High-solids Anaerobic Digestion Research Articles

Design and optimization of a novel intestinal reactor to efficiently improve organic waste uniformity and solid content in high solid organic waste anaerobic digestion

High solid anaerobic digestion (HSAD) technology, a major treatment method for solid organic waste. Existing HSAD reactors comprise primarily vertical and horizontal reactors. Vertical reactors use high-pressure pneumatic stirring, which has high energy consumption and poor stirring performance. Horizontal reactors use long-uniaxial mechanical stirring, which occupies large land areas and has high strength requirements for stirring shaft material. In addition, during the operation of the two reactor types, the continuous liquefaction of the solid waste produces solid-liquid stratification, thus affecting mixing effects and decreasing the reactor volume rate. To solve the above problems, we designed a new simulated intestinal anaerobic digestion reactor with a natural solid-liquid separation system, short-axis mechanical stirring, and a stirring blade optimized through CFD. Use of the solid-liquid separation system increased the solid content in the reactor by 44%–55%. The operation results indicated that the homogenization of the solid waste with the optimal stirring blade was more than 50% greater than that of a conventional vertical reactor. The intestinal reactor solves the problems of poor uniformity and solid content decrease during anaerobic digestion in traditional HSAD, decreases the strength requirements for stirring shaft material, and has potential to increase urban waste treatment capacity.

Ammonia stripping by in situ biogas self-circulation to upgrade continuous thermophilic and mesophilic digestion of hydrothermal high-solid sludge

High-solid anaerobic digestion of hydrothermal sewage sludge has been developed. In order to upgrade the process by focusing on ammonia inhibition, a simply-equipped stripping system without additional alkali or heat supply was introduced by in situ biogas self-circulation. As the determined limit of total ammonia nitrogen at 1500 mg/L and 1000 mg/L for the mesophilic (MAD) and thermophilic anaerobic digestion (TAD) respectively and stripping rate at 5 L/min, continuous MAD and TAD was conducted in parallel. The stripping system successfully polished up the ammonia inhibition, and methanogenic capability of the TAD was promoted to approximately 90.0 % of the potential. Intermittent stripping mode proved usable. More frequent stripping was inevitable for the TAD as compared to the MAD. Hydraulic retention time below 20 d resulted in failure of the stripping mode due to rapid ammonia generation. Overall, this technology was practical in upgrading high-solid sludge digestion by effective ammonia control.

Machine learning for high solid anaerobic digestion: Performance prediction and optimization

Biogas production through anaerobic digestion (AD) is one of the complex non-linear biological processes, wherein understanding its dynamics plays a crucial role towards process control and optimization. In this work, a machine learning based biogas predictive model was developed for high solid systems using algorithms, including SVM, ET, DT, GPR, and KNN and two different datasets (Dataset-1:10, Dataset-2:5 inputs). Support Vector Machine had the highest accuracy (R2) of all the algorithms at 91 % (Dataset-1) and 87 % (Dataset-2), respectively. The statistical analysis showed that there was no significant difference (p = 0.377) across the datasets, wherein with less inputs, accurate results could be predicted. In case of biogas yield, the critical factors which affect the model predictions include loading rate and retention time. The developed high solid machine learning model shows the possibility of integrating Artificial Intelligence to optimize and control AD process, thus contributing to a generic model for enhancing the overall performance of the biogas plant.

Biogas slurry reflux enhances the organic loading rate of high-solid anaerobic digestion of kitchen waste by alleviating fatty acids accumulation

