An Insight into the Anaerobic Co-digestion of Municipal Solid Waste and Food Waste: Influence of Co-substrate Mixture Ratio and Substrate to Inoculum Ratio on Biogas Production

Synergetic anaerobic digestion of food waste for enhanced production of biogas and value-added products: strategies, challenges, and techno-economic analysis

The generation of food waste (FW) is increasing at an alarming rate, contributing to a total of 32% of all the waste produced globally. Anaerobic digestion (AD) is an effective method for dealing with organic wastes of various compositions, like FW. Waste valorization into value-added products has increased due to the conversion of FW into biogas using AD technology. A variety of pathways are adopted by microbes to avoid unfavorable conditions in AD, including competition between sulfate-reducing bacteria and methane (CH4)-forming bacteria. Anaerobic bacteria decompose organic matter to produce biogas, a digester gas. The composition depends on the type of raw material and the method by which the digestion process is conducted. Studies have shown that the biogas produced by AD contains 65-75% CH4 and 35-45% carbon dioxide (CO2). Methanothrix soehngenii and Methanosaeta concilii are examples of species that convert acetate to CH4 and CO2. Methanobacterium bryantii, Methanobacterium thermoautotrophicum, and Methanobrevibacter arboriphilus are examples of species that produce CH4 from hydrogen and CO2. Methanobacterium formicicum, Methanobrevibacter smithii, and Methanococcus voltae are examples of species that consume formate, hydrogen, and CO2 and produce CH4. The popularity of AD has increased for the development of biorefinery because it is seen as a more environmentally acceptable alternative in comparison to physico-chemical techniques for resource and energy recovery. The review examines the possibility of using accessible FW to produce important value-added products such as organic acids (acetate/butyrate), biopolymers, and other essential value-added products.

Anaerobic co-metabolic biodegradation of pharmaceuticals and personal care products driven by glycerol fermentation

Anaerobic digestion in two sequential phases, acidogenesis and methanogenesis, has been shown to be beneficial for enhancing the biomethane generation from wastewater. In this work, the application of glycerol (GOH) as a fermentation co-substrate during the wastewater treatment was evaluated on the biodegradation of different pharmaceuticals and personal care products (PPCPs). GOH co-digestion during acidogenesis led to a significant increase in the biodegradation of acetaminophen (from 78 to 89%), ciprofloxacin (from 25 to 46%), naproxen (from 73 to 86%), diclofenac (from 36 to 48%), ibuprofen (from 65 to 88%), metoprolol (from 45 to 59%), methylparaben (from 64 to 78%) and propylparaben (from 68 to 74%). The heterotrophic co-metabolism of PPCPs driven by glycerol was confirmed by the biodegradation kinetics, in which kbio (biodegradation kinetics constant) values increased from 0.18 to 2.11 to 0.27-3.60 L g-1-VSS d-1, for the operational phases without and with GOH, respectively. The assessment of metabolic pathways in each phase revealed that the prevalence of aromatic compounds degradation, metabolism of xenobiotics by cytochrome P450, and benzoate degradation routes during acidogenesis are key factors for the enzymatic mechanisms linked to the PPCPs co-metabolism. The phase separation of anaerobic digestion was effective in the PPCPs biodegradation, and the co-fermentation of glycerol provided an increase in the generation potential of biomethane in the system (energetic potential of 5.0 and 6.3 kJ g-1-CODremoved, without and with GOH, respectively). This study showed evidence that glycerol co-fermentation can exert a synergistic effect on the PPCPs removal during anaerobic digestion mediated by heterotrophic co-metabolism.

Evaluation of methane generation rate and energy recovery potential of municipal solid waste using anaerobic digestion and landfilling: A case study of Dhanbad, India

In this study, two most common biological waste to energy conversion techniques have been analysed and compared on the basis of methane generation and energy recovery potential. The biogas generation potential has been estimated using anaerobic co-digestion experiment. The main substrate used for this study was food waste, while cow dung and anaerobic digester sludge were used as co-digestion substrates. Food waste was used because of its maximum composition (36% of total wastes) in the study area (Dhanbad city, India) with high biogas generation potential. Cow dung acted as a natural buffer, while anaerobic digester sludge was a source of active inoculum. Based on the maximum biogas yield of 402 mL g^-1 VS and annual food waste generation rate in the study area, the energy recovery potential using anaerobic digestion was estimated to be 6.59 × 10⁶ kWh year^-1. Presently, the wastes in the mixed form are being dumped on a large abandoned land with an approximate area of 93 ha in the outskirts of Dhanbad city in an uncontrolled manner. The annual landfill gas generation from the existing landfill has been estimated using Landfill Gas Emissions Model (LandGEM) based on the waste characteristics, anticipated landfill life and other region-specific parameters of the present study area. The maximum electrical energy recovery potential of 44.62 × 10⁵ kWh from landfill gas has been estimated for the year 2041. Although, the results are based on the waste generation and characteristics of Dhanbad city, the comparison methodology can be applied to other cities.

