Appl. Sci. 2021, 11, 4474. https://doi.org/10.3390/app11104474 www.mdpi.com/journal/applsci
Causes, Factors and Control Measures of Opportunistic
Premise Plumbing Pathogens—A Critical Review
Erin Leslie 1, Jason Hinds 2 and Faisal I. Hai 1,*
1 Strategic Water Infrastructure Laboratory, School of Civil, Mining and Environmental Engineering,
University of Wollongong, Wollongong, NSW 2522, Australia; firstname.lastname@example.org
2 Enware Australia Pty Ltd., Caringbah, NSW 2229, Australia; Jason.Hinds@enware.com.au
* Correspondence: email@example.com
Featured Application: This critical review is highly timely and the need of the hour, given the
prolonged and unwanted building closures during the COVID-19 pandemic. Extended water age
in premise plumbing greatly increases the risk of opportunistic premise plumbing pathogens.
Abstract: This review critically analyses the chemical and physical parameters that influence the
occurrence of opportunistic pathogens in the drinking water distribution system, specifically in
premise plumbing. A comprehensive literature review reveals significant impacts of water age, dis-
infectant residual (type and concentration), temperature, pH, and pipe materials. Evidence suggests
that there is substantial interplay between these parameters; however, the dynamics of such rela-
tionships is yet to be elucidated. There is a correlation between premise plumbing system charac-
teristics, including those featuring water and energy conservation measures, and increased water
quality issues and public health concerns. Other interconnected issues exacerbated by high water
age, such as disinfectant decay and reduced corrosion control efficiency, deserve closer attention.
Some common features and trends in the occurrence of opportunistic pathogens have been identi-
fied through a thorough analysis of the available literature. It is proposed that the efforts to reduce
or eliminate their incidence might best focus on these common features.
Keywords: opportunistic premise plumbing pathogens; legionella; water age; chlorine residual;
The drinking water distribution system (DWDS) is made up of a series of compo-
nents for the storage and conveyance of potable water [1,2]. These engineered systems are
designed to provide an uninterrupted supply of pressurised safe drinking water to all
consumers . The total system can be partitioned into water main and premise plumbing
systems, where the term ‘premise plumbing’ refers to the section of DWDS beyond public
utility main and service lines .
The treatment and delivery of safe potable water by the mitigation of microbiological
hazards has been cited as one of the ten greatest engineering achievements of the 20th
century . The eradication of diseases such as cholera, typhoid fever, and dysentery has
been possible in developed countries through the widespread implementation of filtra-
tion and disinfection . New challenges are emerging in the 21st century with regard to
water quality and public safety. In 2008, the US Centers for Disease Control and Preven-
tion (CDC) recognised that a higher frequency of waterborne disease outbreaks was
caused by pathogens native to the premise plumbing environment rather than traditional
pathogens associated with treatment plant or supply network [7,8].
Citation: Leslie, E.; Hinds, J.; Hai,
F.I. Causes, Factors and Control
Measures of Opportunistic Premise
Plumbing Pathogens—A Critical
Review. Appl. Sci. 2021, 11, 4474.
Academic Editor: Bart Van der
Received: 13 March 2021
Accepted: 9 May 2021
Published: 14 May 2021
Publisher’s Note: MDPI stays neu-
tral with regard to jurisdictional
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Copyright: © 2021 by the authors. Li-
censee MDPI, Basel, Switzerland.
This article is an open access article
distributed under the terms and con-
ditions of the Creative Commons At-
tribution (CC BY) license (http://crea-
Appl. Sci. 2021, 11, 4474 2 of 28
Conventionally, pathogen control is realised at the treatment plant since utilities have
limited control beyond the property line where opportunistic premise plumbing patho-
gens reside and multiply. For this reason, their control is considered a logistical challenge
that will require a shift in the current treatment paradigm. Furthermore, premise plumb-
ing systems involve many components (e.g., water heaters, showers, filters, pipe and fix-
ture materials, and HVAC systems) that have previously failed to demand attention for
their role in amplification and dissemination of opportunistic pathogens . Existing lit-
erature shows end-of-the line plumbing fixtures to be a significant source of microbial
pathogens. The propensity for these to be sources of infection to the end user has also been
established, highlighting the health impacts of plumbing fixtures that harbour pathogens.
The National Research Council (NRC) has identified factors influencing the growth of op-
portunistic pathogens in premise plumbing as high priority for research . Establishing
sound scientific knowledge is important to formulate advice for utilities, property own-
ers/managers, and manufacturers of these systems and components.
Review papers published on the topic to date provide useful accounts of occurrence
[9–11], monitoring , individual factors , guidelines , and different methods of
control [9,15–19] of opportunistic premise plumbing pathogens. The current review paper
fills a gap in that it provides an up-to-date and systematic comprehensive review of
causes, factors, and control measures of opportunistic premise plumbing pathogens in
general and Legionella in particular.
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 disinfectant residuals than municipal mains [20–23]. 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,
disinfectant residues (type and concentration), temperature, pH, and pipe materials are
all important—and there is very likely substantial interplay between these parameters
[1,13,24–27]. Emphasis should be placed on extreme conditions (for example, those in-
duced by water conservation efforts) including stagnation, distribution system materials,
and disinfectant residuals. For example, with national legislative water conservation ini-
tiatives such as Water Efficiency Labelling and Standards (WELS), reducing water use is
now commonplace, and water stagnation risks within premise plumbing systems there-
fore increase. This reinforces the importance of maintaining consistent and regular turno-
ver 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 . It is more precisely defined as a summation of
Appl. Sci. 2021, 11, 4474 3 of 28
residence time from the treatment facility to the water meter of a building (i.e., mains dis-
tribution) and residence time from the water meter to the point of use (i.e., premise plumb-
ing distribution) . Water age can be described primarily as a function of water demand,
system design, and system operation . As demand increases, the time that water is
resident in a system decreases .
2.1. Factors Contributing to Increased Water Age
Increased retention time or stagnation is a common occurrence in drinking water dis-
tribution systems due to demand fluctuation and long intermissions . Particularly
with main distribution, capital planning often dictates that systems be designed to main-
tain pressures and quantities for future demand, which can cause increased water age if
present-day demand is significantly less than that which is forecast .
Dead ends are essentially underutilised or redundant sections of piping where water
tends to stagnate and sediment readily builds up. These can occur in water main and
premise plumbing distribution systems yet can be avoided through proper design and
operation . Notwithstanding this, water age is likely to increase in main and premise
plumbing distribution systems as water conservation practices are adopted at the com-
munity level [3,30].
Water age is also expected to be higher in green building systems when all other
variables are held constant . Preliminary data indicate that water age in modern green
homes averages 250% higher than in conventional residences. Table 1 illustrates a number
of standard practices in green buildings that reduce demand and/or increase total system
volume to yield higher water ages. Table 2 indicates the extent to which such measures
can decrease demand within individual buildings.
Table 1. Examples of water and energy conservation strategies that reduce flow and increase vol-
ume in premise plumbing systems (information source ).
Type of Green Building
zero rainwater office
site storage of up to 11,350 L. Demand was estimated to be
1700 L per month during the spring and up to 5500 L month
during the summer. This resulted in a variable water age be-
tween 2 and 6.7 months.
Leadership in Energy and
Water demand was 60 times less water than equivalent con-
ventional commercial buildings. A water age of 8 days was at-
tributed to very low use at each tap in patient exam rooms,
coupled with large diameter pipes stipulated by plumbing
Net-zero energy house
Used 4 times less water than an equivalent house studied. At
this site, water age was observed before and after installation
of a solar water heater. Hot water storage and age was in-
creased by up to 1.7 days.
Table 2. Example of reduction of water usage (i.e., increased water age) from conservation efforts
in green buildings (data source [21,30]).
