Considerations for Large Building Water Quality after Extended Stagnation

Abstract The unprecedented number of building closures related to the coronavirus disease (COVID‐19) pandemic is concerning because water stagnation will occur in many buildings that do not have water management plans in place. Stagnant water can have chemical and microbiological contaminants that pose potential health risks for occupants. Health officials, building owners, utilities, and other entities are rapidly developing guidance to address this issue, but the scope, applicability, and details included in the guidance vary widely. To provide a primer of large building water system preventative and remedial strategies, peer‐reviewed, government, industry, and nonprofit literature relevant to water stagnation and decontamination practices for plumbing was synthesized. Preventative practices to help avoid the need for recommissioning (e.g., routine flushing) and specific actions, challenges, and limitations associated with recommissioning were identified and characterized. Considerations for worker and occupant safety were also indicated. The intended audience of this work includes organizations developing guidance.


Introduction
In 2020, the global pandemic caused by novel coronavirus (SARS-CoV-2) disease (COVID-19) prompted -stay-at-home‖ orders all over the world that closed or reduced occupancy in many non-essential businesses and other buildings (e.g., education, event, worship, recreation, office, and retail buildings) [1,2]. With more than 5.6 million commercial buildings in the U.S. alone [3], the orders significantly altered drinking water demand patterns at both the water distribution and building system levels [4]. Specifically, many buildings experienced reduced water use, causing increased water stagnation time (i.e., water age). This is problematic as stagnation has been associated with the degradation of water quality in routine settings (on a time scale of hours to days) at the building [5-13] and water distribution system level [14][15][16][17][18][19] and can result in the presence of harmful chemicals (e.g., lead, copper) or harmful organisms (e.g., Legionella pneumophila) in water.
Water quality issues can be prevented or addressed with remedial actions, but the actions needed depend on the conditions of stagnation and on many site-specific factors.
Limited information is available regarding water quality impacts caused by extended stagnation and the effectiveness of plumbing remediation actions. Despite this lack of information, government agencies, water utilities, and private companies rapidly developed guidance to address the widespread building water system closures during the COVID-19 pandemic. Some guidance documents have been cited in this paper, but many are emerging weekly . This review provides an understanding of the challenges, current practices, and knowledge-gaps for maintaining building water systems and restoring building water systems to baseline conditions after extended periods of no or limited water use. This review is not meant to explicitly serve as a step-by-step procedure, rather, it serves as a foundation for the development of step-by-step guidance. The intended audience of this review includes public health officials and other entities that are developing guidance. This may also be of interest to plumbing engineers and building Accepted Article This article is protected by copyright. All rights reserved.
owners who must consider many issues while implementing guidance. Utilities may also be interested to help coordinate their efforts with their customers. Guidance developed to address COVID-19 pandemic stagnation or other prolonged stagnation events should address these considerations.
Finally, the authors' own firsthand experiences assisting building owners to decontaminate and restart plumbing after non-use, conducting plumbing related disease outbreak investigations, and answering questions received from local, state, federal agencies and non-profit organizations about policy were considered. This paper focuses on large buildings and campuses closed in spring 2020 to promote physical distancing (also known as social distancing). In this paper, the term building ‗recommissioning' is used to refer to the reopening of buildings after extended closures and focuses on restoring water quality to baseline conditions. Recommissioning here should not be confused with the recommissioning process featuring water audits and subsequent changes Accepted Article This article is protected by copyright. All rights reserved. made to increase water and energy efficiency in buildings (e.g., changing toilets and fixtures to low-flow) [59]. The focus of this paper is the hot and cold water systems. The authors did not consider other necessary actions unrelated to water quality (e.g., air quality, heating systems) or for other building water systems (e.g., cooling towers) although these likely need attention.
Other considerations will need to be taken for alternative building types (e.g., water shut-offs impacting 15M people due to non-payment [56]). The repurposing of buildings or reopening of medical facilities to expand capacity also received attention during the COVID-19 pandemic, and while these facilities face similar issues as those described here, medical facilities have additional considerations not included in this paper.

