Effects of assimilable organic carbon and free chlorine on bacterial growth in drinking water.

Xiaolu Liu,Xiaoqing He,Weiwen Kong,Yi Jin,Tingting Liu,Bolin Zhang,Jingqi Wang,Rodolfo Paranhos

doi:10.1371/journal.pone.0128825

Abstract

Assimilable organic carbon (AOC) is one of the most important factors affecting the re-growth of microorganisms in drinking water. High AOC concentrations result in biological instability, but disinfection kills microbes to ensure the safety of drinking water. Free chlorine is an important oxidizing agent used during the disinfection process. Therefore, we explored the combined effects of AOC and free chlorine on bacterial growth in drinking water using flow cytometry (FCM). The initial AOC concentration was 168 μg.L-1 in all water samples. Without free chlorine, the concentrations of intact bacteria increased but the level of AOC decreased. The addition of sodium hypochlorite caused an increase and fluctuation in AOC due to the oxidation of organic carbon. The concentrations of intact bacteria decreased from 1.1×105 cells.mL-1 to 2.6×104 cells.mL-1 at an initial free chlorine dose of 0.6 mg.L-1 to 4.8×104 cells.mL-1 at an initial free chlorine dose of 0.3 mg.L-1 due to free chlorine originating from sodium hypochlorite. Additionally, free chlorine might be more obviously affected AOC concentrations than microbial growth did. These results suggested that AOC and free chlorine might have combined effects on microbial growth. In this study, our results showed concentrations determined by FCM were higher than those by HPC, which indicated that some E. coli detected by FCM might not be detected using HPC in drinking water. The level of free chlorine might restrain the consumption of AOC by inhibiting the growth of E. coli; on the other hand, chlorination might increase the level of AOC, thereby increase the potential for microbial growth in the drinking water network.

Highlights

With global industrialization and economization, aquatic pollution has become a problem worldwide [1]
The general microbial quality of drinking water is normally monitored by heterotrophic plate count (HPC)
The results (Fig 1 and supplementary material) indicated that the intact cell counts determined by flow cytometry (FCM) were higher than those by HPC, meaning some VBNC E. coli detected by FCM might not be detected using HPC

Summary

Introduction

With global industrialization and economization, aquatic pollution has become a problem worldwide [1]. Assimilable Organic Carbon and Free Chlorine on Bacterial Growth quality and causes potential risks to human health. Waterborne disease outbreaks caused by pathogenic microorganisms are commonplace [3, 4]. To ensure the safety of drinking water, microbial quality standards have been established globally. United States standards dictate that the heterotrophic plate count must be below 500 CFU.mL-1. The European Union directive states that for every 100 mL of drinking water, no total coliforms should be detected [5], and in China, heterotrophic plate counts in tap water must be lower than 100 CFU. ML-1, while no coliforms or fecal coliforms should be detected in 100 mL of water [6] The European Union directive states that for every 100 mL of drinking water, no total coliforms should be detected [5], and in China, heterotrophic plate counts in tap water must be lower than 100 CFU. mL-1, while no coliforms or fecal coliforms should be detected in 100 mL of water [6]

Methods

Results

Discussion

Conclusion