Recently, I wrote an article regarding potable water and backflow protection. Water is something that most of us take for granted, until we cannot find a clean and reliable source. It is a basic need of most living organisms. Water is one thing we humans cannot live without. Humans can survive longer without food than they can without water.
Water is a major constituent of all living matter. When pure, it is an odorless, tasteless, very slightly compressible liquid oxide of hydrogen (H2O), which appears bluish in thick layers, freezes at 0°C (32°F) and boils at 100°C (212°F), and has a maximum density at 4°C (39.2°F) and a high specific heat. Water is feebly ionized to hydrogen and hydroxyl ions and, in a pure state, is a poor conductor of electricity. Pure water, by itself, is generally considered an incredibly aggressive solvent that begins dissolving and absorbing minerals with anything with which it comes into contact.
The water normally obtained from natural, municipal, or source water usually meets the U.S. Environmental Protection Agency’s (EPA) drinking water requirements. However, it is almost never chemically pure H2O. For those who wish to better understand water treatment, conditioning, and purification, visit ASPE’s Plumbing Engineering Design Handbook, Volume 2, Chapter 11 or read the Read, Learn, and Earn CEU 318 from April 2023.
The Problems with Private Systems
Let’s go back to the earlier article in which I wrote about the case of a campus water system that is fed by two municipal water services and has a 500,000-gallon underground reservoir, which acts as a reserve should they totally lose the municipal supply. The services are served by RPZ (reduced pressure zone) style backflow preventers, located below grade in separate meter pits. It should be noted that RPZs, or most backflow devices, should never be located below a potential flood level. However, the municipality allowed this arrangement since the meter pits were located at the top of a hill and had a gravity waste to the existing stormwater system.
Now let us look at this potable water reservoir and the potential issues that could increase the risk to the users. At the inflow and outflow points, the water velocity can be significant as the 8-inch and 6-inch water services supply the reservoir and the pump suction lines to the distribution system. However, within the reservoir the velocity of the water is almost nonexistent because of the large increase in area within the reservoir. This condition allows for stratification of the water by temperature and density, with almost no mixing occurring. As the velocity within the reservoir is so low, scouring and/or cleaning of the structure surfaces does not occur. Additionally, while the incoming water may contain residual disinfectant, this residual may be consumed within the reservoir. Hence, little to no disinfectant may remain in the water delivered to the campus’s potable water systems.
The facilities manager for this campus told me that they did not need to inspect or clean this reservoir as it was underground and sealed from the surrounding environment. The reservoir was placed into operation in 1998, and it has been opened only once to replace level sensors within the pump wells. These pump wells can be isolated from the reservoir for access and maintenance, which means that even on this one occasion, the bulk of the reservoir was not observed. I asked the manager why this reservoir had never been inspected, cleaned, or maintained and was really surprised by his response: The contractor and engineering firm stated that there was no need as this was potable water, sealed off from any contaminates. I advised the manager that such should not be the case and strongly recommended that the reservoir be scheduled for inspection. After all, it has been in service more than 25 years.
This reservoir is no different than a water tower or aboveground water storage tank—it is just underground. The American Water Works Association (AWWA) publishes many standards and regulations that are used by water purveyors to design, construct, and maintain their systems. Based on these standards, water reservoirs of any type should be visually inspected within the first five years to assess their condition and determine if any remedial work or cleaning should be accomplished. Based on this initial inspection, the purveyor can determine the frequency for future inspection and service. However, this campus reservoir in not part of the municipal system, separated by the RPZ backflow preventers. Hence, the municipality has no concern about contaminating their system or control over the campus’ operation of the reservoir. This is where the protection of the campus water system breaks down as far as its being considered “private.”
Since the private system has no ongoing governmental oversight, it is incumbent on the owner’s facility operations staff to appropriately operate and maintain the water system to protect the employees as well as the patients and public who utilize the campus. It is important for the facility staff to become knowledgeable about this major component of their potable water system. While the staff may be knowledgeable about the “normal” piping, pumps, fixtures, etc., that make up the distribution system, they lack knowledge about the reservoir, although this out-of-sight, out-of-mind component of the potable water system presents a potential high risk to the campus. In my judgment, it is only a matter of time before it could become a significant liability to the campus’ operations.
Potential Concerns with Reservoirs
The reservoir is there to serve the campus in case the municipal system fails. This 500,000-gallon reservoir is intended to provide approximately 96 hours of usable water should the municipal system’s failure occur as part of the emergency plan for the campus.
