Influence of feedstock-to-inoculum ratio on performance and microbial community succession during solid-state thermophilic anaerobic co-digestion of pig urine and rice straw

Effects of long-term exposure to zinc on performances of anaerobic digesters for swine wastewater treatment under various organic loading rates

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.

Impact of using glucose as a sole carbon source to analyze the effect of biochar on the kinetics of biomethane production

The adaptation of biochar in anaerobic digestion (AD) positively influences the conversion of substrate to biomethane and promotes system stability. This study investigated the influence of biochar (BC) doses (0 to 8 g/L) on the Biochemical Methane Potential (BMP) of glucose during a 60-day AD in a mesophilic batch-type reactor. The first 6.5 weeks of the experimentation were dedicated to the microorganism's adaptation to the biochar and degradation of organics from the used inoculum (3 phases of the glucose feeding). The last 2 weeks (4th phase of glucose feeding) represented the assumption, that glucose is the sole carbon source in the system. A machine learning model based on the autoregressive integrated moving average (ARIMA) method was used to model the cumulative BMP. The results showed that the BMP increased with the amount of BC added. The highest BMP was obtained at a dose of 8 g/L, with a maximum cumulative BMP of 390.33 mL CH4/g-VS added. Likewise, the system showed stability in the pH (7.17 to 8.17). In contrast, non-amended reactors produced only 135.06 mL CH4/g-VS and became acidic at the end of the operation. Reducing the influence of carbon from inoculum, sharpened the positive effect of BC on the kinetics of biomethane production from glucose.

Dominant factors analyses and challenges of anaerobic digestion under cold environments

With the development of fermentation technology and the improvement of efficiency, anaerobic digestion (AD) has been playing an increasingly primary role in waste treatment and resource recovery. Temperature is undoubtedly the most important factor because it shapes microbial habitats, changes the composition of the microbial community structure, and even affects the expression of related functional genes. More than half of the biosphere is in a long-term or seasonal low-temperature environment (<20 °C), which makes psychrophilic AD have broad application prospects. Therefore, this review discusses the influencing factors and enhancement strategies of psychrophilic AD, which may provide a corresponding reference for future research on low-temperature fermentation. First, the occurrence of AD has been discussed. Then, the adaptation of microorganisms to the low-temperature environment was analyzed. Moreover, the challenges of psychrophilic AD have been reviewed. Meanwhile, the strategies for improving psychrophilic AD are presented. Further, from technology to application, the current situation of psychrophilic AD in pilot-scale tests is described. Finally, the economic and environmental feasibility of psychrophilic AD has been highlighted. In summary, psychrophilic AD is technically feasible, while economic analysis shows that the output benefits cannot fully cover the input costs, and the large-scale practical application of psychrophilic AD is still in its infancy. More research should focus on how to improve fermentation efficiency and reduce the investment cost of psychrophilic AD.

Influence of Pre-Incubation of Inoculum with Biochar on Anaerobic Digestion Performance

The application of biochar as an additive to enhance the anaerobic digestion (AD) of biomass has been extensively studied from various perspectives. This study reported, for the first time, the influence of biochar incubation in the inoculum on the anaerobic fermentation of glucose in a batch-type reactor over 20 days. Three groups of inoculum with the same characteristics were pre-mixed once with biochar for different durations: 21 days (D21), 10 days (D10), and 0 days (D0). The BC was mixed in the inoculum at a concentration of 8.0 g/L. The proportion of the inoculum and substrate was adjusted to an inoculum-to-substrate ratio of 2.0 based on the volatile solids. The results of the experiment revealed that D21 had the highest cumulative methane yield, of 348.98 mL, compared to 322.66, 290.05, and 25.15 mL obtained from D10, D0, and the control, respectively. Three models-modified Gompertz, first-order, and Autoregressive Integrated Moving Average (ARIMA)-were used to interpret the biomethane production. All models showed promising fitting of the cumulative biomethane production, as indicated by high R2 and low RMSE values. Among these models, the ARIMA model exhibited the closest fit to the actual data. The biomethane production rate, derived from the modified Gompertz Model, increased as the incubation period increased, with D21 yielding the highest rate of 31.13 mL/gVS. This study suggests that the application of biochar in the anaerobic fermentation of glucose, particularly considering the short incubation period, holds significant potential for improving the overall performance of anaerobic digestion.

