WATER SUPPLY BASICS As stated above, the overall configuration of a water supply system is relatively simple and has only a few major components. The first component is the source of the water feeding the supply system. Since a typical American uses about 80-100 gallons of water per day, the source itself must have a yield rate that matches this demand for a community of a particular size. A city of 1 million people would use about 90 million gallons of water per day, equivalent to 62,500 gallons per minute (GPM) or over 1,000 gallons per second (GPS).
For large communities, extensive and even multiple water sources are often required. A well field consisting of an appropriate number of water wells and associated pumps extracting water from a groundwater aquifer is one option. A second option is a surface water source. These include a natural freshwater pond or lake, a man-made reservoir created by a dam, or a free-flowing river. In all cases, the water source has to be able to supply water at the needed rate and have a recharge rate with precipitation and snowmelt sufficient to replenish the water and restore its pre-use volume and surface elevation. Each of the two primary types of water supply—groundwater and surface water—has its advantages and disadvantages. Surface water has a potentially greater flow rate, while groundwater is less prone to water loss via evaporation.
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Once extracted, the water must be delivered to the community via the second component of a water supply service, the conveyance system. These are a series of very large pipelines and associated structures of various designs and sizes referred to as aqueducts. Aqueducts are designed, constructed, and sized to convey the necessary amount of water from the source to the water treatment plant and beyond. Whether they are ancient Roman aqueducts and Persian qanats, pipelines feeding melt to the farmers of California’s Imperial Valley, or the huge underground pipes feeding water to New York City, aqueducts have always been the prerequisite of any civilization.
Conveyance systems begin as large individual pipes and branch out into a series of diverging and looping pipelines. The large pipes are the water mains, designed to carry large amounts of water at high flow rates over long distances. This is the type of pipe used to carry water from the source to the initial treatment facility. From the treatment plant, the conveyance system’s function switches from transport to distribution. Beginning with mains sized from 6 inches to 8, 10, 12, and 16 inches in diameter, the system branches out further to supply pipelines servicing individual homes and businesses. While transmission mains are often made of large diameter reinforced concrete pipes or even concrete-lined tunnels, distribution pipelines are made from a variety of materials such as polyvinyl chloride (PVC), high-density polyethylene (HDPE), cast iron, ductile iron, copper, steel, asbestos cement, and reinforced concrete.
Individual service pipes differ further. Made of lead or type K copper, these pipes have smaller sizes, varying from 0.5 inches to 6 inches in diameter depending on the required flow rate. As it arrives at its end-use point, the water flow passes through a water meter which records its flow rates for billing and record-keeping purposes.
Their structural design varies as much as their material characteristics. Pipe wall thickness along the materials’ modulus of elasticity determines a pipe’s resistance strength against applied loads and internal pressures. External applied loads are a result of overburden pressures (determined by both depth of burial and type of bedding used to reinforce the pipe). Each type of pipe material has a different thickness rating—schedule 40 or 80 for PVC, standard dimensional ration (SDR) for HDPE, iron pipe size (IPS) for iron pipe, etc. Choice of pipe material comes with tradeoffs. Traditional cast iron pipe is cheap but brittle and prone to breakage. Ductile iron pipe is more durable but also more expensive.
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Internal applied pressures are limited by the need to protect supply plumbing used by homes and businesses. This requirement effectively limits the range of internal operating pressure to 50 to 100 psi. The minimum pressure required for discharge points such as faucets is 14 psi for open-air gap discharges or 20 psi for discharge points submerged under an existing tank or body of water.
The third component is the water treatment system mentioned above. This point defines the first stage of the water conveyance system. These are facilities that utilize mechanical and hydraulic processes to remove impurities from the water supply. These impurities include organic debris, silt and sediment, dissolved metals, bacteria and other pathogens, and so on. These methods include chlorination, aeration, coagulation and flocculation, adsorption, filtration with pros media (sand) or membranes, and UV disinfection. The end result is water fit for human consumption.
A fourth component is distributed throughout the conveyance system at optimum points like nodes on a network. These are the short-term storage facilities (towers, tanks, cisterns, secondary reservoirs, lined ponds, etc.) that provide surge capacity to control variations in water demand throughout the day. The surge capacity provided by water storage tanks is necessary since water usage rates never exactly match water supply rates. Tanks are required to safely store water during off-peak hours for later use during periods of peak demand.
