Micro-level evaluation of organic compounds transformation in anaerobic digestion under feast and famine conditions assisted by iron-based materials - Revealing the true mechanism of AD enhancement

Elucidating interactive effects of sulfidated nanoscale zero-valent iron and ammonia on anaerobic digestion of food waste

In our previous study, anaerobic digestion of food waste could be effectively enhanced by adding sulfidated nanoscale zero-valent iron (S-nZVI) under high-strength ammonia concentrations. In this study, in order to further elucidate the specific interactive effects of S-nZVI and ammonia on anaerobic digestion of nitrogen-rich food waste, the methanogenic performance of anaerobic digestion systems respectively added with nanoscale zero-valent iron (nZVI) and S-nZVI were compared and monitored under different ammonia stress conditions. Both nZVI and S-nZVI could effectively stimulate the methanogenesis process among ammonia concentrations ranging from 0 to 3500 mg/L. However, the enhancing effects of S-nZVI and nZVI on anaerobic digestion of food waste were different, in which anaerobic digestion systems added with S-nZVI and nZVI performed best under 2500 mg/L of ammonia and 1500 mg/L of ammonia, respectively. Furthermore, the analysis of microbial communities suggested that ammonia stress enriched acetoclastic methanogens, while adding nZVI and S-nZVI into anaerobic digestions stimulated the process of hydrogenotrophic methanogenesis. Moreover, S-nZVI performed better in promoting the evolution of DIET-related microorganisms than nZVI, resulting in enhanced methane production under high ammonia-stressed conditions. This work provided fundamental knowledge about the interactive effects of S-nZVI and ammonia on the anaerobic digestion of food waste.

Reduction of refractory Maillard reaction products by Fe3+ during thermal hydrolysis pretreatment and enhanced sludge biodegradability

Refractory Maillard reaction products (MRPs) produced during thermal hydrolysis pretreatment (THP) of waste activated sludge (WAS) may negatively impact the performance of downstream anaerobic digestion (AD) and nitrogen removal processes. Operating THP at lower temperature can mitigate the production of MRPs and improve biodegradability of WAS, while solubilization of WAS is reduced. This study intends to develop a method to reduce the refractory MRPs of WAS without compromising on the solubilization. Fe³⁺ was introduced into THP process (165 °C, 30 min) to mitigate Maillard reaction. Effects of Fe³⁺ on solubilization of WAS, reduction of refractory residuals, accumulative methane production, and microbial community shift were studied. Results confirm that solubilization of WAS was improved and refractory residuals were reduced with the amendment of 10 mg-Fe/L FeCl₃. MRPs mitigation mechanisms were investigated and mainly attributed to Fe³⁺-triggered Fenton-like reactions. Methane production was enhanced by 10.4 ± 0.8% and attributed to the improved biodegradability of THP liquor, as well as to the enrichment of protein degradation and methane production related microbial community. This work provides a simple, economical, and safe strategy to reduce refractory residuals discharged from THP-AD system and to enhance methane production for more energy recovery.

Effects of sludge thermal hydrolysis pretreatment on anaerobic digestion and downstream processes: mechanism, challenges and solutions

Thermal hydrolysis pretreatment (THP), as a step prior to sludge anaerobic digestion (AD), is widely applied due to its effectiveness in enhancing organic solids hydrolysis and subsequent biogas productivity. However, THP also induces a series of problems including formation of refractory compounds in THP cylinder, high residual ammonia and organic in the AD centrate, inhibition on downstream nitrogen removal process and reduction in UV-disinfection effectiveness during post-treatment. More attention should be paid on how to mitigate these negative effects. Despite intensive studies were carried out to reduce refractory compounds formation and enhance biological performance, there is limited effort to discuss the solutions to tackle the THP associated problems in a holistic manner. This paper summarizes the solutions developed to date and analyzes their technology readiness to assess application potential in full-scale settings. The content highlights the limitations of THP and proposes potential solutions to address the technological challenges.

Dewaterability improvement and environmental risk mitigation of waste activated sludge using peroxymonosulfate activated by zero-valent metals: Fe0 vs. Al0

