Biomethane Recovery Research Articles

Porous hollow microspheres based on industrial solid waste enhance biomethane recovery from corn straw

The increasing production of industrial solid waste requires better disposal solutions. Porous hollow microspheres (PHM) are small inorganic materials with high surface area and adsorption capacity, but their potential for use in anaerobic digestion (AD) has not been explored. With PHM as additive, the effects of different industrial solid wastes (waste glass, steel slag, and fly ash) with different loadings (2 %–8 %), respectively, on the AD of corn straw were investigated in this study. The results showed that PHM could supplement trace elements and promote biofilm formation, which effectively shortened the lag period (25.00–60.87 %) and increased the methane yield (4.75 %–16.28 %). The 2 % PHM loading based on steel slag gave the highest methane yield (300.16 NmL/g VSadd). Microbial and PICRUSt2 analyses indicated that PHM enriched hydrolytic and acidogenic bacteria, increased the abundance of methanogenesis-related enzyme genes. This study provides a theoretical basis for the comprehensive utilization of coupled industrial and agricultural wastes.

Mechanism underlying the sustained stimulatory effects of energization on biomethane recovery from food waste post-energization cessation

Microbial electrolysis cell-assisted anaerobic digestion represents a promising approach for enhancing methanogenesis. This study investigated the impact of varying energy levels followed by long-term open circuit on biogas recovery from food waste.The results demonstrated that a mild voltage of 0.4 V resulted in 61.7% increase in methane yield, with a methane composition reaching 78.89% vol and a remarkable reduction in digestion time by 8 days. Additionally, the facilitated effects remained after prolonged periods of open-circuit. In-depth study revealed that energization significantly enhanced organic hydrolysis, redox proteins secretion and sludge electro-activity. Microbial communities showed that the ever-present energization enriched the hydrolytic bacterium and electrophiles. Subsequent investigations also revealed the upgradation of enzyme-encoding genes associated with hydrolysis and the synthesis of flavin and its homologs (i.e. ribE, ssuE and nfrA2). These findings collectively demonstrated the enduring benefits of energization were linked to the enhanced hydrolysis and regulated mediator-mediated electron transfer pathway.

Integrated microalgae cultivation and sustainable biofuel production from pretreated food waste

Repurposing of food waste (FW) affords an alternate waste management strategy and sustainable product yield. Thus, this study proposed an integrated biorefinery approach, repurposing pretreated FW hydrolysate for microalgae cultivation and utilizing residual solids for biomethane recovery. FW biomass with different solubilization ranging from (10–15 to 60–65%) obtained during surfactant coupled disperser pretreatment was used as a substrate for microalgae (Chlorella vulgaris) cultivation. A higher microalgae biomass productivity of 0.87 ± 0.003 g/g substrate was obtained from the FW hydrolysate with 40–45% solubilization. The microalgae cultivated in FW hydrolysate with 40–45% solubilization yielded a higher protein, carbohydrate, and lipid accumulation of 380 ± 20 mg/g, 480 ± 30 mg/g and 360 ± 15 mg/g. Besides, the methanogenesis process showed a higher methane yield of 0.294 ± 0.003 L/g COD utilizing residual solids of pretreated food waste as substrate. This integrated biorefinery provides a sustainable platform for completely valorizing food waste biomass and recovery of multiple products.

Biomethane recovery and prokaryotic shifts in anaerobic co-digestion of food waste and paper waste in organic fraction of municipal solid waste: Effect of paper content

Biomethane recovery from paper waste (PW) was achieved by mesophilic co-digestion with food waste. The feeding material containing 0%, 20%, 40% and 50% of PW in total solids (TS) were investigated in the long-term continuous operation. The results showed that the biogas production, pH, alkalinity and biodegradation of volatile solids (79.8 ± 3.6%) were stable for PW contents no more than 50%. The PW = 50% condition was considered the critical limit for the reasons of pump clogging, sufficient alkalinity (2.0 ± 0.3 g-CaCO3/L) and depletion of ammonia. Prokaryotic diversity indices decreased with the increased PW contents. Great shifts were observed in the prokaryotic communities before and after the PW contents reaches 50% as TS (18.4% as total weights). Biomethane recovery yields were deceasing from 445 to 350 NL-CH4/kg-fed-volatile-solids. The PW contents as 40% as TS (13.1% as total weights) obtained the optimal performance among all the feeding conditions.

