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Currently, the volume of waste generation is growing at a high rate. Anaerobic digestion is an effective way to process organic waste to produce biogas. To increase the bioavailability and efficiency of mass transfer between feedstock particles and microorganisms, it is advisable to pretreat organic waste using various methods. One of the most promising and energy-efficient methods for preparing a feedstock for anaerobic bioconversion is its processing in a vortex layer apparatus (VLA). However, there are some limitations in the VLA operation. Therefore, to exclude the stagnant central zone from the volume of the VLA working chamber and increase the magnitude of the magnetic field in the VLA working chamber, a ferromagnetic core in the form of a steel pipe was coaxially mounted. Thus, the purpose of this work is to experimentally study the effect of the ferromagnetic core of the VLA working chamber on the production of biogas during anaerobic bioconversion of a model of organic waste from the agricultural sector. To achieve this goal, an experimental plant was developed and created. The experimental data obtained suggest the high efficiency of pretreatment of the feedstock in VLA with a ferromagnetic core in the working chamber before anaerobic bioconversion. The developed anaerobic bioconversion system made it possible to increase the methane production rate by 287 % at hydraulic retention time (HRT) of 4 days and by 3.5 times at HRT of 2 days compared to the control. At the same time, the methane yield at HRT of 4 days increased by 43 %, and at HRT of 2 days it decreased by 14 %. The hydrogen sulfide content in biogas also increased more than three times, while no hydrogen was detected in the biogas. Thus, pretreatment of the feedstock in VLA with a ferromagnetic core to produce biohythane in a one-stage anaerobic bioconversion system at 55.1 °C is impractical. [ABSTRACT FROM AUTHOR]

The development of net-zero emission fuels is a priority area of modern research due to the imminent reduction of fossil fuel reserves and environmental problems caused by their combustion. One of the promising fuels is hydrogen, which has a high heat of combustion and is eco-friendly, forms water as the only byproduct. Recently, methods of hydrogen production by microorganisms, which use directly the solar energy or utilize the organic waste during fermentation, have been intensively developed and applied. In this review, the basic principles of the main light-dependent (biophotolysis, photofermentation) and light-independent (dark fermentation and microbial electrolysis) methods of biological hydrogen production are discussed. Particular attention is paid to the advantages and disadvantages of these methods, the possibility of combining them into a single system, as well as various strategies for improving biohydrogen production aimed at transition from laboratory research to full-scale application. [ABSTRACT FROM AUTHOR]

The hydrogen-producing bacterium SP-H2 was isolated from a thermophilic acidogenic reactor inoculated with municipal sewage sludge and processing a carbohydrate-rich simulated food waste. Based on the 16S rRNA gene sequence, the bacterium was identified as Thermoanaerobacterium thermosaccharolyticum. The maximum growth rate was observed at 55–60 °C and pH 7.5. The H 2 -producing activity of the bacterium was studied using mono-, di- and tri-saccharides related to both hexoses (maltose, glucose, mannose, fructose, lactose, galactose, sucrose, raffinose, cellobiose) and pentoses (xylose and arabinose), as well as using real wastewaters (cheese whey, confectionery wastewater, sugar-beet processing wastewater). The highest H 2 yield was observed during dark fermentation (DF) of maltose (1.91 mol H 2 /mol hexose or 77.8 mmol H 2 /L). The maximum H 2 production rate was observed during DF of xylose (13.3 ml H 2 /g COD/h) and cellobiose (2.47 mmol H 2 /L/h). The main soluble metabolite products were acetate, ethanol and butyrate. The acetate concentration had a statistically significant positive correlation with the H 2 content in biogas and the specific H 2 yield. Based on the results of the correlation analysis, it was tentatively assumed that in the formic acid (mixed-acid) type fermentation, the rate of H 2 production was higher than in the butyric acid type fermentation. With regard to real wastewater, cheese whey and confectionery wastewater were distinguished by a higher H 2 yield (152 ml H 2 /g COD) and H 2 production rate (0.57 mmol H 2 /L/h), respectively. The highest concentrations of confectionery wastewater and cheese whey, at which the DF process took place, were 5915 and 7311 mg COD/L, respectively. At the same time, SP-H2 dominated in the microbial community, despite the presence of indigenous microorganisms in wastewater. Thus, T. thermosaccharolyticum SP-H2 is a promising strain for DF of carbohydrate-rich unsterile wastewater under thermophilic conditions. [ABSTRACT FROM AUTHOR]

