Carbon-based conductive materials facilitated anaerobic co-digestion of agro waste under thermophilic conditions

Management of agro-waste is a major challenge globally due to inefficient disposal techniques, which concominantly leads pollution and loss of renewable bioenergy. Anaerobic digestion of agro-waste is one of the ways to tackle this problem but hindered by the recalcitrant nature of agro-waste. This study investigated the effect of granular activated carbon (GAC) and granular biochar (GBC) addition to enhance the thermophilic anaerobic co-digestion of wheat husk and sewage sludge. The conductive materials (particle size: 2-5 mm) were added separately at five different concentrations: 10, 20, 30, 40, 50 g/L_inoculum. The findings revealed that samples amended with GAC and GBC at 20 g/L dosage had the highest biogas yield of 263 and 273 mL/gVS_added, respectively, corresponding to 22 and 27% higher yield than the control. Additionally, a shorter lag phase was observed in both cases compared to the Control. However, the GBC amended samples showed relatively stable biogas production compared to GAC and consistent results regarding pH, alkalinity, total volatile fatty acids, and soluble chemical oxygen demand. The preliminary techno-economic analysis indicates that addition of GAC or GBC may not be feasible and require other innovative engineered solutions for the addition of conductive materials. This study confirms that GAC and GBC amendments enhance the biogas productivity and process stability in anaerobic digestion of recalcitrant agro-waste under the high-temperature regime and calls for further research in this direction.

Maximising resource recovery from wastewater grown microalgae and primary sludge in an anaerobic membrane co-digestion pilot plant coupled to a composting process

A pilot-scale microalgae (Chlorella spp.) and primary sludge anaerobic co-digestion (ACoD) plant was run for one year in an anaerobic membrane bioreactor (AnMBR) at 35 °C, 70 d solids retention time and 30 d hydraulic retention time, showing high stability in terms of pH and VFA concentration. The plant achieved a high degree of microalgae and primary sludge substrate degradation, resulting in a methane yield of 370 mLCH₄·gVS_inf^-1. Nutrient-rich effluent streams (685 mgN·L^-1 and 145 mgP·L^-1 in digestate and 395 mgNH₄-N·L^-1 and 37 mgPO₄-P·L^-1 in permeate) were obtained, allowing posterior nutrient recovery. Ammonium was recovered from the permeate as ammonia sulphate through a hydrophobic polypropylene hollow fibre membrane contactor, achieving 99% nitrogen recovery efficiency. However, phosphorus recovery through processes such as struvite precipitation was not applied since only 26% of the phosphate was available in the effluent. Composting process of the digestate coming from the ACoD pilot plant was assessed on laboratory-scale Dewar reactors, as was the conventional sludge compost from an industrial WWTP digestion process, obtaining similar values from both. Sanitised (free of Escherichia coli and Salmonella spp.) and stable compost (respirometric index at 37 °C below 0.5 mgO ₂ g organic matter^-1·h^-1) was obtained from both sludges.

Batch mesophilic anaerobic co-digestion of spent goat batch mesophilic anaerobic co-digestion of spent goat straw bedding and goat cheese whey: Comparison with the mono-digestion of the two sole substrates

Spent livestock bedding is a valuable resource for the production of green energy (methane) in rural areas. Comparison and evaluation of batch anaerobic digestion and co-digestion of different mixtures of goat straw bedding (SGSB) and goat cheese whey were carried out. Biochemical methane potential (BMP) tests of the 100% SGSB, 95% SGSB-5% whey, 90% SGSB-10% whey, 85% SGSB-15% whey and 100% whey were found to be 423 ± 7, 354 ± 9, 371 ± 2, 293 ± 1, 274 ± 2 mL CH₄ g^-1 VS. Two different kinetic models were evaluated. The logistic model revealed a decrease in the maximum methane production rate (R_m) from 34.7 ± 1.5 to 14.1 ± 0.9 mL CH₄ g^-1 VS·d^-1 when the percentage of whey in the mixture increased from 0 to 15% as a consequence of the increased ammonia released during the co-digestion of increased concentrations of whey. The lowest value for the maximum methane production predicted by the model (P) was found for 100% whey (274 ± 10 mL CH₄ g^-1 VS). A two-substrate model was applied to describe the evident existence of rapid and slowly degradable material. Regarding the hydrolysis kinetic constants predicted by this model, considerable increases in the rapid biodegradation stage (k_rapid) were observed when comparing to the values found for the slow (k_slow) biodegradation stage in all the cases tested. The increases between both constants rose from 5 to 42% when the percentage of whey increased.

Environmental consequences of the treatment of corn contaminated by aflatoxin B1 with co-digestion and co-composting in a life cycle perspective

Global environmental performances of anaerobic co-digestion and co-composting of aflatoxin B1 (AFB1) contaminated corn were investigated by a life cycle assessment approach. Anaerobic co-digestion of pig slurry and corn with 25 μgkg^-1 ww AFB1 concentration resulted able to generate 627 NLkgVS^-1 of biogas with a reduction of the AFB1 concentration in the digestate of 44%. At AFB1 concentration of 100 μg kg^-1 ww, the process resulted strongly inhibited with a biogas generation of 122 NLkgVS^-1 and AFB1 concentration reduction in the digestate of 25%. Co-composting of 100 μg kg^-1 dw AFB1 contaminated corn with other substrates as organic fraction of municipal waste, pig slurry, and other lignin-cellulosic residues showed a removal efficiency of AFB1 ranging from about 80 up to 95% depending on the different mixtures adopted. Environmental consequences associated to the removal of 1 mg of AFB1 in different scenarios investigated, including also the use on land of the digestate and of the compost, indicated that global warming was affected equally by co-digestion and co-composting, about 95 kgCO₂eq. Co-digestion showed also the possibility of achieving avoided emissions of about - 0.007 kgNMVOC_eq, - 2.5E-3 kgP_eq, and - 30CTU_e. Benefits concerning resource depletion resulted higher for co-composting due to the high amount of mineral fertilizer replaced. Contribution of AFB1 in the determination of human health (DALY) resulted lower than about 4% for co-digestion and practically negligible for co-composting.

A novel integrated single-stage anaerobic co-digestion and oxidation ditch-membrane bioreactor system for food waste management and building wastewater recycling

This study is to develop a novel integrated single-stage anaerobic co-digestion and oxidation ditch membrane bioreactor (SAC/OD-MBR) for food waste and building wastewater recycling. The co-digestion of food waste (FW) from a canteen with waste sludge (WS) from OD-MBR was performed with the proportion of FW:WS at 10:1 by weight. The liquid digestate from the co-digestion process was further co-treated with building wastewater in the OD-MBR system for water reuse purpose. Maximum methane content of 65.2% in biogas as well as average specific methane yield of 0.24 gCH₄/gVS could be obtained with anaerobic co-digestion of food waste and waste sludge from OD-MBR with HRT of 24 h and horizontal flow velocity of 0.3 m/s. The observed main methanogen species in this co-digestion process were Methanoculleus bourgensis and Methanoculleus palmolei. For co-treatment of liquid digestate and building wastewater with the OD-MBR, it was found that HRT of 24 h and horizontal flow velocity of 0.3 m/s could achieve highest COD and nitrogen removal efficiencies. HRT can be considered as a main key parameter to promote nitrification activity inside the OD-MBR system. Moreover, treated effluent from the SAC/OD-MBR could comply with the water reuse standard for garden and landscape application in the university campus. Furthermore, the techno-economic analysis indicates that this proposed system has a high potential of total cost savings and other indirect benefits. Therefore, the prototype SAC/OD-MBR can be an alternative system for food waste management and wastewater recycling for building application.