Stagnation in large buildings
There are several concerns for water quality that are common in complex large building water systems, which can be made worse by periods of no or low water use [60][61][62]. It is necessary to understand these reactions, the associated health risks, and complexity and variability of building water systems when considering how to maintain water quality. The plumbing and water quality for each building will be unique.
Reactions occurring during stagnation (Figure 1) include: (i) loss of disinfectant residual and decreased disinfectant residual stability, (ii) decreased effectiveness of corrosion control measures, (iii) microbial growth, and (iv) other issues such as taste, odor, and disinfection byproduct formation. Microbial issues include nitrification, growth of harmful organisms (e.g., Legionella pneumophila, Pseudomonas aeruginosa, non-tuberculous mycobacteria, others), and changes in microbial communities. These reactions, most of which have only been studied on a relatively short time-scale, are described in SI-1-Stagnation Issues. It is currently unknown how these reactions may be impacted by long-term stagnation on the order of weeks or months (e.g., growth reaching a carrying capacity with limited introduction of new nutrients).

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Health risks from these reactions can be associated with the ingestion of, inhalation/aspiration of, or contact with contaminated water. Lead, copper, and other metals can leach from pipes or scales that may become unstable during long periods of stagnation.
Increased growth of opportunistic pathogens such as Legionella pneumophila can also occur.
Certain populations are at higher risk for adverse effects (e.g., children for lead exposure, immunocompromised persons for Legionnaire's Disease). COVID-19 patients may also be at risk for co-infection with Legionella pneumophila [63].
Building water systems comprises all the piping, equipment, treatment devices, fixtures, and appliances associated with providing water from the service line point-of-entry (POE) (i.e., where water enters the building) to the point-of-use (POU). The number and type of plumbing components and materials encountered ( Table 1) will depend on the building's water source, design (Figure 2 & S1), and water use applications. For example, water of various qualities (e.g., softened for drinking and unsoftened for handwashing) or temperatures (e.g., cold, multiple hot systems) might have their own parallel piping system through a building. Given the variability and complexity of plumbing, it is difficult to make generalizations.

Water management practices under normal use
Normal building operation can often result in stagnation (e.g., offices over the weekend, unused hotel rooms), and the reactions described in Section 3.1 and SI-1 continuously occur at highly variable rates. Green buildings may be especially impacted by stagnation, since they are designed for lower water use without substantially changing plumbing design [7,10,11]. To manage the water quality issues that occur even with normal use, some buildings (e.g., healthcare) are required to have building water management plans (BWMPs) [50,64,65].
However, in a small survey, nearly 60% of commercial building respondents (n=29) had not heard of building water management plans [66].

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Building water management plans help guide prevention and response to water quality issues, especially opportunistic pathogen growth. The management of building water ultimately requires a diversity of stakeholders to (1) supply water; (2) design, construct, operate, and maintain the system; (3) recommend and implement public health interventions; and (4) enforce applicable codes and regulations, which vary widely by state. Each category has individual building components that require a range of stakeholder involvement and may require one or more external vendors/suppliers to coordinate and manage. Development of BWMPs may also be considered in response to COVID-19 related stagnation ( Table 2, details in Table S2) [25,42]; however, the full implementation of BWMPs can take years and require substantial resources and these documents do not necessarily address long periods of low or no water use explicitly. Building owners and creators of COVID-19 related plumbing guidance may want to instead focus on the most important aspects related to water quality and on quickly achievable goals. This includes keeping water fresh and thermal regulation (i.e., keeping cold water cold and hot water hot). Resources are readily available to aid in the development of these plans [50,54,67].

