Role of pretreatment type and microbial mechanisms on enhancing volatile fatty acids production during anaerobic fermentation of refinery waste activated sludge

A review of volatile fatty acids production from organic wastes: Intensification techniques and separation methods

High value-added products from organic waste fermentation have garnered increasing concern in modern society. VFAs are short-chain fatty acids, produced as intermediate products during the anaerobic fermentation of organic matter. VFAs can serve as an essential organic carbon source to produce substitutable fuels, microbial fats and oils, and synthetic biodegradable plastics et al. Extracting VFAs from the fermentation broths is a challenging task as the composition of suspensions is rather complex. In this paper, a comprehensive review of methods for VFAs production, extraction and separation are provided. Firstly, the methods to enhance VFAs production and significant operating parameters are briefly reviewed. Secondly, the evaluation and detailed discussion of various VFAs extraction and separation technologies, including membrane separation, complex extraction, and adsorption methods, are presented, highlighting their specific advantages and limitations. Finally, the challenges encountered by different separation technologies and novel approaches to enhance process performance are highlighted, providing theoretical guidance for recycling VFAs from organic wastes efficiently.

Exploring nature's filters: Peat-mineral mix for low and high-strength oilfield produced water reclamation

Nature-based solutions are encouraged for treating oilfield produced water from oil and gas extraction, a crucial undertaking that aligns with the Canadian oil sands industry's ambitious goal of zero waste, and the globally recognized Sustainable Development Goals (SDGs) pertaining to water conservation and ecosystem preservation. This study explored the use of peat-mineral mix (PMM), a leftover of inevitable oil sands mining, for treating low and high-strength wastewaters during biofiltration, which contained large molecular weight (44.3 kDa), which include alcohols, aliphatics, aromatics, and ketones, and can impart high toxicity to both fauna and flora (MicroTox: 99 %). The breakthrough curve indicated an effective initial adsorption phase driven by advection within the column dynamics. For complete organics removal and mechanistic insights, the wastewater was re-circulated in a continuous mode for up to 42 days. Here, we found that chemical oxygen demand was reduced from ∼85,000 mg/L to ∼965 mg/L). Kinetics investigations along with physicochemical characterization of PMM and wastewater suggested that chemisorption and anaerobic digestion contributed to the overall removal of contaminants. Chemisorption, led by hydrogen bonding and hydrophobic interactions, was the dominant mechanism, with a limited contribution from physical adsorption (surface area: 2.85 m2/g). The microbial community within the PMM bed was rich/diverse (Shannon > 6.0; Chao1 > 600), with ∼ 50 % unclassified phylotypes representing 'microbial dark matter'. High electric conductivity (332.1 μS cm-1) of PMM and the presence of Geobacter, syntrophs, and Methanosaeta suggest that direct interspecies electron transfer was likely occurring during anaerobic digestion. Both low and high-strength wastewaters showed effective removal of dissolved organics (e.g., naphthenic acids, acid extractable fraction, oil and grease content), nutrients, and potentially toxic metals. The successful use of PMM in treating oilfield produced water offers promising avenues for embracing nature-based remediation solutions at oil refining sites.

Application of calcium peroxide in promoting resource recovery from municipal sludge: A review

The harmless disposal, resource recovery, and synergistic efficiency reduction of municipal sludge have been the research focuses for the last few years. Calcium peroxide (CaO2) is a multifunctional and safe peroxide that produces an alkaline oxidation environment to promote the fermentation of municipal sludge to produce hydrogen (H2) and volatile fatty acids (VFAs), thus realizing sludge resource recovery. This review outlines the research achievements of CaO2 in sludge resource recovery, improvement of sludge dewaterability, and removal of pollutants from sludge in recent years. Meanwhile, the mechanism of CaO2 and its influencing factors have also been comprehensively summarized. Finally, the future development direction of the application of CaO2 in municipal sludge is prospected. This review would provide theoretical reference for the potential engineering applications of CaO2 in improving sludge treatment in the future.

Combined transcriptomic and metabolomic analyses of temperature response of microalgae using waste activated sludge extracts for promising biodiesel production

Waste activated sludge (WAS) as one of the major pollutants with a significant annual production, has garnered significant attention regarding its treatment and utilization. If improperly discharged, it not only caused environmental pollution but also led to the wastage of valuable resources. In this study, the microalgae growth and lipid accumulation using waste activated sludge extracts (WASE) under different temperature conditions were investigated. The highest lipid content (59.13%) and lipid productivity (80.41 mg L-1 d-1) were obtained at cultivation temperatures of 10 and 25 °C, respectively. It was found that microalgae can effectively utilize TN/TP/NH4+-N and other nutrients of WASE. The highest utilization rates of TP, TN and NH4+-N were achieved at a cultivation temperature of 10 °C, reaching 84.97, 77.49 and 92.32%, respectively. The algal fatty acids had carbon chains predominantly ranging from C14 to C18, making them suitable for biodiesel production. Additionally, a comprehensive analysis of transcriptomics and metabolomics revealed up-regulation of genes associated with triglyceride assembly, the antioxidant system of algal cells, and cellular autophagy, as well as the accumulation of metabolites related to the tricarboxylic acid (TCA) cycle and lipids. This study offers novel insights into the microscopic mechanisms of microalgae culture using WASE and approaches for the resource utilization of sludge.

