Membrane biofouling by chlorine resistant Bacillus spp.: effect of feedwater chlorination on bacteria and membrane biofouling

Effect of NaClO and ClO2 on the bacterial properties in a reclaimed water distribution system: efficiency and mechanisms

Extensive application of reclaimed water alleviated water scarcity obviously. Bacterial proliferation in reclaimed water distribution systems (RWDSs) poses a threat to water safety. Disinfection is the most common method to control microbial growth. The present study investigated the efficiency and mechanisms of two widely used disinfectants: sodium hypochlorite (NaClO) and chlorine dioxide (ClO₂) on the bacterial community and cell integrity in effluents of RWDSs through high-throughput sequencing (Hiseq) and flow cytometry, respectively. Results showed that a low disinfectant dose (1 mg/L) did not change the bacterial community basically, while an intermediate disinfectant dose (2 mg/L) reduced the biodiversity significantly. However, some tolerant species survived and multiplied in high disinfectant environments (4 mg/L). Additionally, the effect of disinfection on bacterial properties varied between effluents and biofilm, with changes in the abundance, bacterial community, and biodiversity. Results of flow cytometry showed that NaClO disturbed live bacterial cells rapidly, while ClO₂ caused greater damage, stripping the bacterial membrane and exposing the cytoplasm. This research will provide valuable information for assessing the disinfection efficiency, biological stability control, and microbial risk management of reclaimed water supply systems.

Biofilm enhances the copper (II) adsorption on microplastic surfaces in coastal seawater: Simultaneous evidence from visualization and quantification

Microplastics (MPs) exposed to the urban coastal seawater could form biofilms, which facilitate the adsorption and transportation of hazardous contaminants. However, influence of biofilms on the metal adsorption of MPs, especially the co-existence of biofilm and metals on MPs, is still less known. In this study, the adsorption of copper (Cu) on biofilm-coated MPs (BMPs) was visually analyzed and quantified. The results of scanning electron microscopy in combination with energy dispersive X-ray showed that biofilm and metals co-occurred on MPs in seawater. The nanoscale secondary ion mass spectrometry images further exhibited that the distribution of Cu, chlorine (Cl) and biofilm on MP surfaces was highly consistent. Moreover, the adsorption of Cu(II) on BMPs was enhanced as quantified by inductively coupled plasma-mass spectrometer. Furthermore, different species on BMPs with and without Cu were identified, and their potential functions of metal or Cl metabolism were predicted based on KEGG pathway database. Overall, for the first time, this study provides visual and quantified evidences for the enhancement of Cu(II) adsorption on BMPs based on co-localization, and it may shed a light on the development of methodologies for investigating the interaction among MPs, biofilms and pollutants in marine environment.

Autopsy of Used Reverse Osmosis Membranes from the Largest Seawater Desalination Plant in Oman

The Barka desalination plant, commissioned in 2018, is the largest desalination plant in Oman. It has a capacity of 281 MLD with a reverse osmosis (RO) first-pass recovery rate of 46%. As part of the standard operator practice, a membrane autopsy was conducted to determine the cause of reductions in membrane performance. This study investigated fouled membranes (model No. SW30HRLE-440) from two different locations in the membrane rack. Various analytical methods were used to conduct the membrane autopsy. Field-emission scanning electron microscopy/energy-dispersive X-ray (FESEM/EDS) analyses of membrane samples showed major components of inorganic foulants. Moreover, black and salt-like crystals deposited on the membrane surface revealed significant carbon (C) components and oxygen (O), with a small amount of magnesium (Mg), chloride (Cl), sodium (Na), aluminium (Al), and calcium (Ca), respectively. A Fourier transform infrared (FTIR) analysis revealed the presence of long-chain hydrocarbons, carboxylic acids/esters, carbohydrates/polysaccharides, and inorganic foulants. Thermogravimetric analyses (TGA) of the membranes showed a high initial weight loss due to organic and inorganic fouling. X-ray photoelectron (XPS) analyses further confirmed the presence of inorganic and organic foulants on the membrane surfaces. Bacteria identification results showed the presence of Bacillus cereus and Bacillus marisflavi. This paper offers a detailed analysis of the foulants present on the reverse osmosis membrane surface and sub-surface before and after a cleaning process.