Prevention of water quality issues during periods of low use
Buildings or entire communities can experience long-term periods of low or no use (e.g., schools over the summer, ski resort/snowbird communities, buildings unoccupied between owners, water shut-offs). The buildings impacted by COVID-19 stay-at-home orders may have had either reduced or no water use for weeks to months. The actions described here may be helpful to implement during stagnation and are recommended in some guidance documents ( Table 2).
Routine flushing introduces fresh water to the system regularly to help prevent problems from developing. It could be used to remediate problems if performed frequently, or off-set partial capacity during a ramp-up of economic activity [68]. BWMPs often contain provisions for Accepted Article This article is protected by copyright. All rights reserved.
weekly flushing of -unused‖ or -unoccupied‖ outlets [69] in otherwise occupied buildings, but criteria for its efficacy have not been thoroughly documented. Necessary frequency is especially difficult to determine. Weekly flushing may be insufficient for effective Legionella control due to: (1) plumbing design, hydraulic balancing, or temperature issues; (2) complexity of components such as electronic faucets and thermal mixing valves; (3) stored volume of water relative to water use (i.e., incomplete turnover). In one hospital with hot water recirculating temperature that were inadequate to prevent Legionella growth (<45 °C), a flushing frequency of every two hours was required to reduce culturable Legionella numbers to -acceptable‖ levels [70].This flushing frequency is likely only achievable with auto-flush faucets or solenoid values, and is a clear indication that for some systems, flushing alone may not achieve acceptable results.
Flushing recommendations generally rely upon the assumption that water delivered to the building and used for flushing has a growth deterrent (high temperatures or disinfectant) and a corrosion control component, which may not be the case [7,71,72]. Flushing as a preventative measure in completely unoccupied buildings has not been studied previously. Considerations for the development of plans for flushing (i.e., necessary actions, order of actions, duration of flushing) are discussed in Section 3.5.
Water heater operation can be altered in periods of low or no water use in the building (e.g., turning off a water heater in a summer home during winter). Legionella management in large buildings typically relies on thermal control [69,73,74]. In large buildings, recirculation loops ( Table 1) are often used to move hot water continuously throughout the building, reducing the time for hot water delivery and maintaining high temperatures for stored volumes. For thermal controls to remain effective, both heaters and recirculation lines should maintain high temperatures. However, this approach should be combined with regular flushing of distal pipes to the POU, which can be maintained at ideal opportunistic pathogen growth temperatures during periods of non-use [75]. If water heaters or recirculation pumps are completely shut down, this may save energy and allow systems to cool to sub-optimal growth ranges. The latter

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This article is protected by copyright. All rights reserved. has never been studied, but is recommended by some ( Table 2). If hot water systems are allowed to cool, adequate amounts of flushing should be performed to maintain a disinfectant residual throughout the hot water system, which may be difficult due to disinfectant reactions with plumbing (e.g. water heater sediments).
Draining building water systems is sometimes done when water systems are purposefully shutdown for extended periods (e.g., for construction, summer homes in winter).
This may prevent growth in water but can introduce many other issues. Plumbing is designed to maintain pressure and drainage could introduce backflows and contamination from other water systems such as cooling towers and fire protection systems if effective backflow prevention is not in place. Refilling systems may result in the destabilization of sediments and biofilms or the introduction of external contaminants to the pipes. Shock disinfection may be necessary at startup after depressurization: If depressurization is thought to have occurred in seasonal potable water systems, additional shock disinfection is recommended [51]. While one guidance released regarding COVID-19 building closures advised for draining plumbing [23], another guidance explicitly advises against it due to the pockets of water likely to remain in plumbing [22]. Drained systems likely have different growth conditions, which may induce mold or other organisms to grow. Plumbers are likely needed to safely drain and restart drained plumbing systems and this procedure may not be feasible for many buildings (e.g., with continued occupancy by essential employees).
Water utility distribution networks suffer similar stagnation issues to buildings. Since the efficacy of periodic flushing depends on the water supplied by the water utility, utilities have a role in prevention of building water quality issues. A disinfectant residual should be present but may be harder to achieve with reduced system demand. Utilities may increase the concentration of disinfectant residual in their distribution system, which has precedent in this and other emergency situations [71,72]. This must be weighed against increased disinfection byproduct production formation. Utilities may also more closely review routine water quality monitoring Accepted Article This article is protected by copyright. All rights reserved. data, implement focused flushing efforts [76], or install auto-flushers to increase delivery of disinfectant residual.

