Bacterial antioxidant mechanism in calcium peroxide aided sludge anaerobic fermentation

Comprehensive revealing the destructive effect and inhibitory mechanism of oxytetracycline on aerobic denitrification bacteria Acinetobacter sp. AD1 based on cell state, electron behavior and intracellular environment

The wide application and low utilization rate of oxytetracycline (OTC) make it often detected in wastewater, which may cause harmful effects on microbial denitrification. Aerobic denitrification (AD) as a new microbial denitrification technology has obvious advantages. However, systematic studies on the effects of OTC on it are lacking. In this study, the effect of OTC on AD was comprehensively explored from multiple perspectives, the main results are as follows. From the perspective of bacterial performance, OTC inhibited AD bacteria growth, denitrification efficiency, and caused serious damage to cell morphological structure, results of CCK-8 confirmed that bacterial activity was significantly affected. From the perspective of electron behavior, OTC decreased electron-producing capacity of carbon metabolism, reduced activity of the electron transport system, inhibited the electron consumption of NAR and NIR to varying degrees, thus increased the risk of nitrite accumulation. From the perspective of intracellular environment, OTC broke redox balance and antioxidant mechanism, related carbon and nitrogen cycle functional genes were down-regulated, affected amino acid, organic acid and nucleotide metabolic processes. The above results provide important information for evaluating the potential risks of antibiotics on the application of AD, and provide key background and theoretical support for stabilizing the technology.

Study on short-chain fatty acids production from anaerobic fermentation of waste activated sludge pretreated by alkali-activated ammonium persulfate

As a sustainable method for carbon recovery from waste activated sludge (WAS), anaerobic fermentation to produce short-chain fatty acids (SCFAs) is often limited by disintegration of WAS. A novel pretreatment method of alkaline-activated ammonium persulfate (AP/Alk), employing an initial pH of 10 and an ammonium persulfate dosage of 1 mM/g VSS (mmol per gram volatile suspended solids), was proposed in this study to enhance disintegration of WAS and yield of SCFAs. It was compared with one control and five pretreatment groups including alkali, persulfate, free ammonia, ammonium persulfate, alkali-activated sodium persulfate to elucidate the synergistic effects of free ammonia and radicals in WAS dissolution and acidogenesis within the AP/Alk system. The highest sludge disintegration degree with 30.3 % and maximum SCFAs production with 295.4 mg COD/g VSS were achieved by using the method. Comparative analysis showed that free ammonia primarily disrupted microbial cells to release intracellular organics, while radicals preferentially degraded tightly bound extracellular polymeric substances (TB-EPS) proteins. The synergistic effects of free ammonia and radicals accelerated accumulation of soluble proteins and polysaccharides, improved selectively enrichment of hydrolytic-acidogenic genera (e.g., Macellibacteroides, Proteiniclasticum, Desulfobulbus), and upregulated antioxidant genes to alleviate oxidative stress, but suppressed SCFAs consumers (e.g., unclassified_f__Comamonadaceae) including methanogens (e.g., Methanosaeta), thereby promoting the accumulation of SCFAs and acetic acid proportion. AP/Alk offers a sustainable strategy for WAS utilization and energy recovery.

Coupling of biocarriers and dynamic membrane for an enhanced volatile fatty acids production from sludge anaerobic fermentation

Efficiently and economically recovering volatile fatty acids (VFAs) from sludge anaerobic fermentation (AF) poses a significant challenge. This study discovered a synergistic enhancement effect on VFAs production and membrane fouling control by combining polyethylene (PE) biocarriers and dynamic membrane technology (DM) in an anaerobic bioreactor. The reduced sludge particle size and enhanced hydrolysis efficiency led to a VFAs yield of 1200 mg/L, which is 2.4 times higher than that of traditional AF processes and 1.7 times greater than using the DM module alone. The introduction of PE promoted the enrichment of hydrolytic bacteria, particularly the Christensenellaceae_R-7_group, and facilitated the biotransformation of organic matter. The frictional properties of PE significantly reduced DM fouling, maintaining the transmembrane pressure drop below 30 kPa throughout operation without the need for DM module replacement or cleaning. This study presents a novel approach for resource recovery from sludge through AF, offering new opportunities in the field.

Enhancing waste sludge solubilization through radio frequency treatment perforating bacterial cells

Sludge solubilization is known as a rate-limiting step of anaerobic digestion. Although radio frequency (RF) has been applied for sludge pretreatment due to its similar thermal effect as microwave, the potential non-thermal effects of RF treatment remain controversial. In this study, we demonstrate that RF pretreatment enhances the solubilization and lysis of sludge by 8.02%-19.69% through both thermal and non-thermal mechanisms with less energy input. Scanning electron microscope images provide direct evidence that RF-induced microcurrents penetrated bacterial cells, leading to the release of intracellular substances through formed pores. Additionally, the non-thermal effect of RF treatment which could weaken the cell protection and accelerate the lysis rate involves the disruption of binding forces between extracellular polymeric substances and microbial cells. On average, the utilization of RF at a frequency of 27.12 MHz demonstrates its efficacy as a sludge pretreatment technique, as evidenced by a 13.39% reduction in energy consumption and a 16.9% improvement in treatment performance compared to conductive heating (CH). The findings of this study elucidate the possible mechanism of RF treatment of sludge and could establish a theoretical basis for the practical application of RF treatment in sludge management.