Soil organic matter and nutrient availability affect the applicability of low-carbon energy source in Dehalococcoides-augmented soil

Dehalococcoides is a functional microorganism that completely dechlorinates trichloroethene (TCE). Augmentation with pure Dehalococcoides is important for reducing environmental disturbances that accompany bioaugmentation. However, the applicability of Dehalococcoides-bioaugmentation to contaminated soils is unclear. In this study, seven low-carbon energy sources (methanol, formate, acetate, ethanol, lactate, citrate, and benzoate) were used as electron donors for Dehalococcides to evaluate its applicability in remediating TCE-contaminated soils. Soil microcosms supplemented with ethanol, formate, or lactate showed relatively high dechlorination activity within 111-180 days. The functional gene profiles predicted by PICRUSt2 from 16 S rRNA gene sequences were similar in the proportions of dehydrogenases, which initiate electron donor oxidation, in all soils and did not seem to reflect Dehalococcoides-bioaugmentation applicability. Soils with higher organic matter content (>3.2%; dry weight base) and protein concentration (>1.6 µg/mL) supported complete dechlorination. These results suggest that organic matter and nutrient availability mainly affect successful TCE dechlorination in Dehalococcoides-augmented soils. The study offers significant experimental support for comprehending the suitability of low-carbon energy sources in successful bioaugmentation, aiming to mitigate environmental disturbances associated with the process.

Semi-solid anaerobic co-digestion of source-separated fecal slag and food waste: focusing on methane production, ecological risk assessment, and quality evaluation as fertilizer

Toilet revolution is driven by the urgent need for solutions to improve sanitation and access to high-quality organic fertilizer for rural areas, which is tagged "resource recovery from human waste." This study provides a possible solution via semi-solid anaerobic co-digestion (Aco-D) of source-separated fecal slag (SFS) and food waste (FW) (3:1). A comprehensive investigation of Aco-D at different inoculum/substrate ratios (ISR) was conducted. Results revealed that the reactor with ISR of 1:4 reached the highest methane yield (255.05 mL/gVS), which enhanced Methanosaetaceae, Methanomicrobiales, and Syntrophomonas. Additionally, the reactor with low feedstock (ISR of 1:2) showed higher removal efficiency of antibiotics (74.75%). The ecological risk of digestate decreased to an insignificant hazard quotient level, and the contents of nutrients and heavy metals were in line with the standard requirement for fertilizer. This study could serve as an alternative technology to support further research in SFS management and digestate utilization as fertilizer.

Deeper insights into the effects of substrate to inoculum ratio selection on the relationship of kinetic parameters, microbial communities, and key metabolic pathways during the anaerobic digestion of food waste

The substrate to inoculum ratio (S/I) is a crucial factor that affects not only the stability of the anaerobic digestion (AD) of food waste (FW) but also the methanogenic capacity of the substrate. This is of great significance for the start-up of small-scale batch reactors and the directional regulation of methanogenesi and organic acid production. Most studies have merely clarified the optimal S/I ratio for methane production and revealed the basic composition of microbial communities. However, the mechanism of microbial interactions and the metabolic pathways behind the optimal S/I ratio still remain unclear. Herein, the effects of different S/I ratios (VS basis) on the relationship of kinetic parameters, microbial communities, and metabolic pathways during the AD process of FW were holistically explored. The results revealed that high S/I ratios (4:1, 3:1, 2:1, and 1:1) were prone to irreversible acidification, while low S/I ratios (1:2, 1:3, and 1:4) were favorable for methanogenesis. Moreover, a kinetic analysis demonstrated that the methane yield of S/I = 1:3 were the highest. A bioinformatics analysis found that the diversity of bacteria and archaea of S/I = 1:3 were the most abundant, and the enrichment of Bacteroides and Synergistetes could help to establish a syntrophic relationship with hydrogenotrophic methanogens, which could aid in the fulfillment of a unique niche in the system. In contrast to the findings with the other S/I ratios, the cooperation among microbes in S/I = 1:3 was more apparent. Notably, the abundances of genes encoding key enzymes involved in the methanogenesis pathway under S/I = 1:3 were all the highest. This knowledge will be helpful for revealing the influence mechanism of the ratio relationship between microorganisms and substrates on the biochemical metabolic process of anaerobic digestion, thereby providing effective guidance for the directional regulation of FW batch anaerobic reactors.