Enhanced methane recovery from anaerobic membrane bioreactor coupled with cold plasma pretreatment for rapid hydrolysis and nitrogen removal

Research to increase biomethane recovery efficiency from thickened sewage sludge (TSWS) using sustainable anaerobic digestion (AD) in municipal wastewater treatment plants is ongoing. Pretreating substrates is known to increase organic biodegradation and biomethane conversion rates in AD. Cold plasma (CP), a recently adopted advanced oxidation processes (AOP) has emerged as an alternative to accelerate pretreatment times under different operation variables. This study assessed raw and CP-pretreated TSWS in an anaerobic sequencing batch reactor (ASBR) and anaerobic membrane bioreactor (AnMBR). The effects of incremental organic loading rates (OLR) and nitrogenous compounds concentration on enhanced CH4 bioconversion efficiency were evaluated. We found that the AnMBR outperformed the ASBR, with an overall chemical oxygen demand (COD) conversion rate of 67%, lower total nitrogen (T–N) accumulation (594 mg L−1), and an overall methane yield of 0.24 L CH4 g−1 COD. CP pretreatment improved TSWS AD, resulting in more efficient COD removal and methane recovery. This study suggests that CP technology is a promising pretreatment to improve AD when treating TSWS.

Improved hydrolysis of sewage sludge by air-assisted non-thermal plasma for enhanced biomethane recovery.

Conventional pretreatment technologies have been assessed to resolve the slow hydrolysis of sewage sludge, but high operating costs have prevented their wide use. This study investigated non-thermal plasma (NTP) technologies as an alternative to promote anaerobic digestion (AD). Various contact time (CT) and temperature (T) conditions were used to assess how NTP pretreatment improves the methane conversion of organics in sewage sludge. A multi-response surface model (RSM) using a central composite design (CCD) identified the optimal CT (4.6h) and T (45°C). This statistical optimization of NTP pretreatment led to an enhanced biochemical methane potential of 297 ± 46mL CH4 g-1 COD by reducing operating cost as power consumption as low as 0.08 USD L-1. The result was comparable to those of other advanced oxidation processes (0.14 - 0.60 USD L-1) demonstrating that accelerated hydrolysis of sewage sludge using NTP pretreatment show potential for improving renewable energy recovery from sewage sludge.

Significant enhancement of biomethane recovery in up-flow anaerobic sludge blanket reactor treating starch wastewater through oyster shells conditioning

The up-flow anaerobic sludge blanket (UASB) reactor for treating starch wastewater exhibits poor resistance to organic loading rate shocks and low efficiency in biomethane recovery. This study proposed a solution to address this issue by using oyster shells as an alternative conditioner. Results indicated that the UASB reactor with oyster shells conditioning exhibited a significant increase in methane production from 243 to 435 mL/g-chemical oxygen demand (COD)removed, as the organic loading rate increased from 2.5 to 7.1 kg-COD/(m3·d). This value surpassed that of the control reactor, which experienced a sharp decline in performance. Furthermore, it also exceeded the majority of reported methane yields ranging from 150 to 330 mL/g-CODremoved. The pH adjustment and additional supply of carbon dioxide from oyster shells was primarily responsible for the substantial increase in methane production. Moreover, oyster shell conditioning also resulted in improvements in specific methanogenic activity, extracellular polymeric substances protection, and settling capacity of anaerobic granular sludge, as well as the enrichment in Methanosaeta abundance, and strengthening of methanogenic functional activity. All these factors contributed to the successful methanogenesis of starch wastewater in UASB reactor through oyster shell conditioning. These findings highlighted the potential of using oyster shells as a favorable conditioner for optimizing UASB treatment of easily acidified wastewater (e.g., starch wastewater), which presented promising practical applications.

