Thiosulfate pretreatment enhancing short-chain fatty acids production from anaerobic fermentation of waste activated sludge: Performance, metabolic activity and microbial community

Carbon fixation via volatile fatty acids recovery from sewage sludge through electrochemical-pretreatment-based anaerobic digestion

Capturing the carbon in volatile fatty acids (VFA) produced from the anaerobic digestion (AD) of sewage sludge has the potential to not only provide economic benefits but also reduce greenhouse gas production. This study demonstrates a chemical-free method to collect VFA from an AD instead of methane that involves electrochemical pretreatment (EPT) of sludge. Experimental results show that applying 15 V EPT for 45 min enhances acidogenesis and selectively inhibits methanogenesis, leading to a substantial VFA accumulation (2563.1 ± 307.9 mg COD/L) and achieving 2.5 times more carbon fixation than via methane production. Interfacial thermodynamic analysis shows that EPT induces a decrease in both the repulsive electrostatic energy (from 152.9 kT to 12.2 kT) and the energy barrier (from 57.0 kT to 2.6 kT) in the sludge, leading to increased sludge aggregation and entrapment of microorganisms. Molecular docking sheds lights on how the methanogens interacts with the organic matter released from EPT (e.g., alanine-tRNA ligase), showing that these interactions potentially interfere with the proteins that are associated with the activities of the methanogens and the electron transfer pathways, thereby impeding methanogenesis. Integrating EPT into AD therefore facilitates the recovery of valuable VFA and the capture of carbon from freshwater sludge, providing notable economic and environmental benefits in sewage sludge treatment.

L-tryptophan anaerobic fermentation for indole acetic acid production: Bacterial enrichment and effects of zero valent iron

Indole acetic acid (IAA) as a plant hormone, was one of the valuable products of anaerobic fermentation. However, the enriching method remained unknown. Moreover, whether zero valent iron (ZVI) could enhance IAA production was unexplored. In this work, IAA producing bacteria Klebsiella (63 %) was enriched successfully. IAA average production rate and concentration were up to 3 mg/L/h and 56 mg/L. With addition of 1 g/L ZVI, IAA average production rate and concentration was increased for 2 and 3 folds. Mechanisms indicated ZVI increased Na+K+-ATP activity and electron transport activity for 2 folds and 1 fold. Moreover, macro transcription determined indole pyruvate pathway activity like primary-amine oxidase, indole pyruvate decarboxylase and aldehyde dehydrogenase were increased for 146 %, 187 %, and 557 %, respectively. Therefore, ZVI was suitable for enhancement IAA production from mixed culture anaerobic fermentation.

Deep insights into the roles and microbial ecological mechanisms behind waste activated sludge digestion triggered by persulfate oxidation activated through multiple modes

Persulfate oxidation (PS) is widely employed as a promising alternative for waste activated sludge pretreatment due to the capability of generating free radicals. The product differences and microbiological mechanisms by which PS activation triggers WAS digestion through multiple modes need to be further investigated. This study comprehensively investigated the effects of persulfate oxidation activated through multiple modes, i.e., ferrous, zero-valent iron (ZVI), ultraviolet (UV) and heat, on the performance of sludge digestion. Results showed that PS_ZVI significantly accelerated the methane production rate to 12.02 mL/g VSS. By contrast, PS_Heat promoted the sludge acidification and gained the maximum short-chain fatty acids (SCFAs) yield (277.11 ± 7.81 mg COD/g VSS), which was 3.41-fold compared to that in PS_ZVI. Moreover, ferrous and ZVI activated PS achieved the oriented conversion of acetate, the proportions of which took 73% and 78%, respectively. MiSeq sequencing results revealed that PS_Heat and PS_UV evidently enriched anaerobic fermentation bacteria (AFB) (i.e., Macellibacteroides and Clostridium XlVa). However, PS_Ferrous and PS_ZVI facilitated the enrichment of Woesearchaeota and methanogens. Furthermore, molecular ecological network and mantel test revealed the intrinsic interactions among the multiple functional microbes and environmental variables. The homo-acetogens and sulfate-reducing bacterial had potential cooperative and symbiotic relationships with AFB, while the nitrate-reducing bacteria displayed distinguishing ecological niches. Suitable activation modes for PS pretreatments resulted in an upregulation of genes expression responsible for digestion. This study established a scientific foundation for the application of sulfate radical-based oxidation on energy or high value-added chemicals recovery from waste residues.

Thiosulfate/FeCl3 pre-treatment enhances short-chain fatty acid production and mitigates H2S generation during anaerobic fermentation of waste activated sludge: Performance, microbial community and ecological analyses

Anaerobic fermentation (AF) has been identified as a promising method of transforming waste activated sludge (WAS) into high-value products (e.g., short-chain fatty acids (SCFAs)). This study developed thiosulfate/FeCl3 pre-treatment and investigated the effects of different thiosulfate/FeCl3 ratios (S:Fe = 3:1, 3:2, 1:1, 3:4 and 3:5) on SCFA production and sulfur transformation during the AF of WAS. At a S:Fe ratio of 1:1, the maximal SCFA yield (933.3 mg COD/L) and efficient H2S removal (96.5 %) were obtained. S:Fe ratios ≤ 1:1 not only benefited hydrolysis and acidification but largely mitigated H2S generation. These results were supported by the enriched acidogens and reduced sulfur-reducing bacteria (SRB). Molecular ecological network analysis further revealed that the keystone taxon (g_Saccharimonadales) was found in S:Fe = 1:1, together with reductions in associations among methanogens, acidogens and SRB. This work provides a strategy for enhancing high-value product recovery from WAS and minimising H2S emissions.