Biogas and Volatile Fatty Acid Production During Anaerobic Digestion of Straw, Cellulose, and Hemicellulose with Analysis of Microbial Communities and Functions

The anaerobic digestion efficiency and methane production of straw was limited by its complex composition and structure. In this study, rice straw (RS), cellulose, and hemicellulose were used as raw materials to study biogas production performance and changes in the volatile fatty acids (VFAs). Further, microbial communities and genetic functions were analyzed separately for each material. The biogas production potential of RS, cellulose, and hemicellulose was different, with cumulative biogas production of 283.75, 412.50, and 620.64 mL/(g·VS), respectively. The methane content of the biogas produced from cellulose and hemicellulose was approximately 10% higher than that produced from RS after the methane content stabilized. The accumulation of VFAs occurred in the early stage of anaerobic digestion in all materials, and the cumulative amount of VFAs in both cellulose and hemicellulose was relatively higher than that in RS, and the accumulation time was 12 and 14 days longer, respectively. When anaerobic digestion progressed to a stable stage, Clostridium was the dominant bacterial genus in all three anaerobic digestion systems, and the abundance of Ruminofilibacter was higher during anaerobic digestion of RS. Genetically, anaerobic digestion of all raw materials proceeded mainly via aceticlastic methanogenesis, with similar functional components. The different performance of anaerobic digestion of RS, cellulose, and hemicellulose mainly comes from the difference of composition of raw materials. Increasing the accessibility of cellulose and hemicellulose in RS feedstock by pretreatment is an effective way to improve the efficiency of anaerobic digestion. Since the similar microbial community structure will be acclimated during anaerobic digestion, there is no need to adjust the initial inoculum when the accessibility of cellulose and hemicellulose changes.

Alkaline electro-hydrolysis pretreatment of rice straw for enhanced biogas production under ambient temperature

Biogas production can supplement the renewable energy target of the world. For this abundantly available agricultural waste like wheat and rice straw can be used. Biogas generation using this waste will curb the stubble burning incidences, reduce greenhouse gases, enhance farmer’s income, and strengthen the energy security of many countries. The recalcitrant nature of rice straw is a barrier to its hydrolysis, which is a prerequisite of the anaerobic digestion (AD) process. Alkaline, electro-hydrolysis, and a combination of both pretreatment (alkaline electrohydrolysis) methods are undertaken in the present study. Batch reactors at ambient temperature were used for AD of rice straw at different inoculums to substrate ratios (ISR) of 0.5, 0.75, and 1.0 to observe the effect on biogas/methane yield. Among these, a higher amount of biogas was obtained for ISR of 0.5 in all sets of experiments. The combined pretreatment method yielded biogas of 315.9 mL/gVS (equivalent to methane of 167.4 mL/gVS). For pretreat-ed rice straw by electro-hydrolysis, alkaline, and combined (alkaline and electro-hydrolysis) respectively, the methane yields were 7.03, 18.13, and 49.82% higher than untreated rice straw. The biogas had approximately 53% of methane content. The use of rice straw for biogas production may prove a viable alternative for clean and sustainable energy. In the studies, a first-order kinetic model is found to fit better the experimental results.

Effect of hyperthermophilic pretreatment on methane and hydrogen production from garden waste under mesophilic and thermophilic conditions

Anaerobic digestion of garden waste was investigated in a two-stage process consisting of hyperthermophilic pretreatment followed by mesophilic or thermophilic fermentation. The greatest digestion performance was achieved when the substrates were first treated at 70 °C for 3 days with no inoculation, and then mixed with inoculum (anaerobic sludge) and subjected to anaerobic digestion at 55 °C. Under such conditions, the maximum methane and hydrogen yields from grass were 517 NmlCH₄/kgVS and 52 NmlH₂/kgVS, whereas the corresponding values for leaves were 421 NmlCH₄/kgVS and 23 NmlH₂/kgVS, and these figure were far greater than the yields obtained in experiments with no hyperthermophilic stage. A metagenomic analysis of hyperthermophilic environments revealed the appearance of thermophilic and hyperthermophilic bacteria showing hydrolytic activity against lignocellulosic materials, including Caldicellulosiruptor, Thermovenabulum, Thermoanaerobacter, Moorella, Tepimicrobium, Geobacillus and Thermobacillus at a genus level. A noticeable methane production in the hyperthermophilic stage could be linked to the presence of Methanothermobacter sp.

Strategies to improve solid state anaerobic bioconversion of lignocellulosic biomass: an overview

Solid state anaerobic digestion (SSAD) of lignocellulosic biomass may be attractive solution for its valorisation. Compared to liquid state anaerobic digestion (LSAD), SSAD can handle higher organic loading rates (OLR), requires a less water and smaller reactor volume. It may require lower energy demand for heating or mixing and has higher volumetric methane productivity. Besides numerous benefits of SSAD processes and progress in system design, there are still obstacles, which need to be overcome for its successful implementations. This review aims to compile the recent trends in enhancing the bioconversion of agricultural stubbles in SSAD. Several pretreatment procedures used to breaking lignin and cellulose complex, method to overcome carbon to nitrogen ratio imbalance, use of carbon-based conducting materials to enhance Volatile Fatty Acids (VFA) conversion and additives for achieving nutrient balance will be discussed in this review. Leachate recirculation and its impacts on SSAD of agricultural stubbles are also discussed.

