Fermentative hydrogen and methane co-production from anaerobic co-digestion of organic wastes at high loading rate coupling continuously and sequencing batch digesters

Effects of food wastes based on different components on digestibility and energy recovery in hydrogen and methane co-production

This study was conducted for four organic fractions (carbohydrates, proteins, cellulose, lipids) at an inoculum concentration of 30 % and a total solid (TS) of 8 % to investigate the effect of the main components of food waste on the performance of the two-stage anaerobic digestion. The results showed that the gas phase products were closely related to the composition of the substrate, with the carbohydrate and lipid groups showing the best hydrogen (154.91 ± 2.39mL/gVS) and methane (381.83 ± 12.691mL/gVS) production performance, respectively. However, the increased protein content predisposes the system to inhibition of gas production, which is mutually supported by changes in the activity of dehydrogenase and coenzyme F420. Butyric acid (53.19 %) dominated the liquid phase products in both stages, indicating that all four organic fractions were butyric acid-based fermentation and that the final soluble chemical oxygen demand degradation reached 72.97 %-82.86 %. The carbohydrate and cellulose groups achieved the best energy recovery performance, with conversion rates exceeding 65 %. The above results can provide a useful reference for the resource utilization of food waste.

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

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

Anaerobic Digestion of Agri-Food Wastes for Generating Biofuels

Presently, fossil fuels are extensively employed as major sources of energy, and their uses are considered unsustainable due to emissions of obnoxious gases on the burning of fossil fuels, which can lead to severe environmental complications, including human health. To tackle these issues, various processes are developing to waste as a feed to generate eco-friendly fuels. The biological production of fuels is considered to be more beneficial than physicochemical methods due to their environmentally friendly nature, high rate of conversion at ambient physiological conditions, and less energy-intensive. Among various biofuels, hydrogen (H₂) is considered as a wonderful due to high calorific value and generate water molecule as end product on the burning. The H₂ production from biowaste is demonstrated, and agri-food waste can be potentially used as a feedstock due to their high biodegradability over lignocellulosic-based biomass. Still, the H₂ production is uneconomical from biowaste in fuel competing market because of low yields and increased capital and operational expenses. Anaerobic digestion is widely used for waste management and the generation of value-added products. This article is highlighting the valorization of agri-food waste to biofuels in single (H₂) and two-stage bioprocesses of H₂ and CH₄ production.

High-solid digestion from cellulosic ethanol stillage with activated sludge of simultaneous propionate degradation and methanogenesis

High solid anaerobic digestion (HSAD) was an emerging bioconversion technology which had the advantages of small digester, less digestate and low heating energy. A one-stage anaerobic system in CSTR by inoculating activated sludge of simultaneous propionate degradation and methanogenesis was proposed to improve the high-solid digestion performance and to stabilize the reaction process. Semi-continuous mode was successfully used to perform HSAD from cellulosic ethanol whole stillage at an initial substrate loading of 15.4% (w/w) dry matter content with different OLRs from 1.5 to 5.0 gVS·L^-1 d^-1 at an HRT of 30 days. The average methane yield during whole digestion reached 349.9 mL⋅gVS^-1 with a total VS removal rate of 61.3%. The acclimation mechanism of multifunctional activated sludge was also explored by analyzing the functional property, physiological activity and microbial community structure. The results indicated the feasibility and efficiency of multifunctional activated sludge in a semi-continuous high-solid stirred tank reactor system.

Sustainable strategy for municipal solid waste disposal in Hong Kong: current practices and future perspectives

Hong Kong (HK) is confronted by increasing problems of solid waste disposal, as it is an overpopulated city with limited land resources. Currently, solid waste disposal mainly relies on three landfills located in the New Territories. However, the current waste treatment facilities and policies cannot appropriately control and manage increments of solid waste. The primary reason is the increased amount of municipal solid waste (MSW) caused by the growth of the population and the economy, with food waste accounting for the largest proportion of MSW in HK. The secondary reason is that the capacity of existing landfills will be exhausted in the near future as the level of waste generated continues to grow. To deal with these problems, in this paper, we propose five approaches with the aim of identifying the most sustainable strategy for efficient solid waste disposal in HK: a food waste recycling program; an MSW charging scheme; the implementation of incineration plants (i.e., waste to energy); black soldier fly bioconversion and a waste trading scheme; and black soldier fly bioconversion and a hybrid anaerobic digestion system. This is followed by a detailed demonstration of each approach, particularly focusing on the benefits, limitations, and implementation of each in the case of HK. The results of this study may shed light on how to effectively and sustainably manage the increasing amount of solid waste in HK.

