## 3 Factors Governing Survival and Occurrence of Opportunistic Pathogens in Premise Plumbing Systems
### Temperature Control  
As previously outlined, many studies have established that water temperatures between certain thresholds correlate with OPPP colonisation [148; 149; 150]. Accordingly, decontamination of premise hot water systems frequently occurs by 'thermal pasteurisation' or a'superheat-and-flush' approach. This involves raising hot water heater temperatures to around 70 \({}^{\circ}\)C for a period of time usually around 24 h before 'flushing' at each outlet for a minimum of 5 min. The water temperature should reach at least 60 \({}^{\circ}\)C at the outlet. For this treatment method to be effective, dead-legs must be minimised or eliminated. This can be achieved during the design phase by shutting off valves in existing systems. Without first completing this step, there is a significant risk that these stagnant areas can re-seed the system following treatment [9]. There should be consideration of potential scale build-up inside the piping of older buildings. Under these conditions, scale functions as an insulator to protect the _Legionella_ bacteria buried within from the temperatures required for killing. Additional flushing time could be the solution; however, protocols are required to manage the inadvertent risks of scaling and flooding. A minimum of eight hours is recommended to allow the entire scale layer to become heated to over 60 \({}^{\circ}\)C [33]. In general, superheat-and-flush should only be implemented as a component for the short-term management of premise plumbing pathogens, since it can cause infrastructure damage, does not prevent re-seeding, and can exacerbate other water quality issue including tastes and odours. For example, several studies have observed recolonisation by _Legionella_ within weeks to months after treatment [15; 43; 151]. [PERSON] et al. [152] also demonstrated the ability of _Legionella pneumophila_ to rapidly proliferate after temperatures were lowered and suggested this was a microbial response to nutrients released by newly killed biofilm. It is likely for these reasons that ASHRAE [153], WHO [154], and others recommend this type of thermal disinfection as an emergency measure only. Conversely, a study by [PERSON] et al. [68] indicated that thermal shock yielded persistent benefits (of greater than 6 months) at metered faucets initially colonised by _Pseudomonas aeruginosa_. Plumbing codes and guidelines in Canada and Australia recommend maintaining hot water distribution temperatures above 60 \({}^{\circ}\)C. This is a strategy also favoured by the WHO to limit pathogen growth [5; 154]. Unfortunately, this can be viewed as a contradiction to energy savings goals and scaling prevention, and it can also cause scaling issues. For these reasons, the US EPA recommends cooler temperatures in residences, which are less likely to be effective in pathogen control [4]. On the other hand, ASHRAE Standard 188 has been developed for _Legionella_ control specifically in at risk buildings. It mandates temperatures above 60 \({}^{\circ}\)C at heater outlets and hot water temperatures above 51 \({}^{\circ}\)C throughout the distribution system [153; 155]. Despite this, there is no detailed discussion of inherent conflicts with scaling or energy savings goals for premise plumbing. There is certainly a need to explicitly acknowledge that the risk of colonisation must be balanced with other risks including infrastructure damage (e.g., dissolution, corrosion, and scaling of plumbing materials) and scaling. [PERSON] and [PERSON] [4] discusses the potential of water softening and/or anti-scaling chemicals to reduce or avoid permanently damaging the infrastructure when raising the temperatures in hot water systems above approximately 45 \({}^{\circ}\)C. Standard 188 further mandates temperatures below 25\({}^{\circ}\)C at all locations of cold water systems, which is again a worthy goal; however, this threshold is not always achievable and often exceeded.