Type of Facility Water/Energy Conservation Strategies That Can Increase
Commercial buildings A high number of fixtures increases system stagnation
Rainwater harvesting requires adequate storage to ensure ade-
quate supply during droughts
Decreased water use through behavioral changes results in
high water age
Appl. Sci. 2021, 11, 4474 4 of 28
Efficient fixtures reduce flow up to three times increasing wa-
ter age proportionally
Residential buildings Solar water heaters can double hot water storage volume
Rainwater harvesting reduces tap water used for non-
Distribution system water age is higher because water utilities
sell less water than 10 years ago
Efficient fixtures reduce flow up to three times, increasing wa-
ter age proportionally
Using rainwater or reclaimed water concurrently decreases demand and increases
the overall system volume. Since large storage volumes are necessary to endure drought,
tanks are often sized to satisfy weeks or even months of demand. Furthermore, the quality
and safety of rainwater used for potable water uses remains relatively less studied [31,32].
Many hot water system configurations require large tank sizes in open-loop systems that
provide increased system volumes and water ages. Water-efficient fixtures also signifi-
cantly reduce flow and therefore demand . For example, an inefficient showerhead
can use between 15 and 25 L of water per minute, while an efficient showerhead can use
between 6 and 8 L of water per minute. As showers tend to run for more than 3 min, there
may be enough time to replenish the old water in the supply pipes leading to that shower.
The situation is more critical for basins that are regulated between 4 and 6 L per minute
but have a usage run time that is often less than 10 s. This pulls ‘slugs’ of heated water
into a premise system without it ever actually reaching the end of line, providing new
nutrients to enhance biofilm.
2.2. Expected Changes in Water Quality Resulting from Elevated Water Age
Water age is a major factor in water quality deterioration within distribution systems,
which occurs via reactions within the bulk water and/or interactions between plumbing
materials and the water . As water is conveyed through the system, it is subject to
various chemical, physical, and aesthetic transformations, which will proceed to a greater
or lesser extent according to factors such as water flow rate, finished water quality, plumb-
ing materials, and deposited materials.
Evidence strongly indicates the potential for high water age to negatively impact the
quality of drinking water in main and premise plumbing distribution systems. It is asso-
ciated with problems including disinfectant stability, corrosion of plumbing components,
scaling, development of tastes and odours, and microbial (re)growth [19,25,30,33,34].
Symptoms of high water age are often diagnosed via consumer complaints. Monitoring
of various chemical and biological water quality parameters might also reveal high water
age, for example, lower than expected disinfectant residuals, elevated levels of disinfect-
ant by-products, and elevated bacterial counts .
Water quality concerns that can be caused or worsened by increased detention time
in distribution systems, with implications on public health, are summarised in Table 3
Table 3. Chemical, biological, and physical water quality issues worsened by high water age.
Chemical Issues Biological Issues Physical Issues
Disinfection decay and
by-product formation Microbial proliferation Temperature fluctua-
tions; taste and odour
Corrosion of fixtures and
leaching of metals from
Appl. Sci. 2021, 11, 4474 5 of 28
2.2.1. Loss of Disinfectant Residual and Microbial Ramifications
Disinfectant decay is more likely to occur in premise plumbing systems than in main
distribution due to higher pipe surface area-to-water volume ratios, more frequent stag-
nation points, longer detention times, higher temperatures, and lower disinfectant resid-
Traditionally, the control of pathogens by water utilities has been achieved by coag-
ulation, filtration, and disinfection at the point of treatment prior to distribution . Free
chlorine and monochloramine are the two main disinfectants preferred by utilities .
Mounting evidence suggests that this is no longer a sufficient approach, especially for
systems challenged by high water ages, including green building designs.
The purpose of a secondary disinfectant residual in water supplied by utilities is to
protect the consumer against pathogens and bacterial regrowth [3,21]. The selected disin-
fectant must ultimately inactivate microorganisms in bulk water, control or remove bio-
film, and inactivate microorganisms associated with that biofilm . Unlike free chlorine
and monochloramine, ozone and ultraviolet light are not effective as residual and, there-
fore, they are effective only at the point of use [3,16,28,40–43].
Rhoads and Edwards  discuss how residuals can disappear as a result of abiotic
and biotic reactions within the bulk water and/or between plumbing surfaces and the wa-
ter. Factors that affect the persistence of disinfectant residuals include water quality,
plumbing materials (including adhering biofilms), and system operation. In their survey
of green building water systems, Rhoads et al.  found that chlorine and chloramine
residuals were often completely absent in the green building systems, decaying up to 144
times faster in premise plumbing with high water age when compared to distribution
Water quality decreases with increasing distance from the point of treatment as dis-
infectants decay and residual concentrations fall below adequate levels. This inevitably
results in a shift towards rapid bacterial growth [3,36,44]. The efficacy of various disinfec-
tion methods applied for the control of opportunistic premise plumbing pathogens is de-
tailed in Section 4, which includes a discussion of the role of in building disinfection sys-
2.2.2. Formation of Disinfectant By-Products
Organic and inorganic disinfection by-products (DBPs) form as disinfectants react
with naturally occurring materials in potable water distribution systems . DBP for-
mation potential varies within and between systems and is a function of chemical and
physical characteristics including pH, temperature, type and level of organic matter, type
and level of disinfectant residual, and contact time. Increased potential for DBP formation
has been linked to increased water ages or contact times. Resulting changes in water qual-
ity could cause DBP reactions to proceed faster and go further. The challenge is to inter-
rupt the cycle induced by the requirement for higher disinfectant dosages as decay occurs,
thereby increasing DBP formation potential.
More than 600 DBPs have been identified in chlorinated tap water, including haloa-
cetic acids (HAAs) and trihalomethanes (THMs) . The USEPA describes how people
who drink water containing HAAs and THMs in excess of maximum contaminant levels
(MCLs) for a prolonged number of years have an increased risk of getting cancer, or ex-
perience problems with their liver, kidneys, or central nervous system . However, the
WHO recommends that “efficient disinfection must never be compromised” and “micro-
biological quality must always take precedence” when a choice must be made between
meeting either microbiological guidelines or guidelines for disinfectants and disinfectant
by-products . Thus, it might be concluded that waterborne pathogens pose a more
serious and immediate threat to public health than DBPs.
Appl. Sci. 2021, 11, 4474 6 of 28
2.2.3. Corrosion Control Effectiveness
Phosphates are often added to drinking water supplies to minimise the corrosion of
piping materials [21,28,47]. Increased water age influences the effectiveness of such cor-
rosion control inhibitors by the provision of poorly buffered waters, which challenges pH
management [48,49]. Corrosion can reduce the lifetime of premise plumbing infrastruc-
ture and cause leaching of lead and copper into the water . In addition, although there
appears to be substantial interplay between corrosion control and disinfection, implica-
tions for microbial control are not fully understood.
Corrosion products react with some disinfectants to enhance or reduce their impact
depending on the exact water chemistry and pipe materials [21,50]. For example, Al-Jasser
 conducted a study showing that metallic pipes (cast iron and stainless steel) con-
sumed more chlorine as they aged, which was likely due to the accumulation of corrosion
products. Conversely, plastic pipes (polyvinylchloride and medium-density polyeth-
ylene) consumed less chlorine as they aged and exerted no demand after a decade of ser-
‘Blue water syndrome’ i.e., blue staining occurs in waters with high levels of soluble
and/or particulate copper. Although elevated levels of copper in water are not known to
cause long-term health effects, it has been linked to gastrointestinal upset and exacerba-
tion of problems associated with nitrate ingestion, especially in children . Such occur-
rences are expected to be more frequent in certain situations with water conservation prac-
Copper corrosion failure (often referred to as pinhole leaks, and as non-uniform or
pitting corrosion) is strongly attributed to frequent stagnation, as well as accumulation of
debris during installation, and microbial activity [44,51]. Therefore, occurrence might be
more frequent in green buildings associated with low flow velocities and low water use.