The stabilization and dewaterability of waste activated sludge (WAS) are essential factors for downstream disposal or reuse. Herein, two types of zero-valent metals, zero-valent iron (Fe⁰) and zero-valent aluminum (Al⁰), were compared for their ability to activate peroxymonosulfate (PMS) during the WAS conditioning process, with the effects of PMS activation by these two metals on WAS dewaterability and the potential environmental risks evaluated. Results showed that compared to Al⁰/PMS treatment, Fe⁰/PMS treatment achieved superior WAS dewaterability and reduced operational costs. Using PMS combined with Fe⁰ and Al⁰ treatments under optimal conditions, the water content (Wc) of dewatered sludge decreased to 55.7 ± 2.7 wt% and 59.4 ± 1.3 wt%, respectively. Meanwhile, application of the Fe⁰/PMS treatment system reduced the total annual cost by approximately 33.1%, compared to the Al⁰/PMS treatment. Analysis of the dewatering mechanism demonstrated that in the Fe⁰/PMS treatment, Fe³⁺/Fe²⁺ flocculation played an important role in the enhancement of WAS dewatering, while sulfate radical (SO₄^•-) oxidation was the dominant factor for WAS dewaterability improvement in Al⁰/PMS treatment. The greater enhancement of WAS dewaterability by Fe⁰/PMS treatment, was mainly attributed to more efficient reduction of hydrophilic extracellular polymeric substances (EPS) and an increase in surface charge neutralization. Environmental risk evaluation results indicated that Fe⁰/PMS and Al⁰/PMS treatments both effectively alleviated the environmental risks of heavy metals and faecal coliforms in dewatered sludge. Overall, this study proposes a novel perspective for the selection of an optimal PMS activator in sludge treatment.

Mechanistic insights into a novel nitrilotriacetic acid-Fe0 and CaO2 process for efficient anaerobic digestion sludge dewatering at near-neutral pH

Traditional Fenton or Fenton-like oxidation has been widely studied for waste activated sludge dewaterability. However, the narrow pH range (2.0-4.0) and the instabilities of Fe²⁺ and H₂O₂ have hindered its commercial application. Owing to the high alkalinity of anaerobic digestion (AD) sludge, traditional Fenton or Fenton-like oxidation is economically unfeasible for its dewatering. In this study, we successfully demonstrated a novel and feasible method that used nitrilotriacetic acid (NTA)-Fe⁰ combined with CaO₂ (NTA-Fe⁰/CaO₂) at near-neutral pH (∼6.0) (a slight pH adjustment) in which capillary suction time ratio (CST₀/CST) and centrifuged weight reduction (CWR) improved by 6 folds and 42.98 ± 0.37%, respectively, under the optimal conditions. The presence of NTA accelerated the Fe⁰ corrosion, Fe²⁺ stability and turnover between Fe²⁺ and Fe³⁺. As such, Fe⁰ could effectively catalyze CaO₂ to produce hydroxyl radicals (•OH) under near-neutral conditions. Accordingly, various molecular weight hydrophilic compounds in different extracellular polymeric substances fractions were significantly reduced after treatment. The hydrophilic functional groups especially protein molecules were largely reduced. Consequently, the viscosity of sludge and particle size effectively decreased, while the release of bound water, surface charge, flocculation, and flowability of sludge were improved. The cost-benefit analysis further demonstrated the NTA-Fe⁰/CaO₂ treatment has high reusability and stability and is also more economical over the FeCl₃/CaO and Fenton's reagent/CaO treatments. In summary, the NTA-Fe⁰/CaO₂ process is a cost-effective and practically feasible technology for improving AD sludge dewaterability.

Direct interspecies electron transfer (DIET) can be suppressed under ammonia-stressed condition - Reevaluate the role of conductive materials

Thermal hydrolysis pretreatment (THP) and anaerobic digestion (AD) integrated (THP-AD) process is a promising process for sludge management. However, the high ammonia production during the THP-AD process severely affects system's stability and performance. Conductive materials are widely reported to stimulate AD, thus they are potentially helpful in alleviating ammonia inhibition. This study investigated the effects of three widely studied conductive materials, i.e. zero-valent iron (ZVI), magnetite nanoparticles (Mag.) and powder activated carbon (PAC), on THP-AD process. Results showed that all the tested materials could effectively stimulate methanogenesis process under non-ammonia inhibition conditions. However, upon ammonia stress, these materials behaved distinctively with the best methanogenic performance in ZVI group followed by Mag. Group, and even worsened inhibition occurred in PAC group. The mechanisms behind were investigated from two levels-the reaction kinetics of each anaerobic digestion step and the responses of intracellular metabolism. It is revealed that ZVI effectively promoted all AD reactions, especially the energy unfavorable propanoate and butanoate metabolism and overall methanogenesis. In addition, ZVI likely acted as intracellular electron shuttles, and the conjunction point of ZVI to electron transfer system was identified as EtfAB: quinone oxidoreductase. On the contrary, the declined methanogenic performance in PAC group was attributed to selectively stimulated the growth of acetoclastic methanogen - Methanosaeta, which is sensitive to ammonia toxicity. The proteomic information further revealed that ammonia stress was unfavorable to the formation of direct interspecies electron transfer between syntrophic anaerobes. Overall, the present study provides fundamental knowledge about the role of different conductive materials in AD systems from intracellular proteomic level.