Batch and semi-continuous anaerobic co-digestion of goose manure with alkali solubilized wheat straw: A case of carbon to nitrogen ratio and organic loading rate regression optimization

Influences of straw alkaline pretreatment on biogas production and digestate characteristics: artificial neural network and multivariate statistical techniques

Anaerobic digestion is one of the best options for producing valuable end products (biogas and biofertilizer). The aim of this study was to investigate the influences of thermoalkaline pretreatment of wheat straw on biogas production and digestate characteristics from codigestion with waste-activated sludge. Different alkaline conditions (NaOH, KOH and Na2CO3) and pretreatment durations (1, 3 and 5 h) were used for straw pretreatment. Batch anaerobic codigestion of sludge and pretreated straw was conducted under different pretreatment conditions. A feedforward neural network (FFNN) model, logistic model and statistical analysis were applied to the experimental data to predict biogas and investigate the significance and relationships among the variables. NaOH pretreatment for 5 h showed the best treatment conditions: biogas yield was 6.59 times higher than that without treatment. Moreover, the proportions of total solids, total volatile solids, chemical oxygen demand and microbial count removed reached 63.52%, 74.60%, 78.15% and 82.22%, respectively. The methane content was 67.50%, indicating that the biogas had a high quality. The thermoalkaline pretreatment significantly affected biogas production and digestate characteristics, allowing it to be used as a biofertilizer. Experimental data were successfully modelled for predicting biogas production using the applied models. The R2 values reached 0.985 and 0.999 for the logistic and FFNN models, respectively.

Influence of microwave temperature and power on the biomethanation of food waste under mesophilic anaerobic conditions

Food waste is an attractive feedstock for Anaerobic Digestion due to its high biodegradability and moisture content. Nevertheless, due to its complex structure and composition, methane yield is typically compromised with 50-60% of the theoretical maximum obtained. The well-known limitation of the hydrolysis step can be circumvented by adopting feedstock pre-treatments, such as microwave irradiation. It improves solubilization of various FW components making them more readily available for the microorganisms and reducing AD process duration. In this work different heating rates (7.8, 3.9 and 1.9 °C/min) and temperatures (85, 115, 145, 175 °C) were applied when pre-treating food waste as a substrate for AD. Increase in the solubilization of organic matter in the form of Soluble Chemical Oxygen Demand was the most significative change in FW characteristics after pre-treatment, with final temperature of 175 °C and heating rate of 3.9 °C showing a 73.19% increment. Nevertheless, process performance of AD of MW FW was optimum at 85 °C 7.8 ramp, showing no intermediate products accumulation, up to 77% more methane produced in the first week of digestion compared to the other conditions tested and reduction of 96.36% on the lag phase duration, compared to the control. On the other hand, samples treated at 175 °C, regardless of heating rate, consistently showed poor process performance, with low methane yield, possibly due to the formation of hard-to-digest compounds. This work underlines the importance of adjusting microwave temperature and power when pre-treating FW for biomethane production so the process is optimized.

Current development and perspectives of anaerobic bioconversion of crop stalks to Biogas: A review

As one of the most abundant biomass resources, crop stalks are great potential feedstock available for anaerobic digestion (AD) to produce biogas. However, the specific physical properties and complex chemical structures of crop stalks form strong barriers to efficient AD bioconversion. To overcome these problems, many efforts have been made over the past few years. This paper reviewed recent research in the evolving field of anaerobic bioconversion of crop stalks and was focused on three critical aspects affecting AD performance: various pretreatment methods and their effects on the improvement of crop stalk biodegradability, determination of specific AD operation parameters for crop stalks, and development of AD technologies. Finally, recommendations on the future development of crop stalk AD were proposed.

A review on anaerobic digestion with focus on the role of biomass co-digestion, modelling and optimisation on biogas production and enhancement

The status, recent trends and future perspectives in modelling and optimisation of anaerobic co-digestion is investigated. Areas that can be focused on and those which need further research towards enhancing biogas production are pointed out. Co-digestion, modelling and optimisation of anaerobic digestion as well as techno-economic aspects are reviewed in this paper. It was noted that co-digestion requires more research into a variety of bio-resources and their specific blend proportions. Modelling and optimisation of co-digestion with substrate seasonal fluctuations has not been addressed in previous studies. Controlling key process factors including temperature, pH, and carbon to nitrogen ratio is critical in improving biogas yield. Biogas hybridisation is yet to be explored in depth. The majority of researches are focused on mono-digestion, feedstock co-digestion, modelling, and optimisation of anaerobic digestion needs significant further investigations. A multi-objective approach taking all technical and economic parameters in the modelling and optimization is essential.