Research by Lytle and Schock  determined free chlorine to be an important factor to
induce pitting under certain conditions. Severe pitting corrosion can jeopardise the integ-
rity of an entire plumbing system, for which costs of repair or replacement can be sub-
Lead is a neurotoxin that can cause permanent, irreversible damage when consumed
and is therefore a recognised threat to public health in water supply [53,54]. The corro-
sivity of the supply water is an important driver for lead into building plumbing systems
. System design and operation can also influence the rate of release . Prolonged
periods of stagnation and high-water age increase the contact time between water and
lead-based plumbing components or solders, which can increase the rate of metal release
. Lytle and Schock  observed an exponential increase in lead levels with stagnation
time in the first 20–24 h of exposure.
Lead pipe plumbing is not widespread in Australian homes relative to Europe and
the US, where infrastructure is more dated . Nowadays, the installation of lead-based
piping and the use of lead-based solders is largely banned for new constructions and ren-
ovations. Despite this, the risk of lead exposure remains [60–65]. A field study by Elfland
et al.  revealed that premise plumbing lines in green buildings with relatively low
water demand had very high lead leaching from brass and bronze devices with lead coat-
2.3. Impact of Water and Energy-Efficiency Initiatives
As noted above, specific elements designed to achieve net zero or energy-efficient
buildings have recently been subject to scrutiny for their potential to increase pathogen
growth and aerosolisation. For example, multiple studies have demonstrated that me-
tered faucets dispense higher levels of P. aeruginosa and L. pneumophilia than conventional
faucets [67–70]. When the metered faucets are hands free, additional problems can arise
due to the solenoid valve used to control the water flow. Such solenoid valves, when ac-
tivated, force a soft polymer diaphragm against a sealing face to close the water supply.
Appl. Sci. 2021, 11, 4474 7 of 28
This soft ‘rubberised’ material can provide an ideal surface for colonisation as to the small
volume of stagnant water beneath the diaphragm needed for it to operate. Notably, since
the introduction of WELS in Australia, every tap now includes a mesh capture point on
its outlet, which is suspected to be an ideal breeding ground for bacteria. The mechanisms
driving these trends in outlet flow control devices in tap need to be better studied .
Solar water heaters and rainwater tanks require large storage volumes to meet sus-
tainability goals, which increases holding time and microbial risk. Reducing hot water
system temperatures in an attempt to conserve energy can also support conditions for
pathogen growth in hot and cold-water systems [21,30,51]. Accordingly, critics of the
Leadership in Energy and Environmental (LEED) rating system devised by the United
States Green Building Council (USGBC) have reworked the acronym to stand for “Le-
gionella Enabled Engineering Design”. As noted earlier, Green Star and the WELS rating
system are the equivalent benchmarks for water efficiency drives in Australia.
The main benchmarks for sustainability, Green Star and the WELS rating system in
Australia, have been aiming for simplicity in order to maximise their reach and subse-
quent adoption. This simplicity, combined with the current approach to sustainability as
a kind of box to tick, has led to a disconnection between the design and construction of a
building and its ongoing occupancy and management. While the performance of a build-
ing is a priority across all levels of Green Star, these benchmarks have created unforeseen
consequences for the well-being of building users by failing to demonstrate an under-
standing of the knock-on effects when a building is not managed correctly. The current
water efficiency solutions under sustainability benchmarks, combined with a lack of in-
formation available within building management services have created environments
perfect for the growth and transmission of opportunistic pathogens in premise plumbing
Green design principles are pivotal to sustainable development. It would be unwise
to abandon water and energy conservation efforts. Instead, researchers and stakeholders
associated with the drinking water distribution system should continue to advance their
understanding of potential water quality issues and public health concerns to formulate
better policies, codes, standards, risk assessment and management approaches.
3. Factors Governing Survival and Occurrence of Opportunistic Pathogens in Premise
Evidence is emerging that both the number of opportunistic premise plumbing path-
ogens in drinking water and the number of individuals who are at risk of infection by
these pathogens are increasing . Representative opportunistic pathogens of concern in
premise plumbing include Legionella pneumophila, which causes Legionnaires Disease and
Pontiac Fever; Mycobacterium avium, which causes respiratory illness; and Pseudomonas ae-
ruginosa [14,72]. Opportunistic premise plumbing pathogens do not tend to cause disease
in a healthy host; however, they can be fatal to individuals with a compromised immune
system such as the elderly, HIV-infected persons, or hospitalised patients [2,10,72]. Dis-
eases caused by these pathogens does not spread person-to-person or through the direct
consumption of drinking water. Rather, considerable evidence indicates that individuals
become ill when exposed to airborne water droplets that have been seeded by the patho-
gens. Activities that can lead to aspiration include showering and hand washing.
3.1. Legionella Pneumophila
Legionella is part of the bacterial genus that includes several well-known pathogenic
species including Legionella pneumophilia. It was first isolated as the causative agent in an
outbreak of severe respiratory illness following the 1976 American Legion convention in
Philadelphia, PA [73,74]. Around 16% of cases acquired from this convention resulted in
fatality, and the illness henceforth became known as Legionnaires’ disease . At the
time, it was hypothesised that the bacterium was harbored in cooling towers and air con-
ditioning systems within proximity to the affected population. For decades, Legionella and
Appl. Sci. 2021, 11, 4474 8 of 28
Legionnaires’ disease were strongly associated with these origins. In more recent times,
with developments in detection and analytical methods, premise plumbing has become
recognised as another important source [13,76]. Reported outbreaks of Legionnaires’ dis-
eases have been linked to water systems in hotels, cruise ships, industrial facilities, public
buildings, and residences; however, the majority have occurred in hospitals, healthcare
facilities, and nursing homes .
The growth and survival of Legionella in premise plumbing is not the sole requisite
for disease. Rather, the organism must penetrate the deep alveolar region of the lungs
within a susceptible host [6,78]. When airborne, virulent Legionella are inhaled into this
region; they infect and replicate within alveolar macrophages to cause disease. To become
airborne, Legionella must enter the bulk water and exit the system as bioaerosol. Aerosoli-
sation is considered the primary mode of Legionella transmission [79,80]. Microscopic wa-
ter droplets created at outlets can readily evaporate to yield only small infectious particles.
These particles can travel great distances (up to 3 km under extreme circumstances), hav-
ing complex dispersion patterns that are a function of many variables [76,81]. Pruden et
al.  suggests that Legionella-parasitised protozoa and Legionella-containing protozoan
vacuoles may be similarly released and disseminated. It has been shown that changes in
operation of the premise system can dislodge Legionella colonised biofilms to increase the
concentration of Legionella in bulk water and dispersed aerosols by association [12,82].
Aspiration and the instillation of contaminated water into the lung during respiratory
tract manipulation also present possible routes for infection [6,13].
Legionella can be introduced into premise plumbing systems in relatively small num-
bers carried via high-quality finished water, or in relatively large numbers via contamina-
tion from non-potable sources (e.g., backflow from fixtures, poorly installed or maintained
cross-connections, etc.) and from disruptions in the supply water distributions system
[6,13]. Once introduced, premise plumbing provides conditions unique from main water
distribution systems that stimulate colonisation. These conditions include high surface to
volume ratios, excessive water age, water temperatures within optimal growth ranges,
and inadequate or absent disinfectant residuals [83,84]. The role of biofilms, which de-
velop on most surfaces in contact with non-sterile water, is considered fundamental for
the chronic colonisation of premise plumbing by Legionella . Biofilms containing amoe-
bic host organisms facilitate replication of Legionella by providing protection from chemi-
cal and heat treatments [86,87].