In-situ alkaline enhanced two-stage anaerobic digestion system for waste cooking oil and sewage sludge co-digestion

Anaerobic digestion is a promising way for resource recovery from waste cooking oil (WCO) due to its high bio-methanation potential. In-situ mild alkaline (pH 8) enhanced two-stage continuous stirred tank reactors (ALK-2-CSTRs) were implemented to explore its efficiency in co-digesting WCO and sewage sludge with stepwise increase of WCO in the co-substrates. Results demonstrate that the ALK-2-CSTRs effectively promoted methane yield from the co-substrates via promoting hydrolysis, long chain fatty acids (LCFAs) degradation and protecting methanogens from exposure to high concentration of LCFAs directly. The maximum methane yield of the ALK-2-CSTRs is 39.2% higher than that of a single stage CSTR system at the optimal feed mixture of 45:55 (WCO:SS [VS]). The thermophilic operation applied to the stage-1 of the ALK-2-CSTRs failed to improve the methane yield when the methanogenic performance was stable; while upon WCO overloaded, the elevated temperature mitigated the deterioration of methanogenesis by stimulating the bioconversion of the toxic LCFAs, especially the unsaturated oleic acid. Microbial community analysis reveals the ALK-2-CSTRs stimulated the growth of lipolytic bacteria and hydrogenotrophic methanogens, which suggests the hydrogenotrophic methanogenic pathway was promoted. Cost evaluation demonstrates the economical superiority of the ALK-2-CSTR over the prevailing strategies developed for enhancing methane yield from the co-substrates.

The effect of brewery spent grain application on biogas yields and kinetics in co-digestion with sewage sludge

The present study examines the effect of introducing dried brewery spent grain (BSG), known as the main solid by-product of the brewery industry on biogas yields and kinetics in co-digestion with sewage sludge (SS). The experiment was conducted in semi-continuous anaerobic reactors (supplied once a day) operating under mesophilic conditions (35°C) at different hydraulic retention times (HRT) of 18 and 20 d. In co-digestion runs, the BSG mass to the feed volume ratio was constant and maintained 1:10.The results indicated that the addition of BSG did not influence the biogas production, by comparison with SS mono-digestion (control run). At HRT of 18 d, in the co-digestion run, the average methane yield was 0.27 m³ kg/VS_added, while in the control run the higher value of 0.29 m³ kg/VS_addedwas observed. However, there was no difference in terms of statistical significance. At HRT of 20 d, the methane yield was 0.21 m³ kg/VS_added for both mono- and co-digestion runs. In the BSG presence, the decrease in kinetic constant values was observed. As compared to SS mono-digestion, reductions by 21 and 35% were found at HRT of 20 and 18 d, respectively. However, due to the supplementation of the feedstock with BSG rich in organic compounds, the significantly enhanced energy profits were achieved with the highest value of approx. 40% and related to the longer HRT of 20 d. Importantly, the mono- and co-digestion process proceeded in stable manner. Therefore, the anaerobic co-digestion of SS and BSG might be considered as a cost-effective solution that could contribute to the energy self-efficiency of wastewater treatment plants (WWTPs) and sustainable waste management for breweries.

Anaerobic co-digestion of corn stover and chicken manure using continuous stirred tank reactor: The effect of biochar addition and urea pretreatment

The performance of biochar mediated anaerobic co-digestion (co-AD) of corn stover (CS) and chicken manure (CM) using continuous stirred tank reactor (CSTR) was studied. Results showed that urea pretreated CS (UPCS) and biochar addition in anaerobic digestion (AD) system can improve co-AD. The effect of urea pretreatment is similar to that of biochar addition, and their synergistic effect was apparent under medium and high OLR conditions. When the OLR was 4.2 and 6.3 g VS/L/d, the biochar mediated UPCS/CM co-AD operated stably with the VMP of 2.160 and 1.616 L/L/d, and VMP of the biochar mediated UPCS /CM were 32.8%-89.6% and 27.8%-96.4% higher than other reactors, respectively. The results reveal that urea pretreatment and biochar addition promoted AD process through strengthening the buffer capacity system established by ammonia nitrogen and volatile fatty acids and improving the degradation of lignocellulosic biomass.

Understanding the fate and impact of capsaicin in anaerobic co-digestion of food waste and waste activated sludge

Anaerobic co-digestion is an attractive option to treat food waste and waste activated sludge, which is increasingly applied in real-world situations. As an active component in Capsicum species being substantially present in food waste in many areas, capsaicin has been recently demonstrated to inhibit the anaerobic co-digestion. However, the interaction between capsaicin and anaerobic co-digestion are still poorly understood. This work therefore aims to deeply understand the fate and impact of capsaicin in the anaerobic co-digestion. Experiment results showed that capsaicin was completely degraded in anaerobic co-digestion by hydroxylation, O-demethylation, dehydrogenation and doubly oxidization, respectively. Although methane was proven to be produced from capsaicin degradation, the increase in capsaicin concentration resulted in decrease in methane yield from the anaerobic co-digestion. With an increase of capsaicin from 2 ± 0.7 to 68 ± 4 mg/g volatile solids (VS), the maximal methane yield decreased from 274.6 ± 9.7 to 188.9 ± 8.4 mL/g VS. The mechanic investigations demonstrated that the presence of capsaicin induced apoptosis, probably by either altering key kinases or decreasing the intracellular NAD⁺/NADH ratio, which led to significant inhibitions to hydrolysis, acidogenesis, and methanogenesis, especially acetotrophic methanogenesis. Illumina Miseq sequencing analysis exhibited that capsaicin promoted the populations of complex organic degradation microbes such as Escherichia-Shigella and Fonticella but decreased the numbers of anaerobes relevant to hydrolysis, acidogenesis, and methanogenesis such as Bacteroide and Methanobacterium.

Biohythane production and microbial characteristics of two alternating mesophilic and thermophilic two-stage anaerobic co-digesters fed with rice straw and pig manure

To investigate biohythane production and microbial behavior during temperature-phased (TP) anaerobic co-digestion (AcD) of rice straw (RS) and pig manure (PM), a mesophilic-thermophilic (M1-T1) AcD system and a thermophilic-mesophilic (T2-M2) AcD system were continuously operated for 95 days in parallel. The maximal ratio (8.44%v/v) of produced hydrogen to methane demonstrated the feasibility of biohythane production by co-digestion of RS and PM. T2-M2 exhibited higher hydrogen (16.68 ± 1.88 mL/gVS) and methane (197.73 ± 11.77 mL/gVS) yields than M1-T1 (3.08 ± 0.39 and 109.03 ± 4.97 mL/gVS, respectively). Methanobrevibacter (75.62%, a hydrogenotrophic methanogen) dominated in the M1 reactor, resulting in low hydrogen production. Hydrogen-producing bacteria (Thermoanaerobacterium 32.06% and Clostridium_sensu_stricto_1 27.33%) dominated in T2, but the abundance of hydrolytic bacteria was low, indicating that hydrolysis could be a rate-limiting step. The thermophilic acid-producing phase provided effective selective pressure for hydrogen-consuming microbes, and the high diversity of microbes in M2 implied a more efficient pathway of methane production.