Valorization of Fourth-Range Wastes: Evaluating Pyrolytic Behavior of Fresh and Digested Wastes

Changes in daily habits and a stressful lifestyle create modifications in consumer preferences and open opportunities to new market products. This is the case of fourth-range products in which the industrial sector generates a waste stream of high quality. Valorization of this type of waste as a single stream is desirable to avoid lowering quality with other low-grade materials. Anaerobic digestion of fourth-range wastes was studied under discontinuous and semi-continuous conditions. A high carbon content characterizes the organic material composed of fruit and vegetable wastes. The fast degradation of the substrate indicated no limitations associated with the hydrolysis stage, as observed from kinetic parameters estimated from batch assays. However, the easiness of degradation did not translate into short hydraulic retention times when operating under semi-continuous conditions. Additionally, the insufficient amount of nutrients prevented the development of a well-balanced digestion process. Specific methane production was 325 mL CH4/g VS added at a hydraulic retention time of 30 days. However, solid accumulation was observed at the end of the experiment, indicating that conditions established did not allow for the complete conversion of the organic material. Digestate evaluation using thermal analysis under inert conditions showed a thermal profile evidencing the presence of complex components and a high tendency to char formation.

Biogas production from the landfilled easily degradable fraction of municipal solid waste: mining strategy for energy recovery

The growing demand for space and financial resources to manage current and new municipal solid waste (MSW) landfills has become a massive challenge for several countries. Additionally, landfills contribute to adverse environmental impacts such as pollution and CO2 (carbon dioxide) and CH4 (methane) emissions. This paper has analyzed the possibility of producing biogas from landfilled MSW. An easily degradable fraction of landfilled MSW with 8 years of landfilling was mined and subjected to chemical characterization and elemental composition analysis. The abbreviation for the study sample was called ED8 - Mined. The low values of lignin (24.5%) and nitrogen content (0.7%) and high values of holocellulose (75.9%) and C/N (46.1%) on dry basis were obtained resulting in materials with the potential to be used for biogas generation. Recalcitrant materials were found in greater amounts than easily biodegradable fresh MSW fractions. The reuse of energy from landfilled MSW can contribute positively to the country's environment and economy, reducing environmental liabilities and generating energy in a controlled way. In Delta A Sanitary Landfill, Southeastern Brazil, the recovery of the ED8 - Mined would reflect a significant recovery of about 100,000 tonnes of landfilled materials for annual MSW cells of about 450,000 tonnes, allowing recovery of materials and space expansion for rejects.

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Estimation of methane production through the anaerobic digestion of greenhouse horticultural waste: A real case study for the Almeria region

The methane production of greenhouse horticultural waste (GHW) from Almeria (Spain), from where fruits and vegetables are exported to all parts of Europe, was calculated in this work through a combination of experimental and theoretical methods. To this end, eight samples of GHW were collected and characterized in a waste treatment plant. The collection of samples was fairly distributed throughout the year to ensure a representative characterization. The amount of methane produced in a hypothetical anaerobic digestion process was predicted through empirical models fed by experimental data. The experimental characterization revealed that GHW contained an adequate content of volatile matter (65.72% TS), but a high value for total dry matter (53.46%) and lignin content (9.36%), as well as a low moisture content (46.54%) and C/N ratio (17.46). Inhibiting compounds were also observed in the characterization, such a S (0.43%) and Cl (1.41%). The methane production predicted was 0.229 Nm³ CH₄/kg volatile matter, which may seem low in comparison to other waste potentially usable for anaerobic digestion. Nonetheless, the co-digestion of GHW with other waste could be an interesting alternative to enhance methane production and solve seasonality issues. Suitable pre-treatment can be also explored to increase the usability of GHW in anaerobic digestion. All in all, this work establishes a theoretical basis for potential solutions to manage the GHW produced in Almeria.

