► Solid state anaerobic digestion (SS-AD) of eight types of lignocellulosic biomass. ► Liquid anaerobic digestion (L-AD) of eight biomass feedstocks was compared with SS-AD. ► No significant ...difference in methane yield between SS-AD and L-AD. ► Volumetric biogas productivity of SS-AD was 2- to 7-fold greater than that with L-AD. ► Methane yields from crop residues were higher than those from woody biomass.
Lignocellulosic biomass feedstocks (switchgrass, corn stover, wheat straw, yard waste, leaves, waste paper, maple, and pine) were evaluated for methane production under liquid anaerobic digestion (L-AD) and solid-state anaerobic digestion (SS-AD). No significant difference in methane yield between L-AD and SS-AD, except for waste paper and pine, were found. However, the volumetric productivity was 2- to 7-fold greater in the SS-AD system compared with the L-AD system, except for paper. Methane yields from corn stover, wheat straw, and switchgrass were 2–5 times higher than those from yard waste, maple, and pine biomass. Waste paper had a methane yield of only 15L/kg VS caused by souring during SS-AD due to organic overloading. Pine also had very low biogas yield of 17L/kg VS, indicating the need for pretreatment prior to SS-AD. The findings of this study can guide future studies to improve the efficiency and stability of SS-AD of lignocellulosic biomass.
Solid-state anaerobic digestion (SS-AD) generally occurs at solid concentrations higher than 15%. In contrast, liquid anaerobic digestion (AD) handles feedstocks with solid concentrations between ...0.5% and 15%. Animal manure, sewage sludge, and food waste are generally treated by liquid AD, while organic fractions of municipal solid waste (OFMSW) and lignocellulosic biomass such as crop residues and energy crops can be processed through SS-AD. Some advantages of SS-AD include smaller reactor capacity requirements, less energy used for heating, and no processing energy needed for stirring. Due to its lower water content, the digestate of SS-AD is much easier to handle than the effluent of liquid AD. However, SS-AD systems also have disadvantages such as larger amounts of required inocula and much longer retention time.
The principles and applications of the SS-AD process are reviewed in this paper. The variation in biogas production yields of different feedstocks is discussed as well as the need for pretreatment of lignocellulosic biomass to enhance biogas production. The effects of major operational parameters, including C/N ratio, solids content, temperature, and inoculation on the performance of SS-AD are summarized. While an increase in operating temperature can improve both the biogas yield and the production efficiency, other practices such as using AD digestate or leachate as an inoculant or decreasing the solid content, may increase the biogas yield but have negative impact on production efficiency. Different reactor configurations used in current commercial scale SS-AD systems and the impact of economics on system selection are also discussed.
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•Biochar simultaneously promoted methane production and phosphorus transformation.•300 °C and 0.075 g∙g−1 of biochar were the optimal temperature and dosage.•BC300 promoted an 18% ...increase in the proportion of active phosphorus.•Biochar may facilitate the hydrogenotrophic methanogenic pathway.•Synergistic effect of microorganisms increased the proportion of active phosphorus.
Biochar has been proved to improve methane production in high solids anaerobic co-digestion (HS-AcoD) of dewatered sludge (DS) and food waste (FW), but its potential mechanism for simultaneous methane production and phosphorus (P) transformation has not been sufficiently revealed. Results showed that the optimal preparation temperature and dosage of sludge-based biochar were selected as 300 °C and 0.075 g·g−1, respectively. Under this optimized condition, the methane production of the semi-continuous reactor increased by 54%, and the active phosphorus increased by 18%. The functional microorganisms, such as Methanosarcina, hydrogen-producing, sulfate-reducing, and iron-reducing bacteria, were increased. Metabolic pathways associated with sulfate reduction and methanogenesis, especially hydrogenotrophic methanogenesis, were enhanced, which in turn promoted methanogenesis and phosphorus transformation and release. This study provides theoretical support for simultaneously recovery of carbon and phosphorus resources from DS and FW using biochar.
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•Helical impeller improves mixing & reduces stratification vs. pitched-blade impeller.•Pitched-blade impeller diminishes bacterial diversity, no effect on methanogens.•Pitched-blade ...impeller enhances digestate dewaterability in lab reactors.•Impeller geometry – a new perspective to enhance digestate dewaterability.
The efficiency of anaerobic digestion (AD) processes is intricately tied to mixing quality. This research investigates the influence of two impeller types, namely a helical ribbon impeller (HRI) and a pitched-blade impeller (PBI), on key aspects of AD. The investigation encompassed mixing dynamics, methane production, microbial communities, and the previously unexplored impact on digestate dewaterability. Results show that agitation with the PBI exhibited stratification, with bottom layer total solids (TS) values of 3.1% for the PBI and 2.6% for the HRI. Nevertheless, methane yield remained unchanged, averaging 286 LN/kg volatile solids (VS)added. Slower mixing with the HRI achieved more uniform mixing and reduced energy requirements. Additionally, impeller type significantly affected digestate dewaterability, leading to a 3.8% increase in TS of the dewatered sludge when using the PBI. These findings highlight the importance of considering mixing not only for methane production and reduced maintenance but also for achieving optimal digestate dewaterability.