Monitoring of Food Waste Anaerobic Digestion Performance: Conventional Co-Substrates vs. Unmarketable Biochar Additions

This study proposed the selection of cost-effective additives generated from different activity sectors to enhance and stabilize the start-up, as well as the transitional phases, of semi-continuous food waste (FW) anaerobic digestion. The results showed that combining agricultural waste mixtures including wheat straw (WS) and cattle manure (CM) boosted the process performance and generated up to 95% higher methane yield compared to the control reactors (mono-digested FW) under an organic loading rate (OLR) range of 2 to 3 kg VS/m³·d. Whereas R3 amended with unmarketable biochar (UBc), to around 10% of the initial fresh mass inserted, showed a significant process enhancement during the transitional phase, and more particularly at an OLR of 4 kg VS/m³·d, it was revealed that under these experimental conditions, FW reactors including UBc showed an increase of 144% in terms of specific biogas yield (SBY) compared to FW reactors fed with agricultural residue. Hence, both agricultural and industrial waste were efficacious when it came to boosting either FW anaerobic performance or AD effluent quality. Although each co-substrate performed under specific experimental conditions, this feature provides decision makers with diverse alternatives to implement a sustainable organic waste management system, conveying sufficient technical details to draw up appropriate designs for the recovery of various types of organic residue.

Effects of different pretreatment methods on biogas production and microbial community in anaerobic digestion of wheat straw

The pretreatment of wheat straw has been recognized to be an essential step prior to anaerobic digestion, owing to the high abundance of lignocellulosic materials. In order to choose economical and effective techniques for the disposal of wheat straw, effects of five pretreatment methods including acid, alkali, co-pretreatment of acid and alkali, CaO₂, and liquid digestate of municipal sewage sludge on anaerobic digestion of wheat straw were investigated by analyzing biogas production and organic matter degradation in the study. The results showed that among these pretreatment methods, the methane yield was highest in the liquid digestate pretreated-wheat straw with 112.6 mL g_TS^-1, followed by the acid, alkali, and CaO₂ pretreatments, and the lowest was observed in the co-pretreatment of acid and alkali. Illumina MiSeq sequencing of the microbial communities in the anaerobic digesters revealed that the genera Ruminiclostridium including Ruminiclostridium and Ruminiclostridium 1, Hydrogenispora, and Capriciproducens were the main hydrolytic bacteria, acidogenic bacteria, and acetogenic bacteria, respectively, in the anaerobic digesters. Capriciproducens and Hydrogenispora dominated in the first and the later stages, respectively, in the anaerobic digesters, which could work as indicators of the anaerobic co-digestion stage of sludge and wheat straw. The total solid and SO₄^2--S contents of the solid digestate and the NH₄⁺-N concentration of the liquid digestate had a significant influence on the microbial community in the digesters. These findings indicated that liquid digestate pretreatment was a potential option to improve the anaerobic digestion of wheat straw, due to the low cost without additional chemical agents.

Synergistic digestion of banana pseudo-stems with chicken manure to improve methane production: Semi-continuous manipulation and microbial community analysis

This study aimed to investigate the interactions between banana pseudo-stems (BPS) and chicken manure (CM) during anaerobic co-digestion (AcoD) in batch and semi-continuous experiments. The batch experiments results showed that the methane yield was the highest (193.7 mL/g VS) in AcoD with BPS: CM ratio of 4:1, which was increased by 57.2% and 66.1%, respectively. Semi-continuous experiments revealed that AcoD resulted in higher methane production. Monitoring of the system parameters indicated that AcoD could better adapt to the increasing organic loading rate, with better system stability and methane production efficiency. The microbial analysis illustrated that AcoD increased the relative abundance of hydrolytic bacteria such as Firmicutes, Patescibacteria, and Bacteroidetes. With regard to archaea, AcoD improved the abundance of Methanosaeta, the major acetoclastic methanogens. These changes in the microbial flora allowed AcoD to remain stable while efficiently producing methane and improved the BPS and CM processing efficiency.

