Application of nano-scale transition metal carbides as accelerants in anaerobic digestion

Four types of nano-scale transition metal carbides (HfC, SiC, TiC, and WC), used as accelerants in anaerobic digestion (AD) with cattle manure, were investigated through batch experiments under mesophilic conditions (37 ± 1 °C). The AD system with four carbide accelerants showed a higher biogas yield (463–499 mL/g TS), chemical oxygen demand (COD) degradation rate (58.62–78.90%) and total Kjeldahl nitrogen (TKN) concentrations (905.0–1077.0 mg/L) as compared with control check (CK, 294 mL/g TS, 46.99%, 290 mg/L). All of the digestate samples from the AD systems using four carbide accelerants showed not only higher degradation of organic compounds during thermal analysis, but also stronger fertilizer values. The use of transition metal compounds (TMCs) as accelerants in AD can efficiently improve the conversion of waste resources into biogas and fertilizers, which can potentially open new avenues for the use of TMCs in upcoming research on biomass energy.     

Energizing marginal soils – The establishment of the energy crop Sida hermaphrodita as dependent on digestate fertilization,NPK, and legume intercropping

Growing energy crops in marginal, nutrient-deficient soils is a more sustainable alternative to conventional cultivation. The use of energy-intensive synthetic fertilizers needs to be reduced, preferably via closed nutrient loops in the biomass production cycle. In the present study based on the first growing season of a mesocosm experiment using large bins outdoors, we evaluated the potential of the energy plant Sida hermaphrodita to grow in a marginal sandy soil. We applied different fertilization treatments using either digestate from biogas production or a commercial mineral NPK-fertilizer. To further increase independence from synthetically produced N-fertilizers, the legume plant Medicago sativa was intercropped to introduce atmospherically fixed nitrogen and potentially facilitate the production of additional S. hermaphrodita biomass. We found digestate to be the best performing fertilizer because it produced similar yields as the NPK fertilization but minimized nitrate leaching. Legume intercropping increased the total biomass yield by more than 100% compared to S. hermaphrodita single cropping in the fertilized variants. However, it negatively influenced the performance of S. hermaphrodita in the following year. We conclude that a successful establishment of S. hermaphrodita for biomass production in marginal soils is possible and digestate application formed the best fertilization method when considering a range of aspects including overall yield, nitrate leaching, nitrogen fixation of M. sativa, and sustainability over time.     

Fate of Plant Available Nutrients during Hydrothermal Carbonization of Digestate

Products from the hydrothermal carbonization of two digestates have been analyzed in order to investigate the fate of nitrogen, phosphorus, and potassium in specific plant available speciations. The availability of phosphate and potassium ions was significantly increased due to hydrothermal carbonization of a nutrient‐rich digestate. In contrast, the availability of NH₄⁺ and PO₄^3? in the process liquor was decreased to < 10 % of the original value in the case of a nutrient‐poor digestate. With the exception of nitrate, process parameters did not show an influence on the recovery of the investigated ions with the HTC process liquor.     

Cascaded production of biogas and hydrochar from wheat straw: Energetic potential and recovery of carbon and plant nutrients

The increasing interest in biogas production has brought notable attention to lignocellulosic wastes as a promising and yet unexploited feedstock. As these materials are usually highly recalcitrant the energetic efficiency of biogas production, however, is comparatively low. With the aim to overcome this drawback, a novel cascaded approach was investigated that combines anaerobic digestion with hydrothermal carbonization (HTC). The latter is used to convert the digestate into a carbon-rich product termed hydrochar. An energetic evaluation of this cascaded treatment shows that the energy recovery can be nearly doubled compared to single anaerobic digestion.Furthermore, systematic HTC experiments with both fresh and digested wheat straw and with reaction temperatures of 190 °C, 210 °C, 230 °C, and 250 °C revealed an effect of reaction temperature on carbon, nitrogen, and phosphorus concentration in the final hydrochar. Carbon, nitrogen and phosphorus are primarily retained in the hydrochar, which could favor its use as soil ameliorant instead of an energy carrier.     