Biorefinery concept comprising acid hydrolysis, dark fermentation, and anaerobic digestion for co-processing of fruit and vegetable wastes and corn stover

A new biorefinery conceptual process is proposed for biohydrogen and biomethane production from a combination of fruits and vegetable wastes (FVW) and corn stover (CS). The objective of this work was to perform the acid hydrolysis (HCl 0.5% v/v, 120 °C, 2 h) of the FVW and CS at 3:1 dry basis ratio, and to process its main physical phases, liquid hydrolyzates (LH) and hydrolyzed solids (HS), by mesophilic dark fermentation (DF) and anaerobic digestion (AD), respectively. In DF of LH as carbon source, hydrogen was produced at maximum rate of 2.6 mL H₂/(g_glucose h) and maximum accumulation of 223.8 mL H₂/g_glucose, equivalent to 2 mol H₂/mol_glucose, in a butyric-pathway-driven fermentation. HS were digested to methane production assessing inoculum to substrate ratios in the range 2-4 g_inoculum/g_VS. The main results in AD were 14 mmol CH₄/g_vs. The biorefinery demonstrated the feasibility to integrate the acid hydrolysis as pretreatment and subsequently use the LH for hydrogen production, and the HS for methane production, with an energy yield recovery of 9.7 kJ/g_vs, being the energy contribution from anaerobic digestion 8-fold higher than of dark fermentation.

Effect of yeast addition on the biogas production performance of a food waste anaerobic digestion system

Food waste contains numerous easily degradable components, and anaerobic digestion is prone to acidification and instability. This work aimed to investigate the effect of adding yeast on biogas production performance, when substrate is added after biogas production is reduced. The results showed that the daily biogas production increased 520 and 550 ml by adding 2.0% (volatile solids; VS) of activated yeast on the 12th and 37th day of anaerobic digestion, respectively, and the gas production was relatively stable. In the control group without yeast, the increase of gas production was significantly reduced. After the second addition of substrate and yeast, biogas production only increased 60 ml compared with that before the addition. After fermentation, the biogas production of yeast group also increased by 33.2% compared with the control group. Results of the analysis of indicators, such as volatile organic acids, alkalinity and propionic acid, showed that the stability of the anaerobic digestion system of the yeast group was higher. Thus, the yeast group is highly likely to recover normal gas production when the biogas production is reduced, and substrate is added. The results provide a reference for experiments on the industrialization of continuous anaerobic digestion to take tolerable measures when the organic load of the feed fluctuates dramatically.

Mathematical Modeling and Stability Analysis of a Two-Phase Biosystem

We propose a new mathematical model describing a biotechnological process of simultaneous production of hydrogen and methane by anaerobic digestion. The process is carried out in two connected continuously stirred bioreactors. The proposed model is developed by adapting and reducing the well known Anaerobic Digester Model No 1 (ADM1). Mathematical analysis of the model is carried out, involving existence and uniqueness of positive and uniformly bounded solutions, computation of equilibrium points, investigation of their local stability with respect to practically important input parameters. Existence of maxima of the input–output static characteristics with respect to hydrogen and methane is established. Numerical simulations using a specially elaborated web-based software environment are presented to demonstrate the dynamic behavior of the model solutions.

Hyperthermophilic hydrogen production in a simplified reaction medium containing onion wastes as a source of carbon and sulfur

In this study, the hyperthermophilic dark fermentation of onion wastes (OW) for hydrogen production was investigated. OW were used at different proportions in mixed fruit and vegetable wastes (FVW) to evaluate their effect on hydrogen production by Thermotoga maritima. Fermentations were performed in a pH-controlled batch stirred tank reactor (BSTR) using seawater as a simplified reaction medium. Results showed that increasing OW proportions in total fruit and vegetable wastes (tFVW) improved H₂ production. Therefore, increasing the OW to tFVW ratio from 0 to 0.8 increased the cumulative H₂ production from 109 to 223.6 mmol/L. The H₂ productivity was also improved from 7.3 to 28.82 mmol/h.L. In fact, OW contain carbohydrates, sulfur compounds, and other nutrients, which were used as a carbon source and energetic substrate for H₂ production by the halophilic bacterium T. maritima in seawater without additional chemical compounds. Then, a H₂ yield of 3.36 mol H₂/mol hexose was achieved using 200 mL of OW, containing 55 mmol/L of carbohydrates. A concept of H₂ production from FVW at high proportions of OW in a simplified reaction medium was proposed. It allowed a H₂ yield of 209 LH₂/kg volatile solids which could be an interesting future alternative to the current fossil fuel.

