartment. A novel configuration of the UAHB reactor, composed of two three-phase separators (3PHS), was proposed to evaluate the biogas production in the moving and fixed bed in the treatment of synthetic sewage at a temperature range of 14-21 °C and hydraulic retention time (HRT) of 12, 10 and 8 h. The bench-scale reactor was operated in three different phases with organic loading rate (VOLR) of 0.6 (0.3-0.7), 0.7 ± 0.2, and 1.1 ± 0.1 kg COD m^-3 d^-1, respectively, for 225 days. The average removal efficiency of chemical oxygen demand (COD_t) was 78 (42-89)%, and the total biogas yield was 3090 (1704-4782) mL d^-1, with 66% of the lower 3PHS (moving bed) and 34% of the upper 3PHS (fixed bed). However, no significant difference was observed between the biogas yield on the 3PHS (p-value = 0.5048), thus confirming the influence of temperature in the biogas production. The average percentage of methane was 76 (60-82)% for both beds, and the filter media increased the production by 21%. Thus, it can be concluded that the fixed bed suppressed the instability of the moving bed regarding the biogas production and contributed to the final quality of the effluent.

Supplementation of KOH to improve salt tolerance of methanogenesis in the two-stage anaerobic digestion of food waste using pre-acclimated anaerobically digested sludge by air-nanobubble water

Air-nanobubble water (NBW) was applied to pre-acclimate anaerobically digested sludge that was then used as the inoculum in the two-stage anaerobic digestion (AD) of high saline (20 g NaCl/L) food waste (FW) to optimize NBW application in the AD of high saline FW. K⁺ was simultaneously supplemented during the methanogenic stage to resist the inhibition of salt on methanogens. Results showed that after the second pre-acclimation cycle, the inoculum activity was increased 27% in the Air-NBW supplemented reactor in comparison to the deionized water (DW) supplemented one. In the first-stage AD, H₂ yield was enhanced by 46% in the Air-NBW pre-acclimated sludge reactor compared with the DW pre-acclimated sludge reactor. Besides, supplementation of KOH in the methanogenic stage could enhance methane production by 17-25% in the DW reactors at initial pH 7.5, 8.0, and 9.0 when compared to the control reactor (using NaOH adjusted initial pH to 7.5), respectively.

A review on biogas upgrading in anaerobic digestion systems treating organic solids and wastewaters via biogas recirculation

Biogas upgrading is an essential process for efficient and safe utilization of biogas produced from anaerobic digestion (AD), a cost-effective and environmentally friendly technology for bioenergy recovery from organic wastes. Biogas recirculation in AD reactors has been recently reported as a cost-effective and promising method to enhance methane content in biogas. This review aimed to summarize the state-of-the-art of biogas recirculation-based AD systems to better understand the possible mechanisms and main factors relating to in-situ biogas upgrading. It shows that biogas recirculation in the AD reactor can not only enhance methane content via both physicochemical and biological effects, but also help establish a robust AD system with high buffering capacity for highly efficient treatment of various organic wastes. More research works are demanding for a better understanding of the mechanisms and the optimization of the whole AD system, targeting its further development for high-calorie bioenergy production.

Valorization of carbon dioxide and waste (Derived from the site of Eutrophication) into syngas using a catalytic thermo-chemical platform

Global warming increases a chance of eutrophication, and such fact offers that unhygienic organic waste materials (OWMs) in water must be treated. Hence, this study laid emphasis on the thermal-chemical (pyrolysis) process to establish a rapid valorization platform for OWMs. Indeed, OWMs were collected from the eutrophication site, and OWMs were mainly comprised of lignocellulosic biomass, microalgae (cyanobacteria) and the diverse types of bacteria (commonly observed from livestock waste). In an attempt to offer more sustainable valorization route for OWMs, CO₂ was used as a raw material in pyrolysis process. From the CO₂-assisted pyrolysis, the conversion of CO₂ and OWMs into gaseous fuel (CO) was observed. A cheap Ni-based catalyst was used in pyrolysis of OWMs as a strategic practice to promote conversion of CO₂ into CO. Indeed, syngas production (38 %) was enhanced from catalytic pyrolysis over Ni/SiO₂ under CO₂ condition as compared to inert condition (N₂).

Influence of cell lysis by Fenton oxidation on cryptic growth in sequencing batch reactor (SBR): Implication of reducing sludge source discharge

The cell lysis-cryptic growth was implemented by Fenton oxidation in sequencing batch reactor. Optimizing sludge lysis condition could maximize the release of nutrients and sludge disintegration degree. After Fenton oxidation, the extracellular polymeric substance was obviously destroyed with the sludge average particle decreased from 64 μm to 36 μm. After 5% of the settled sludge in sequencing batch reactor (SBR) was oxidized by Fenton and then returned to SBR, the mixed liquor suspended solids (MLSS) decreased by 19.3% at the end of 35 days operation, the average mixed liquor volatile suspended solids/mixed liquor suspended solids (MLVSS/MLSS) was promoted by 13.3% during the entire operation. Returning lysed sludge had no significant influence on the organics and nitrogen removal, but the total phosphorus removal was distinctly enhanced by generating FePO4 precipitate. Additionally, returning lysed sludge suppressed nitrifying bacteria and promoted denitrifying bacteria slightly. Consequently, the cell lysis-cryptic growth for reducing sludge source discharge from wastewater biological treatment could be achieved on the premise of ensuring effluent quality.

Performance and stability of biogas recirculation-driven anaerobic digestion system coupling with alkali addition strategy for sewage sludge treatment

Wastewater treatment plants are particularly challenging with the treatment and disposal of sewage sludge produced from the treatment units due to its high costs and environmental hazards. In this study, a biogas recirculation-driven anaerobic digestion (AD) system was developed with upward shear force being provided by biogas recirculation coupled with the alkali addition strategy, targeting biogas upgrading, sludge stabilization, and sludge flocculation simultaneously, thus reducing the sludge management costs. Compared to the conventional AD system, the novel biogas recirculation-driven AD system could achieve biogas upgrading with 10% higher CH₄ content. Besides, the combination of NaOH and Ca(OH)₂ addition strategy obviously improved sludge settleability and dewaterability compared to the single NaOH 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, fix partial CO₂ with less CO₂ emission, and bridge with some sludge flocs. Moreover, 12.6% lower net cost for sludge management was achieved by this biogas recirculation-driven AD system together with the combination alkali addition strategy, which is regarded as a promising integrated multi-purpose system for sludge treatment.

In situ Biogas Upgrading by CO2-to-CH4 Bioconversion

Biogas produced by anaerobic digestion is an important renewable energy carrier. Nevertheless, the high CO₂ content in biogas limits its utilization to mainly heat and electricity generation. Upgrading biogas into biomethane broadens its potential as a vehicle fuel or substitute for natural gas. CO₂-to-CH₄ bioconversion represents one cutting-edge solution for biogas upgrading. In situ bioconversion can capture endogenous CO₂ directly from the biogas reactor, is easy to operate, and provides an infrastructure for renewable electricity storage. Despite these advantages, several challenges need to be addressed to move in situ upgrading technologies closer to applications at scale. This opinion article reviews the state of the art of this technology and identifies some obstacles and opportunities of biological in-situ upgrading technologies for future development.