Enzyme-enhanced acidogenic fermentation of waste activated sludge: Insights from sludge structure, interfaces, and functional microflora

Anaerobic fermentation is widely installed to recovery valuable resources and energy as CH4 from waste activated sludge (WAS), and its implementation in developing countries is largely restricted by the slow hydrolysis, poor efficiency, and complicate inert components therein. In this study, enzyme-enhanced fermentation was conducted to improve sludge solubilization from 283 to 7728 mg COD/L and to enhance volatile fatty acids (VFAs) yield by 58.6 % as compared to the conventional fermentation. The rapid release of organic carbon species, especially for tryptophan- and tyrosine-like compounds, to outer layer of extracellular polymeric substance (EPS) occurred to reduce the structural complexity and improve the sludge biodegradability towards VFAs production. Besides, upon enzymatic pretreatment the simultaneous exposure of hydrophilic and hydrophobic groups on sludge surfaces increased the interfacial hydrophilicity. By quantitative analysis via interfacial thermodynamics and XDLVO theory, it was confirmed that the stronger hydrophilic repulsion and energy barriers in particle interface enhanced interfacial mass transfer and reactions involved in acidogenic fermentation. Meanwhile, these effects stimulate the fermentation functional microflora and predominant microorganism, and the enrichment of the hydrolytic and acid-producing bacteria in metaphase and the proliferation of acetogenic bacteria, e.g., Rubrivivax (+9.4 %), in anaphase also benefits VFAs formation. This study is practically valuable to recovery valuable VFAs as carbon sources and platform chemicals from WAS and agriculture wastes.

Promoting short-chain fatty acids production from sewage sludge via acidogenic fermentation: Optimized operation factors and iron-based persulfate activation system

Promoting short-chain fatty acids (SCFAs) production and ensuring the stability of SCFAs-producing process are becoming the two major issues for popularizing the acidogenic fermentation (AF). The key controlling operating and influencing factors during anaerobic fermentation process were thoroughly reviewed to facilitate better process performance prediction and to optimize the process control of SCFAs promotion. The wide utilization of iron salt flocculants during wastewater treatment could result in iron accumulating in sewage sludge which influenced AF performance. Additionally, appropriate ferric chloride (FC) could promote the SCFAs accumulation, while poly ferric sulfate (PFS) inhibited the bioprocess. Iron/persulfate (PS) system was proved to effectively enhance the SCFAs production while mechanism analysis revealed that the strong oxidizing radicals remarkably enhanced the solubilization and hydrolysis. Moreover, the changes of oxidation-reduction potential (ORP) and pH caused by iron/PS system exhibited more negative effects on the methanogens, comparing to the acidogenic bacteria. Furthermore, performance and mechanisms of different iron species-activating PS, organic chelating agents and iron-rich biochar derived from sewage sludge were also elucidated to extend and strengthen understanding of the iron/PS system for enhancing SCFAs production. Considering the large amount of generated Fe-sludge and the multiple benefits of iron activating PS system, carbon neutral wastewater treatment plants (WWTPs) were proposed with Fe-sludge as a promising recycling composite to improve AF performance. It is expected that this review can deepen the knowledge of optimizing AF process and improving the iron/PS system for enhancing SCFAs production and provide useful insights to researchers in this field.

Enhancing the production of volatile fatty acids by potassium permanganate from wasted sewage sludge: A batch test experiment

Recovering resources from wastewater treatment is vital for the transition from a linear to a circular economy model in the water sector. Volatile Fatty Acids (VFAs) are valuable products among the possible recovered resources. This study investigates the influence of potassium permanganate (KMnO4) addition during acidogenic fermentation of waste activated sludge for enhancing VFAs production. Specifically, different fermentation batch tests with and without KMnO4 addition were carried out using two distinctive sewage sludges as feedstocks. Results showed that KMnO4 addition increased the VFAs yield up to 144 and 196 mgCOD/g VSS for the two sludges. When KMnO4 was used as pre-treatment, 55 % of sCOD were VFAs. This latter result was mainly debited to the recalcitrant organics' disruption promoted by the oxidative permanganate ability.

Role of potassium ferrate in anaerobic digestion of waste activated sludge: Phenotypes and genotypes

The specific effects of potassium ferrate (PF) on acid and methane production in anaerobic digestion need further exploration. This study comprehensively investigated the role of PF in organic matter conversion in waste activated sludge (WAS) digestion. Due to the high pH produced by PF self-decomposition, the hydrolysis of organic matter was promoted, whereas the methanogenesis was inhibited. PF could further directly oxidize protein and polysaccharides released by hydrolysis to produce volatile fatty acids (VFAs) and involve in the transformation of ammonia nitrogen. PF could induce the enrichment of functional genes related to fermentation pathways and lessen those related to methanogenesis, and the phylum resistant to PF oxidation and the strains capable of producing VFAs were enriched, resulting in VFAs accumulation. This study analyzed the participation way of PF in anaerobic digestion and provided a theoretical basis for the application of PF in promoting VFAs recovery from sludge digestion.