It has been more than 30 years since the enactment of the Safe Drinking Water Act (SDWA). Much has changed since then — most of it for the better. However, despite improvements in the quality of water, serious drinking water problems remain, particularly when it comes to water storage. As the population in the United States approaches 300 million people, more and more stresses are being put on the ecosystems that provide our raw water and the processing and distribution systems that deliver drinking water to the general public.
On September 30, 2003, the Environmental Protection Agency issued a Strategic Plan for 2003-2008. In that plan Acting Administrator Marianne Lamont Horinko said, "This strategic plan offers a new, more workable approach to our environmental protection efforts in the near future. We have established five new long-term, results-based goals to replace the 10 goals of our previous plans." The second of those goals is "Clean and Safe Water."
In 2003, the United States stated that 266.9 million Americans were relying on more than 160,000 different water systems to provide their safe drinking water. In that same publication, the EPA restates one of its core strategic objectives, "EPA Strategic Objective 2.1: Protect Public Health, Sub-Objective 2.1.1: Water Safe to Drink." By 2008, the EPA said it expects 95 percent of the population serviced by community water systems will receive drinking water that meets all applicable health-based drinking water standards through effective treatment and source water protection.
On the international front, the 2002 World Summit on Sustainable Development chose as a goal to reduce the number of people lacking access to safe drinking water by 50 percent by the year 2015 through a program that would "promote best practices and support capacity-building for water and sanitation infrastructure" globally.
As the world develops, population expectations drive the desire for readily available, pure, clean drinking water. Utilities and government agencies are scrambling to meet these needs. As they do though, they encounter several problems with the distribution of this vital resource. For years, people in the potable water industry have known that extended detention time of water in distribution system storage tanks leads to stagnation, reduced disinfectant levels, and poor/fluctuating water quality.
Water Storage Issues
Storage capacity is essential in water distribution systems to maintain hydraulic flexibility, peak shaving, and fire/emergency storage. But, it is well known that there are inherent problems with the quality of water in storage if not sufficiently turned over.
Some of the symptoms associated with poor water quality in storage include:
Problem/Symptom 1: Water stored over a period of time can stagnate due to stratification. Stratification, as well as a host of other factors, can cause stagnation.
When water is stored for an extended period of time, the water tends to stratify. Older water tends to float atop newer water. In this condition, the water does not mix well and may become stagnant. The stratification process can be exacerbated by the geometry of the tank, tank inlet/outlet locations, temperature profiles, and insufficient drafting or other operational factors. While many tank operators believe that the simple addition and removal of water in a tank mixes the water, it has been shown that even with regular draw and fill cycles, water can remain stratified. Although this problem is present in most tanks, it is particularly prevalent in standpipes.
Problem/Symptom 2: Disinfectant residual levels can drop to very low levels in the water in the tank.
When water is stored for an extended period of time, the disinfectant concentration is reduced. An example of this is water disinfected with chlorine gas. For example:
- When chlorine gas is injected into filtered water as a disinfectant, the diatomic chlorine gas molecules (Cl2) go into solution interspersed between the water molecules (H2O). Because the water is in a distribution system and under pressure, the chlorine gas stays in solution in the water. When the water is released into a water storage tank, the pressure of the water distribution line is lost because the water is now at atmospheric pressure. With the water no longer under pressure, the chlorine gas comes out of solution. (This principle is the same principle that allows CO2 to come out of solution when carbonated beverages are opened.)
- When the ambient temperature is warmer than the temperature of the water, the heating of the water in storage causes the gas to expand and come out of the water (diffusion). The longer the water remains in the tank, the greater the extent of diffusion. (This principle is the same principle that causes a warm can of carbonated beverage to open with a greater expulsion of gas.)
- The larger the surface area of the water in a tank, the more contact between the surface of the water and the atmospheric air. This increased surface-area interaction increases the opportunity for the entrained gas to escape. (This principle is the same principle that causes a glass filled with carbonated beverage to go flat faster than an open bottle of a carbonated beverage.)
