Impacts of chlorination on the population dynamics, pathogenicity, community structures and functional genes of sewage microbiomes

Abstract

Effluent discharge is a major source of pathogenic contamination to the environment. To protect public health, sewage is often chlorinated to remove pathogens before discharge. However, there are some concerns about the safety of chlorinated effluents. Microbes which have high resistance to chlorine stress may survive in the chlorinated effluents. Conventional culture-based enumerations of fecal indicator bacteria (FIB) for monitoring the effluent sanitary quality cannot detect viable but non-culturable (VBNC) cells. Also, there is a lack of information about the fate of microbes in chlorinated effluents after discharge into the receiving aquatic environment. Therefore, a holistic understanding of the impacts of chlorination on the microbiome is needed to evaluate the disinfection outcomes and the performance of monitoring schemes. In this study, effluents were collected from local sewage treatment facilities to examine the alterations of physiological states, community structures, pathogenicity, functional repertoire and gene expression of microbes under chlorination. The results revealed that large proportions of VBNC FIB cells persisted in the chlorinated effluents, indicating that culture-based enumerations are prone to the overestimation of disinfection efficacy. Also, numerous pathogens remained viable after chlorination. The pathogen loads had stronger correlations with the concentrations of FIB detected by PMA-qPCR method than the culture-based methods. Microcosm experiments showed that the bacterial communities from seawater-based effluents survived better in seawater than those from freshwater-based effluents. High chlorine dosages reduced the chance of bacterial regrowth in seawater. On the other hand, metagenomics and metatranscriptomics analyses indicated that the alterations of genetic functions and expression of bacteria under chlorination differed by the types of effluents. Certain types of effluents might have higher risks of causing diseases. The strength of indicator bacteria in detecting different chlorine-resistant bacteria varied with the effluent types. These findings highlighted the importance of tailoring disinfection and monitoring strategies according to the effluent characteristics.

Date of Award	2023
Original language	English
Awarding Institution	The Hong Kong University of Science and Technology
Supervisor	Stanley Chun Kwan LAU (Supervisor)