Process Optimization and Biomethane Recovery from Anaerobic Digestion of Agro-Industry Wastes

Among the sustainable initiatives for renewable energy technologies, anaerobic digestion (AD) is a potential contender to replace fossil fuels. The anaerobic co-digestions of goat manure (GM) with sorghum (SG), cotton gin trash (CGT), and food waste (FW) having different mixing ratios, volumes, temperatures, and additives were optimized in single and two-stage bioreactors. The biochemical methane potential assays (having different mixing ratios of double and triple substrates) were run in 250 mL serum bottles in triplicates. The best-yielding ratio was up-scaled to fabricated 2 L bioreactors. The biodegradability, biomethane recovery, and process efficacy are discussed. The co-digestion of GM with SG in a 70:30 ratio yielded the highest biomethane of 239.3 ± 15.6 mL/gvs, and it was further up-scaled to a two-stage temperature-phased process supplemented with an anaerobic medium and fly ash (FA) in fabricated 2 L bioreactors. This system yielded the highest biomethane of 266.0 mL/gvs, having an anaerobic biodegradability of 67.3% in 70:30 GM:SG co-digestion supplemented with an anaerobic medium. The BMP of the FA-amended treatment may be lower because of its high Ca concentration of 205.74 ± 3.6. The liquid fraction of the effluents can be applied as N and P fertigation. The Ca concentration was found to be 24.3, 25.1, and 6.3 g/kg in GM and GM:SG (TS) and SG solid fractions, respectively, whereas K was found to be 26.6, 10.8, and 7.4 g/kg. The carbon to nitrogen ratio of solid fraction varied between 2.0 and 24.8 for return to the soils to enhance its quality. This study involving feedstock acquisition, characterization, and their anaerobic digestion optimization provides comprehensive information and may assist small farmers operating on-farm anaerobic digesters.

Exploring the environmental consequences of roadside grass as a biogas feedstock in Northwest Europe

The Russo-Ukrainian war has highlighted concerns regarding the European Union's (EU) energy security, given its heavy dependence on Russian natural gas for electricity and heating. The RePowerEU initiative addresses this challenge by targeting a significant increase in biomethane production (up to 35 billion m3 by 2030) to replace natural gas, aligning with the EU methane strategy's emission reduction and air quality improvement goals. However, the use of energy crops as biogas feedstock has raised land-use concerns, necessitating a policy shift towards alternative sources such as agro-residues, livestock manure, and sewage sludge.This study investigates the environmental impacts of using roadside grass clippings (RG) as an alternative feedstock for biogas production, focusing on selected regions in Northwest Europe (Belgium, Netherlands). The aim is to evaluate the environmental performance of RG as a mono- or co-substrate for biogas production, comparing it to the current practice of composting. Additionally, the study assesses the environmental impacts associated with biogas end-use in these regions.The results indicate that co-digestion of RG with pig manure offers a more environmentally friendly alternative compared to mono-digestion of RG or the existing composting practice. This finding is primarily attributed to the avoided emissions resulting from conventional pig manure management. Furthermore, in terms of climate change impacts concerning biogas end-use, the study identifies that combined heat and power (CHP) systems are preferable to biomethane recovery in regions with a natural gas-based electricity mix. However, for reducing fossil resource use, biomethane recovery emerges as the preferred option.By providing insights into the environmental performance of RG as a biogas feedstock and evaluating the impacts of different biogas end-use options, this study offers insights to policymakers for the development of sustainable energy strategies in Northwest Europe.

Application of Anaerobic Co-digestion of Brewery by-Products for Biomethane and Bioenergy Production in a Biorefinery Concept

This study investigated the anaerobic co-digestion (AcoD) of brewery by-products for biomethane and bioenergy recovery, focusing on operational performance evaluation, kinetic analysis, microbial metataxonomic, and metabolic function prediction. The biochemical methane potential was conducted under mesophilic (35 °C) and methanogenic conditions (pH 7.5) by mixing brewery wastewater and sludge from the brewery wastewater treatment plant (1:1, v/v), following the addition (2.5 – 12.5 %, w/v) of brewer’s spent grains (BSG). The results demonstrate that the highest methane yield (88.02 mL CH4/g TVS) was obtained with 12.5 % BSG, which was 20.66-fold higher than the control reactor operated with wastewater and sludge (4.26 mL CH4/g TVS). The bioenergy recovery from biomethane could generate electricity (0.348 kWh/kg TVS) and heat (1556 MJ/kg TVS), avoiding greenhouse gas emissions (0.114 kg CO2-eq/kg TVS). The microbial community dynamics revealed a predominance of Halobacterota, Chloroflexi, and Euryarchaeota phylum. The genera Methanosaeta and Methanobacterium, and the Anaerolineaceae family predominated in the AcoD process. The metabolic function prediction showed the presence of genes (K01895, K00193, K00625, and K00925) associated with the direct activation of acetate in the acetoclastic pathway and methane production. Finally, the data obtained provide a perspective on using brewery by-products for bioenergy production in a biorefinery concept, reducing the environmental impacts and contributing to the circular bioeconomy transition of the beer industry.Graphical abstract