Effect of low digestate recirculation ratio on biofuel and bioenergy recovery in a two-stage anaerobic digestion process

A relatively high (0.2–4.3) digestate recirculation ratio (RR) is typically adopted to raise the pH and provide the dark fermentation reactor (DF) with alkalinity and hydrogen-producing microorganisms in a two-stage anaerobic digestion process. This study examined the production of bio-H₂ and bio-CH₄ from readily biodegradable organic waste in a large scale recirculated two-stage thermophilic anaerobic system to determine the effect of low RR on biofuel and bioenergy recovery. The performance of the two-stage system was evaluated at 2 hydraulic retention times (HRT) (1.1 and 2.5 d) in DF and 4 RR (0, 0.11, 0.18 and 0.25). The pH in DF was not controlled and ranged from 3.8 to 4.2. Hydrogen yield was negatively affected by digestate recirculation, while CH₄ yield, as well as H₂ and CH₄ production rates, first tended to increase and then decrease with increasing RR. Overall, biofuel and bioenergy were best recovered at an RR of 0.11, namely 1.48 L H₂/L/d, 0.88 L CH₄/L/d, 106.2 mL H₂/g VS_init.,161.3 mL CH₄/g VS_init., 7.7 kJ/g VS_init. and 88.2 kJ/L/d were obtained depending on HRT in DF. It has been shown that a low RR can improve the performance of the two-stage anaerobic digestion process.     

Improving methane production from wheat straw by digestate liquor recirculation in continuous stirred tank processes

Wheat straw is an abundant, cheap substrate that can be used for methane production. However, the nutrient content in straw is inadequate for methane fermentation. In this study, recycling digestate liquor was implemented in single-stage continuous stirred tank processes for enrichment of the nutrient content of straw with the aim of improving the methane production. The VS-based organic loading rate was set at 2 g/(L d) and the solid retention time at 40 days. When wheat straw alone was used as the substrate, the methane yields achieved with digestate liquor recycling was on average 240 ml CH₄/g VS giving a 21% improvement over the processes without recycling. However, over time, the processes suffered from declining methane yields and poor stability evidenced by low pH. To maintain process stability, wheat straw was co-digested with sewage sludge or supplemented with macronutrients (nitrogen and phosphorous). As a result, the processes with digestate liquor recycling could be operated stably, achieving methane yields ranging from 288 to 296 ml CH₄/g VS. Besides, the processes could not be operated sturdily with supplementation of macronutrients without digestate liquor recycling. The highest methane yield (296 ± 16 ml CH₄/g VS) was achieved by co-digestion with sewage sludge plus recycling of digestate liquor after filtration (retention of nutrients and microorganisms). This was comparable to the maximum expected methane yield of 293 ± 13 ml CH₄/g VS achieved in batch test. The present study therefore demonstrated that digestate liquor recycling could lead to a decreased dilution of vital nutrients from the reactors thereby rendering high process performance and stability.     

Environmental assessment of electricity generation from an Italian anaerobic digestion plant

A standard ISO Life Cycle Assessment study was carried out to evaluate the environmental sustainability of electricity production from an anaerobic digestion (AD) plant using a mixture of dedicated energy crops, agricultural residues and livestock effluents as input materials. The functional unit was 1 MJ of electricity. System boundaries were from cradle to grave and covered all the phases from energy crops cultivation to the production of biogas and its use in a Combined Heat and Power plant to produce electricity. Liquid and solid digestate storage and spreading on agricultural land were included. Primary data were collected from the AD plant for all the above phases. Since heat produced is used only internally, no allocation was applied in the study. As regards digestate management, CH₄ emissions were calculated from literature, whereas four literature methods were applied for calculation of nitrogen emissions with the goal to perform a sensitivity analysis on LCA results. ILCD Handbook impact assessment methodologies were used. Results show that the main hotspots are energy crops cultivation and the management of digestate, mainly because of both nitrogen and methane emissions, affecting Global Warming, Acidification, Marine and Freshwater Eutrophication. Finally, a detailed Monte Carlo analysis, was carried out to evaluate the results uncertainty. The study represents the state of the art about the environmental performance of the AD plant with the use of sensitivity and uncertainty analysis, which both improve the reliability of results, and allows drawing general conclusions on how to mitigate the environmental impacts of AD process.     

