Assessing Intermediate Formation and Electron Competition during Thiosulfate-Driven Denitrification: An Experimental and Modeling Study

Tourmaline mediated enhanced autotrophic denitrification: The mechanisms of electron transfer and Paracoccus enrichment

Autotrophic denitrification (AD) without carbon source is an inevitable choice for denitrification of municipal wastewater under the carbon peaking and carbon neutrality goals. This study first employed sulfur-tourmaline-AD (STAD) as an innovative nitrate removal trial technique in wastewater. STAD demonstrated a 2.23-fold increase in nitrate‑nitrogen (NO3--N) removal rate with reduced nitrite‑nitrogen (NO2--N) accumulation, effectively removing 99 % of nitrogen pollutants compared to sulfur denitrification. Some denitrifiers microorganisms that could secrete tyrosine, tryptophan, and aromatic protein (extracellular polymeric substances (EPS)). Moreover, according to the EPS composition and characteristics analysis, the secretion of loosely bound extracellular polymeric substances (LB-EPS) that bound to the bacterial endogenous respiration and enriched microbial abundance, was produced more in the STAD system, further improving the system stability. Furthermore, the addition of tourmaline (Tm) facilitated the discovery of a new genus (Paracoccus) that enhanced nitrate decomposition. Applying optimal electron donors through metabolic pathways and the microbial community helps to strengthen the AD process and treat low carbon/nitrogen ratio wastewater efficiently.

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.

Effects of microplastics on denitrification and associated N2O emission in estuarine and coastal sediments: insights from interactions between sulfate reducers and denitrifiers

Global estuarine and coastal zones are facing severe microplastics (MPs) pollution. Sulfate reducers (SRB) and denitrifiers (DNB) are two key functional microorganisms in these zones, exhibiting intricate interactions. However, whether and how MPs modulate the interactions between SRB and DNB, with implications for denitrification and associated N2O emissions, remains poorly understood. Here, we simultaneously investigated the spatial response patterns of SRB-DNB interactions and denitrification and associated N2O emissions to different MPs exposure along an estuarine gradient in the Yangtze Estuary. Spatial responses of denitrification to polyvinyl chloride (PVC) and polyadipate/butylene terephthalate (PBAT) MPs exposure were heterogeneous, while those of N2O emissions were not. Gradient-boosted regression tree and multiple regression model analyses showed that sulfide, followed by nitrate (NO3-), controlled the response patterns of denitrification to MPs exposure. Further mechanistic investigation revealed that exposure to MPs resulted in a competitive and toxic (sulfide accumulation) inhibition of SRB on DNB, ultimately inhibiting denitrification at upstream zones with high sulfide but low NO3- levels. Conversely, MPs exposure induced a competitive inhibition of DNB on SRB, generally promoting denitrification at downstream zones with low sulfide but high NO3- levels. These findings advance the current understanding of the impacts of MPs on nitrogen cycle in estuarine and coastal zones, and provide a novel insight for future studies exploring the response of biogeochemical cycles to MPs in various ecosystems.

Physics-informed neural network-based serial hybrid model capturing the hidden kinetics for sulfur-driven autotrophic denitrification process

Sulfur-driven autotrophic denitrification (SdAD) is a biological process that can remove nitrate from low carbon/nitrogen (C/N) ratio wastewater. Although this process has been intensively researched, the mechanism whereby its intermediates (i.e., elemental sulfur and nitrite ions) are generated and accumulated remains elusive. Existing mathematical models developed for SdAD cannot accurately predict the intermediates in SdAD because of the incomplete knowledge of process kinetic resulting from changes in the environmental conditions and electron competition during SdAD. To address this limitation, we proposed a novel serial hybrid model structure based on a physics-informed neural network (PINN) to capture the dynamics of the process kinetics and predict the substrate concentrations in SdAD. In this study, we evaluated the model through numerical experiments and applied it to real case studies involving batch and continuous-flow reactor scenarios. By leveraging the PINN approach, the hybrid model yielded accurate predictions at both the state (i.e. substrate concentration) and kinetic levels in the numerical experiments and performed better than both mechanistic and purely data-driven models in the case studies. Furthermore, we used the trained hybrid model to design control strategies for SdAD and a novel integrated process involving SdAD and anammox for energy-efficient nitrogen removal. Finally, we discuss the advantages and application scope of the PINN-based hybrid model.

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

A novel strategy based on thiosulfate pretreatment for enhancing short-chain fatty acids (SCFAs) from anaerobic fermentation (AF) of waste activated sludge (WAS) was proposed in this study. The results showed that the maximal SCFA yield increased from 206.1 ± 4.7 to 1097.9 ± 17.2 mg COD/L with thiosulfate dosage increasing from 0 to 1000 mg S/L, and sulfur species contribution results revealed that thiosulfate was the leading contributor to improve SCFA yield. Mechanism exploration disclosed that thiosulfate addition largely improved WAS disintegration, due to thiosulfate serving as a cation binder for removing organic-binding cations, especially Ca²⁺ and Mg²⁺, dispersing the extracellular polymeric substance (EPS) structure and further entering into the intracellularly by stimulated carrier protein SoxYZ and subsequently caused cell lysis. Typical enzyme activities and related functional gene abundances indicated that both hydrolysis and acidogenesis were remarkably enhanced while methanogenesis was substantially suppressed, which were further strengthened by the enriched hydrolytic bacteria (e.g. C10-SB1A) and acidogenic bacteria (e.g. Aminicenantales) but severely reduced methanogens (e.g. Methanolates and Methanospirillum). Economic analysis confirmed that thiosulfate pretreatment was a cost-effective and efficient strategy. The findings obtained in this work provide a new thought for recovering resource through thiosulfate-assisted WAS AF for sustainable development.