Identification of synergistic impacts during anaerobic co-digestion of organic wastes

Fractional calculus as a generalized kinetic model for biochemical methane potential tests

This study presents a fractional calculus model as a generalized kinetic model for estimating the maximum methane yield and degradation kinetics in biomethane potential (BMP) assays, a key analytical method in anaerobic digestion research and application. The fractional model outperformed common first-order kinetic models by yielding superior data fitting and properly managing substrate heterogeneity. The fractional model showed robust performance in mono-digestion, co-digestion and pre-treatment BMP assays with or without presence of large tailing or sigmoidal patterns in the BMP curve. The main advantage of the fractional model over other models is its ability to capture the complexities of the methane production process without losing model accuracy. Assessment of the mathematical model revealed that for fractional orders greater than 0.8 the Mittag-Leffler sequence could be transformed into a more computationally efficient exponential function.

Impacts of seasonal variation on volatile fatty acids production of food waste anaerobic fermentation

This study aims to investigate the influence of seasonal variations on Volatile fatty acids (VFAs) production from food waste (FW) and to quantify their impact. Results of batch experiments with external pH control indicated that the properties of FW exhibited significant seasonal variations and were markedly different from kitchen waste (KW). The spring group demonstrated the highest VFA concentration and VFA/SCOD, at 31.24 g COD/L and 92.20 % respectively, which were 1.22 and 1.27 times higher than those observed in the summer season. The combined proportion of acetic acid and butyric acid accounted for 81.10 % of the total VFAs in spring, suggesting the highest applicability to the carbon source. The VFA content of all seasonal groups in descending order was butyric acid, propionic acid and acetic acid. Carbohydrates, along with spicy and citrusy substances, promoted the conversion of total VFA and butyric acid, while proteins and lipids favored the production of acetic acid and propionic acid.

Enhanced digestion of sludge via co-digestion with food waste in a high-solid anaerobic membrane bioreactor: Performance evaluation and microbial response

A 15 L high-solid mesophilic AnMBR was operated for the digestion of food waste, primary sludge and excess sludge. The digestion performance was evaluated from the perspective of methane generation, permeate quality and organic reduction. Furthermore, the change in the microbial community was investigated by 16S rRNA gene analysis. The results showed that the introduction of sludge decreased the H2S levels in biogas compared with the mono-digestion of food waste and the co-digestion with food waste increased biogas and methane production compared with the mono-digestion of sludge. A substitution ratio of 25 % became a turning point of permeate composition and reaction rates. The energy recovery ratios of the mesophilic AnMBR were over 75 % based on stoichiometric analysis. In reaction kinetics analysis, hydrolysis as the first step of anaerobic digestion was found to be most influenced by the composition of the substrate. Finally, the microbial community structures were stable under tested conditions while the evolutionary relationships within the dominant phyla were observed. In the archaea community, Methanosaeta was the dominant methanogen regardless sludge ratio in the substrate.

Effect of alkaline pre-treatment on hydrolysis rate and methane production during anaerobic digestion of paunch solid waste

Paunch is comprised of the partially digested feed contained in cattle or sheep and contributes 20-50% of organic waste produced at red meat processing facilities. Anaerobic digestion has been identified as a promising technology for paunch treatment, however treatment times can be long and when combined with the moderate degradability of paunch this results in high treatment costs that need to be improved. Pre-treatment was investigated as a strategy to improve AD of paunch, alkaline treatment (NaOH or KOH) was selected due to the high lignin content. A range of alkaline loadings (1-20 g 100gTS-1) were tested with an equivalent hydroxide molar concentration of 9-250 mM [OH-]. Alkaline pre-treatment improved both the hydrolysis rate and the overall degradability of paunch solid by up to 4.4 times and 60%, respectively. The enhanced hydrolysis rate and methane yield was correlated to changes in material composition during pre-treatment. While alkaline concentration was an important factor, there were no significant improvements at alkaline concentrations above 12 g 100gTS-1 (150 mM [OH-]).

A Review on Performance Improvement of Anaerobic Digestion Using Co-Digestion of Food Waste and Sewage Sludge

Anaerobic co-digestion (AcoD) is a vital technology in the decarburization of the economy because of its ability to process organic waste, recover nutrients, and create biogas as a sustainable biofuel all at the same time. This attribute also makes this technology a viable partner in pursuing a circular economic model. However, the poor biogas output of typical substrates like sewage sludge and animal manure and the hefty installation costs limit its viability. This review paper with literature analysis provides a good grasp of the anaerobic co-digesting process with diverse food digestion methods. In this survey, we have analyzed the Anaerobic Digestion of water waste, food waste, and animal manure and the anaerobic co-digestion of animal waste with water waste and food waste with water waste. This analysis demonstrates that anaerobic co-digestion produces more methane biogas than anaerobic digestion. Also, it has been shown that by adjusting the ratio of food and animal waste to water waste, we can produce more methane. In the future, we would like to supplement anaerobic co-digestion by altering the proportion of different wastes that are mixed with water waste in order to increase methane production.

Systematical analysis of sludge treatment and disposal technologies for carbon footprint reduction

This study aims to comprehensively analyze the Greenhouse Gases (GHGs) emissions from current sewage sludge treatment and disposal technologies (building material, landfill, land spreading, anaerobic digestion, and thermochemical processes) based on the database of Science Citation Index (SCI) and Social Science Citation Index (SSCI) from 1998 to 2020. The general patterns, spatial distribution, and hotspots were provided by bibliometric analysis. A comparative quantitative analysis based on life cycle assessment (LCA) put forward the current emission situation and the key influencing factors of different technologies. The effective GHG emissions reduction methods were proposed to mitigate climate change. Results showed that incineration or building materials manufacturing of highly dewatered sludge, and land spreading after anaerobic digestion have the best GHG emissions reduction benefits. Biological treatment technologies and thermochemical processes have great potential for reducing GHGs. Enhancement of pretreatment effect, co-digestion, and new technologies (e.g., injection of carbon dioxide, directional acidification) are major approaches to facilitate substitution emissions in sludge anaerobic digestion. The relationship between the quality and efficiency of secondary energy in thermochemical process and GHGs emission still needs further study. Solid sludge products generated by bio-stabilization or thermochemical processes are considered to have a certain carbon sequestration value and can improve the soil environment to control GHG emissions. The findings are useful for future development and processes selection of sludge treatment and disposal facing carbon footprint reduction.