Although two-stage anaerobic digestion (TSAD) is not a novel process, information of the process stability, microbial community and effective operating conditions is limited and often contradictory. In this work, the influence of different carrier materials (polyurethane foam, carbon felt, Raschig rings and a combination of carbon felt and Raschig rings) on the performance of the thermophilic TSAD of dairy wastewater was studied. The organic loading rate (OLR) in the acidogenic reactor (RH) was gradually increased from 13.74 to 32.56 g COD/(L d), and from 0.64 to 11.46 g COD/(L d) in the methanogenic reactors by correspondingly reducing the hydraulic retention time (HRT). The highest hydrogen production rate of 1280.3 mL/(L·d) and hydrogen yield of 93.2 mL/g COD were achieved at OLR of 13.74 g COD/(L·d), but hydrogen production stopped at higher OLR. The main soluble metabolites in RH were butyrate and lactate, and the microbial community was dominated by Streptococcus , Thermoanaerobacterium , Veillonellales-Selenomonadales and Pseudomonas. The highest methane production rate (2674 mL/(L·d) at HRT of 0.5 days) was observed in the reactor with polyurethane foam, while the highest methane yield (305.5 mL/g COD at HRT of 1.5 days) was obtained in a reactor containing carbon felt. Spirochaetaceae , Desulfomicrobium , Anaerolineaceae , Candidatus Caldatribacterium and Cloacimonadaceae W5 were linked to these materials and explained the highest methanogenic performance. [ABSTRACT FROM AUTHOR]

Biohythane production through anaerobic digestion of organic waste in a single reactor is a potentially more cost-effective process compared to the traditional two-stage scheme. In this work, the production and composition of biohythane obtained from complex organic waste in a semi-continuous thermophilic anaerobic bioreactor was studied with a gradual increase in hydraulic retention time (HRT) from 2 to 5 days and a corresponding decrease in the organic loading rate from 12.27 to 4.8 g VS/(L·d). At low HRT, a significant predominance of hydrogen content in biogas was observed. However, with an increase in HRT to 5 days, the volumetric and specific yield of methane increased. The modified Gompertz model and the first order model showed a high coefficient of determination (R2 = 0.97–1) for the calculated kinetic parameters of hydrogen and methane formation. At all values of HRT, the predominance of the hydrogen-producing genus Thermoanaerobacterium was observed in the microbial community. When HRT was increased to 5 days, representatives of methanogenic archaea of the genera Methanosarcina and Methanosaeta actively developed. The hydrogen and methane content in the final biogas mixture was approximately 37 % and 30 %, respectively. Potential research avenues for improving biohythane production in a single reactor have been considered. [ABSTRACT FROM AUTHOR]

Agricultural waste, called biomass, is of great interest as a renewable energy source. The purpose of this work was to determine the optimal recirculation rate (RR) values of the solid digestate when applying a new approach to the intensification of anaerobic conversion, providing a high quality biogas yield. This approach includes solid digestate recirculation and pre-processing a mixture of raw waste and solid digestate in a vortex layer apparatus. For this, the operation of an experimental biogas plant without recirculation and with recirculation was considered, starting from very low RR values, from 0.034 to 1, to ensure a high methane production rate (MPR) and methane yield (MY). From the point of view of MPR and MY, the recirculation ratios of 0.034 and 0.071 look the most attractive, however, with a recirculation ratio of 0.5, the content of hydrogen sulfide de-creased to 14 ppm with a simultaneous increase in MPR by 12 % compared to the control. [ABSTRACT FROM AUTHOR]