Legionella can withstand a wide range of temperatures depending on system condi-
tions and available nutrients. For example, Konishi et al.  documented survival be-
tween 20 and 50 °C in the presence of iron and L-cysteine. Yee and Wadowsky  ob-
served optimal growth with low levels of nutrients, low flow, and stagnant water between
32 and 42 °C. Tepid hot water systems resulting from improper operation in attempt to
reduce energy consumption are particularly vulnerable to colonisation . Although
some studies have shown survival in temperatures of up to 70 °C, it is widely accepted
that temperatures exceeding 55 °C provide acceptable control [89,90].
As is further detailed in Section 4, chemical disinfectant residuals can be applied to
control Legionella in building water systems. Disinfection efficacy is a function of many
variables including system conditions and the extent of Legionella colonisation. It has been
observed that Legionella are comparatively less susceptible to chlorination than Escherichia
Coli [17,42]. Therefore, disinfection might be expected to reduce competition for nutrients,
enhancing the growth and survival of Legionella.
Legionella are facultative intracellular parasites . This means that some Legionellae
are not dependent upon a host for survival and are able to survive outside freely in envi-
ronments that support their fastidious growth requirements. Despite this, Legionella
within biofilms and amoebae hosts have proven to be more resistant to disinfection than
free-floating, planktonic Legionella [86,87].
Legionella can enter the viable but nonculturable (VBNC) state in response to envi-
ronmental stressors [91,92]. A VBNC organism remains able to infect a susceptible host
Appl. Sci. 2021, 11, 4474 9 of 28
but cannot be grown on culture media. Common distribution system disinfectants, includ-
ing monochloramines, can induce Legionella into the VBNC state. Some Legionellae that
cannot be cultivated on media can multiply in hosts and can be grown in species of FLA
. This relationship was first noted by Rowbotham  and has been described by Buse
and Ashbolt  as a defining aspect of the Legionella lifecycle. To illustrate, Acanthamoeba
(a genus of amoebae) are able to graze on Legionella at temperatures below 22 °C without
repercussion; however, Legionella may bypass this process at higher temperatures to rap-
idly multiply, increase in virulence, and kill the amoebae host [95–97].
3.2. Mycobacterium Avium
Non-tuberculosis mycobacteria (NTM) are a group of opportunistic pathogens found
in water and soil, including building water systems [2,72,98].
The most frequently isolated mycobacteria of the approximately 150 known species
are displayed in Table 4.
Table 4. Frequently isolated non-tuberculous mycobacteria (NTM) [6,15].
Slowing Growing Species Rapidly Growing Species
Mycobacterium intracellulare Mycobacterium chelonae
Mycobacterium kansasii Mycobacterium fortuitum
Factors that influence Mycobacterium avium growth in pipes include temperature, wa-
ter flow, nutrients, pipe material and condition, and residual disinfectant. A study by
Schulze-Robbecke and Buchholtz  demonstrated the ability of Mycobacterium avium to
survive at 50 °C for up to 60 min. Similar results are widely replicated and can explain the
residence particularly in domestic hot water heaters and pipes . Another study by
Lewis and Falkinham  observed the tolerance of Mycobacterium avium during periods
of stagnancy, which is common in the operation of premise plumbing. This study also
showed growth at 6% and 12% oxygen as well as in air (21% oxygen). Another study by
Falkinham et al.  revealed a correlation between concentrations of Mycobacterium
avium and organic carbon concentrations.
Mycobacterium avium is relatively chlorine resistant and can survive concentrations
otherwise able to destroy or inactivate indicator bacteria (e.g., Escherichia coli) . Ad-
ditionally, chlorine resistance is higher in water than in culture medium, and most stains
appear more tolerant of chloramine than free chlorine disinfection. Although all NTM
species are at least 100 times more resistant to chlorine and other disinfectants compared
to Escherichia coli, not all withstand chlorine disinfection equally [19,103–106]. Pruden et
al.  suggest that turbidity may be an important factor during the inactivation of plank-
tonic NTM in drinking water. The mechanism by which disinfection selects for NTM in
drinking water distribution (i.e., by reducing numbers of competing organisms) is well
documented in several studies [102,107].
Pilot system studies have been designed to simulate and study the behaviour of NTM
in the drinking water distribution system. One such study demonstrated the ability of
Appl. Sci. 2021, 11, 4474 10 of 28
Mycobacterium avium to form biofilms in pipe under different concentrations of organic
matter and disinfectant residuals . Furthermore, Mycobacterium avium is resistant to
killing by amoebae and does not undergo phagocytosis but rather multiplies within these
hosts . There is mounting evidence that suggests this as another factor contributing
to its residence in drinking water systems and premise plumbing.
Drinking water contains both rapid and slow-growing mycobacterial species. Alt-
hough their rate of growth in rich laboratory media can be very low (at 1 generation per
day), Mycobacterium avium complex (MAC) and NTM are generally well adapted to life in
aqueous, flowing environments, owing to their physiological properties [6,102]. NTM
possess a waxy, hydrophobic cell surface, which is a consequence of long-chain (C40-C80)
mycolic acids . Researchers often credit these cell envelopes as the basis for antibiotic
and disinfectant resistance . The waxy, hydrophobic surface of NTM cells has also
been linked to increased surface adherence and biofilm formation [111,112]. The enrich-
ment of NTM in biofilms is a particularly important aspect for slow-growing species in
flowing systems. Hydrophobicity also makes NTM susceptible to dispersion and
transport as aerosols [113,114]. Showerheads, swimming and therapeutic pools, hot tubs,
and spas are all known settings for NTM exposure via aerosols [115,116].
3.3. Pseudomonas aeruginosa
Pseudomonas aeruginosa is a naturally occurring bacterium in soil and water; however,
it has also been isolated from drinking water distribution systems, antimicrobial soaps,
and disinfectants [117–121]. Although rarely carried by healthy individuals, Pseudomonas
aeruginosa has been recovered in up to 60% of hospitalised patients and is therefore con-
sidered an opportunistic pathogen . Pseudomonas aeruginosa has been identified as the
cause of hospital- and community-acquired cases of life-threatening pneumonia. Commu-
nity-acquired infections, within a relatively healthy and normal population, are more
likely linked with recreation water (e.g., contaminated swimming pools, hot tubs, and
whirlpools) and characterised by relatively minor eye, ear, and skin conditions.
Tap water and premise plumbing have been established as a source of Pseudomonas
aeruginosa disease, particularly in healthcare settings [123,124]. Pseudomonas aeruginosa
strains present within tap water samples from sinks within intensive care units have been
indistinguishable by molecular typing to strains identified within infected patients .
In addition, Pseudomonas aeruginosa has been found to colonise faucets for more than two
years despite its absence from the mains supplying the sinks. Several studies have demon-
strated that the pathogen is transmissible via direct contact and/or indirect contact. Direct
contact may occur through bathing or the ingestion of contaminated water. Indirect con-
tact may occur through contamination of a device or fomite, inhalation of aerosols dis-
persed from contaminated water sources, and aspiration of contaminated water or aero-
The minimal nutritional requirements of Pseudomonas aeruginosa enable its survival
in a range of natural and artificial environments. It persists in distilled or deionised wa-
ters, as well as aquatic environments of moderate salinity and even in high nutrient envi-
ronments [129,130]. The growth of Pseudomonas aeruginosa is dependent on system condi-
tions including temperature, pH, and oxygen conditions system. Although able to survive
a wide range of temperatures (from 4 to 42 °C) common in premise plumbing, virulence
decreases below 30 °C. Optimal growth in rich medium and suspended form is between
30 and 37 °C . Pseudomonas aeruginosa can grow well in microaerobic (i.e., not com-
pletely anaerobic) conditions expected in stagnant waters associated with low flows in
premise plumbing. Growth also becomes significantly limited under acidic conditions.
