• Energy Research

Among agricultural residues, lignocellulosic materials (LMs) are highly attractive substrates for anaerobic digestion (AD), given their high availability, low cost and no direct competition with food and feed production. Hemp (Cannabis sativa L.) is a multipurpose crop, and its cultivation has boosted again in the last years. Nevertheless, due to contrasting legislation, in some European Countries the harvesting and manufacturing of plant components (e.g. leaves and inflorescences) has stopped, resulting in a massive amount of lignocellulosic biomass to be diversely disposed. This research explores the valorization of hemp biomass (whole stalk, bast fiber, decorticated hurds, and a mixture of leaves and inflorescences) by AD, as a first major step for the establishment of a wider, future biorefinery platform. Both physical (grinding) and chemical (acid and alkali) pretreatments have been investigated to break down the lignocellulosic matrix of different hemp biomass components. Their biomethane yield has been then evaluated through batch biochemical methane potential (BMP) tests performed in 100 mL serum bottles under controlled mesophilic (37°C) conditions for 45 days. The highest cumulative biomethane production (422 mL CH4/g VS) was obtained with the raw bast fiber, while the BMP of the raw whole stalk and decorticated hurds reached 271 and 240 mL CH4/g VS, respectively. The alkali pretreatment with NaOH increased the BMP of the hurds by 15% obtaining a final biomethane yield of 277 mL CH4/g VS. The mixture of leaves and inflorescences resulted in the lowest BMP values, with the NaOH-pretreated material reaching approximately 150 mL CH4/g VS.

Abstract In the emerging context of circular bioeconomy, industrial hemp (Cannabis Sativa L.) biomass is a valuable resource for the sustainable implementation of second-generation biorefineries. Potentially, all the main hemp components can find application within different biorefinery approaches, adding value to the conventional production of hemp fibres and seeds. Hurds, leaves and inflorescences, constituting most of the hemp plant biomass, and often considered as low-value residues, can indeed play a key role in the sustainable production of both bioenergy and high-value bioproducts. The present article reviews the advances and outlines the potential future perspectives of hemp-based biorefineries. After critically overviewing some of the most established applications of hemp, spanning from soil bioremediation to bioenergy and biofuel production, particular attention is given to novel valorisation schemes to synthetize highly demanded bioproducts such as microbial protein and biopolymers. Our preliminary calculations show that hemp biomass can sustain high biodiesel yield (1.6 g/g VS (volatile solids)) and related revenues (510–868 €/ha•year), while bioethanol production can yield 0.10–0.33 mL/g VS, profiting between 75–325 €/ha•year. Moreover, hemp suits biomethane production by yielding and profiting 98–426 mL/g VS and 60–380 €/ha•year, respectively. High yields of polyhydroxybutyrate (0.13 g/g VS) can be obtained, albeit high production costs might restrain their marketability. Finally, the biomethane-to-microbial protein pathway can yield and profit 0.03–0.15 g/g VS and 141–893 €/ha•year, respectively, while the volatile fatty acids-to-microbial protein pathway 0.04 g/g VS and 91–362 €/ha•year.

Resources in used water are at present mainly destroyed rather than reused. Recovered nutrients can serve as raw material for the sustainable production of high value bio-products. The concept of using hydrogen and oxygen, produced by green or off-peak energy by electrolysis, as well as the unique capability of autotrophic hydrogen oxidizing bacteria to upgrade nitrogen and minerals into valuable microbial biomass, is proposed. Both axenic and mixed microbial cultures can thus be of value to implement re-synthesis of recovered nutrients in biomolecules. This process can become a major line in the sustainable "water factory" of the future.

The swift and successful transition towards a fossil fuel-free economy is amongst the most complex challenges ever faced by humanity, implicating intricate connections and trade-offs with the so-called water–energy–food nexus [...]

Biological nitrogen assimilation is an emerging technique for the removal and upcycling of nitrogen from wastewater in the form of microbial protein (MP). For this purpose, a mixed hydrogen-oxidizing bacteria (HOB) culture capable of growing mixotrophically was evaluated for the treatment of synthetic and real wastewater under different carbon-to-nitrogen (C/N) ratios. The same mixed HOB culture was grown under heterotrophic conditions to compare treatment and process performances in terms of ammonium nitrogen (N-NH4+) removal and assimilation, chemical oxygen demand (COD) removal, CO2 release, biomass concentration, yield and protein content. Under mixotrophic conditions, the highest biomass concentration was 342.5 mg VSS∙L−1, doubling that obtained under heterotrophic conditions (161.2 mg VSS∙L−1). Discharge limits for both COD and total N were met under mixotrophic conditions, with nitrogen removal and assimilation into protein-rich biomass achieving up to 99 %. On the contrary, under heterotrophic conditions, the high content of residual nitrogen as nitrite (up to 26 mg∙L−1 of N-NO2−) did not allow to meet discharge limits for total N. The mixed HOB culture gave promising results in terms of biomass yield (0.32 g VSS·g CODH2+acetate−1) and protein content (up to 56 % of VSS) when grown mixotrophically. Under heterotrophic conditions, instead, the biomass yield (0.25 g VSS·g CODacetate−1) and protein content (35 %) were substantially lower. This study suggests that the H2-driven assimilatory mixotrophic metabolism can be successfully applied for wastewater treatment to produce effluents that meet discharge limits while mitigating greenhouse gas emissions (CO2 and N2O) and upcycling nitrogen into MP.

The global protein demand is rapidly increasing at rates that cannot be sustained, with projections showing 78% increased global protein needs by 2050 (361 compared to 202 million ton protein /year in 2017). In the absence of dedicated mitigation strategies, the environmental effects of our current food production system (relying on agriculture) are expected to surpass the planetary boundaries—the safe operating space for humanity—by 2050.

AbstractNitrogen is the most crucial element in the production of nutritious feeds and foods. The production of reactive nitrogen by means of fossil fuel has thus far been able to guarantee the protein supply for the world population. Yet, the production and massive use of fertilizer nitrogen constitute a major threat in terms of environmental health and sustainability. It is crucial to promote consumer acceptance and awareness towards proteins produced by highly effective microorganisms, and their potential to replace proteins obtained with poor nitrogen efficiencies from plants and animals. The fact that reactive fertilizer nitrogen, produced by the Haber Bosch process, consumes a significant amount of fossil fuel worldwide is of concern. Moreover, recently, the prices of fossil fuels have increased the cost of reactive nitrogen by a factor of 3 to 5 times, while international policies are fostering the transition towards a more sustainable agro‐ecology by reducing mineral fertilizers inputs and increasing organic farming. The combination of these pressures and challenges opens opportunities to use the reactive nitrogen nutrient more carefully. Time has come to effectively recover used nitrogen from secondary resources and to upgrade it to a legal status of fertilizer. Organic nitrogen is a slow‐release fertilizer, it has a factor of 2.5 or higher economic value per unit nitrogen as fertilizer and thus adequate technologies to produce it, for instance by implementing photobiological processes, are promising. Finally, it appears wise to start the integration in our overall feed and food supply chains of the exceptional potential of biological nitrogen fixation. Nitrogen produced by the nitrogenase enzyme, either in the soil or in novel biotechnology reactor systems, deserves to have a ‘renaissance’ in the context of planetary governance in general and the increasing number of people who desire to be fed in a sustainable way in particular.