Thermally enhanced solubilization and anaerobic digestion of organic fraction of municipal solid waste

A comprehensive review on anaerobic digestion with focus on potential feedstocks, limitations associated and recent advances for biogas production

With the escalating energy demand to accommodate the growing population and its needs along with the responsibility to mitigate climate change and its consequences, anaerobic digestion (AD) has become the potential approach to sustainably fulfil our demands and tackle environmental issues. Notably, a lot of attention has been drawn in recent years towards the production of biogas around the world in waste-to-energy perspective. Nevertheless, the progress of AD is hindered by several factors such as operating parameters, designing and the performance of AD reactors. Furthermore, the full potential of this approach is not fully realised yet due the dependence on people's acceptance and government policies. This article focuses on the different types of feedstocks and their biogas production potential. The feedstock selection is the basic and most important step for accessing the biogas yield. Furthermore, different stages of the AD process, design and the configuration of the biogas digester/reactors have been discussed to get better insight into process. The important aspect to talk about this process is its limitations associated which have been focused upon in detail. Biogas is considered to attain the sustainable development goals (SDG) proposed by United Nations. Therefore, the huge focus should be drawn towards its improvements to counter the limitation and makes it available to all the rural communities in developing countries and set-up the pilot scale AD plants in both developing and developed countries. In this regard, this article talks about the improvements and futures perspective related to the AD process and biogas enhancement.

Meso- and thermophilic posttreatment of press water coming from a thermophilic municipal solid waste digester

An efficient biogas production out of organic (waste) materials is important to contribute to a carbon-neutral future. In this study, thermophilic press water (PW) coming from an organic fraction of the municipal solid waste digester was further digested in a thermo- and mesophilic posttreatment approach using two semicontinuous 14 L digesters. The results showed that the PW can still have considerable high biogas potential-at least during the touristic high season in central Europe. The change in temperature led to an increase in volatile fatty acid concentrations and a decrease in biogas production in the mesophilic approach in the first days. However, the losses in biogas production at the beginning could be compensated thus there were no considerable differences in biogas production between thermo- and mesophilic posttreatment at the end of incubation. This can most probably be contributed to a change in the microbial community, and potentially problematic intermediates like valerate could be better degraded in the mesophilic reactor. Especially the abundance of representatives of the phylum Bacteroidota, like Fermentimonas spp., increased during mesophilic anaerobic digestion.

Semi-continuous anaerobic co-digestion of thermal and thermal-alkali processed organic fraction of municipal solid waste: Methane yield, energy analysis, anaerobic microbiome

The semi-continuous anaerobic co-digestion (AcoD) of thermal and thermal-alkali pretreated organic fraction of municipal solid waste (OFMSW) and sewage sludge (SS) was studied under varying hydraulic retention times (HRT) and organic loading rates (OLR Three semi-continuous digesters were operated under control (non-pre-treated), thermally pretreated (125 °C), and thermal-alkali pretreated (125°C-3g/L NaOH) conditions at variable OLRs at 2.5, 4.0, 5.1, and 7.6 kgVS/m3.d and corresponding HRTs of 30, 20, 15, and 10 days. The 10 and 43% higher methane yield (0.445 m3/kgVS) and 11 and 57% higher VS removal (52%) was achieved for thermal-alkali pretreated digester at 5.1 kgVS/m3.d OLR over thermally pretreated (0.408 m3/kgVS, 45% VS removal) and control digesters (0.310 m3/kgVS, 33% VS removal), respectively. Thermal and thermal-alkali digesters failed on increasing the OLR to 7.6 kgVS/m3.d, whereas the control digester becomes upset at 5.1 kgVS/m3.d OLR. The metagenomic study revealed that Firmicutes, Bacteroidetes, Chloroflexi, Euryarchaeota, Proteobacteria, and Actinobacteria were the predominant bacterial population, whereas Methanosarcina and Methanothrix dominated the archaeal community. Energy balance analysis revealed that thermal alkali pretreatment showed the highest positive energy balance of 114.6 MJ/ton with an energy ratio of 1.25 compared with thermally pretreated (81.5 MJ/ton) and control samples (-46.9 MJ/ton). This work pave the way for scaleup of both thermal and thermal-alkali pre-treatment at 125 °C to realize the techno-economic and energy potential of the process.