Advanced methanogenic performance and fouling mechanism investigation of a high-solid anaerobic membrane bioreactor (AnMBR) for the co-digestion of food waste and sewage sludge

Disposal of the increasingly huge amounts of sewage sludge (SeS) has become an impending problem worldwide. To solve this problem, a high-solid anaerobic membrane bioreactor (AnMBR) was used for the anaerobic co-digestion (AcoD) of SeS and food waste (FW). This study investigated the effects of SeS ratio on the methanogenic performance of the AcoD with a gradual increase value from 0 to 25%, 50%, 75% and 100% (total solids based). The results showed that the highest methanogenic performance was achieved at mono FW digestion. As for the co-digestion, the optimal FW/SeS ratio for methanogenic performance was 75%:25% among all the mixing ratios. The COD based biogas yield and methane yield were 0.498 L-biogas/g-COD_fed and 0.295 L-CH₄/g-COD_fed at this optimal mixing ratio, which were 67.7% and 67.6% higher than those of the mono SeS digestion, respectively. The upgraded values were attributed to the improved hydrolysis ratio (by 8.14%) and the balanced carbon-to-nitrogen (C/N) ratio by co-digestion with FW, which synergistically stimulated methanogenesis ratio by 81.0%. The continuous membrane filtration property was investigated and four typical trans-membrane pressure (TMP) variation curves at different fouling degrees were determined. The membrane could sustainably operate at a flux of 6 L/m²/h (LMH) at the mixed liquor total solids (MLTS) concentration of 25 - 30 g/L. The combination of continuous membrane filtration property, particle size distribution of the mixed liquor and the external forces analysis was firstly applied to unravel the membrane fouling mechanism of a high-solid AnMBR. The result of this study will contribute to the establishment of an efficient FW and SeS treatment strategy.

Functional characteristic of microbial communities in large-scale biotreatment systems of food waste

In order to evaluate microbial community structure dominated metabolic function profiles in large-scale food waste (FW) biotreatment systems, bacterial, archaeal and fungal community associated with metabolic function in high-temperature aerobic fermentation (AF) and anaerobic co-digestion (AcoD) processes were comprehensively investigated in this study. The qPCR results showed the higher gene copies of bacteria and fungi in initial and AF-treated FW compared with AcoD-treated FW, as well as bacteria and archaea in AcoD-treated FW were highly abundant among detected samples. Furthermore, the total abundances of archaea ((1.18-4.88) × 10⁶ copies/ng DNA) in AcoD system were 2-3 orders of magnitude higher than that in other samples (P < 0.01), indicating active archaeal activity in AcoD system. Correlation analysis of microbial community and metabolic function indicated that the higher abundances of Kazachstania, Pyrobaculum, Sulfophobococcus, Lactobacillus and Candida in initial FW had close linkages with lipid metabolism (P < 0.05). Abundant Aspergillus, Staphylococcus, Pelomonas, Corynebacterium, Faecalibacterium, Methanobacterium and Xeromyces in AF system were positively and significantly correlated with high metabolic activities of energy metabolism, carbohydrate metabolism, amino acid metabolism, fatty acid metabolism, glycosaminoglycan degradation, sulfur metabolism and nitrogen metabolism. As for AcoD system, dominant genera Methanosaeta, Methanoculleus, Methanobacterium, Fastidiosipila, Rikenellaceae RC9, Bifidobacterium and Xeromyces had close relationships with metabolism of cofactors and vitamins, energy metabolism, methane metabolism, carbohydrate metabolism and glycosaminoglycan degradation (P < 0.05). These results are expected to improve the metabolic efficiency by functional microorganism in different large-scale FW treatment systems.

Enhanced anaerobic co-digestion of waste activated sludge and food waste by sulfidated microscale zerovalent iron: Insights in direct interspecies electron transfer mechanism

The enhancement of zerovalent iron (ZVI) on anaerobic digestion (AD) has been proved, but there are still some problems that constrain the large-scale application of ZVI, such as the destruction of cell membrane and the inhibition of methanogenesis led by rapid H₂ accumulation. Aiming at these problems, sulfidated microscale zerovalent iron (S-mZVI) was employed to evaluate its effect on anaerobic co-digestion (AcoD) of waste activated sludge (WAS) and food waste (FW). Experimental results showed that S-mZVI promoted the direct interspecies electron transfer (DIET) between specific bacteria and methanogens, resulting in higher methane yield. At S-mZVI 10 g/L, the cumulative methane yield and ETS activity reached 264.78 mL/g-VS and 24.62 mg INTF/(g-TS h), which was 1.33 and 1.83 times that of blank. Microbiological analysis demonstrated that the abundance of DIET-related microorganisms such as Syntrophomonas, Methanosarcina and Methanobacterium increased with the increasing dosage of S-mZVI.

Fate of integrons, antibiotic resistance genes and associated microbial community in food waste and its large-scale biotreatment systems

The prevalence and dissemination of antibiotic resistance genes (ARGs) have been globally gained increasing concerns. However, the fate and spread of ARGs in food waste (FW) and its large-scale biotreatment systems are seldomly understood. Here, we investigated the initial and biologically treated FW in two major FW treatment systems of aerobic fermentation (AF) and anaerobic co-digestion (AcoD) processes. The total relative abundances of integrons and ARGs significantly increased from initial FW to treated FW. Among targeted ARGs, ermB and strB were predominant ARGs, which accounted for 52.58-95.28% of total abundance across all samples. Mantel test indicated that integrons (intl1 and intl2) were positively and significantly correlated with detected ARGs (Mantel test, r = 0.24, p < 0.05), suggesting integrons display significant contributions on driving ARG alteration during FW treatment processes. RDA results indicated that bla_OXA, strB and bla_TEM were more likely to be proliferated by potential host of Firmicutes (96.55-99.77%) in initial FW, while bla_CTX-M and mefA were potentially enriched by Proteobacteria (17.12-49.82%) in AF system and ermB, sul1, aadA and tetQ were possibly enhanced by Bacteroidetes (27.43-43.71%) in AcoD system. Consideration of the higher enriched abundance of total ARGs (66.88 ± 87.34 times) and the used inoculum sludge in AcoD-treated system, the resource utilization of anaerobically digested products should draw our more attentions. These findings would deepen our understanding of prevalence and proliferation of ARGs in FW treatment systems and serve as a foundation for guiding the application of biologically treated FW.

First approaches to valorizate fat, oil and grease (FOG) as anaerobic co-substrate with slaughterhouse wastewater: Biomethane potential, settling capacity and microbial dynamics

Anaerobic digestion (AD) is the biological preferred treatment applied to Slaughterhouse wastewaters (SWW) due to its effectiveness. The aim of the study is to investigate the effect of different percentages of fats, oil and grease (FOG) on biomethane production in anaerobic co-digestion with slaughterhouse wastewater using BMP tests under mesophilic conditions (35 °C). For this purpose, three percentages of FOG from 1% to 10% were tested. Biodegradability, biomethane production and the microbial population were studied. In addition, settling capacity has been evaluated at different conditions: i) before and after anaerobic co-digestion; ii) at different temperature 25 °C and 35 °C. The settling rates as well as the characterization of the digestate were recorded. Experimental results showed that all the co-digestion mixtures (FOG percentages = 1-10%) enhanced biomethane production and biodegradability compared to AD of sole SWW. The best conditions were achieved at 5-10% of FOG, showing biodegradability of 66-70% CODt_removal and specific biomethane productions of 562 and 777 mL_CH4·g^-1_CODsremoved, respectively. Regarding microbial dynamics, Eubacteria was reduced with the increase in %FOG but Acetate utilizing methanogens was increased. Regarding settling capacity, mesophilic temperatures (35 °C) increased the settling rate of digestate in 1.76 times and reduced the lag-phase to 0.92 min; obtaining a more concentrated sludge and leaving a clarified whose TSS represent only 8% of TS.

In search of the optimal inoculum to substrate ratio during anaerobic co-digestion of spent coffee grounds and cow manure

The inoculum to substrate (I:S) ratio is a crucial operating parameter during the start-up period of anaerobic digestion (AD) processes and this ratio shows high differentiation with respect to substrate composition. While spent coffee grounds (SCG) have started to gain attraction in AD as a co-substrate due to their vast production and promising methane potential, there is still not enough information on the operative environment of SCG-based biogas reactors. This study investigated the optimal I:S ratio during anaerobic co-digestion of SCG and cow manure. Biochemical methane potential tests were conducted at mesophilic conditions and the influence of I:S ratio on methane production and digestion stability was evaluated at a wide range of I:S ratios from 0.5:1 to 4:1 (volatile solids (VS) basis). Methane yields increased gradually starting from the I:S ratio of 0.5:1 up to 3:1 and the highest methane yield (225 mlCH₄ gVS^-1) was achieved at the I:S ratio of 3:1. Comparatively lower methane yields were obtained at the ratios of 3.5:1 and 4:1. Instable AD conditions were established at the lowest I:S ratio examined (0.5:1), which caused volatile fatty acid (VFA) accumulation. The results highlighted that anaerobic co-digestion of SCG and cow manure is a promising approach, while the I:S ratio should be well-maintained due to the high potential risk of rapid and/or excess VFA production of these feedstocks.