Combined fungal and bacterial pretreatment of wheat and pearl millet straw for biogas production - A study from batch to continuous stirred tank reactors

In present study, chitinolytic bacteria were employed to bioaugment the biogas production from fungal pretreated agricultural residues. The fungal pretreatment of wheat and pearl millet straw was done by Chaetomium globosporum. Pretreated straw were digested anaerobically at batch scale with and without the presence of chitinolytic bacteria. Contrary to untreated samples, the addition of chitionolytic bacteria with pretreated wheat and pearl millet straw provided 41 and 57% higher biogas yield. The study was further upscaled to continuous stirred tank reactors. At continuous scale too, wheat straw pretreated with Chaetomium globosporum combined with chitinolytic bacteria resulted in 16% higher biogas yield in contrast to untreated straw. Higher abundance of methanogens was detected in reactors running with pretreated wheat straw during microbial community analysis. The identified bacteria belonged mostly to Firmicutes, Bacteroidetes, Proteobacteria phyla.

Biogas Production from Anaerobic Co-Digestion of Spent Mushroom Substrate with Different Livestock Manure

Spent mushroom substrate (SMS) is defined as the biomass waste generated during industrial mushroom cultivation. Utilization of SMS has been extensively researched and has immense potential as a sustainable substrate for generating biogas that can offset fossil fuel use. This closed loop energy generation process that can be set up in mushroom plants will reduce the dependence on fossil fuels and has the potential to reduce greenhouse gas emissions, which will benefit the environment. Anaerobic co-digestion of SMS with different agricultural wastes such as livestock manure would result in enhanced biogas production. In this study, the anaerobic co-digestion of SMS was carried out by combing yellow back fungus SMS along with chicken, dairy and pig manure. SMS combined with chicken manure yielded a slightly higher cumulative methane yield when compared with the combination of dairy manure and pig manure. Factors such as the total solids (TS) and the relative ratio of manure to SMS loading had a significant impact on the cumulative methane yield, volatile solids removal, with a particularly prominent synergistic effect. The synergistic effect was also closely related to the C/N ratio, and under experimental conditions (TS = 15%, SMS relative ratio of 50% and C/N ratio = 25.6), the cumulative methane yield of SMS with chicken manure (CM) was increased by 414% compared with that obtained using SMS or CM separately. We carried out a multiple linear regression (MLR) analysis, a statistical technique that uses several explanatory variables to predict the outcome of a response variable. Our analysis concluded that by using operating conditions (TS = 15%, and SMS ratio = 38.9), we were able to achieve the maximum cumulative methane yield (CMY).

Effects of amino-modified biofilm carriers on biogas production in the anaerobic digestion of corn straw

This paper studied the property of three different biofilm carriers added into the anaerobic digestion systems, a granular activated carbon, a polyacrylonitrile, and a polyacrylonitrile modified with diethylenetriamine (PAN-NH₂). The PAN-NH₂ system kept the maximum biogas and methane production, which were 42.69% and 37.29% higher than the control system, respectively. The value of pH and chemical oxygen demand, the content of total solid and volatile solid, volatile fatty acids concentration, coenzyme F₄₂₀ concentration, and microbial community analysis were investigated during the anaerobic digestion process. The PAN-NH₂ system had the highest removal efficiency of the pollutants and regulated the pH of the system better than other systems. The result of high-throughput sequencing analysis showed that the addition of biofilm carriers and mediation with amino-groups adjusted system pH and improved biogas and CH₄ production by reducing the relative abundance of bacteria in the hydrolysis/acidogenesis stages. Methanosarcina gradually replaced other methanogens during the experimental runs and was the dominant methanogen at the end of the anaerobic digestion process.

Enhancement of biogas production from rape straw using different co-pretreatment techniques and anaerobic co-digestion with cattle manure

The present study investigated the possibility of valorizing rape straw through anaerobic digestion and the possibility of improving biomethane yield by pretreatment with H₂SO₄, combined H₂SO₄ with steam explosion (SE) and SE combined with superfine grinding (SFG). To evaluate the pretreatment method efficiency, several analytical techniques were applied. Additionally, the performance of co-digesting of cattle manure (CM) with pretreated rape straw (PRS) at different ratios was evaluated. The results showed that combined pretreatment could dissolve the lignocellulosic fiber structure, which positively stimulated methane yield. The highest cumulative CH₄ yield (CMY) of 305.7 mLg^-1VS was achieved by combined SE at 180 °C for 5 min with SFG, which was 77.84% higher than the untreated. The CMY was further improved by 11.4-59% higher than the control (CM) using co-digestion. This study confirmed that, under optimal parameters of AD, pretreatment with SEG180 could significantly boost the CMY from co-digestion of CM and PRS.