Valorization of strawberry extrudate waste: Recovery of phenolic compounds by direct-hydrothermal treatment and subsequent methane production by mesophilic semi-continuous anaerobic digestion

Strawberry extrudate (SE) is an underused by-product from strawberry industry. Recovery of the phenolic compounds present in SE would represent a very interesting valorisation option. Two main challenges need to be solved, firstly, the solubilisation and recovery of the phenolic compounds contained in SE, and, after that, the stabilisation of the resulted de-phenolized SE. The present research evaluates the potential of a biorefinery process combining a hydrothermal pre-treatment, followed by a phenolic extraction process and, finally, the anaerobic digestion of the remaining SE for producing energy that will contribute to compensate the energy requirements of the whole system. Following the hydrothermal pre-treatment at 170 °C for 60 min, an extraction of 0.6 ± 0.1 g of gallic acid per kilogram of SE was achieved using an adsorbent resin, representing a recovery rate of 64 %. Long-term semi-continuous anaerobic digestion of de-phenolized SE was evaluated at different organic loading rates to evaluate the stability of the process. The anaerobic digestion of pre-treated SE achieved a stable methane production value of 243 ± 34 mL CH4·g volatile solids-1·d-1 at an organic loading rate (ORL) of 1.25 g volatile solids·L-1·d-1. During the operation at this ORL, the control parameters including pH, alkalinity, soluble chemical organic demand (sCOD), and volatile fatty acid (VFA) remained stable and consistently constant. Specifically, the VFA in the reactor during this stable period achieved a value of 102 ± 128 mg O2/L. Also, an economic balance showed that the minimal price of the generated phenolic extract for having benefited from the proposed biorefinery system was 0.812 €·(g of gallic acid equivalents)-1, a price within the range of phenolic compounds used in the food industry.

Energy recovery from agro-forest wastes through hydrothermal carbonization coupled with hydrothermal Co-gasification: Effects of succinic acid on hydrochars and H2 production

Aiming to upgrade agro-forest wastes into value-added solid and gaseous fuels in the present investigation, hydrothermal carbonization (HTC) of spruce (SP), canola hull (CH), and canola meal (CM) was optimized in terms of operating conditions, maximizing the higher heating value of hydrochars. The optimal operating conditions were achieved at HTC temperature, reaction time, and solid-to-liquid ratio of 260 ^°C, 60 min, and 0.2 g mL^-1, respectively. At the optimum condition, succinic acid (0.05-0.1 M) was used as HTC reaction medium to investigate the effects of acidic medium on the fuel characteristics of hydrochars. The succinic acid assisted HTC was found to eliminate ash-forming minerals e.g., K, Mg, and Ca from hydrochar backbones. The calorific values, H/C and O/C atomic ratios of hydrochars were in the range of 27.6-29.8 MJ kg^-1, 0.8-1.1, and 0.1-0.2, respectively, indicating the biomass upgrading into coal-like solid fuels. Finally, hydrothermal gasification of hydrochars with their corresponding HTC aqueous phase (HTC-AP) was assessed. Gasification of CM resulted in a relatively high H₂ yield of 4.9-5.5 mol kg^-1 followed by that for SP with 4.0-4.6 mol H₂ per kg of hydrochars. Results suggest that hydrochars and HTC-AP have a great potential for H₂ production via hydrothermal co-gasification, while suggesting HTC-AP reuse.

Environment friendly emerging techniques for the treatment of waste biomass: a focus on microwave and ultrasonication processes

In the recent past, an increasing interest is mostly observed in using microwave and ultrasonic irradiation to aid the biological conversion of waste materials into value-added products. This study is focused on various individual impacts of microwaves and ultrasonic waves for the treatment of biomass before the synthesis of value-added products. Following, a comprehensive review of the mechanisms governing microwaves and ultrasonication as the treatment methods, their effects on biomass disruption, solubilization of organic matter, modification of the crystalline structure, enzymatic hydrolysis and production of reducing sugars was performed. However, based on the lab-scale experiments evaluated, microwaves and ultrasonication were studied to be economically and energetically ineffective despite their beneficial effects on the waste biomass. This article reviews some of the difficulties associated with using microwaves and ultrasonic irradiation for the efficient processing of waste biomasses and identified some potential directions for future study.

A strategic review on sustainable approaches in municipal solid waste management andenergy recovery: Role of artificial intelligence,economic stability andlife cycle assessment

The consumption of energy levels has increased in association with economic growth and concurrently increased the energy demand from renewable sources. The need under Sustainable Development Goals (SDG) intends to explore various technological advancements for the utilization of waste to energy. Municipal Solid Waste (MSW) has been reported as constructive feedstock to produce biofuels, biofuel carriers and biochemicals using energy-efficient technologies in risk freeways. The present review contemplates risk assessment and challenges in sorting and transportation of MSW and different aspects of conversion of MSW into energy are critically analysed. The circular bioeconomy of energy production strategies and management of waste are also analysed. The current scenario on MSW and its impacts on the environment are elucidated in conjunction with various policies and amendments equipped for the competent management of MSW in order to fabricate a sustained environment.