Impact of primary treatment methods on sludge characteristics and digestibility, and wastewater treatment plant-wide economics

Biogas production from anaerobic sludge digestion plays a central role for wastewater treatment plants to become more energy-efficient or even energy-neutral. Dedicated configurations have been developed to maximize the diversion of soluble and suspended organic matter to sludge streams for energy production through anaerobic digestion, such as A-stage treatment or chemically enhanced primary treatment (CEPT) instead of primary clarifiers. Still, it remains to be investigated to what extent these different treatment steps affect the sludge characteristics and digestibility, which may also impact the economic feasibility of the integrated systems. In this study, a detailed characterization has been performed for sludge obtained from primary clarification (primary sludge), A-stage treatment (A-sludge) and CEPT. The characteristics of all sludges differed significantly from each other. The organic compounds in primary sludge consisted mainly of 40% of carbohydrates, 23% of lipids, and 21% of proteins. A-sludge was characterized by a high amount of proteins (40%) and a moderate amount of carbohydrates (23%), and lipids (16%), while in CEPT sludge, organic compounds were mainly 26% of proteins, 18% of carbohydrates, 18% of lignin, and 12% of lipids. The highest methane yield was obtained from anaerobic digestion of primary sludge (347 ± 16 mL CH₄/g VS) and A-sludge (333 ± 6 mL CH₄/g VS), while it was lower for CEPT sludge (245 ± 5 mL CH₄/g VS). Furthermore, an economic evaluation has been carried out for the three systems, considering energy consumption and recovery, as well as effluent quality and chemical costs. Energy consumption of A-stage was the highest among the three configurations due to aeration energy demand, while CEPT had the highest operational costs due to chemical use. Energy surplus was the highest by the use of CEPT, resulting from the highest fraction of recovered organic matter. By considering the effluent quality of the three systems, CEPT had the highest benefits, followed by A-stage. Integration of CEPT or A-stage, instead of primary clarification in existing wastewater treatment plants, would potentially improve the effluent quality and energy recovery.

A comparative life cycle assessment of anaerobic mono- and co-digestion of livestock manure in Bangladesh

Proper management of biogenic residues, particularly livestock manure and food waste, is a major challenge for Bangladesh. While mono-digestion has traditionally been used on farms for treating manure, inadequate energetic output limits its applicability. Food waste, however, is typically landfilled in current practice. Co-digestion of biowaste emerged as an alternative due to synergistic yield and capacity to handle multiple waste streams. However, its environmental performance is underreported, particularly in developing countries. This study aimed to compare the environmental implications of co-digestion and mono-digestion of livestock manure (poultry and cow manure) with food waste from a life cycle assessment perspective for the regional context of Bangladesh. Two inventory cases were considered, accounting for mechanistically calculated (case M) and experimentally reported synergistic biogas yield (case E). Co-digestion scenarios showed net benefits by reducing three of the five impact categories-considerably reducing climate change (up to 117%), eutrophication potential, and terrestrial ecotoxicity in both cases (54.5 % and 55.7 %, respectively). The highest decrease occurred for climate change by diverting food waste landfilling. However, when synergistic biogas yield was considered, acidification potential and malodor air emissions increased by co-digestion owing to a higher amount of hydrogen sulfide and ammonia in the produced gas, thus entailing significant environmental burdens. The key hotspot in most categories was open storage of digestate, necessitating appropriate post-treatment.

Anaerobic digestion of fruit and vegetable waste: a critical review of associated challenges

The depletion of fossil fuels coupled with stringent environmental laws has encouraged us to develop sustainable renewable energy. Due to its numerous benefits, anaerobic digestion (AD) has emerged as an environment-friendly technology. Biogas generated during AD is primarily a mixture of CH₄ (65-70%) and CO₂ (20-25%) and a potent energy source that can combat the energy crisis in today's world. Here, an attempt has been made to provide a broad understanding of AD and delineate the effect of various operational parameters influencing AD. The characteristics of fruit and vegetable waste (FVW) and its feasibility as a potent substrate for AD have been studied. This review also covers traditional challenges in managing FVW via AD, the implementation of various bioreactor systems to manage large amounts of organic waste and their operational boundaries, microbial consortia involved in each phase of digestion, and various strategies to increase biogas production.

The characteristics of solid-phase substrate during the co-fermentation of lignite and straw

Co-fermentation of lignite and biomass has been considered as a new approach in achieving clean energy. Moreover, the study of the characteristics of solid phase in the synergistic degradation process is of great significance in revealing their synergistic relationship. Accordingly, in order to produce biogas, lignite, straw, and the mixture of the two were used as the substrates, the solid phase characteristics of which were analyzed before and after fermentation using modern analytical methods. The results revealed that the mixed fermentation of lignite and straw promoted the production of biomethane. Moreover, the ratios of C/O and C/H were found to be complementary in the co-fermentation process. Furthermore, while the relative content of C-C/C-H bonds was observed to be significantly decreased, the aromatics degree of lignite was weakened. Also, while the degree of branching increased, there found to be an increase in the content of cellulose amorphous zone, which, consequently, led to an increase in the crystallinity index of the wheat straw. Hence, the results provide a theoretical guidance for the efficient utilization of straw and lignite.

Anaerobic Co-Digestion with Food Waste: A Possible Alternative to Overcome the Energy Deficit of Sludge Thermal Pretreatment

Thermal pretreatment (TP) was an effective method to improve the anaerobic digestion of waste-activated sludge. In order to balance the energy consumption of sludge TP integrated with anaerobic digestion, food waste was introduced as a co-substrate to achieve an energy self-sustainable sludge treatment system. An anaerobic biodegradability test was performed using thermal pretreated sludge and food waste in order to clarify the kinetics and mechanism of co-digestion, especially the synergetic effect on specific methane yield. The prominent synergetic effect was an initial acceleration of cumulative methane production by 20.7-23.8% observed during the first 15 days. The modified Gompertz model presented a better agreement of the experimental results, and it was a suitable tool for methane production prediction of mono- and co-digestion. The energy assessment showed that co-digestion with food waste was a sustainable solution. When the moisture content of the TP sludge was 80-90%, the energy compensation required was about 0.04-0.22 t VS_Foodwaste/t VS_Sludge, which could maintain the integration of neutral or even positive energy between TP and anaerobic digestion.

Kinetic model discrimination on the biogas production in thermophilic co-digestion of sugarcane vinasse and water hyacinth

Co-digestion between sugarcane vinasse (Vn) and water hyacinth (WH) at various mixing ratios of 0:1, 1:0, 1:3, 3:1, and 1:1 was carried out under thermophilic conditions (55 °C) for 60 days. The effect of various mixing ratios on the pH changes, soluble chemical oxygen demand (sCOD) reduction, and cumulative biogas production was investigated. The first order, modified Gompertz, and logistic function kinetic models were selected to fit the experimental data. Model discrimination was conducted through the Akaike Information Criterion (AIC). The study revealed that co-digestion shows better performance compared to the mono-digestion of both substrates. Vn:WH mixing ratio 1:1 with inoculum to substrate ratio (ISR) of 0.38 g VS_inoculum/g VS_substrate is the most favorable ratio, achieving sCOD reduction efficiency and cumulative biogas production of 71.6% and 1229 mL, respectively. Model selection through AIC revealed that ratio 1:1 was best fitted to the logistic function kinetic model (R² = 0.9897) with Y_m and K values of 1232 mL and 31 mL/day, respectively.