Effect of ozonation on biodegradability enhancement and biomethanation potential of pretreatment process wastewaters from 2G ethanol production

AbstractBACKGROUNDThe production of second‐generation (2G) ethanol from lignocellulosic biomass generates large quantities of high‐strength complex wastewaters from various stages of the process. An effective pretreatment is necessary in order to overcome the low biodegradability of these wastewaters for further biomethane recovery. The present study reports the effect of ozonation on the biodegradability enhancement and biomethanation potential of complex 2G ethanol pretreatment process wastewaters, from acid (H2SO4) and alkali (NaOH) pretreatment process.RESULTSOzonation (9.33 g m−3) was performed for both acid and alkali pretreatment wastewaters at a flow rate of 10 L min−1 for 1 h. Ozonation of both wastewaters increased biodegradability index (BI) up to 36% along with minimal chemical oxygen demand (COD; 7 ± 3%), color (10 ± 4%), and toxicity (22.5 ± 7.5%) reduction. Biomethane potential tests were carried out for the control and ozonated wastewaters having BI > 0.4. The acid‐pretreatment wastewater indicated poor biomethanation potential due to high presence of sulfates (COD/SO4 ~ 3.0). However, ozonation of alkali‐pretreatment wastewater facilitated up to twofold enhancement in the cumulative methane volume, while the enhancement in the rate of methane production was found to be up to 47%. Fourier transform infrared spectroscopic analysis of control and ozonated wastewaters revealed the breakdown of complex organic molecules and aromatics to readily biodegradable intermediates. The kinetics of methane generation could be predicted by the modified Gompertz model.CONCLUSIONThe study demonstrated the efficacy of ozonation as an effective pretreatment option to facilitate biodegradability enhancement of 2G ethanol pretreatment process wastewaters. The enhancement in biodegradability was observed to enable subsequent biogas recovery via anaerobic digestion along with COD removal. © 2023 Society of Chemical Industry (SCI).

Techno-Economic Assessment of On-Site Production of Biomethane, Bioenergy, and Fertilizer from Small-Scale Anaerobic Digestion of Jabuticaba By-Product

Bioenergy recovery from biomass by-products is a promising approach for the circular bioeconomy transition. However, the management of agri-food by-products in stand-alone treatment facilities is a challenge for the low-capacity food processing industry. In this study, the techno-economic assessment of a small-scale anaerobic digestion process was evaluated for the management of jabuticaba by-product and the production of biomethane, electricity, heat, and fertilizer. The process was simulated for a treatment capacity of 782.2 m3 y−1 jabuticaba peel, considering the experimental methane production of 42.31 L CH4 kg−1 TVS. The results of the scaled-up simulated process demonstrated the production of biomethane (13,960.17 m3 y−1), electricity (61.76 MWh y−1), heat (197.62 GJ y−1), and fertilizer (211.47 t y−1). Economic analysis revealed that the process for biomethane recovery from biogas is not profitable, with a net margin of −19.58% and an internal rate of return of −1.77%. However, biogas application in a heat and power unit can improve project feasibility, with a net margin of 33.03%, an internal rate of return of 13.14%, and a payback of 5.03 years. In conclusion, the application of small-scale anaerobic digestion can prevent the wrongful open-air disposal of jabuticaba by-products, with the generation of renewable energy and biofertilizer supporting the green economy toward the transition to a circular economy.