The inhibition of fatty acids metabolism is a key limiting factor for organic loading rate (OLR) enhancement during the high-solid anaerobic digestion (HSAD) of kitchen waste (KW). Biogas slurry reflux (BSR) has been demonstrated to enhance system stability and biogas production. However, the mechanism through which BSR modulates fatty acids metabolism, thereby increasing OLR, remains unclear. Therefore, by conducting thermodynamic evaluation and multi-omics analyses, this study explored the feasibility of BSR for enhancing OLR in HSAD and its regulatory role in fatty acids metabolism throughout a semi-continuous long-term (363-day) pilot experiment. In the first OLR increase stage, the system operated stably when the OLR was below 6.0 g·VS/L·d, and the average daily methane yield exceeded 400 mL/g VS. Moreover, functional genes related to fatty acids degradation were enriched. However, fatty acids metabolism was inhibited at an OLR of 6.0 g·VS/L·d, resulting in a noticeable accumulation of fatty acids, leading the system to the brink of collapse. Notably, the implementation of BSR promoted an OLR increase, reaching levels of 7.5 g·VS/L·d. Thermodynamic analyses demonstrated that BSR promoted the thermodynamic degradation of propionate and butyrate under equivalent OLR conditions. Particularly, BSR enhanced the enrichment of Syntrophaceticus and Syntrophomonas, thereby promoting syntrophic acetate oxidizing-hydrogenotrophic methanogenesis. Metagenome and metabolome analyses suggested that BSR promoted medium and long-chain fatty acids degradation and methanogenesis under high OLR (OLR exceeding 6.0 g·VS/L·d). These findings provide systematic insights into the regulation of fatty acids biochemical metabolism through BSR, thus promoting an increase in OLR within plug-flow HSAD.

Adding Granular Activated Carbon and Zerovalent Iron to the High-Solid Anaerobic Digestion System of the Organic Fraction of Municipal Solid Waste: Anaerobic Digestion Performance and Microbial Community Analysis.

Anaerobic digestion (AD) performance and microbial dynamics were investigated in a high-solid anaerobic digestion (HSAD) system of the organic fraction of municipal solid waste (OFMSW). 1, 5, 10, and 15% (w/w, dry weight of the OFMSW) of granular activated carbon (GAC) and zerovalent iron (ZVI) were added to the HSAD system. The results showed that adding ZVI and GAC can improve the methane yield of the OFMSW. Notably, R-(GAC + ZVI) exhibited the highest cumulative methane yield of 343.0 mL/gVS, which was 57.1% higher than that of the R-control. At the genus level, the dominant bacteria included norank_f__norank_o__MBA03, norank_f__norank_o__norank_c__norank_p__Firmicutes, Fastidiosipila, norank_f__Rikenellaceae, and Sphaerochaeta, while Methanoculleus, Methanobacterium, and Methanosarcina were the dominant archaea. The highest relative abundance of norank_f__norank_o__norank_c__norank_p__Firmicutes was 30.8% for the R-(GAC + ZVI), which was 71.4% higher than that of the R-control. The relative abundance of Methanoculleus and Methanobacterium for the R-(GAC + ZVI) and the R-control group accounted for 79.0 and 90.8% of the total archaeal abundance, respectively. Additionally, the relative abundance of Methanosarcina was 10.6% for R-(GAC + ZVI), which was higher than that of the R-control (1.1%). After the addition of GAC and ZVI, the electron transfer capacity of the HSAD system was enhanced, resulting in promoted methane production. Thus, the simultaneous addition of GAC and ZVI to the HSAD system can be an effective strategy to promote the cumulative methane yield of the OFMSW.

Super diluted mixture alkali (SDMA) pretreatment for cornstalk anaerobic Digestion: A green strategy for High-Solid biogas production

Biogas production from lignocellulose by anaerobic digestion has made significant progress lately. However, the bioconversion of lignocellulose is still hindered by its recalcitrant and heterogeneous structure. To overcome this challenge, a novel method of super diluted mixture alkali (SDMA) pretreatment was developed to enhance the biogas production from cornstalk in high-solid anaerobic digestion. The cornstalks treated with 2 g/L NH3 and 1 g/L NaOH achieved a cumulative methane yield of 241.50 mL/g VS, which was 184.65 % and 55.6 % higher than that of the untreated and 2 % NaOH treated cornstalks, respectively. The system stability was verified by a scale-up experiment in a 10 L bioreactor. Remarkably, the entire process does not generate any wastewater or chemical pollution. Moreover, the resultant biogas slurry could be directly applied as a concentrated bio-fertilizer. The findings are potentially beneficial for the development of the second-generation biogas industry from lignocellulosic biomass, which can provide both clean and economical bioenergy.