The hydrogen-producing bacteria SP4 and SP6 were isolated from the compost and identified by 16S rRNA gene sequencing as Clostridium butyricum and Clostridium beijerinckii , respectively. A comparative study on the biohydrogen-producing activity of the isolated strains was carried out using mono-, di- and tri-saccharides belonging to both hexoses (maltose, glucose, mannose, fructose, lactose, galactose, sucrose, raffinose, cellobiose) and pentoses (xylose). To assess the biotechnological significance, real wastewater rich in sugars (cheese whey, confectionery wastewater, sugar beet processing wastewater) was also used as a substrate. C. butyricum SP4 fermented sugars with a yield of 0.93–1.52 mol H 2 /mol hexose (pentose); the maximum yield was obtained from fructose, the minimum – from raffinose and cellobiose. The most preferred substrate for C. beijerinckii SP6 was sucrose with a yield of 1.76 mol H 2 /mol hexose, while cellobiose yielded only 0.64 mol H 2 /mol hexose. Overall, the efficiency of converting wastewater to H 2 by C. butyricum SP4 was also slightly lower (66–93 ml H 2 /g chemical oxygen demand (COD)) than that of C. beijerinckii SP6 (76–103 ml H 2 /g COD). Even though the main soluble metabolite products (SMPs) for both isolates were acetate and butyrate, C. butyricum SP4 also produced a significant amount of ethanol (up to 21.5% of SMPs) and formate (up to 32.5% of SMPs), and C. beijerinckii SP6 – lactate (up to 25% of SMPs). A distinctive feature of C. beijerinckii SP6 was a significantly lower (almost 2 times) yield of SMPs, while C. butyricum SP4 had a higher rate of H 2 production according to the results obtained from the kinetic study using the modified Gompertz equation and the first order equation. Analysis of Spearman's rank correlation coefficients revealed a statistically significant relationship between the kinetic parameters of H 2 production and the concentration of butyrate and the final pH of the medium for C. butyricum SP4, and with the concentration of ethanol for C. beijerinckii SP6. These findings provide valuable information on the metabolic capabilities of the most studied hydrogen-producing representatives of the Clostridium genus for their use in optimizing the technology for biohydrogen production by dark fermentation of various organic wastes. [ABSTRACT FROM AUTHOR]

Two-stage anaerobic digestion is a promising way to increase the efficiency of decomposition of organic waste. However, spatial separation of stages makes this technology less economically attractive. In this work, we studied a model of two-stage anaerobic digestion with stages separated in time in one reactor. To implement this model, we used a combination of (1) bioaugmentation with a hydrogen-producing Thermoanaerobacterium thermosaccharolyticum SP-H2, resistant to low pH, (2) setting a low initial pH (5.5) to reduce the activity of hydrogenotrophic methanogens, (3) supplementing additives in the form of soluble iron (II) and granular activated carbon (GAC) to stimulate hydrogenase activity and direct interspecies electron transfer (DIET), respectively. For comparison, two methanogenic inoculums with different dominant microbial groups and initial iron concentrations were used. With the simultaneous addition of GAC and iron (II), sequential production of hydrogen and methane was observed for both methanogenic inoculums. Without the addition of GAC, methane formation was practically not observed, which, apparently, indicated the impossibility of syntrophic degradation of acidogenesis products due to the high partial pressure of hydrogen and the absence of conductive material for activation of DIET. Significant differences in hydrogenase activity and the kinetics of hydrogen and methane formation according to the modified Gompertz equation depending on the iron content in the system may indicate the importance of this additive. Further studies should be aimed at determining the optimal initial pH, concentrations of stimulating additives, bioaugmentation and methanogenic cultures, as well as confirming the stability of the proposed single-reactor two-stage anaerobic digestion system in semi-continuous mode. [ABSTRACT FROM AUTHOR]

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-H 2 and bio-CH 4 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 CH 4 yield, as well as H 2 and CH 4 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 H 2 /L/d, 0.88 L CH 4 /L/d, 106.2 mL H 2 /g VS init. ,161.3 mL CH 4 /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. [ABSTRACT FROM AUTHOR]

The work investigated the characteristics of the dark fermentation (DF) process of a number of simple (starch, sunflower oil, peptone, both separately and mixed) and complex (dog food, pig feed, sewage sludge) substrates using a mixed culture of microorganisms, with a controlled pH (5.5), at 55 °C. Peptone and sunflower oil were characterized by the lowest production of H 2 , namely 5.0 and 2.3 ml H 2 /g COD, respectively. The specific hydrogen yield from starch was 1.55 mol H 2 /mol hexose. The addition of peptone and sunflower oil to starch reduced the specific yield of hydrogen from starch by 23%. A large difference in hydrogen production was observed during DF of complex substrates. The specific hydrogen yield from dog food was 46.5 ml H 2 /g COD or 143.4 ml H 2 /g carbohydrates; from pig feed – 32.1 ml H 2 /g COD or 91.6 ml H 2 /g carbohydrates; and from sewage sludge – 9.3 ml H 2 /g COD or 98.0 ml H 2 /g carbohydrates. Possible relationships between the biopolymer composition of substrates and characteristics of the DF process were analyzed using Spearman's rank correlation coefficients. The concentration of carbohydrates, as well as the ratio of carbohydrates/proteins and carbohydrates/fats, were the main factors influencing the high specific yield of H 2 , its content in biogas, as well as the ratio of H 2 /soluble metabolites. The concentration of proteins had a statistically significant positive effect on the accumulation of acetate and succinate, and carbohydrates - on the accumulation of caproate. [ABSTRACT FROM AUTHOR]