Hydrogen and methane production through two stage anaerobic digestion of straw residues

: Anaerobic digestion of agricultural waste can contribute to the European renewable energy needs. The 71% of the 20,000 anaerobic digestion plants in operation already uses these agro-waste as feedstock; part of these plants can be converted into two stage processes to produce hydrogen and methane in the same plant. Biomethane enriched in hydrogen can replace natural gas in grids while contributing to the sector decarbonisation. Straw is the most abundant agricultural residue (156 Mt/y) and its conventional final fate is uncontrolled soil disposal, landfilling, incineration or, in the best cases, composting. The present research work focuses on the fermentation of spent mushroom bed, an agricultural lignocellulosic byproduct, composed mainly from wheat straw. The substrate has been characterized and semi-continuous tests were performed evaluating the effect of the hydraulic retention time on hydrogen and volatile fatty acids production. It was found that all the tests confirmed the feasibility of the process even on this lignocellulosic substrate, and also, it was identified HRT 4.0 d as the best option to optimize the productivity of volatile fatty acids (17.09 gCODVFAs/(KgVS*d)), and HRT 6.0 d for hydrogen (7.98 LH2/(KgVS*d)). The fermentation effluent was used in biomethanation potential tests to evaluate how this process affects a subsequent digestion phase, reporting an increase in the energetical feedstock exploitation up to 30%

Single Cell Proteins production from food processing effluents and digestate

: The increase in human population determines a higher proteins request to sustain the feed demand for animals and aquaculture. Single Cell Proteins (SCPs) consist of mixed protein from pure and mixed culture of bacteria, fungi, algae, and yeast, which are grown and harvested to accomplish the food requirement of human and animals. This work investigated the production of Saccharomyces cerevisiae to be used as SCPs for animal feeding. The effluent of candies production process, rich in sugars, about 40 g/L, and agricultural digestate rich in nitrogen and other macro and micronutrients, were used for the yeast's growth. Preliminary batch tests demonstrated that aerobic conditions optimized the biomass growth. Then, continuous aerobic tests were conducted at different dilution rates. The dilution rate of 0.50 d-1, corresponding to a hydraulic retention time of 2 days, optimized both the biomass productivity of 0.25 g/L per day and the protein content of 28% w/w. The analysis of the aminoacidic profile demonstrated that obtained SCPs could be used as an integrator of feed for fish and monogastric animals. On the contrary, they were not suitable for pet feed as all the amino acids concentrations were lower than required standards. These results suggested that anaerobic digesters in the agricultural sector can be transformed into small biorefineries for microbial protein production

Volatile fatty acids production from waste rich in carbohydrates: Optimization of dark fermentation of pasta by products

Pasta is one of the most appreciated Italian products around the world. The 31% of the global production of pasta is made by Italian companies: here, about 120 companies assure an annual production of 5.3 million tons of pasta. The pasta not compliant with the required selling standards (for example size, weight) is usually crushed, grounded to power, and used for animals feeding. This study offered a more advantageous alternative for the valorization of pasta by-products, whose mono-fermentation for Volatile Fatty Acids (VFAs) production was not investigated before. In particular, we considered pasta residues to optimize the operational parameters both in batch and in continuous conditions. The first round consisted in batch tests at different Total Solids (TS) con- centration (1.5–10% TS w/w). The 2% TS w/w emerged as the best one, able to assure a VFAs yield close to the 80%. Consequently, this concentration was chosen for the conduction of continuous tests dedicated to the optimization of the best HRT and SRT for the process. The HRT of 3 d and a SRT of 4.4 d allowed the best process performances with a VFAs productivity of 3.48 gVFAs_COD per L of reactor and per day. The optimization of the process conditions for VFAs production led to a greater concentration of butyric acids (about 10% w/w) instead of the mere acetic acid production. Because of the economic value of butyric acid, this result is of particular interes

A critical review on the techno-economic feasibility of nutrients recovery from anaerobic digestate in the agricultural sector

