• Energy Research

The perennial rhizomatous grass giant reed (Arundo donax L.) can be exploited to produce hydrogen by dark fermentation. This implies a high availability of simple sugars, like glucose and xylose, and, thus, a pre-treatment is necessary to remove lignin and expose the holocellulose to enzymatic attack. This study aimed at evaluating the hydrogen production from giant reed hydrolysates. Giant reed dry meal was pre-treated with diluted NaOH (1.2% weight/weight), then the solid fraction was separated from the alkaline black liquor by filtration, enzymatically hydrolyzed with a cellulase blend (Cellic CTec2), and fermented in mesophilic batch conditions with a microbial consortium derived from pig slurry. The impact on hydrogen yield of initial pH was evaluated by comparing the hydrogen production from hydrolysates with not adjusted (5.3) or adjusted initial pH (8.7) using NaOH or alkaline black liquor. The highest hydrogen yield, 2.0 mol/mol of hexoses, was obtained with alkaline initial pH 8.7, regardless of how the pH adjustment was managed. The yield was 39% higher than that obtained in reactors with initial pH 5.3. In conclusion, thermo-alkaline pre-treatment followed by enzymatic saccharification and initial pH adjustment at 8.7 with the black liquor remaining after pre-treatment is a promising strategy to produce hydrogen from giant reeds in dark fermentation.

AbstractThe replacement of silage maize with giant reed as energy crop has been proposed as a mean for reducing the need of irrigation water as well as monetary and environmental costs of cultivation. Little is known about giant reed response to within‐season harvesting, and its effect on the methane production in anaerobic digestion. The effect of three harvest schedules on yield, biomass composition and methane production of giant reed was evaluated at one site of the Po Valley, northern Italy, for three consecutive years. In a completely randomized block design with four replicates the treatments applied annually were: (i) double harvest 1: first cut at the end of June + second cut at the beginning of October (DH1); (ii) double harvest 2: first cut at the end of July + second cut at the beginning of October (DH2); (iii) single harvest at the beginning of October (SH). The crop stand was established in the year 2015 and treatments were repeatedly applied in the years 2016, 2017 and 2018. The SH treatment determined the highest average annual dry matter (DM) yield (59.0 Mg DM ha−1). The DM yield for treatments with double harvest was significantly lower, that is, −30% for DH1 and −15% for DH2. In terms of specific methane yield there was little advantage in harvesting biomass twice during the growing season. The average specific methane yield varied substantially between years (i.e. from 144 to 233 ml CH4 g−1 VS), and in every year the values tended to decrease with the ageing of harvested biomass. Methane yield per hectare, however, was driven by DM yield, thus SH also determined the highest average value (9110 m3 CH4‐STP ha−1). In conclusion, the single annual harvest at the end of the growing season is an ideal strategy for maximizing methane production from giant reed.

In the last five years, the use of hydrogen as an energy carrier has received rising attention because it can be used in internal combustion and jet engines, and it can even generate electricity in fuel cells. The scope of this work was to critically review the methods of H2 production from renewable and non-renewable sources, with a focus on bio-H2 production from the perennial grass giant reed (Arundo donax L.) due to its outstanding biomass yield. This lignocellulosic biomass appears as a promising feedstock for bio-H2 production, with a higher yield in dark fermentation than photo-fermentation (217 vs. 87 mL H2 g−1 volatile solids on average). The H2 production can reach 202 m3 Mg−1 of giant reed dry matter. Assuming the average giant reed dry biomass yield (30.3 Mg ha−1 y−1), the attainable H2 yield could be 6060 m3 ha−1 y−1. A synthetic but comprehensive review of methods of H2 production from non-renewable sources is first presented, and then a more detailed analysis of renewable sources is discussed with emphasis on giant reed. Perspectives and challenges of bio-H2 production, including storage and transportation, are also discussed.

Protein recovery from dairy waste leaves large amounts of deproteinized cheese whey, which could be further exploited for chemicals and energy carrier production in anaerobic digestion. The aim of this study was to evaluate the effect of deproteinized cheese whey (scotta or scotta permeate) in co-digestion with pig slurry at different initial pH values on biogas and volatile fatty acid production. Five levels of dairy waste (0, 25%, 50%, 75%, 100%, with amounts of pig slurry complementary to 100) were tested, in factorial combination with five values of initial pH (6.5, 7.0, 7.5, 8.0, 8.5), in laboratory mesophilic, in-batch, static conditions. The presence of dairy waste in the recipe induced anaerobic fermentation and a drastic drop in pH. The addition of pig slurry allowed the accumulation of large amounts of volatile fatty acids (up to 35–40 g l−1, at neutral pH, in the recipe with 25% dairy waste/75% pig slurry), especially propionic and valeric. Methanogenesis began when hydrogen production had stopped, and after pH adjustment at neutrality. The formulation 75% dairy waste/25% pig slurry had the highest methane (CH4) yield per volume unit of feedstock (10.3 ml CH4(stp) ml−1, on average). Increasing percentages of pig slurry reduced the CH4 yield per volume unit of feedstock, while increasing the specific CH4 yield. The addition of pig slurry to reactors digesting undiluted deproteinized dairy wastes, although inappropriate for optimizing CH4 yield, nevertheless allows to obtain high concentrations of volatile fatty acids.

