Changes in microbial community during biohydrogen production using gamma irradiated sludge as inoculum

Fruit peel crude enzymes for enhancement of biohydrogen production from synthetic swine wastewater by improving biohydrogen-formation processes of dark fermentation

Biohydrogen is a promising clean fuel but with a low yield. This study aims to enhance biohydrogen production from synthetic swine wastewater by employing crude enzymes obtained from different fruit peels (orange, mandarin, and banana) to improve the biohydrogen-formation processes of dark fermentation. Results indicated that dosing with crude enzymes affected volatile fatty acids (VFAs) and biogas composition insignificantly, while increased biohydrogen yield from 1.62 ± 0.00 (blank) to 1.90 ± 0.08 (orange peel), 2.01 ± 0.00 (mandarin peel), and 1.96 ± 0.01 (banana peel) mol H₂/mol glucose, respectively. Banana peel crude enzyme was the most effective additive, with 1 g/L protein improving 97.41 ± 3.72 % of biohydrogen yield. The crude enzymes wielded less influence on acetic acid and butyric acid pathways but enhanced other biohydrogen production pathways. These observations demonstrated that fruit peel-based crude enzymes as additives are advantageous to improving biohydrogen yield towards higher biohydrogen production.

Investigating changes in the characteristics of antibiotic resistance genes at different reaction stages of high solid anaerobic digestion with pig manure

Contamination of antibiotic resistance genes (ARGs) from animals is a serious issue as they may spread to human pathogenic bacteria. The reduction of ARG contamination from livestock waste is thus essential. High solid anaerobic digestion (HSAD) is a new and effective technology although some aspects, such as change characteristics of ARGs at different reaction stages, have not been fully investigated. This study focused firstly on the variations in ARGs at different reaction stages in HSAD systems with five different starting methods: 1 natural start (the control) and 4 rapid starts by changing leachate reflux forms. The results showed that the rapid starting methods could accelerate start-up and increase biogas production by 312.5%. The starting and acidification stages showed higher microbial richness and diversity compared with the other stages. ARGs found to be reduced at acidification stage. Variation in ARGs at the starting and acidification stages was mainly driven by a combination of microbial community, mobile genetic elements (MGEs), and environmental factors; while the main contributory factors at the gas production stage were biomass and several unexplained factors. At the ending stage, the main driving factors were biomass and microbial communities. Most of the potential hosts (16/20) of the ARGs belonged to the Firmicutes phylum, which showed the lowest connections with the ARGs at the gas production stage.

Inhibitory effect of acetic acid on dark-fermentative hydrogen production

This study examined the working mechanisms of acetic acid inhibition on dark fermentative hydrogen production. It was found that undissociated acetic acid (UAA) concentration was the critical factor in acetic acid inhibition. Hydrogen production activity decreased by 50 % and 90 % when UAA concentrations was 76.3 mg/L (1.27 mmol/L) and 686.7 mg/L (11.44 mmol/L), respectively. Dominant microbes were changed from genus Clostridium_sensu_stricto_1 to genus Inhella, Aquabacterium and Caulobacter under the stress of acetic acid inhibition. Functional enzyme analysis showed that acetic acid inhibited the hydrogen production by activating the lactate formation pathway when UAA concentration was below the inhibition threshold, while by impairing most hydrogen-producing pathways when UAA concentration was over the inhibition threshold. In brief, acetic acid inhibited the hydrogen production by altering the dominant microbial community and regulating the metabolic pathways, controlling the UAA concentration would be a good strategy to alleviate the acetic acid inhibition.