Layered Antibiofouling Composite Membrane for Quenching Bacterial Signaling

Bacterial quorum quenching (QQ) media with various structures (e.g., bead, cylinder, hollow cylinder, and sheet), which impart biofouling mitigation in membrane bioreactors (MBRs), have been reported. However, there has been a continuous demand for membranes with QQ capability. Thus, herein, we report a novel double-layered membrane comprising an outer layer containing a QQ bacterium (BH4 strain) on the polysulfone hollow fiber membrane. The double-layered composite membrane significantly inhibits biofilm formation (i.e., the biofilm density decreases by ~58%), biopolymer accumulation (e.g., polysaccharide), and signal molecule concentration (which decreases by ~38%) on the membrane surface. The transmembrane pressure buildup to 50 kPa of the BH4-embedded membrane (17.8 h ± 1.1) is delayed by more than thrice (p < 0.05) of the control with no BH4 in the membrane’s outer layer (5.5 h ± 0.8). This finding provides new insight into fabricating antibiofouling membranes with a self-regulating property against biofilm growth.

Chlorine Tolerance and Cross-Resistance to Antibiotics in Poultry-Associated Salmonella Isolates in China

Chlorine disinfectants have been widely used in the poultry supply chain but this exposure can also result in the development of bacterial tolerance to chlorine and this is often linked to antibiotic cross-resistance. The objectives of this study were to investigate sodium hypochlorite (NaClO) tolerance of Salmonella isolated from poultry supply chains and evaluate cross-resistance. We collected 172 Salmonella isolates from poultry farms, slaughter houses and retail markets in China during 2019–2020. We found that S. Enteritidis, S. Kentucky, and S. Typhimurium constituted &gt; 80% of our Salmonella isolates. Overall, 68% of Salmonella isolates were resistant to &gt; 3 antibiotics and S. Kentucky displayed a significantly (p &gt; 0.05) higher frequency (93.2%) of multidrug resistance than the other serovars. Tolerance to chlorine at MIC &gt; 256 mg/L was detected in 93.6% of isolates (161/172) and tolerant isolates displayed higher decimal reduction times (D value) and less ultrastructural damage than did the suspectable strains under chlorine stress. Spearman analysis indicated significant positive correlations between chlorine tolerance (evaluated by the OD method) and antibiotic resistance (p &lt; 0.05) to ceftiofur, tetracycline, ciprofloxacin and florfenicol and this was most likely due to efflux pump over-expression. The most frequently detected chlorine resistance gene was qacEΔ1 (83.1%, n = 143) and we found a positive correlation between its presence and MIC levels (r = 0.66, p &lt; 0.0001). Besides, we found weak correlations between chlorine-tolerance and antibiotic resistance genes. Our study indicated that chlorine disinfectants most likely played an important role in the emergence of chlorine tolerance and spread of antibiotic resistance and therefore does not completely control the risk of food-borne disease. The issue of disinfectant resistance should be examined in more detail at the level of the poultry production chain.

Assessment of physiological responses of bacteria to chlorine and UV disinfection using a plate count method, flow cytometry and viability PCR

AIMS

This study aimed to investigate the physiological responses of two gram-negative bacteria (Escherichia coli and Pseudomonas aeruginosa) and two gram-positive bacteria (Enterococcus faecalis and Bacillus sphaericus) to ultraviolet (UV) and chlorine disinfection.

METHODS AND RESULTS

Bacterial inactivation by UV and chlorine disinfection were evaluated with a plate count method for culturability, FCM and PMA-qPCR for membrane integrity and DyeTox13-qPCR for enzymatic activity, respectively. Both UV and chorine disinfection caused complete loss of culturability while membrane integrity remained intact after UV disinfection. Both DyeTox13-qPCR and PMA-qPCR showed high ΔC_t values up to 8.9 after chlorine disinfection, indicating that both methods were able to distinguish non-treated from chlorine-treated cells. Although PMA-qPCR could not differentiate membrane integrity of cells on UV exposure, DyeTox13-qPCR showed significant differences in ΔC_t values of 5.05 and 10.4 for gram-negative (E. coli) and gram-positive (Enterococcus) bacteria, respectively. However, DyeTox13-qPCR for gram-negative bacteria displayed relatively small differences in ΔC_t values compared with gram-positive bacteria.