Review on anaerobic digestion of rice straw for biogas production

India is an agrarian country producing a large amount of rice straw as an agricultural residue. These residues are burnt openly leading to severe environmental pollution and health hazards. Among several options available, anaerobic digestion of rice straw into biomethane gas and digestate is a promising technology. The current paper reviews the characteristics, principles of rice straw and the process variables (temperature, volatile fatty acids, and pH, carbon to nitrogen ratio, metal elements and organic loading rate) that affect the performance of the rice straw digestion and process strategies which may alleviate the barriers and may improve the biomethane yield. Co-digestion of rice straw with nitrogen-rich substrates is proven to be an effective way to balance the carbon to nitrogen ratio, in turn, leads to nutrient balance and enhance the biomethane yields of anaerobic co-digestion system. Moreover, pretreatment is another effective strategy; physical, chemical and biological pretreatments are reviewed in the article which improved the performance of digester. The utilisation of rice straw along with other co-substrates and appropriate pretreatment may be a recommended sustainable solution for preventing environmental and health hazards.

Perspectives on microbial community in anaerobic digestion with emphasis on environmental parameters: A systematic review

This paper review is aiming to comprehensively identify and appraise the current available knowledge on microbial composition and microbial dynamics in anaerobic digestion with focus on the interconnections between operational parameters and microbial community. We systematically searched Scopus, Web of Science, pubmed and Embase (up to August 2019) with relative keywords to identify English-language studies published in peer-reviewed journals. The data and information on anaerobic reactor configurations, operational parameters such as pretreatment methods, temperature, trace elements, ammonia, organic loading rate, and feedstock composition and their association with the microbial community and microbial dynamics were extracted from eligible articles. Of 306 potential articles, 112 studies met the present review objectives and inclusion criteria. The results indicated that both aceticlastic and hydrogenotrophic methanogenesis are dominant in anaerobic digesters and their relative composition is depending on environmental conditions. However, hydrogenotrophic methanogens are more often observed in extreme conditions due to their higher robustness compared to aceticlastic methangoens. Firmicutes and Bacteroidetes phyla are most common fermentative bacteria of the acidogenic phase. These bacteria secrete lytic enzymes to degrade organic matters and are able to survive in extreme conditions and environments due to their spores. In addition, among archaea Methanosaeta, Methanobacterium, and Methanosarcinaceae are found at high relative abundance in anaerobic digesters operated with different operational parameters. Overall, understanding the shifts in microbial composition and diversity as results of operational parameters variation in anaerobic digestion process would improve the stability and process performance.

Effect of Biochar Addition on the Microbial Community and Methane Production in the Rapid Degradation Process of Corn Straw

Anaerobic digestion with corn straw faces the problems of difficult degradation, long fermentation time and acid accumulation in the high concentration of feedstocks. In order to speed up the process of methane production, corn straw treated with sodium hydroxide was used in thermophilic (50 °C) anaerobic digestion, and the effects of biochar addition on the performance of methane production and the microbial community were analyzed. The results showed that the cumulative methane production of all treatment groups reached over 75% of the theoretical methane yield in 7 days and the addition of 4% biochar increased the cumulative methane production by 6.75% compared to the control group. The addition of biochar also decreased the number of biogas and methane production peaks from 2 to 1, and had a positive effect on shortening the digestion start-up period and reducing the fluctuation of biogas production during the digestion process. The addition of 4% biochar increased the abundance of the bacterial family Peptococcaceae throughout the digestion period, promoting the hydrolysis rate of corn straw. The dominant archaeal genus Methanosarcina was significantly more abundant at the peak stage and the end of methane production with 4% biochar added compared to the control group.

