• Energy Research

Arundo donax (giant reed) hydrolysate was exploited as a substrate for anaerobic digestion aimed to the production of biogas and biohydrogen. A mixed culture adapted from a primary sludge digester was used as inoculum. Besides the biogas, the anaerobic fermentation products were ethanol, acetic acid and butyric acid. Together, the soluble products accounted for about 51 % of degraded carbon. The produced biogas was 5 % H2, 41 % CO2 and 54 % CH4 by volume. Fermentation kinetics was slower and biogas yields were lower than those found with glucose fermentation with same mixed culture. Further optimization of the process is envisaged in order to improve biohydrogen yield.

Anaerobic co-digestion of mixed feedstocks improves the biogas yields due to a better balance of nutrients in the digestion medium. A suitable choice for improving biogas yields from the anaerobic digestion of municipal solid wastes is the co-digestion with lignocellulosic materials. The growing exploitation of the giant reed in several industrial fields motivated a preliminary investigation on the anaerobic co-digestion of steam-exploded giant reed and the organic fraction of municipal solid wastes (OFMSW).The anaerobic digestion was carried out at 37 °C, in batch operation mode. Biogas volumes produced and the concentration–time profiles of volatile fatty acids were analysed for different initial ratios of the mixed feedstock. All the mixtures performed better than the single feedstock. The optimal biogas yield was obtained with the co-digestion of a mixture containing 75% OFMSW and 25% giant reed, which produced 236 mL CH4/g VS with a 1.5-fold increase respect to the digestion of OFMSW alone.

The natural microbial activity in the unsaturated soil is vital for protecting groundwater in areas where high loads of biodegradable contaminants are supplied to the surface, which usually is the case for airports using aircraft de-icing fluids (ADF) in the cold season. Horizontal and vertical distributions of microbial abundance were assessed along the western runway of Oslo Airport (Gardermoen, Norway) to monitor the effect of ADF dispersion with special reference to the component with the highest chemical oxygen demand (COD), propylene glycol (PG). Microbial abundance was evaluated by several biondicators: colony-forming units (CFU) of some physiological groups (aerobic and anaerobic heterotrophs and microscopic fungi), most probable numbers (MPN) of PG degraders, selected catabolic enzymatic activities (fluorescein diacetate (FDA) hydrolase, dehydrogenase, and β-glucosidase). High correlations were found between the enzymatic activities and microbial counts in vertical soil profiles. All microbial abundance indicators showed a steep drop in the first meter of soil depth. The vertical distribution of microbial abundance can be correlated by a decreasing exponential function of depth. The horizontal trend of microbial abundance (evaluated as total aerobic CFU, MPN of PG-degraders, and FDA hydrolase activity) assessed in the surface soil at an increasing distance from the runway is correlated negatively with the PG and COD loads, suggesting the relevance of other chemicals in the modulation of microbial growth. The possible role of potassium formate, component of runway de-icers, has been tested in the laboratory by using mixed cultures of Pseudomonas spp., obtained by enrichment with a selective PG medium from soil samples taken at the most contaminated area near the runway. The inhibitory effect of formate on the growth of PG degraders is proven by the reduction of biomass yield on PG in the presence of formate.

The dark fermentation of lignocellulose hydrolysates is a promising process for the production of hydrogen from renewable sources. Nevertheless, hydrogen yields are often lower than those obtained from other carbohydrate sources due to the presence of microbial growth inhibitors in lignocellulose hydrolysates. In this study, a microbial consortium for the production of hydrogen by dark fermentation has been obtained from a wild methanogenic sludge by means of thermal treatments. The consortium has been initially acclimated to a glucose-based medium and then used as inoculum for the fermentation of Arundo donax hydrolysates. Hydrogen yields obtained from fermentation of A. donax hydrolysates were lower than those obtained from glucose fermentation using the same inoculum (0.30 ± 0.05 versus 1.11 ± 0.06 mol of H2 per mol of glucose equivalents). The hydrogen-producing bacteria belonged mainly to the Enterobacteriaceae family in cultures growing on glucose and to Clostridium in those growing on A. donax hydrolysate. In the latter cultures, Lactobacillus outcompeted Enterobacteriaceae, although Clostridium also increased. Lactobacillus outgrowth could account for the lower yields observed in cultures growing on A. donax hydrolysate.

Biological treatments focused on stabilizing and detoxifying olive mill wastewater facilitate agronomic reuse for irrigation and fertilization. Anaerobic digestion is particularly attractive in view of energy recovery, but is severely hampered by the microbial toxicity of olive mill wastewater. In this work, the addition of biochar to the digestion mixture was studied to improve the stability and efficiency of the anaerobic process. Kinetics and yields of biogas production were evaluated in batch digestion tests with biochar concentrations ranging from 0 to 45 g L−1. The addition of biochar reduced sensibly the lag phase for methanogenesis and increased the maximum rate of biogas generation. Final yields of hydrogen and methane were not affected. Upon addition of biochar, soluble COD removal increased from 66% up to 84%, and phenolics removal increased from 50% up to 95%. Digestate phytotoxicity, as measured by seed germination tests, was reduced compared to raw wastewater. Addition of biochar further reduced phytotoxicity and, furthermore, a stimulatory effect was observed for a twenty-fold dilution. In conclusion, biochar addition enhances the anaerobic digestion of olive mill wastewaters by effectively reducing methanogenesis inhibition and digestate phytotoxicity, thus improving energy and biomass recovery.

The exploitation of mixed cultures for the dark fermentation of hydrolysates of lignocellulosic biomass is hindered by low hydrogen yields due to the presence of inhibitors of bacterial growth (phenols, furans) and to proliferation of bacteria not involved in the production of hydrogen (e.g. lactic acid bacteria) or consuming hydrogen (methanogens, acetogens). Initially, a non-methanogenic anaerobic consortium has been tested for the the dark fermentation of a synthetic medium with glucose as sole carbon source (yields average at 0.66 mol H2/mol glucose). The effect of single inhibitors (p-coumaric acid, furfural) added to the synthetic medium has been evaluated in terms of hydrogen yields and kinetics. 0.2 g/L of p-coumaric acid limited both kinetics and yield of H2 production (dropping to 0.16 mol H2/mol glucose), whereas 2.0 g/L furfural prolonged the lag phase of about one day without significantly affecting the overall yield. The control of medium pH has then been exploited for the enhancement of hydrogen yields. Keeping pH at 6.0-6.5 has shown to be an efficient way to prevent the outgrowth of lactic acid bacteria during dark fermentation of glucose, improving H2 yields (1.33 mol H2/mol glucose). Finally, comparable hydrogen yields (1.40 mol H2/mol glucose equivalents) have been observed during the dark fermentation of Arundo donax hydrolysate with pH control, notwithstanding the presence of inhibitor compounds in the medium. Proceedings of the 24th European Biomass Conference and Exhibition, 6-9 June 2016, Amsterdam, The Netherlands, pp. 622-626