Biomethane recovery through co-digestion of cheese whey and glycerol in a two-stage anaerobic fluidized bed reactor: Effect of temperature and organic loading rate on methanogenesis

Anaerobic digestion for CH4 recovery in wastewater treatment has been carried out with different strategies to increase process efficiency, among which co-digestion and the two-stage process can be highlighted. In this context, this study aimed at evaluating the co-digestion of cheese whey and glycerol in a two-stage process using fluidized bed reactors, verifying the effect of increasing the organic loading rate (OLR) (2–20 g-COD.L−1.d−1) and temperature (thermophilic and mesophilic) in the second stage methanogenic reactor. The mesophilic methanogenic reactor (R-Meso) (mean temperature of 22 °C) was more tolerant to high OLR and its best performance was at 20 g-COD.L−1.d−1, resulting in methane yield (MY) and methane production (MPR) of 273 mL-CH4.g-COD−1 and 5.8 L-CH4.L−1.d−1 (with 67% of CH4), respectively. Through 16S rRNA gene massive sequencing analysis, a greater diversity of microorganisms was identified in R-Meso than in R-Thermo (second stage methanogenic reactor, 55 °C). Firmicutes was the phyla with higher relative abundance in R-Thermo, while in R-Meso the most abundant ones were Proteobacteria and Bacteroidetes. Regarding the Archaea domain, a predominance of hydrogenotrophic microorganisms could be observed, being the genera Methanothermobacter and Methanobacterium the most abundant in R-Thermo and R-Meso, respectively. The two-stage system composed with a thermophilic acidogenic reactor + R-Meso was more adequate for the co-digestion of cheese whey and glycerol than the single-stage process, promoting increases of up to 47% in the energetic yield (10.3 kJ.kg-COD−1) and 14% in organic matter removal (90.5%).

Ultrafiltration as a pre-treatment technology to improve vinasse biomethanation

Vinasse is a residue that remains after the fractional distillation of fermented juice to obtain ethanol. Intended to extend the opportunities for by-product recovery, vinasse could be used as feedstock for biomethane production, departing from the linear focus of vinasse production, treatment, and disposal, to vinasse production and reuse. However, the high concentrations of sulfate and its inhibitory effect on biological activity could lower the yield of biomethane production. To overcome that, it was studied the potential of ultrafiltration membranes to alleviate the concentrations of sulfate from vinasse and concentrate the organic matter, intended to enhance the biomethane production potential. Raw vinasse, ultrafiltration concentrate, and ultrafiltration permeate were subjected to anaerobic digestion experiments and the cumulative biomethane yield was monitored over 70 days. The ultrafiltration concentrate presented the highest degradation rate (μm: 2.83 ± 0.24 mL/g-VS.d.mL-sample) and longest lag-phase time (λ: 12.31 ± 0.83 d) compared with raw sugarcane vinasse (μm: 1.44 ± 0.11 mL/g-VS.d.mL-sample; λ: 6.57 ± 0.93 d) and ultrafiltration permeate (μm: 1.14 ± 0.04 mL/g-VS.d.mL-sample; λ: 2.92 ± 0.74 d). The potential for biomethane recovery was complemented by an energetic analysis involved in biomethane production and processing. The biomethane recovery from ultrafiltration concentrate presented the lowest specific energy consumption (3.34 MJ/Nm³-CH4), followed by raw sugarcane vinasse (3.51 MJ/Nm³-CH4) and ultrafiltration permeate (3.64 MJ/Nm³-CH4). The remaining digested streams could still be potentially applied to soils for better use of their nutrients, however without an excessive load of organic matter already consumed during anaerobic digestion. Overall, the study demonstrated an effective alternative to salinity relief in raw sugarcane vinasse by ultrafiltration membranes and an alternative of higher vinasse valorization.

Enhancement of biomethane recovery from batch anaerobic digestion by exogenously adding an N-acyl homoserine lactone cocktail

Biomethane recovered through anaerobic digestion (AD) is a renewable, sustainable, and cost-effective alternative energy source that has the potential to help address rising energy demands. Efficient bioconversion during AD depends on the symbiotic relationship between hydrolytic bacteria and methanogenic archaea. Interactions between microorganisms occur in every biological system via a phenomenon known as quorum sensing (QS), in which signaling molecules are simultaneously transmitted and detected as a mode of cell-to-cell communication. However, there's still a lack of understanding on how QS works in the AD system, where diverse bacteria and archaea interact in a complex manner. In this study, different concentrations (0.5 and 5 μM) of signaling molecules in the form of an N-acyl homoserine lactone cocktail (C6-, C8-, C10-, and 3-oxo-C6-HSL) were prepared and introduced into anaerobic batch reactors to clearly assess how QS affects AD systems. It was observed that the methane yield increased with the addition of AHLs: a 5 μM AHL cocktail improved the methane yield (341.9 mL/g-COD) compared to the control without AHLs addition (285.9 mL/g-COD). Meanwhile, evidence of improved microbial growth and cell aggregation was noticed in AHLs-supplemented systems. Our findings also show that exogenously adding AHLs alters the microbial community structure by increasing the overall bacterial and archaeal population counts while favoring the growth of the methanogenic archaea group, which is essential in biomethane synthesis.