Assessment of biofertilizer quality and health implications of anaerobic digestion effluent of cow dung and chicken droppings

Anaerobic digestate have been identified as a rich source of essential plant nutrients. Nevertheless, its safety measured by the concentration of pathogen present is of great concern to end users. This research explored the efficiency of the mesophilic biodigestion process in the stabilization and sanitization of cow dung and chicken droppings. Six (6) kg each of cow dung and chicken droppings were collected fresh and free from impurities, pre-fermented, mixed with water in the ratio 1:1 w/v to form slurry, fed into the respective reactors and digested for 30 days at an average ambient temperature of 30 ± 2 °C. The pH of the medium fluctuated between 6.5 and 8.0. The analysis of the feedstock and effluent of the digesters showed that a total solids reduction of 75.3% and 60.1% were recorded for cow dung and chicken droppings while the reduction in total coliforms was 95% and 70% respectively for the dung and droppings. Microbial analysis of the biofertilizer produced reveals both aerobic and anaerobic organisms which include species of Pseudomonas, Klebsiella, Clostridium, Bacillus, Bacteroides, Salmonella, Penicillum and Aspergillus. Escherichia coli and Shigella spp. were removed while species of Salmonella and Klebsiella were still present in the digestate. Notwithstanding these results, the digestate still requires further treatment for it to be suitable for application on unrestricted crops either as fertilizer; otherwise a health problem would be created as attempt is made to improve soil fertility.     

Modeling of air gasification of dark fermentation digestate in a downdraft gasifier

The paper presents the results of numerical simulation of the gasification process in a downdraft gasifier to produce syngas with high hydrogen content. For the first time, the possibility of using dark fermentation digestate as a feedstock for thermochemical conversion using air as an oxidizer at equivalence ratio (ER) of 0.45, 0.55 and 0.65 was investigated. Modeling of the gasification process was carried out in the software package Comsol Multiphysics. As a result of numerical studies, the concentrations of the main components of the syngas were obtained. The syngas yield at air gasification was 1.8 m³/kg. At the same time, the combustion heat of the generated gas varied from 3.1 to 3.9 MJ/m³ with the molar ratio (MR) being in the range from 3.1 to 3.9. The maximum content of hydrogen (26.94%) in syngas was achieved at an ER of 0.45. The hydrogen production efficiency HPE ranged from 23.8 to 27.3%. The thermal power that can be obtained from the syngas ranges from 47 to 59 kW. Carbon conversion efficiency coefficient (CCE) was 23.6–28.8%. Based on the design calculation, the main geometric parameters of a downdraft gasifier for the production of syngas from anaerobic digestates were obtained.     

Development of a new cleaner production process for cassava ethanol

A new cleaner production process for cassava ethanol has been developed, in which the thin stillage by-product was treated initially by anaerobic digestion, and the digestate further processed by hydrogen-form cation exchange resin before being recycled as process water to make mash for the next ethanol fermentation batch. Thuswastewaterwas eliminated and freshwater and energy consumptionwas significantly reduced. To evaluate the new process, ten consecutive batches of ethanol fermentation and anaerobic digestion at lab scale were carried out. Average ethanol production in the recycling batches was 11.43% (v/v) which was similar to the first batch,where deionized (DI)waterwas used as process water. The chemical oxygen demand (COD) removal rate reached 98% and the methane yield was 322 ml per gramof COD removed, suggesting an efficient and stable operation of the anaerobic digestion. In conclusion, the application of the new process can contribute to sustainable development of the cassava ethanol industry.