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Drinking water enters buildings with a given microbiological community composition. Within premise plumbing systems, the drinking water is subject to very different conditions and temperatures. Whereas part of the water stays cold, another part is heated to provide hot water. In this study, drinking water samples were taken at different locations in four buildings that had central heating circles and that were equipped with ultrafiltration modules. The latter were intended to keep bacterial numbers low. When studying the increase in bacterial concentrations in these water samples using regrowth tests at different incubation temperatures, a temperature-dependence could be observed. Bacteria in cold water samples propagated best when incubated at 22°C, but often poorly at 36°C and not at all at 50°C. Bacteria in hot water samples showed the reverse behavior and grew best when incubated at 50°C, whereas growth at 22°C was poor or associated with a long growth lag. Water samples from distal taps in periphery locations used for retrieving both cold and hot water showed intermediate growth behaviors. Results suggest the existence of different temperature-adapted bacterial populations within domestic drinking water systems. The finding was supported by sequence data revealing distinct differences in the microbiomes between cold and hot water samples. Abundant bacterial groups in hot water included Deinococci, Kryptonia, Ignavibacteria, Nitrospiria, Gemmatimonadetes and different genera of Gammaproteobacteria. Stagnation of hot water at 50°C, 55°C, or 60°C furthermore shaped the microbiome in different ways indicating that small temperature differences can have a substantial impact on the bacterial communities.

This review discusses the relationship between dissolved oxygen and Legionella growth. Growth of Legionella in building water systems is considered to pose a health risk and controlling it could benefit public health by reducing the number of healthcare and community acquired Legionnaires’ disease cases. Conditions known to encourage Legionella growth include low disinfectant concentrations, warm temperature, and water stagnation. Legionella is an obligate aerobe meaning its cellular respiration is inhibited under anaerobic conditions. Despite evidence from earlier published studies the importance of dissolved oxygen for the survival and growth of Legionella has received little attention from researchers and public health authorities. Modern measurement devices can reliably determine if dissolved oxygen concentrations in potable water and other building water systems are at levels capable of supporting Legionella growth or inhibiting its amplification. Removing dissolved oxygen from water can be achieved using gas transfer membrane contactors. Water with low dissolved oxygen levels interferes with Legionella’s cellular respiration by oxygen starvation, whereas disinfectants achieve the same effect by interfering with oxygen transport across the cell membrane. Ultimately, the same effect on Legionella growth may be achieved by reducing dissolved oxygen as treatment with oxidants, while avoiding the drawbacks of corrosion and disinfectant byproducts.

This review discusses Legionella, among the most prolific and publicly well-known waterborne pathogens, and advances in potential treatment technologies. The number of cases associated with Legionella continues to rise, as does its public awareness. Currently, cases associated with premise plumbing account for the largest number of legionellosis cases in the United States. So, while it is important to understand Legionella as such, it is also important to investigate how to treat drinking water in premise plumbing for Legionella and other waterborne pathogens. While there are currently several methods recognized as potential means of inactivating waterborne pathogens, several shortcomings continue to plague its implementation. These methods are generally of two types. Firstly, there are chemical treatments such as chlorine, chlorine dioxide, monochloramine, ozone, and copper-silver ionization. Secondly, there are physical treatments such as thermal inactivation and media filtration. Their shortcomings range from being labor-intensive and costly to having negative health effects if not properly operated. Recently developed technologies including ultraviolet (UV) irradiation using light emitting diodes (LEDs) and innovative carbon nanotube (CNT) filters can better control waterborne pathogens by allowing for the simultaneous use of different treatment measures in plumbing systems.