Investigating microbial dynamics and potential advantages of anaerobic co-digestion of cheese whey and poultry slaughterhouse wastewaters

Resource recovery and prevention of environmental pollution are key goals for sustainable development. It is widely reported that agro-industrial activities are responsible for the discharge of billions of liters of wastewater to the environment. Anaerobic digestion of these energy rich agro-industrial wastewaters can simultaneously mitigate environmental pollution and recover embedded energy as methane gas. In this study, an assessment of mono- and co-digestion of cheese whey wastewater (CWW) and poultry slaughterhouse wastewater (PSW) was conducted in 2.25-L lab-scale anaerobic digesters. Treatment combinations evaluated included CWW (R1), PSW (R2), 75:25 CWW:PSW (R3), 25:75 CWW:PSW (R4), and 50:50 CWW:PSW (R5). The digestion efficiencies of the mixed wastewaters were compared to the weighted efficiencies of the corresponding combined mono-digested samples. R4, with a mixture of 25% CWW and 75% PSW, achieved the greatest treatment efficiency. This corresponded with an average biodegradability of 84%, which was greater than for R1 and R2 at 68.5 and 71.9%, respectively. Similarly, R4 produced the highest average cumulative methane value compared to R1 and R2 at 1.22× and 1.39× for similar COD loading, respectively. The modified Gompertz model provided the best fit for the obtained methane production data, with lag time decreasing over progressive treatment cycles. PCoA and heatmap analysis of relative microbial abundances indicated a divergence of microbial communities based on feed type over the treatment cycles. Microbial community analysis showed that genus Petrimonas attained the highest relative abundance (RA) at up to 38.9% in the first two cycles, then subsequently decreased to near 0% for all reactors. Syntrophomonas was highly abundant in PSW reactors, reaching up to 36% RA. Acinetobacter was present mostly in CWW reactors with a RA reaching 56.5%. The methanogenic community was dominated by Methanothrix (84.3–99.9% of archaea). The presence of phosphate and Acinetobacter in CWW feed appeared to reduce the treatment efficiency of associated reactors. Despite Acinetobacter being strictly aerobic, previous and current results indicate its survival under anaerobic conditions, with the storage of phosphate likely playing a key role in its ability to scavenge acetate during the digestion process.

Current Progress in Optimising Sustainable Energy Recovery From Cattle Paunch Contents, a Slaughterhouse Waste Product

Paunch contents are the recalcitrant, lignocellulose-rich, partially-digested feed present in the rumen of ruminant animals. Cattle forage in Europe is primarily from perennial and Italian ryegrasses and/or white clover, so paunch contents from forage-fed cattle in Europe is enriched in these feedstuffs. Globally, due to its underutilisation, the potential energy in cattle paunch contents annually represents an energy loss of 23,216,548,750–27,804,250,000 Megajoules (MJ) and financial loss of up to ~€800,000,000. Therefore, this review aims to describe progress made to-date in optimising sustainable energy recovery from paunch contents. Furthermore, analyses to determine the economic feasibility/potential of recovering sustainable energy from paunch contents was carried out. The primary method used to recover sustainable energy from paunch contents to-date has involved biomethane production through anaerobic digestion (AD). The major bottleneck in its utilisation through AD is its recalcitrance, resulting in build-up of fibrous material. Pre-treatments partially degrade the lignocellulose in lignocellulose-rich wastes, reducing their recalcitrance. Enzyme systems could be inexpensive and more environmentally compatible than conventional solvent pre-treatments. A potential source of enzyme systems is the rumen microbiome, whose efficiency in lignocellulose degradation is attracting significant research interest. Therefore, the application of rumen fluid (liquid derived from dewatering of paunch contents) to improve biomethane production from AD of lignocellulosic wastes is included in this review. Analysis of a study where rumen fluid was used to pre-treat paper sludge from a paper mill prior to AD for biomethane production suggested economic feasibility for CHP combustion, with potential savings of ~€11,000 annually. Meta-genomic studies of bacterial/archaeal populations have been carried out to understand their ruminal functions. However, despite their importance in degrading lignocellulose in nature, rumen fungi remain comparatively under-investigated. Further investigation of rumen microbes, their cultivation and their enzyme systems, and the role of rumen fluid in degrading lignocellulosic wastes, could provide efficient pre-treatments and co-digestion strategies to maximise biomethane yield from a range of lignocellulosic wastes. This review describes current progress in optimising sustainable energy recovery from paunch contents, and the potential of rumen fluid as a pre-treatment and co-substrate to recover sustainable energy from lignocellulosic wastes using AD.

Evaluation of methane production from the anaerobic co-digestion of manure of guinea pig with lignocellulosic Andean residues

The objective of this research was to evaluate anaerobic co-digestion of guinea pig manure (GP) with Andean agricultural residues such as amaranth (AM), quinoa (QU) and wheat (TR) in batch biodigesters under mesophilic conditions (37 ⁰C) for 40 days. As microbial inoculum, sewage treatment sludge was used in two inoculum/substrate ratios (ISR of 1 and 2). In terms of methane production, the best results occurred in treatments containing AM and QU as co-substrate and an ISR of 2. Thus, the highest methane production yield in the GP:AM biodigesters (25:75) and GP:QU (25:75) with 341.86 mlCH₄/g VS added and 341.05 mlCH₄/g VS added, respectively. On the other hand, the results showed that methane production with an ISR of 2 generated higher yields for guinea pig waste and the methane fraction of the biogas generated was in a range from 57 to 69%. Methane production kinetics from these raw materials was studied using five kinetic models: modified Gompertz, logistic equation, transfer, cone and Richards. The cone model adjusted best to the experimental values ​​with those observed with r² of 0.999 and RMSE of 1.16 mlCH₄/g VS added. Finally, the highest biodegradability (experimental yield/theoretical yield) was obtained in the GP-AM biodigesters (25:75) with 67.92%.