Co-treatment of potassium ferrate and peroxymonosulfate enhances the decomposition of the cotton straw and cow manure mixture

Since there is high lignocellulose content in the cotton straw and cow manure mixture (MCC), the appropriate MCC pretreatment is important to promote the anaerobic digestion (AD) hydrolysis. This study mainly explored the effect of potassium ferrate (PF) and peroxymonosulfate (PMS) pretreatments on MCC decomposition. PMS + PF co-treatment showed a higher reduction of total solid and volatile solid than PF pretreatment and PMS pretreatment. Hydrolysis of treated MCC indicated that the PF pretreatment was more effective to the release of organics than the PMS pretreatment and the PMS + PF co-treatment. However, the PMS + PF co-treatment resulted in a higher lignin removal rate (40.4%-50.5%) than the PMS pretreatment (30.8%) and the PF pretreatment (21.4%). The PMS₁ + PF₂ co-treatment (molar ratio of 1:2) acquired the optimal lignin removal rate and the release of organics among the PMS + PF co-treatment with different dosing ratio. Potential mechanism was that PF reduction products activated PMS to produce free radicals (SO₄^-, OH), which attacked lignocellulosic components and promoted MCC decomposition. The PMS₁ + PF₂ co-treatment was deduced to be the optimal pretreatment method when considering MCC decomposition, biodegradability, and mass transfer in the bioreactor.

Temperature-phased anaerobic co-digestion of food waste and paper waste with and without recirculation: Biogas production and microbial structure

Two temperature-phased anaerobic digestion (TPAD) systems (55 °C in the first reactor and 35 °C in the second reactor) with and without recirculation were operated in parallel for the co-digestion of food waste and paper waste. A long-term experiment was carried out for these two systems with the paper waste ratios elevated from 0 to 50%. The removal efficiencies of COD, TS, VS, carbohydrate and protein in the recirculated TPAD system were higher than those of the non-recirculated system. The successful acclimation of thermophilic cellulose-degrading bacteria in the first reactor (RT1), partly due to recirculation, ensured the effective degradation of cellulose when the paper waste ratio was higher than 40%, resulting in the production of large amounts of hydrogen in reactor RT1. In the absence of recirculation, the main substance produced in the first reactor of the non-recirculated system (T1) was lactic acid. This gradually led to over-acidification and a low degradation efficiency and no methane or hydrogen was produced in T1. Recirculation helped to establish a stable bacterial community capable of producing bio-hydrogen in reactor RT1. The relatively low pH of 5.5 in the RT1 inhibited the activity of hydrogenotrophic archaea without consuming hydrogen, facilitating high hydrogen production levels.

Effects of liquid digestate pretreatment on biogas production for anaerobic digestion of wheat straw

In this study, the effects of liquid digestate pretreatment of wheat straw on its biogas production efficiency and anaerobic digestion were investigated using a laboratory-scale biochemical methane potential test. Scanning electron microscope, X-ray diffraction analysis and Fourier transform infrared spectroscopy were used to analyze the changes in main compositions and physico-chemical structure of wheat straw after different pretreatment durations (3 days, 5 days, and 7 days). The results evidenced that liquid digestate pretreatment can dissolve the lignocellulosic structure, which positively promoted anaerobic biogas production. Liquid digestate pretreatment achieved a satisfactory anaerobic digestion efficiency with wheat straw as the substrate. Furthermore, the production of biogas and methane were also found to increase after pretreatment. Thus, it was concluded that a duration of 5-day liquid digestate pretreatment was optimal as it yielded 39.76% more biogas production, and 49.58% more biomethane yield, in 35.71% shorter digestion time (T₈₀) as compared to raw wheat straw. The modified Gompertz model was able to explain the kinetic behavior during anaerobic digestion of wheat straw. Based on the aforementioned experimental observations, a comparison between effects of equivalent NH₄⁺-N concentration of ammonia solution and liquid digestate pretreatment was conducted, which indicated that ammonolysis played crucial role in the enhancement of biogas production process by pretreated liquid digestate.