Recovering plumbing after periods of low or non-use
If no preventative actions are applied, a process called recommissioning may be needed. No consensus was found in literature for the length of time a building can remain unoccupied or with low occupancy before it should be formally recommissioned, or the extent of actions that should be performed. Several actions described below are suggested for the annual restart of seasonal potable water systems (e.g., campgrounds, fairgrounds) and as part of initial building commissioning if building occupancy is delayed ( Table 3). Specific care must be taken in adapting these documents, as the complexities and variability of large building water systems may not be considered. Moreover, the continued occupancy of some buildings during prolonged stagnation (e.g., essential staff) may not be compatible with certain actions.
System integrity should be checked before taking any remedial actions. For buildings, this could involve inspection of mechanical and plumbing components ( Table 1) to identify leaks, depressurization, adequate backflow prevention, and assess functionality (e.g., hot water supply and return temperatures, on-site disinfectant dosing correctly). The performance of routine maintenance or start-up procedures if equipment was taken offline can also be considered.
Recommissioning flushing reducing has similar goals and considerations as routine flushing. There is precedent for using flushing alone to restart water systems that have experienced extended stagnation but maintained pressure [49,51]. However, this flushing only strategy is targeted at reduction of coliforms (Total Coliform Rule [77]) or lead (flushing for schools [78]), which can be removed relatively easily. Flushing cannot eliminate biofilms where opportunistic pathogens can continue to grow, and it is unclear how flushing alone would impact opportunistic pathogen recurrence. Especially with a slow ramp-up of building activity (i.e.,

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phase plan for COVID-19 pandemic recovery [68]), initial flushes might need to be paired with routine flushing to introduce more fresh water to pipes. The development of a flushing plan is discussed in Section 3.5.
Flushing after long stagnation also requires the additional consideration of worker safety.
Initial flushes of stagnant water can sometimes release high concentrations of chemical and microbiological contaminants due to high shear stress [79] and in-situ reactions (SI-1). To reduce exposure risk, workers responsible for flushing can be screened for preexisting conditions that may make them vulnerable (e.g., to opportunistic pathogens), and/or use personal protective equipment (PPE) ( Legionella or other pathogen-containing aerosols can accumulate in the room as flushing is performed. The CDC recommended to authors of this study that local health authorities should be consulted to determine appropriate personal protective equipment [80]. The global shortages in critical PPE (e.g., gloves, masks) [81] surrounding the COVID-19 pandemic must also be considered. To further reduce exposure, flushing can also be conducted in a manner that reduces water splashing and aerosolization [82] (e.g., hoses to connect spigots to drains, towels and bags placed over faucets and showerheads, covering toilets). Increasing ventilation can also help to reduce aerosols during flushing activities.
It may be beneficial to coordinate recommissioning flushing with actions taken by the local utility. Customers could coordinate building flushing with utility hydrant flushing efforts or conduct flushing during periods when utilities temporarily boost disinfectant levels. It is possible that many buildings flushing concurrently could impact local buried water distribution system pressure [83].