Let’s focus on the reservoir. It is constructed from reinforced, poured-in-place concrete. Internally, concrete columns support the reinforced concrete lid. While this is not an unusual construction method for this type of reservoir, it does have many opportunities for debris accumulation: squared corners, columns that add additional squared edges along the bottom of the reservoir. As this is a reservoir, not a pressurized vessel, it must be open to the atmosphere to allow for the changes in water level. You must remember, concrete is porous and should not be considered “waterproof,” so over time, water will be absorbed into the surface. This can result in spalling of the concrete and expose the rebar to the water, which is unique to concrete vs. a steel ground-mounted storage tank or elevated water tower. Regardless of the material of construction, all of these vessels will require periodic inspection, cleaning, and maintenance.
Following is a partial list of concerns related to these systems:
- Water could become stagnate due to the very low level of velocity within the storage space.
- Stratification of the water could occur due to differing temperatures within the water column.
- The chemical disinfectants contained in the potable water can also be consumed by the “still” water as it interacts with any contaminates contained in the water or introduced from the environment, as well as settle in various stratified layers of the water column.
- As there is almost no mixing of the water within the large area of the reservoir, some water may “age” more than other parts of the water as water forms a “channel” between the higher velocity inlets and outlets that serve the reservoir.
- Any debris or contaminates carried in from the municipal system will settle out within the reservoir and accumulate along squared edges where there is almost zero water movement.
- Atmospheric vents (generally screened) still allow air contaminates to enter the reservoir along with insects and sometimes small critters such as snakes, mice, etc.
As you can see, these reservoirs can allow for the development of potential pathogenic growth to occur within the stored water volume. Hence, inspection, maintenance, and cleaning are needed to ensure that the stored water remains safe for potable use.
How Design Professionals Can Work with Facilities Staff to Ensure Their Water Storage Remains Potable
As our profession learns more about the potential for pathogenic growth in potable water distribution systems (such as eliminating dead ends, keeping water in motion to minimize biofilm growth, and maintaining the water within established temperatures to again minimize the potential for pathogenic growth), it has had to adjust how we design and operate systems. The profession has come to realize that the potable water needs to remain “fresh,” have a reasonable exchange via movement or use (not allowed to become stagnant), and maintain a temperature below or above the growth range of known pathogens.
While these exterior water vessels, reservoirs, ground-mounted water storage tanks, or elevated water towers are outside the facility water distribution system, as design professionals we must take them into consideration when they are under the owner’s control. It becomes the responsibility of the facility staff and ownership to ensure that their system remains healthy and clean.
The following is a list of design considerations to address the potential issues discussed above. This list is based on the 500,000-gallon, below grade, concrete reservoir described in this article, but most of these would apply to all bulk storage vessels within a water system.
- Ensure a water exchange rate to minimize the aging of water within the vessel
- Provide interior mechanical circulation within the vessel to minimize stratification of the water by temperature and disinfectant levels
- Consider monitoring the level of residual disinfectant contained within the stored volume of water, which may require multiple sensor points
- Consider supplemental disinfectant, but remember that the level of disinfectant must be distributed throughout the volume of water. Injecting disinfectant or placing a basket of disinfectant pellets in the water will not distribute it without water movement.
- Minimize squared corners where debris can collect along the walls and around columns. It is better to use coved transitions from vertical to horizontal as well as slope the floor to a common collection point. Again, water movement will be needed to minimize debris buildup and accumulation. This debris buildup can promote the growth of biofilm and pathogenic colony growth.
- Establish a maintenance and inspection program that allows the staff to determine the frequency of these inspections based on data collected over time. At a minimum, I would recommend an initial inspection within five years of initial service. The next inspection would be driven by what was found from that initial inspection.
- Keep records of these inspections, such as video and photographs along with written documentation. This is an insurance policy should a pathogenic outbreak occur, but it also assists in planning for future improvements and maintenance within the vessel.
This by no means is a complete list, but from a design perspective the most important considerations are to ensure that the volume within the vessel continually turns over (minimize water aging), ensure sufficient water movement to “scour” surfaces within the vessel, monitor the residual disinfectant levels throughout the vessel, and minimize stratification within the water column.
While not everyone might consider this “plumbing,” it is part of the domestic water system when it is not part of the municipal provider’s network. As design professionals, we must always protect the public’s health, safety, and welfare.
About the Author
David D. Dexter, FNSPE, FASPE, CPD, CPI, LEED BD+C, PE, is a registered Professional Engineer, Certified Plumbing Inspector, and Certified Plans Examiner with more than 40 years of experience in the installation and design of plumbing systems. He specializes in plumbing, fire protection, and HVAC design as well as forensics related to mechanical system failures. Dave serves as Chair of ASPE’s Main Design Standards Committee, Chair of the Bylaws Committee, Co-Chair of the College of Fellows Selection Committee, and Co-Chair of the Professional Engineer Working Group. He also was the 2008–2009 President of the Engineering Foundation of Ohio, 2010–2011 President of the Ohio Society of Professional Engineers, and 2012–2014 Central Region Director for the National Society of Professional Engineers.
The opinions expressed in this article are those of the author and not the American Society of Plumbing Engineers.