LAWS, REGULATIONS, LOCAL CODES, AND BACKFLOW PREVENTION The need to maintain safe water supplies and prevent contaminant backflow is outlined in the Code of Federal Regulation, including that buildings and facilities “provide that there is not backflow from, or cross-connection between, piping systems that discharge waste water or sewage and piping systems that carry water for food or food manufacturing.” (Source: “Food and Drugs,” Food and Drug Administration, Code of Federal Regulations, title 21, sec. 110.37, www.fda.gov).
The overall protection of our clean water supply is entrusted to the United States Environmental Protection Agency (USEPA). This authority is given to it by the Safe Drinking Water Act (SDWA) which authorizes the USEPA to set national health-based standards for public drinking water. The USEPA sets the technical standards and regulatory requirements for water supply systems, which are further enforced by state and local governments. At the local level, there are usually requirements for annual testing and certification of backflow prevention devices attached to local and individual water supply pipelines. As with most environmental regulations, the Federal government can grant enforcement primacy to local and state governments, provided that they adopt and enforce drinking water standards that are at least as stringent as the basic Federal regulations. Federal law allows state and local governments to go even further, letting them establish and adopt additional standards so long as they do not conflict with Federal regulations.
THE PHYSICS AND THE MECHANICS OF SIPHONAGE AND BACKFLOW Siphons seem to defy gravity. Science has several explanations for this behavior. A siphon’s configuration consists of a reservoir of liquid exposed to ambient air pressure, a tube-shaped in a hump with a high point, which discharges into another container at a lower elevation that is also exposed to air pressure. The height differential results in the tube being shorter at the intake end before the high point in the hump and longer at the discharge end after the high point.
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For the most part, siphons are driven by the force of air pressure. The liquid is initially sucked into the tube, completely filling it over its whole length. The liquid in the longer discharge end weighs more and drains out of the tube. As it does so, it creates a zone of low pressure at the top of the hump. When it does, the decreased pressure causes the air pressure acting on the surface of the water in the storage reservoir to push the liquid up into the tube towards the hump’s low-pressure zone. In doing so the water flows over the hump and continues down the longer end of the tube to the discharge point.
In addition to air pressure, the cohesion of the liquid adds to its movement through the siphon tube. Tests of siphons in a vacuum (the absence of any air pressure) still work. As gravity pulls the liquid down from the longer discharge end, liquid cohesion allows it to pull additional liquid from behind it, creating a chain of discharging liquid.
Backflow is the result of either back pressure or siphonage. Of the two, siphonage is more common, since back pressure requires that the system pressure increases so as to overcome the supply pressure, and there are relatively few mechanisms that can cause this to happen. Back pressure is analogous to blowing liquid into a straw. Something similar happens in back siphonage, only instead of an increase in system pressure, there is a decrease in supply pressure. This can occur when a supply is interrupted, drained down, etc. Siphonage is analogous to sucking liquid out of a straw.
The mechanics of backflow require a more detailed explanation. The physical movement of backflow occurs when liquids within the pipeline distribution network flow backward against the system’s designed flow direction. Physical failure (a breakage or crack in the pipeline) can result in loss of upstream pressure. These breaks can be caused by shifting foundations, vibration from vehicle traffic or equipment operations, valves and fixtures that break or wear out over time, or a simple mechanical failure of a key pump providing needed pressure head.
The reverse flow can create a suction that effectively pulls contaminant-containing water back into the pipeline itself. The resulting impact on water supply is called “indirect cross-contamination.” Back pressure, on the other hand, results in contaminants being forced into the pipeline by exterior pressure. This mechanism is referred to as “direct cross-contamination.” A true siphon can occur in water supply systems when an open faucet or other discharge point that is left open and submerged in a body of water, such as a sink or storage tank, gets subjected to a sudden loss of pressure in the supply line.
Once inside the water supply system, another mechanism occurs at the molecular level to ensure that the contaminants get widely spread. This is diffusion through water either spreading while it is not moving or being carried by flows when the system is delivering water. Even small quantities of pollutants can have a serious impact on both the real quality of the water supply and the public’s perception of its cleanliness. With strict federal and state regulatory standards, even a small amount of contamination can render a water supply undrinkable.
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