Anaerobic co-digestion of corn stover and wastewater from hydrothermal carbonation

This study aimed to investigate the interactions between wastewater of hydrothermal carbonation (W-HTC) and corn stover (CS) during anaerobic co-digestion. The results showed the maximum cumulative methane production of co-digestion was 280.7 ± 3.2 mL/g VS, and it increased by 5.84% and 10.69% compared with mono-digestion of CS and W-HTC, respectively. Increasing the HTC temperature and excess addition of W-HTC inhibits early and middle stage of co-digestion due to toxic organic inhibitors, and the negative effect of phenols is substantially more than furans. The microbial analysis illustrated the addition of W-HTC can promote the growth of Clostridia and Bacteroidia. The growth of Methanomassiliicoccus and Methanosarcina was more vigorous in most of co-digestions, which was positively correlated with methane production. The study concluded methanogenesis can be enhanced by the co-digestion of W-HTC and CS, which provide optimization of process conditions and some reaction mechanism for application of W-HTC in anaerobic digestion.

Biomethane production from anaerobic co-digestion and steel-making slag: A new waste-to-resource pathway

A proof of concept of using steel-making slag to upgrade biogas to biomethane is demonstrated in this study. Biogas is generated from the anaerobic co-digestion of sewage sludge and beverage waste. The CO₂ capture capacity of an alkaline liquor derived from the release of calcium from the steel-making slag is comparable to that of the commercial adsorbent monoethanolamine. Although only 5% of Ca in the steel-making slag was released to the alkaline liquor, 1 ton of steel-making slag could be capable of upgrading 10 m³ of biogas to over 90% methane content. The results also show that pH can be used as a surrogate parameter to monitor and control biogas upgrading. Further research to improve the release of calcium is essential for the acceleration of the weathering process of steel-making slag for subsequent construction applications.

Synergistic Effects of Anaerobic Co-Digestion of Pretreated Corn Stover with Chicken Manure and Its Kinetics

The pretreatment effects and synergistic effects of anaerobic co-digestion of pretreated corn stover (CS) with chicken manure (CM) were studied. Results showed that the NaOH-H₂O₂ pretreatment effect on CS was better than urea pretreatment in terms of anaerobic digestion promotion. The highest cumulative methane yield of 332.7 mL/g VS added was obtained from the CS (NaOH-H₂O₂ pretreated)/CM ratio of 1:3, and the highest cumulative methane yield of 319.7 mL/g VS added was obtained from the CS (urea pretreated)/CM ratio of 1:2. Synergistic effects were found in CS (NaOH-H₂O₂ pretreated)/CM ratios of 2:1, 1:2, 1:3 and CS (urea pretreated)/CM ratios of 1:1, 1:2. Synergistic effect was not found at CS (unpretreated)/CM ratios of 1:2 and 1:3. Pretreatment of CS can produce synergistic effect on anaerobic co-digestion and increase cumulative methane yield by 6.54-24.65%. Among the four kinetic models, modified Gompertz model was best fitted in describing the methane production during anaerobic co-digestion (R² = 0.9845-0.9988).

Co-Digestion Biomethane Production and the Effect of Nanoparticle: Kinetics Modeling and Microcalorimetry Studies

To improve the production rate of methane, powder-activated carbon (PAC), granule activated carbon (GAC), titanium dioxide-anatase (TiO₂), and synthesized zeolite (permutit) were added in the co-digestion process. The co-substrates were corn stover (CS) and pig manure (PM) mixed in the ratio of 1:2 (w/w). The kinetic analysis model and ADM1da model were applied to obtain the kinetic parameters of the process. Besides, the heat flow analysis of the co-digestion process was determined using isothermal microcalorimetry. The addition of the PAC, GAC, TiO₂, and synthesized zeolite improved the methane cumulative yield by 40.12, 31.25, 31.17, and 43.74% respectively, as compared with the control reactor. The kinetic analysis and ADM1da model results indicated that the overall rate constant of the co-digestion process increased by 1.5 times averagely because of the effect of these materials. It was also observed that much higher heat energy released from the experimental sample compared with the control reactor, which indicated that the improvement of the metabolic process of the AcoD system. The addition of TiO₂-anatase improved methane production by 31.17%, which could be a promising method to improve the biomethane in a large-scale due to its availability and accessibility.

Anaerobic co-digestion of hydrolysate from anaerobically digested sludge with raw waste activated sludge: Feasibility assessment of a new sewage sludge management strategy in the context of a local wastewater treatment plant

Sustainable sewage sludge management is a worldwide issue in wastewater treatment plants (WWTPs). This work developed a new strategy for sewage sludge treatment involving the integration of hydrothermal treatment (HT) with anaerobic co-digestion (AcoD), particularly on the feasibility of mesophilic AcoD of anaerobically digested sludge (DS) hydrolysate and waste activated sludge (WAS). Results show that AcoD of DS hydrolysate from HT 170℃ for 30 min with WAS achieved the highest CH₄ production of 205.39 mL CH₄/g-VS_fed. By adopting the new AD-HT170-AcoD strategy, 61.88 mL CH₄/g-tVS_fed higher CH₄ yield and 22.2% more total solids (TS) reduction were obtained in addition to much better sludge settleability and 7.6% wt. less sludge cake production compared to the conventional mono AD. Although negative energy gain was obtained, the proposed AD-HT170-AcoD strategy is promising, economically feasible, and sustainable when the final sludge disposal of WAS is concerned in the context of whole WWTP.

Synergistic effects of rice straw and rice bran on enhanced methane production and process stability of anaerobic digestion of food waste

This study investigated the synergistic effects of rice straw (RS) and rice bran (RB) addition on methane production and process stability of anaerobic digestion of food waste (FW). Positive synergistic effect (Synergy index (SI) = 1.03-1.24 > 1) was noticed in all the co-digestion reactors. The optimum mixing ratio of FW:RS:RB (volatile solid (VS) basis) was 60:10:30 with the maximum SI (1.24), achieving 27.4% increase in methane yield (235.4 mL/g-VS) and around 5 days shorter of λ (3.7 days) compared to the mono-digestion of FW (184.8 mL/g-VS and 8.2 days). Remarkably high concentration of volatile fatty acids (VFAs) was also accumulated in the mono-digestion of FW, especially propionic acid, which to a great extent caused the methane production to stagnate. Results from this study demonstrate that co-digestion of FW and RS with RB has high potentials for energy recovery from AD of the mixed feedstocks and its stable operation.

Powdered activated carbon facilitates methane productivity of anaerobic co-digestion via acidification alleviating: Microbial and metabolic insights

Low methanogenic efficiency caused by excess acidification is a challenge during anaerobic digestion. This study indicated that both granular activated carbon (GAC) and powdered activated carbon (PAC) promoted the start-up of methanogenesis and methane output in anaerobic co-digestion of food waste and fruit-vegetable waste. Moreover, PAC performed better than GAC. Specifically, the highest cumulative methane yield and shortest lag phase were observed in 5 g/L PAC and 10 g/L PAC group, 22.0% higher and 62.5% shorter than that without activated carbon supplementation, respectively. PAC facilitated the methane productivity by effectively accelerating volatile fatty acids (VFAs) consumption and thereby alleviating acidification. Syntrophic VFAs oxidizing bacteria (Gelria and Syntrophomonas) and direct interspecies electron transfer related microorganisms (Geobacter and Methanosarcina) were remarkably enriched by PAC. Furthermore, metagenomic analysis showed that both PAC and GAC might facilitate the electron transfer between microbes by acting as the electrical bridge and enhance both hydrogenotrophic and aceticlastic pathways.