An integrated system combining Tetrasphaera-dominated enhanced biological phosphorus removal with sulfur autotrophic denitrification to enhance biological nutrients removal

This study developed a two-stage process, including Tetrasphaera-dominated enhanced biological phosphorus-removal (EBPR(T)) sequencing batch reactor (SBR), followed by sulfur autotrophic denitrification (SADN) SBR, to achieve advanced nutrients removal from low VFAs wastewater. The removal efficiencies of nitrogen and phosphorus (PO43--P) reached 99 % with effluent PO43--P and total inorganic nitrogen (TIN) below 0.5 mg/L and 1 mg/L in EBPR(T) and SADN SBR, respectively. Mechanism analysis indicated that as increasing drainage ratio and complex carbon sources, free amino acids, glycogen, and PHA served as the endogenous carbon sources of Tetrasphaera to store energy. SADN contributed to approximately 80 % of nitrogen removal. DNA and cDNA results indicated Tetrasphaera was shifted from clade 2 to clade 1 after increasing the drainage ratio and the complexity of the carbon source, and Tetrasphaera (50.95 %) and Ca. Accumulibacter (9.12 %) were the most important functional microorganisms synergized to remove phosphorus at the transcriptional level in EBPR(T). Thiobacillus (45.97 %) and Sulfuritalea (9.24 %) were the dominant sulfur autotrophic denitrifiers at gene and transcriptional level in SADN. The results suggested that the EBPR(T) - SADN SBRs have great nutrient removal performance in treating low VFAs wastewater without additional carbon sources.

Polyethylene microplastic-induced microbial shifts affected greenhouse gas emissions during litter decomposition in coastal wetland sediments

Microplastic contamination has become increasingly aggravated in coastal environments, further affecting biogeochemical processes involved with microbial community shifts. As a key biogeochemical process mainly driven by microbiota in coastal wetland sediments, litter decomposition contributes greatly to the global greenhouse gas (GHG) budget. However, under microplastic pollution, the relationship between microbial alterations and GHG emissions during litter decomposition in coastal wetlands remains largely unknown. Here, we explored the microbial mechanism by which polyethylene microplastic (PE-MP) influenced greenhouse gas (i.e., CH4, CO2 and N2O) emissions during litter decomposition in coastal sediments through a 75-day microcosm experiment. During litter decomposition, PE-MP exposure significantly decreased cumulative CH4 and CO2 emissions by 41.07% and 25.79%, respectively. However, there was no significant change in cumulative N2O emissions under PE-MP exposure. The bacterial, archaeal, and fungal communities in sediments exhibited varied responses to PE-MP exposure over time, as reflected by the altered structure and changed functional groups of the microbiota. The altered microbial functional groups ascribed to PE-MP exposure and sediment property changes might contribute to suppressing CH4 and CO2 emissions during litter decomposition. This study yielded valuable information regarding the effects of PE-MP on GHG emissions during litter decomposition in coastal wetland sediments.

Differences in greenhouse gas emissions and microbial communities between underground and conventionally constructed wastewater treatment plants

Large quantities of greenhouse gases (GHGs) are emitted into the atmosphere during wastewater treatment. In this study, GHG and microbial samples were collected from four wastewater treatment plants (WWTPs), and their differences and relationships were assessed. The study showed that, compared with conventionally constructed WWTPs, well-established gas collection systems in underground WWTPs facilitate comprehensive collection and accurate accounting of GHGs. In aboveground WWTPs, capped anoxic ponds promote methane production releasing it at 2-8 times the rate of uncapped emissions, in contrast to nitrous oxide emissions. Moreover, a stable subsurface environment allows for smaller fluctuations in daily GHG emissions and higher microbial diversity and abundance. This study highlights differences in GHG emission fluxes and microbial communities in differently constructed WWTPs, which are useful for control and accurate accounting of GHG emissions.

Synergic effects of free ammonia and sodium percarbonate for enhancing short-chain fatty acid production during sludge fermentation: Effectiveness assessment and mechanism elucidation

The production of short-chain fatty acids (SCFAs) from waste activated sludge (WAS) via anaerobic fermentation is typically restricted by poor sludge decomposition capacity and low substrate-availability. Therefore, the free ammonia (FA)‑sodium percarbonate (SPC) technology was presented to successfully overcome the limitation while addressing unsatisfactory acidogenic fermentation pretreated with sole FA or SPC. It revealed that FA + SPC co-pretreatment could boost the SCFA concentration to 347.1 mg COD/g VSS at 180 mg/L FA and 0.15 g/g TSS SPC. In-depth studies demonstrated that FA + SPC pretreatment greatly improved sludge disintegration, biodegradability of substrates and acidification of hydrolysis products. Furthermore, FA + SPC co-pretreatment stimulated the activity of hydrolytic and acidogenic enzymes but inhibited methanogenic enzymes while changing the microbial structure and promoting the enrichment of fermentation microorganisms. The synergistic effect of FA and SPC in this work improves the yield of SCFAs from WAS and facilitates the study of WAS carbon resource recovery.