Microbial community assembly and dynamics in Granular, Fixed-Biofilm and planktonic microbiomes valorizing Long-Chain fatty acids at 20 °C

Distinct microbial assemblages evolve in anaerobic digestion (AD) reactors to drive sequential conversions of organics to methane. The spatio-temporal development of three such assemblages (granules, biofilms, planktonic) derived from the same inoculum was studied in replicated bioreactors treating long-chain fatty acids (LCFA)-rich wastewater at 20 °C at hydraulic retention times (HRTs) of 12-72 h. We found granular, biofilm and planktonic assemblages differentiated by diversity, structure, and assembly mechanisms; demonstrating a spatial compartmentalisation of the microbiomes from the initial community reservoir. Our analysis linked abundant Methanosaeta and Syntrophaceae-affiliated taxa (Syntrophus and uncultured) to their putative, active roles in syntrophic LCFA bioconversion. LCFA loading rates (stearate, palmitate), and HRT, were significant drivers shaping microbial community dynamics and assembly. This study of the archaea and syntrophic bacteria actively valorising LCFAs at short HRTs and 20 °C will help uncover the microbiology underpinning anaerobic bioconversions of fats, oil and grease.

Methanogenic performance and microbial community during thermophilic digestion of food waste and sewage sludge in a high-solid anaerobic membrane bioreactor

The methanogenic performance and microbial community of the thermophilic anaerobic mono-digestion and co-digestion of food waste and sewage sludge in a high-solid membrane bioreactor were investigated by a continuous experiment. The methane recovery rate of the system reached 98.0% and 89.0% when the substrate was pure food waste and 25% sewage sludge substitution, respectively. Kinetics characterization showed that hydrolysis was the rate-limiting step in both mono-digestion and co-digestion while methanogenic performance and microbial community were significantly affected by feed condition. The dominant archaea for methane generation shifted from Methanothermobacter thermophilus (72.82%) to Methanosarcina thermophila (96.25%) with sewage sludge gradually added from 0% to 100% in the substrate. The relationships between digestion performance, such as the accumulation of soluble proteins in the reactor, and functional microbial groups were also carefully analyzed. Finally, reasonable metabolic pathways for mono-digestion and co-digestion were summarized.

Optimizing ADM1 Calibration and Input Characterization for Effective Co-Digestion Modelling

Anaerobic co-digestion in wastewater treatment plants is looking increasingly like a straightforward solution to many issues arising from the operation of mono-digestion. Process modelling is relevant to predict plant behavior and its sensitivity to operational parameters, and to assess the feasibility of simultaneously feeding a digester with different organic wastes. Still, much work has to be completed to turn anaerobic digestion modelling into a reliable and practical tool. Indeed, the complex biochemical processes described in the ADM1 model require the identification of several parameters and many analytical determinations for substrate characterization. A combined protocol including batch Biochemical Methane Potential tests and analytical determinations is proposed and applied for substrate influent characterization to simulate a pilot-scale anaerobic digester where co-digestion of waste sludge and expired yogurt was operated. An iterative procedure was also developed to improve the fit of batch tests for kinetic parameter identification. The results are encouraging: the iterative procedure significantly reduced the Theil’s Inequality Coefficient (TIC), used to evaluate the goodness of fit of the model for alkalinity, total volatile fatty acids, pH, COD, volatile solids, and ammoniacal nitrogen. Improvements in the TIC values, compared to the first iteration, ranged between 30 and 58%.

A simplified model for simulating anaerobic digesters: Application to valorisation of bagasse and distillery spent wash

Current anaerobic digestion (AD) design methods rely on crude empirical models or sophisticated anaerobic digestion models (like ADM1) requiring a large number of parameters which are difficult to obtain experimentally. A simplified model for simulating AD was developed in this work. The model requires knowledge of CH₄/CO₂ ratio in biogas or indigestible fraction in substrate and batch biomethane potential (BMP) data for estimating three kinetic parameters (maximum specific growth rate, half velocity constant and cell death rate). Reported lab scale BMP data of sugarcane bagasse and spent wash were used to first estimate the kinetics and then to simulate corresponding largescale AD. Simulated results of specific methane yield and digester performance were consistent with available largescale AD data. The potential of the model to simulate single and multi-stage AD were illustrated. The presented approach and model will be useful for effectively valorising a variety of complex biomass substrates to biogas.

Localized mixing of anaerobic plug flow reactors

An innovative localized-mixing concept was tested in an Anaerobic Plug Flow Reactor (AnPFR) treating Food Waste (FW) mixed with municipal Wastewater (WW). The proposed concept consists of placing propellers along the shaft of the AnPFR at key points that represent the mid-region of each of the anaerobic digestion stages: hydrolysis, acidogenesis, and methanogenesis. First, the need for and efficiency of localized mixing (the new concept suggested by the authors) were investigated. While the main benefit of localized mixing is the reduction of energy demand associated with (conventional) uniform mixing (i.e., throughout the longitudinal axis), the system can also benefit from synergetic reactions in non-mixed zones. In fact, at a Total Solid (TS) content of 15% (Organic Loading Rate (OLR) of 4.2 g VS.L ^- 1.d ^- 1) and a Hydraulic Retention Time (HRT) of 28 days, the mixing pattern was sufficient to maintain stable operation, with high removal rates (up to 96% of solids) and high biogas generation (1128 ± 55 ml.g VS_fed^-1, of which 68.9% consisted of CH₄); but when mixing was halted, the system's performance deteriorated. Second, the loading capacity of the locally-mixed AnPFR was investigated by subjecting it to different TS content (10%, 15%, 20%, and 22.5%, corresponding to OLRs of 2.8, 4.2, 6.3, and 7.9 g VS.L ^- 1.d ^- 1, respectively) while operating under the same HRT. It was found that the system can adequately sustain a feed with a maximum TS of 20% while achieving removal rates up to 92% for solids and a CH₄ yield of 613 ml.g VS_fed^-1. The digester was simulated using computational fluid dynamics. The outputs revealed: (1) highest radial mixing at the center of the methanogenesis zone where the propeller is located and (2) low longitudinal mixing before and after the propeller of the methanogenesis stage, implying the presence of sedimentation zones that was visually verified. The former is assumed to favor better dispersion of inhibitors and improved stability, while the latter is expected to provide stagnant areas for enhanced biochemical synergies.

Anaerobic co-digestion of dairy manure and Japanese knotweed (Fallopia japonica) under thermophilic condition: Optimal ratio for biochemical methane production

Anaerobic co-digestion of animal manure and lignocellulosic biomass is a potent approach for sustainable biomethane production. Co-digestion of dairy manure (DM) and Japanese knotweed (JK), which was collected from a riverbank, was investigated at five different DM-to-JK mixing ratios (100:0, 90:10, 80:20, 60:40, and 0:100; wet weight basis) under thermophilic condition. The results showed that the methane yields obtain from the co-digestion of DM and JK were much higher than that obtained from JK alone (104 ml/gVS), which indicates the synergistic effect and the benefits of co-digesting JK with DM. The highest methane yield (232 ml/gVS) was obtained from the DM-to-JK ratio of 90:10, which was 14.9% and 123.1% higher than that from DM and JK alone, respectively. It also showed the highest synergistic effect (61 ml/gVS). However, further increase in JK ratios led to the decrease in methane yield and synergistic effect. Therefore, applying the co-digestion of DM and JK at a ratio of 90:10 is recommended for biomethane production.

