Two-step anaerobic digestion of rice straw with nanobubble water

Metagenomic analysis reveals metabolic mechanism of enhancing lignocellulosic anaerobic digestion mediated by CO2/O2-nanobubble water

Nanobubble water (NW) has been reported to enhance anaerobic digestion (AD), but its influence on the metabolic pathways of microorganisms remains unclear. In this study, the specific methane yields of rice straw in the CO2NW and O2NW treatments increased by 6.9% and 18.3%, respectively. The electron transport system (ETS) and coenzyme F420 activities were enhanced by the addition of NW. Metagenomic analysis showed that the abundances of most enzymes in the acidification were significantly increased by both CO2NW and O2NW. Regarding methanogenesis, CO2NW promoted the expression of genes encoding enzymes of hydrogenotrophic methanogenesis, while O2NW stimulated both the acetoclastic and hydrogenotrophic methanogenesis. With the addition of O2NW, the expressions of modules related to the tricarboxylic acid (TCA) cycle and oxidative phosphorylation were enhanced, resulting in increased ATP production. This study provided fundamental evidence of the metabolic pathways of microorganisms mediated by NW at each stage of AD.

Effects of nanobubble water on digestate soaking hydrolysis of rice straw

This study investigated the performance of combined nanobubble water (NW) and digestate in the soaking hydrolysis process. Two types of NW (CO2NW and O2NW) with digestate were used to soak rice straw for 1, 2, 3, 5, and 7 days. During soaking process, the volatile fatty acids (VFA) concentration in the treatment with O2NW and digestate for 3 days (O2NW-3 d) reached 7179.5 mg-HAc/L. Moreover, the highest specific methane yield (SMY) obtained in this treatment could reach 336.7 NmL/gVS. Although the addition of NW did not significantly increase SMY from digestate soaking, NW could accelerate the rate of methane production and reduce digestion time of T80. The enrichment of Enterobacter in the soaking process was observed when using CO2NW and O2NW as soaking solutions which played important roles in VFA production. This study provides a new insight into environment-friendly enhanced crop straw pretreatment, combining NW and digestate soaking hydrolysis.

Effect of nanobubble water on medium chain carboxylic acids production in anaerobic digestion of cow manure

Nanobubble water promotes the degradation of difficult-to-degrade organic matter, improves the activity of electron transfer systems during anaerobic digestion, and optimizes the composition of anaerobic microbial communities. Therefore, this study proposes the use of nanobubble water to improve the yield of medium chain carboxylic acids produced from cow manure by chain elongation. The experiment was divided into two stages: the first stage involved the acidification of cow manure to produce volatile acidic fatty acids as electron acceptors, and the second phase involved the addition of lactic acid as an electron donor for the chain elongation. Three experimental groups were established, and air, H2, and N2 nanobubble water were added in the second stage. Equal amounts of deionized water were added in the control group. The results showed that nanobubble water supplemented with air significantly increased the caproic acid concentration to 15.10 g/L, which was 55.03 % greater than that of the control group. The relative abundances of Bacillus and Caproiciproducens, which are involved in chain elongation, and Syntrophomonas, which is involved in electron transfer, increased. The unique ability of air nanobubble water supplemented to break down the cellulose matrix resulted in further decomposition of the recalcitrant material in cow manure. This effect subsequently increased the number of microorganisms associated with lignocellulose degradation, increasing carbohydrate metabolism and ATP-binding cassette transporter protein activity and enhancing fatty acid cycling pathways during chain elongation. Ultimately, this approach enabled the efficient production of medium chain carboxylic acids.

Enhancement of anaerobic digestion of rice straw by amino acid-derived ionic liquid

This study proposes a novel method to enhance methane production from anaerobic digestion using an amino acid-derived ionic liquid, glycine hydrochloride, ([Gly][Cl]), as an exogenous additive. After 40 days of digestion with 5% [Gly][Cl], the cumulative methane production was 115.56 mL/g VS, which was 73% higher than that of the control group (without additive). Specifically, the peak activities of cellulase, xylanase, and lignin peroxidase were significantly higher than those of the control group. The addition of [Gly][Cl] increased bacterial diversity and reduced archaeal diversity. Synergistota represented by Syner-01, Fibrobacterota represented by BBMC-4, Bacteroides, and unclassified_f__Lachnospiraceae significantly increased in relative abundance. It suggested that [Gly][Cl] stimulated the activities of protein-hydrolyzing and acid-producing bacteria. [Gly][Cl] also increased the abundance of methanogens and archaea, converting more lignocellulose to methane. Methanobacterium, that metabolizes H₂ and CO₂ to CH₄, was more abundant. Therefore, [Gly][Cl] can improve methane yield as an anaerobic digestion additive.