Pseudomonas aeruginosa displays relative resistance against common disinfectants
used in water treatment [130,132]. In hospital and care facility settings, infections have
been linked to the pathogen’s presence in solutions used for surface sterilisation, broncho-
scopes washed with non-sterile water or disinfectant solutions, and in-dwelling catheters
Appl. Sci. 2021, 11, 4474 11 of 28
The persistence and growth of Pseudomonas aeruginosa in engineered distribution sys-
tems and premise plumbing is largely credited for its habitation in biofilms [134,135].
Through this mechanism, Pseudomonas aeruginosa colonise biofilm in plumbing fixtures,
including faucets and showerheads, where risk of exposure to susceptible persons is high
[123,136]. The production of extracellular polymeric substances (EPS) by Pseudomonas ae-
ruginosa affords it additional protection from environmental stressors by aiding the colo-
nisation of and organisation within biofilms.
3.4. Common Features of Opportunistic Premise Plumbing Pathogens
Premise plumbing has several unique features including high surface to volume ra-
tios, unique and varied pipe materials, low levels of organic carbon, and periods of stag-
nation that select for certain opportunistic premise plumbing pathogens, which adapt for
survival, growth, and persistence in such systems/environments much unlike ‘classic’ wa-
terborne pathogens such as Salmonella and Escherichia coli. Based on the available litera-
ture, it appears reasonable to assume that such pathogens that are native to premise
plumbing must share common characteristics . The following section draws parallels
between these examples, and Table 5 gives an overview of common qualities as indicated
by the current body of research/literature. As previously stated, this knowledge will allow
for implementation of the appropriate engineered controls to protect public health and
Table 5. Summary of features common to opportunistic premise plumbing pathogens.
Infection of human hosts—particularly the young, elderly, immunosuppressed, and im-
Persistence in drinking water distribution systems (i.e., mains and premise plumbing)
Slow growth and regrowth in drinking water distribution systems
Growth within amoebae (i.e., resistance to phagocytosis)
Survival at low oxygen during periods of stagnation
Opportunistic premise plumbing pathogens (OPPPs), as opportunistic pathogens, in-
fect individuals having one or more risk factors making them more susceptible than the
general population to these bacteria [2,137]. OPPPs do not typically cause disease in a
healthy host; however, they can be fatal to individuals with a compromised immune sys-
tem such as the elderly, HIV-infected persons, or hospitalised patients. This will have im-
plications for risk assessment and the management of OPPPs, particularly for hospitals
and healthcare facilities.
Many of the diseases contracted by human hosts caused by OPPPs are difficult to
treat due to their relative resistance to antibiotics. Mycobacterium avium has a thick, wax,
and lipid-rich outer membrane that is not penetrated by most commonly used antibiotics
. Pseudomonas aeruginosa isolates also have unique barriers to entry . Growth in
biofilms and amoebae acts as additional barriers to the entry of antibiotics . Based on
this knowledge, it would be extremely difficult, if at all possible, to develop strategies for
treating infections .
Secondly, all OPPPs are relatively resistant to chlorine and various other disinfect-
ants used in water treatment. Falkinham et al.  collated information regarding the chlo-
rine resistance of waterborne pathogens relative to Escherichia coli. This is presented below
in Table 6. The issue is compounded, since OPPPs can not only survive exposure to resid-
ual disinfectant levels but also thrive as competitors for nutrients. This is an important
Appl. Sci. 2021, 11, 4474 12 of 28
consideration when considering drinking water treatment systems. Current practice ap-
parently encourages reductions in population diversity and allows for the amplification
of numbers of a smaller group of microorganisms .
Table 6. Chlorine resistance of opportunistic premise plumbing pathogens relative to Escherichia
Genus/Species CT99.9 % 1 Reference
Escherichia coli 0.09 (reference) 
51 (567-fold) [103,111]
Pseudomonas aeruginosa 1.92 (21-fold) 
1 CT99.9 % represents the disinfectant concentration (mg/L) multiplied by the contact time (min) re-
quired to kill 99.9% of cells.
Future practice should be guided by a comprehensive reassessment of risks and treat-
ment priorities—in many instances, the control and eradication of OPPPs could take prec-
The distribution of OPPPs throughout drinking water systems is another point of
difference with other waterborne microorganisms. The concentration of ‘classic’ patho-
gens such as Escherichia coli will fall as they move from the source due to the dilution and
the absence of growth [102,141]. Conversely, OPPPs are native to drinking water distribu-
tion systems, and their numbers will increase beyond the treatment plant. To further com-
plicate the matter, numbers of OPPPs do not correspond with numbers of Escherichia coli,
faecal coliforms, or other measures of microbial water quality and can otherwise be diffi-
cult to measure.
Slow growth is considered to be an attribute that contributes to this lack of OPPP
detection and its persistence in drinking water distribution systems. For example, it takes
up to 14 days for the first appearance of Mycobacterium avium colonies incubated at 37 °C
. This is considered as an advantage in the sense that a slow growth means a slow
death. This is supported by Table 6, which shows increased chlorine resistance for cells of
Legionella pneumophila and Mycobacterium avium, which are adapted to drinking water.
Additional common features include biofilm formation, resistance to killing by phag-
ocytic amoebae, thermal tolerance, and survival under stagnancy (i.e., low levels of oxy-
gen). Attachment to biofilms and growth within amoebae appear to provide OPPPs with
increased protection against these harsh conditions. Biofilm formation also prevents mi-
croorganisms from being washed out of flowing pipe systems .
Some opportunistic pathogens, such as Mycobacterium avium, can withstand assimi-
lable organic carbon concentrations as low as 50 μg/L . The ability of Legionella to
grow in low organic concentrations is believed to depend largely on its relationship with
host amoebae. On the other hand, Pseudomonas aeruginosa is so good as persisting without
readily available nutrients that it has been noted to grow in distilled water. It is important
to acknowledge that the overall performance of these pathogens are not the same. Falkin-
ham  writes that Mycobacterium avium is clearly the most resistant to chlorine and only
Pseudomonas aeruginosa can grow under anaerobic conditions .
Overall, a review of common features and trends in the occurrence of opportunistic
pathogens supports the view that they are well suited for growth in premise plumbing
systems. Efforts to reduce or eliminate their incidence might best focus on these common
features and on conditions within the premise plumbing environment.
Appl. Sci. 2021, 11, 4474 13 of 28
4. Evaluation of Specific Engineered Controls for Opportunistic Pathogens in Premise
In order to reduce or eliminate the risk of disease caused by OPPPs, it is necessary to
minimise their concentrations in the affected systems . With this considered, the fol-
lowing sections evaluate the effectiveness of specific treatment methods and approaches.
4.1. Source Water Treatment and Distribution System Maintenance
The function of source water treatment and distribution system maintenance is lim-
ited in the context of OPPP control. Pruden et al.  has noted the emphasis of water
treatment and distribution system operation more for control of amoebic pathogens (e.g.,
Naegleria fowleri and Acanthamoeba). Despite the propensity for such organisms (including
Legionella, NTM, and Pseudomonas aeruginosa) to amplify and proliferate beyond the prop-
erty line, US EPA standards mandate Legionella removal from water treatment plants. Alt-
hough it remains imperative that water provided up until this point is of a high quality
with acceptable disinfectant residual, focus might be more wisely placed on control
It appears that coagulation and filtration are effective in the removal of certain free-
living amoebae species, including Naegleria fowleri and Acanthamoeba [145–147]. It is not
yet clear how this will impact the previously described relationships between free-living
amoebae and Legionella, NTM, and Pseudomonas aeruginosa downstream. The positive out-
come in terms of OPPP control would involve a reduction in numbers by the removal of
host protection. On the other hand, it may lead to reduced competition for nutrients and
therefore increased OPPP growth. Chlorination and other drinking water distribution sys-
tem maintenance practices do also appear effective in the control of some amoebae.
