• Energy Research

In the framework of sustainability, water shortages and water pollution are two important aspects to be considered. Proposing efficient and low-impact technologies is of paramount importance to promote circular economies associated with the use of water in the industrial context, especially in the textile industry. In this work, the application of a set of magnetic nanostructured adsorbents (MNAs) to cleanse metal ions from textile wastewaters was studied and analyzed. MNAs were generated with a low-cost process, involving iron (II/III) salts (e.g., chlorides), sodium or ammonium hydroxide solutions, and graphene oxide, obtained from graphite by a modified Hummers’ method at room temperature. The shape and the size were studied with transmission electron microscopy. Adsorbents were tested with different metal ions (e.g., copper, chromium (III), and nickel). Metal ion concentrations were analyzed by means of inductively coupled plasma optical emission spectroscopy (ICP-OES), and adsorption isotherms were characterized. From the results, the MNAs exhibited the capability of removing metal ions up to a yield of 99% for Cr3+, 94.7% for Cu2+, and 91.4% for Ni2+, along with adsorption loads up to 4.56 mg/g of MNAs.

The use of energy crops in the treatment of wastewaters is of increasing interest, particularly in view of the widespread scarcity of water in many countries and the possibility of obtaining renewable fuels of vegetable origin. The aim of this study was to evaluate the feasibility of landfill leachate phytotreatment using sunflowers, particularly as seeds from this crop are suitable for use in biodiesel production. Two different irrigation systems were tested: vertical flow and horizontal subsurface flow, with or without effluent recirculation. Plants were grown in 130L rectangular tanks placed in a special climatic chamber. Leachate irrigated units were submitted to increasing nitrogen concentrations up to 372mgN/L. Leachate was successfully tested as an alternative fertilizer for plants and was not found to inhibit biomass development. The experiment revealed good removal efficiencies for COD (η>50%) up until flowering, while phosphorous removal invariably exceeded 60%. Nitrogen removal rates decreased over time in all experimental units, particularly in vertical flow tanks. In general, horizontal flow units showed the best performances in terms of contaminant removal capacity; the effluent recirculation procedure did not improve performance. Significant evapo-transpiration was observed, particularly in vertical flow units, promoting removal of up to 80% of the inlet irrigation volume.

Spent coffee grounds (SCGs) are potentially optimal substrates for methane production but the content of organic compounds refractory to anaerobic digestion reduces the yield of the process. Alkaline pre-treatment was applied to enhance the methane recovery from SCGs through anaerobic digestion. NaOH was applied with different loadings, namely 2, 4, 6, 8% w/w for 24 h, to assess the efficiency of the process and the optimal amount of the basifying solution applied. The highest concentration of NaOH (8% w/w) lead to the best anaerobic digestion performances (392 mLCH4/gVS) as a consequence of the slightly higher lignin degradation which was 24% higher than that of the untreated substrate, and of the higher dissolved organic carbon concentration.

Aerobic biodegradation of biomass can release considerable heat, reaching temperatures of up to 65 °C. This heat can be recovered and used for domestic purposes through the implementation of Compost Heat Recovery System (CHRS). In this study, data were collected from a full-scale CHRS, fed with tree-pruning residues, installed in a farmhouse located in Northern Italy. The CHRS (2.75 kW average heating power) worked in conjunction with a pellet combustor for one year.Energy and carbon balances were analyzed and compared (over a 15-year life-time) with combinations of alternative heating systems (both traditional and green ones). The real case study provided a heat supply at a competitive cost (0.087 € kWh−1). A scenario with two CHRSs would further decrease costs (0.074 € kWh−1). In terms of the carbon balance, a CHRS can save up to 0.252 kgCO2-eq kWh−1 of energy produced, compared to a fossil-fuel alternative (natural-gas), while promoting carbon storage for around 0.05 kgCO2-eq kWh−1 in agricultural soils by compost amendment. Over a 15-year period, each module can potentially substitute fossil-derived heat for around 264 MgCO2-eq, while increasing soil carbon pool by around 20 MgCO2-eq, as C-stock calculated on a medium-term scenario (100-years).CHRSs have great potential to furnish renewable heat at competitive prices, while providing other ecosystem services, such as carbon storage and nutrients cycling to soil. Economic valorization of tree-pruning residues could also be an incentive for the implementation of agroforestry practices and landscape features. Further studies are needed in this relatively unexplored field, which might be of interest in the context of EU regulatory frameworks such as the EU Directive 2018/2001 and the upcoming Common Agricultural Policy (CAP) 2021 – 2027.