Enhanced medium-chain fatty acids production from Cephalosporin C antibiotic fermentation residues by ionizing radiation pretreatment

Antibiotic fermentation residues (AFRs) have been classified as hazardous waste in China. Anaerobic fermentation may be a good approach for AFRs treatment, through which value-added chemicals could be obtained simultaneously. This study firstly explored medium-chain fatty acids (MCFAs) production from AFRs through two-stage anaerobic fermentation, and gamma radiation was adopted for AFRs pretreatment. The results showed that both antibiotics removal and MCFAs production from AFRs were significantly promoted by gamma radiation pretreatment. No residual Cephalosporin C (CEP-C) was detected in gamma radiation treated groups after fermentation. Highest MCFAs concentration of 90.55 mmol C/L was obtained in 50 kGy treated group, which was 2.22 times of the control group. Genera that were positively correlated with MCFAs production were enriched in gamma radiation treated groups, like genus Paraclostridium, Terrisporobacter, Caproiciproducens and Sporanaerobacter, while genera that were negatively correlated with MCFAs production were diminished during the chain elongation process, like genus Bacteroides and NK4A214_group. Enzymes analysis suggested that the promoted MCFAs production was induced by the enrichment of functional enzymes involved in Acetyl-CoA formation and RBO pathway. This work suggested that gamma radiation pretreatment and two-stage anaerobic fermentation could achieve the dual benefits of AFRs treatment and value-added chemicals recovery.

Improved biohydrogen evolution through calcium ferrite nanoparticles assisted dark fermentation

Dark fermentation (DF) is a green hydrogen (H₂) production process, but it is far below the theoretical H₂ yield. In this study, calcium ferrite nanoparticles (CaFe₂O₄ NPs) were produced to augment H₂ yield via DF. The highest H₂ yield of 250.1 ± 6.5 mL/g glucose was achieved at 100 mg/L CaFe₂O₄ NPs. Furtherincreasein CaFe₂O₄ NPs above 100 mg/L, such as 600 mg/L, would slightly lower H₂ yield to 208.6 ± 2.6 mL/g glucose. The CaFe₂O₄ NPs in DF system released calcium and iron ions, promoting granular sludge formation andDF microbial activity. Soluble metabolites revealed that butyric acid was raised by CaFe₂O₄ NPs, which indicated the improved metabolic pathway for more H₂. Microbial structure composition further illustrated that CaFe₂O₄ NPs could increase the abundance of dominant microbial populations, with the supremacy of Firmicutes up to 71.22 % in the bioH₂ evolution group augmented with 100 mg/L CaFe₂O₄ NPs.

Rhizosheath: An adaptive root trait to improve plant tolerance to phosphorus and water deficits?

Drought and nutrient limitations adversely affect crop yields, with below-ground traits enhancing crop production in these resource-poor environments. This review explores the interacting biological, chemical and physical factors that determine rhizosheath (soil adhering to the root system) development, and its influence on plant water uptake and phosphorus acquisition in dry soils. Identification of quantitative trait loci for rhizosheath development indicate it is genetically determined, but the microbial community also directly (polysaccharide exudation) and indirectly (altered root hair development) affect its extent. Plants with longer and denser root hairs had greater rhizosheath development and increased P uptake efficiency. Moreover, enhanced rhizosheath formation maintains contact at the root-soil interface thereby assisting water uptake from drying soil, consequently improving plant survival in droughted environments. Nevertheless, it can be difficult to determine if rhizosheath development is a cause or consequence of improved plant adaptation to dry and nutrient-depleted soils. Does rhizosheath development directly enhance plant water and phosphorus use, or do other tolerance mechanisms allow plants to invest more resources in rhizosheath development? Much more work is required on the interacting genetic, physical, biochemical and microbial mechanisms that determine rhizosheath development, to demonstrate that selection for rhizosheath development is a viable crop improvement strategy.

Semi-solid state promotes the methane production during anaerobic co-digestion of chicken manure with corn straw comparison to wet and high-solid state

Total solid content (TS) is an important factor for biogas production during anaerobic digestion. In this study, we explored the influence of different TS (5% wet, 15% semi-solid and 25% solid state) on the relative cumulative methane production (RCMP) during anaerobic co-digestion of chicken manure with corn straw. Results showed that total ammonium nitrogen and free ammonia nitrogen concentration increased with the increase of TS. Ammonium nitrogen in treatments at 15% TS was 2.25-2.76 times as high as that at 5% TS, which was below 3 times. The highest chemical oxygen demand removal and RCMP were obtained in the treatment of 15% TS with a ratio of 2:1 chicken manure: corn straw (based on TS). The RCMP in the treatments of 15% TS were 3.63-4.59 times higher than that of 5% TS based on the volume of substrates. The abundance of Caldicoprobacter improving the degradation of corn straw was significantly positively correlated with the RCMP, and the average abundance of Caldicoprobacter at 15% TS was 8.33 and 7.02 times higher than that at 5% and 25% TS, respectively. Structural equation models analysis suggested that TS significantly impacted the RCMP by indirectly impacting free ammonia nitrogen and microbial abundance. These findings indicated semi-solid state (15% TS) decreased ammonia nitrogen releasing and improved the abundance of Caldicoprobacter, and increased RCMP during anaerobic co-digestion of chicken manure with corn straw.