The effect of microwave pretreatment on anaerobic co-digestion of sludge and food waste: Performance, kinetics and energy recovery

This paper studied the effect of microwave (MW) pretreatment on anaerobic co-digestion of sludge (SS) and food waste (FW). Using SS and FW as digestive substrates, the MW pretreatment method was used to determine the changes in the substrate matrix by means of batch anaerobic digestion at 37 °C. The kinetics of methane production were calculated, and the changes in organic matter during anaerobic co-digestion, the properties of the anaerobic-digested effluent, and the net energy output of the co-digestion system were determinated. The results showed that MW pretreatment was beneficial to the dissolution of organic matter, conversion of protein to NH₄⁺-N, cumulative methane production, unit biomethane yield, and reaction rate of methane production in the SS and FW anaerobic co-digestion system. The highest cumulative methane production in the co-digestion system reached 3446.3 ± 172.3 mL (35 days), which was 19.93% higher than that of the control. Furthermore, MW pretreatment significantly increased the accumulation of VFAs and the content of butyric acid in the anaerobic-digested effluent, which was beneficial to the methanogenesis process. The MW pretreatment of all co-digested substrates produced a greater net energy output than the control, and the MW-SS + MW-FW group yielded the highest net energy output, which was 76.25 kJ/g Fed VS. The results indicated that MW pretreatment prior to SS and FW anaerobic co-digestion is an effective way to improve the anaerobic digestion efficiency and energy recovery rate.

Anaerobic co-digestion of sewage sludge with cellulose, protein, and lipids: Role of rheology and digestibility

Rheology is known to have an impact on the performance of digesters, but the effect of additional substrates (co-digestion) is poorly understood. The main objective of this study was to investigate the effects of the addition of cellulose, protein and lipids to substrates on the rheological behaviour and biogas production of the mixture of primary sludge (PS) and waste-activated sludge (WAS) in a batch system. A mixture of PS and WAS to form the main substrate was anaerobically co-digested with different types of organic matter (cellulose, protein and lipids) as co-substrates at different co-substrate to main substrate ratios of 2-8 (wt%) under mesophilic conditions and below ammonia inhibition levels. Yield stress (τ_y) and the flow consistency index (k) of the combined feed in the case of cellulose and protein were significantly dependent on the amount of co-substrate added, while there was an insignificant impact on these properties when lipids were added. Cellulose significantly increased τ_y and k in the feed, which resulted in poor fluidity and the improper homogenisation of the digester content, and consequently decreased the biogas yield. In contrast, the biogas yield was improved through the addition of 2% to 6% protein despite an increase in τ_y and k of the feed, but the methane yield decreased at 7% and 8% levels of protein concentration. This observation indicates that the threshold for τ_y and k of the digester media depends on the organic nature and digestibility of the substrate. There was no significant impact on the flow properties of the initial mixture when lipids were added, and their addition increased the biogas yield. A first-order kinetic reaction model was used for predicting the yield of methane from these digesters. The rate constant values revealed an increasing trend, with the highest for protein then lipids then cellulose.

Fat, oil, and grease (FOG) deposits yield higher methane than FOG in anaerobic co-digestion with waste activated sludge

The formation of fat, oil, and grease (FOG) deposits in sewers is a global challenge for the maintenance of sewer collection systems. Tons of FOG deposits (FDs) are removed from sewer systems every year and present an opportunity for increased methane production via anaerobic co-digestion with waste activated sludge (WAS) at water resource recovery facilities with existing anaerobic digesters. We hypothesized that FDs have higher biomethane potential than that of FOG (e.g., FOG collected in grease interceptors), because of the reduction of inhibition of long chain fatty acids due to saponification. In this study, substantially enhanced methane production was found in anaerobic co-digestion of WAS with FDs within the substrate to inoculum (S/I) ratio range of 0.25-1.2, and the maximum ultimate methane production (685.7 ± 24.1 mL/gVS_added, at S/I = 0.5) was 4.0 times higher than in the control (with WAS only) after 42 days of incubation. Although the lag phase period was longer in FD co-digestion (S/I = 0.5) than in FOG co-digestion (S/I = 0.5) under the same organic loading (gVS) and two times the COD loading, the daily methane production rate became higher after Day 15 in FD co-digestion. Significantly higher cumulative methane production (10.2%, p < 0.05) was obtained in FD co-digestion than in FOG co-digestion after 42-days. Microbial community analysis revealed higher levels of Geobacter in FD co-digestion, possibly suggesting a role for direct interspecies electron transfer (DIET) between Methanosaeta and Geobacter. This work provides fundamental insights supporting anaerobic co-digestion of FDs with WAS, demonstrating the advantages of FDs compared to FOG as co-substrate for enhanced biomethane recovery.

Methane yields during anaerobic co-digestion of animal manure with other feedstocks: A meta-analysis

Anaerobic co-digestion of animal manure with other feedstocks (aka co-digestion) is increasingly being used to enhance methane yield and organic waste management. The benefits accruing from co-digestions compared to mono-digestions, however, vary greatly in the literature. The goal of this research was to use meta-analysis to critically compare methane yields between mono- and co-digestions and identify relevant factors (co-substrate type, substrate dose, carbon to nitrogen (C/N) ratio, volatile solids (VS), substrate pH, organic loading rate (OLR), and hydraulic retention time (HRT)) contributing to methane yield. Published studies (n = 64 representing 384 case-studies) with sufficient detail on methane yield were identified for the meta-analysis. Analysis indicated that co-digestion of animal manure with other feedstocks significantly increased methane yield (249 L kg^-1[VS]), compared with anaerobic mono-digestion of animal manure (171 L kg^-1[VS]). Similar methane yields increases (47-57 L kg^-1[VS]) were obtained from co-digestions in batch reactors of swine (238-287 L kg^-1[VS]), poultry (213-260 L kg^-1[VS]), and cattle manure (147-204 L kg^-1[VS]). In continuous digesters of cattle manure (175-299 L kg^-1[VS]) co-digestion had the greatest methane yield improvement of 124 L kg^-1[VS], swine manure (212-322 L kg^-1[VS]) co-digestion ranked second with 110 L kg^-1[VS], and poultry manure ranked third with 70 L kg^-1[VS]. Improved methane yield were obtained at optimum C/N ratio ranging from 26 to 34. The respective optimum OLR for co-digestion of swine, poultry, and cattle manure were 1.2, 1.4 and 3.4 kg VS m^-3 d^-1, while the recommended HRT was 30-40 d. Taken together, anaerobic co-digestion of animal manure with other feedstock significantly improved anaerobic digestion. Factors contributing to methane yields included: substrate-type and dose, VS, C/N, OLR, and HRT.

Modelization of anaerobic processes during co-digestion of slowly biodegradable substrates

The influence of the soluble substrates over the anaerobic processes has been extensively investigated, but little is known about the effects of particulate substrate. The biodegradation of these substrates starts with the hydrolytic step, this process is slower than the other ones involved in the biodegradation of particulate substrates and usually becomes the rate-limiting step. This study investigate the effect of the initial total solids (TS) concentration on the anaerobic co-digestion of two slowly biodegradable organic substrates. The wastes mixtures were prepared at different dilutions in the range from 10% to 28% TS. From these experiments it was observed that as TS concentration increased, the methane production decreased. These results were modelled and it was observed that neither hydrolysis nor fermentation stages controlled the methane production rate. Being a substrate inhibition event experienced at the methanogenic stage the responsible of the lower methane production when operating at high TS concentrations.