Effect of alkali pretreatment time on kitchen waste anaerobic digestion performance enhanced by alkali pretreatment combined with bentonite: performance enhancement, microbial community structure, and functional gene analysis.

Kitchen waste was mainly composed of carbohydrates, lipids, and proteins. Anaerobic digestion (AD) of kitchen waste usually occurred acidification and further deteriorated. In our previous study, alkali pretreatment combined with bentonite (AP/Be) treatment was proved to enhance high solid AD of kitchen waste. However, effects of AP time on AP/Be were not yet studied. This study investigated the effects of AP time on AP/Be treatment on enhancing high solid AD. The results showed that compared with the control group, the cumulative methane production rate could be increased by 3.30 times (149.7mL CH4/g VS) and the volatile solids (VS) reduction rate increased by 63.36%. Microbial community analysis showed that the relative abundance of Methanosarcina and Methanosaeta were increased from 6.49 and 7.83% to 47.14 and 16.39% respectively. Predictive functional analysis showed that AP/Be treatment increased the abundance of energy production and conversion, coenzyme transport, and metabolism. This study revealed the potential mechanism of AP/Be enhanced kitchen waste AD performance and AP/Be was a potential strategy to strengthen AD.

Enhancing High Solid Anaerobic Digestion of Kitchen Waste with Red Mud Addition: Performance and Microbial Community

Enhancing High Solid Anaerobic Digestion of Kitchen Waste with Red Mud Addition: Performance and Microbial Community

Metagenomes and metagenome-assembled genomes from a sequentially fed anaerobic digester treating solid organic municipal waste.

We present a data set of four metagenomes and 281 metagenome-assembled genomes describing the microbial community of a laboratory-scale high solids anaerobic digester. Our objective was to obtain information on the coding potential of the microbial community and draft genomes of the most abundant organisms in the digester.

Enhanced methane production during the anaerobic digestion of chicken manure through the addition of pristine and recovered biochar

This study investigates the impact of a 50:50 mixture of pristine and recovered biochar (CBC) on high-solids anaerobic digestion of chicken manure, comparing its performance with pristine biochar (PBC), recovered biochar (RBC), and a Control group. The experiment, conducted under mesophilic conditions using a 15% total solid content and 5% biochar dosage, showed that CBC treatment achieves the highest cumulative biogas production (314 mL g−1-VS) and the highest cumulative methane production o(112 mL g−1-VS) over 21 days. Further, digesters containing CBC exhibited the highest rate of COD removal (80.8%), surpassing digesters amended with PBC (71.1%), RBC (69.2%) and Control (57.8%). Although digesters amended with PBC showed a higher TAN adsorption capacity of 48 mg g−1 compared to the 32 mg g−1 of CBC, both TAN adsorption and the existing microbial population of 1.2 × 1014 microbes per gram, have allowed CBC to exhibit superior digester performance. These findings offer valuable insights for optimising anaerobic digestion processes and cutting operational costs in waste-to-energy projects incorporating biochar as an additive.

Comparative study of high-solid anaerobic digestion at laboratory and industrial scale – Process performance and microbial community structure

High-solid anaerobic digestion (HSD) for biogas production, compared with wet digestion, is attracting interest due to advantages such as reduced fresh water usage, improved digestate quality and potential for high organic loading rates. However, the underlying processes are not well described and evaluated for HSD. In this study, two laboratory-scale reactors (46 L) of plug-flow type were designed to simulate an industrial-scale HSD process co-digesting food waste, agricultural waste and garden residues under thermophilic conditions. Performance of the laboratory-scale HSD process under stable and disturbed conditions was compared with that in industrial-scale reactors. The results showed that the laboratory- and industrial-scale processes had similar efficiency (93 %) and VS-reduction (43 % and 41 %, respectively) and relatively similar specific methane production (339 and 366 NL CH4/kg VS, respectively). Results from tracer studies combined with chemical analyses showed no phase-separation or plug-flow behaviour along the horizontal axis in either laboratory- or industrial-scale reactors, indicating a need for further process optimisation. Analyses of microbial community structure showed high similarity between laboratory- and industrial-scale, but with some differences caused by downscaling. During the experiment, the laboratory- and industrial-scale processes both showed signs of disturbance, i.e. VFA accumulation at NH4+-N levels > 4 g/L, accompanied by a shift in microbial community structure at both scales, with significant increases in relative abundance of e.g. genera Defluviitoga and Methanothermobacter. In conclusion, this study confirmed the validity of simulating HSD at laboratory scale, thus providing valuable insights into biogas production from high-solid substrates, both in laboratory- and industrial-scale processes.