The exacerbation of soil acidification correlates with structural and functional succession of the soil microbiome upon agricultural intensification

Agricultural intensification driven by land-use changes has caused continuous and cumulative soil acidification (SA) throughout the global agroecosystem. Microorganisms mediate acid-generating reactions; however, the microbial mechanisms responsible for exacerbating SA feedback remain largely unknown. To determine the microbial community composition and putative function associated with SA, we conducted a metagenomic analysis of soils across a chronosequence that has elapsed since the conversion of rice-wheat (RW) to rice-vegetable (RV) rotations. Compared to RW rotations, soil pH decreased by 0.50 and 1.56 units (p < 0.05) in response to 10-year and 20-year RV rotations, respectively. Additionally, acid saturation ratios were increased by 7.3% and 36.2% (p < 0.05), respectively. The loss of microbial beta-diversity was a key element that contributed to the exacerbation of SA in the RV. Notably, the 20-year RV-enriched microbial taxa were more hydrogen (H⁺)-, aluminium (Al³⁺)-, and nitrate nitrogen (NO₃^--N) -dependent and contained more genera exhibiting dehydrogenation functions than did RW-enriched taxa. "M00115, M00151, M00417, and M00004" and "M00531 and M00135" that are the "proton-pumping" and "proton-consuming" gene modules, respectively, were linked to the massive recruitment of acid-dependent biomarkers in 20-year RV soils, particularly Rhodanobacter, Gemmatirosa, Sphingomonas, and Streptomyces. Collectively, soils in long-term RV rotations were highly acidified and acid-sensitive, as the enrichment of microbial dehydrogenation genes allowing for soil buffering capacity is more vulnerable to H⁺ loading and consequently promotes the colonization of more acid-tolerant and acidogenic microbes, and ultimately provide new clues for researchers to elucidate the interaction between SA and the soil microbiome.

Hydroxyapatite Fabrication for Enhancing Biohydrogen Production from Glucose Dark Fermentation

Hydroxyapatite (HA) had the effect of maintaining the pH balance of the reaction system and promoting enzyme activity. In this work, hydroxyapatite was synthesized by coprecipitation and characterized for biohydrogen (bioH₂) production from glucose. The highest bioH₂ yield obtained was 182.33 ± 2.41 mL/g glucose, amended with an optimal dosage of 400 mg/L HA, which was a 55.80% higher bioH₂ yield compared with the control group without any addition. The results indicated that HA facilitated the deterioration of organic substances and increased the concentration of soluble microbial products (SMPs). Microbial community analysis revealed that HA significantly increased the abundance of Firmicutes from 35.27% (0 mg/L, HA) to 76.41% (400 mg/L, HA), which played an essential role in bioH₂ generation. In particular, the abundance of Clostridium sensu stricto 1 increased from 15.33% (0 mg/L HA) to 45.17% (400 mg/L HA) and became the dominant bacteria. The results also indicated that HA likely improves bioH₂ production from organic wastewater in practice.