Giant reed is a non-food, tall, rhizomatous, spontaneous perennial grass that is widely diffused in warm-temperate environments under different pedo-climatic conditions. In such environments, it is considered one of the most promising energy crops in terms of economic and environmental sustainability, as it can also be cultivated on marginal lands. Owing to its complex and recalcitrant structure due to the lignin content, the use of giant reed as a feedstock for biogas production is limited. Thus, pre-treatment is necessary to improve the methane yield. The objective of this review was to critically present the possible pre-treatment methods to allow the giant reed to be transformed in biogas. Among the studied pre-treatments (i.e., hydrothermal, chemical, and biological), alkaline pre-treatments demonstrated better effectiveness in improving the methane yield. A further opportunity is represented by hybrid pre-treatments (i.e., chemical and enzymatic) to make giant reed biomass suitable for bio-hydrogen production. So far, the studies have been carried out at a laboratory scale; a future challenge to research is to scale up the pre-treatment process to a pilot scale.

The biogas production through the anaerobic digestion (AD) of giant reed (Arundo donax L.) biomass has received increasing attention. However, due to the presence of lignin, a low CH4 yield can be obtained. Aiming to improve the CH4 yield from giant reed biomass, the effectiveness of a thermo-chemical pre-treatment based on KOH was evaluated in this paper. The usefulness of a washing step before the AD was also assessed. The pre-treatment led to a specific CH4 yield up to 232 mL CH4 g−1 VS which was 21% higher than that from untreated biomass; the maximum daily rate of production was improved by 42%, AD duration was reduced by 10%, and CH4 concentration in the biogas was increased by 23%. On the contrary, the washing step did not improve the AD process. Besides, washing away the liquid fraction led to biomass losses, reducing the overall CH4 production. The use of a KOH-based pre-treatment appears as a good option for enhancing the AD of giant reed, also presenting potential environmental and agronomical benefits, like the avoidance of salty wastewater production and the likely improvement of the digestate quality, due to its enriched K content.

Abstract In this study, the biogas production from two fermentation wastewaters deriving from the cultivation of two strains of lactic acid bacteria on ricotta cheese exhausted whey was evaluated in mono-digestion or co-digestion with pig slurry. Lactic acid bacteria were: Lactococcus lactis subsp. lactis and Streptococcus thermophilus. Anaerobic digestion was carried out in laboratory mesophilic, batch, static conditions. Biogas production was monitored until exhaustion. Methane yield of the fermentation wastewaters in mono-digestion was, on average, 372 mL CH4 g−1 VS, and 416 mL CH4 g−1 VS in co-digestion. These values are comparable with that of other feedstocks currently used for biogas production. The co-digestion halved the duration of anaerobic digestion. The fermentation wastewaters are suitable feedstocks for biogas production, and it is possible to directly mix them with pig slurry in existing plants. The digestate could be used as crop fertilizer from a renewable source.

This study carried out for the first time a comparison among ligninolytic (white-rot) and cellulosolytic or xylanolytic (Trichoderma) pre-treated wheat straw, for biogas production, potential, without or with pig slurry in co-digestion. Methane (CH4) production from wheat straw pre-treated for 4 and 10 weeks with seven different fungal isolates was preliminarily measured. Then, the effects on biogas yield of the co-digestion with pig slurry were checked on straw pre-treated with 3 selected fungal strains. The maximum production of CH4 from pre-treated straw with Ceriporiopsis subvermispora (SUB) for 4 and 10 weeks was higher than the control (16% and 37%, respectively). The accumulation daily rate was higher than control (42% and 81%, respectively). A positive correlation between CH4 accumulation daily rate and straw enzymatic digestibility was found. In co-digestion with pig slurry, SUB pre-treated straw for 10 weeks showed an accumulation daily rate of 17.4 mL d-1 g-1 VS, significantly higher (17%) than that of the control. The time to reach the maximum CH4 production was shortened on average from 34 to 21 days in co-digestion with pig slurry, in comparison with pre-treated mono-digested wheat straw. The biological pre-treatment with selected white-rot fungi appears a promising technology to increase methane production from wheat straw.