Effects of organic loading rates on high-solids anaerobic digestion of food waste in horizontal flow reactor: Methane production, stability and mechanism

Continuous augmentation of anaerobic digestion with electroactive microorganisms: Performance and stability

The performance and stability of a bioelectrochemical anaerobic digester (BeAD), continuously augmented with electroactive microorganisms (EAMs), were investigated. The BeAD showcased superior performance, sustaining the high COD removal efficiency and methane production rate of 76.5 % and 0.67 L/(L.d), respectively, in a stable state. Prominently, it exhibited remarkable resilience under hydraulic and organic shock loads, adeptly recuperating from disturbances up to 1000 % of its stable condition. This resilience of up to 300 % shock load was driven by increased levels of electron transport components such as quinones and riboflavins, which act as electron shuttles. However, after extreme shock exposures from 500 % to 1000 %, despite the spike in inhibitory by-products such as humic acids and ammonia, the upregulation of the mtr complex was pivotal in recovering and sustaining methane production. These insights emphasize the BeAD's capability to bolster both performance and stability, thereby providing a potent strategy for practical application of bioelectrochemical systems.

Innovative system to maximize methane production from fruit and vegetable waste

Anaerobic digestion of fruit and vegetable waste (FVW) offers an environmentally friendly alternative for waste disposal, converting it into methane for energy recovery. Typically, FVW digestion is conducted in a continuously stirred tank reactor (CSTR) due to its ease of use and stability with solid concentrations between 5 and 10%. However, CSTRs are limited to organic loading rates (OLRs) of about 3 kg COD/m3.day, resulting in large reactor volumes, low methane productivity, and costly wet digestate handling. This work introduces a novel method for methane production from FVW using a high-rate reactor system. The proposed approach involves grinding, centrifuging, and/or pressing the FVW to separate it into liquid and solid phases. The liquid phase is then digested in an up-flow anaerobic sludge blanket (UASB) reactor, while the solid phase undergoes digestion in a dry methanization reactor. A model incorporating all biological reactors was implemented in the Anaerobic Digestion Model 1 (ADM1) to provide a theoretical basis for the experimental development of this system. The current simulation scenarios offer initial references for operating the experimental system, which will, in turn, generate data for further model refinement. For instance, constrained liquid-gas mass transfer was considered for dry fermentation, with additional potential biochemical kinetic limitations to be incorporated following on experimental evidence. The success of this system could enable energy recovery in 72 Central Wholesale Markets across Brazil, offering a critical tool for planning, operating, and optimizing such systems.

Modeling and forecasting biogas production from anaerobic digestion process for sustainable resource energy recovery

Anaerobic digestion (AD) is one of the most extensively accepted processes for organic waste cleanup, and production of both bioenergy and organic fertilizer. Numerous mathematical models have been conceived for modeling the anaerobic process. In this study, a new modified dynamic mathematical model for the simulation of the biochemical and physicochemical processes involved in the AD process for biogas production was proposed. The model was validated, and a sensitivity analysis based on the OAT approach (one-at-a-time) was carried out as a screening technique to identify the most sensitive parameters. The model was developed by updating the bio-chemical framework and including more details concerning the physico-chemical process. The fraction XP was incorporated into the model as a particulate inert product arising from biomass decay (inoculum). New components were included to distinguish between the substrate and inoculum, and a surface-based kinetics was used to model the substrate disintegration. Additionally, the sulfate reduction process and hydrogen sulfide production have been included. The model was validated using data extracted from the literature. The model's ability to generate accurate predictions was testified using statistical metrics. The model exhibited excellent performance in forecasting the parameters related to the biogas process, with measurements falling within a reasonable error margin. The relative absolute error (rAE) and root mean square error (RMSE) were both less than 5 %, indicating a high ability of the current model in comparison with the literature. Additionally, the scatter index (SI) was below 10 %, and the Nash-Sutcliffe efficiency (NES) approached one, which affirms the model's accuracy and reliability. Finally, the model was applied to investigate the performances of the AD of food waste (FW). The findings of this study support the robustness of the developed model and its applicability as a virtual platform to evaluate the efficiency of the AD treatment and to forecast biogas production and its quality, CO2 emission, and energy potential across various organic solid waste types.