Influence of perfluorooctanoic acid on alkaline anaerobic fermentation of waste activated sludge: Perspective from volatile fatty acids production and sludge reduction

Alkaline anaerobic fermentation is an effective approach for resource utilization and reduction of waste activated sludge (WAS). Perfluorooctanoic acid (PFOA) is widespread in WAS, however, its potential impact on alkaline anaerobic fermentation of WAS remains largely unknown. Hence, this study focused on investigating the influence of PFOA on volatile fatty acids (VFAs) production and sludge reduction during alkaline anaerobic fermentation (pH = 10 ± 0.1), as well as the critical mechanisms. Results demonstrated that low PFOA concentration (5 mg/kg-TS) raised VFAs yield to 109.37%, while high levels of PFOA (25 and 50 mg/kg-TS) remarkably decreased VFAs production to 89.55% and 80.44% of the control. Mechanism exploration revealed that PFOA facilitated the solubilization process, and low PFOA level enhanced the accumulation of VFAs via increased bioavailable substrates and the activities of enzymes and microorganisms. On the contrary, high levels of PFOA were substantial biotoxicity, inducing excessive ROS production, causing oxidative damage, and reducing enzyme activity and functional microbial abundance, thereby decreasing VFAs production. Additionally, further analysis of sludge physicochemical properties confirmed that the effect of PFOA on WAS reduction exhibited the same trend as VFAs production. This work provides a basis for PFOA environmental risk assessment and WAS resource utilization.

Chlorinated organophosphorus flame retardants induce the propagation of antibiotic resistance genes in sludge fermentation systems: Insight of chromosomal mutation and microbial traits

Waste activated sludge (WAS) is a critical reservoir for antibiotic resistance genes (ARGs) due to the prevalent misuse of antibiotics. Horizontal gene transfer (HGT) is the primary mechanism for ARGs spread through mobile genetic elements (MGEs). However, the role of non-antibiotic organophosphorus flame retardants (Cl-OFRs) in ARG transmission in the WAS fermentation system remains unclear. This study examines the effects of tris(2-chloroethyl) phosphate (TCEP), a representative Cl-OFR, on ARG dynamics in WAS fermentation using molecular docking and metagenomic analysis. The results showed a 33.4 % increase in ARG abundance in the presence of TCEP. Interestingly, HGT did not appear to be the primary mechanism of ARG dissemination under TCEP stress, as evidenced by a 2.51 % decrease in MGE abundance. TCEP binds to sludge through hydrogen bonds with a binding energy of - 3.6 kJ/mol, leading to microbial damage and an increase in the proportion of non-viable cells. This interaction prompts a microbial shift toward Firmicutes with thick cell walls, which are significant ARG carriers. Additionally, TCEP induces chromosomal mutations through oxidative stress and the SOS response, contributing to ARG formation. Microorganisms also develop multidrug resistance mechanisms to expel TCEP and mitigate its toxicity. This study provides a comprehensive understanding of Cl-OFRs effects on the ARGs fates in WAS fermentation system and offers guidance for the safe and efficient treatment of Cl-OFRs and WAS.

Oxidative stress responses in biofilms

Oxidizing agents are low-molecular-weight molecules that oxidize other substances by accepting electrons from them. They include reactive oxygen species (ROS), such as superoxide anions (O2-), hydrogen peroxide (H2O2), and hydroxyl radicals (HO-), and reactive chlorine species (RCS) including sodium hypochlorite (NaOCl) and its active ingredient hypochlorous acid (HOCl), and chloramines. Bacteria encounter oxidizing agents in many different environments and from diverse sources. Among them, they can be produced endogenously by aerobic respiration or exogenously by the use of disinfectants and cleaning agents, as well as by the mammalian immune system. Furthermore, human activities like industrial effluent pollution, agricultural runoff, and environmental activities like volcanic eruptions and photosynthesis are also sources of oxidants. Despite their antimicrobial effects, bacteria have developed many mechanisms to resist the damage caused by these toxic molecules. Previous research has demonstrated that growing as a biofilm particularly enhances bacterial survival against oxidizing agents. This review aims to summarize the current knowledge on the resistance mechanisms employed by bacterial biofilms against ROS and RCS, focussing on the most important mechanisms, including the formation of biofilms in response to oxidative stressors, the biofilm matrix as a protective barrier, the importance of detoxifying enzymes, and increased protection within multi-species biofilm communities. Understanding the complexity of bacterial responses against oxidative stress will provide valuable insights for potential therapeutic interventions and biofilm control strategies in diverse bacterial species.

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.

Enhancement of sewer sediment control and disruption of adhesive gelatinous sediment structure using low-dose calcium peroxide

Large quantities of sediments in urban sewer systems pose significant risk of pipe clogging and corrosion. Owing to their gel-like structure, sewer sediments have strong resistance to hydraulic shear stress. This study proposed a novel approach to weaken the erosion resistance of sewer sediments by destroying viscous gel-like biopolymers in sediments with low doses of calcium peroxide (CaO2). After treatment with 10-50 mg g-1 TS of CaO2, the critical erosion shear stress was significantly reduced by 25.7%-59.9%. The sediment aggregates gradually disintegrated into small diameter particles with increasing CaO2 dosage. Further analysis showed that the strong oxidizing and alkaline environment induced by CaO2 treatment led to cell lysis and changes in the composition and property of extracellular polymeric substances (EPS). After CaO2 treatment, aromatic proteins and humic acid-like substances associated with adhesion translocated from the inner EPS layers to outer layers while being disintegrated into small organic molecules. Concomitantly, CaO2 treatment disrupted the main functional groups (-OH, COO-, C-N, CO, and CN) in inner EPS layers, thus weakening EPS adhesion. Analysis of protein secondary structure and zeta potential reflected the reduced aggregation capacity of sediment microorganisms and loosening of sediment structure after CaO2 treatment. Thus, CaO2 treatment facilitated fragmentation and disaggregation of the gelatinous structure of sewer sediments. Such green strategy decreased the cost of sewer sediment disposal by 42.10-68.95% when compared to water flushing, and it would improve the self-cleaning capacity of sewer system and efficiency of dredging equipment.