## 2 Characteristics of Premise Plumbing Systems and Risk of Opportunistic Pathogens  
The distinction between main and premise distribution is marked by a number of characteristics specific to each section--for example, premise plumbing has higher surface area to volume ratios, lengthier stagnation periods, more diverse plumbing materials, and lower disinfect residuals than municipal mains [20; 21; 22; 23]. The other major factor here is that the risk is elevated in the extremities of the system because the end-of-line fittings are the interface between the human user/consumer of the delivered water and the fact that these fittings have many different capture points that can enhance bacterial growth. There is a significant body of literature over the past several decades, which indicates how these characteristics can impact upon water quality within the distribution system.  
It is important to understand the extent to which chemical and physical parameters influence the occurrence of opportunistic pathogens in the drinking water distribution system and specifically in premise plumbing. Although opportunistic pathogens tend to occur in buildings, beyond the property line and out of utility control, the quality of water delivered up until that point can be influential for their growth. The impacts of water age, disinfect residues (type and concentration), temperature, pH, and pipe materials are all important--and there is very likely substantial interplay between these parameters [13; 14; 24; 25; 26; 27]. Emphasis should be placed on extreme conditions (for example, those induced by water conservation efforts) including stagnation, distribution system materials, and disinfectant residuals. For example, with national legislative water conservation initiatives such as Water Efficiency Labelling and Standards (WELS), reducing water use is now commonplace, and water stagnation risks within premise plumbing systems therefore increase. This reinforces the importance of maintaining consistent and regular turnover of water flow throughout the premise plumbing system as well as utilising suitable materials for the manufacture of end-of-line plumbing products that can resist bacterial colonisation that may be encouraged due to stagnation and low water flow. Information about these aspects will provide utilities with greater knowledge of how their treatment trains and distribution infrastructure impact the occurrence of opportunistic pathogens and how they might best be modified for improved control.  
One of the most important characteristics that can affect the growth and proliferation of opportunistic pathogens in premise plumbing systems is water age. Water age is a term that represents the average time taken for water to reach its point-of-use from its point-of-entry within a distribution system [28]. It is more precisely defined as a summation of residence time from the treatment facility to the water meter of a building (i.e., mains distribution) and residence time from the water meter to the point of use (i.e., premise plumbing distribution) [21]. Water age can be described primarily as a function of water demand, system design, and system operation [29]. As demand increases, the time that water is resident in a system decreases [28].