• Energy Research

An experimental apparatus was constructed to perform hybrid solid anaerobic digestion batch processing of the organic fraction of municipal solid waste. The preliminary process was carried out with a high total solids concentration of about 33% w w−1 and with an initial organic load of about 340 kg VS kg−1. The fresh organic fraction to inoculum ratio used to enhance the anaerobic process start-up was 0.910 kg VS kg VS−1. The process was conducted by spreading the percolate on top of the mixture. The percolate was stored in a separate section of the apparatus with a mean hydraulic retention time of about 1 day. During the process, acetate, butyrate and propionate in the percolate reached concentrations ranging from 3000 to 11 000 mg L−1. In spite of these high concentrations, the biomethane produced from both the solid and the percolate was quite high, at about 210 NL kg VS−1. The digestate obtained at the end of the run showed rather good features for being classified as an organic fertilizer according to Italian law. However, a residual phytotoxicity level was detected by a standardized test showing a germination index of about 50%.

Solid Batch Anaerobic Digestion (SADB) is an interesting process that can lead to the production of quite high bio-methane from several biodegradable substrates. In particular the limited production of waste liquids discharged by the reactors, along with light pre-treatment requirement, makes this process suitable for a high spreading potential in many European regions. Unfortunately, some inhibition phenomena occur when the anaerobic digestion is performed with high Total Solids concentration, as in the SADB. These phenomena can affect negatively both the stability and the viability of the process. Among the different solution exploitable for managing these phenomena, the one based on percolate recirculation seems to be very interesting. The experimental runs conducted exploiting the Organic Fraction of Municipal Solid Waste, shows that the recirculation leads to a significant reduction of the Volatile Fatty Acids concentration along with an higher biogas production rate and stability. The biogas produced in the test with percolate recirculation is double and the VFA concentration results to be significantly lower.

The co-digestion of a variable amount of fruit and vegetable waste in a waste mixed sludge digester was investigated using a pilot scale apparatus. The organic loading rate (OLR) was increased from 1.46 kg VS/m(3) day to 2.8 kg VS/m(3) day. The hydraulic retention time was reduced from 14 days to about 10 days. Specific bio-methane production increased from about 90 NL/kg VS to the maximum value of about 430 NL/kg VS when OLR was increased from 1.46 kg VS/m(3) day to 2.1 kg VS/m(3) day. A higher OLR caused an excessive reduction in the hydraulic retention time, enhancing microorganism wash out. Process stability evaluated by the total volatile fatty acids concentration (mg/l) to the alkalinity buffer capacity (eq. mg/l CaCO3) ratio (i.e. FOS/TAC) criterion was 2.46 kg VS/m(3) day, GI decreased rapidly. This corresponding trend between FOS/TAC and GI was further investigated by the definition of the GI ratio (GIR) parameter. Comparison between GIR and FOS/TAC suggests that GI could be a suitable criterion for evaluating process stability.

A Pre-Industrial Scale Anaerobic Digestion Cell, working in batch mode in the Solid State (i.e. Total Solids content higher than 25% w/w) was built for the co-treatment of Animal and Agricultural Residues. The anaerobic cell has a total volume of about 170m3 and is able to treat about 95 tonnes of biomass per cycle. The aim is to perform the Anaerobic Digestion process with a high solids content to reduce, or completely eliminate, the waste liquid fraction usually produced by other types of Anaerobic Digestion technology. This could lead to the production of renewable electrical energy from animal manure and other agricultural residues in a more sustainable way. The cell was built on an existing Italian farm that raises about 135 tonnes per year of live weight swine, and also produces other agricultural residues. The problem of the starting the Anaerobic Digestion inside the cell without inoculum was analysed and different strategies were evaluated. The one based on aerobic pre-treatment of the mixture before the anaerobic process inside the cell seems to give interesting results both for the rapidity of starting the methanogens phase as well as for the quality of the biogas produced. The results show that 15 days of aerobic pre-treatment led to the production of a biogas with a biomethane concentration of about 50%v/v after 10 days of anaerobic treatment. Further, the whole anaerobic process can be considered practically finished after about 65 days.

