• Energy Research

Abstract Biofuels are fast advancing as alternative sources of renewable energy due to their non-polluting features and cost-competitiveness in comparison to fossil fuels. However, in order to fast-track their development, focus is shifting towards the use of technologies that will maximize their yields. Nanoparticles are gaining increasing interest amongst researchers due to their exquisite properties, which enable them to be applied in diverse fields such as agriculture, electronics, pharmaceuticals and food industry. They are also being explored in biofuels in order to improve the performance of these bioprocesses. This review critically examines the various studies in literature that have explored nanoparticles in biofuel processes such as biohydrogen, biogas, biodiesel and bioethanol production, towards enhancing their process yields. Furthermore, it elucidates the different types of nanomaterials (metallic, nanofibers and nanotubes) that have been used in these bioprocesses. It also evaluates the effects of immobilized nanoparticles on biofuels such as biodiesel, and the ability of nanoparticles to effectively suppress inhibitory compounds under certain conditions. A short section is included to discuss the factors that influence the performance of nanoparticles on biofuels production processes. Finally, the review concludes with suggestions on improvements and possible further research aspects of these bioprocesses using nanoparticles.

Production of biohydrogen and related metabolic by-products was investigated in Solid State Dark Fermentation (SSDF) of food waste (FW) and wheat straw (WS). The effect of the total solids (TS) content and H2 partial pressure (ppH2), two of the main operating factors of SSDF, were investigated. Batch tests with FW at 10, 15, 20, 25 and 30% TS showed considerable effects of the TS on metabolites distribution. H2 production was strongly inhibited for TS contents higher than 15% with a concomitant accumulation of lactic acid and a decrease in substrate conversion. Varying the ppH2 had no significant effect on the conversion products and overall degradation of FW and WS, suggesting that ppH2 was not the main limiting factor in SSDF. This study showed that the conversion of complex substrates by SSDF depends on the substrate type and is limited by the TS content.

Dark fermentation of organic biomass, i.e. agricultural residues, agro-industrial wastes and organic municipal waste is a promising technology for producing renewable biohydrogen. In spite of its potential, this technology needs further research and development to improve the biohydrogen yield by optimizing substrate utilization, microbial community enrichment and bioreactor operational parameters such as pH, temperature and H2 partial pressure. On the other hand, the technical and economic viability of the processes need to be enhanced by the use of valuable by-products from dark fermentation, which mostly includes volatile fatty acids. This paper reviews a range of different organic biomasses and their biohydrogen potential from laboratory to pilot-scale systems. A review of the advances in H2 yield and production rates through different seed inocula enrichment methods, bioreactor design modifications and operational conditions optimization inside the dark fermentation bioreactor is presented. The prospects of valorizing the co-produced volatile fatty acids in photofermentation and bioelectrochemical systems for further H2 production, methane generation and other useful applications have been highlighted. A brief review on the simulation and modeling of the dark fermentation processes and their energy balance has been provided. Future prospects of solid state dark fermentation are discussed.

Sustainable development is driving a rapid focus shift in the wastewater and organic waste treatment sectors, from a "removal and disposal" approach towards the recovery and reuse of water, energy and materials (e.g. carbon or nutrients). Purple phototrophic bacteria (PPB) are receiving increasing attention due to their capability of growing photoheterotrophically under anaerobic conditions. Using light as energy source, PPB can simultaneously assimilate carbon and nutrients at high efficiencies (with biomass yields close to unity (1 g CODbiomass·g CODremoved-1)), facilitating the maximum recovery of these resources as different value-added products. The effective use of infrared light enables selective PPB enrichment in non-sterile conditions, without competition with other phototrophs such as microalgae if ultraviolet-visible wavelengths are filtered. This review reunites results systematically gathered from over 177 scientific articles, aiming at producing generalized conclusions. The most critical aspects of PPB-based production and valorisation processes are addressed, including: (i) the identification of the main challenges and potentials of different growth strategies, (ii) a critical analysis of the production of value-added compounds, (iii) a comparison of the different value-added products, (iv) insights into the general challenges and opportunities and (v) recommendations for future research and development towards practical implementation. To date, most of the work has not been executed under real-life conditions, relevant for full-scale application. With the savings in wastewater discharge due to removal of organics, nitrogen and phosphorus as an important economic driver, priorities must go to using PPB-enriched cultures and real waste matrices. The costs associated with artificial illumination, followed by centrifugal harvesting/dewatering and drying, are estimated to be 1.9, 0.3-2.2 and 0.1-0.3 $·kgdry biomass-1. At present, these costs are likely to exceed revenues. Future research efforts must be carried out outdoors, using sunlight as energy source. The growth of bulk biomass on relatively clean wastewater streams (e.g. from food processing) and its utilization as a protein-rich feed (e.g. to replace fishmeal, 1.5-2.0 $·kg-1) appears as a promising valorisation route.