Coffee is the world’s second most traded commodity and the most renowned drink worldwide. The increasing production of coffee has been accompanied by a rise in consumption, and consequent increment in the amount of spent coffee grounds (SCGs) remaining as a solid residue from coffee brewing. In view of the high content of biodegradable compounds, if disposed, SCGs will certainly need to be biostabilized, although they should preferably be exploited in a biorefinery chain scheme. A wide range of alternative options is available for use in recycling SCGs as a valuable resource: food additives, pharmaceutical components, bio-sorbents, bio-fuels, and bio-products. The option of producing biogas from SCGs was tested and lab-scale bio-methane potential experiments were performed using different substrate to inoculum (S/I) ratios, namely 0.5, 1, and 2. A S/I ratio of 2 was found to be the optimal condition, resulting in a methane yield of 0.36 m3CH4/kgVS.

Compost Heat Recovery Systems (CHRS) sustainably capture heat from composting waste biomass, helping reduce greenhouse gas emissions and fossil fuel reliance. The choice of feedstock affects the performance of CHRSs as it controls the microbial activities and the amount of heat generated. This review evaluates plant-based, animal-derived, and non-agricultural feedstocks to optimize CHRS energy recovery. A systematic review of 244 studies, published from 1996 to 2023 and available on Scopus, Web of Science, and external databases, categorized feedstocks based on properties like carbon-nitrogen ratio (C/N), moisture content, bulk density, and heating value to assess their impact on energy recovery and compost quality. The review followed the PRISMA guidelines, excluding irrelevant documents and those that lacked quantitative data. Animal-based materials, which have high levels of moisture and nutrients, such as nitrogen (14.50–32.20 g/kg TS) and phosphorus (13.0–13.5 g/kg TS), promote rapid growth of microbes and consistent heat production supported by their stable carbon content (353.8–450.0 g/kg TS) and optimal C/N ratios (5.90–28.90). On the other hand, plant-based materials that are rich in volatile solids (327.2–960.0 g/kg TS) and lignin (36.7–290.0 g/kg TS) offer a steady and prolonged release of heat but decompose more slowly.

The concept of biorefinery expands the possibilities to extract value from organic matter in form of either bespoke crops or organic waste. The viability of biorefinery schemes depends on the recovery of higher-value chemicals with potential for a wide distribution and an untapped marketability. The feasibility of biorefining organic waste is enhanced by the fact that the biorefinery will typically receive a waste management fee for accepting organic waste. The development and implementation of waste biorefinery concepts can open up a wide array of possibilities to shift waste management towards higher sustainability. However, barriers encompassing environmental, technical, economic, logistic, social and legislative aspects need to be overcome. For instance, waste biorefineries are likely to be complex systems due to the variability, heterogeneity and low purity of waste materials as opposed to dedicated biomasses. This article discusses the drivers that can make the biorefinery concept applicable to waste management and the possibilities for its development to full scale. Technological, strategic and market constraints affect the successful implementations of these systems. Fluctuations in waste characteristics, the level of contamination in the organic waste fraction, the proximity of the organic waste resource, the markets for the biorefinery products, the potential for integration with other industrial processes and disposal of final residues are all critical aspects requiring detailed analysis. Furthermore, interventions from policy makers are necessary to foster sustainable bio-based solutions for waste management.