- Another reason disinfectant levels tend to drop in tanks is because of the tendency of bio-film to form. Between disinfectant molecules and biological or chemical agents naturally introduced at the water surface (stagnation zone), interactions occur that result in the formation of disinfection byproducts (DBPs); such as trihalomethanes (THM) and haloacetic acids (HAA5) in the water.
Hence, the age of the water in the tank is a good predictor of its quality.
Mixing of water in tanks does not prevent the disinfectant from coming out of solution nor does it introduce more disinfectant. However, it assures that the disinfectant concentration of the water in the tank is well distributed and as high as it can be. Consistent blending minimizes the formation of DBPs in water storage tanks, thus helping utilities deal with regulatory concerns. As new water is added to the tank, the overall level of disinfectant is raised. Uniformity and consistency are the keys to keeping water at its peak quality. Continual blending and mixing of ALL of the water during BOTH the fill cycle AND the draft cycle is essential to assuring the end user the best possible quality of water.
Problem/Symptom 3: Short-circuiting of water directly between the tank inlet and the outlet, or even in tanks with a single inlet/outlet can lead to a false sense of security about water mixing in a tank.
Short-circuiting is a term that refers to the phenomenon that occurs when a fluid flows as a stream through another fluid. In the case of water tanks, the influent water creates flow streams through the water in the tank directly from the tank”s inlet to its outlet. This can occur for several reasons. The design of the tank can create physical pathways in which the stream can easily run. Thermal properties of the water that is introduced to the tank can force water in a given direction. As an example, if the sun warms the water in the tank, the cooler water from the underground pipelines can hug the bottom of the tank and proceed directly from the inlet to the outlet; even around complex baffles. The results of this short-circuiting are that some regions of the tank not near this stream can become stagnant dead spots. Drafting the tank to a very low level or even totally draining the water may minimize the volume of stagnant water; unfortunately this can put the stagnant water into the system.
Continual whole-tank mixing and blending eliminates short-circuiting and the dead spots that can form in storage tanks.
Problem/Symptom 4: The hydraulic constraints of daily operations often prohibit the operators from regularly drafting the tank to the appropriate levels to maintain optimum water quality in the tank.
As populations grow, and as more and more consumers are added to water systems, the need to provide adequate supplies and adequate pressures becomes more acute. These needs force utilities to operate with large volumes of stored water as safety reserves. Tank levels are often kept high for extended periods of time to maintain street pressure. Limitations created by required system pressures often reduce the ability to draft storage tanks. While this practice maintains an adequate, albeit low quality reserve water supply, it prevents the utilities from regularly drafting a tank to clear any aged water. The ability to use draft/fill cycles to change over stratified or stagnant water is reduced. As the water ages, it tends to stratify; and more disinfectant is released from the system. Together, these occurrences can lead to problems with the water quality and complaints from customers. Limitations created by required system pressures reduce the ability of utilities to draft storage tanks.
Mixing of the water during all fill and draft cycles alleviates these concerns for tank operators, utility owners, and ultimately the users.
Problem/Symptom 5: The need to maintain a minimum amount of water for fire and safety reasons can force utilities to maintain a minimum amount of water in a tank, thus prohibiting them from regularly drafting the tank to remove aged water.
An important component to water management for any utility is to provide a margin of water safety for the community served. This means that the system must always have a certain reserve on hand in case of a major fire or disaster. As the population of an area increases, the need to maintain these levels of safety increases as well. These factors often force utilities to maintain high levels of water in tanks. These levels may be required for extended periods. This practice maintains adequate pressure in the system and adequate supplies of water, but it often prohibits the city from drafting a tank on a regular basis. Again, the ability to change out the water on a regular basis is reduced, the water ages, stratifies, and more disinfectant is lost from the system.
Regular mixing with every fill and draft cycle assures that all of the water is as good as it can be all of the time.
Problem/Symptom 6: Long-term storage of water can increase the costs associated with disinfection — either by increasing the disinfectant required at the water purification plant or by requiring a secondary dosing facility in the field.