Biogas Production by Co-Digestion of Canteen Food Waste and Domestic Wastewater under Organic Loading Rate and Temperature Optimization

The objective of this study was to characterize biogas production performance from the co-digestion of food waste and domestic wastewater under mesophilic (35 ± 1 °C) and thermophilic (55 ± 1 °C) conditions. The food waste used as a co-substrate in this study was collected from a main canteen at the Hatyai campus of Prince of Songkla University, Songkhla Province, Thailand. The optimum co-digestion ratio and temperature conditions in a batch experiment were selected for a semi-continuous experiment. Organic loading rates (OLRs) of 0.66, 0.33, and 0.22 g volatile solid (VS) L−1 d−1 were investigated in a semi-continuous experiment by continuously stirring a tank reactor (CSTR) for biogas production. The highest biomethane potential (BMP, 0.78 ml CH4 mg−1 VS removal) was achieved with a ratio of food waste to domestic wastewater of 10:90 w/v at a mesophilic temperature. An OLR of 0.22 g VS L−1 d−1 of co-digestion yielded positive biogas production and organic removal. The findings of this study illustrate how biogas production can be used for operating feed conditions and control for anaerobic co-digestion of domestic wastewater and food waste from a university canteen.

A comprehensive review of anaerobic digestion of organic solid wastes in relation to microbial community and enhancement process

Organic solid wastes (OSWs) should be regarded as valuable resources rather than dead-end landfill waste that causes public health and odor concerns. Anaerobic digestion (AD) is an ideal approach for managing organic solid waste issues and involves using a group of anaerobic microorganisms to transform OSWs into useful products. In this review, over 100 publications related to AD of OSWs have been compiled, discussed, and analyzed. A comprehensive analysis of the environmental and safety impacts of AD, its key environmental factors, co-digestion, and pretreatment, as well as the AD of OSWs by various anaerobic microbes uncovered by high throughput sequencing-based approaches, is presented. The purpose of this review is to provide an outline of the current knowledge of AD processes from a multi-angle perspective. A comprehensive understanding of AD of OSWs and genome-enabled biology development could be helpful for providing up-to-date knowledge of AD, developing it, overcoming its drawbacks and, ultimately, improving global waste control for more efficient environmental management. © 2018 Society of Chemical Industry.

Mesophilic anaerobic co-digestion of residual sludge with different lignocellulosic wastes in the batch digester

Co-digestion of residual sludge (RS) and different lignocellulosic wastes (LWs) including greening waste (GW), decocted Chinese herbs waste (DCHW) and sugarcane bagasse waste (SCBW) was investigated in batch digester. Results show that the application of GW presented the highest specific methane yield (161 mL CH₄/g VS_added) due to its high carbohydrate fraction and more balanced C/N ratio in co-substrate mixture. Buswell equation was applied and it is found that biodegradability index (BI) for co-digestion varied from 68.1% to 74.2% (53.0% for RS mono-digestion) depending on the lignin fractions of the LWs. Variation of pH, VFAs, alkalinity and ammonia throughout the digestion were also examined. The addition of LWs induced VFAs formation, as well as their conversion to methane. The higher microbial diversity in RS/LWs co-digestion further confirmed the positive effect of LWs addition in co-digestion.

Improved process performance of the acidification phase in a two-stage anaerobic digestion of complex organic waste: Effects of an iron oxide-zeolite additive

Improvement of the acidification phase is an attractive alternative to break through the rate-limiting step in the two-phase anaerobic digestion of complex organic wastes. An additive named iron oxide-zeolite was introduced into the acidification phase at mesophilic and room temperature. By virtue of the additive supplemented, significantly improved hydrolysis/fermentation in regard to higher soluble chemical oxygen demand (sCOD) concentration (21.53-27.30%) and better lignocellulosic degradation at both temperatures has been obtained. Furthermore, an optimized volatile fatty acid formation type (more favorable acetate and less undesired propionate) has been achieved. The favorable environment of the acidogenic effluent facilitated the subsequent methanogenesis. The increased microbial community abundances of some hydrolytic, acetogenic and cellulolytic bacteria may provide the explanation on the promoted hydrolysis/acidogenesis. The results in this study suggested that supplementation of the iron oxide-zeolite into the acidification phase is a valuable alternative to improve hydrolysis/acidogenesis of the complex substrates.

