Revealing impacts of operational modes on anaerobic digestion systems coupling with sulfate reduction

Combined semi-continuous feeding and stripping: Improving volatile fatty acid production and sulfate reduction in a two-phase anaerobic sulfate reduction system

This study developed a two-phase anaerobic sulfate reduction system with semi-continuous feeding, integrated with vapor stripping, to assess its treatment performance at varying chemical oxygen demand (COD)/SO42- ratios. At a COD/SO42- ratio of 2, the system achieved high removal efficiencies of 91.85 % for COD and 92.70 % for sulfate, respectively. Vapor stripping effectively maintained low free H2S concentrations, remaining below 50 mg/L in the first-phase reactor (Ra) and below 30 mg/L in the second-phase reactor (Rm). Additionally, the scouring effect of vapor stripping altered the functional group composition on the sludge surface, promoting the formation of sulfate-reducing granular sludge. Microbial analysis revealed a synchronized enrichment of Desulfovibrio and Enterococcus in Ra, as well as Methanosaeta and Enterococcus in Rm under a semi-continuous feeding regime combined with vapor stripping. Conductivity measurements and correlation analyses suggested that electroactive Methanosaeta and Enterococcus engaged in syntrophic metabolism via extracellular electron transfer, which was particularly beneficial for organic removal under low COD/SO42- ratio conditions. At a COD/SO42- ratio of 2, dissimilatory sulfate reduction (DSR) prevailed in Ra, whereas assimilatory sulfate reduction (ASR) dominated in Rm. This balance optimized sulfate removal efficiency while mitigating sulfide toxicity toward methanogens. Overall, this study presents a promising approach for the efficient treatment of high-sulfate organic wastewater with low COD/SO42- ratio.

Strategies for enriching targeted sulfate-reducing bacteria and revealing their microbial interactions in anaerobic digestion ecosystems

Deciphering relationships between sulfate-reducing bacteria (SRB) and other microorganisms is crucial for stable operation of anaerobic digestion systems when treating sulfate-containing wastewater. However, few studies have differentiated the incomplete oxidizing SRB (IO-SRB) and complete oxidizing SRB (CO-SRB) in anaerobic digestion ecosystems. Four ethanol-fed bioreactors were operated under two operational modes (sequencing batch reactor, SBR; and continuous-flow reactor, CFR) and two chemical oxygen demand (COD) to sulfate ratios (1 and 2) to systematically explore strategies for enriching IO-SRB and/or CO-SRB and their microbial interactions with other microorganisms. Compared to SBRs, CFRs could enhance sulfate removal and demonstrated higher microbial activities in sulfate and ethanol degradation. IO-SRB competed with ethanol oxidizing bacteria in all reactors, and IO-SRB's contribution to ethanol degradation increased from 62.9 %-67.1 % to 69.0 %-82.1 % as the COD/sulfate ratio decreased from 2 to 1. Moreover, CO-SRB competed acetotrophic methanogens exclusively in CFRs, as CO-SRB could not be efficiently enriched in SBRs. Low COD/sulfate ratios facilitated the enrichment of Desulfococcus (CO-SRB), and the CFR operational mode further strengthened its enrichment. Additionally, hydrogenotrophic SRB outperformed hydrogenotrophic methanogens in all four reactors. In general, IO-SRB and CO-SRB possessed distinct microbial interactions with methanogens, with potential syntrophic relationships between IO-SRB and acetotrophic methanogens while competitive relationships between CO-SRB and acetotrophic methanogens.