Effect of the Substrate to Inoculum Ratios on the Kinetics of Biogas Production during the Mesophilic Anaerobic Digestion of Food Waste

This study evaluates the effects of the varying substrate to inoculum ratios (S:I) of 0.5, 1, 2, 3, 4, 5, and 6 (volatile solids/VS basis) on the kinetics of biogas production during batch mesophilic (35 ± 1 °C) anaerobic digestion (AD) of simulated food waste (FW), using anaerobic digestate as the inoculum. Kinetic parameters during biogas production (scrubbed with NaOH solution) are predicted by the first-order and the modified Gompertz model. The observed average specific biogas yields are in descending order corresponding to the S:I ratios 1, 2, 4, 6, 3, 5, and 0.5, respectively, and the significant effect of the S:I ratio was observed. The tests with the S:I of 1 have the maximum average biogas production rates of 88.56 NmL/gVS.d, whereas tests with the S:I of 6 exhibited the lowest production rates (24.61 NmL/gVS.d). The maximum biogas yields, predicted by the first order and the modified Gompertz model, are 668.65 NmL/gVS (experimental 674.40 ± 29.10 NmL/gVS) and 653.17 NmL/gVS, respectively. The modified Gompertz model has been proven to be suitable in predicting biogas production from FW. VS removal efficiency is greater in higher S:I ratios, with a maximum of 78.80 % at the S:I ratio of 6, supported by the longer incubation time. Moreover, a significant effect of the S:I ratio is seen on kinetics and energy recovery from the AD of FW.

Synergistic digestion of banana pseudo-stems with chicken manure to improve methane production: Semi-continuous manipulation and microbial community analysis

This study aimed to investigate the interactions between banana pseudo-stems (BPS) and chicken manure (CM) during anaerobic co-digestion (AcoD) in batch and semi-continuous experiments. The batch experiments results showed that the methane yield was the highest (193.7 mL/g VS) in AcoD with BPS: CM ratio of 4:1, which was increased by 57.2% and 66.1%, respectively. Semi-continuous experiments revealed that AcoD resulted in higher methane production. Monitoring of the system parameters indicated that AcoD could better adapt to the increasing organic loading rate, with better system stability and methane production efficiency. The microbial analysis illustrated that AcoD increased the relative abundance of hydrolytic bacteria such as Firmicutes, Patescibacteria, and Bacteroidetes. With regard to archaea, AcoD improved the abundance of Methanosaeta, the major acetoclastic methanogens. These changes in the microbial flora allowed AcoD to remain stable while efficiently producing methane and improved the BPS and CM processing efficiency.

Enhanced lignocellulosic component removal and biomethane potential from chestnut shell by a combined hydrothermal-alkaline pretreatment

This study proposes new perspectives for the management and biorefinery of wastes deriving from the agri-food sector such as chestnut shell (CS), which was here used as an organic feedstock for biomethane production through anaerobic digestion (AD). 1-5% alkaline (i.e. NaOH and KOH), hydrothermal (i.e. at 100 °C) and combined hydrothermal-alkaline pretreatments were employed to enhance the CS biodegradability prior to biochemical methane potential (BMP) tests conducted under mesophilic conditions. The hydrothermally-pretreated CS with 3% NaOH achieved the highest biomethane yield of 253 (±9) mL CH₄·g VS^-1 coupled to a volatile solid reduction of 48%. The hydrothermal-alkaline pretreatment positively affected both delignification and hemicellulose polymerization, promoting an approximately 2.4-fold higher substrate biodegradability compared to the untreated CS, which only reached a CH₄ production of 104 (±5) mL CH₄·g VS^-1. AD proceeded via volatile fatty acid accumulation, subsequently followed by methane production that was effectively simulated via the modified Gompertz kinetic having a R² of 0.974-0.999. Among the physical-chemical parameters characterizing the CS, the soluble chemical oxygen demand (sCOD) was highly correlated with the BMP showing a Pearson coefficient of 0.952. The cumulative biomethane yield, the sCOD and the cellulose, hemicellulose and lignin amount of CS were also processed through the least square method, obtaining a useful regression equation to predict the BMP. The economic assessment indicated that the hydrothermal-alkaline pretreatment is a cost-effective method to improve the BMP of CS, also for future full-scale applications.