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•The combination of AC and CaO2 achieved superior anaerobic digestion performance.•AC/CaO2 induced more ROS production and bolstered metabolic activity.•AC/CaO2 promoted the formation ...of bio-aggregates.•AC/CaO2 enabled improved mutualism between facultative bacteria and methanogens.•AC/CaO2 built a robust enzymatic defense system and unique damage repair strategy.
This study examined the combination of activated carbon and magnetite with calcium peroxide in enhancing the anaerobic digestion (AD) performance of food waste (FW). The individual mechanisms of these two approaches were also clarified. The results indicated that AC/CaO2 achieved the highest specific methane yield of 434.4 mL/g VS, followed by Fe3O4/CaO2 (416.9 mL/g VS). Both were significantly higher than other groups (control, AC, Fe3O4, and CaO2 were 330.1, 341.4, 342.8, and 373.2 mL/g VS, respectively). Additionally, compared to Fe3O4/CaO2, AC/CaO2 further increased reactive oxygen species (ROS), thereby enhancing the hydrolytic acidification process. Simultaneously, the higher ROS levels of Fe3O4/CaO2 and AC/CaO2 promoted the formation of microbial aggregates and established a more robust enzymatic defense system and unique damage repair strategy. The research comparatively analyzed the synergistic mechanism of iron-based and carbon-based conductive materials with CaO2, providing new perspectives for optimizing the AD of FW.
The long-term performance of anaerobic digestion (AD) often decreases substantially when treating swine wastewater contaminated with heavy metals. However, the toxicological characteristics and ...mechanisms of continuous exposure to heavy metals under different organic loading rates (OLR) are still poorly understood. In these semi-continuous AD experiments, it was found that zinc concentrations of 40 mg/L only deteriorated the reductive environments of AD. In comparison, a concentration of 2.0 mg/L probably facilitated the reproduction of microorganisms in the operating digesters with a constant OLR of 0.51 g COD/(L·d). Nevertheless, when the OLR was increased to 2.30 g COD/(L·d), 2.0 mg/L zinc inhibited various life activities of microorganisms at the molecular level within only 10 days. Hence, even though 2.0 mg/L zinc could promote AD performances from a macroscopic perspective, it had potential inhibitory effects on AD. Therefore, this study deepens the understanding of the inhibitions caused by heavy metals on AD and the metabolic laws of anaerobic microorganisms in swine wastewater treatment. These results could be referred to for enhancing AD in the presence of zinc in practical swine wastewater treatment.
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•High organic loading rates increased toxicity of zinc on microorganisms.•20 and 40 mg/L zinc severely damaged the integrity of the cell membrane.•40 mg/L zinc caused almost complete cell membrane damage by day 70.•2.0 mg/L zinc inhibited the activities of various anaerobic microorganisms.•40 mg/L zinc severely impaired sludge structure stability.
An effective pretreatment is the first step to enhance the digestibility of lignocellulosic biomass – a source of renewable, eco-friendly and energy-dense materials – for biofuel and biochemical ...productions. This review aims to provide a comprehensive assessment on the advantages and disadvantages of lignocellulosic pretreatment techniques, which have been studied at the lab-, pilot- and full-scale levels. Biological pretreatment is environmentally friendly but time consuming (i.e. 15–40 days). Chemical pretreatment is effective in breaking down lignocellulose and increasing sugar yield (e.g. 4 to 10-fold improvement) but entails chemical cost and expensive reactors. Whereas the combination of physical and chemical (i.e. physicochemical) pretreatment is energy intensive (e.g. energy production can only compensate 80% of the input energy) despite offering good process efficiency (i.e. > 100% increase in product yield). Demonstrations of pretreatment techniques (e.g. acid, alkaline, and hydrothermal) in pilot-scale have reported 50–80% hemicellulose solubilisation and enhanced sugar yields. The feasibility of these pilot and full-scale plants has been supported by government subsidies to encourage biofuel consumption (e.g. tax credits and mandates). Due to the variability in their mechanisms and characteristics, no superior pretreatment has been identified. The main challenge lies in the capability to achieve a positive energy balance and great economic viability with minimal environmental impacts i.e. the energy or product output significantly surpasses the energy and monetary input. Enhancement of the current pretreatment techno-economic efficiency (e.g. higher product yield, chemical recycling, and by-products conversion to increase environmental sustainability) and the integration of pretreatment methods to effectively treat a range of biomass will be the steppingstone for commercial lignocellulosic biorefineries.