In this study, the life cycle assessment (LCA) method has been used to evaluate the environmental impacts of various municipal solid waste (MSW) management system scenarios in Banepa municipality, Nepal, in terms of global warming potential (GWP), acidification potential (AP), eutrophication potential (EP), human toxicity potential (HTP), abiotic depletion potential (ADP), and photochemical ozone creation potential (POCP). There are at least six possible scenarios of MSW management in Banepa: the current or baseline scenario (Scenario 1); composting with landfilling (Scenario 2); material recovery facility (MRF) recycling, composting, and landfilling (Scenario 3); MRF and anaerobic digestion (AD); composting, and landfilling (Scenario 4); MRF, composting, AD, and landfilling (Scenario 5); and, finally, incineration with landfilling (Scenario 6). Using both information from Ecoinvent 3.6 (2019) and published research articles, a spreadsheet tool based on the LCA approach was created. The impact of the recycling rate on each of the six abovementioned scenarios was evaluated using sensitivity analysis, which showed that the recycling rate can considerably decrease the life-cycle emissions from the MSW management system. Scenario 3 was found to have the least overall environmental impact with a GWP of 974.82 kg CO2 eq. per metric ton (t), EP of 0.04 kg PO4 eq./t, AP of 0.15 kg SO2 eq./t, HTP of 4.55 kg 1,4 DB eq./t, ADP of −0.03 kg Sb eq./t, and POCP of 0.06 kg C2H4 eq./t. By adoption of MRF and biological treatments such as composting and AD, environmental impact categories such as AP, EP, HTP, ADP, POCP, and GWP can be significantly reduced. The findings of this study can potentially serve as a reference for cities in the developing world in order to aid in both the planning and the operation of environmentally friendly MSW management systems.

This work aimed to investigate the effect of the initial pH, combination of food to microorganism ratio (F/M) and initial pH, substrate pre-treatment and different inoculum sources on the dark fermentative biohydrogen (H2) yields. Three model complex waste biomasses (food waste, olive mill wastewater (OMWW) and rice straw) were used to assess the effect of the aforementioned parameters. The effect of the initial pH between 4.5 and 7.0 was investigated in batch tests carried out with food waste. The highest H2 yields were shown at initial pH 4.5 (60.6 ± 9.0 mL H2/g VS) and pH 5.0 (50.7 ± 0.8 mL H2/g VS). Furthermore, tests carried out with F/M ratios of 0.5, 1.0 and 1.5 at initial pH 5.0 and 6.5 revealed that a lower F/M ratio (0.5 and 1.0) favored the H2 production at an initial pH 5.0 compared to pH 6.5. Alkaline pre-treatment of raw rice straw using 4% and 8% NaOH at 55°C for 24h, increased the H2 yield by 26 and 57-fold, respectively. In the dark fermentation of OMWW, the H2 yield was doubled when heat-shock pre-treated activated sludge was used as inoculum in comparison to anaerobic sludge. Overall, this study shows that the application of different operating parameters to maximize the H2 yields strongly depends on the biodegradability of the substrate.

Abstract In recent years, much attention has been directed towards the integration of dark fermentation process into a biorefinery concept to enhance the energetic gains, thereby improving the competitiveness of this process. The volatile fatty acids (VFAs) from dark fermentative H2-producing processes serve as precursors for the microbial synthesis of a broad spectrum of biotechnologically-important products such as biofuels and biocommodities. These products are desirable substrates for secondary bioprocesses due to their biodegradable nature and affordability. This short review discusses the use of acidogenic-derived VFAs in the production of value-added compounds such as polyhydroxyalkanoates (PHAs) alongside the microbial-based fuels (hydrogen, biogas, and electricity), and other valuable compounds (succinic acid, citric acid, and butanol). The review also highlights the strategies that have been used to enhance the extraction of VFAs from acidogenic effluents and other related waste streams. The application of novel enhancement techniques such as nanoparticles during VFAs recovery is also discussed in this work. Furthermore, the work highlights some of the recent advances in dark fermentation-based biorefinery, particularly the development of pilot-scale processes. Finally, the review provides some suggestions on the advancement of dark fermentation-based biorefineries using VFAs that are derived from acidogenic processes.