Biohydrogen production by co-fermentation of antibiotic fermentation residue and fallen leaves: Insights into the microbial community and functional genes

This investigation explored the co-fermentation of antibiotic fermentation residue (AFR) and fallen leaves for enhancing biohydrogen production, and analyzed the mechanism from the aspects of microbial activity, microbial community and functional genes. The results showed that the optimal mixing ratio of AFR to leaves was 25:75 (VS basis), which balanced the substrate condition and synergistically enhanced the biohydrogen productivity, and the hydrogen yield was 37.45 mL/g-VS_added, which was 438.8% and 9.2% higher compared to the sole AFR fermentation and the sole leaves fermentation, respectively. The co-fermentation also improved the organics utilization and induced a more effective metabolic pathway. Further microbiology analysis found that the co-fermentation promoted the microbial activity, enriched more hydrogen-producing bacteria (Clostridium sensu stricto 1), and enhanced the expression of hydrogen-producing functional genes (e.g. genes encoding ferredoxin hydrogenase (EC 1.12.7.2) and pyruvate-ferredoxin oxidoreductase (EC 1.2.7.1)), which were fundamentally responsible for the synergistic biohydrogen fermentation.

Progress in osmotic membrane bioreactors research: Contaminant removal, microbial community and bioenergy production in wastewater

Renewable energy, water conservation, and environmental protection are the most important challenges today. Osmotic membrane bioreactor (OMBR) is an innovative process showing superior performance in bioenergy production, eliminating contaminants, and low fouling tendency. However, salinity build-up is the main drawback of this process. Identifying the microbial community can improve the process in bioenergy production and contaminant treatment. This review aims to study the recent progress and challenges of OMBRs in contaminant removal, microbial communities and bioenergy production. OMBRs are widely reported to remove over 80% of total organic carbon, PO₄^3-, NH₄⁺ and emerging contaminants from wastewater. The most important microbial phyla for both hydrogen and methane production in OMBR are Firmicutes, Proteobacteria and Bacteroidetes. Firmicutes' dominance in anaerobic processes is considerably increased from usually 20% at the beginning to 80% under stable condition. Overall, OMBR process has great potential to be applied for simultaneous bioenergy production and wastewater treatment.

Unexpected high production of biohydrogen from the endogenous fermentation of grape must deposits

The aim of this work was to assess the performances of wine byproduct biomass for hydrogen production by dark fermentation. Grape must deposits from two grape varieties (Pinot Gris and Chardonnay) were considered, either with external microbial inoculum or without. We show that grape must residues contain endogenous microflora, well adapted to their environment, which can degrade sugars (initially contained in the biomass) to hydrogen without any nutrient addition. Indeed, hydrogen production during endogenous fermentation is as efficient as with an external heat-treated inoculum (2.5 ± 0.4 L_H2.L^-1_reactor and 1.61 ± 0.41 mol_H2.mol^-1_{consumed hexose}, respectively) with a lower energy cost. Hydrogen-producing bacteria were selected from the endogenous microflora during semi-batch bioreactor operation, as shown by T-RFLP profiles and 16S rRNA sequencing, with Clostridium spp. (butyricum, beijerinckii, diolis, roseum) identified as the major phylotype. Such hydrogen production efficiency opens new perspectives for innovating in the valorization of winery by-products.

Dynamics of bacterial communities and substrate conversion during olive-mill waste dark fermentation: Prediction of the metabolic routes for hydrogen production

The aim of this work was to study the biological catalysts and possible substrate conversion routes in mesophilic dark fermentation reactors aimed at producing H₂ from olive mill wastewater. Bacillus and Clostridium were the most abundant phylotypes during the rapid stage of H₂ production. Chemical analyses combined with predictive functional profiling of the bacterial communities indicated that the lactate fermentation was the main H₂-producing route. In fact, during the fermentation process, lactate and acetate were consumed, while H₂ and butyrate were being produced. The fermentation process was rich in genes that encode enzymes for lactate generation from pyruvate. Lactate conversion to butyrate through the generation of pyruvate produced H₂ through the recycling of electron carriers via the pyruvate ferredoxin oxydoreductase pathway. Overall, these findings showed the synergy among lactate-, acetate- and H₂-producing bacteria, which complex interactions determine the H₂ production routes in the bioreactors.