A critical review on pretreatment and detoxification techniques required for biofuel production from the organic fraction of municipal solid waste

The organic fraction of municipal solid waste (OFMSW) is a widely-available promising feedstock for biofuel production. However, the presence of different inhibitors originating from fruit and food/beverage wastes as well as recalcitrant lignocellulosic fractions hampers its bioconversion. This necessitates a pretreatment to augment the biodigestibility and fermentability of OFMSW. Hence, this review aims to provide the in-vogue inhibitory compound removal and pretreatment techniques that have been employed for efficient OFMSW conversion into biofuels, i.e., hydrogen, biogas, ethanol, and butanol. The techniques are compared concerning their mode of action, chemical and energy consumption, inhibitor formation and removal, economic feasibility, and environmental sustainability. This critique also reviews the existing knowledge gap and future perspectives for efficient OFMSW valorization. The insights provided pave the way toward developing energy-resilient cities while addressing environmental crises related to generating OFMSW.

Novel insight on ferric ions addition to mitigate recalcitrant formation during thermal-alkali hydrolysis to enhance biomethanation

Thermal-chemical pre-treatment has proven to facilitate the solubilization of organics and improvement in biogas generation from the organic fraction of municipal solid waste (OFMSW). However, the production of recalcitrant is inevitable when OFMSW is pretreated at high temperatures and alkali dosage. This study develops a strategy to use Fe³⁺ to reduce the formation of recalcitrant compounds, i.e., 5-HydroxyMethyl Furfural (5-HMF), furfurals, and humic acids (HA) during thermal-alkali pre-treatment. It was postulated that the formation of the recalcitrant compound during pre-treatment can be reduced by Fe³⁺ dosing to oxidize intermediates of Maillard reactions. A decrease in 5-HMF (45-49%) and furfurals (54-66%) was observed during Fe³⁺ (optimum dose: 10 mg/L) mediated thermal-alkali pre-treatment owing to the Lewis acid behavior of FeCl₃. The Fe³⁺ mediated assays show a substantial improvement in VS removal (28%) and biogas yield, i.e., 31% (292 mL/gVS_added) in 150 °C + 3 g/L NaOH, 34% (316 mL/gVS_added) in 175 °C + 3 g/L NaOH, and 36% (205 mL/gVS_added) in 200 °C + 3 g/L NaOH assays, over their respective controls (no Fe³⁺ dosing). The reducing property of Fe³⁺ rendered a low ORP (-345 mV) in the system than control, which is beneficial to the anaerobic microbiome. Electrical conductivity (EC) also shows a three-fold increase in Fe³⁺ mediated assays over control, promoting direct interspecies electron transfer (DIET) amongst microbes involved in the electrical syntrophy. The score plot and loading plots from principal component analysis (PCA) showed that the results obtained by supplementing 10 mg/L Fe³⁺ at 150, 175, and 200 °C were significantly different. The correlation of the operational parameters was also mutually correlated. This work provides a techno-economically and environmentally feasible option to mitigate the formation of recalcitrant compounds and enhance biogas production in downstream AD by improving the degradability of pretreated substrate.

Biomethane enhancement using reduced graphene oxide in anaerobic digestion of municipal solid waste

The present research investigated the impact of reduced graphene oxide (rGO) addition on the semi-continuous anaerobic digestion of the organic fraction of municipal solid waste (OFMSW) in the range of 0.5-10 gVolatileSolid(VS)/L_reactorday organic loading rates (OLR). Adding rGO enhanced the rate and yield of biomethane production, and the maximum biomethane increment rate was obtained as 110% at an OLR of 4.0 gVS/L_reactorday. However, after increasing the OLR to 6 gVS/L_reactorday, there was a dramatic decrease in biomethane production because of volatile fatty acid (VFA) accumulation. Methanotrix is the predominant archaeal genus at OLRs lower than 6 gVS/L_reactorday in reactors (89-97%). An increment in biomethane production was associated with the higher abundance of the Methanothrix genus in the rGO-supported reactor (rG) than in the control reactor (rC).