Solid-state anaerobic co-digestion of organic fraction of municipal waste and sawdust: impact of co-digestion ratio, inoculum-to-substrate ratio, and total solids

Municipal solid waste contains mainly organic wastes that can be a good source for anaerobic digestion. Solid-state anaerobic digestion is an affordable and suitable technique to mitigate the organic fraction of municipal solid waste (OFMSW). However, as the organic loading of OFMSW is high, co-digestion with other materials can improve the system's performance. This study aimed to investigate the performance of the co-digestion of OFMSW and sawdust and study the parameters affecting its performance. Based on the experiments, the optimum sawdust/OFMSW ratio was achieved 1:2 with the methane production of 0.3 L/g VS. In addition, the inoculum-to-substrate ration (I/S) was investigated at 1:4, 1:2, 1:1, 2:1 ratios. The best result was obtained at 2:1 ratio with a total methane yield of 0.28 L/g VS. The results also indicated that I/S ratios less than 1:1 led to fatty acid accumulation and acidic pH condition. The effect of total solids content on the co-digestion process was also examined in this study. According to the results, as the total solids increased, the biomethane yield decreased while the biogas content increased.

Post-treatment of dewatered digested sewage sludge by thermophilic high-solid digestion for pasteurization with positive energy output

This study investigated the possibility to use thermophilic anaerobic high solid digestion of dewatered digested sewage sludge (DDS) at a wastewater treatment plant (WWTP) as a measure to increase total methane yield, achieve pasteurization and reduce risk for methane emissions during storage of the digestate. A pilot-scale plug-flow reactor was used to mimic thermophilic post-treatment of DDS from a WWTP in Linköping, Sweden. Process operation was evaluated with respect to biogas process performance, using both chemical and microbiological parameters. Initially, the process showed disturbance, with low methane yields and high volatile fatty acid (VFA) accumulation. However, after initiation of digestate recirculation performance improved and the specific methane production reached 46 mL CH₄/g VS. Plug flow conditions were assessed with lithium chloride and the hydraulic retention time (HRT) was determined to be 19-29 days, sufficient to reach successful pasteurization. Degradation rate of raw protein was high and resulted in ammonia-nitrogen levels of up to 2.0 g/L and a 30% lower protein content in the digestate as compared to DDS. Microbial analysis suggested a shift in the methane producing pathway, with dominance of syntrophic acetate oxidation and the candidate methanogen family WSA2 by the end of the experiment. Energy balance calculations based on annual DDS production of 10000 ton/year showed that introduction of high-solid digestion as a post-treatment and pasteurization method would result in a positive energy output of 340 MWh/year. Post-digestion of DDS also decreased residual methane potential (RMP) by>96% compared with fresh DDS.

Composition-oriented estimation of biogas production from major culinary wastes in an anaerobic bioreactor and its associated CO2 reduction potential

Despite the wide applications of dry anaerobic digestion (AD), a number of fundamental issues, such as composition-oriented estimation of biogas production and CO₂ reduction potential, were not well understood yet. The objective of this study was to establish composition-oriented models for prediction of biogas production and the associated shift of microbial communities. Three important factors regarding feedstock, including loading, carbon-to-nitrogen ratio, and solid-to-liquid ratio, were found to significantly affect the biogas production. The biogas production and digestion kinetics were evaluated with the response surface methodology. The major contribution to biogas production was found to be hydrogenotrophic methanogens (82.6 ± 0.4%). The net CO₂ reduction potential was assessed from the life-cycle approach, and a substantial amount of CO₂ generation (i.e., 2.8-6.7 tonne/tonne-VS) could be reduced by AD, compared to incineration, revealing that dry AD for food waste treatment should be one of the essential practices in the portfolio of global CO₂ mitigation.

Eco-compatible biochar mitigates volatile fatty acids stress in high load thermophilic solid-state anaerobic reactors treating agricultural waste

A high concentration of accumulated volatile fatty acids (VFAs) is one of the most important factors resulting in reactor failure during solid-state anaerobic digestion. In this study, the feedstock-to-inoculum (F/I) ratio (0.5, 2, 3, 4 and 6) and the recovery method after failure (biochar addition or inoculum addition) were investigated in batch solid-state anaerobic digestion fed with rice straw and pig urine. An F/I ratio of 3 was the threshold for stable operation, while the reactors failed at F/I ratios of 4 and 6 because of high accumulated VFAs concentrations (above 30 g HAc/kg). Biochar addition (10% or 20% (wet weight) of the mixture) was as effective as inoculum addition (by adjusting the F/I ratio to 2 or 3) in promoting VFAs degradation in failed reactors within a short period (<1 day). The buffering capacity of biochar was important in promoting VFAs degradation.