Fate of antibiotic resistance genes during high-solid anaerobic co-digestion of pig manure with lignite

Lignite could be used to promote methane production during high-solid anaerobic co-digestion (HS-AcoD) of pig manure, however, the effects of lignite amendment on the fate of ARGs during HS-AcoD are unknown. Here, we explored the influence of lignite (0%, 8%, 16%, 32%, and 64%) on the fate of ARGs during HS-AcoD of pig manure. The results showed that 16% lignite reduced the absolute abundance of ARGs by 28.71% compared with the 0% lignite treatment. Variation partitioning analysis suggested the combined effect of microbial community, mobile genetic elements (MGEs) and environmental factors was the major driver shaping the pattern of ARGs. The potential hosts of ARGs were Bifidobacterium, Lactobacillus, Tissierella and Streptococcus. Structural equation models analysis suggested lignite indirectly impacted the pattern of ARGs by significantly reducing the abundance of microbial community and MGEs. These findings give an insight into the mechanistic understanding of the lignite influence on the reduction of ARGs during HS-AcoD.

Promoting the granulation process of aerobic granular sludge in an integrated moving bed biofilm-membrane bioreactor under a continuous-flowing mode

This investigation demonstrated that aerobic granular sludge (AGS) could be cultivated rapidly in a single continuous-flowing membrane bioreactor (MBR) by introducing freely moved bio-carriers with a filling ratio of 10%. By operating the bioreactor for 28 days, AGS was successfully cultivated and kept stable for >2 months with a compact structure and clear shape, in which, extracellular polymeric substances played a key role in maintaining the stability of granular sludge structure. The microbial composition between AGS and attached biofilm was quite different, which indicated that the introduced bio-carriers improved the biodiversity within the bioreactor. Additionally, an explicit internal circulation was formed by the introduced bio-carriers, which was the main reason leading to the rapid formation of AGS. This is an interesting discovery and a novel approach to promote the rapid granulation of biomass in an MBR. Moreover, combining the biodegradation of AGS and filtration of membrane module, the bio-reactor achieved an excellent performance in removing COD_Cr (>90%) and TN (>85%) during the whole process.

The reconstitution mechanism of napier grass microiota during the ensiling of alfalfa and their contributions to fermentation quality of silage

To reveal the reconstitution mechanism of exogenous microbiota and their contributions to fermentation quality during the early stage of alfalfa ensiling. The chopped alfalfa was treated with the following: distilled water (A1); napier grass microbiota (A1N); γ-ray radiation + distilled water (A0); γ-ray radiation + napier grass microbiota (A0N). Inoculating napier grass microbiota to non-irradiated alfalfa decreased the LA concentration, while enhanced the LA production of irradiated alfalfa during the 7 d of ensiling. Inoculating napier grass microbiota increased AA and ammonia-N contents and enhanced the decline of WSC for both non-irradiated and irradiated alfalfa silages. Enterococcus and Pediococcus dominated A1 silage. Leuconostocs and Lactobacillus constituted the majority of bacterial community in A0N, Lactobacillus rapidly became the predominated genera, while Lactobacillus, Leuconostocs, Enterococcus, and Pediococcus constituted the majority of bacterial community in A1N. Thus forage microbiota transplantation may be a potential practice to improve fermentation quality of less readily fermentable forages.

Antibiotic fermentation residue for biohydrogen production using different pretreated cultures: Performance evaluation and microbial community analysis

Antibiotic fermentation residue produced from pharmaceutical plants has been listed as a "Hazardous Waste", however it contains various substrates which can be used for biofuel production. In this study, the possibility of biohydrogen production from antibiotic fermentation residue was evaluated, the process efficiency and microbial community dynamics with five different inoculum pretreatments (alkaline, γ-radiation, heat-shock, aeration and acid) were assessed. Results showed that alkaline pretreatment was most efficient for hydrogen fermentation, and the hydrogen yield, volatile solids (VS) removal and maximal hydrogen production rate reached 17.8 mL/g-VS_added, 17.8% and 3.79 mL/h, respectively. Different inoculum pretreatments led to a obvious variation in the fermentation pathway and microbial community structure. The highest content of hydrogen-producing bacteria, especially Clostridium, essentially contributed to the highest hydrogen fermentation efficiency for the system with alkaline pretreatment. This investigation suggested that antibiotic fermentation residue is a potential feedstock for hydrogen production through dark fermentation.