A new integrated circular economy index and a combined method for optimization of wood production chain considering carbon neutrality

The wood industry is potentially advantageous to applying the concepts of circular economy for sustainable development and can contribute to the commitment of carbon neutrality. This study developed an integrated circular economy index based on five different quantitative indicators for assessment of the wood production chain: heat recovery rate, CO2 sequestration rate, fossil fuel substitution rate, renewable electricity usage rate, and revenue increase from the by-products. A combination of best-worst method (BWM) and linear goal programming (LGP) techniques was investigated to develop an optimal circular economy model of wood processing chain for reduction in CO2 emission. The integrated circular economy index and the combined method were tested in a case-study of a rubberwood processing chain in Vietnam. The proposed model suggests that the woodchips and biomass from the harvesting and processing of rubberwood could be collected and treated using microwave thermolysis techniques; the enzyme hydrolysis technique is appropriate for bioethanol and biomethane recovery from the sawdust; and the hot air technique is preferable in the drying process. The proposed model could result in a significant reduction of the total net carbon emission from +552,750 tons CO2eq to −1,145,940 tons CO2eq per year. This could support the achievement of Vietnam's zero CO2 emission goal and hopefully contribute to the country's commitment to carbon emission neutrality by the year 2050.

Artificial intelligence-based modeling and optimization of microbial electrolysis cell-assisted anaerobic digestion fed with alkaline-pretreated waste-activated sludge

Microbial electrolysis cell-assisted anaerobic digestion (MEC-AD) is a promising emerging strategy to enhance simultaneously waste treatment and biomethane recovery from various biowastes, particularly waste-activated sludge (WAS). However, MEC-AD is still in the early stages of development, with numerous experimental studies but no modeling or optimization. Thus, to provide an effective modeling and optimization tool for this process, this study proposed applying artificial intelligence for the first time. The literature-based experimental data of MEC-AD fed with alkaline-pretreated waste-activated sludge (al-WAS) were used for this purpose. Accordingly, a two-hidden-layer artificial neural network (ANN) with topology 2–25–34–6, obtained from the response surface methodology, showed the best agreement between actual and predicted data with a low mean squared error of 0.0579 and a high R-value of 0.9870. This best ANN model was then optimized by particle swarm optimization. As a result, an Eapp of 0.63 V was found to be optimal for al-WAS-fed MEC-AD with a highest net energy output of 41.3 KJ/L-reactor (∼2.6 MJ/kg-WAS) and highest net monetary value of 0.72 $/L-reactor (∼45 $/kg-WAS), which enhanced around 160 % and 300 % compared to AD alone. These findings can support decision-making for managers and operators in wastewater treatment, biomass waste management, and renewable energy sectors.

The relationship between quorum sensing dynamics and biological performances during anaerobic membrane bioreactor treatment

Anaerobic membrane bioreactors (AnMBRs) enhance carbon neutrality with biomethane recovery from wastewater; however, microbial signaling, which may affect biological performances, was poorly understood. Here, we thus evaluate quorum sensing (QS) dynamics while monitoring acyl-homoserine lactones (AHLs) and autoinducer-2 (AI-2) levels during long-term AnMBR operations after sludge inoculation. Significant organic removal and methane production were achieved with the reactor startup. Signal molecule levels varied with transient organic loading rates, depending on their types. A starving condition may cause an increase in short- and medium-chain AHLs and AI-2. Biopolymers, biosolids, volatile fatty acids, and alkalinity levels had positive correlations with short- and medium-chain AHLs and AI-2, whereas methane production had positive correlations with long-chain AHLs. The principal component analysis of QS signal composition and biological performance data explains their interconnectivity. The findings of this study help to understand that QS signals regulate metabolic pathways in addition to microbial group behaviors.

