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One of the objectives of the European Union (EU) 2030 strategy is the sustainable growth through a series of measures focused on the application of the “Circular Economy” (ec.europa.eu, 2020) with many benefits also for climate, environment, and society. Food waste and sewage sludge management is part of this actions plan. Both streams are ideal source for microbial valorization processes to produce biofuels in a sustainable way. Anaerobic digestion (AD) is the benchmark robust technology for biogas production; however, the dark fermentation (DF) is a higher-rate process which offers the possibility to accumulate important building blocks (volatile fatty acids; VFA) and hydrogen from renewable resources, at reduced volumetric impact compared to AD. Focusing on this goal, the effectiveness of the hydrogenase enzymes needs to be favoured; some strategies have been already adopted: maintaining pH between 5.0-6.0; limiting the activities of methanogenic bacteria, not responsible for VFA and H2 accumulation, by decreasing the hydraulic retention time (HRT). Mesophilic and thermophilic bio-H2 and VFA production by DF process from food waste (alone or mixed with sewage sludge) has been studied in this work through batch assays. Pursuing the interest of studying interactions between adsorbents materials and bacteria, some trials were amended with zeolite (Z; chabazite type) at zeolite/inoculum ratio (Z/I) of 0.20 g Z/g VSinoculum. The zeolite’s presence brought the H2 yield up to 0.04 m³ H2/kg VS in thermophilic trials, in parallel to the increased acetic acid production (close to 50% of the organic acids, including lactic and VFA). The rate of H2 production also increased with Z addition, up to 27.3 mmol/(L d), 23% higher than control without Z. The positive effect of zeolite could be related to its associated surface area (available for bacterial adhesion), or ability to adsorb/exchange protons. These results open novel perspectives not only in bio-H2 production but also in the study of interactions between bacteria and inorganicporous materials. [ABSTRACT FROM AUTHOR]

This study explores the potential of utilizing tannery sludge for short-chain fatty acids (SCFAs) production through fermentation, incorporating zeolites in the process. The study involved a mild thermal pretreatment of the sludge as well, aimed at enhancing the subsequent fermentation process by increasing the solubilization of the organic matter. Six thermophilic continuously stirred tank reactor runs with varying hydraulic retention times (HRT) and zeolite addition are conducted. Zeolites, known for their low cost and absorption properties, significantly impacted chromium concentrations. SCFA production levels remain relatively consistent across tests. However, the acidification efficiency, as reflected in the SCFAs/sCOD ratios, shows improvement in the presence of Chabazite. Specifically, the acidification efficiency is highest in test RB8 (0.92 COD/COD). This suggests that the addition of Chabazite enhances acidification efficiency in tannery sludge applications. The findings highlight zeolites' potential to absorb heavy metals and improve acidification efficiency, indicating promising practical applications for tannery sludge treatment. [ABSTRACT FROM AUTHOR]

The tannery industry is a very lucrative and widespread sector in Italy, and it is yet one of the most polluting, mainly due to the tannery sludge's disposal in landfills, as it is considered a special residue by Italian legislation. An evaluation of the performance of the anaerobic fermentation process to obtain biogas and short-chain fatty acids (SCFAs) has been performed in this paper in different conditions concerning temperature, total solids content, and oxidizing and/or thermal pretreatments. The batch test trials revealed that the hydrogen peroxide pretreatment proved to be effective in increasing the biogas production, already at low doses but reaching the highest amount of 204 mL with the dose of 0.6 g H2O2/gTS. Regarding the SCFAs production, the combined microwave and hydrogen peroxide (MW-H2O2) pretreatment followed by thermophilic conditions gave the best results, with maximal SCFA concentration above 24 g CODSCFA/L. In the tests conducted without pretreatment, the mesophilic temperature seem preferable since the acidification performances were comparable to or even better than their thermophilic counterparts while being less energy intensive. The obtained results proved that tannery sludge can be employed in different ways and provide a viable alternative to landfilling, to handle this waste in a greener way, in a circular economy approach. [ABSTRACT FROM AUTHOR]