Hyperthermophilic Treatment of Grass and Leaves to Produce Hydrogen, Methane and VFA-Rich Digestate: Preliminary Results

In this study, the feasibility of hydrogen and methane production from grass and leaves via hyperthermophilic anaerobic digestion was investigated. The hyperthermophilic treatment of grass at 70 °C resulted in the highest concentrations of volatile fatty acids (TVFA) and reducing sugars in the supernatant of over 21 and 6.5 g/L reported on day 3 and 4 of the experiment. In contrast, hydrolysis and acidification of leaves performed slower and with lower efficiency, as the peak concentrations of TVFA and reducing sugars were observed at the end of the process. However, the highest cumulative hydrogen and methane yields of 69.64 mLH2/gVS and 38.63 mLCH4/gVS were reported for leaves digested at 70 °C, whereas the corresponding maximum productions observed for grass were 50 mLH2/gVS and 1.98 mLCH4/gVS, respectively. A temperature increase to 80 °C hampered hydrogen and methane production and also resulted in lower yields of volatile fatty acids, reducing sugars and ammonia as compared to the corresponding values reported for 70 °C.

Methane production from the co-digestion of pig manure and corn stover with the addition of cucumber residue: Role of the total solids content and feedstock-to-inoculum ratio

This study investigated performance and stability of increasing total solids (TS) content (10-30%) and feedstock-to-inoculum (F/I) ratios (1, 2) on anaerobic co-digestion of agricultural wastes. The cumulative methane yields generally decreased with the increasing TS content except for the TS content of 30% at F/I ratio of 1 and TS content of 10% at F/I ratio of 2. This was consistent with the maximum methane production rate (Rmax) and rate of the hydrolysis (Kh) stage in reactors. The pH, VFAs and NH4+-N content were positively correlated with increasing TS contents and F/I ratios. Economic analysis results indicated the net present value generally increased with increasing TS contents and TS content of 30% at F/I ratio of 1 had the highest net present value (5.7 million US$) and internal rate of return (41.9%). This indicated solid-state anaerobic digestion was financially attractive under analyzed conditions.

Comparison of mesophilic and thermophilic methane production potential of acids rich and high-strength landfill leachate at different initial organic loadings and food to inoculum ratios

Landfill leachate (LL), which can contaminate both ground and surface water is a major global environmental issue. The aim of the present study was to investigate the biomethane potential (BMP) of a high-strength LL with low pH (5.0), high solids concentration (16%), and high organic matter (170 g/L of chemical oxygen demand (COD); 55 g/L of volatile fatty acids (VFA)) with ammonia nitrogen (NH₃-N) (17 g/L). We investigated the BMP of LL at four different initial organic loadings (IOL) of 170 g/L, 85 g/L, 42.5 g/L and 21 g/L of COD and Food to inoculum (F/I) ratios of 0.5; 1; 2 and 3 at mesophilic (35 ± 2 °C) and thermophilic temperatures (55 ± 2 °C). We found that the highest cumulative CH₄ could be obtained at an IOL of 42.5 g/L of COD regardless of the F/I ratio and temperature. The highest methane content results in biogas at an IOL of 42.5 g/L were 72% and 74% at mesophilic and thermophilic temperatures respectively. About 80-100% of cumulative methane was produced within 15 days in thermophilic reactors, and 40-72% in mesophilic reactors. The kinetic study revealed a fourfold reduction of lag phase in thermophilic compared to mesophilic reactors. The methane yield and organic matter removal rate increased as the concentration of IOL in LL decreased from 170 g/L to 21 g/L regardless of temperature. There exists an inverse correlation between IOL and organic matter removal efficiency. About 80% COD reduction was obtained at mesophilic temperature, and 90% at thermophilic temperature, at an IOL of 42.5 g/L and 21 g/L of COD. The modified Gompertz model showed a good fit to the experimental data, with R² > 0.98 in all cases. Overall, the findings of this study conclude that treatment of acids rich and high-strength LL both at mesophilic and thermophilic temperature is feasible at an optimum IOL of 42.5 g/L of COD. However, treatment of LL at thermophilic temperature outperformed compared to mesophilic over the digestion time.

Coupling of biochar-mediated absorption and algal-bacterial system to enhance nutrients recovery from swine wastewater

Algal-bacterial system (ABS) used in treatment of high-strength ammonium wastewaters receives more and more attentions. In this paper, biochar-mediated absorption (BMA) and ABS were applied to recover nutrients from swine wastewater (SW) with high-strength ammonium, respectively. The results showed that the BMA could recover ammonium from the SW, which mitigated ammonia toxicity to the ABS. The bacterial community diversity containing four phyla of bacteria was identified for the first time during nutrients recovery from the SW by the ABS. Proteobacteria and Firmicutes were the two most abundant phyla. A novel scheme for nutrients recovery from the SW by the coupled BMA-ABS method was proposed and evaluated. Nutrients recovery was obviously improved by the coupled BMA-ABS method with biomass concentration of 1.97 g L^-1, and a NH₄⁺-N recovery efficiency of 96%, a total nitrogen recovery efficiency of 95%, a total phosphorus recovery efficiency of 96%, and a chemical oxygen demand recovery efficiency of 99%. The coupled BMA-ABS method could enhance nutrients recovery from the SW.

