Water systems: the next installment. (Building Principles).
In the last column (September/October 2002 issue), we discussed water production and common ways that water systems supply pressure. In this column, we will clarify several basic issues concerning potable water quality including disinfection, taste and odor, and a few different configurations of distribution systems. It is important to understand that each system is unique--from the chemistry of the water to the topography of the land--and as such, each system has to be studied individually. In short, there are no one-size-fits-all solutions.
All of the supply components of the water supply system must be disinfected. This includes the well itself, the well pump, the storage tanks, and the inside of the distribution pipelines. Often this is done when a new component is first installed. It's important to note that many systems "winterize" by draining the system to prevent damage from freezing. Under no circumstances should the pipeline be left in a condition where debris or animals could enter the pipe. Bach time that a drained system is refilled, the inside of the pipe should be flushed clear and then decontaminated by introducing a concentrated chlorine solution to the pipe and letting it stand for 24 hours. Normally, the residual chlorine level (which is discussed later) and bacteriological counts are confirmed by a lab before the system is put into service and water used for drinking.
Most often, a chlorine compound is used to disinfect the water. It can be injected into the water as a liquid or a gas. In the proper concentration, disinfection should be complete after the water has been in contact with the chlorine about twenty minutes. A small amount of free chlorine should remain in the solution at the farthest reaches of the system to continue to combat most biological pathogens that could find their way into the water.
Note that the previous paragraph emphasizes most and does not claim to inactivate all potential pathogens. Recently, Giardia Lamblia and cryptosporidium have caused outbreaks of illness in public water supplies all across the nation, spurring "boil water" and "no use" orders by the municipal supplier. These situations have occurred because the treatment facilities through which the raw water flowed were not prepared to expose these organisms to lethal doses of chlorine. To inactivate Giardia cysts, the lethal chlorine concentration is more than 10 ml /liter or 10,000 parts per million (ppm) for more than thirty minutes. Contrast that with a typical health department swimming pool chlorine residual requirement range between 1.5 and 5 ppm. As you can see, chlorine is no "silver bullet." In the case of these two contaminants, preventing the supply from being contaminated is the best answer.
Ultraviolet fight and ozone
Although ultraviolet light and ozone can be used very effectively to disinfect water, these alone are not typically accepted methods of distribution system disinfection since once the water has passed the treatment units, there is no remaining disinfecting agent in the water. Any small pinholes or leaks in the system could allow pathogens into the pipeline where they could multiply with impunity
Sampling and testing
At a minimum, most camp and conference center water systems are typically required to perform basic disinfection procedures and collect regular samples. These are tested in a laboratory to confirm that the supply does not contain live organic pathogens and that the most remote taps contain sufficient residual disinfectant to be effective. Most often, the lab provides data on "total coliforms" and "fecal coliforms." In and of themselves, these organisms are typically not dangerous, but are used as indicators of the health of the water environment. If the disinfection system is working properly, most of the organisms, which can make consumers sick, should be neutralized.
It is important to note that the health department or state mandated tests usually DO NOT require routine operational checks for contaminants that could represent a health threat to consumers--such as lead or copper. These may be required tests if there have been past occurrences of sickness or other water standard violations. Keep in mind that groundwater conditions change with time and that as the supplier, the camp or conference center cannot rely on the minimum testing standards to support a legal defense if someone gets sick. In short, a full range of tests at least yearly will help ensure that the system is healthy.
Water Quality: Taste and Odor
Obviously, this is a very broad topic, and both the degree of problem and the degree of acceptable solution are pretty subjective. Water Treatment Plant Design (American Society of Civil Engineers & American Water Works Association, 1990) describes the situation well--"Because little is known about the exact chemical cause of a taste or odor problem, the treatment has historically been more of an art than a science. "The book goes on to describe two categories of treatment--chemical transformation and removal. This column focuses more on the transformation methods since these represent the most common small-scale treatment techniques. There are some conditions that may have simple solutions to help make the water more palatable. Please note that "simple" is not necessarily the same as "easy" or "cheap."
Many people incorrectly interchange "water softening" and "water treatment." Softening is actually a form of treatment. Water softening is a process by which dissolved minerals are removed from the solution. Common household softeners use beds of salt in a tank through which the water flows. As it does, the electrochemical properties of the salt attract certain minerals (most notably calcium carbonate molecules) using a process referred to as "ion exchange." The water that comes out the other side of the salt bed has a mineral content much lower than the water that went in. This makes the water feel less sticky and hence the term "softer." As more and more calcium carbonate builds up on the surface of the salt crystals. the exchange rate decreases, and the unit becomes less efficient. Modern household softeners have an automatic backwash or regeneration cycle to clean off the salt and allow it to resume its task. Along with the mineral content that is washed off, some of the salt goes, too, and so more must b e added occasionally.