Enhancement of anaerobic digestion by co-digesting food waste and water hyacinth in improving treatment of organic waste and bio-methane recovery

In Kenya, 57% of the municipal solid waste generated is Food waste (FW) which has high organic content. However, the treatment and bioconversion of FW to biogas have always been challenging due to its rapid biodegradation, resulting from rapid hydrolysis and accumulation of volatile fatty acids and lowering pH in the bioreactor. In this study, the anaerobic digestibility of FW as a mono substrate was compared to co-digestion of FW with water hyacinth (WH) for improved biogas production and organic matter removal efficiency in a laboratory batch reactor. Different mix proportions of FW and WH were co-digested under mesophilic conditions (37 °C) at a dilution of 6% (w/v) Total Solids (TS) content. The TS of the substrates (Food waste and Water Hyacinth) were pre-processed to have a concentration of TS at 6% (60 g/L) to operate a wet AD which requires the substrate to be less than 15% TS. The proportions of WH: FW (v/v) were 100:0, 85:15, 70:30, 55:45, 30:70, 15:85, and 0:100. In the batch rectors the anaerobic co-digestion was conducted with Substrate to Inoculum (S/I) ratio of 1:1. FW is generally considered to have high volatile solids which hydrolyze rapidly lowering pH arising from excess production of Hydrogen which in presence of CO2 and acetogenic bacteria leads to more production of acetate, formate and other long chain fatty acids which inhibits methanogenesis as a result of rapid acidification. The rapid acidification of the bioreactors that are used to treat FW results in the inhibition of the methanogenesis process. The co-digestion of the substrates could have improved the process parameters by reducing acidity caused by the high C/N ratio, reducing the inhibitory range, and increasing the buffer capacity which enhanced the bio-methane potential and the microbial activity. The batch experiments were set in triplicate for both cases of FW, WH, mixtures, and Inoculum. The results showed that the average gas yields after 81 days for the various mix proportions were 256.27and 357.69 ml/g-VS for mono-digestion of WH and FW respectively. For the mixtures of WH: FW the average reported biogas production were 305.01, 280.27, 548.91,616.01 and 270.87 ml/g-VS for mixtures of 15:85, 30:70, 55:45,70:30 and 85:15 respectively. The modified Gompertz model showed that the digesters with WH and FW alone had lag times of 2.599 and 1.052 days respectively. The mix substrates of WH: FW 85:15, 70:30, 55:45, 30:70 and 15:85 shown lag times of 2.456, 3.777, 2.574, 1.956 and 1.75 days respectively. A mix (WH: FW) of 70:30 had the highest maximum specific biogas production Rmax and the maximum biogas production potential of 18.19 mlCH4/gVS per day and 607.7mlCH4/gVS respectively. The R2 and RSME values ranged from 0.9867 to 0.9963 and 2.663 to 9.359 respectively in all the digesters. The study shows that the co-digestion of WH and FW in the mix ratio of 70:30 improved the volume of biogas produced and organic matter removal efficiency reached 79%.

Environmental impacts of dry anaerobic biorefineries in a Life Cycle Assessment (LCA) approach

The organic fraction of municipal solid waste (OFMSW) represents the 34% of European waste and the overall share is going to increase since separate collection will be implemented across the member states. There is still lack of studies on environmental impacts of dry anaerobic biorefineries applying innovative solvents to produce biopolymers (e.g., polyhydroxyalkanoates – PHA) from the OFMSW. A comparative Life Cycle Assessment (LCA) analysis was applied to assess the environmental impacts of a conventional management strategy to recover biomethane and compost from the OFMSW with a dry anaerobic biorefinery recovering biomethane, fertilizers and PHAs. Four approaches to recover PHAs were included in the study, comprising two innovative and environmentally sustainable extraction methods (i.e., solvent extraction with ethyl acetate (EA) and ionic liquids (ILs)). The inventory data were derived from an Italian case study plant, while the data for modelling the PHA selection/accumulation and PHA extraction processes were retrieved from the literature. The comparison revealed that, at the current technological readiness level (TRL), the conventional management strategy has better environmental performances than the novel approach producing PHAs. For example, biomethane recovery, for which a substitution with natural gas was assumed, showed the highest avoided impacts for abiotic depletion of fossil fuel (65.5%), global warming (57.4%) and ozone layer depletion (65.4%) categories. PHA recovery scenarios showed high environmental impacts in all categories linked to natural resource depletion. PHA extraction with EA proved to be the most environmentally sustainable and promising technique towards an industrial application.