Insight into the mechanism of alkali-thermal pretreatment of food-waste solid residue through fluorescence spectroscopy coupled with parallel factor analysis

Food-waste solid residue is the remaining solid after food waste treatment, with high yield, high solid content, high protein and fiber content. Effective pretreatment is necessary to improve the efficiency of hydrolysis and acidification for anaerobic digestion of food-waste solid residue. In this study, fluorescence spectroscopy coupled with parallel factor analysis were used to insight into the mechanism of food-waste solid residue during three pretreatments (alkali, thermal and alkali-thermal). Pretreatments increased the solubility of lignocellulosic substrate and destroyed structure of starch, while lignocellulosic analogs were effectively cracked, changing the composition and improving the degradability. Soluble chemical oxygen demand, soluble protein and soluble polysaccharide concentrations were increased by 144.60%, 350.57% and 138.72% after pretreatment under the condition of 120 °C + 2% CaO, respectively. Three-dimensional fluorescence spectra showed the region of maximum fluorescence intensity under alkali-thermal pretreatments, indicating chemical bonds (such as OC-C) were easier broken and the solubility of organic substances were increased. Three main fluorescence components were obtained by parallel factor analysis, which were humic acid-like, lignocellulose-like and protein-like, respectively, while the lignocellulose-like had the maximum Fmax value. The fluorescence intensity of samples under alkali-thermal pretreatment varied in the range from 59.48 × 105 to 13.18 × 106, which was an increase of 174.27%-507.74% over the control (21.68 × 105), indicating that alkali-thermal pretreatment observably accelerated the breaking of chemical bonds, and thus promoted the dissolution of organic matter. This study deeply revealed the mechanism of alkali-thermal pretreatment of food-waste solid residue, which is of great significance for efficient resource utilization of food waste and food-waste solid residue.

Deciphering the impact of organic loading rate and digestate recirculation on the occurrence patterns of antibiotics and antibiotic resistance genes in dry anaerobic digestion of kitchen waste

Organic loading rate (OLR) is crucial for determining the stability of dry anaerobic digestion (AD). Digestate recirculation contributes to reactor stability and enhances methane production. Nevertheless, the understanding of how OLR and digestate recirculation affect the abundance and diversity of antibiotics and antibiotic resistance genes (ARGs), as well as the mechanisms involved in the dissemination of ARGs, remains limited. This study thoroughly investigated this critical issue through a long-term pilot-scale experiment. The metabolome analyses revealed the enrichment of various antibiotics, such as aminoglycoside, tetracycline, and macrolide, under low OLR conditions (OLR ≤ 4.0 g·VS/L·d) and the reactor instability. Antibiotics abundance decreased by approximately 19.66-31.69 % during high OLR operation (OLR ≥ 6.0 g·VS/L·d) with digestate recirculation. The metagenome analyses demonstrated that although low OLR promoted reactor stability, it facilitated the proliferation of antibiotic-resistant bacteria, such as Pseudomonas, and triggered functional profiles related to ATP generation, oxidative stress response, EPS secretion, and cell membrane permeability, thereby facilitating horizontal gene transfer (HGT) of ARGs. However, under stable operation at an OLR of 6.0 g·VS/L·d, there was a decrease in ARGs abundance but a notable increase in human pathogenic bacteria (HPB) and mobile genetic elements (MGEs). Subsequently, during reactor instability, the abundance of ARGs and HPB increased. Notably, during digestate recirculation at OLR levels of 6.0 and 7.0 g·VS/L·d, the process attenuated the risk of ARGs spread by reducing the diversity of ARGs hosts, minimizing interactions among ARGs hosts, ARGs, and MGEs, and weakening functional profiles associated with HGT of ARGs. Overall, digestate recirculation aids in reducing the abundance of antibiotics and ARGs under high OLR conditions. These findings provide advanced insights into how OLR and digestate recirculation affect the occurrence patterns of antibiotics and ARGs in dry AD.

