Transport of Enterococcus faecalis in granular activated carbon column: Potential energy, migration, and release

https://doi.org/10.1016/j.colsurfb.2019.110415Get rights and content

Highlights

Abstract

An increasing number of water purification plants use granular activated carbon filtration as an advanced treatment technology. One of the main constraints of carbon filtration is bacterial leakage, which can impact public drinking water safety. In this study, Enterococcus faecalis, commonly detected in natural water, was employed as the target bacteria for investigating the mechanism of deposition and migration of bacteria in granular activated carbon medium. The repulsive barrier, secondary potential well and potential energy change curve under various conditions were depicted by DLVO theory. Moreover, the influence, including ionic strength, ionic charge and flow rate, on bacterial transport was comprehensively discussed. The Enterococcus faecalis penetration curve was in accordance with the van der Waals force and electrostatic repulsion force under different conditions. Finally, 8%–11% of Enterococcus faecalis was released into the effluent when ionic strength declined.

Introduction

The safety of drinking water quality is closely related to people's life and physical health, and microbes are one of the key factors affecting human health [1,2]. According to reports, the population drinking faecal-contaminated water sources in 2012 was approximately 1.5 billion [3], and approximately 88% of diarrhoeal diseases worldwide and 1.7 million deaths are due to unsafe drinking water, sanitation and personal hygiene [4]. As domestic sewage, hospital sewage, livestock manure and landfill leachate contain a large number of pathogenic microorganisms, they can enter the aquatic environment and pose a potential threat to the safety of drinking water quality [5]. Enterococcus faecalis is commonly found in the intestines of humans and animals and its detection rate in faeces is second only to Escherichia coli [6]. It can survive in a variety of environments and has certain drug resistance, and as a pathogenic bacteria, can cause endocarditis, bacteremia, urinary tract infections, meningitis, and so on [7].

Granular activated carbon filtration has become one of the mainstream processes for advanced treatment of drinking water in China due to the high efficiency in removing pollution [8]. For example, in Hunan province, China, carbon filtration is the most popular choice for waterworks quality reconstruction. In general, the granular activated carbon filtration process occurs at the back end of the water treatment process. The filtered water flows into a disinfection pool and then enters the municipal pipe network and user taps [9]. Regardless of whether the pre-ozone oxidation unit is set, the granular activated carbon filtration effluent contains a certain amount of bacteria (including pathogenic bacteria), i.e., the "bio-leakage" phenomenon. The leakage of pathogenic bacteria can increase consumption of disinfectants and their metabolites might generate carcinogens [10]. The problem of microbial leakage in carbon filtration essentially involves two processes: the adsorption and deposition of the granular activated carbon on the influent microorganisms in the carbon filter. Under desorption and along with the non-adsorbed deposition of microorganisms, micro floc particles are carried with the flow of water in the biological activated carbon migration process. In summary, the biological leakage during the activated carbon process in the water plant is related to the safety of public drinking water, and it involves the deposition and migration mechanism of bacteria in the granular activated carbon medium.

To study the migration of bacteria in the formation, many scholars have studied the deposition and migration mechanisms of bacteria in porous media such as quartz sand, soil and glass beads. The results show that the migration of microorganisms in porous media is affected by the nature of the microorganisms (surface properties [11], shape [12] and nutritional status [13]), porous media properties (particle size [14], saturation [15] and the influence of organic matter composition [16,17]) and environmental factors (hydraulic conditions [18], ionic strength [19], pH [20] and temperature [21]). However, the law of deposition and migration of bacteria in granular activated carbon columns has not been comprehensively analysed. In this work, the migration and transformation of Enterococcus faecalis in granular activated carbon column were investigated. The impact of ionic strength, ionic charge and flow rate on the bacteria transport and deposition were studied. The DLVO theory was employed to explore the mechanism of bacteria migration.

Section snippets

Materials

The Enterococcus faecalis strain, purchased by the China Centre of Industrial Culture Collection, was stored in a preservation tube in a -80 ℃ refrigerator to retain its biological properties. To conduct the experiment, the Enterococcus faecalis was first placed in a 37 ℃ constant temperature water bath for 30 min, and then inoculated and transferred to the LB medium. The Enterococcus faecalis in the late logarithmic growth, was employed in a further study [21]. A spectrophotometry method was

Potential energy of Enterococcus faecalis and granular activated carbon

The zeta potentials on the surface of Enterococcus faecalis and granular activated carbon are shown in Table 1. When the ionic strengths were 0 mmol/L, 10 mmol/L, 20 mmol/L NaCl and 10 mmol/L CaCl2, the zeta potentials of Enterococcus faecalis and granular activated carbon were negative, indicating that the electrostatic repulsion force existed under those conditions. When the concentration of the NaCl solution increased from 0 mmol/L to 20 mmol/L, the zeta potential of Enterococcus faecalis

Conclusion

Studying the migration mechanism of Enterococcus faecalis in granular activated carbon medium can lead to a better understanding of the leakage of pathogenic bacteria in activated carbon filter, which is very important for ensuring the safety of drinking water and human health. When a higher ionic concentration was employed, the Enterococcus faecalis tend to be disordered. The energy of the repulsive barrier and secondary potential well disappeared under the 20 mmol/L NaCl background solution.

Acknowledgments

This work was financially supported by the Natural Science Foundation of Hunan province, China (Grants 2017JJ2030).

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