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•Cost-competitiveness remains the challenge for full-scale lignocellulosic biorefinery.•Pretreatment techniques have shown great efficiency in lab-scale investigations.•Combined pretreatment techniques create a synergistic effect on sugar conversion.•Recycling chemicals and enzymes from lignocellulosic pretreatment enhance cost benefits.•Pretreatment is an integral part of breakthrough lignocellulosic biofuel plants in operation.
With the development of the world economy and society, the living standards of residents have been improved, along with a large amount of food waste and carbon dioxide (CO2) emissions. In the face of ...global warming and energy shortages, food waste can be used as high-value bio-energy raw materials which is also an effective way to reduce CO2 emissions. Therefore, this paper proposes a novel anaerobic digestion and CO2 emissions efficiency analysis based on a Slacks-Based Measure integrating Data Envelopment Analysis (SBM-DEA) model to evaluate and optimize the process structure of anaerobic treatment of food waste. The total feed volume and the discharge volume of liquid digestate are taken as inputs, and the total methane (CH4) production volume is taken as the desirable output and CO2 emissions are regarded as the undesirable output to build the biogas production and CO2 emissions evaluation model during the anaerobic digestion process. Finally, the proposed method is used in the actual anaerobic digestion process. The results show that the overall efficiency values in January, April, May, and June in 2020 are higher than those in other months. At the same time, due to the optimal allocation of slack variables of inputs and undesirable outputs, the efficiency values of other inefficient anaerobic digestion days can be improved.
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•Novel anaerobic digestion and CO2 emissions efficiency analysis method is proposed.•Inefficient and efficient anaerobic digestion days of food waste treatment can be effectively distinguished.•CO2 emissions and CH4 production optimization of food waste treatment are obtained.•Production efficiency of inefficient production configuration can be improved.
Microalgal-based wastewater treatment plants are conceived as low cost and low energy consuming systems. The operation of these plants involves the management of primary sludge and microalgal ...biomass. The aim of this study is to analyse the anaerobic co-digestion of both by-products in terms of biogas production and contaminants of emerging concern removal. The co-digestion of microalgae and primary sludge (25/75% on a volatile solids basis) was investigated in continuous reactors and compared to microalgae mono-digestion at a hydraulic retention time of 20days. Results showed how the co-digestion enhanced the anaerobic digestion of microalgal biomass, since primary sludge is a more readily biodegradable substrate, which increased the methane production by 65% and reduced the risk of ammonia toxicity. Regarding the contaminants, musk fragrances (galaxolide and tonalide) and triclosan showed the highest abundance on primary sludge (0.5–25μg/g TS), whereas caffeine, methyl dihydrojasmonate and triphenyl phosphate were barely detected in both substrates (<0.1μg/g TS). The removal of these contaminants was compound-depending and ranged from no removal to up to 90%. On the whole, microalgae mono-digestion resulted in a higher removal of selected contaminants than the co-digestion with primary sludge.
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•Co-digestion of microalgae and primary sludge was evaluated in continuous reactors.•Occurrence and fate of the most common emerging organic contaminants was evaluated.•Methane yield increased by 65% with co-digestion versus microalgae mono-digestion.•An energy assessment shows that co-digestion generated 4.5-fold the energy consumed.•Removal of emerging organic contaminants ranged from no removal to 90%.
In light of the substantial global production of biomass waste, effective waste management and energy recovery solutions are of paramount importance. Hydrothermal liquefaction (HTL) and anaerobic ...digestion (AD) have emerged as innovative techniques for converting biomass waste into valuable resources. Their integration creates a synergistic framework that mitigates inherent limitations, leading to improved efficiency, enhanced product quality, and the comprehensive utilization of biomass. This review paper investigates the integration of HTL and AD, highlighting its significance and potential benefits as well as the optimal sequencing (HTL followed by AD and AD followed by HTL). The review encompasses experimental procedures, factors influencing both sequencing options, energy recovery characterizations, final product outcomes, as well as toxicological assessments and discussions on reduction. Additionally, it delves into the transition towards a circular bioeconomy and discusses the challenges and opportunities intrinsic to these processes. The findings presented in this review offer valuable insights to shape future research in this evolving field.
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•The integration of AD and HTL offers a complementary approach to waste treatment.•Our review examines both sequencing approaches: HTL followed by AD, and vice versa.•Key parameters and pretreatments were assessed to enhance overall energy recovery.•Integrating AD and HTL aligns with the circular bioeconomy for waste valorization.•Innovative methods for toxicity reduction in the HTL-AD systems were reviewed.
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