Evolution of quorum sensing process and their regulatory role on biochemical metabolism during the organic loading rate increase in dry anaerobic digestion

The organic loading rate (OLR) is a critical parameter affecting the stability of dry anaerobic digestion (AD) of kitchen waste (KW), and significantly impacting the variations in physicochemical parameters and microbial communities. However, the evolution of quorum sensing (QS) and their role on anaerobic biochemical metabolism during the increase in OLR in dry AD remain unknown. Therefore, this study systematically elucidated the matter through multi-omics analysis based on a pilot-scale dry AD of KW. The results demonstrated that fluctuations in the OLR significantly influenced the microbial QS in dry AD. When the OLR ≤4.0 g·VS/L·d, the system operated stably, and methane production increased. The enrichment of Proteobacteria was crucial for sustaining high levels of functional genes associated with various types of QS, including acyl-homoserine lactones (AI-1), autoinducer-2 (AI-2), autoinducer-3 (AI-3), and gamma-aminobutyric acid (GABA). This enabled cooperative communication among microbes under low OLR. Furthermore, most genes associated with these QS processes positively affected hydrolysis, acidogenesis, and methanogenesis. When the OLR increased to 6.0 g·VS/L·d, the fatty acids and hydrogen partial pressure increased significantly. The autoinducing peptides (AIP)-type became the predominant QS and was positively correlated with fatty acids abundance. Syntrophaceticus and Syntrophomonas may promote syntrophic oxidation of acetate at high OLR through AIP-type QS. These findings provided new insights into the QS processes of microbes during dry AD of KW and a theoretical foundation for optimizing biochemical metabolic processes in dry AD through QS.

Modelling of technical, environmental, and economic evaluations of the effect of the organic loading rate in semi-continuous anaerobic digestion of pre-treated organic fraction municipal solid waste

The study concerned technical feasibility, economic profitability, and carbon footprint (CF) analysis of semi-continuous anaerobic digestion (sAD) of organic fraction of municipal solid waste (OFMSW). The research assessed the pre-treatment effect on sAD by varying organic loading rates (OLR) from 3.38 to 6.75 kgvs/m3d. Three sAD configurations were investigated: hydrodynamic-cavitated (HC-OFMSW), enzymatically pre-treated (EN-OFMSW), and non-pre-treated (AD-OFMSW). Principal Component Analysis and Supervised Kohonen's Self-Organizing Maps combined the experimental, economic, and environmental evaluations. The sAD configurations were grouped predominantly according to the OLR however, within each OLR group the configurations were clustered according to the pre-treatments. The finding highlighted that pre-treatments offset inhibition in sAD of OFMSW due to the OLR increase, being economically profitable and CF negative up to 4.50 kgvs/m3d for EN-OFMSW and to 5.40 kgvs/m3d for HC-OFMSW. Whereas sAD-OFMSW remained economically and environmentally viable only up to 3.87 kgvs/m3d. HC-OFMSW reached the highest performance. In detail, for HC-OFMSW the NPV and CF ranged from 17679.30 to 43827.12 euros and from -51.08 to -407.210 kg CO2eq/1 MWh daily produced, by decreasing the OLR from 5.40 to 3.87 kgvs/m3d. These results are fundamental since pre-treatment is usually expensive due to additional energy or chemical requirements.

Performance of high solids enzymatic hydrolysis and bioethanol fermentation of food waste under the regulation of saponin

Bioethanol recovery from food waste through high solids enzymatic hydrolysis (HSEH) and high solids bioethanol fermentation (HSBF) alleviate the energy crisis. However, this cause decreased glucose and bioethanol yields due to the high solids content. In this study, saponin was introduced into food waste HSEH and HSBF systems to enhance the product yields. Under the regulation of saponin, the substrate released >90% of the theoretical reducing sugar. The glucose concentration increased by 137.41 g/L after 24 h of HSEH with 2.0% saponin. The bioethanol titer reached 73.2 g/L (1.0%-saponin). Untargeted metabolomics illustrating that saponin had higher antifungal properties at lower concentrations (0.5%-saponin) that caused a decrease in bioethanol yield. The addition of saponin concentrations of 1.0%∼3.0% promoted HSEH, HSBF, and the metabolism of Saccharomyces cerevisiae; thus, 1.0% was suggested for practical use. This study deepened the understanding of saponin in enhancing HSBF and provides theoretical support for further application.