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Shock disinfection introduces a high concentration of disinfectant or high temperature for a relatively short period of time to reduce the presence of microorganisms in the system. This procedure may reduce biofilms, but is not designed to eliminate biofilms. The practice is common for initial building commissioning and remediation of Legionella colonization. The American Society of Heating Refrigeration and Air-conditioning Engineers Standard 188 (ASHRAE 188 [50]) outlines that shock disinfection after construction should occur within three weeks of planned occupancy. If occupancy is delayed more than four weeks, another shock disinfection is required prior to occupancy. It is unclear how this would apply for buildings that have already been occupied or have continued low occupancy. Another challenge is that the ASHRAE 188 action thresholds (e.g., 3, 4 weeks) are not based on peer-reviewed studies with supporting evidence.
Free chlorine, chlorine dioxide, chloramines, and thermal shock have been used successfully for remediating Legionella growth [69,84]. Some COVID-19 guidance recommends shock-disinfection ( Table 2) [85]. Targeted approaches to disinfect plumbing associated with high-exposure activities for inhalation of aerosols (e.g., showering, Jacuzzis) have been used in the past, and are suggested by the CDC in their COVID-19 building system guidance [42].
The implementation of shock disinfection likely requires the assistance of professionals to ensure efficacy and safety. Disinfection recommendations for commissioning procedures would have to be adapted (Table 4). In short, all parts of the system should be exposed to water with an inhibitory temperature or with disinfectants for recommended durations to achieve sufficient contact time. Flushing is recommended both before and after the procedure to remove loose deposits and high levels of chemical disinfectants. A building water system risk assessment (done by professionals) can help identify any secondary issues with water system operation (e.g., unbalanced hot water system [69,73,74,86]) thus maximizing procedure efficiency. Material compatibility can be an issue, with shock disinfection causing plumbing leaks and damage [87][88][89][90]. To avoid dermal and inhalation exposure it may be necessary to prevent

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building entrance or post clear signage to warn of harmful chemicals or temperatures. Disposal of water with high chemical concentrations may require pretreatment or coordination with local wastewater authorities. Lower dosage limits combined with increased contact time may be desirable to limit potential issues with high chemical doses (i.e., pipe damage, disposal).
Testing, while not typically required for occupied buildings, is the most definitive way to ensure that water in a building is ready for use. Requiring testing after disasters has precedent: when volatile organic carbons were discovered in drinking water after a wildfire in Paradise, CA, the local health authority required tests prior to occupancy [91,92]. Moreover, some North American authorities require testing of lead and copper in schools [93,94]. Testing has been recommended in some COVID-19 guidance documents ( Table 2) to validate flushing and recommissioning practices. If conducted, testing should focus on relevant water quality parameters. Testing for disinfectant residual, which is required with the startup of seasonal potable water systems, can easily be done on-site with inexpensive handheld instruments.
Testing for metals (e.g., lead, copper) can be accomplished using a certified lab. Total coliform or E. coli testing, frequently used to certify seasonal systems or buildings for occupancy after construction [48,49,51], has low relevance in occupied pressurized buildings. General bacteria testing (e.g., heterotrophic plate count, HPC) may be more relevant to determine the success of shock disinfection, but HPC results are difficult to interpret, since normal use results in high and variable counts in buildings [6,12, [95][96][97][98]. Importantly, HPC levels have not been associated with any known health impacts. Testing for opportunistic pathogens is more relevant for understanding health risks and comparing measured concentrations with concentration limits recommended for reducing risk [99,100]. However, choosing which one(s) to test for, how/where/when to collect samples (e.g., first flush vs. after flushing for several minutes), and how to interpret results requires professional assistance due to the variability in water quality within a building [5,7,98]. Advice for regular Legionella sampling in other countries may be a useful start point [69]. Testing should be conducted through a certified lab, and results are not

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This article is protected by copyright. All rights reserved. available for 7 days or more. Testing recommendations in guidance documents often lack necessary specificity, especially regarding where to test ( Table 2).