Biogas Production from Oil Palm Empty Fruit Bunches and Palm Oil Decanter Cake using Solid-State Anaerobic co-Digestion

Oil palm empty fruit bunches (EFB) and palm oil decanter cake (DC) were used to investigate biogas production by using solid-state anaerobic co-digestion (SS-AcoD) with 15% total solid (TS) content. Solid state anaerobic digestion (SS-AD) using substrate to inoculum (S:I) ratio of 3:1, methane yields of 353.0 mL-CH4/g-VS and 101.5 mL-CH4/g-VS were respectively achieved from mono-digestion of EFB without oil palm ash (OPA) addition and of DC with 10% OPA addition under mesophilic conditions 35 °C. By adding 5% OPA to SS-AD using 3:1 S:I ratio under thermophilic conditions (55 °C), mono-digestion of EFB and DC provided methane yields of 365.0 and 160.3 mL-CH4/g-VS, respectively. Furthermore, SS-AcoD of EFB:DC at 1:1 mixing ratio (volatile solid, VS basis), corresponding to carbon to nitrogen (C:N) ratio of 32, gathering with S:I ratio of 3:1 and 5% ash addition, synergistic effect is observed together with similar methane yields of 414.4 and 399.3 mL-CH4/g-VS, achieved under 35 °C and 55 °C, respectively. According to first order kinetic analysis under synergistic condition, methane production rate from thermophilic operation is 5 times higher than that from mesophilic operation. Therefore, SS-AcoD could be potentially beneficial to generate biogas from EFB and DC.

Study of an enhanced dry anaerobic digestion of swine manure: Performance and microbial community property

Anaerobic digestion could treat organic wastes and recovery energy. Dry anaerobic digestion had advantages of low heating energy, small digester and less digestate, but its methane production was poor. In this study, an enhanced dry anaerobic digestion of swine manure (thermal treatment + dry anaerobic digestion) was proposed, and its feasibility was investigated via semi-continuous experiment. Results showed that methane production rates were 314.6, 416.0, 298.0 and 69.9 mL CH₄/g VS at solid retention time (SRT) of 41 d, 35 d, 29 d and 23 d. Volatile solids (VS) removal rate and methane production rate could reached 71.4% and 416.0 mL CH₄/g VS respectively at SRT of 35 d. Methane production rate of the enhanced dry anaerobic digestion was 390% higher than that of dry anaerobic digestion. Microbial study indicated that hydrogenotrophic methanogens predominated with the abundance of 90.2%, while acetoclastic methanogens were not detected. This process was feasible, and was of great practical importance.

Perspectives on variabilities in biomethane potential test parameters and outcomes: A review of studies published between 2007 and 2018

Biomethane Potential (BMP) test continues to be a useful and inexpensive assay to estimate the digestibility and maximum methane production of various organic substrates in anaerobic digestion or co-digestion processes. Despite its usefulness and several published efforts toward standardizing it, the BMP test still do not follow a universally accepted standard protocol. This makes the comparison of results among studies quite challenging. In this context, this paper analyzes 78 peer-reviewed BMP studies published between 2007 and 2018 that used the BMP test primarily to assess methane potential of commonly digested substrates, such as food waste, wastewater sludge and manure. We focused on the similarities and differences in the methodologies used and, where possible, the results obtained from these studies were compared and discussed. It was observed that many studies do not provide adequate information on salient aspects of the BMP methodology, and results are sometimes reported in different units of measurements. The inoculum to substrate ratio (ISR), substrate concentration and/or load should be clearly indicated in future studies, and positive controls should be included to validate BMP results. It is recommended that more studies assess the impact of nutrient addition, potential effects of continuous and intermittent mixing and mixing intensities and the influence of reactor size and headspace volume on BMP results.