Unlocking soil revival: the role of sulfate-reducing bacteria in mitigating heavy metal contamination

Soil contamination with heavy metals from industrial and mining activities poses significant environmental and public health risks, necessitating effective remediation strategies. This review examines the utilization of sulfate-reducing bacteria (SRB) for bioremediation of heavy metal-contaminated soils. Specifically, it focuses on SRB metabolic pathways for heavy metal immobilization, interactions with other microorganisms, and integration with complementary remediation techniques such as soil amendments and phytoremediation. We explore the mechanisms of SRB action, their synergistic relationships within soil ecosystems, and the effectiveness of combined remediation approaches. Our findings indicate that SRB can effectively immobilize heavy metals by converting sulfate to sulfide, forming stable metal sulfides, thereby reducing the bioavailability and toxicity of heavy metals. Nevertheless, challenges persist, including the need to optimize environmental conditions for SRB activity, address their sensitivity to acidic conditions and high heavy metal concentrations, and mitigate the risk of secondary pollution from excessive carbon sources. This study underscores the necessity for innovative and sustainable SRB-based bioremediation strategies that integrate multiple techniques to address the complex issue of heavy metal soil contamination. Such advancements are crucial for promoting green mining practices and environmental restoration.

Deciphering Fe@C amendment on long-term anaerobic digestion of sulfate and propionate rich wastewater: Driving microbial community succession and propionate metabolism

This study evaluated the reflection of long-term anaerobic system exposed to sulfate and propionate. Fe@C was found to efficiently mitigate anaerobic sulfate inhibition and enhance propionate degradation. With influent propionate of 12000mgCOD/L and COD/SO42- ratio of 3.0, methane productivity and sulfate removal were only 0.06 ± 0.02L/gCOD and 63 %, respectively. Fe@C helped recover methane productivity to 0.23 ± 0.03L/gCOD, and remove sulfate completely. After alleviating sulfate stress, less organic substrate was utilized to form extracellular polymeric substances for self-protection, which enhanced mass transfer in anaerobic sludge. Microbial community succession, especially for alteration of key sulfate-reducing bacteria and propionate-oxidizing bacteria, was driven by Fe@C, thus enhancing sulfate reduction and propionate degradation. Acetotrophic Methanothrix and hydrogenotrophic unclassified_f_Methanoregulaceae were enriched to promote methanogenesis. Regarding propionate metabolism, inhibited methylmalonyl-CoA degradation was a limiting step under sulfate stress, and was mitigated by Fe@C. Overall, this study provides perspective on Fe@C's future application on sulfate and propionate rich wastewater treatment.

Mechanistic understanding of acclimation and energy metabolism of acetoclastic methanogens under different substrate to microorganism ratios

Mechanistic understanding of acetoclastic methanogenesis is pivotal for optimizing anaerobic digestion for efficient methane production. In this study, two different operational modes, continuous flow reactor (CFR) and sequencing batch reactor (SBR), accompanied with solids retention times (SRT) of 10 days (SBR10d and CFR10d) and 25 days (SBR25d and CFR25d) were implemented to elucidate their impacts on microbial communities and energy metabolism of methanogens in acetate-fed systems. Microbial community analysis revealed that the relative abundance of Methanosarcina (16.0%-46.0%) surpassed Methanothrix (3.7%-22.9%) in each reactor. SBRs had the potential to enrich both Methanothrix and Methanosarcina. Compared to SBRs, CFRs had lower total relative abundance of methanogens. Methanosarcina exhibited a superior enrichment in reactors with 10-day SRT, while Methanothrix preferred to be acclimated in reactors with 25-day SRT. The operational mode and SRT were also observed to affect the distribution of acetate-utilizing bacteria, including Pseudomonas, Desulfocurvus, Mesotoga, and Thauera. Regarding enzymes involved in energy metabolism, Ech and Vho/Vht demonstrated higher relative abundances at 10-day SRT compared to 25-day SRT, whereas Fpo and MtrA-H showed higher relative abundances in SBRs than those in CFRs. The relative abundance of genes encoding ATPase harbored by Methanothrix was higher than Methanosarcina at 25-day SRT. Additionally, the relative abundance of V/A-type ATPase (typically for methanogens) was observed higher in SBRs compared to CFRs, while the F-type ATPase (typically for bacteria) exhibited higher relative abundance in CFRs than that in SBRs.