Bioaugmentation strategy for enhancing anaerobic digestion of high C/N ratio feedstock with methanogenic enrichment culture

To investigate whether bioaugmentation could improve the digestion performance of high C/N ratio feedstock without co-digestion with nitrogen-rich substrate, different forms of enriched methanogenic culture were introduced to the continuous feed digesters. The performance efficiency of bioaugmentation on digestion improvement was compared. The effect of bioaugmentation on microbial community composition was revealed as well. Results demonstrated that routine bioaugmentation with liquid culture (containing the microbes and the medium remains) showed the best performance, with the organic loading rate (OLR), methane percentage, volumetric methane production (VMP) and volatile solid methane production (VSMP) higher at 1.0 g L^-1 d^-1, 24%, 0.22 L L^-1 d^-1 and 0.23 L g^-1 VS d^-1 respectively, compared to the non-bioaugmentation control. Whole genome pyrosequencing analysis suggested that consecutive microbial consortium addition could reconstruct the methanogens community by increasing the populations of acetoclastic methanogens Methanothrix, which could accelerate the degradation of acetate and methane production.

Anaerobic digestion performance of sweet potato vine and animal manure under wet, semi-dry, and dry conditions

Sweet potato vine (SPV) is an abundant agricultural waste, which is easy to obtain at low cost and has the potential to produce clean energy via anaerobic digestion (AD). The main objectives of this study were to reveal methane production and process stability of SPV and the mixtures with animal manure under various total solid conditions, to verify synergetic effect in co-digestion of SPV and manure in AD systems, and to determine the kinetics characteristics during the full AD process. The results showed that SPV was desirable feedstock for AD with 200.22 mL/g VS_added of methane yield in wet anaerobic digestion and 12.20 L_methane/L_{working volume} in dry anaerobic digestion (D-AD). Synergistic effects were found in semi-dry anaerobic digestion and D-AD with each two mixing feedstock. In contrast with SPV mono-digestion, co-digestion with manure increased methane yield within the range of 14.34–49.11% in different AD digesters. The values of final volatile fatty acids to total alkalinity (TA) were below 0.4 and the values of final pH were within the range of 7.4–8.2 in all the reactors, which supported a positive relationship between carbohydrate hydrolysis and methanogenesis during AD process. The mathematical modified first order model was applied to estimate substrate biodegradability and methane production potential well with conversion constant ranged from 0.0003 to 0.0953 1/day, which indicated that co-digestion increased hydrolysis efficiency and metabolic activity. This work provides useful information to improve the utilization and stability of digestion using SPV and livestock or poultry manure as substrates.

Effect of organic loading rate on anaerobic co-digestion of rice straw and pig manure with or without biological pretreatment

In this study, rice straw (RS) and pig manure (PM) mixtures with or without bio-pretreatment were used as the substrates and digested in a 9 L of anaerobic reactor at Organic loading rates (OLRs) of 0.4-3.1 kg COD/(m³ d). The volumetric methane production rate (VMPR), methane yield and anaerobic stability were comparatively investigated. The results showed the co-anaerobic digestion processes of RS and PM mixture after biological pretreatment were very stable at OLRs of 0.4-2.5 kg COD/(m³ d), and its optimal VMPR and methane yield could reach 0.64 L CH₄/(L d) and 0.4557 L CH₄/g COD_removed at OLR of 2.5 kg COD/(m³ d), which were 62.4% and 37.8% higher than those of the control under the same OLR condition. This study indicated the biological pretreatment with a cellulolytic microbial consortium own great potential in improving the methane yield and productivity of RS and PM wastes.

Different organic loading rates on the biogas production during the anaerobic digestion of rice straw: A pilot study

The aim of this work was to investigate the mesophilic methane fermentation of rice straw at different organic loading rates (OLRs) in a 300m³ bioreactor. It was found that biogas production increased when the OLR was below 2.00kg VS_substrate/(m³·d). The average volumetric biogas production reached 0.86m³/(m³·d) at an OLR of 2.00kg VS_substrate/(m³·d). Biogas production rate was 323m³/t dry rice straw over the whole process. The pH, chemical oxygen demand, volatile fatty acid, and NH₄⁺-N concentrations were all in optimal range at different OLRs. High-throughput sequencing analysis indicated that Firmicutes, Fibrobacteres, and Spirochaetes predominated in straw samples. Chloroflexi, Proteobacteria, and Planctomycetes were more abundant in the slurry. The hydrogenotrophic pathway was the main biochemical pathway of methanogenesis in the reactor. This study provides new information regarding the OLR and the differences in the spatial distribution of specific microbiota in a rice straw biogas plant.