Meta-omics based analyses of microbiome involved in biomethanation of rice straw in a thermophilic anaerobic bioreactor under optimized conditions

Biomethanation of rice straw was performed at 55 °C without thermochemical pretreatment using cattle dung supplemented with Methanothermobacter thermautotrophicus strains. Methane yield of 323 ml g^-1 VS obtained under optimized conditions such as particle size (1 mm), carbon to nitrogen ratio (15:1), substrate to inoculum ratio (1:1), organic loading rate (7.5% w/v) and hydraulic retention time (20 days), was one of the highest ever reported from rice straw. Metagenome analysis revealed several putative novel taxa among resident microbes. The genomes of Clostridium, Hungateiclostridium, Alkaliphilus, Anaerocolumna, Olsenella, Paenibacillus, Pseudoclostridium, Tepidanaerobacter and Turicibacter were recovered as metagenome assisted genomes. Clostridium spp. and M. thermautotrophicus were the dominant hydrolytic and methanogenic microbes, respectively. Syntrophic acetate oxidation coupled to hydrogenotrophic methanogenesis was found to be the major pathway for methane production. Efficient thermophilic biomethanation of rice straw without thermochemical pretreatment using cattle dung supplemented with M. thermautotrophicus is reported for the first time.

Novel insight into high solid anaerobic digestion of swine manure after thermal treatment: Kinetics and microbial community properties

Compared to traditional anaerobic digestion (AD), high solid anaerobic digestion (HSAD) had the advantages of small digester, low heating energy and less digestate. However, the methane production was poor. In our previous study, thermal treatment (70 ± 1 °C, 3 days) without any dilution could satisfactorily enhance the methane production rate of HSAD by up to 39.5%. However, effects of solid content on HSAD after thermal treatment were not yet studied. In this study, HSAD was conducted at 11.7-17.6% solid content, and the control experiment was conducted at low solid content (4.4% solid content). Results showed that HSAD's methane production rate was the highest at 11.7% solid content (158 mL CH₄/g VS), and could reach up to 89.2% of that at 4.4% solid content. The utilization of organics was revealed by kinetics analysis that the readily biodegradable organics could be utilized at increasing solid content with decreasing hydrolysis rate. Furthermore, it was notable that methylotrophic methanogens predominated in HSAD with the abundance of 82.6%. This was quite unique from the general belief that AD system was usually dominated by acetoclastic or hydrogenotrophic methanogenic pathways. In this study, the microbial community structure of HSAD after thermal treatment was firstly studied, its unique specific methanogenic pathways was firstly revealed.

Comparison of leachate percolation and immersion using different inoculation strategies in thermophilic solid-state anaerobic digestion of pig urine and rice straw

Heterogeneous distribution of substrate and microorganisms and low mass transfer limit methane production dramatically in solid-state anaerobic digestion (SS-AD). To overcome this challenge, this study determined the optimal inoculation strategy (complete premix/slurry application) for reusing solid digestate as inoculum and the optimal leachate circulation method (percolation/immersion) using batch digestion. Initially, percolation and immersion (1 h per 3 days) were compared and the result shows that immersing rice straw into leachate was superior to leachate percolation in methane production. Effect of the immersion period (24, 48 and 72 h) in each circulation cycle on methane production was then evaluated for each inoculation strategy. Methane production increased until the immersion period up to 24 h and then decreased, while the average cumulative methane yield with an immersion period of 24 h was (180 mL/g volatile solids). Slurry application with an immersion period 24 h is recommended as the optimum operating condition.

Impact of turning waste on performance and energy balance in thermophilic solid-state anaerobic digestion of agricultural waste

Mixing is an important operation in solid-state anaerobic digestion (SS-AD) to improve the mass transfer of the solid phase. This study proposed simple turning by loader in common garage-type digester without commonly used mixer or percolation system (simplified SS-AD). In simplified-SS-AD, turning is conducted in open condition. Thus, oxidation of anaerobic sludge during turning would influence digestion performance. Therefore, in this study, the effect of turning wastes by mixing during digestion on a simplified SS-AD fed with rice straw and pig urine was investigated. Four different mixing frequency levels-no mixing (M0) and mixing once a day (M-1/1), once every 3 days (M-1/3) and once a week (M-1/7)-were conducted. Methane yields of M0, M-1/3 and M-1/7 were comparable with each other. Methane yield and lag period of M-1/1 were approximately 61% and 155% of M0 (351.2  mL/g VS and 4.7 days), respectively. Furthermore, the chemical oxygen demand (COD) of acetate accumulated in the digestate of M-1/1 was comparable to the difference in the COD of methane production between M-1/1 and the other treatments. Mixing every day also resulted in a higher oxidation-reduction potential and carbon dioxide content. These findings suggest that methanogenesis was inhibited in M-1/1 by frequent mixing in the atmosphere. Net energy analysis of SS-AD plant operation showed that M0 can obtain the highest net energy gain, whereas net energy production of M-1/7 was reduced by rewarming after mixing. Therefore, no mixing is the most effective approach for the proposed simplified process.