Facilitating effects of the synergy with zero-valent iron and peroxysulfate on the sludge anaerobic fermentation system: Combined biological enzyme, microbial community and fermentation mechanism assessment

This study evaluated a synergetic waste activated sludge treatment strategy with environmentally friendly zero-valent iron nanoparticles (Fe0) and peroxysulfate. To verify the feasibility of the synergistic treatment, Fe0, peroxysulfate, and the mixture of peroxysulfate and Fe0 (synergy treatment) were added to different sludge fermentation systems. The study demonstrated that the synergy treatment fermentation system displayed remarkable hydrolysis performance with 435.50 mg COD/L of protein and 197.67 mg COD/L of polysaccharide, which increased 1.13-2.85 times (protein) and 1.12-1.49 times (polysaccharide) for other three fermentation system. Additionally, the synergy treatment fermentation system (754.52 mg COD/L) exhibited a well acidification performance which was 1.35-41.73 times for other systems (18.08-557.27 mg COD/L). The synergy treatment fermentation system had a facilitating effect on the activity of protease, dehydrogenase, and alkaline phosphatase, which guaranteed the transformation of organic matter. Results also indicated that Comamonas, Soehngenia, Pseudomonas, and Fusibacter were enriched in synergy treatment, which was beneficial to produce SCFAs. The activation of Fe0 on peroxysulfate promoting electron transfer, improving the active groups, and increasing the enrichment of functional microorganisms showed the advanced nature of synergy treatment. These results proved the feasibility of synergy treatment with Fe0 and peroxysulfate to enhance waste activated sludge anaerobic fermentation.

Deciphering anaerobic ethanol metabolic pathways shaped by operational modes

Efficient anaerobic digestion requires the syntrophic cooperation among diverse microorganisms with various metabolic pathways. In this study, two operational modes, i.e., the sequencing batch reactor (SBR) and the continuous-flow reactor (CFR), were adopted in ethanol-fed systems with or without the supplement of powdered activated carbon (PAC) to examine their effects on ethanol metabolic pathways. Notably, the operational mode of SBR and the presence of CO2 facilitated ethanol metabolism towards propionate production. This was further evidenced by the dominance of Desulfobulbus, and the increased relative abundances of enzymes (EC: 1.2.7.1 and 1.2.7.11) involved in CO2 metabolism in SBRs. Moreover, SBRs exhibited superior biomass-based rates of ethanol degradation and methanogenesis, surpassing those in CFRs by 53.1% and 22.3%, respectively. Remarkably, CFRs with the extended solids retention time enriched high relative abundances of Geobacter of 71.7% and 70.4% under conditions with and without the addition of PAC, respectively. Although both long-term and short-term PAC additions led to the increased sludge conductivity and a reduced methanogenic lag phase, only the long-term PAC addition resulted in enhanced rates of ethanol degradation and propionate production/degradation. The strategies by adjusting operational mode and PAC addition could be adopted for modulating the anaerobic ethanol metabolic pathway and enriching Geobacter.

New insights into syntrophic ethanol oxidation: Effects of operational modes and solids retention times

Anaerobic ethanol oxidation relies on syntrophic interactions among functional microorganisms to become thermodynamically feasible. Different operational modes (sequencing batch reactors, SBRs, and continuous flow reactors, CFRs) and solids retention times (SRT, 25 days and 10 days) were employed in four ethanol-fed reactors, named as SBR25d, SBR10d, CFR25d, and CFR10d, respectively. System performance, syntrophic relationships, microbial communities, and metabolic pathways were examined. During the long-term operation, 2002.7 ± 56.0 mg COD/L acetate was accumulated in CFR10d due to the washout of acetotrophic methanogens. Microorganisms with high half-saturation constants were enriched in reactors of 25-day SRT. Moreover, ethanol oxidizing bacteria and acetotrophic methanogens with high half-saturation constants could be acclimated in SBRs. In SBRs, Syner-01 and Methanothrix dominated, and the low SRT of 10 days increased the relative abundance of Geobacter to 38.0%. In CFRs, the low SRT of 10 days resulted in an increase of Desulfovibrio among syntrophic bacteria, and CFR10d could be employed in enriching hydrogenotrophic methanogens like Methanobrevibacter.