4.2. Temperature Control
As previously outlined, many studies have established that water temperatures be-
tween certain thresholds correlate with OPPP colonisation [148–150]. Accordingly, decon-
tamination 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 °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 °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. With-
out first completing this step, there is a significant risk that these stagnant areas can re-
seed the system following treatment .
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 scalding and flooding. A minimum of eight hours is recommended to allow the
entire scale layer to become heated to over 60 °C .
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 includ-
ing tastes and odours. For example, several studies have observed recolonisation by Le-
gionella within weeks to months after treatment [15,43,151]. Temmerman et al.  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 , WHO , and others
recommend this type of thermal disinfection as an emergency measure only. Conversely,
a study by van der Mee-Marquet et al.  indicated that thermal shock yielded persistent
Appl. Sci. 2021, 11, 4474 14 of 28
benefits (of greater than 6 months) at metered faucets initially colonised by Pseudomonas
Plumbing codes and guidelines in Canada and Australia recommend maintaining
hot water distribution temperatures above 60 °C. This is a strategy also favoured by the
WHO to limit pathogen growth [5,154]. Unfortunately, this can be viewed as a contradic-
tion to energy savings goals and scalding 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 . On the other hand, ASHRAE Standard
188 has been developed for Legionella control specifically in at risk buildings. It mandates
temperatures above 60 °C at heater outlets and hot water temperatures above 51 °C
throughout the distribution system [153,155].
Despite this, there is no detailed discussion of inherent conflicts with scaling or en-
ergy 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 scalding. Brazeau and Edwards  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 °C. Standard 188
further mandates temperatures below 25° C at all locations of cold water systems, which
is again a worthy goal; however, this threshold is not always achievable and often ex-
In Australia, Environmental Health Standing Committee (enHealth) Guidelines for
managing Legionella in health and aged care facilities recommend that premise plumbing
systems maintain circulated heated water above 60 °C while also ensuring that the cold
water remains below 20 °C . Reducing the flow rate of end-of-line fixtures and asso-
ciated pipeworks reduces the ability for the circulated heated water (60 °C) to travel to the
outlet, regularly pasteurising supply lines and replenishing them with new water. For
example, over 50% of all water use activations within a working hospital are between 4
and 24 s where the hot water never reaches the outlet. Maintaining 60 °C heated water is
important, but it is only effective if water flow reaches the outlet regularly.
Maintaining regular flow is just as critical on the cold supply side. There is evidence
of legionella survival in cold water below 20 °C. Furthermore, cold water in premise
plumbing water systems may follow the ambient temperature conditions of the adjacent
environment, which typically resides between 22 and 26 °C due to air conditioning sys-
tems or thermal conditions in plumbing ducts. Maintaining good regular flow helps en-
sure that the cold water does not increase in temperature and hence reducing colonisation
conditions and bacterial risk. To combat both these potential risks, the enHealth guide
lines ask for all fixtures that remain unused for 7 days be flushed. If facility managers do
not monitor fixture use, they flush every tap once a week (resulting in enormous water
wastage and recourse cost). If they do not flush any of the taps, it increases risk and makes
them vulnerable to litigation if a health issue occurs. Thus, the effort to design a more
water-efficient facility by reducing water flow may end up wasting more water than con-
ventionally designed and operated facilities.
Secondary disinfection (chlorination) performed by utilities can assist in the control
of many opportunistic premise plumbing pathogens, as well as traditional pathogens.
Chloramine in particular is valued for its potential to reduce the risk of Legionnaires’ dis-
ease and the occurrence of P. aeruginosa at a community level [37,157]. Despite this, the
absence of disinfectant residuals and OPPP regrowth observed towards the end of main
distribution systems is reason to question the suitability of this strategy [36,44].
In-building and on-site chemical disinfection is recommended for facilities fre-
quented by at-risk populations, such as hospitals and aged care facilities . Chlorine,
chloramine, and chlorine dioxide are commonly applied chemical disinfectants in such
Appl. Sci. 2021, 11, 4474 15 of 28
systems. Indeed, many facility owners and operators choose to install supplemental dis-
infection treatment systems on the basis of disease prevention for economic or insurance
reasons. More often, the decision to employ additional treatment is motivated by an out-
break of disease or by the detection of OPPPs in building water samples. Particularly for
small facilities, installation, operation, and maintenance can be too costly and complicated
for facility owners and operators.
As mentioned in Section 2, the application of chlorine, and possibly chloramine can
be detrimental to premise plumbing infrastructure, with potential to cause serious pinhole
leaks in copper and stress corrosion failure of stainless steel . An investigation of con-
tinuous chlorination revealed an increase in the incidence of pinhole leaks by up to 30
The physical and chemical characteristics of the water entering the premise system
will also influence the effectiveness of disinfection technologies to varying degrees
[15,159]. For example, increased water temperatures have little impact on the effectiveness
of copper or silver ions relative to chlorine and chlorine dioxide disinfectant residuals,
which are lessened as reactions with organic materials and/or pipe surfaces proceed
quicker and further. The acidity or alkalinity of finished water can also determine the ef-
fectiveness of chlorine, monochloramine, and copper ions to a greater extent than that of
chlorine dioxide and silver ions. Many other physical and chemical parameters can also
impact the performance of specific disinfectants for the control of opportunistic patho-
Another major weakness in this method results from the disturbance of biofilm pop-
ulations and subsequent release of opportunistic pathogens. Furthermore, not all disin-
fectants are equally effective against bacteria within biofilms [44,160,161]. In addition to
the protection provided by biofilms, the prolonged exposure to chlorine may select for
chlorine or even drug resistance of certain OPPPs, including Mycobacterium Avium .
Laboratory and full-scale studies have been conducted to assess the effectiveness of
chlorine against legionella across a range of physical and chemical conditions such as dose
and residual levels, temperature, and pH [44,160,161]. In general, it was found that higher
doses of chlorine in the order of 2–6 mg/L were needed for continuous control. Chlorine
was proven to be more effective on control at higher temperatures; however, it also de-
cayed faster. The fact that higher temperatures cause decay of chlorine residual further
reinforces the need to ensure regular replenishment of water within premise plumbing
systems to ensure that the disinfectant can reach the system extremities (end-of-line fit-
The bactericidal action of the chlorine was also found to be enhanced at lower pH
levels. Turbidity is suspected to interfere with the disinfection process and may need to
be addressed prior to disinfection. One study found that the linkage between Legionella
pneumophila and protozoa including amoebae required much higher doses of chlorine for
Monochloramine (NH2Cl) has a more persistent and stable disinfectant residual than
chlorine and does not cause undesirable tastes and odours to the same extent as other
disinfectants . The mechanism for inactivation by monochloramine differs from that of
chlorine, which can be consumed in irrelevant reactions. Although it has a much lower
disinfection efficiency, it is able to control bacterial regrowth and biofilm formation via its
ability to penetrate the biofilm. Current practice is to use a chlorine-to-ammonia ratio be-
tween 3:1 and 5:1 to produce monochloramine . Furthermore, several case studies rec-
ommended maintaining a residual in the order or 1–2 mg/L as an effective means for con-
taining biofilm growth, minimising Legionella colonisation and preventing outbreaks.
Appl. Sci. 2021, 11, 4474 16 of 28
Flushing and frequent monitoring are essential to demonstrate acceptable ammonia and
chlorine residual levels.