The Effect of Ammonia Toxicity on Methane Production of a Full-Scale Biogas Plant—An Estimation Method

Ammonia accumulation in biogas plants reactors is becoming more frequently encountered, resulting in reduced methane (CH4) production. Ammonia toxicity occurs when N-rich substrates represent a significant part of the biogas plant’s feedstock. The aim of this study was to develop an estimation method for the effect of ammonia toxicity on the CH4 production of biogas plants. Two periods where a biogas plant operated at 3200 mg·L−1 (1st period) and 4400 mg·L−1 (2nd period) of ammonium nitrogen (NH4+–N) were examined. Biomethane potentials (BMPs) of the individual substrates collected during these periods and of the mixture of substrates with the weight ratio used by the biogas plant under different ammonia levels (2000–5200 mg·L−1 NH4+–N) were determined. CH4 production calculated from the substrates’ BMPs and the quantities used of each substrate by the biogas plant was compared with actual CH4 production on-site. Biogas plant’s CH4 production was 9.9% lower in the 1st and 20.3% in the 2nd period in comparison with the BMP calculated CH4 production, of which 3% and 14% was due to ammonia toxicity, respectively. BMPs of the mixtures showed that the actual CH4 reduction rate of the biogas plant could be approximately estimated by the ammonia concentrations levels.

Insights into organic loading rates of anaerobic digestion for biogas production: a review

Anaerobic digestion (AD) for biogas production is affected by many factors that includes organic loading rate (OLR). This OLR appears to be closely linked to various other factors and understanding these linkages would therefore allow the sole use of OLR for process performance monitoring, control, as well as reactor design. This review's objective is to collate the various AD factor specific studies, then relate these factors' role in OLR fluctuations. By further analyzing the influence of OLR on the AD performance, it would then be possible, once all the other factors have been determined and fixed, to manage an AD plant by monitoring and controlling OLR only. Decisions on reactor design, process kinetics, biogas yield and process stability can then be made much more quickly and with minimal troubleshooting steps.

A new approach to enhance the conventional two-phase anaerobic co-digestion of food waste and sewage sludge

BACKGROUND

Two-phase anaerobic co-digestion (TAcoD) is a versatile technology for the simultaneous treatment of organic materials and biogas production. However, the produced digestate and supernatant of the system contain heavy metals and organic substances that need to be treated prior to discharge or land application. Therefore, in this study, an innovative TAcoD for organic fertilizer and high supernatant quality achievement was proposed.

METHODS

In the conventional TAcoD, mixed sewage sludge (SS) and food waste (FW) were first hydrolyzed in the acidogenic reactor, and then the hydrolyzate substrate was subjected to the methanogenic reactor (TAcoD 1). In the modified TAcoD (TAcoD 2), only FW was fed into the acidogenic reactor, and the produced hydrolyzed solid was directly converted to the organic fertilizer, while the supernatant with high soluble chemical demand (SCOD) concentration was further co-digested with SS in the methanogenic reactor.

RESULTS

Although TAcoD 1 produced bio-methane yield and potential energy of 56.18% and 1.6-fold higher than TAcoD 2, the economical valorization of TAcoD 2 was 9-fold of that from TAcoD 1. The supernatant quality of TAcoD 2 was far better than TAcoD 1, since the SCOD, total nitrogen (TN), and total phosphor (TP) removal in TAcoD 2 and TAcoD 1 were 94.3%, 79.4%, 90.7%, and 68.9%, 28%, 46%, respectively. In terms of solid waste management, the modified TAcoD converted FW to organic fertilizer and achieved a solid reduction of 43.62% higher than that of conventional TAcoD.

CONCLUSIONS

This new modification in two-phase anaerobic co-digestion of food waste and sewage sludge provides a potentially feasible practice for simultaneous bio-methane, organic fertilizer, and high supernatant quality achievement.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s40201-020-00603-8.

Optimization and Analysis of Liquid Anaerobic Co-Digestion of Agro-Industrial Wastes via Mixture Design

Anaerobic co-digestion (AcoD) is a widely employed technique to produce biogas from simultaneous digestion of various biomasses. However, the selection of the optimal proportions of the substrates in the mixtures presents a challenge. This research used a mixture design to investigate the interactions between the liquid fraction of piglet manure (PM), cow manure (CWM), and starch wastewater (SWW). A modified Gompertz model was used to identify the statistically significant parameters of the methane production curves. The optimal compositions of the mixtures were identified based on multi-objective optimization of the maximal methane yield (YCH4) and maximal methane specific production rate (rCH4) parameters. The study was validated using a double mixture of PM and CWM and a triple mixture. The estimated degradation rates for both mixtures were faster than the predicted ones. The absolute relative errors of rCH4 were 27.41% for the double mixture and 5.59% for the triple mixture, while the relative errors of YCH4 were 4.64% for the double mixture and 10.05% for the triple mixture. These relative errors are within the normal limits of a process with high variability like AD. Thus, mixture design supported by the tested models is suitable for the definition of practically advisable mixtures of substrates.

Anaerobic co-digestion of fruit and vegetable waste: Synergy and process stability analysis

Anaerobic mono- and co-digestion of fruits and vegetable waste (FVW), slaughterhouse waste (SHW), and cattle manure (CM) under mesophilic conditions (35°C) were conducted through biochemical methane potential tests to investigate how the FVW in a co-substrate formulation improves the methane yield, the degradative synergy between substrates, and especially the stability of the process. The co-digestion of FVW:SHW and FVW:CM were evaluated with volatile solids (VS) ratios of 1:2, 1:1, and 2:1. The results indicated that the highest synergistic effect was found in the co-digestion FVW:CM at 1:1 VS ratio. However, the co-digestion FWV:SHW at 1:2 VS ratio increased the methane yield by 74.2% compared to the mono-digestion of FVW (776.3 mL CH₄ g VS_added^-1). As a critical condition in these processes, the stability was evaluated using the early warning indicator VFA/TA (volatile fatty acids/total alkalinity). The co-substrate SHW promotes greater stability in methane production as the soluble carbohydrate content in FVW increases. It was proposed that the high protein (49.04 ± 0.96% VS) and ammonia content (693 ± 3 mg L^-1) of SHW leads to the formation of a dampening system known as a carbonate-acetic buffer. It was concluded that balanced distribution between carbohydrates, proteins, and lipids is crucial to increase methane yields, and the low methane productions were associated with low N-NH₄⁺ concentrations (FVW:CM co-digestions). The results obtained in this study can serve as a basis to design full-scale digesters under similar operating conditions and with the same substrate:co-substrate relationships.Implications: The production of methane from the anaerobic digestion process of food, and lose waste presents a viable alternative of valorization and could help to mitigate environmental impacts. However, anaerobic digestion from these substrates carries high instabilities and low methane yields. The need to increase these yields and contribute to process stability must be considered in the selection of a co-substrate. In this context, this work aims to evaluate the best fruits and vegetable waste: co-substrateformulation, that promotes higher methane yield, a synergy between substrates, and to improve the AD process stability in the presence of perturbations in the substrate composition. We believe that our results could be helpful for the design processes for methane production from fruit and vegetable waste, to contribute to competitiveness with conventional energies and promote the sustainability of these processes.