Historically, utilities have ensured adequate concentrations of disinfectants in two ways:
Disinfectants have been added to the water in the primary water treatment plants in sufficient dosages to remain disinfected until the water reaches the final consumer; and in large, zoned systems, disinfectants have been added at secondary injection/treatment sites in the field. This has often been done at or near storage facilities in the distribution systems. This method of operation can be costly and/or increase potential liability on the part of the utility.
In the first instance, the need to disinfect water to an extent that ensures quality to the end of the system forces some utilities to dose the water to a greater degree than necessary to allow for the loss of disinfectant during transit and storage. This can be a costly proposition.
In the second case, the field disinfecting process can be dangerous, expensive, and may expose the general public to disinfectants and processes that are normally found only in highly restricted plant environments. This increases costs and potential liability.
Mixing and blending of the water at every opportunity ensures the highest available concentration of disinfectant in all of the water all of the time and can reduce the need for secondary treatment, thus reducing treatment costs and their incumbent added liability.
Problem/Symptom 7: Ice damage occurs in colder climates when ice forms in above ground storage tanks, resulting in damage to structures, cathodic protection systems, and internal coatings on surfaces.
It is well know that ice forms in storage tanks during winter in cold climates. It is also well known that this ice formation can wreak havoc with tank structures. Many ice related events have been documented such as collapsed tanks, collapsed risers, and severe damage to internal structures and coatings. At particular risk are expensive cathodic protection systems. Ice is never "friendly" to tanks. The weight of the ice and its ability to cling to parts of the tank causes failure of many tank structures. As the water level rises and falls, the ice floats up and down within the tank, scouring the walls of their coatings. With the coatings removed, the ice then etches and scratches the surface of the metal where it begins to rust. The ice also captures rust, coating debris, and other matter in the water — all to be subsequently released in spring.
Mixing/blending of water with every fill and draft cycle reduces the possibility of ice formation. This provides several dramatic benefits:
- Internal structures remain unharmed, thus reducing the cost to repair.
- Cathodic protection systems are unscathed, reducing replacement costs.
- Tank linings are preserved, extending useful life and increasing times between refurbishments — reducing long-term operating costs.
- Tank life is extended by reducing the stresses on the tank and its components.
Attempted Solutions
The issues associated with the storage of water in tanks have been known for years. And, over the years many attempts have been made to rectify and/or prevent these problems. Most of them have met with some limited success, though none of them have proved to be a total solution.
One of the first attempts at solving the problem was to change the locations and sizes of the inlets and the outlets. It was originally thought that this approach would solve all of the problems. While the situation was improved, certainly all of the problems were not rectified.
Another approach was to use mechanical agitation or stirring. Because this approach had worked well in smaller industrial process tanks, it was assumed that it would work in larger water tanks as well. While the agitation worked to some degree, the mixers often simply created specific streamlines in the water that resulted in areas of the tank remaining unmixed. Agitation often simply provided localized turbulence, which increased the rate of disinfectant escape. In addition, these systems proved to be both costly to operate and difficult/costly to maintain and repair.
Yet another attempted solution involved the introduction of pump recirculation systems. This process used a pump to move water from one part of the tank to another part of the tank. Technologies tried have incorporated both submersible and external pumps, complex piping, jet nozzles, and restrictive orifices. While this methodology worked to some degree, it proved maintenance intensive, costly to operate, and difficult and costly to repair.
Lastly, various operational procedures were developed to ensure continual good quality water. One strategy requires the regular and frequent deep drafting of a tank to remove as much water as possible and then refilling it with water from the system. Another strategy that has been employed has been to pump through the tank, changing the water though plug flow. Hence, tank influent and effluent flows are the same. While these methods have met with some success, there is no question that they are, at times, impractical or impossible. Other constraints and operational policies of the system prohibit these techniques from always being implemented.