Enhancing of pretreatment on high solids enzymatic hydrolysis of food waste: Sugar yield, trimming of substrate structure

The development of high solids enzymatic hydrolysis (HSEH) technology is a promising way to improve the efficiency of bioenergy production from solid waste. Pretreatment methods such as ultrasound (USP), freeze-thaw (FTP), hydrothermal (HTP), and dried (DRD) were carried out to evaluate the effect and mechanism of the pretreatment methods on the HSEH of FW. The reducing sugar of HTP and DRD reached 94.75% and 94.92% of the theoretical value. HTP and DRD could reduce the crystallinity of FW. DRD resulted in lower alignment and the occurrence of fractures of the substrate and exposed the α-1,4 glycosidic bond of starch. The high destructive power of HTP and DRD reduced the obstacles caused by the high solid content. Moreover, DRD consumed only 27.62% of the total energy of HTP. DRD could be a promising pretreatment methods for glucose recovery for its high product yield, significant substrate destruction, and economic feasibility.

Biogas production of food waste with in-situ sulfide control under high organic loading in two-stage anaerobic digestion process: Strategy and response of microbial community

A two-stage anaerobic digestion process utilizing food waste was investigated in this study, without any additive and co-digestion. Solid content, temperature and pH value were key controlling factors for hydrolysis, which results the optimized food waste hydrolysate with COD/VS_{food waste} of 2.67. Efficient biogas production was maintained in long-term operation (>150 d) without any additive, and methane production yields up to 699.7 mL·gVS^-1·d^-1 was achieved under organic loading rate (OLR) of 31.0 gVS·d^-1. Methane production can be recovered (70.4 %) after temperature shock within 30 days. This study confirmed the possibility to establish two-stage food waste anaerobic digestion system under high organic load. pH, OLR, and temperature are key factors to maintain stable biogas production, while pH control was performed as a in situ sulfide control technology (75.8 % sulfide reduction). This study provides practical strategies for food waste utilization and decreasing carbon footprint.

Strategies for enhancing the efficacy of anaerobic digestion of food industry wastewater: An insight into bioreactor types, challenges, and future scope

Food waste have become a growing concern worldwide with raising population and economic growth. Wastewater discharged from food industries contains many valuable and toxic components that have a negative impact on the ecological system. Large amounts of wastewater are discharged from the food industry, which necessitates the creation of effective technologies. Wastewater from the food industry can be seen as a rich source of energy and a primary source for generating valuable products. Waste disposal and resource recovery are sustainably valued by anaerobic digestion of wastewater from the food sector. The characteristics, composition, and nature of wastewater produced from various food sectors are elaborated upon in this review. An overview of the anaerobic digestion process for wastewater treatment in the food industry is included. Enhancement strategies for the anaerobic digestion process have been discussed in detail. In addition, various types of reactors utilized for performing anaerobic digestion is illustrated. Though anaerobic digestion process possesses advantages, the challenges and future scope are examined for improving the outcome.

The effect of heat pre-treatment on the anaerobic digestion of high-solid pig manure under high organic loading level