Comparison of Biological Efficiency Assessment Methods and Their Application to Full-Scale Biogas Plants

For calculation of biological efficiency of a biogas plant (BP), it is required to determine the specific methane potential (SMP) of the substrate. A study comparing available methods for determination of SMP and the comparison with data of full-scale BPs is missing but necessary according to the differences in process conditions between both. Firstly, mass and mass associated energy balances of 33 full-scale BPs were calculated and evaluated. The results show plausible data for only 55% of the investigated BPs. Furthermore, conversion and yield efficiencies were calculated according to six different methods for SMP determination. The results show a correlation between the measured on-site specific methane yield and the calculated SMP by methods based on biological degradability. However, these methods underestimate the SMP. Calculated SMPs based on calorific values are higher, but less sensitive. A combination of biochemical and energetical methods is a promising approach to evaluate the efficiency.

Advances towards understanding long chain fatty acids-induced inhibition and overcoming strategies for efficient anaerobic digestion process

The inhibition of the anaerobic digestion (AD) process, caused by long chain fatty acids (LCFAs), has been considered as an important issue in the wastewater treatment sector. Proper understanding of mechanisms behind the inhibition is a must for further improvements of the AD process in the presence of LCFAs. Through analyzing recent literature, this review extensively describes the mechanism of LCFAs degradation, during AD. Further, a particular focus was directed to the key parameters which could affect such process. Besides, this review highlights the recent research efforts in mitigating LCFAs-caused inhibition, through the addition of commonly used additives such as cations and natural adsorbents. Specifically, additives such as bentonite, cation-based adsorbents, as well as zeolite and other natural adsorbents for alleviating the LCFAs-induced inhibition are discussed in detail. Further, panoramic evaluations for characteristics, various mechanisms of reaction, merits, limits, recommended doses, and preferred conditions for each of the different additives are provided. Moreover, the potential for increasing the methane production via pretreatment using those additives are discussed. Finally, we provide future horizons for the alternative materials that can be utilized, more efficiently, for both mitigating LCFAs-based inhibition and boosting methane potential in the subsequent digestion of LCFA-related wastes.

Simultaneous Synergy in CH4 Yield and Kinetics: Criteria for Selecting the Best Mixtures during Co-Digestion of Wastewater and Manure from a Bovine Slaughterhouse

Usually, slaughterhouse wastewater has been considered as a single substrate whose anaerobic digestion can lead to inhibition problems and low biodegradability. However, the bovine slaughter process generates different wastewater streams with particular physicochemical characteristics: slaughter wastewater (SWW), offal wastewater (OWW) and paunch wastewater (PWW). Therefore, this research aims to assess the anaerobic co-digestion (AcoD) of SWW, OWW, PWW and bovine manure (BM) through biochemical methane potential tests in order to reduce inhibition risk and increase biodegradability. A model-based methodology was developed to assess the synergistic effects considering CH4 yield and kinetics simultaneously. The AcoD of PWW and BM with OWW and SWW enhanced the extent of degradation (0.64–0.77) above both PWW (0.34) and BM (0.46) mono-digestion. SWW Mono-digestion showed inhibition risk by NH3, which was reduced by AcoD with PWW and OWW. The combination of low CH4 potential streams (PWW and BM) with high potential streams (OWW and SWW) presented stronger synergistic effects than BM-PWW and SWW-OWW mixtures. Likewise, the multicomponent mixtures performed overall better than binary mixtures. Furthermore, the methodology developed allowed to select the best mixtures, which also demonstrated energy and economic advantages compared to mono-digestions.

Wastes to profit: a circular economy approach to value-addition in livestock industries

The livestock sector is a fundamental part of the modern global economy and provides food, clothing, furnishings, and various other products. So as to ensure its resilience to changes in consumer expectations, cost of production, and environmental sustainability, the sector must shift to a circular economy model. Current strategies to recover value from wastes and low-value co-products from livestock industries yield limited value; hence, new technologies are required to upgrade wastes and co-products, and generate high-value products that can feed into the livestock value chain. Anaerobic digestion can convert high organic-content waste to biogas for energy and a stable nutrient-rich digestate that can be used as fertiliser. Microbial technologies can transform wastes to produce nutritionally advanced feeds. New materials from waste can also be produced for livestock industry-specific applications. While aiming to add commercial value, the successful implementation of these technologies will also address the environmental and productivity issues that are increasingly valued by producers and consumers.

Management and valorization of waste from a non-centrifugal cane sugar mill via anaerobic co-digestion: Technical and economic potential

The main sugarcane wastes from the non-centrifugal cane sugar (NCS) agro-industry, agricultural crop residue (ACR) and sugarcane scum (SCS), were used to produce biogas in a bench-scale semi-continuous anaerobic tubular digester. A two-stage strategy was proposed to achieve the appropriate operability and stability of the digester. In the first stage, the operability of the digester was achieved with ACR mono-digestion. In the second stage, the digester feed was changed until it reached an ACR:SCS ratio (co-digestion) of 75:25, based on volatile solids, and until stability was achieved. The strategy was successful, and specific biogas production of 0.132 m³ kg^-1VS with a methane content of 50.4% was achieved, confirming the technical feasibility of the process. Economic viability was established through a case study at a typical NCS mill. Therefore, anaerobic co-digestion can be consolidated as a technological alternative for the treatment of ACR + SCS and the sustainable benefit of the NCS agro-industry.

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).