Considerations for flushing plans
Guidance for flushing, whether for routine or recommissioning flushing, must account for variability in building water systems. The development of site-specific flushing plans is necessary. Estimating the total volume in the water system, or diagnostic testing (disinfectant residual and/or temperature) can help determine how much water to flush at each location. All flushing procedures should begin by establishing fresh water at the POE (i.e., adequate flushing to clear the service line and any stagnant water in the distribution system) and then progressing through the system in a -downstream‖ fashion, as described below. An inventory of water outlets and devices will help ensure water movement at all taps. Flushing plans may vary slightly if they are done periodically versus after extended stagnation; for instance, recommissioning flushing may include draining and cleaning of storage tanks, whereas routine flushing may not. The time, effort, and cost (e.g., water price [117]) of flushing will vary considerably amongst buildings.
Service lines provide water to the building for cold, hot and other property water systems (e.g., sprinkler systems [101], cooling towers). The water volume stored in a commercial building service line can range from 10s of gallons to 1000s of gallons [102] depending on the property design (Figure 4). It is critical that water is flushed at the building POE first to avoid drawing stagnant or potentially contaminated water into plumbing. Multiple POEs may also exist in a building. Conducting POE flushing in conjunction with hydrant flushingeither by the utility or with privately owned fire hydrants (e.g., on campuses) may be considered to clear service lines [102]. It is advisable to confirm the presence of disinfectant residual at the POE, as distribution system water quality may be degraded.

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Mechanical plumbing equipment located in mechanical spaces and -upstream‖ of the main building piping network (Table 1) must be considered. Bacterial growth, including pathogens, has been associated with this equipment and with subsequent disease cases [103][104][105][106]. Volume stored in the devices must be overturned, which can be difficult due to nonideal flow through them (e.g., water heaters [107]). Since buildings have a wide variety of devices, general guidance should require inventorying devices. Several guidance documents recently released for homes [108,109] and buildings ( Table 2) with stagnant/shutoff water fail to account for all devices. In the case of recommissioning, additional action may be needed, including ensuring the equipment is still functioning or draining/cleaning. Manufacturer guidance does not typically cover prolonged stagnation events. Routine maintenance and initial startup procedures (e.g., softener resin replacement or cleaning and disinfection, filter replacement) can serve as a starting point. Adapting existing recommendations should focus on ways to fully overturn the storage volume and remove accumulated sediment/biofilm [66,110].
Plumbing configuration, including pipe length, diameter, and layout, can vary greatly (Figure 2). Site-specific configurations will affect the water volume (and time) needed to flush each tap, toilet, or device and the order in which outlets should be flushed. Typical nonresidential plumbing systems in large buildings have a trunk-and-branch design with one or more risers and headers with branches to individual outlets, but may have much more complexity (e.g., multiple pressure zones with tanks in high rise buildings, Table 1). Smaller systems may also have a manifold design. Implementing effective protocols may require access to plumbing plans (or as-built drawings, if available) and/or building personnel knowledge of system design and operation. As-built construction drawings may also be useful in inventorying every water outlet (e.g., outdoor spigots, forgotten taps) so that every pipe and location is flushed. Dead ends (pipes that lead to nowhere) can also exist in buildings, especially if buildings have been remodeled or had a change in use. Dead end pipes will require Accepted Article This article is protected by copyright. All rights reserved.
professional help to flush (e.g., with depressurization); it is best to identify and cap them as close to main branches as possible.
