## 3 Factors Governing Survival and Occurrence of Opportunistic Pathogens in Premise Plumbing Systems
### In-Building Filtration  
Point-of-use (POU) devices are installed to provide treated water where needed. They are typically attached to faucets and shower heads or under the counter of a kitchen or bathroom sink. Microfiltration, ultrafiltration, nanofiltration, and reverse osmosis process are all membrane filtration technologies capable of removing contaminants. According to [PERSON] et al. [9], these systems need to be vacuum or pressure driven. _Pseudomonas aeruginosa_ measures 0.5 to 0.8 micrometres by 1.5 to 3.0 micrometres, and the average free-living _Legionella_ cell is approximately 1.0 to 3.0 micrometres by 0.5 to 1.0 micrometres [172].  
The pore size on microfiltration ranges from 0.1 to 0.5 micrometres and would likely be suitable for the removal of most bacteria. As the one with the largest pore size among the four main membrane types mentioned above, it can be operated under relatively low pressure and therefore low energy, so it is the cheapest [173]. Finer pore size should lead to higher removal efficiency; however, maintenance requirements and associated costs will also increase. For these reasons, hospital operators, building owners, hotels, and nursing home owners tend to use POU filtration devices as a proactive measure or in response to emergency situations rather than as a single measure.  
A hospital study by [PERSON] et al. [174] demonstrated the ability of POU filters at faucets to achieve greater than 99% reduction in concentration of _Legionella pneumophilia_ and _Mycobacterium_ isolates compared to control faucets through seven days of use. Based on this positive performance, the authors recommend that studies of a prolonged duration be conducted to systematically and scientifically evaluate efficacy on a large scale. Although POU filtration may be an effective option to limit the exposure of pathogens to high-risk patients, application is constrained due to their short maximum lifetime and membrane clogging. These sentiments are reflected by several other studies [9].  
[PERSON] et al. [6] described the potential impacts of three common types of filters (whole building granular activated carbon (GAC), end of faucet, and end of showerhead filters) beyond simply their intended capacity to physically remove OPPPs from flow. These filters can potentially concentrate biofilms and nutrients, thereby increasing the likelihood of regrowth within the filter. Particularly with end of faucet and showerhead filters, there is the risk that biofilm shearing will increases the potential exposure to contaminated aerosols. Several studies have linked POU filters to the increased occurrence of OPPPs, particularly _Pseudomonas aeruginosa_[15; 123; 136]. GAC filters demonstrate a tendency to deplete disinfect residuals and therefore increase the possibility of downstream seeding. An investigation of one building water system following an outbreak of _Legionella_ revealed that the chloramine residual has been reduced to virtually zero due to the placement of a GAC filter at the point of entry [175]. Although this raises a new set of concerns about such treatments, this situation is generally avoided by filters applied immediately at the point of use.