Microbial community in an anaerobic membrane bioreactor and its performance in treating organic solid waste under controlled and deteriorated conditions

The adoption of anaerobic membrane bioreactors (AnMBRs) for organic solid waste management is important for the recovery of energy and high-quality treated water. However, few studies have focused on AnMBR treatment of high-strength organic solid waste and the microorganisms involved under deteriorated operating conditions. In the present study, a 15-L bench-scale AnMBR was operated using a model slurry of high-strength organic solid waste with the organic loading rate (OLR) increasing from 2.3 g chemical oxygen demand (COD) L^-1 day^-1 (represented as a controlled condition) to 11.6 g COD L^-1 day^-1 (represented as a deteriorated condition), and microbial community dynamics over 120 days of operation were analyzed. The abundances of methanogens and bacteria that were dominant under the controlled condition decreased as a result of both high organic loading and sludge withdrawal under the deteriorated condition and did not recover thereafter. Instead, numbers of putative volatile fatty acid (VFA)-producing bacterial operational taxonomic units (OTUs) related to the genus Prevotella increased rapidly, reaching a relative abundance of 43.2%, leading to the deterioration of methanogenic AnMBR operation. Considering that the sequences of these OTUs exhibited relatively low sequence identity (91-95%) to those of identified Prevotella species, the results strongly suggest that the accumulation of VFAs by novel VFA-producing bacteria in the digestion sludge promotes the disruption of the methanogen community under deteriorated conditions.

Anaerobic Co-Digestion of Cheese Whey and Industrial Hemp Residues Opens New Perspectives for the Valorization of Agri-Food Waste

Cheese whey (CW) and hemp hurds (HH) represent typically overabundant biowastes of food and agricultural production, and their circular management is crucial to improve both sustainability and profitability of the agri-food chain. By combining experimental biochemical methane potential (BMP) tests and literature data, the techno-economic aspects of a possible future bioenergy valorization of CW and HH through anaerobic digestion (AD) and co- digestion (coAD) were analyzed. Along the 42-days, BMP assays, CW, and HH alone rendered BMP values of 446 ± 66 and 242 ± 13 mL CH4·g VS−1, respectively. The application of coAD with CW and HH at a 70:30 ratio allowed to enhance the biomethane production by 10.7%, as compared to the corresponding calculated value. In terms of economic profitability, the valorization of HH as biomethane in a dual-purpose hemp cultivation could potentially enable net profits of up to 3929 €·ha−1, which could rise to 6124 €·ha−1 in case of coAD with CW. Finally, by projecting the biomethane potential from current and future available CW and HH residues in the national context of Italy, a total biomethane yield of up to 296 MNm3·y−1 could be attained, offering interesting perspectives for the sustainability of key sectors such as transportation.

Perspective on enhancing the anaerobic digestion of waste activated sludge

Anaerobic digestion (AD) of waste activated sludge (WAS) is an important bio-energy strategy that has been hindered by low conversion efficiency. This paper presents a comprehensive review of research on the sludge's property and enhancing AD of WAS, and proposes two perspectives of material structure and microbial activity on improving AD efficiency. In the first part of this review, the key principle problems for hindering AD efficiency are identified based on the concept of AD. Then, the possibility that the complex microstructure and composition of WAS are responsible for poor biodegradability is considered and main methods for enhancing AD are summarized. In the third part, according to the published works, the main knowledge gaps in research are recognized as the identification and specific activity adjustment of functional microbes, the understanding of key constituents of WAS and their interactions, the deciphering of complex structure of sludge organic substance, and the revealing of relationships between complex nature of WAS and biodegradability. Further discussions reveal that to enhance AD more studies should be centered on the sludge's structure and properties in future. However, this review is expected to provide the clear and accurate research directions for enhancing AD efficiency of WAS.

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.

Study of the digestion process at a full-scale solid-state biogas plant by using ORWARE: Model modification and implementation

The configuration of the reactor influences the digestion process and thus the product yields; other factors such as the rate of biogas production or biogas loss also affect the process specifically with high solid configuration. With these in mind, the ORganic WAste REsearch (ORWARE) anaerobic digestion sub-model was modified to be able to study solid-state anaerobic digestion (SS-AD) (using plug-flow reactor). The simulation results from the updated model agreed with the operational data with respect to methane yield, digestate yield and energy turnover. The model was found to be sensitive to changes in feedstock composition but to a lesser extent to changes in process temperature and retention time. By applying the model on several cases of liquid anaerobic digestion (L-AD), it was noticed that L-AD at mesophilic condition with 25 retention days seemed to be superior to other cases of L-AD with regard to energy turnover. However, even if similar methane production were observed for L-AD and SS-AD, the model suggested higher energy turnover for the case of SS-AD at thermophilic condition, being 10% more in average in comparison with cases of L-AD.

Feeding control of anaerobic co-digestion of waste activated sludge and corn silage performed by rule-based PID control with ADM1

Anaerobic co-digestion (AcoD) of corn silage (CS) and waste activated sludge (WAS) co-substrates, compared with anaerobic digestion (AD) of mono-substrate WAS, was simulated under mesophilic conditions with the adapted IWA Anaerobic Digestion Model No. 1 (ADM1), and a rule-based PID control system for control of the AcoD of CS and WAS, through control of their ratios in the feed, was developed, implemented with the model as a test platform. Tests on AcoD of co-substrates were conducted at the COD mass-based feeding ratios of CS to WAS 1:2.5, 1:2.0 and 1:1.2. The maximal biogas production was 0.94 m³/kgVS·d at the feeding ratio 1:1.2. The ADM1 was adapted, and the high-sensitivity kinetic parameters were calibrated and optimised using the data from the tests of steady state mono-digestion of WAS and AcoD of CS and WAS. The simulated data of biogas and methane production, methane content, VFA and pH agreed well with the test data. The rule-based PID control was developed with an additional expert system, in which the lower level controller operated the level of VFA/TIC and the upper level controller manipulated the setpoints of methane production. The feeding ratio of CS to WAS was used as a manipulated variable. With the constraint boundaries, the test on the control system showed that it could keep methane production stable to the setpoint and maximise methane production while resisting the disturbances to AcoD by adjusting the feeding ratios of CS to WAS.

Co-digestion of by-products and agricultural residues: A bioeconomy perspective for a Mediterranean feedstock mixture

This study focused on applying batch and continuous co-digestion approaches to investigate the effects of a feedstock mixture (FM) constituted by ten Mediterranean feedstocks highly available in the Mediterranean area (i.e., olive pomace, olive mill wastewater, citrus pulp, poultry litter, poultry and cattle manure, whey and cereal straw) on methane production for bioenergy generation. For the same feedstock mixture (FM), two different anaerobic digestion (AD) tests were carried out to evaluate the possible inhibitory effects of some biomasses on the biological process. The first AD test showed a methane yield equal to 229 Nm³CH₄/tVS (27% lower than that measured during the batch test). During the second AD test, the specific production was 272 m³CH₄/tVS. Both tests showed a similar methane content of methane in the biogas, equal to about 57%. The first AD test showed an inhibition effect of the process: total conversion of the organic matter into biogas was not ended. The second batch test demonstrated that the selected FM could be viable to carry out the co-digestion and could provide a flexible solution to generate advanced biofuels in biogas plants located in the Mediterranean area.