Nutrient balance for anaerobic co-digestion of tannery wastes: Energy efficiency, waste treatment and cost-saving

The macronutrients ratio present in tannery wastes is normally not ideal for anaerobic digestion (AD). In most cases, it is necessary to add nutrients to obtain a more balanced AD process and to ensure favorable conditions for the growth and metabolism of microorganisms. The aim of this study was to verify the influence of the components of nutrient solution added to AD of tannery wastes with regard to the energy and waste treatment efficiency and to the cost-saving analysis. The findings provides better understanding of the nutritional requirement of co-AD of tannery wastes, with the highest value of biogas production of 30.14 mL/g of added VSS, besides representing a step in the search for a more balanced, efficient and viable process. The results obtained proving the competitiveness of co-AD of tannery wastes instead of disposing it in landfills (saving off about 71% in terms of electric consumption).

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.

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.

Microbial community dynamics during anaerobic co-digestion of corn stover and swine manure at different solid content, carbon to nitrogen ratio and effluent volumetric percentages

The methane production and the microbial community dynamics of thermophilic anaerobic co-digestion (AD) of corn stover, swine manure and effluent were conducted at total solid (TS) content of 5%, 10% and 15%, the carbon to nitrogen ratio (C/N) of 20, 30 and 40 and the effluent volumetric percentage (EVP) of 20%, 40% and 60%. For batches with 5% TS, the highest methane yield of 238.5-283.1 mL g^-1 volatile solid (VS) and the specific methane productivity of 138.5-152.2 mL g^-1 initial VS were obtained at the C/N ratios of 20 and 30. For the mixtures with 10% and 15% TS, the highest methane yield was 341.9 mL g^-1 VS and 351.2 mL g^-1 VS, respectively, when the C/N ratio of 20% and 60% EVP conditions were maintained. Co-digestion of swine manure with corn stover caused an obvious shift in microbial population, in which the archaeal population changed from 0.3% to 2.8% and the bacterial community changed from 97.2% to 99.7%. The experimental batches with the highest relative abundance of the archaeal population (2.00% of total microbial population for 5% TS, 1.74% for 10% TS and 2.76% for 15% TS) had the highest rate of methanogenesis subsequently enhancing methane production (283.08 mL g^-1 VS for 5% TS, 341.91 mL g^-1 VS for 10% TS and 351.23 mL g^-1 VS for 15% TS). The results of microbiome analysis enabled understanding the key populations in biomethane generation.

Acid-fermented fish by-products broth: An influence to sludge reduction and biogas production in an anaerobic co-digestion

This study investigates the effects of anaerobic co-digestion of a fishery by-products broth (FFB) mixed with sewage sludge on biogas production and sludge reduction. A 5:5 mixing ratio of sewage sludge and FFB generated the highest removal efficiency of volatile solids (VS) (51.3%), total solids (TS) (48.7%) and chemical oxygen demand (COD) (51.3%). In addition, the highest biogas production (0.585 L/g·VS_in·d) was also achieved with a mixing ratio of 5:5, followed 0.305 L/g·VS_in·d for 7:3. The methane content in produced biogas was associated more than 70% with ratios of 7:3 and 5:5. Moreover, the 5:5 ratio produced the greatest amount of the energy recovered at 4.1 kWh, followed by the 7:3 mixing ratio at 3.4 kWh. Therefore, the mixing ratio from 7:3 to 5:5 for anaerobic co-digestion of sewage sludge and FFB may be suitable for biogas production and organic matter removal. However, a 5:5 mixing ratio is recommended for anaerobic digestion of sewage sludge and FFB because it has the highest digestion efficiency, can treat larger quantities of fish by-products, and can recover the most energy. Biogas produced by mixing FFB with sewage sludge opens up the possibility of biogas production using organic waste because of the higher methane content.

Synergistic effect of sugarcane scum as an accelerant co-substrate on anaerobic co-digestion with agricultural crop residues from non-centrifugal cane sugar agribusiness sector

Anaerobic co-digestion (AcoD) of the main residues from the non-centrifugal cane sugar (NCS) making process, agricultural crop residues (ACR) and sugarcane scum (SCS), was evaluated using biochemical methane potential tests. Substrates were pretreated: ACR through particle size reduction, and SCS with dilution. The maximum methane yield of 0.276 Nm³ CH₄ kg^-1 VS_added occurred at an ACR of 2 mm and at 12.5% dilution of SCS, at a ratio of 75:25 based on volatile solids, which was 30.2% and 5.9% higher compared to SCS and ACR in mono-digestion, respectively. ACR was a substrate of adequate buffer capacity for the AcoD stability, while the SCS, in addition to helping accelerate the process, also helped improve the inoculum's methanogenic and hydrolytic activity. The first-order kinetic and dual-pool two-step models were suitable to describe methane yield. AcoD of ACR with SCS is a good option for the treatment of streams in the NCS agribusiness sector.

Interactions between substrate characteristics and microbial communities on biogas production yield and rate

In this study, leather fleshing waste (LFW) and a complementary substrate (food waste leachate; FWL) were co-digested. The main focus of research was to study effects on biogas production caused by interactions between co-substrates when combined in different mixing ratios and changes on microbial community structures. Due to a positive effect of co-digestion (i.e., establishing nutrient equilibrium), the adjusted substrates for optimum C/N ratio by blending LFW and FWL resulted in significantly higher biodegradability and biomethane production (375.5-520.8 mL CH₄ g^-1 VS) than the mono-digestion of each substrate. According to co-digestion of LFW and FWL, microbial communities became more diverse and the changes of microbial structure influenced the biomethane production performance. Among the co-digesting conditions, the biomethane production yield and rate of the samples were in reverse order and the results were firmly associated with the relative richness of lipids (yield-related) and proteins (rate-related) in the co-substrates.

Interpretable machine learning for predicting biomethane production in industrial-scale anaerobic co-digestion

The objective of this study is to apply machine learning models to accurately predict daily biomethane production in an industrial-scale co-digestion facility. The methodology involved applying elasticnet, random forest, and extreme gradient boosting to input-output data from an industrial-scale anaerobic co-digestion (ACoD) facility. The models were used to predict biomethane for 1-day, 3-day, 5-day, 10-day, 20-day, 30-day, and 40-day time horizons. These models were fit on four years of operational data. The results showed that elastic net (a model with assumptions of linearity) was clearly outperformed by random forest and extreme gradient boosting (XGBoost), which had out-of-sample R²values ranging between 0.80 and 0.88, depending on the time horizon. In addition, feature importance and partial dependence analysis demonstrated the marginal and interaction effects on biomethane of selected biowaste inputs. For instance, food waste co-digested with percolate were shown to have strong positive interaction effects. One implication of this study is that XGBoost and random forest algorithms applied to industrial-scale ACoD data provide dependable prediction results and may be a useful complement for experimental and mechanistic/theoretical models of anaerobic digestion, especially where detailed substrate characterization is difficult. However, these models have limitations, and suggestions for deriving additional value from these methods are proposed.

Molecular weight distribution of pretreated thickened waste activated sludge and fat, oil, and grease

Co-digestion samples containing thickened waste activated sludge and fat, oil and grease were subjected to three different pretreatment methods, i.e., microwave at 175 °C, hyper-thermophilic stage at 70 °C, and conventional heat at 70 °C. The soluble matter extracted from the un-pretreated and pretreated samples were subjected to an ultrafiltration (UF) process using four different membrane sizes (300, 100, 10, and 1 kDa) for molecular weight distribution analysis. Every pretreatment method had a different effect on the solubilization and redistribution of the soluble matter (SCOD and TVFA). For example while MW pretreatment resulted in a significant increase in the SCOD at the lowest molecular weight (< 1 kDa) and at the highest molecular weight (> 300 kDa), Hyper pretreatment caused the majority of the SCOD ( ̴ 62.7% of total SCOD) to be concentrated at the smaller molecular weight range (< 10 kDa). The MW and hyper-thermophilic pretreatments were much more effective in increasing samples solubilization and biogas production compared to the conventional heat pretreatment. The hyper-thermophilic samples had the maximum improvement in cumulative biogas production from all the molecular weights compared to MW- and Heat-pretreated samples; Hyper-pretreated samples achieved 86.5% higher cumulative biogas production compared to the control.