Mechanisms of enhanced biohydrogen production from macroalgae by ferrous ion: Insights into correlations of microbes and metabolites

This study explored the mechanisms of the enhanced hydrogen production from macroalgae by Fe²⁺ supplementation. Highest hydrogen yield of 19.47 mL/g VS_added was achieved at Fe²⁺ supplementation of 400 mg/L, which was 6.25 times of the control test. In depth analysis of substrate degradation, microbial distribution and metabolites formation was conducted. The results showed that Fe²⁺-supplemented group was dominated by Clostridium butyricum (67.2%) and Ruminococcus gnavus (24.2%), which stimulated hydrogen generation and volatile organic acids accumulation. In contrast, Fe²⁺-deficient group had a microbial community dominated by Exiguobacterium sp. (29.0%), Acinetobacter lwoffii (24.5%) and Clostridium stricto 13 (23.4%), which induced higher efficiency of both biomass hydrolysis and mineralization. Microbes from a single system were mutually cooperative, while microbes from Fe²⁺-deficient and those from Fe²⁺-supplemented systems were mutually exclusive. This study suggested that Fe²⁺ is critical in macroalgae fermentation system to affect the microbial community structure and subsequently switch the metabolic pathways.

Enhanced biohydrogen production from macroalgae by zero-valent iron nanoparticles: Insights into microbial and metabolites distribution

In this work, effect of Fe⁰ nanoparticles (Fe⁰ NPs) on macroalgae fermentation was explored. Hydrogen production was significantly enhanced by 6.5 times comparing with control test, achieving 20.25 mL H₂/g VS_added with addition of 200 mg/L Fe⁰ NPs. In-depth analysis of substrate conversion showed that both hydrogen generation and acids accumulation were promoted with Fe⁰ NPs supplementation. Microbial analysis demonstrated that both hydrogen-producing strains belonging to genus Clostridium and Terrisporobacter sp. favorable for acids formation were enriched with Fe⁰ NPs supplementation, while species Acinetobacter lwoffii beneficial to organics mineralization was eliminated. Complex substrate compositions resulted in more prevalent cooperative relationships among species in the system. This study suggested that Fe⁰ NPs plays a crucial role in macroalgae fermentation by affecting the microbial distribution, subsequently influencing the products distribution and energy conversion.

Dark fermentation metabolic models to study strategies for hydrogen consumers inhibition

A Flux Balance Analysis (FBA) metabolic model of dark fermentation was developed for anaerobic mixed cultures. In particular, the model was applied to evaluate the effect of a specific inoculum pre-treatment strategy, addition of waste frying oil (WFO) on H₂-producing and H₂-consuming metabolic pathways. Productions of volatile fatty acid (VFAs), CO₂, H₂ and CH₄ measured through triplicate batch experiments, were used as constraints for the FBA model, to compute fluxes trough different metabolic pathways. FBA model could estimate the effect of pre-treatment with WFO on major microbial populations present in the mixed community (H₂ producing bacteria, homoacetogen and methanogens). Results revealed that low concentrations of WFO did not completely inhibited hydrogenotrophic methanogens. FBA showed that acetoclastic methanogens were more sensitive to WFO, in comparison to hydrogenotrophic methanogens. The proposed model can be used to study H₂ production by any other mixed microbial culture with similar substrates.

Pretreatment of macroalgal Laminaria japonica by combined microwave-acid method for biohydrogen production

Suitable pretreatment can effectively enhance the fermentative hydrogen production from algae biomass. In this study, combined microwave-acid pretreatment was applied to disintegrate the biomass of macroalgae L. japonica, and dark fermentation in batch mode was conducted for hydrogen production. The results showed that combining microwave pretreatment at 140 °C and 2450 MHz with 1% H2SO4 for 15 min could effectively disrupt macroalgal cells and release the organic matters, and soluble chemical oxygen demand (SCOD) concentration increased by 1.92-fold and achieved 5.12 g/L. During the fermentation process, both polysaccharides and proteins were consumed. Hydrogen production process was dominated by acetate-type fermentation, and the dominance of genus Clostridium contributed to more efficient hydrogen production. After the pretreatment, hydrogen yield increased from 15 mL/g TSadded to 28 mL/g TSadded, and energy conversion efficiency increased from 9.5% to 23.8%. Combined microwave-acid pretreatment is potential in enhancing hydrogen production from the biomass of L. japonica.