4.3.3. Chlorine Dioxide
Chlorine dioxide is a water-soluble gas having superior penetration in biofilms than
elemental chlorine gas. Multiple studies have confirmed that its correct usage renders it
effective in incapacitating certain bacterial pathogens, viruses, and protozoan pathogens,
including Legionella . Chlorine dioxide disinfection systems have been installed in hos-
pitals to control Legionella and biofilm in hot and cold-water systems. Although a vast
majority of laboratory and pilot-scale tests have determined chlorine dioxide to be effec-
tive against Legionella, a few studies argued that it was not suitable at high levels as would
be applied in shock treatment or emergency remediation to inactivate Legionella pneumoph-
ila [3,28]. Systems that successfully perform their function have reported dosage rates be-
tween 0.4 and 0.7 mg/L, while a residual between 0.1 and 0.5 mg/L at the tap is considered
as usually sufficient to control Legionella—naturally higher residuals may be required in a
system experiencing severe colonisation .
4.3.4. Copper–Silver Ionisation
Copper–silver ionisation has been used to control Legionella and other bacteria in var-
ious settings . However, some researchers argue that it is most effective when con-
centrations can continually be monitored and adjusted, requiring special equipment and
expertise. Both copper and silver have biocidal activity, which results in a synergistic ef-
fect in the lysing of bacteria and protozoa cells, and denaturing of their proteins [17,42].
Recommended effective concentrations of copper and silver range from 0.2 to 0.4 mg/L
and 0.02–0.08 mg/L, respectively , and they tend to be higher in larger systems. Other
studies have shown the ability of these metals to penetrate biofilms. The overall efficiency
of copper–silver ionisation is largely impacted by water pH and TDS. Silver will precipi-
tate when subject to waters with high TDS concentrations and become unavailable for
disinfection. Rohr et al.  warns of the potential for microbial resistance to silver over
the time span of years. Evidence in support of in-building copper-silver ionisation origi-
nates predominately from field studies, with experts calling for additional data from con-
trolled laboratory investigations. For example, Legionella outbreaks have been reported in
buildings featuring copper–silver ionisation installations . Application of this method
has been severely restricted by authorities in the US and parts of Europe on this basis.
The application of ozone and ultraviolet light (UV) disinfection techniques is limited
to direct local application due to the absence of a residual effect unlike chlorine [17,42,43].
Ozone must be produced on site due to its short half-life. These systems are considered
difficult to retrofit, technically challenging to maintain, and expensive . The major ben-
efit of ozone appears to be its consistent performance, which requires a short contact time
and is not generally affected by temperature, pH, or turbidity [40,43]. A batch-reactor
study by Domingue et al.  demonstrated CT 99% reduction of L. pneumophila within
five minutes. Similarly, Muraca et al.  achieved a 5 log (CT 99.999%) reduction of Le-
gionella pneumophilia with 0.5 mg ozone/L within five hours. Thomas et al.  further
demonstrated the ability of ozone to reduce established biofilms at a concentration of 0.5
mg ozone/L. However, more research is required to determine the long-term effects (e.g.,
corrosivity) and efficacy of ozone.
4.3.6. UV Disinfection
UV disinfection is another treatment technology for the inactivation of pathogens
and is commonly employed by utilities in Australia, the US, and Europe as a final polish-
Appl. Sci. 2021, 11, 4474 17 of 28
ing step in the drinking water treatment train. UV light irradiation does not kill microor-
ganisms; rather, it damages their DNA, thereby disrupting their ability to reproduce and
preventing infectivity. Optimum disinfection is achieved at a wavelength of 254 nanome-
tres; however, efficacy of treatment can be significantly impeded by high turbidity
[18,167,168]. High concentrations of particulate matter and certain dissolved species in-
hibit the effectiveness of UV disinfection by impairing the transmission of light to the tar-
get microorganisms . Some studies indicate that high water temperatures can reduce
the longevity of UV reactors and equipment; however other studies insist that tempera-
ture fluctuations have little or no impact on disinfection efficacy. UV disinfection does not
support the formation of DBPs at doses applied to drinking water, nor does it change pH
or treated water quality to increase corrosivity .
In support of UV disinfection for the control of OPPPs, it was showed that relatively
low UV doses (mJ/cm2) achieved a 2-log (99%) reduction in six different Legionella species
. Muraca et al.  showed that higher UV doses achieved a 5-log (99.999%) reduction
in 20 min in a recirculating model premise plumbing under various test conditions. At a
higher UV dose again (90 mJ/cm2), Miyamoto et al.  showed that Legionella species
were inactivated within three minutes of exposure. In general, UV light is most effective
on protozoa followed by bacteria and least effective against viruses. In regard to the rela-
tionship between OPPPs and free-living amoebae, Cervero-Aragó et al.  determined
that higher fluence was required for a 4-log (99.99%) reduction in Legionella species when
co-cultured with amoeba. UV disinfection has been successfully applied to control Le-
gionella in building water systems [167,171].
Commercial in-building UV systems are available across a wide price range and can
treat between 3.7 and 1900 L/min with minimum set-up requirements (i.e., basic plumbing
skills and an electrical outlet) . The most significant ongoing maintenance costs and
efforts typically include the annual replacement of UV bulbs and the quarterly cleaning
of the quartz sleeve through which the water flows. Due to the lack of residual provided
by UV, downstream growth of microorganisms is a concern with this method. In addition,
the application of UV may diminish disinfectant residual . As a result, it is often used
to supplement other treatment options and/or as near as possible to the POU. UV treat-
ment is not always possible where systems are susceptible to downstream contamination
4.4. 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 pro-
cess are all membrane filtration technologies capable of removing contaminants. Accord-
ing to Springston et al. , these systems need to be vacuum or pressure driven. Pseudo-
monas aeruginosa measures 0.5 to 0.8 micrometres by 1.5 to 3.0 micrometres, and the aver-
age free-living Legionella cell is approximately 1.0 to 3.0 micrometres by 0.5 to 1.0 micro-
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 . 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 nurs-
ing 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 Sheffer et al.  demonstrated the ability of POU filters at fau-
cets 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
Appl. Sci. 2021, 11, 4474 18 of 28
be conducted to systematically and scientifically evaluate efficacy on a large scale. Alt-
hough 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 .
Pruden et al.  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 con-
taminated aerosols. Several studies have linked POU filters to the increased occurrence of
OPPPs, particularly Pseudomonas aeruginosa [15,123,136]. GAC filters demonstrate a ten-
dency to deplete disinfectant residuals and therefore increase the possibility of down-
stream 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 . Although this raises a new set of
concerns about such treatments, this situation is generally avoided by filters applied im-
mediately at the point of use.
4.5. Plumbing Materials
Extensive research has been conducted to determine the relationships between ma-
terials commonly applied in drinking water distribution systems and the occurrence of
OPPPs . In general, evidence supports the hypothesis that copper does not encourage
growth to the extent of other materials . This appears particularly true for the rela-
tionship between copper and Legionella . It is proposed that this inhibiting action ex-
erted by copper on the growth of OPPPs may not be long term. Rather, it is likely deter-
mined by the concentration of aqueous Cu2+ ions, which is a complicated function of water
quality parameters, including pH and water age . Multiple studies from Italy have
correlated lower levels of Legionella colonisation with copper concentrations above 50
Despite its potential, copper presents a number of dichotomies that must be consid-
ered prior to its selection as part of a holistic management strategy for the control of
OPPPs . For example, Nguyen et al.  describes the mechanism of copper-acceler-
ated decay of chlorine and chloramine disinfectant. Others have observed the role of cop-
per as a trace micronutrient for certain bacteria or the adaption of certain bacteria to higher
copper concentrations [41,52,83].