Synergistic effect from anaerobic co-digestion of food waste and Sophora flavescens residues at different co-substrate ratios

When food waste (FW) undergoes anaerobic digestion, the hydrolysis rate is rapid, and thus causes system instability. Sophora flavescens residues (SFRs) are rich in complex hydrolysed substances, such as lignocellulosic material. When combined FW and SFRs can effectively improve the stability of digestion systems and increase biogas yields. In this work, batch anaerobic experiments were conducted at different co-substrate ratios to investigate the performance of co-digestion and the synergistic effect of FW and SFRs. The co-digestion of the two substrates exerted synergistic effects on biogas production and the highest synergy was 120.8%. After digestion, the ratio of hydrolysed chemical oxygen demand (COD) to the entire COD (RCOD_H) of the co-digestion group was 1.08 times that of the single FW group, which indicated the co-digestion promoted the hydrolysis of substrates. Moreover, the hydrolysis rate constant (k_h) of co-digestion group increased by 4.10 times in comparison with that of the single FW group, which indicated the co-digestion increased the hydrolysis rate. In other words, the synergistic effect mainly occurred in the hydrolysis acidification process.

Semi-continuous anaerobic co-digestion of flotation sludge from broiler chicken slaughter and sweet potato: Nutrients and energy recovery

Energy production based on the proper allocation of environmental liabilities is in line with the concept of sustainability. Flotation sludge (S) is a type of waste derived from the physical treatment of the wastewater generated in significant quantities during chicken slaughter in Brazil. If not treated, this wastewater may contribute to pollution, but further treatment provides clean energy and nutrient recycling. The present study aimed at evaluating the reduction of (S) organic load by means of mono and co-digestion with sweet potatoes (P) while promoting its conversion into energy (methane) and nutrients (digestate). Semi-continuous reactors (60 L capacity) were used with a hydraulic retention time of 25 days. The reactors were fed daily with 2.4 L consisting of 60% digestate recirculation, 40% non-chlorinated water and 4.5% total solids (TS). Using nine reactors and six progressive periods, eleven conditions were evaluated with three replicates each. The percentages of (P) and (S) varied from 0 to 100. The best observed condition in terms of energy recovery and TS removal was 60% of P + 40% of S (p ≤ 0.05), as it presented values of at least an increase of 92% in total biogas volume, an increase of 123% in specific methane production, an increase of 98% in specific methane yield and an increase of 44% in TS removal efficiency compared to mono-digestions. The fertilizer potential of the digestate generated in the different conditions was calculated and evaluated according to the area of (P) production. The results varied from 3.6 to 10.8 ha of (P) using 100 m³ of digestate. A multivariate analysis showed that higher amounts of (P) in substrate composition favor energy recycling while higher concentrations of (S) enhance the production of a digestate with valuable agronomic characteristics.

Addressing the synergy determination in anaerobic co-digestion and the inoculum activity impact on BMP test

Anaerobic mono-digestion and co-digestion are nowadays widely used in wastewater treatment plants (WWTP). However, the data processing of the conventional biochemical potential test (BMP) carried out to assess potential substrates should be enhanced to reduce the uncertainty of the results. In this study, two methodologies aiming to improve the data processing in anaerobic digestion studies were proposed. The methodologies aimed at the estimation of synergy in anaerobic co-digestion of organic waste and the standardization of the BMP test results by considering the activity of the inoculums under mono-digestion conditions. Both methodologies comprise the application of the Gompertz equation. For the first methodology, four cosubstrates and two types of substrates were used. Regarding synergy estimation, the cosubstrates dairy whey and grease sludge had an impact on the degradation kinetic. In regard to the second methodology, the results indicate that the activity of the inoculums exerts an influence on the BMP analysis, and it should be considered. This can be meaningful when comparing results among studies when different inoculums are used or even for studies where the same inoculum is used but it is taken at different reactor operational moments.

Interactive Effects of Chemical Composition of Food Waste during Anaerobic Co-Digestion under Thermophilic Temperature

The effects of chemical composition (carbohydrates, lipids, and protein) on the anaerobic co-digestion performance of food wastes (FW) were investigated from the viewpoints of methane production, dynamic parameters, and microbial community structure. The results of this study showed that a notable gasification rate was positively correlated with the proportion of the composition. A T2 reactor, which consisted of 60% carbohydrates, 20% lipids, and 20% protein, held a higher gasification rate of 65.09% compared to other groups, while its process parameters showed some deficiency regarding the stability of digestion, especially for low biochemical methane potential (BMP), which was not beneficial for the actual practice. A T4 reactor, with a highest gasification rate of 70.68%, held the maximum BMP (497.44 mL/g VS). The stable chemical parameters achieved the optimal proportion, consisting of 40% carbohydrates, 40% lipids, and 20% protein. Furthermore, its microbial populations were rich and achieved a balance of the two main dominant communities of acetoclastic methanogens and hydrogenotrophic methanogens, whose relative abundance was close. It was obvious that interactive effects were caused by different proportional composition, which led to constantly changing chemical parameters and microbial community.

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.

Optimizing biomethane production of mesophilic chicken manure and sheep manure digestion: Mono-digestion and co-digestion kinetic investigation, autofluorescence analysis and microbial community assessment

Optimization of mesophilic methane production from Chicken manure (CM) and Sheep manure (SM) at total solid (TS) of 8% and 1.6% were obtained by sequence tests in mono-digestion. However, the positive synergy of co-digestion with an optimum CM/SM of 2.5 (310 mLCH₄/gVS_added) resulted in a high hydrolytic capacity and methane production. The modified Gompertz model (R² > 0.98) and modified Aiba model (R² > 0.88) illustrated co-digestion significantly improved the methane generation rate with strong ammonia tolerance. Dissolved Organic Matter (DOM) variation in response to the metabolic rate of microbial community illustrated that the SMP-like and protein-like components half-split by EEM-PARAFAC were significantly negative corresponded to bio-methane production. Moreover, the canonical correlation analysis (CCA) resulted a significant difference between the substrate and DOM composition. Potential functional metabolic illustrated statistically significance difference between mono and co-digestion, however, Methanosaeta and Syntrophobacter predominated the syntrophic methanogenesis. The constructed complex metabolic cooperation caused the co-digestion stable and high efficiency.