CONCLUSION

UV and chlorine disinfection led to changes in physiological state of gram-negative and gram-positive bacteria. Particularly, UV disinfection could induce active but non-culturable (ABNC) for gram-negative bacteria and dormant cell for gram-positive bacteria where intact cells no longer showed the enzymatic activity.

SIGNIFICANCE AND IMPACT OF THE STUDY

UV and chlorine are commonly used to disinfect water, food and fomites to inactivate pathogenic bacteria. However, a viable but non-culturable (VBNC) state of bacteria induced by disinfection may underestimate the health risks because of the potential resuscitation of VBNC cells. This study highlighted that bacteria could undergo different physiological (ABNC or dormant) states during UV and chlorine disinfection. In addition, viability PCR techniques could provide insight into the changes in physiological states during disinfection processes.

Physicochemical Properties of Extracellular Polymeric Substances Produced by Three Bacterial Isolates From Biofouled Reverse Osmosis Membranes

This work describes the chemical composition of extracellular polymeric substances (EPS) produced by three bacteria (RO1, RO2, and RO3) isolated from a biofouled reverse osmosis (RO) membrane. We isolated pure cultures of three bacterial strains from a 7-year-old biofouled RO module that was used in a full-scale seawater treatment plant. All the bacterial strains showed similar growth rates, biofilm formation, and produced similar quantities of proteins and polysaccharides. The gel permeation chromatography showed that the EPS produced by all the strains has a high molecular weight; however, the EPS produced by strains RO1 and RO3 showed the highest molecular weight. Fourier Transform Infrared Spectroscopy (FTIR), Proton Nuclear Magnetic Resonance (¹H NMR), and Carbon NMR (¹³C NMR) were used for a detailed characterization of the EPS. These physicochemical analyses allowed us to identify features of EPS that are important for biofilm formation. FTIR analysis indicated the presence of α-1,4 glycosidic linkages (920 cm^-1) and amide II (1,550 cm^-1) in the EPS, the presence of which has been correlated with the fouling potential of bacteria. The presence of α-glycoside linkages was further confirmed by ¹³C NMR analysis. The ¹³C NMR analysis also showed that the EPS produced by these bacteria is chemically similar to foulants obtained from biofouled RO membranes in previous studies. Therefore, our results support the hypothesis that the majority of substances that cause fouling on RO membranes originate from bacteria. Investigation using ¹H NMR showed that the EPS contained a high abundance of hydrophobic compounds, and these compounds can lead to flux decline in the membrane processes. Genome sequencing of the isolates showed that they represent novel species of bacteria belonging to the genus Bacillus. Examination of genomes showed that these bacteria carry carbohydrates-active enzymes that play a role in the production of polysaccharides. Further genomic studies allowed us to identify proteins involved in the biosynthesis of EPS and flagella involved in biofilm formation. These analyses provide a glimpse into the physicochemical properties of EPS found on the RO membrane. This knowledge can be useful in the rational design of biofilm control treatments for the RO membrane.

Evaluating method and potential risks of chlorine-resistant bacteria (CRB): A review

Chlorine-resistant bacteria (CRB) are commonly defined as bacteria with high resistance to chlorine disinfection or bacteria which can survive or even regrow in the residual chlorine. Chlorine disinfection cannot completely control the risks of CRB, such as risks of pathogenicity, antibiotic resistance and microbial growth. Currently, researchers pay more attention to CRB with pathogenicity or antibiotic resistance. The microbial growth risks of non-pathogenic CRB in water treatment and reclamation systems have been neglected to some extent. In this review, these three kinds of risks are all analyzed, and the last one is also highlighted. In order to study CRB, various methods are used to evaluate chlorine resistance. This review summarizes the evaluating methods for chlorine resistance reported in the literatures, and collects the important information about the typical isolated CRB strains including their genera, sources and levels of chlorine resistance. To our knowledge, few review papers have provided such systematic information about CRB. Among 44 typical CRB strains from 17 genera isolated by researchers, Mycobacterium, Bacillus, Legionella, Pseudomonas and Sphingomonas were the five genera with the highest frequency of occurrence in literatures. They are all pathogenic or opportunistic pathogenic bacteria. In addition, although there are many studies on CRB, information about chlorine resistance level is still limited to specie level or strain level. The difference in chlorine resistance level among different bacterial genera is less well understood. An inconvenient truth is that there is still no widely-accepted method to evaluate chlorine resistance and to identify CRB. Due to the lack of a unified method, it is difficult to compare the results about chlorine resistance level of bacterial strains in different literatures. A recommended evaluating method using logarithmic removal rate as an index and E. coli as a reference strain is proposed in this review based on the summary of the current evaluating methods. This method can provide common range of chlorine resistance of each genus and it is conducive to analyzing the distribution and abundance of CRB in the environment.