Improving mechanisms of biohydrogen production from grass using zero-valent iron nanoparticles

This paper investigated the improving mechanisms and microbial community dynamics of using zero-valent iron nanoparticles (Fe⁰ NPs) in hydrogen fermentation of grass. Results showed that Fe⁰ NPs supplement improved microbial activity and changed dominant microbial communities from Enterobacter sp. to Clostridium sp., which induced a more efficient metabolic pathway towards higher hydrogen production. Meanwhile, it is also proposed that Fe⁰ NPs could accelerate electron transfer between ferredoxin and hydrogenase, and promote the activity of key enzymes by the released Fe²⁺. The maximal hydrogen yield and hydrogen production rate were 64.7 mL/g-dry grass and 12.1 mL/h, respectively at Fe⁰ NPs dosage of 400 mg/L, which were 73.1% and 128.3% higher compared with the control group. Fe⁰ NPs also shorten the lag time and facilitated the hydrolysis and utilization of grass. This study demonstrated that Fe⁰ NPs could effectively improve hydrogen production and accelerate the fermentation process of grass.

Pretreatment of grass waste using combined ionizing radiation-acid treatment for enhancing fermentative hydrogen production

In this study, the combined ionizing radiation-acid pretreatment process was firstly applied to enhance hydrogen fermentation of grass waste. Results showed that the combined pretreatment synergistically enhanced hydrogen fermentation of grass waste. The SCOD and soluble polysaccharide contents of grass waste increased by 1.6 and 2.91 times after the combined pretreatment, respectively. SEM observation and crystallinity test showed the combined pretreatment effectively disrupted the grass structure. Owing to the more favorable substrate conditions, the hydrogen yield achieved 68 mL/g-dry grass_added after the combined pretreatment, which was 161.5%, 112.5% and 28.3% higher than those from raw, ionizing radiation pretreated and acid pretreated grass waste, respectively. The VS removal also increased from 13.9% to 25.6% by the combined pretreatment. Microbial community analysis showed that the abundance of dominant hydrogen producing genus Clostridium sensu stricto 1 increased from 37.9% to 69.4% after the combined pretreatment, which contributed to more efficient hydrogen fermentation.

Kinetics and microbial community analysis for hydrogen production using raw grass inoculated with different pretreated mixed culture

In this study, five pretreatment methods (heat shock, acid, base, aeration and gamma radiation) were applied for enriching hydrogen producers from anaerobically digested sludge, aiming to compare their hydrogen fermentation performance using raw ryegrass as substrate. Results showed that various pretreatment methods caused great variations on grass hydrogen fermentation performance. Acid pretreatment was most efficient compared with other tested pretreatment methods, with relevant hydrogen yield of 64.4mL/g dry grass and organics removal of 31.4%. Kinetics results showed that the first-order kinetic model fitted hydrogen evolution better than the modified Gompertz model. Microbiological analysis showed that various pretreatment methods caused great variations on microbial activity and microbial community composition. Clostridium and Enterococcus were two dominant genera, while relative abundances of these two genera varied greatly for different pretreated samples. Difference in microbial activity and microbial community distribution induced by the pretreatment methods might directly cause different ryegrass fermentation performance.

Pre-treatment technologies for dark fermentative hydrogen production: Current advances and future directions

Hydrogen is regarded as a clean and non-carbon fuel and it has a higher energy content compared to carbon fuels. Dark fermentative hydrogen production from organic wastes is the most promising technology for commercialization among chemical and biological methods. Using mixed microflora is favored in terms of easier process control and substrate conversion efficiencies instead of pure cultures. However, mixed cultures should be first pre-treated in order to select sporulating hydrogen producing bacteria and suppress non-spore forming hydrogen consumers. Various inoculum pre-treatments have been used to enhance hydrogen production by dark fermentation including heat shock, acid or alkaline treatment, chemical inhibition, aeration, irradiation and inhibition by long chain fatty acids. Regarding substrate pre-treatment, that is performed with the aim of enhanced substrate biodegradability, thermal pre-treatment, pH adjustment using acid or base, microwave irradiation, sonication and biological treatment are the most commonly studied technologies. This article reviews the most investigated pre-treatment technologies applied for either inoculum or substrate prior to dark fermentation, the long-term effects of varying pre-treatment methods and the subsequently feasibility of each method for commercialization.