A model-based approach for evaluating the effects of sludge rheology on methane production during high solid anaerobic digestion: Focusing on mass transfer resistance

Mass transfer process is vital for biochemical reactions, which is related to sludge rheology. But this inherent interaction has not yet been established for sewage sludge anaerobic digestion (AD). This work investigated the role of sludge rheology in bio-kinetics of AD. AD of sodium acetate and microcrystalline cellulose under different total solids (TS of 6.29%, 7.63%, 9.02%, 11.06%) of digested sludge was set up to identify the role of sludge rheology. Results showed that rheological properties, e.g., viscosity and viscoelasticity, increased with the increase of TS content. These resulted in negative effect on methane production of sodium acetate with lag phase increased from 0.93 d to 1.57 d, but almost no effect was observed for microcrystalline cellulose. An extended ADM1 model was developed, which revealed that enhanced sludge rheological properties increased mass diffusion resistance and reduced uptake rate of acetate. This effect can be described explicitly by mass transfer coefficient (kla_Sac). However, from the original ADM1, this effect was indistinctly lumped in apparent half-saturation coefficient (ksac). Based on the role of sludge rheology in mass transfer and bio-kinetics, it is suggested that modeling the methane production based on TS contents and sludge rheology is possible, which should be considered in giving an insight into the micro-scale phenomena (e.g. mass transport, bio-kinetics) in high solid AD.

Iron-carbon micro-electrolysis material enhanced high-solid anaerobic digestion: Performance and microbial mechanism

Iron-carbon micro-electrolysis (ICME) materials are widely used as composite conductive materials for wastewater treatment to promote the degradation of volatile solids (VS), but there is virtually no research on their effect on high-solid anaerobic digestion (HSAD). This study aimed to investigate the effect of 10–50 g/L ICME dosages on HSAD and to reveal the microbial enhancement mechanism. The batch experiments (working volume of 400 mL) were conducted in a constant temperature water bath shaker. The results showed that the ICME materials promoted methane production by HSAD. The highest methane production rate was 297.79 mL/ g VS at a dosage of 50 g/L, which was 28% higher than that of the control. Scanning electron microscopy results showed that the ICME materials increased the attachment area of microorganisms. Further analyses of microbial community indicated that ICME materials facilitate symbiotic metabolism of syntrophic acetate-oxidizing bacteria (Synergistaceae) and hydrogenotrophic methanogens (Methanospirillum). This potentially promotes the conversion of the acetoacetic methanogenesis pathway to the syntrophic acetate oxidation and hydrogenotrophic methanogenesis pathways. This study revealed ICME material enhancement in HSAD as well as new insights into the microbial mechanisms.

Composite nanofiber membranes to enhance the performance of high solids anaerobic digestion of organic rural household waste resources

This study investigated the effect of using nanofiber membrane composites containing Prussian blue-like compound nanoparticles (PNPs) with a stainless steel mesh as the carrier (NMCs) to relieve ammonia nitrogen inhibition of rural organic household waste during high-solid (total solid content of 15 %) anaerobic digestion (AD) and increase methane production. NMCs with different PNP contents were added to high-solid AD. When NMC with 15 % PNPs was added, the concentrations of volatile fatty acids and ammonia nitrogen were lower than those of the blank control group, and the methane yield was the highest (301.31 mL/g volatile solids [VS]), at 2.8 times higher than that of the blank control group. The R2 value of the modified Gompertz equation was 0.997. During digestion, the activities of extracellular protease and coenzyme F420 were higher than those of the control group. NMCs were successfully recovered at the end of the experiment, and confocal scanning three-dimensional electron micrographs indicated that NMC with 15 % PNPs of NMCs had stronger biological activity; microbial community structure analysis indicated increased abundance of hydrogenotrophic methanogens in this group. The study provides a new solution for improving the efficiency of biogas production and the recycling of exogenously added materials.