Boosting oxygen reduction and permeability properties of doped iron-porphyrin membrane cathode in microbial fuel cells

To achieve a membrane cathode with excellent performance, iron-porphyrin (Fe(por)) was doped to boost the catalytic and permeability properties in microbial fuel cell (MFC). The membrane cathode with the optimal 0.05 g of Fe(por) (denoted as Fe(por)-0.05) had the highest current density of 10.3 A m^-2 and the lowest charge transfer resistance of 12.6 ± 0.3 Ω. The ring-disk electrode (RDE) results further proved that the oxygen reduction reaction (ORR) occurred on the Fe(por)-0.05 through a direct four-electron transfer pathway. Moreover, the membrane cathode performed better permeability properties under electric filed and the Fe(por)-0.05 + E (E was electric field) obtained the lowest flux attenuation ratio of 14.1 ± 0.2%, which was related to its superior hydrophilicity and strong electrostatic repulsion force. Iron-porphyrin can simultaneously enhance the ORR activity and permeability of membrane cathode, providing a new direction for the practical application in MFCs.

Comparison between a Conventional Anti-Biofouling Compound and a Novel Modified Low-Fouling Polyethersulfone Ultrafiltration Membrane: Bacterial Anti-Attachment, Water Quality and Productivity

In this work, the efficiency of a conventional chlorination pretreatment is compared with a novel modified low-fouling polyethersulfone (PES) ultrafiltration (UF) membrane, in terms of bacteria attachment and membrane biofouling reduction. This study highlights the use of membrane modification as an effective strategy to reduce bacterial attachment, which is the initial step of biofilm formation, rather than using antimicrobial agents that can enhance bacterial regrowth. The obtained results revealed that the filtration of pretreated, inoculated seawater using the modified PES UF membrane without the pre-chlorination step maintained the highest initial flux (3.27 ± 0.13 m³·m⁻²·h⁻¹) in the membrane, as well as having one and a half times higher water productivity than the unmodified membrane. The highest removal of bacterial cells was achieved by the modified membrane without chlorination, in which about 12.07 × 10⁴ and 8.9 × 10⁴ colony-forming unit (CFU) m⁻² bacterial cells were retained on the unmodified and modified membrane surfaces, respectively, while 29.4 × 10⁶ and 0.42 × 10⁶ CFU mL⁻¹ reached the filtrate for the unmodified and modified membranes, respectively. The use of chlorine disinfectant resulted in significant bacterial regrowth.

Effect of pre-oxidation on low pressure membrane (LPM) for water and wastewater treatment: A review

Low pressure membrane (LPM) filtration is a promising technology for drinking water production, wastewater reclamation as well as pretreatment for seawater desalination. However, wider implementation of LPM is restricted by their inherent drawbacks, i.e., membrane fouling and insufficient rejection for dissolved contaminants. Pretreatment of feed water is a major method to improve the performance of LPM, and pre-oxidation has gained extensive attention because it can significantly alter compositions and properties of feed water through chemical reactions. This paper attempts to systematically review efficiency and mechanisms of pre-oxidation in membrane fouling control and permeate water quality improvement. On the basis of briefly discussing major foulants and fouling mechanisms of LPM, advantages and disadvantages of pre-oxidation in mitigating organic fouling, inorganic fouling and biofouling are discussed in detail. Impacts of pre-oxidation on removal of micropollutants, bulk organic matter and inorganic pollutants are summarized, and potential by-products of different oxidants are presented. As a prerequisite for the integration of chemical oxidation with LPM filtration, compatibility of membrane with oxidants at low concentration and long exposure time are highlighted. Finally, the existing challenges and future research needs in practical application of chemical oxidation to improve performance of LPM are also discussed.