Hydrogen and methane production by co-digesting liquid swine manure and brewery wastewater in a two-phase system

Co-digesting liquid swine manure and brewery wastewater for hydrogen and methane production was studied using an integrated, two-phase system with different organic loading rates (OLR) under mesophilic conditions. The highest volumetric hydrogen and methane production rates achieved were 294.06 ± 3.06 mL H₂ L^-1 d^-1 and 497.94 ± 10.01 mL CH₄ L^-1 d^-1, respectively, at the OLR of 8613.6 mg COD L^-1 d^-1, together with the highest hydrogen-COD and -TVS yields (34.14 ± 0.36 mL g^-1 COD and 45.46 ± 0.71 mL g^-1 TVS) and the maximum methane-TVS yield (261.42 ± 8.41 mL g^-1 TVS). The highest hydrogen and methane concentrations in the biogas (31.86 ± 0.68% and 82.66 ± 0.33%, respectively) were also obtained at this OLR. The maximum methane-COD yield (270.3 ± 3.93 mL g^-1 COD), however, was from the lowest OLR (1148.6 mg COD L^-1 d^-1). The two-phase system removed 75.54 ± 0.19% of COD from the influent.

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.

Revealing the correlation of biomethane generation, DOM fluorescence, and microbial community in the mesophilic co-digestion of chicken manure and sheep manure at different mixture ratio

Batch co-digestion tests of chicken manure (CM) and sheep manure (SM) at different ratio (R_s/c) were conducted under mesophilic condition (35 °C). Batch kinetic analysis of bioCH₄ production, excitation-emission matrix (EEM) fluorescence of dissolved organic matter (DOM), and microbial community were investigated. The well-fitted modified Gompertz model (R², 0.98-0.99) resulted that the co-digestion markedly improved the methane production rate and shortened the lag phase time. The highest bioCH₄ yield of 219.67 mL/gVS_add and maximum production rate of 0.378 mL/gVS_add/h were obtained at an optimum R_s/c of 0.4. Additionally, a significant variation of DOM was detected at the R_s/c of 0.4 with a consistent degradation of soluble microbial byproduct-like and protein-like organics. The positive synergy effects of co-digestion conspicuously enhanced the bioCH₄ production efficiency. FI₃₇₀ and NADH were significantly correlated to R_s/c (p < 0.05). Moreover, the correlations among process indicator, EEM-peaks and different environmental parameters were evaluated by Pearson correlation analysis. The high diversity of acetoclastic methanogens and hydrogenotrophic methanogens in the co-digestion improved the stability of process. Graphical Abstract.

Harvesting zero waste from co-digested fruit and vegetable peels via integrated fermentation and pyrolysis processes

The aim of this study is to assess an innovative economic approach for the production of both fermentative hydrogen and biochar from fruit and vegetable peels (FVPs) via fermentation/pyrolysis process. Firstly, in fermentation batches, multi-fermentation of FVPs positively affected the harvested hydrogen yield and COD reduction efficiency, which reached their maximal values of 3.9 ± 0.6 mmol/g_COD and 56.2 ± 4.6% at batch of 25% pea + 25% tomato + 25% banana + 25% orange (M4). Secondly, digestates produced from all batches were pyrolyzed at 500 °C for investigating the potential for biochar production. Based on the characteristics of the pyrolyzed digestate, biochar produced from S1 (spinach) exhibited the highest specific surface area, density, pore volume, biochar production yield, and pyrolysis profit of 28.43 ± 3.95 m²/g, 1.93 ± 0.18 g/cm³, 0.59 ± 0.08 cm³/g, 59.04 ± 2.36%, and 3.66 $/kg_feedstock, respectively. However, the maximum overall profit from both fermentation and pyrolysis processes was 5.21 $/kg_feedstock and was denoted for M4.