Autoclaved sludge as the ideal seed to culture anammox bacteria: Reactor performance and microbial community diversity

Reducing activity of commensal bacteria in inocula may enhance anammox bacteria proliferation and realization of anammox process. Fast start-up of anammox process in an UASB reactor was successfully achieved by using autoclaved sludge (anaerobic granular sludge pretreated by autoclaving) and 0.3% active anammox sludge as inoculum. Continuous experiments indicated that R2 (autoclaved sludge addition) could shorten the start-up period from 72days to 63days. The first 50days anammox population specific growth rates (μ) of R1 (the control) and R2 were determined to be 0.014d^-1 and 0.045d^-1 using q-PCR assays. Analysis of coefficient of variations of nitrogen removal performance during days 96-225 indicated that R2 was more stable than R1. The Illumina MiSeq sequencing showed that autoclaving could decrease microbial diversity of sludge and enhance the abundance of anammox bacteria. Furthermore, PICRUSt community functions forecast and c-di-GMP measure illuminated the result of higher stability in R2.

Co-fermentation of sewage sludge with ryegrass for enhancing hydrogen production: Performance evaluation and kinetic analysis

The low C/N ratio and low carbohydrate content of sewage sludge limit its application for fermentative hydrogen production. In this study, perennial ryegrass was added as the co-substrate into sludge hydrogen fermentation with different mixing ratios for enhancing hydrogen production. The results showed that the highest hydrogen yield of 60mL/g-volatile solids (VS)_added was achieved when sludge/perennial ryegrass ratio was 30:70, which was 5 times higher than that from sole sludge. The highest VS removal of 21.8% was also achieved when sludge/perennial ryegrass ratio was 30:70, whereas VS removal from sole sludge was only 0.7%. Meanwhile, the co-fermentation system simultaneously improved hydrogen production efficiency and organics utilization of ryegrass. Kinetic analysis showed that the Cone model fitted hydrogen evolution better than the modified Gompertz model. Furthermore, hydrogen yield and VS removal increased with the increase of dehydrogenase activity.

Biological caproate production by Clostridium kluyveri from ethanol and acetate as carbon sources

Caproate is a valuable industrial product and chemical precursor. In this study, batch tests were conducted to investigate the fermentative caproate production through chain elongation from acetate and ethanol. The effect of acetate/ethanol ratio and initial ethanol concentration on caproate production was examined. When substrate concentration was controlled at 100mM total carbon, hydrogen was used as an additional electron donor. The highest caproate concentration of 3.11g/L was obtained at an ethanol/acetate ratio of 7:3. No additional electron donor was needed upon an ethanol/acetate ratio ≥7:3. Caproate production increased with the increase of carbon source until ethanol concentration over 700mM, which inhibited the fermentation process. The highest caproate concentration of 8.42g/L was achieved from high ethanol strength wastewater with an ethanol/acetate ratio of 10:1 (550mM total carbon). Results obtained in this study can pave the way towards efficient chain elongation from ethanol-rich wastewater.

Fast atrazine degradation by the mixed cultures enriched from activated sludge and analysis of their microbial community succession

In this study, fast atrazine degradation by the mixed bacterial cultures from sewage sludge was investigated. The acquired activated cultures showed great capability in atrazine degradation. The biodegradation process was well fitted into a pseudo-first reaction kinetic model. Atrazine could inhibit the propagation of ammonium oxidation bacteria and nitrite oxidation bacteria, decreasing the ammonium removal rate and the accumulation of nitrite. Only 162-172 reads of Nitrosomonadaceae and no Nitrospirales were detected after atrazine was exposed to the mixed cultures. The bacterial community structures in the cultures under different inoculation conditions (with or without atrazine) were investigated to explore the mechanism of atrazine degradation. Our results show that the genera Thiobacillus and Ferruginibacter were the most possible candidates responsible for the degradation of atrazine.