Conclusions drawn from studies focused on the influence of plastic materials mainly
advance the hypothesis that pathogen growth is supported, even encouraged in the case
of Legionella and Pseudomonas aeruginosa, due to the release of organics from these materi-
als [13,135]. Again, the extent of colonisation appears to be a function of water quality
parameters, and under some circumstances, rubber-coated valves, ethylene propylene
diene monomer (EPDM), and polyurethane surfaces can strongly encourage growth
4.6. Flow Patterns and Reducing Water Age
According to Pruden et al. , water flow patterns control the duration and timing
of stagnation events, internal pipe velocities, presence of stagnation zones, and use of re-
circulation pumps. Water flow patterns in premise plumbing can influence the formation
and shearing of biofilms, the transport or nutrients and disinfectants, temperature profiles
throughout the system, the rate of bulk water chemical reactions including disinfectant
decay, and, therefore, the occurrence of OPPPs . Inherent to the number of influential
factors, it is difficult to establish direct correlations between the effects of flow variations
on the growth and survival of OPPPs.
Appl. Sci. 2021, 11, 4474 19 of 28
Understanding how to reduce water age within a distribution network is imperative
to ensuring the quality of drinking water supplied. To facilitate this, additional research
is required to determine with greater certainity what range of water ages are problematic
and under what circumstances. Specifically, literature directly describing the relationship
between water age, and the occurrence of opportunistic premise plumbing pathogens
such as legionella spp. is lacking. Exceeding this knowledge deficiency may result in a
wider range of preventative and remedial actions available to lessen the effects of high
water age in distribution systems. Methods currently available during design and opera-
tion include regular flushing, correct sizing, and the elimination of dead ends.
Given that water age is a function of demand, there would likely be substantial ben-
efit in performing comprehensive modelling of occupant behaviour and consumption.
This should extend to assessing the accuracy and validity of future demand prediction for
the sizing of new builds. Rhoads et al.  recommends an increased monitoring of green
buildings to identify and resolve problems as they are encountered and hence establish
any trends. Then, this information might be incorporated into building codes and stand-
ards outline or circumvent any negative and unintended consequences involved.
For example, these codes and standards could mandate size requirements for plumb-
ing system volumes according to demand—this might be validated by the knowledge that
a higher premise plumbing water age will result whenever there are significant reductions
to potable water demand without proportionally reducing the total system size. This ap-
proach would be most suited to the design and construction phases and would be ex-
tremely difficult, if at all possible, beyond such times. For existing pipe networks within
existing structures, flushing is likely to be a more effective solution . This strategy
seems counterproductive to green building principals and offers the impression of water
and energy wastage. Although water can be recovered for use in non-potable applications,
some must be expended, especially if flushing is combined with thermal shock treatment.
4.7. Challenges Inherent to the Current Water Treatment Paradigm
The current water treatment doctrine originates from the need to combat waterborne
pathogens such as Salmonella, Escherichia coli, and Vibrio cholerae, primarily from faecal
contamination of supplies. The successful control of these disease-causing pathogens has
been achieved and can be largely attributed to their natural attenuation over the time of
exposure to drinking water, by treatment steps including disinfection, and because they
do not multiply outside of their mammalian hosts. It has been established that in contrast,
opportunistic premise plumbing pathogens are adapted to growth and persistence in
drinking water, especially in building plumbing systems, and therefore tolerate disinfect-
ant residuals. This explains why numbers of Legionella pneumophilia, Mycobacterium avium,
and Pseudomonas aeruginosa, actually increase with distance from the treatment plant
Falkinham et al.  describes the three main challenges that this poses to the current
paradigm for water treatment. Firstly, it is believed that source tracking within a water
distribution system is of limited value, since amplification and the likelihood of detection
increases in remote parts of the system. Secondly, using disinfection guidance established
for the eradication of Escherichia coli can select for the prevalence of opportunistic patho-
gens by reducing competition for nutrients. Instead, new principals might be developed
according to the susceptibility of these disinfectant-resistant opportunistic pathogens.
Lastly, locations for regulatory compliance sampling tend to be at the treatment plant ef-
fluent, which is the least likely place where opportunistic premise plumbing pathogens
will be detected. Unfortunately, utilities are not obligated to sample stagnant water in
buildings where detection is most likely [13,15].
Table 7 summarises the additional problems associated with both traditional patho-
gens and OPPPs. There is evidently interplay, be it synergistic or antagonistic, amongst
these various problems, which complicates risk assessment and management strategies.
Most notably, direct conflicts exist between the control of OPPPs and the realisation of
Appl. Sci. 2021, 11, 4474 20 of 28
other public safety and sustainability goals (e.g., prevention of scalding and disinfection
by-products, energy conservation, water conservation, and corrosion control).
Table 7. Comparing and contrasting problems of traditional pathogens to opportunistic premise
Re-growth in the distribution system Generally, no Yes
Risk of disease
significantly influenced by
immune status and/or age No Yes
Risk significantly reduced by
treatment (e.g., source water protection,
primary disinfection, filtration etc)
Risk reduced by secondary disinfection
and residual Yes No
Likelihood of disease increased by water
use patterns, plumbing hydraulics, build-
ing design and operation, plumbing mate-
rials, and hot water system settings
Generally, no Yes
Route/mode of exposure Ingestion Inhalation, ingestion, skin
Water nutrient levels (organic carbon, ni-
trogen, phosphorus, trace nutrients) influ-
Conflicts between risk reduction and other
, public safety, sustainability
Possible conflict with
limiting DBP for-
Major conflict with DBP
rmation, scalding, energy
and water conservation,
Influence by water chemistry (tempera-
ture, pH, dissolved oxygen) in main and
premise distribution system
Little to none Yes
Fundamentals of conventional pathogen control that are transferable to the control
of OPPPs include principles of disinfection, filtration, and public education. Beyond this,
there is little overlap in regard to responsible parties and solutions. Although this paper
previously highlighted the common characteristics of opportunistic premise plumbing
pathogens, it is not realistic that one measure will reduce numbers of all species. In fact, it
might never be practical or at all possible to completely eradicate all opportunistic patho-
gens from premise plumbing in homes, condominiums, apartments, hospitals, and office
buildings [13,15]. Furthermore, it is not likely that control measures will have the same
outcomes in treatment plants, distribution systems, and premise plumbing as conditions
within and between systems vary widely. Therefore, case-specific strategies need to be
It is important to elucidate the extent to which chemical and physical parameters in-
fluence the occurrence of opportunistic pathogens in the drinking water distribution sys-
tem and specifically in premise plumbing. The impacts of water age, disinfectant residual
(type and concentration), temperature, pH, and pipe materials are not well-defined, and
there is very likely substantial interplay between these conditions to complicate this task.
There is a link between premise plumbing system characteristics, including those featur-
Appl. Sci. 2021, 11, 4474 21 of 28
ing water and energy conservation measures, and increased water quality issues and pub-
lic health concerns. Other interconnected issues exacerbated by high water age, such as
disinfectant decay and reduced corrosion control efficiency, deserve unique attention.
Furthermore, the relationships between these other issues and the occurrence of oppor-
tunistic pathogens must be better understood. It may not be possible to completely elimi-
nate or manage all of these risks; hence, risk characterisation and prioritisation will be of
Author Contributions: E.L.: Methodology, Investigation, Data curation, Formal analysis, Visualisa-
tion, Writing—Original draft; J.H.: Formal analysis, Writing—Review and editing; F.I.H.: Concep-
tualisation, Supervision, Methodology; Data curation, Formal analysis, Project administration,
Funding acquisition, Writing—Review and editing. All authors have read and agreed to the pub-
lished version of the manuscript.
Funding: This work was co-funded by the Faculty of Engineering and Information Sciences at the
University of Wollongong, Australia and Enware Australia Pty Ltd.
Acknowledgments: The authors are indebted to William E. Price of University of Wollongong, Aus-
tralia for his insightful comments on the manuscript.
Conflicts of Interest: E.L. and F.I.H. declare no conflict of interest. There are no direct conflicts of
interest; however, Jason Hinds would like to declare that he is the R&D Manager at Enware Aus-
tralia Pty Limited, who are a manufacturer of commercial plumbing products.
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