Cold isostatic pressing-study on methane production and microbial communities in anaerobic digestion of wheat straw with high-solids

In this study, the effect of cold isostatic pressing (CIP) pretreatment on the physicochemical properties and subsequent anaerobic digestion (AD) performance of wheat straw (WS) was explored, which did not have additional chemical reagent over the whole process. The WS was subjected to CIP pretreatment by pressures from 350 to 700 MPa, respectively, while AD was carried out at mesophilic temperature with total solids (TS) 7% and 14%. The mapping indicated that the CIP pretreatment promoted the hydrolysis of WS by destroying the dense structure. The CIP pretreatment changed pH and volatile fatty acids (VFAs) in the AD system. Compared with the control group, the cumulative methane production of CIP pretreatment group with TS 14% increased by 22.07%–56.18%. The acetic acid concentration peaked on 4th day and 14th day with 3943.23 mg/L and 3815.21 mg/L, respectively, in H600, and propionic acid concentration also reached two peaks at the same time with 2014.92 mg/L and 2405.20 mg/L, respectively. It was also observed that the CIP pretreatment maintained the pH in the range of 6.44–7.40, maintaining the stability of the overall system. It was also found that CIP pretreatment increased 9 species of hydrolysis and acidogenic bacteria, and methanogen Methanosarcina, thereby facilitating the hydrolysis and methanogenic stages. Predictive functional profiling comfirmed that CIP pretreatment could promote glycolysis/gluconeogenesis, fatty acid metabolism, and fatty acid degradation, whose relative abundance increased from 1.10% to 1.13%, 0.60%–0.62%, and 0.19%–0.23%. Methanogenic capacity of the system was improved because of CIP pretreatment enhancing the accessibility of microorganisms by disrupting the microstructure of WS. The energy conversion analysis showed that this method without chemical reagent addition is feasible for high-solids anaerobic digestion.

Advanced steam explosion as thermal hydrolysis process for high solids anaerobic digestion and enhanced methane yield: Proof of concept

Sewage sludge management is one of the major issues faced by wastewater treatment plant (WWTP) operators. Though anaerobic digestion (AD) is one of the most preferred sludge management techniques, the complex microbial floc structure of the sludge creates physical and chemical barriers that limit the rate of microbial degradation resulting in a longer hydraulic retention time (HRT) of 25–30 days. The thermal hydrolysis process (THP) can be used as a pretreatment technique to enhance the rate of biodegradation which results in reduced retention time and increased methane yields. This work investigated the effects of the THP on the semi-continuous anaerobic digestion of high solids sludge (15% total solids, TS) obtained from a municipal wastewater treatment plant (WWTP) operating at higher solids retention time (SRT) of 40 days. The thermal hydrolysis process achieved the effective release of organics into the liquid phase resulting in 37% chemical oxygen demand (COD) solubilization. Two semi-continuous digesters, one fed with untreated sludge (control digester, 5% TS) to mimic the conventional anaerobic digester and the other fed with THP sludge (160 °C, 30 min, 6 bar, 10% TS) were operated under a mesophilic regime and a hydraulic retention time (HRT) of 15 days. The methane yield from the THP digester (0.366 ± 0.010 m3/kg VSadded, 70 ± 1.13% CH4) was 47% higher than the control digester (0.248 ± 0.013 m3/kg VSadded). The volatile solids removal (VSR) was 51 ± 0.6% in the THP digester, i.e., 43% higher over control digester (35.7 ± 1.3%). The fecal coliforms concentration in the digestate of control and THP digesters were 9200 and 200 MPN/g, respectively. Thus, the THP sludge meets the Class A biosolids criteria (<1000MPN/g). THP integrated anaerobic digestion achieved higher solids reduction with enhanced energy recovery, high solids loading with shorter retention time, and generated Class A biosolids.

Mass Transfer Enhancement in High-Solids Anaerobic Digestion of Organic Fraction of Municipal Solid Wastes: A Review.