Although some folks would claim that sulfur water is highly nutritive and beneficial (I really knew someone who liked his sulfur well!), most people find the smell and taste of sulfur generally unappealing. In most cases, the sulfur is being dissolved and carried by water moving through the ground on its way to the well. It can often be removed from solution by mixing it with lots of air under pressure. This procedure, known as "aeration," requires some specialized equipment including automatic compressors, storage tanks, and mixers. In addition, once the sulfur has been moved from the solution into a gaseous form, the stinky stuff from the water is now stinky stuff in the air. You've really only moved the problem.
Most often, a mossy taste or odor is the result of algae somewhere in the system. Both the algae itself and the chemicals used to control it can be the source for unpleasant tasting water. Algae requires sunlight, so most frequently, this is a problem when surface supplies are used for drinking. Where there is a mossy taste but the supply is a well, it's likely that the well has been contaminated by surface runoff containing algae or algae byproducts. As suggested earlier, this should be a loud signal to the operator that bacteriological testing should be done to insure that other contaminants like Giardia and crypto haven't entered with the algae.
Iron, manganese, and metallic taste
This condition will require a small amount of detective work to determine the source. First, it's critically important to determine whether the mineral taste is inherent to the groundwater or whether it's being generated in the distribution system. iron and its oxides are common in the earth's crust and are generally pretty soluble in water. If the groundwater you're pumping has iron compounds in it, they can be often removed with one of several chemical applications. Oddly enough, just chlorinating the water may remove enough iron from the solution to make the water much more palatable. Remember that what was dissolved, or in solution, now settles out of solution (precipitates). The iron particles will need to be filtered out--adding another step to the treatment process.
Recall from basic chemistry that pH is a measure of how many hydrogen ions are floating around looking to attract oppositely charged particles. Lacking other particles dissolved in the water, these renegade hydrogens may work to remove the interior coatings of the pipe itself Elements that can be freed from the pipe and its joints include iron, copper, and lead. All of these elements have been identified as potential health hazards, and the EPA has set maximum contaminant levels (MCLs) for potable water. If the source of the taste problem is corrosive water, manipulation of the pH is not a difficult task. But Rite the other treatment methods, it entails some specialized equipment and operator knowledge in order to achieve the desired results.
A stale taste of water is usually indicative of water that has been in storage too long either in a storage tank or in the distribution system. If the water has been in the system too long, it's likely that the chlorine residual is also no longer up to the required level. This problem is best corrected by modifying the distribution network, perhaps using one of the configurations that follow.
There are two methods of moving water from a source or storage spot to the points of use. Their names describe what they would look like from above--branched and looped. The chart above shows the exact same location of demands for both systems, and illustrates how each configuration could be used.
In a branched system, a single pipe feeds smaller pipes along the way much like a tree trunk feeds the branches. A simple looped system has an interconnected system of pipes that can feed water from at least two directions. Remember that water will move according to differences in pressure and doesn't "know" that there is a short route and a long route to the tap at location 1. When the user at 1 opens the faucet, water will flow in both directions (though more will flow toward 1 than 2).
In our practice, we see branched systems in 75 percent or more of the camp and conference center systems. This is because as new areas are developed, a water line is extended from the main to serve the new buildings or facilities. Although this is often the most expedient method of solving the problem, it has significant implications in terms of service, operation, and the quality of water delivered to the tap.
Pressure and volume
In a branched system, supply water has only one path to reach the user. This translates into a lower net service pressure when there are multiple demands along the trunk or main. In a looped system, water can flow through both branches to serve a single demand or multiple simultaneous users. Because of this, the diameter of the pipe in a looped system can usually be smaller than a branched main.
The concentration of chlorine fed into the system at the well is typically higher with a branched system than with a looped system. This is to compensate for the potential for long periods of non-use at the far end of the line, As seen previously, looped systems can rely on at least a small amount of flow through both branches.
If the water main in a branched system requires work, all of the users downstream from the point of repair will have to be shut off until the system is reconnected. If a looped system is equipped with strategically located valves, part of the system can be taken off line and serviced while other portions remain live.
There axe some economic issues that may seem to make the looped system impractical or too expensive. However, if allowed to solely govern the network selection, these issues could actually cost more in chemical costs, service, and quality issues over the long run. When it comes to your water supply system, your design professional should help you consider all of the alternatives and the potential impacts of each choice.
Rick Stryker is a professional engineer with Camp Facilities Consulting providing study, design, permitting, and construction consultation services to the camp and conference center community. Camp personnel may contact him at 570-296-2765 or by e-mail at email@example.com.
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|Date:||Nov 1, 2002|
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