Improving performance and phosphorus content of anaerobic co-digestion of dairy manure with aloe peel waste using vermiculite

Phosphorus content of the digestate is crucial for evaluating its fertilizer utilization in anaerobic digestion system. The vermiculite containing rich-phosphorus is firstly used as an accelerant in anaerobic batch co-digestion system of aloe peel waste and dairy manure. After introducing vermiculite, the cumulative biogas production (295.14-353.96 mL/g VS), chemical oxygen demand removal rate (45.53%-71.03%), and volatile solid removal rate (50.70%-52.76%) are remarkably higher than those of reference reactor (234.08 mL/g VS, 39.38%, 45.10%). The thermal and fertility analyses manifest the digestates with vermiculite possess superior stability, admirable fertilizer values (5.97%-6.81%), and excellent total phosphorus content (11.44-13.29 g/kg). The improved co-digestion performance can be attributed to the addition of vermiculite. This work introduces a novel approach for improving the performance of anaerobic co-digestion and the fertilizer utilization of digestate in the co-digestion systems.

Co-digestion of palm oil mill effluent for enhanced biogas production in a solar assisted bioreactor: Supplementation with ammonium bicarbonate

Lack of sufficient nitrogenous substrate and buffering potential have been acknowledged as impediments to the treatment of palm oil mill effluent through co-digestion processes. In this study, ammonium bicarbonate was used to provide the nitrogenous substrate and buffering potential. To regulate the impact of ammonium bicarbonate toxicity on the anaerobic co-digestion system, dosages from 0 to 40 mg/L were supplemented. The biogas yield was used to indicate the effects of NH₄⁺ toxicity. In a solar-assisted bioreactor, solar radiation was first collected by a solar panel and converted into electricity, which was then used to heat a mixture of palm oil mill effluent and cattle manure to maintain the reactor in the mesophilic temperature range. This co-digestion operation was performed semi-continuously and was analyzed at a 50:50 mixing ratio of palm oil mill effluent and cattle manure. The results indicate that the additional dosing of ammonium bicarbonate can significantly enhance biogas production. Maximum cumulative biogas and methane productions of 2034.00 mL and 1430.51 mL, respectively, were obtained with the optimum addition of 10 mg/L ammonium bicarbonate; these values are 29.80% and 42.30% higher, respectively, than that obtained in the control co-digestion operation without addition of ammonium bicarbonate. Utilization of a mathematical equation (G = Gm^k/t) to describe a kinetic analysis of the biogas yield also indicated that the optimum ammonium bicarbonate dose was 10 mg/L. The results of this study suggest that supplementation with ammonium bicarbonate doses of up to 40 mg/L can be used to provide nitrogenous substrates and buffering potential in anaerobic co-digestion processes. The determination of the optimal dose provides an alternative and efficient option for enhanced biogas production, which will have obvious economic advantages for feasible industrial applications.

Methane production by co-digestion of poultry manure and lignocellulosic biomass: Kinetic and energy assessment

Six typical Brazilian lignocellulosic biomasses (rice straw, corn cob, peanut shell, sawdust, coffee husk and sugarcane bagasse) were evaluated for methane production by solid-state anaerobic co-digestion with poultry manure. The results showed the highest methane production was obtained with corn cob and poultry manure (126.02 Nm³ CH₄. ton residue^-1) using a food to inoculum ratio of 0.5, which lowered volatile fatty acids accumulation. In this condition, the thermal energy production (1.73 MJ.kg live chicken^-1) would be able to replace 53.2% of the energy with firewood in poultry farming. The high hemicellulose and low lignin content in corn cob seem to explain the biomethanation of such biomass, and this agrees with the microbial analysis which revealed the predominance of bacteria related to plant polysaccharides hydrolysis and carbohydrate conversion in the inoculum. The methane production was best modelled by Groot's multi-stage model, and the microbial adaptation to lignin might explain this.

Evaluation of biomethanization during co-digestion of thermally pretreated microalgae and waste activated sludge, and estimation of its kinetic parameters

The maximum methane yield that can be obtained from anaerobic co-digestion of microalgae and waste activated sludge (WAS) mixtures, after thermal pretreatment at 65 °C during 4 h, was investigated. Furthermore, the fitting of the experimental data by five kinetic models (first-order, second-order, modified Gompertz, Logistic, and two-substrate) was evaluated. Thermal pretreatment increased the methane yield of single microalgae and WAS digestion by ≈ 44 and by ≈ 52%, respectively. The results also showed that up to 60% of WAS can be co-digested with microalgae without impairing the methane yield, producing up to 338 mL_CH4 g_VS^-1. Data from digestion of non-pretreated microalgae and WAS were well described by all kinetic models, but digestion of thermally pretreated microalgae, WAS, and their co-digestion mixtures, was best fitted by means of a two-substrate model, indicating that after pretreatment it is necessary to take into account the contribution of both rapidly and slowly biodegradable fractions.

The effects of thiosulfinates on methane production from anaerobic co-digestion of waste activated sludge and food waste and mitigate method

Thiosulfinates, a natural antibiotic, existed in all parts of Allium, therefore might be accumulated in large amounts in food waste (FW). FW was often added into waste activated sludge (WAS) anaerobic digestion process as a kind of supplement for nutrition balance. However, the impact of thiosulfinates on methane production and the possible approach to mitigate its inhibition on the co-digestion process could be available in few literatures. This work was carried out in a series of batch experiment at pH 7.0 ± 0.2 and 35 ± 1.0 ℃ to promote the further understanding of this process. The experimental results showed that the methane accumulation decreased from 270.6 ± 13.4 to 16.7 ± 7.0 mL/g VSS (volatile suspended solids) when the initial concentration of thiosulfinates increased from 0 to 2.5 μg/g VSS. The activities of functional enzymes (F₄₂₀ and CoM) were inhibited by 99.06% and 99.82% compared with control group when reactor contained 2.5 μg/g VSS thiosulfinates. Furthermore, different temperature, pH, and combination pretreat were applied to impair the inhibition of thiosulfinate. Compared with no pretreatment group, methane yield was increased by 2.26, 32.18 and 42.2-fold, respectively which group was under pretreatment method of heat (100 ℃), alkali (pH 9) and combination.

Anaerobic co-digestion of fish processing waste with a liquid fraction of hydrothermal carbonization of bamboo residue

The effect of different mixing ratios of fish processing waste (FPW) with a liquid fraction (LF) of hydrothermal carbonization (HTC) of bamboo residues on biogas and methane yield was investigated. The different mixing ratios (FPW + LF) and HTC temperature (200-280 °C) had significant effects on biogas and methane production. The anaerobic co-digestion of the various mixing ratio of FPW and LF of bamboo residues did not enhance the methane yield compared to the AD of FPW alone. However, a mixture of 75FPW + 25LF(2 2 0) presented a comparable methane production (133 mL/g VS) to that achieved with 100FPW (142 mL/g VS), which represents an increase of only 6.4%. The ratio of 75FPW + 25LF(2 2 0) increased the biogas yield by 81% compared to the control group of 100LF(2 2 0). The mixing ratio of 75 FPW + 25LF(2 2 0) did not require clean water input to dilute FPW for biogas production and can be a practical waste management method.