The unsustainable environmental impact of fossil derived fertilizers and the depletion of raw materials for their production make the research of alternative sources for fertilizers production one of the main priorities of the international agenda. Agricultural digestate, especially if derived from livestock manure, is considered a potential candidate for the recovery of bio-based fertilizers: 180 M tonnes of digestate, which contain high concentrations of nitrogen (2–5 kg/m3) and phosphorous (0.5–1.5 kg/m3), are produced annually in the EU. This paper overviews different technologies, their robustness and yields in terms of nutrients recovery for different biobased fertilizers. Ammonia stripping and struvite precipitation are the technologies more developed at full scale allowing an average removal and recovery yield of 80–90 % for nitrogen and phosphorous, respectively. Due to their high degree of development, costs for ammonia stripping and struvite precipitation are relatively low, ranging from 2 to 7 € per kg of nutrient recovered. However, other technologies are rapidly developing and spreading. Pressure-driven membrane technologies (ultrafiltration and reverse osmosis) are emerging as important processes for nutrient recovery, although not selective. Nitrogen and phosphorous removal efficiencies for these technologies are around 75–95 % and 85–99 %, respectively. The main drawback of membrane processes is represented by the membrane fouling which prevents their large diffusion at full scales because of management difficulties. Moreover, the operational cost of this process is higher compared to other available technologies, ranging from 4 to 12 € per m3 of digestate. Full-scale applications for membrane technologies are available, but still limited, while several studies at laboratory and pilot scale are currently under development. Membrane contactors are receiving attention for the possibility to strip ammonia directly in the membrane with high ammonia recovery (over the 95 %). But this technology is currently developed at low TRL in the EU with several studies at laboratory and pilot scale. Ion exchange, adsorption, evaporation, and electrodialysis are also discussed in this review paper. The difficulty to have solid matrices able to assure a selective nitrogen/phosphorous compound adsorption and the need for regeneration has limited the ion exchange process at full TRL for treatment of agricultural digestate. However, some pilot scale applications of this technology have been investigated for wastewater treatment. Similarly, the high economic and energetic costs for evaporation and electrodialysis have prevented further applications of these techniques for pilot and full scale operation

Decarbonization of the European natural gas grid using hydrogen and methane biologically produced from organic waste: A critical overview

The global CO2 emissions are above 35 GtCO2/year. Recently, the European Commission approved the Green Deal with the aim to reduce the greenhouse gases emissions. Hydrogen, because of its capacity to burn without CO2 formation, is considered one of principal energy option for the future to obtain decarbonization. In 2020, total hydrogen demand was estimated at 8.6 Mt but over 90% of all hydrogen production plants use fossil fuels as feedstock. In perspective, a relatively small part of this hydrogen can be of biological origin. Anaerobic digestion, in particular two stages processes, can assure biological hydrogen and methane productions using organic waste and waste effluents as feedstock. Carbon dioxide in biogas can be removed to obtain a gas blend of methane and hydrogen with typical concentrations of 90:10–70:30% v/v. This review aims to provide an overview on biohythane production in Europe at Technology Readiness Level greater or equal to 5 (technology validated in relevant environment). At the moment, there are few examples of biohythane plants at pilot or higher scales, mainly located in Italy, France and United Kingdom. Here reactors with volumes in the range 20–350 L and 300-1000 L were able to assure a stable hydrogen and methane, respectively, production. The highest volumes of 700 and 3800 L for acidogenic and methanogenic phases, respectively, were successfully tested close Milan (Italy)

A groundbreaking biorefinery loop for the valorization of cigarette butts into fermentable sugars and bioethanol

Cigarette Butts are one of the most diffused and toxic waste in urban contexts. They are composed by a cellulose acetate filter, used to retain toxic compounds during the smoking of cigarette, and eventual tobacco's residues. This work had the ambitious to exploit cigarette butts in a biorefinery scheme to produce fermentable sugars and bioethanol. Cigarette butts followed different sequential operations. Deacetylation of cellulose acetate was optimized by a lipase treatment, which played an important role in the biological degradation and deacetylation of cellulose acetate. The following cellulase addition allowed the conversion of the deacetylated cellulose into fermentable sugars. In particular, the best performances were obtained with a cellulase addition of 10% of the cigarette butts, which led to a final fermentable sugars' concentration of about 12 g/L, corresponding to a cigarette butts' conversion of about 70% w/w. Then, the fermentable sugars were used as substrates for bioethanol production by three different yeasts: Metschnikowia pulcherrima MALV5, Lachancea fermentati LS16 and Saccharomyces cerevisiae EC1118. Metschnikowia pulcherrima MALV5, Lachancea fermentati LS16 achieved similar results with a bioethanol production of 2.14 and 2.44 g/L, respectively, corresponding to a fermentable sugars conversion of about 20% w/w

Toward the Transition of Agricultural Anaerobic Digesters into Multiproduct Biorefineries

Anaerobic digestion allows for the proper management of agro-waste, including manure. Currently, more than 18,000 anaerobic digestion plants are under operation in EU, 80% of which are employed in the rural context. Tariff schemes for power generation from biogas produced during anaerobic digestion of agricultural feedstocks in Germany, Italy and Austria are coming to an end and new approaches are needed to exploit the existing infrastructures. Digesters in the rural context can be implemented and modified to be transformed into sustainable multi-feedstock and multi-purpose biorefineries for the production of energy, nutrients, proteins, bio-chemicals such as carboxylic acids, polyesters and proteins. This paper describes how the transition of agricultural anaerobic digesters into multi-products biorefineries can be achieved and what are the potential benefits originating from the application of a pilot scale platform able to treat cow manure and other crop residues while producing volatile fatty acids, polyhydroxyalkanoates, microbial protein material, hydrogen, methane and a concentrated liquid stream rich in nitrogen, potassium and phosphorus