End use devices (i.e., appliances, Table 1) also have internal water storage and many become colonized by pathogens [111][112][113][114][115][116]. Yet, they are absent from some COVID-19 inspired building water system flushing guidance ( Table 2). These devices' volumes can be overturned by running them or manual action (e.g., discarding several batches of new ice). For recommissioning, routine maintenance of all devices (e.g., replacement of filters) should also be considered. Medical and dental facilities with specific sterile water or special application appliances (e.g., dialysis, heater-cooler machines) must be particularly carefully maintained and cleaned. Procedures and manufacturer protocols for these devices should always be consulted [112,[117][118][119][120].
Fixtures such as faucets, aerators, fountains (bubblers), thermostatic mixing valves, showerheads, and shower hoses can be relatively easily removed, cleaned and/or discarded.
Pathogen growth and heavy metal accumulation (e.g., particles of copper, iron, lead) have been associated with these plumbing components [121][122][123][124][125][126][127][128][129][130][131][132]. Thermostatic mixing valves, used in showers and faucets to mix hot and cold water to prevent scalding, have been identified as particularly problematic for growth of Legionella [133][134][135]. Cleaning such devices is recommended for normal maintenance [66,69,136,137], and recommissioning may be an opportune time for these practices. At a minimum, these devices should be checked for functionality, as the release of sediment during flushing can cause them to leak or become plugged, potentially creating a cross-connection between hot and cold water systems. While not directly part of water delivery, sink drains can be a source of pathogens, contaminating faucet aerators in hospitals [138], and thus cleaning and disinfection should be considered.
Waste water generated during flushing must also be considered. If the building utilizes an onsite septic system, special care must be taken not to overload and flood the system, as this can permanently damage the tank and leaching field. Any flushing procedure should ensure Accepted Article This article is protected by copyright. All rights reserved. that drainage capacity can be met, and flushing must be monitored to avoid flooding (i.e., with drain blockage) and cross-connections.
Flowrate during flushing is an important consideration, especially as it dictates how long it will take to remove water from plumbing. Some experts suggest low flowrates at taps to minimize aerosolization [139]. Flushing at high flowrates can mobilize loose deposits and biofilm from pipe walls, which may be desirable during recommissioning, but requires several considerations. There is no consensus amongst experts, and a mixed approach (i.e., starting low and increasing flowrate as water quality improves) could be used. Diagnostic testing can check that that flushing is complete -that all stagnant water is removed from the system and growth deterrent is delivered to all taps. Diagnostic tests including turbidity, pH, temperature, specific conductance, and disinfectant residual are suggested by the USEPA to determine where water is originating from during flushing [144]. Temperature stabilization (<0.1 °C or <0.2 °F change [144]) is easy and may indicate that water is coming from the distribution network, but local climate considerations should be taken into account (i.e., water will not get cold in hot climates in summer). USEPA-approved and inexpensive handheld disinfectant residual monitors can also be used. Disinfectant residual test strips should be used with caution. Care should be taken to test for the correct disinfectant (i.e., free and total chlorine for systems using chlorine vs. monochloramine and total chlorine for systems using chloramines; total chlorine alone can be a useful indictor for either system).
A disinfectant residual is unlikely to persist through water heaters, so temperature is used as a flushing diagnostic in hot water systems. This is more difficult than in cold systems, but has been successful [74,86]. A high stable temperature (i.e., 55° C, recirculating temperature) may be reached for several minutes without drawing significantly from the water heater (i.e., 60° C, heater set point). If the system is not properly balanced, steady-state temperatures can vary substantially throughout the building. Changes in building heating and cooling or hot water system operation during shutdown may also affect flushing temperature profiles. Installation of temperature probes may enable building managers to better understand their building water during flushing and normal use.

