• Energy Research

Starting from (but not limited to) their importance in the Italian context, three agricultural substrates, two of fruit origin (grape seeds and plum stones) and one of herbaceous origin (woad), were comparatively tested for both biomethane production and digestate characterization. The anaerobic digestion tests showed that grape seeds had the highest net methane production of 253.0 NmL g volatile solids (VS)(-1), followed by plum stones, whose best resulting net methane production was 174.7 NmL gVS(-1), and finally by woad with a net methane production of 153.1 NmL gVS(-1). Interestingly, the best methane productions of the fruit substrates were obtained with different substrate to inoculum ratios (on a VS basis), 1:1 for grape seeds but 2:1 for plum stones. On the other hand, a three-month ageing of woad caused a limited reduction of methane production. The estimation of obtained degrees of conversion, carried out on a chemical oxygen demand (COD) basis for the specific tests achieving the respective best methane productions, gave values of 48%, 31%, and 33% for grape seeds, plum stones, and woad, respectively. The estimated degrees of conversion were evaluated along with the respective methane productions and substrate COD/VS ratios. The comparison of Fourier transform infrared (FT-IR) spectra and differential thermal analysis (DTA) profiles, carried out for selected digestates in pairs, revealed some distinctive differences in the relative intensities or presence and absence of particular peaks in the FT-IR spectra and in the relative intensities of the exothermic peaks or horizontal curve shifting of the DTA profiles.

A combined experimental evaluation of methane production (obtained by anaerobic digestion) and detailed digestate characterization (with physical-chemical, thermo-gravimetric and mineralogical approaches) was conducted on two organic substrates, which are specific to Italy (at regional and national levels). One of the substrates was grape seeds, which have an agricultural origin, whereas the other substrate was vegetable-tanned leather dust, which has an industrial origin. Under the assumed experimental conditions of the performed lab-scale test series, the grape seed substrate exhibited a resulting net methane production of 175.0 NmL g volatile solids (VS)(-1); hence, it can be considered as a potential energy source via anaerobic digestion. Conversely, the net methane production obtained from the anaerobic digestion of the vegetable-tanned leather dust substrate was limited to 16.1 NmL gVS(-1). A detailed characterization of the obtained digestates showed that there were both nitrogen-containing compounds and complex organic compounds present in the digestate that was obtained from the mixture of leather dust and inoculum. As a general perspective of this experimental study, the application of diversified characterization analyzes could facilitate (1) a better understanding of the main properties of the obtained digestates to evaluate their potential valorization, and (2) a combination of the digestate characteristics with the corresponding methane productions to comprehensively evaluate the bioconversion process.

Because home composting is a prevention option in managing biowaste at local levels, the objective of the present study was to contribute to the knowledge of the process evolution and compost quality that can be expected and obtained, respectively, in this decentralized option. In this study, organized as the research portion of a provincial project on home composting in the territory of Pesaro-Urbino (Central Italy), four experimental composters were first initiated and temporally monitored. Second, two small sub-sets of selected provincial composters (directly operated by households involved in the project) underwent quality control on their compost products at two different temporal steps. The monitored experimental composters showed overall decreasing profiles versus composting time for moisture, organic carbon, and C/N, as well as overall increasing profiles for electrical conductivity and total nitrogen, which represented qualitative indications of progress in the process. Comparative evaluations of the monitored experimental composters also suggested some interactions in home composting, i.e., high C/N ratios limiting organic matter decomposition rates and final humification levels; high moisture contents restricting the internal temperature regime; nearly horizontal phosphorus and potassium evolutions contributing to limit the rates of increase in electrical conductivity; and prolonged biowaste additions contributing to limit the rate of decrease in moisture. The measures of parametric data variability in the two sub-sets of controlled provincial composters showed decreased variability in moisture, organic carbon, and C/N from the seventh to fifteenth month of home composting, as well as increased variability in electrical conductivity, total nitrogen, and humification rate, which could be considered compatible with the respective nature of decreasing and increasing parameters during composting. The modeled parametric kinetics in the monitored experimental composters, along with the evaluation of the parametric central tendencies in the sub-sets of controlled provincial composters, all indicate that 12-15 months is a suitable duration for the appropriate development of home composting in final and simultaneous compliance with typical reference limits.

In an urban or suburban area, the sustainability of a waste management process is expected to be closely related to the territorial context and the local citizens’ behaviour. From this perspective, the implementation of the peculiar proximity waste collection system in a small town in Central Italy (San Costanzo) was considered. As compared to the previous road collection system in the same municipality, its environmental performance in terms of Source Separation Level (SSL), Waste Generation or Collection Rate (WGR or WCR) and Interception Rate (IR) was evaluated. An original analysis of the citizens’ monthly frequency and of their participation rate both in relation to the setting out of the Unsorted Residual Waste (URW) was also carried out. Following the full implementation of the new waste collection scheme, the SSL achieved almost 79%, the WCR of the URW decreased by about 82% and the most IR values resulted above 83%. From a social point of view, the study further highlighted that, with the new waste collection scheme active, more than 50% of users were accustomed to set out the URW at most 5 times per month and the corresponding participation rate to set out the URW was around 62%.

In the Marche Region (Central Italy), the residual municipal waste (RMW) is commonly processed in mechanical biological treatment (MBT) systems. In these systems, following a first mechanical selection, the undersize organic fraction from RMW (us-OFRMW) undergoes a partial aerobic biological treatment before being landfilled as a biostabilised fraction (bios-OFRMW) without dedicated energy or material recovery. Alternative us-OFRMW management scenarios have been elaborated for this region, at both present (reference year 2019) and future (reference year 2035) time bases. In the first scenario, the potential bioenergy recovery through anaerobic digestion (AD) from the us-OFRMW was evaluated. The second scenario aimed at evaluating the residual methane generation expected from the bios-OFRMW once landfilled, thus contributing also to the potential environmental impact connected with landfill gas (LFG) diffuse emissions from the regional landfills. The diversion to AD, at the present time, would allow a potential bioenergy recovery from the us-OFRMW equal to 4.35 MWel, while the alternative scenario involves greenhouse gas (GHG) emissions equal to 195 kg CO2 eq. per ton of deposited bios-OFRMW. In the future, the decreased amount of the us-OFRMW addressed to AD would still contribute with a potential bioenergy recovery of 3.47 MWel.