The increasing global population and urbanization have led to a pressing need for effective solutions to manage the organic fraction of municipal solid waste (OFMSW). High-solids anaerobic digestion (HS-AD) has garnered attention as a sustainable technology that offers reduced water demand and energy consumption, and an increased biogas production rate. However, challenges such as rheology complexities and slow mass transfer hinder its widespread application. To address these limitations, this review emphasizes the importance of process optimization and the mass transfer enhancement of HS-AD, and summarizes various strategies for enhancing mass transfer in the field of HS-AD for the OFMSW, including substrate pretreatments, mixing strategies, and the addition of biochar. Additionally, the incorporation of innovative reactor designs, substrate pretreatment, the use of advanced modeling and simulation techniques, and the novel conductive materials need to be investigated in future studies to promote a better coupling between mass transfer and methane production. This review provides support and guidance to promote HS-AD technology as a more viable solution for sustainable waste management and resource recovery.

Two-stage (liquid-solid) anaerobic mono-digestion of ammonia-rich chicken manure at low-temperature: Operational strategy and techno-economic assessment

This study investigated mono-digestion of ammonia-rich chicken manure (CM) in a high-solids anaerobic-digester (HSAD) at low-temperature (20±1°C) adopting the recirculation-percolation mode of operation using liquid-inoculum. HSAD was operated for about 282 days in batch-runs under different organic loading rates (4.3-8.7 gVS/L.d), total solids (65-70%) and total kjeldahl nitrogen (TKN) (23.3- 32.8 g/L) concentrations. It was noted that acclimation of the liquid inoculum to a high concentration of TKN (6.9 g/L) facilitated CM digestion, thereby reducing the start-up phase. The maximum cumulative biogas production rate and specific methane yield were found to be 20 L/d and 0.80±0.12 LCH4g/VSfed, respectively with a methane content over 60% without any signs of inhibition. Operating HSAD at low temperature prevented the accumulation of free-ammonia, which is toxic to methanogens. A techno-economic analysis was also conducted as part of this study to determine the profitability of the system and a 9-year tentative payback period was estimated.

Pilot-scale investigation of conductive carbon cloth amendment for enhancing high-solids anaerobic digestion and mitigating antibiotic resistance

This study examined the effectiveness of introducing conductive carbon cloth into a pilot-scale high-solids anaerobic digestion (HSAD) system. Adding carbon cloth increased methane production by 22 % and improved the maximum methane production rate by 39 %. Microbial community characterization indicated a possible direct interspecies electron transfer–based syntrophic association among microbes. Using carbon cloth also enhanced microbial richness, diversity, and evenness. Carbon cloth effectively reduced the total abundance of antibiotic resistance genes (ARGs) by 44.6 %, mainly by inhibiting horizontal gene transfer, as shown by the significant decrease in the relative abundance of integron genes (particularly intl1). The multivariate analysis further demonstrated strong correlations of intl1 with most of the targeted ARGs. These findings suggest that carbon cloth amendment can promote efficient methane production and attenuate the spread of ARGs in HSAD systems.

Insights into high-solids anaerobic digestion of food waste concomitant with sorbate: Performance and mechanisms

High-solids anaerobic digestion (HS-AD) of food waste is increasingly applied commercially. Sorbate, a food preservative extensively used in the food industry, induces potential environmental risks. Results indicated sorbate at 0–10 mg/g volatile solids (VS) slightly inhibited methane production, and the cumulative methane yield suggested a negative correlation with 25 mg/g VS sorbate, with a reduction of 15.0% compared to the control (from 285.7 to 253.6 mL CH4/g VS). The reduction in methane yield could be ascribed to the promotion of solubilization and inhibition of acidogenesis and methanogenesis with sorbate addition. Excessive sorbate (25 mg/g VS) resulted in the inhibition of aceticlastic metabolism and the key enzymes activities (e.g., acetate kinase and coenzyme F420). This study deeply elucidated the response mechanism of HS-AD to sorbate, supplemented the potential ecological risk assessment of sorbate, and could provide insights to further prevent the potential risk of sorbate in anaerobic digestion of food waste.