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Flushing duration is extremely difficult to generalize in flushing plans. Widely issued time-based flushing protocols will be ineffective for some buildings due to variability in building water systems. For example, using some prescribed times recommended in COVID-19 building water guidance documents ( Table 2)

Communication
No regulations were found that required building owners to notify building occupants about building water quality. However, a proactive approach to addressing and communicating water quality issues in buildings is generally recommended at both the utility and facility level [146]. If a communication program is pursued, several items should be considered.
Communications about building water health risk should be coordinated with local public health authorities. Example communication messages for building owners, public health authorities, and water utilities related to building water can be found in the SI section (SI-2).
Information developed for utilities may be informative for developing materials for boil water notices, as well as do not drink, and do not use notices in escalating order of severity is also available [152], but building owners should be aware that these warnings are focused on meeting primary drinking water regulations and are not necessarily protective of public health with respect to opportunistic pathogens. easily identified. One large building owner who contacted the authors has been posting -drinking water out of service signs‖ at faucets where no residual can be found, which also reduces touch-points for COVID-19 transmission. In extreme contamination situations, building owners could perform a -lockout and tagout‖ of the affected water fixture or building area [156].

Conclusions
This review was conducted to inform the development of guidance to address water quality concerns in fully or partially shutdown buildings, and the reopening/repurposing of other buildings. Reduced or no water use in buildings may present both chemical (lead, copper) and microbiological (opportunistic pathogens) health risks. However, the unprecedented nature of widespread, long-term building closures has never been studied and health risks have not been quantified with respect to specific plumbing designs, plumbing features, or operational parameters. Building water quality is the responsibility of the building owner, though codes require that the local health authority (generally referred to in the UPC and IPC [52,53]) make decisions about building water system commissioning, and similar responsibility may be conferred for recommissioning. The delivery of high quality water requires the cooperation of several diverse stakeholders.
Several efforts were recognized as requiring future investigation:  Evaluate the effectiveness of specific recommissioning actions or series of actions in reducing health risks across plumbing configurations. Documenting success or failure of guidance will allow for improved guidance that minimizes risks and costs.  Develop methods for determining the frequency, number, and location of representative water samples for a building and the necessary chemical and microbiological analyses needed to adequately assess health risks and inform remedial actions.

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 Investigate the factors that control chemical and microbiological water quality characteristics under prolonged stagnation (i.e., months) and strategies to prevent water quality deterioration.
The COVID-19 response provides an opportunity for health officials, building owners, and utilities to proactively reduce building water system health risks. Coordination of efforts between these entities will enhance success. Evidence-based standards and guidance are lacking and needed to address routine building water system maintenance (e.g., flushing), monitoring, and recommissioning procedures [157]. In absence of those standards, information contained in this review can help inform and guide health authorities and building officials make building water system and public health decisions. In writing guidance for buildings impacted by the COVID-19 pandemic, several key facts should be considered:  Guidance must allow for site-specific variation in buildings and allow for tailored plans and actions.
 Stagnation duration and severity will vary (e.g., by length of stay-at-home order, type of business, and plan for building re-occupancy), and actions may need to be tailored.
More complex building plumbing, particularly with higher-risk occupants, may require more intensive preventative or remedial measures.   With a slow ramp-up of building occupancy, water stagnation will continue. Repeated actions (e.g., routine flushing) may be necessary to prevent water quality degradation and plumbing damage.

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This article is protected by copyright. All rights reserved.  Emergency preparedness requires forethought. If preventative actions (i.e., routine flushing guidelines) are developed now, they can be implemented now and in response to future disasters that prompt low occupancy or building shutdowns.

Funding
Funding for this work was partially supported by each organization as well as a U.S.

Conflicts of Interest
The authors declare no competing interest.

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This article is protected by copyright. All rights reserved. Figure 1: Potential chemical and microbial water quality impacts associated with prolonged stagnation in chlorine (Cl 2 ) and chloramine-based (NH 2 Cl) disinfectant drinking water systems.

Figure 3:
Considerations for recommissioning guidance in six major categories: system integrity, recommissioning necessity, informing occupants, flushing considerations, disinfection considerations, and finally, readiness of water for use. 1 ASHRAE 188 is an adoptable standard focused on Legionella contamination and is the only guidance regarding length of closure that may prompt the recommendation for recommissioning actions; it may not apply to all contaminants discussed; 2 multifamily residential, >10 stories tall, healthcare facility, patient stays >24 hours, housing or treating immunocompromised individuals, housing >65 years old occupants.

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This article is protected by copyright. All rights reserved.     Point-of-use devices On-faucet treatment system, under sink treatment system. * ASHRAE 188 defines the delivery system for hot and cold water as the -potable‖ water system [50], and it is sometimes referred to as -domestic‖ water. Some of the components contain both metal and plastic subparts. These include gaskets, polysulfone or PEX dip tubes, liners and coatings such as glass, ceramic, epoxy, polyurethane, polyurea, and fiberglass. Gaskets may be ethylene propylene diene monomer (EPDM) (sulfur or peroxide crosslinked), butyl rubber (BR), natural rubber (NBR), neoprene, styrene butadiene rubber (SBR), synthetic rubber.