Recent Progress
Recently, the advantages of continually mixing water in storage tanks have come to be realized. In the mid 1990s, several projects were proposed in which the concepts of advanced hydraulic engineering and computer-aided design were melded to create an elegant solution to problems encountered in the industry. Since these engineering solutions were envisioned and implemented, the water quality issues of hydraulically required storage are no longer a disadvantage. Where storage was previously seen as a necessary but problematic part of the water distribution system, it can now be seen as an integral component in assuring high-quality water within a distribution system.
Using new technology based on advanced fluid dynamics, water can be totally mixed in the tank with every fill and draft cycle. Unique computer-aided designs can capture and use the potential energy already available in the system. By converting the potential energy of the water under pressure to kinetic energy utilities can mix the fluid without increasing head pressure (the cost of pumping) or the costs associated with storing and mixing water.
This technology takes water mixing in storage tanks to a new level of sophistication by incorporating an ultra-low-profile valve constructed from stainless steel and UHMWPE (ultra-high molecular-weight polyethylene). The technology, used in both new construction and routine tank rehabilitation projects, can yield significant water quality improvements by eliminating stagnation, stratification, short-circuiting, and winter ice. It can also lead to continual peak water quality and maintenance of adequate water supply. Using this methodology, water quality was not only maintained, but also quickly reacquired, after complete catastrophic system failures during last year’s blackout in the Northeast and Midwest. All the tanks with this new mixing technology recovered their disinfectant residuals in effluent samples within hours.
Applying fluid dynamics principles, the new technology noted here thoroughly mixes the water from different regions and levels of the tank during both fill and draft cycles. It uses the available energy in the inlet pipe to mix the water during filling, gravity and the atmospheric pressure at the storage facility to mix during drafting. This is done with a minimal backpressure on the pumps of less than one PSI. Some mixing occurs with the introduction of the water into the tank through multitude ports. This introduction influences a limited region near the inlet ports and is most effective when the water level is low. Most of the mixing during fill is created by three dimensional streamline currents that influence each other in such a manner that the water is blended throughout the tank — even the tank walls are utilized.
The mixing system also works during the draft cycle. It does not constrain the water to leave the tank from a single point or a small number of points. Gravity forces the water back into the distribution system through multitude ports from different regions and elevations within the tank. During the draft cycle, each port draws water from regions above that port’s location. These regions are different from the regions influenced by the fill cycle, which are more horizontal in nature. This combined mixing eliminates short-circuiting, stratification and stagnation.
Since the device introduces and removes water from all regions of the tank, the need to drop the water level in tanks to extreme levels is eliminated. Regular water inflows and outflows maintain water quality, with reduced reliance on extreme tank drafting. Keeping stored, treated water moving ensures uniformity of quality in water storage tanks and in the rest of the distribution system.
About the Author
Larry Rice is the global product development project manager for BIF, a provider of flow measurement systems for water applications. He earned his undergraduate degree in Biochemistry and an MBA from Case Western Reserve University. Mr. Rice has worked internationally as a senior manager in product development and project management in more than 70 countries, with particular emphasis on field instrumentation and control. He holds several patents and has written extensively on the topic of process control. He can be reached at [email protected] or 440 519-2431.
For More Information: www.bifwater.com
References
- U.S. Environmental Protection Agency. September, 2003. Strategic Plan, 2003-2008. EPA XXX/K-02-02. Washington, DC: U. S. Government Printing Office. Page 4. Available online at www.epa.gov/ocfo/plan/2003sp.pdf. Date of access June 1, 2004
- U.S. Environmental Protection Agency, Office of Ground Water and Drinking Water Accessing Drinking Water Data in SDWIS/Fed (Safe Drinking Water Information System/Federal Version Web Site), www.epa.gov/safewater/data/pdfs/factoids_2003.pdf
- United Nations. 2002. Report of the World Summit on Sustainable Development: Johannesburg, South Africa, 26 August 26-September 4, 2002. Page 11. Available online at www.joburgsummit2002.com/Jowsco/docs/plan_final.pdf. Date of access June 1, 2004
