Do particles in wastewater protect pathogens from disinfection

Author: Charndeep Chahal

Chahal, Charndeep, 2020 Do particles in wastewater protect pathogens from disinfection, Flinders University, College of Medicine and Public Health

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Abstract

Ultraviolet (UV) radiation and chlorination are widely used in drinking water and recycled water disinfection. Both organic and inorganic particles can limit disinfection efficiency by protecting pathogens in various ways, such as physical shielding and increasing disinfectant demand. The negative impact of particles is well documented, and the particle size required to offer protection differs for and is dependent on each pathogen type. This increases the disinfectant demand and can require additional treatment, such as filtration, thereby increasing the treatment costs. The extent of association of pathogens (bacteria, viruses or protozoans) with the particles vary widely and is dependent on factors such as the nature (organic or inorganic) and size of particles, wastewater environment and treatment type. A better understanding of the various pathogen associations and their impact on disinfection kinetics is required to devise alternate cost-effective treatment options. This thesis investigates the association behaviour of indicator organisms and the effect of particles on UV and chlorine disinfection in treated wastewater effluent from South Australian and Victorian wastewater treatment plants.

Scanning electron microscopy coupled with elemental analyser was used to analyse the shape, structure and composition of various particles. Additional parameters, such as nutrients, solids, and particle surface charge, were monitored to characterise the water quality and particulate environment. The surrogate organisms used in this study were E. coli, FRNA bacteriophage (or MS-2 phage) and anaerobic bacterial spores, and the fraction of organisms associated with particles was determined using centrifugation, serial filtration and homogenisation (for spores and E. coli only). The association behaviour of C. parvum oocysts was also studied using the same techniques and compared with anaerobic spores. Disinfection experiments using UV irradiation and chlorination were conducted to determine if there were any differences in the inactivation of particle-associated and free organisms, or if particles interfered in disinfection.

Unique particulate environments displaying specific elemental, size and charge characteristics for particles in the different effluents were identified. A new SEM protocol was developed to semi-quantitative the contribution of biological (organic) and inorganic particles in samples. Adelaide lagoon effluent had the highest monthly average number of particles in all the size classes, which were mostly inorganic in nature (70 %) compared to the Adelaide secondary effluent and Melbourne lagoon effluent. Adelaide lagoon effluent had higher numbers of larger particles than the other two samples. E. coli and anaerobic spores were associated with 1.2 and 10 ┬Ám particles respectively. FRNA bacteriophage did not show any association with particles. Anaerobic spores were not ideal surrogates for C. parvum oocysts; the oocysts showed some evidence of particle association, but the extend of removal due to potential particle association was much lower than that observed for spores. The presence of particles negatively impacted disinfection efficiency.

I conclude that particle association is greatly dependent on the characteristics of the pathogen / surrogate, particle size, and potentially other factors that impact particle charge and surface interactions. Homogenisation and filtration can be used as effective methods for particle removal or particle / pathogen disaggregation. For the doses tested, particles negatively impacted the efficiency of UV disinfection, and filtration greatly increased the efficiency disinfection by removing particles. Our findings will help selective designing of effective filtration methods for pathogen removal.

Keywords: particles, pathogen, association, disinfection

Subject: Microbiology & Infectious Diseases thesis

Thesis type: Doctor of Philosophy
Completed: 2020
School: College of Medicine and Public Health
Supervisor: Fiona Young