Exploration of how UV disinfection affects the microbial community in drinking water

Abstract

UV irradiation is getting increasingly popular as a disinfection step in drinking water production. Ringsjöverket, a drinking water treatment plant in Sweden installed a full-scale UV aggregate in December 2016. However, the effect of UV irradiation on drinking water has mainly been tested on specific pathogens in labscale environment. Heterotrophic plate counts are often used when analyzing drinking water despite the fact that the results may not always relate to the total bacteria population or the existence of harmful pathogens. This study investigated the effect of UV on the whole bacterial community with heterotrophic plate counts (HPCs), flow cytometry (FCM) and qPCR.
Untreated water directly before Ringsjöverket UV aggregate, water treated with Ringsjöverket ́s UV aggregate and water treated with a labscale UV aggregate was analyzed. Water was stored in light-impermeable 20 l water containers at 7 °C and 22 °C. The water was then analyzed as a function of days after UV irradiation up to 10 days.
HPC resulted in that the UV treated samples showed less diversity in terms of morphology of CFUs, but also had more CFUs. This could imply that UV has a selective effect. FCM analysis of intact cells pointed to that the UV treated samples did not grow back, but also did not die off completely. This can be compared with an untreated reference where bacteria count increased from initial levels. The results from the intact cell analysis implies that UV inhibits the growth of bacteria but does not necessarily have a direct killing effect. Flow cytometric analysis pointed towards a decrease in the fraction of cells with high nucleic acid content compared to untreated reference. At higher temperature the decrease was followed by an increase a couple of days later. This implies that UV initially changes the bacteria community composition but that the effect is not permanent due to repair mechanisms or that some bacteria does not get affected by UV and then start growing. The instant effects of UV were not detected by HPC or FCM but seen with qPCR targeting the 16S rRNA gene. This method showed that untreated samples had more DNA that could be amplified, and that Ringsjöverket UV seemed to result in less DNA damage than the labscale aggregate.
Further studies could be directed at investigating different dyes for flow cytometric analysis, such as dyes that targets bacteria at different rates depending on metabolic activity. Also, detection of UV damage through qPCR could be improved through using longer amplicon length. To get even further insight in how the taxonomic composition of bacteria is changed by UV, the amplicons from qPCR can be sequenced.

Popular Abstract

Safe drinking water is essential for human survival, society and countless industries. The water that comes from the taps in Sweden is treated in large scale drinking water treatment plants to ensure this. The treatment plants use a multiple barrier approach to take care of different aspects, one of the more important being treatment of microbiological properties. However, these barriers are in continuous need of improvement due to increasing population and changes in climate. In 2011 there was a Cryptosporidium outbreak in Östersund with estimated costs for the society up to 220mSEK. Parasites such as Cryptosporidium and Giardia are resistant to chlorine, a method commonly used for drinking water disinfection.
The parasites Cryptosporidium and Giardia and other pathogens can however effectively be killed of with UV irradiation. This has led to an increase of the usage of UV in drinking water treatment plants over the world. Ringsjöverket, a drinking water treatment plant that provide water for parts of northwestern Skåne in Sweden, installed its very own UV aggregate as a part of the treatment process in 2016. UV has proven to be effective as a disinfection method and has the positive effect of not using chemicals. However, research has mainly been directed at investigating how specific pathogens behave after UV irradiation. Often the research has been conducted in labscale environments.
The aim of this thesis was to explore how the whole bacterial community is affected by UV irradiation. Water from Ringsjöverket, that had either been irradiated with Ringsjöverket UV aggregate or a labscale aggregate as well as untreated reference water was taken in early autumn 2017 and then stored at either 7 °C or 22 °C. The water was analyzed with three methods to see what happens up to ten days after UV irradiation. The first method, Heterotrophic plate count (HPC), is a method that counts the number of culturable bacteria in a water sample. It is a standard method for analyzing drinking water, but this has been debated recently since neither the total bacteria count nor the existence of harmful microorganisms can always be related to the results of HPC.
The second method that was used, flow cytometry (FCM), is FCM can accurately detect the number of bacteria in a sample through staining the DNA of the cells which then will emit a detectable light signal when shot with a laser. This method also allows detection of changes in the community since the relative amount of DNA for each cell also can be detected. FCM is increasing in popularity when it comes to analyzing drinking water due to its easiness to use and robust results. The last method, qPCR, analyzes the amount of DNA. This method is of great interest in detecting UV-related damage since UV irradiation damages the DNA and in theory makes it less detectable in qPCR.
Interestingly the HPC showed that the UV irradiated samples had more regrowth than the untreated reference. These results contradicted results from both FCM and qPCR that yielded higher amounts of intact cells and DNA respectively in the untreated reference after a couple of days. Elevated storage temperature of the samples had the effect of both increasing growth of the untreated reference but also contributing to a decline of bacteria in the UV irradiated samples. The instant effect of UV irradiation could only be detected with qPCR. The labscale aggregate proved to have a slightly larger effect compared to the full scale aggregate at Ringsjöverket. With increasing use of UV in water treatment plants further optimization of these methods is a good idea in order to ensure that the effectiveness of UV irradiation is sufficient.

• author: Ekman, Sakarias
• supervisor: Kristjan Pullerits; Peter Rådström
• organization: Applied Microbiology
• course: KMBM05 20172
• year: 2018
• type: H2 - Master's Degree (Two Years)
• subject: Technology and Engineering; Biology and Life Sciences
• keywords: heterotrophic plate count, qPCR, flow cytometry, drinking water, UV disinfection, bacterial community, applied microbiology, teknisk mikrobiologi
• language: English
• id: 8938992
• date added to LUP: 2018-05-15 14:05:36
• date last changed: 2018-05-15 14:05:36