Climate warming is predicted to raise the mean global temperature by 1 °C in the next 50 years, and this change is believed to be capable of affecting soil organic matter cycling and nutrient availability. With the aim of increasing knowledge on the response of forest soils to the ongoing climate change, we used altitude as a proxy for temperature change and studied chemical and biochemical properties of European beech (Fagus sylvatica L.) forest soils at two altitudes (800 and 1000 m) from central Apennines (Italy). Results showed that 1 °C of mean annual air temperature difference between the sites at the two altitudes had greater effect on the mineral horizons than on the organic horizons. At higher altitude, mineral soil had limited development, higher pH, and higher organic matter content due to the lower efficiency of the microbial community. Enzymatic activities of the organic horizons were generally not affected by altitude. Conversely, we observed a higher activity of xylosidase, β-glucosidase, alkaline phosphomonoesterase, arylsulfatase, and leucine-aminopeptidase in the sub-superficial horizons (Bw1 and Bw2) of the soils at 1000 m. We hypothesized that, as a response to environmental and climatic constraints occurring at higher altitude, plant roots increase the production of enzymes directly and/or indirectly by triggering the microbial community through exudation.

Plant communities, through species richness and composition, strongly influence soil microorganisms and the ecosystem processes they drive. To test the effects of other plant community attributes, such as the identity of dominant plant species, evenness, and spatial arrangement, we set up a model mesocosm experiment that manipulated these three attributes in a full factorial design, using three grassland plant species (Anthoxanthum odoratum, Plantago lanceolata, and Lotus corniculatus). The impact of the three community attributes on the soil microbial community structure and functioning was evaluated after two growing seasons by ester-linked phospholipid fatty-acids analysis, substrate-induced respiration, basal respiration, and nitrogen mineralization and nitrification rates. Our results suggested that the dominant species identity had the most prevalent influence of the three community attributes, with significant effects on most of the measured aspects of microbial biomass, composition and functioning. Evenness had no effects on microbial community structure, but independently influenced basal respiration. Its effects on nitrogen cycling depended on the identity of the dominant plant species, indicating that interactions among species and their effects on functioning can vary with their relative abundance. Systems with an aggregated spatial arrangement had a different microbial community composition and a higher microbial biomass compared to those with a random spatial arrangement, but rarely differed in their functioning. Overall, it appears that dominant species identity was the main driver of soil microorganisms and functioning in these model grassland communities, but that other plant community attributes such as evenness and spatial arrangement can also be important.

Abstract Anaerobic digestion is an environmentally sustainable way to manage organic waste, and it is able to enhance the recovery of organic carbon and nutrients in agricultural soils and to produce renewable energy. Solid-state anaerobic digestion (S-SAD) is a technology that permits the treatment of different type of residues, but is characterized by inhibition phenomena, resulting in a low operational stability. An experimental apparatus, equipped with a recirculation system for the digestate liquid fraction (percolate), was used to optimize the S-SAD system. Different frequencies of recirculation, one, two or four per day, were carried out to investigate how recirculation might affect the quality of the liquid fraction as well as the possible effects on biogas production and on the obtained solid digestate quality. Biogas production was positively affected by percolate spreading, especially when recirculation was performed 4 times per day. As shown by percolate chemical analyses, recirculation avoided the accumulation of volatile fatty acids in the liquid fraction, resulting in a better process stability. In addition, recirculation induced a large consumption of readily available compounds in the percolate, as shown by the depletion of water extractable organic C and total reducing sugars. The quality of the digested solid fraction was also improved by percolate recirculation in terms of the C/N ratio and organic N parameters. These findings showed that daily repeated recirculation of the liquid fraction is suitable to avoid inhibition phenomena during S-SAD and to improve the quality of the digestate solid fraction.