Since more and more large-scale farms appear in China and changes in fecal sewage source disposal, the production of high-concentration solid manure waste is also increasing, and its conversion and utilization are gaining attention. This study investigated the effect of heat pre-treatment (HPT) on the thermophilic anaerobic digestion (AD) of high-solid manure (HSM). Pig manure (PM) feed with a total solids of 13% was used for the HPT and subsequent anaerobic digestion (AD) test. The HPT was carried out at 60°C, 80°C, and 100°C, respectively, for 15 min after the heating reached the set temperature. The results show that HPT led to PM feed COD solubilization, observing a maximum increase of 24.57% after pretreated at 100°C, and the treated PM feed under this condition received the maximum methane production potential of 264.64 mL·g−1 VS in batch AD test, which was 28.76% higher than that of the untreated group. Another semi-continuous AD test explored the maximum volume biogas production rate (VBPR). It involves two organic loading rates (OLR) of 13.4 and 17.8 g VSadded·L−1·d−1. The continuous test exhibited that all the HPT groups could produce biogas normally when the OLR increased to the high level, while the digester fed with untreated PM showed failure. The maximum VBPR of 4.71 L L−1·d−1 was observed from PM feed after pre-treated at 100°C and running at the high OLR. This reveals that thermal treatment can weaken the impact of a larger volume of feed on the AD system. Energy balance analysis demonstrates that it is necessary to use a heat exchanger to reuse energy in the HPT process to reduce the amount of energy input. In this case, the energy input to energy output (Ei/Eo) ranged from 0.34 to 0.55, which was much less than one, suggesting that biogas increment due to heat treatment can reasonably cover the energy consumption of the pre-treatment itself. Thus combining HPT and high-load anaerobic digestion of PM was suitable.

Effect of optimized intermittent mixing during high-solids anaerobic co-digestion of food waste and sewage sludge: Simulation, performance, and mechanisms

Inadequate mixing has been proven to be a major cause of anaerobic digester failure. This study revealed the mechanism of mixing intervals on high-solids anaerobic co-digestion (HS-AcoD) of food waste (FW) and sewage sludge (SS). Optimized intermittent mixing time (15 min/h) was determined through computational fluid dynamics (CFD) simulation. Experimental results indicated that the simulated intermittent mixing could shorten digestion time and increase cumulative methane output (366.8 mL/gVS) compared with continuous mixing and unmixing. Mixing could considerably accelerate substrate solubilization and hydrolysis. Maximum rates of acidogenesis (53.35 %) and methanogenesis (49.41 %) were observed with an optimized intermittent mixing (15 min/h). Vigorous mixing induced apoptosis and disrupted syntrophic metabolism, whereas intermittent mixing promoted the syntrophic metabolism between Syntrophomonas and Methanobacterium, and led to an enrichment of genes involved in acidogenic and methanogenic pathways. These findings have important implications for the development of an optimized intermittent mixing strategy for maximizing HS-AcoD efficiency of FW and SS.

Propionate-cultured sludge bioaugmentation to enhance methane production and micropollutant degradation in landfill leachate treatment

This research investigates the use of propionate-cultured sludge to enhance methane (CH₄) production and micropollutant biodegradation in biochemical methane potential (BMP) experiment treating landfill leachate. The experiments were carried out using non-acclimatized and acclimatized seed sludge with variable food to microorganism ratios of 1:1 and 1:2. Under the propionate-cultured sludge bioaugmentation, the concentrations of propionate-cultured sludge were varied between 10, 20, and 30 % (v/v). The acclimatized seed sludge exhibited high microbial abundance and diversity which promoted the CH₄ production and micropollutant biodegradation. The modified Gompertz model indicated that the optimal condition was the acclimatized seed sludge with 30% (v/v) propionate-cultured sludge, achieving the lag time (λ), maximum CH₄ production rate (R_max), and maximum CH₄ potential yield (P_max) of 0.57 day, 17.35 NmL/h, and 140.58 NmL/g COD. The research novelty lies in the use of propionate-cultured sludge bioaugmentation in landfill leachate treatment to enhance CH₄ production and micropollutant biodegradation.

Insights into high-solids anaerobic digestion of food waste enhanced by activated carbon via promoting direct interspecies electron transfer

High-solids anaerobic digestion (HS-AD) of food waste frequently confronted the acidification and failure under high organic loading rates (OLRs). Results indicated powdered activated carbon (PAC) addition significantly enhanced methane production and process stability than granular activated carbon, and columnar activated carbon at higher OLRs via accelerating the propionate consumption. Potential direct interspecies electron transfer (DIET) partners, including various syntrophic oxidation bacteria and methanogens, were enriched with the activated carbon (AC) addition. Furthermore, DIET contribution to methane production was 35% by PAC, predicated by the modified Anaerobic Digestion Model No.1 (ADM1). This study deeply elucidated the DIET mechanism and offered the potential foundations for the selection and applications of AC-based materials in HS-AD of food waste.