Insights into the roles of humic acids in facilitating the anaerobic digestion process

Humic acid-anchored hydrochar for enhancing methane production in anaerobic digestion of cow manure

This study assesses the effectiveness of two hydrochar variants-humic acid-anchored hydrochar and sodium hydroxide-modified hydrochar-in enhancing biogas production from high-solids anaerobic digestion of cow manure. The purpose of humic acid modification is that its abundant oxygen-containing functional groups promote direct interspecies electron transfer and improve microbial efficiency in anaerobic digestion. Humic acid-anchored hydrochar was prepared by anchoring humic acid to hydrochar. To further optimize the electron transfer capacity and structural properties of the hydrochar, the humic acid-anchored hydrochar was subsequently treated with sodium hydroxide to produce sodium hydroxide-modified hydrochar. The alkali modification effectively removes pore impurities and enhances the redox properties of the material, thereby improving the electron exchange between microorganisms. Experiments were conducted in 500 mL anaerobic serum bottles at a total solids content of 10 %. In the control group, high ammonia nitrogen concentrations inhibited methane production, yielding only 49.54 mL/g volatile solids. In contrast, the addition of sodium hydroxide-modified hydrochar increased cumulative methane production by 80.13 %, reaching 112.38 mL/g VS. Additionally, electron transfer system activity and coenzyme F420 levels increased 94.13 % and 96.58 %, respectively. Microbial analysis revealed an enrichment of bacteria involved in direct interspecies electron transfer and an optimized community structure. Correlation analysis demonstrated a significant positive relationship between enhanced interspecies electron transfer capacity and methane production. The incorporation of modified hydrochar enabled the anaerobic digestion system to maintain high methane yields despite elevated ammonia nitrogen levels. These findings offer valuable insights for improving livestock and poultry manure management and advancing environmental protection efforts.

Collaborative or competitive interactions between bacteria and methanogens on the biocorrosion of Q235A steel

Bio-corrosion of Fe (0) metals in the actual environments results from the combined action of multiple microbes rather than the single action of one type of microbe. Nevertheless, the interspecies interactions between the corrosive microorganism and co-existing microbes, as well as their effects on the bio-corrosion of Fe (0) metals, remain unclear, especially for the interspecies interactions between methanogens and co-existed bacteria in microbiota in the absence of sulfate. Herein, the interspecies interactions between methanogens and co-existed bacteria in three different kinds of methanogenic microbiota (Methanothrix, Methanospirillum, or Methanobacterium dominant) and their effects on methanogens-influenced corrosion of Q235A steel were investigated. The initial results showed that competitive interactions existed between Methanothrix/Methanospirllum and fermentative acetogenic bacteria (Clostridiaceae_1, Family_XI, Peptostreptococcaceae, Pirllulaceae, and Tannerellaceae), while collaborative interactions existed between Methanobacterium and acetate-oxidizing bacteria (Synergistaceae and Spirochaetaceae). Further analysis demonstrated that the competitive interactions obstructed the attachment of Methanothrix/Methanospirllum and promoted the formation of dense corrosion products layer on the steel surface, thereby inhibiting Methanothrix/Methanospirllum-influenced corrosion. Contrarily, the collaborative interactions promoted the attachment of Methanobacterium and the formation of porous and loose corrosion products layer on the steel surface, thereby promoting Methanobacterium-influenced corrosion. Ultimately, the corrosion rate of steel induced by the Methanobacterium dominant microbiota (0.216 ± 0.042 mm/y) was much higher than by the Methanothrix/Methanospirllum dominant microbiota (0.009-0.046 mm/y). This work provided new insights into the understanding of the effects of co-existed bacteria on the corrosion of Fe (0) metals induced by methanogens in microbiota.

The collaboration and competition between indigenous microorganisms and exogenous anaerobic digester sludge in anaerobic treatment of pickled mustard wastewater at different salinities

The highly concentrated pickled mustard wastewater presents significant potential for energy recovery, but the stress effect of high osmotic pressure on cell integrity and activity seriously impedes the methane production by anaerobic microorganisms. The survival ability of indigenous microorganisms (IM) in pickled mustard wastewater supports the establishment of anaerobic treatment. Moreover, inoculation of anaerobic digester sludge is a common start-up strategy. However, the effects of exogenous anaerobic sludge on IM are unclear, especially in hypersaline environment. This research aimed to investigate the influence of exogenous anaerobic sludge on the construction, performance, and microbiota at 3% and 5% salinity. And the research focused on the collaboration and competition between exogenous anaerobic sludge and IM. The neutral community model (which explains the formation and evolution of biological communities) indicated that the interaction between exogenous digester sludge microorganisms and IM dominated community assembly. At 3%, the digester sludge collaborated with IM to increase daily COD reduction and biogas production compared with IM group. However, at 5%, the competitive relationship reduced daily COD reduction and biogas production compared with IM group. This study provides a new perspective for the selection of inoculation strategies for exogenous anaerobic digester sludge under different salinity, in order to realize energy conversion from salinity organic wastewater.

Continuous augmentation of anaerobic digestion with electroactive microorganisms: Performance and stability

The performance and stability of a bioelectrochemical anaerobic digester (BeAD), continuously augmented with electroactive microorganisms (EAMs), were investigated. The BeAD showcased superior performance, sustaining the high COD removal efficiency and methane production rate of 76.5 % and 0.67 L/(L.d), respectively, in a stable state. Prominently, it exhibited remarkable resilience under hydraulic and organic shock loads, adeptly recuperating from disturbances up to 1000 % of its stable condition. This resilience of up to 300 % shock load was driven by increased levels of electron transport components such as quinones and riboflavins, which act as electron shuttles. However, after extreme shock exposures from 500 % to 1000 %, despite the spike in inhibitory by-products such as humic acids and ammonia, the upregulation of the mtr complex was pivotal in recovering and sustaining methane production. These insights emphasize the BeAD's capability to bolster both performance and stability, thereby providing a potent strategy for practical application of bioelectrochemical systems.

Formation/characterization of humin-mediated anaerobic granular sludge and enhanced methanogenic performance

This study presents a novel method for accelerating the granulation of methanogenic anaerobic granular sludge (AnGS) in an upflow anaerobic sludge blanket (UASB) reactor using solid-phase humin (HM). The results demonstrated that HM-mediated AnGS (HM-AnGS) formed rapidly within 50 days. The increase in particle size, settling velocity and mechanical strength was attributed to the rapid granulation of the HM-AnGS. The maximum methane yield of the HM-AnGS was 5-fold higher than that of the control group. This is consistent with the findings, which showed that HM-AnGS had 3.2-3.4 times more methyl-coenzyme M reductase (Mcr) activity and 2.4-2.9 times more adenosine triphosphate (ATP) than control groups. Molecular analyses indicate that HM most likely accelerated interspecies electron transfer (IET) in HM-AnGS (e.g., from Enterococcus to Methanosaeta). Furthermore, the HM-AnGS was effective in recovering energy from actual slaughterhouse wastewater.