Sulfate in anaerobic co-digester accelerates methane production from food waste and waste activated sludge

The presence of sulfate in food waste (FW) and waste activated sludge (WAS) threatens the anaerobic co-digestion for methane production. In this study, methane production from the anaerobic co-digestion of FW and WAS at sulfate concentrations of 50, 100, and 400 mg S/L was not affected, but instead deteriorated at 200 and 300 mg S/L. However, a model-based kinetic analysis reveals that sulfate can significantly promote the conversion of rapidly biodegradable substrates by up to 93%. From a point of thermodynamic view, the presence of sulfate can stimulate sulfate-reducing bacteria acting as acetogens to convert propionate to acetate, providing an alternative metabolic pathway for methanogenesis. In the anaerobic co-digestion, regulation of sulfate can be a potential strategy to improve the efficiency of methane production. However, more research is needed to optimize the sulfate concentration and substrate types in the anaerobic co-digester.

Increased loading stress leads to convergence of microbial communities and high methane yields in adapted anaerobic co-digesters

Enhancing biogas production, while avoiding inhibition of methanogenesis during co-digestion of grease interceptor waste (GIW), can help water resource recovery facilities reduce their carbon footprint. Here we used pre-adapted and non-adapted digesters to link microbial community structure to digester function. Before disturbance, the pre-adapted and non-adapted digesters showed similar methane production and microbial community diversity but dissimilar community composition. When exposed to an identical disturbance, the pre-adapted digester achieved better performance, while the non-adapted digester was inhibited. When re-exposed to disturbance after recovery, communities and performance of both digesters converged, regardless of the temporal variations. Co-digestion of up to 75% GIW added on a volatile solids (VS) basis was achieved, increasing methane yield by 336% from 0.180 to 0.785 l-methane/g-VS-added, the highest methane yield reported to date for lipid-rich waste. Progressive perturbation substantially enriched fatty acid-degrading Syntrophomonas from less than 1% to 24.6% of total 16S rRNA gene sequences, acetoclastic Methanosaeta from 2.3% to 11.9%, and hydrogenotrophic Methanospirillum from less than 1% to 6.6% in the pre-adapted digester. Specific hydrolytic and fermentative populations also increased. These ecological insights demonstrated how progressive perturbation can be strategically used to influence methanogenic microbiomes and improve co-digestion of GIW.

Multi-hydrolytic enzyme accumulation and microbial community structure of anaerobic co-digestion of food waste and waste-activated sludge

The accumulation of multi-hydrolytic enzyme through anaerobic co-digestion of waste-activated sludge (WAS) and food waste (FW) was studied by regulating temperature, pH and the mass ratio of FW to WAS (F/W). Experimental results showed that temperature had a profound effect on the activity of the enzyme and the most suitable temperatures for the accumulation of amylase and protease were 37°C and 50°C, respectively. The highest activity of amylase and protease accumulated reached 10.29 and 19.23 U/mL at an F/W ratio of 2:1. The addition of anaerobic co-digestion solution enriching protease and amylase had positive effects on the hydrolysis of WAS. In addition, the Illumina high-throughput sequencing demonstrated that the bacterial diversity decreased, but the bacterial abundance increased during the co-digestion process of WAS and FW. The predominant strains for secreting amylase were Lactobacillus and Clostridium-sensu-strito-1, and Aeromonas was the dominant strain for secreting protease.

Microbial community acclimatization for enhancement in the methane productivity of anaerobic co-digestion of fats, oil, and grease

The methane productivity and long chain fatty acids (LCFAs) degradation capability of unacclimatized seed sludge (USS) and acclimatized seed sludge (ASS) at different substrate ratios of fats oil and grease (FOG) and mixed sewage sludge were investigated in this study. Biogas produced in ASS in initial phase of anaerobic digestion had higher methane content (65-76%) than that in USS (26-73%). The degradation of major LCFAs in the ASS was 22-80%, 33-191%, and 7-64% higher for the substrate ratios of 100:10, 100:20, and 100:30, respectively, as compared to the LCFAs' degradation in USS. Microbial acclimatization increased the population of Firmicutes (40%), Bacteroidetes (32%), Synergistetes (10%), and Euryarchaeota (8%) in ASS, which supported the faster rate of LCFAs degradation for its later conversion to methane. The significant abundance of Syntrophomonas and Methanosarcina genera in ASS supported faster generation rate of methane in an obligatory syntrophic relationship.

Enhanced anaerobic co-digestion of fat, oil, and grease by calcium addition: Boost of biomethane production and microbial community shift

This work focused on the application of calcium (0.1-1% w/v) to overcome the inhibition caused by the high loadings (2% v/v) of fat, oil, and grease (FOG) in the context of biomethane production, organic removal, and microbial community shift. Addition of 0.5% calcium showed maximum biomethane production (6-fold increase); biomethane production decreased following the addition of calcium (>0.5%). The highest organic removal rates were 83 and 89% upon the addition of 0.3 and 0.5% calcium, respectively. Addition of calcium facilitated the growth of bacteria of phylum Firmicutes from the Clostridium, Syntrophomonas, and Sedimentibacter genera. The population of members from the genus Methanosaeta increased after the addition of 0.5% calcium, which is one of the factors responsible for high biomethane production. This study demonstrated that addition of calcium is an attractive strategy to avoid the inhibition of the growth of anaerobic microflora due to the presence of high FOG concentrations.

Anaerobic digestion of mixed urban biowaste: The microbial community shift towards stability

Anaerobic digestion is applied worldwide to treat food waste (FW) with the aim of obtaining renewable bioenergy by exploiting the methane gas produced. However, there are several problems in practical applications, primarily due to system instability. Although exhaustive knowledge regarding anaerobic microbial community composition has been established, few studies have investigated long-term correlations between microbial consortia, operative conditions and feedstock characteristics. Here, microbial community shifts as a response to feedstock variations were investigated in long-term semi-continuous systems, which were evaluated by an in situ cell detection method and 16S rRNA gene amplicon sequencing. FW digestion showed progressive system instability caused by the inhibition of methanogens, which resulted in volatile fatty acid accumulation and process failure at the low organic loading rate (OLR). Conversely, by co-digesting FW with waste-activated sludge (WAS), a stable process with methane yields of up to 0.27 Nm³  kg^-1VS_fed for OLR = 1.7 gVS L^-1d^-1 was achieved. This stabilizing effect was not related to the buffering capacity of WAS, but to its capacity to avoid volatile fatty acid accumulation and falls in pH by overcoming methanogenic activity inhibition. WAS addition promoted the establishment of a stable and active archaeal population in anaerobic co-digestion (AcoD) reactors. The continuous supply of trace elements together with the seeding of microbial functional groups were the main drivers that positively affected process stability.

A simultaneous assessment of organic matter and trace elements bio-accessibility in substrate and digestate from an anaerobic digestion plant

This study evaluates a simultaneous assessment of organic matter (OM) and trace elements (TE) bio-accessibility in substrate and digestate from a full-scale anaerobic digester by a sequential OM extraction method. Simultaneous release of TE was determined along with the extraction of different OM fractions and the effects of extracting reagents on characteristics of OM were evaluated by nuclear magnetic resonance (NMR) spectroscopy. The reagents used for sequential extraction of OM were not enough selective. However, proteins were particularly removed by 0.1 M NaOH, while 72% H₂SO₄ mainly extracted hemicellulose and cellulose. The OM fractionation allowed for simultaneous extraction of >60% of total As, Cd, Co, Fe, Mn, Ni and Zn, while the extraction was limited for Al, Cr, Cu, Mo, and Pb. In substrate, >50% of total As, Co, Mn and Ni and <40% of total Fe, Zn and Mo were identified in bio-accessible fractions. In digestate, all elements demonstrated poor bio-accessibility except for As.