Diauxie Studies in Biogas Production from Gelatin and Adaptation of the Modified Gompertz Model: Two-Phase Gompertz Model

The anaerobic degradation of gelatin results in a two-phase cumulative biogas production curve, i.e., diauxie behaviour. The modified Gompertz model is normally used to fit these curves but due to the diauxie it would result in a less accurate representation. Furthermore, this inhibition slows down the production of biogas in batch reactors. This study adapted the modified Gompertz model to fit cumulative biogas production curves with diauxie behaviour and to investigate the inhibition that leads to this diauxie. Results show that the two-phase Gompertz model can fit diauxie curves with high accuracy and that diauxie curves are not a direct consequence of the accumulation of volatile fatty acids produced in the process of anaerobic digestion.

Waste-to-energy nexus: A sustainable development

An upsurge in global population due to speedy urbanization and industrialization is facing significant challenges such as rising energy-demand, enormous waste-generation and environmental deterioration. The waste-to-energy nexus based on the 5R principle (Reduce, Reuse, Recycle, Recovery, and Restore) is of paramount importance in solving these Gordian knots. This review essentially concentrates on latest advancements in the field of 'simultaneous waste reduction and energy production' technologies. The waste-to-energy approaches (thermal and biochemical) for energy production from the agricultural residues are comprehensively discussed in terms environmental, techno-economic, and policy analysis. The review will assess the loopholes in order to come up with more sophisticated technologies that are not only eco-friendly and cost-effective, but also socially viable. The waste-to-energy nexus as a paradigm for sustainable development of restoring waste is critically discussed considering future advancement plans and agendas of the policy-makers.

Interactive effects of carbohydrate, lipid, protein composition and carbon/nitrogen ratio on biogas production of different food wastes

The anaerobic co-digestion (AcoD) of FWs produces variable methane yields, mainly due to variable carbon/nitrogen (C/N) ratio and proportions of lipids (L), carbohydrates (C), and proteins (P) in different FW samples. In this study, a significant interaction between C/N ratio and LCP composition was found and contributed to the differing trends between special (SMP) and theoretical methane production. The highest SMP of 595 mL CH₄ gVS^-1 occurred at C/N of 25 and LCP of 63.25:22.62:14.13, followed by 592 mL CH₄ gVS^-1 at C/N of 30 and LCP of 48.94:39.74:11.32, which also reflected their interaction. Attributing to their interactive effect on obtaining optimal process parameters and microbial community, the inhibition threshold of lipid as well as the methane yield was increased. Understanding the interaction between C/N ratio and LCP composition is an effective and promising way to obtain suitable mixture ratios of organic wastes under AcoD.

Effect of pH, substrate loading, oxygen, and methanogens inhibitors on volatile fatty acid (VFA) production from citrus waste by anaerobic digestion

Citrus waste from e.g., juice production is a potential substrate for anaerobic digestion (AD). However, due to the toxic citrus peel oil content, citrus waste has several challenges in biogas production. Hence, volatile fatty acids (VFAs) are very interesting intermediate products of AD. This paper was aimed to investigate VFA production from citrus wastes by boosting its production and inhibiting methane formation. Therefore, the effects of inoculum to substrate ratio (ISR), O₂ presence, pH, and inhibitor for methanogens, in VFA production from citrus waste through acidification process were studied. The addition of 2 g/L methanogens inhibitor and the presence of O₂ in the reactors were able to reduce methane production. The highest yield of VFA (0.793 g VFA/g VS_added) was achieved at controlled pH at 6 and low substrate loading (ISR 1:1). Acetic acid (32%), caproic acid (21%), and butyric acid (15%) dominate the VFA composition in this condition.

Effects of temperature and inoculation ratio on methane production and nutrient solubility of swine manure anaerobic digestion

This study investigated the effects of temperature and volatile solids (VS) ratio of feedstock to inoculum (F/I ratio) on methane (CH₄) production and the solubility of nitrogen (N), phosphorus (P), cooper (Cu), and Zinc (Zn) after anaerobic digestion (AD) of swine manure. The highest cumulative CH₄ yield of 470 L/kgVS_feed was obtained with F/I ratios of 2.0 and 3.0 with mesophilic (37 °C) temperature, and methane production rate decreased with the increase of F/I ratio. As F/I ratio increased from 0.5 to 4.0, the lag phase for methane production increased from 1.02 days to 13.52 days, indicating an initial inhibition at high F/I ratios. AD increased the concentrations of ammonia, Cu and Zn in the AD effluent supernatant, while decreased total and water extractive P concentrations. The changes of ammonia, Cu, Zn, and P concentrations were more significant with the increase of F/I ratio.