In-situ sulfite treatment promotes solid reduction during aerobic digestion of waste activated sludge: Feasibility for small-scale wastewater treatment plants

Aerobic digestion remains the preferred choice for small-scale wastewater treatment plants (WWTPs) in some developing countries, largely due to economic viability and operational simplicity. The escalating production of waste activated sludge (WAS) has prompted small-scale WWTPs to improve efficiency. To address this issue, this study employed an in-situ sulfite treatment as a non-intrusive method to augment aerobic digestion. With sulfite-enhanced solubilization and hydrolysis, a 3.6-fold increase in degradation was achieved. Both sludge dewatering properties and pathogens inactivation were improved. Microbial community analysis revealed a preferential enrichment of Actinobacteriota and Firmicutes during sulfite treatment. The desktop scaling-up estimation suggests that implementing this treatment yielded operational cost savings exceeding 40 %. In summary, in-situ sulfite treatment offers a cost-effective strategy for WAS management in small-scale WWTPs.

Long-term effects of thiosulfate on the competition between sulfur-mediated bacteria and glycogen accumulating organisms in sulfate-rich carbon-deficient wastewater

Sewage nutrient (e.g., nitrogen and phosphorus) biological removal performance is often limited by the deficient carbon source and undesirable glycogen accumulating organisms (GAOs), even in sulfate-containing wastewater. Thiosulfate (S2O32-) as a bioavailable, environmentally-benign, metastable and cost-effective agent has been regarded as electron carriers that induces high sulfur-mediated bacterial activity for nutrient removal from wastewater. In this study, the long-term effects of thiosulfate on the competition between sulfur-mediated bacteria (SMB, including sulfur-reducing bacteria (SRB) and sulfur-oxidizing bacteria (SOB)) and GAOs were explored to further close the gap of our knowledge on the control of GAOs under carbon deficient wastewater. Three reactors were continuously operated for over 100 days and were fed with 200 mg acetate-COD/L and 20 (R1), 50 (R2) and 80 (R3) mg S/L thiosulfate respectively. The results revealed that adding thiosulfate at the beginning of the anoxic phase promoted sulfur metabolism and increased the proliferation of SRB (mainly Desulfobacter) and SOB (mainly Chromatiaceae). Correspondingly, the relative abundance of GAOs (mainly Candidatus_Competibacter) decreased. After the carbon source was reduced, the abundance of GAOs increased and the competitive activity of SRB was weakened, resulting in the reduced sulfate reduction, which could be attributed to the fact that GAOs had a higher carbon source competitiveness than SRB under low carbon source conditions. While SOB maintained a high abundance due to the addition of thiosulfate as an additional electron donor, which enhanced the denitrification efficiency. Additionally, the dominant SOB shifted from Thiobacillus to Chromatiaceae during the long-term operation, indicating that Chromatiaceae had a higher competitive advantage for reduced sulfur (e.g., S2O32-, Polysulfide (Poly-S)) and nitrate compared to Thiobacillus. Furthermore, microbial functional genes revealed that S metabolism was enhanced during long-term operation. The potential mechanism and optimization strategy regarding the competition between sulfur-mediated bacteria and GAOs were revealed.

Effect of mixing intensity on volatile fatty acids production in sludge alkaline fermentation: Insights from dissolved organic matter characteristics and functional microorganisms

Alkaline fermentation for volatile fatty acids (VFAs) production has shown potential as a viable approach to treat sewage sludge. The hydrolysis and acidogenesis of sludge are greatly influenced by mixing. However, the effects of mixing intensity on VFAs production in sludge alkaline fermentation (SAF) remain poorly understood. This study investigated the impacts of mixing intensity (30, 90 and 150 rpm continuous mixing, and 150 rpm intermittent mixing) on VFAs production, dissolved organic matter (DOM) characteristics, phospholipid fatty acid profiles and microbial population distribution in SAF. Results showed that 150 rpm continuous and intermittent mixing enhanced the hydrolysis of sludge, while 150 rpm intermittent mixing resulted in the highest VFAs production (3886 ± 266.1 mg COD/L). Analysis of fluorescent and molecular characteristics of DOM revealed that 150 rpm intermittent mixing facilitated the conversion of released DOM, especially proteins-like substances, into VFAs. The abundance of unsaturated and branched fatty acids of microbes increased under 150 rpm intermittent mixing, which could aid in DOM degradation and VFAs production. Firmicutes and Tissierella were enriched at 150 rpm intermittent mixing, which favored the maximum VFAs yield. Moreover, Firmicutes were found to be the key functional microorganisms influencing the yield of VFAs during SAF. This study provides an understanding about the mixing intensity effects on VFAs production during SAF, which could be helpful to improve the yield of VFAs.