Recalcitrant compounds formation, their toxicity, and mitigation: Key issues in biomass pretreatment and anaerobic digestion

Increasing energy demands and environmental issues have stressed the importance of sustainable methods of energy production. Anaerobic digestion (AD) of the biodegradable waste, i.e., agricultural residues, organic fraction of municipal solid waste (OFMSW), sewage sludge, etc., results in the production of biogas, which is a sustainable and cost feasible technique that reduces the dependence on fossil fuels and also overcomes the problems associated with biomass waste management. To solubilize the organic matter and enhance the susceptibility of hardly biodegradable fraction (i.e., lignocellulosic) for hydrolysis and increase methane production, several pretreatments, including physical, chemical, biological, and hybrid methods have been studied. However, these pretreatment methods under specific operating conditions result in the formation of recalcitrant compounds, such as sugars (xylose, Xylo-oligomers), organic acids (acetic, formic, levulinic acids), and lignin derivatives (poly and mono-phenolic compounds), causing significant inhibitory effects on anaerobic digestion. During the scaling up of these techniques from laboratory to industrial level, the focus on managing inhibitory compounds formed during pretreatment is envisaged to increase because of the need to use recalcitrant feedstocks in anaerobic digestion to increase biogas productivity. Therefore, it is crucial to understand the production mechanism of inhibitory compounds during pretreatment and work out the possible detoxification methods to improve anaerobic digestion. This paper critically reviews the earlier works based on the formation of recalcitrant compounds during feedstocks pretreatment under variable conditions, and their detrimental effects on process performance. The technologies to mitigate recalcitrant toxicity are also comprehensively discussed.

Carbon based conductive materials mediated recalcitrant toxicity mitigation during anaerobic digestion of thermo-chemically pre-treated organic fraction of municipal solid waste

High-temperature thermal pretreatment alone or in conjugation with chemical pretreatment (highly acidic or alkaline) produced recalcitrant compounds, which inhibits the anaerobic digestion (AD) process performance. This study aims to develop a strategy to use carbon-based conductive materials to mitigate the recalcitrant toxicity and enhance the methane generation in the downstream AD. The formation of recalcitrant compounds, mainly the furan derivatives, i.e., furfural and 5-HydroxyMethyl furfurals (5-HMF) during thermo-chemical pretreatment of OFMSW at 150 °C, 175 °C, 200 °C with 3 g/L-NaOH dose, and the alleviation of their inhibitory effects by adding 25 g/L of each of granular activated carbon (GAC) and granular biochar (GBC) during mesophilic AD were studied. The addition of conductive materials resulted in the highest biogas yield of 462 mL/gVS_added (GAC) and 449 mL/gVS_added (GBC) for 175°C-3g/L-NaOH pretreatment, which was >45% higher over control. The highest improvement of >65% in biogas yield was observed for 200°C-3g/L-NaOH pretreatment despite the lower biogas yield. The conductive materials amended digester shows a significant decrease in the 5-HMF and furfurals concertation. The highest reduction in 5-HMF (44%) and furfural (51%) concentrations were observed for 200°C-3g/L-NaOH pretreatment, and 25 g/L GBC amended tests. The score plots from the principal component analysis (PCA) of the characterization of the digestate showed that the data were significant, whereas the loading plots depicted the correlation of different experimental parameters studied (like fate of recalcitrant, biogas yield and other parameters post AD of OFMSW when aided with conductive materials). Application of regression models in all the batch assays depicted that a lag phase of 2-4 days was observed in Modified Gompertz Model (MGM), 4-5 days in Logistic Model (LM) and a rapid hydrolysis was proven with the value of hydrolysis coefficient being between 0.003 and 0.029 from the first-order (FO) model.

The Energetic Aspect of Organic Wastes Addition on Sewage Sludge Anaerobic Digestion: A Laboratory Investigation

One of the possibilities to achieve energy neutrality of wastewater treatment plants (WWTPs) is the implementation of the anaerobic co-digestion strategy. However, a key factor in its successful implementation on the technical scale is the application of components with complementary composition to sewage sludge (SS). In the 7resent study, the influence of adding various co-substrates on the energy balance of anaerobic digestion was evaluated. The following organic wastes were used as additional components to SS: organic fraction of municipal solid waste (OFMSW) and distillery spent wash (DW) applied in two- and three-component systems. The experiments were performed in semi-flow anaerobic reactors with the volume of 40 L under mesophilic conditions (35 °C) at hydraulic retention time (HRT) of 20, 18, and 16 d. The application of substrates to SS resulted in enhancements of methane yields as compared to SS mono-digestion. The statistically significant differences were observed in tertiary mixtures at both HRT of 18 and 16 d. Therein, average values were 0.20 and 0.23 m3 kg−1VSadd at HRT of 18 and 16 d, respectively. Among all co-digestion series, the most beneficial effect on energy balance was found in 20% v/v DW presence in both two- and three-component systems at HRT of 16 d.