Environmental Science & Engineering - www.esemag.com - November 2004
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The quality of Canadian drinking water:
Does mixing hold the key?

By Larry Rice,B.S., M.B.A., and Bruce Morgan, B.Tech.

The majority of Canadians simply turn on the water tap and draw a tall glass of water with absolutely no thought as to the quality, much less the age of the water. After all, we don’t live in some uncivilized, third-world outpost, we live in Canada, and our water is certainly pure and trustworthy. Or is it?

During the past few years and after a couple of widely publicized “accidents”, the safety of drinking water in the developed countries of the world has come under great scrutiny. Recent research has begun to explain the relationship between stagnation and water age. As late as the September issue of the AWWA Journal, Dr. Walter Grayman warned: “As water ages, the potential for bacterial regrowth and the formation of disinfectant by-products increases while the disinfectant levels decrease.”

In an earlier published work, Rice wrote; “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.”

So, what is the key to maintaining peak water quality in distribution storage? Without question, there is a need for adequate mixing and blending of all drinking water in distribution systems to ensure adequate treatment and disinfection. As Grayman wrote in his conclusion: “To discourage the formation of zones of stagnant, older water, distribution system storage facilities should be designed and operated to encourage good mixing.”

How does the water lose its disinfectant in distribution storage? Think about the production of carbonated beverages. When they are manufactured, CO2 gas is injected into the liquid to carbonate the beverage. When the container of beverage is sealed, the CO2 gas molecules are trapped between the liquid molecules. They remain in solution as long as the container is sealed. When the container is opened the CO2 gas immediately begins to escape and the beverage is said to “go flat”. The beverage is still the same, but the CO2 gas molecules have escaped.

Similarly, diatomic chlorine gas molecules are injected into pure, filtered water as a disinfectant. These molecules begin to escape the water as the water enters distribution storage tanks. Storage tanks are at atmospheric pressure. The water was under pressure in the piping of the distribution system because the pumps pressurized the water to move it, and the pipelines kept the water constrained and under pressure. This pressure keeps the chlorine molecules in solution just like the pressure in a carbonated beverage container keeps the carbon dioxide in solution. As the water moves from the pipeline (an area of pressurization) into non-pressurized tanks, the chlorine gas molecules escape the water, thus reducing the disinfectant.

Adequately mixing and blending of the water in distribution helps to assure continued quality by always mixing new water with stored water, thus maintaining adequate disinfectant levels, reducing the level of disinfectant by-products, and in general reducing the average age of the water in the system.

So, what is the right way to mix the water in water storage facilities? Several technologies have been tried in the past several years. Unfortunately, some have serious drawbacks or are expensive to operate and maintain.

To adequately mix the water, and ultimately reduce water age, engineers must address specific problems in a specific tank and take into account the water quality needs of the utility and their customers. In addition, the operational parameters of the tank must be considered. Simply “throwing hardware” at the problem and then boasting about a mixing “system” is not enough. The tank must be closely evaluated and an engineered system must be designed specifically for that tank to truly solve the specific problems of each tank. Water storage tanks are like snowflakes, no two are alike.

Various technologies have been tried. These include: mechanical mixers, pump-arounds, baffles, valves or orifices, and most recently “ported-conduit” systems.

Mechanical mixers are just what you would expect. An electric motor, suspended about the water surface, turns an impeller or propeller mounted on a shaft in the water. Though these will “agitate” the water locally, they can only truly mix the water if the tank is of a relatively small size. However, far worse is the sense that a problem is being addressed when the mixer may, in fact, be exacerbating the problem. Think of a soft drink—if you shake the can, more of the CO2 will escape than if you simply open the can. In the same way, water will lose more of its disinfectant (Cl2) if it is agitated and more of the water’s surface area is subjected to the atmosphere. The disadvantage of mechanical mixers is in the cost of the energy required and the labor needed to maintain the motors and provide regular inspections, etc. These mixers only mix the water in the tank, and have no way to force new water to blend with old water.

Pump-arounds have been around for years. Water is taken from one part of the tank and pumped to another area of the tank where it is reinjected. Pump-arounds mix the water to a certain extent, particularly when all the water is approximately the same temperature. However, when water is a different temperature in different parts of the tank (the usual situation) this practice can lead to short circuiting, a situation where a stream of colder water taken from the bottom of a tank is pumped around and put back into the top of the tank where it simply “channels” directly down through the warm water and back to the bottom of the tank. This is especially true in the summer months. The disadvantages include the energy required for the pumps and the labor to maintain pumps, provide regular inspections, etc. All of this contributes to a high life cycle cost for this technology. As with mechanical mixers, these mixers only mix the water in the tank, and have no way to force new water to blend with old water.

Baffling is simply an arrangement of a series of labyrinthine walls of various shapes that attempt to mix the water as the water flows between the inlet and the outlet of a storage tank by forcing the water to flow around these shapes. Unfortunately, when there is a temperature differential between the inlet water and the stored water, the water simply “snakes” around the baffles, finding the outlet. The warmer water in the upper strata of the tank may not contact the new water under these circumstances.

Drafting tubes are the simplest of all attempts at water mixing. A section of pipe is mounted vertically several inches off the floor of the tank and directly above the inlet pipe. Theoretically, as the water flows out of the inlet and up through the section of pipe suspended above it, this new water pulls older water already in the tank up through the draft tube thus mixing new water with old water.

Valves or orifice nozzles are the most complex of the mixing techniques, and inherently require the greatest maintenance and inspection burden. In addition, these systems may result in loss of head, increased pumping costs, and reduced fill times. The water is directed up from the inlet through a header “standpipe” extending high into the tank, and out through a limited number of valves or restrictive orifices.

Ported-conduit systems have proven to be a cost-effective solution to the problem. They also assure mixing and blending of the water during both the fill and draft cycles. As water enters a storage tank through the inlet, it is directed through a series of conduits to various areas of the tank. The water exits these conduits through hundreds of ports located on the sides of the conduits. These ports create streamlines that take the water to the far walls of the tank. These streamlines cause the "new" water to infiltrate the ambient water, causing total mixing.

As the water reaches a storage tank it is under pressure. This pressure drives the water into the tank. This water, under pressure, possesses potential energy. The storage tank is at atmospheric pressure. As this water, with its potential energy, reaches an area of atmospheric pressure, the potential energy is converted into kinetic energy. This energy differential (the difference between the potential energy and kinetic energy) is the energy that is utilized to cause the water to flow throughout all areas of the tank. So, for the fill cycle, the secret is to harness the energy to horizontally direct the water to all areas of the tank. During the normal drafting cycle, ported-conduit systems draft the water back through the same port and conduit system. The water enters the ports from areas immediately above the ports, and continues along the conduits to the distribution system. Gravity pushes the water into the ports as water is taken by the conduits back into the distribution system. During the draft cycle, the secret is to use gravity to vertically direct the water from all areas of the tank back into the distribution system.

Conclusion
It is absolutely essential that all water storage tanks are operated in a way that ensures that the water is mixed during every fill and every draft cycle; and that the age of the water is continually minimized. This practice is necessary to ensure adequate levels of disinfectant and to assure continued peak water quality.

Larry Rice, BIF WATER.
Contact email: lrice@bifwater.com.
Bruce Morgan, Metcon Sales & Engineering.
Contact e-mail: brucem@metconeng.com.
Larry Rice Bruce Morgan

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