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A water tap
Close-up of tap water
Tap water (running water) is part of indoor plumbing, which became available in the developed world in the late 19th century and common in the mid-20th century.

The provision of tap water is a massive infrastructure of piping, pumps, and water purification works. The direct cost of the tap water alone, however, is a small fraction of that of bottled water, which can cost from 240 to 10,000 times as much for the same amount.

The availability of clean tap water brings major public health benefits. Usually, the same administration that provides tap water is also responsible for the removal and treatment beforedischarge or reclamation of wastewater.

In many areas, chemicals containing fluoride are added to the tap water in an effort to improve public dental health. In some countries, this remains a controversial issue for a portion of the population. See water fluoridation controversy.

Tap water may contain various types of natural but relatively harmless contaminants such as scaling agents like calcium carbonate in hard water and metal ions such as magnesium and iron, and odoriferous gases such as hydrogen sulfide. Local geological conditions affecting groundwater are determining factors of the presence of these substances in water.

Occasionally, there are health concerns regarding the leakage of dangerous biological or chemical contaminating agents into local water supplies when people are advised by public health officials not to drink the water, and stick to bottled water instead. An example is the recent discovery of potentially hazardous nitrates in the public water supply in Kingmanmarker, Arizonamarker.

Tap water uses

According to a 1999 American Water Works Association study on residential end uses of water in the United States, Americans drink more than 1 glass of tap water per day (the daily human drinking water requirement being 2-3 U.S. quarts (1.9-2.8 litres). Daily indoor per capita water use in a typical single family home is , falling into the following categories: Of all water supplied to studied homes annually, for perspective, 42 percent was for indoor purposes and 59 percent for outdoor purposes.

Experimental attempts have been made to introduce non-potable greywater or rainwater for secondary uses such as toilets in order

Potable water supply

This supply may come from several possible sources.

Domestic water systems have been evolving since the first thinking man located his home near a running water supply, e.g. a stream or river. The water flow also allowed sending waste water away from his domicile.

Modern indoor plumbing delivers clean, safe, potable water to each service point in the distribution system. It is imperative that the clean water not be contaminated by the waste water (disposal) side of the process system. Historically, this contamination of drinking water has been the largest killer of humans.

Hot water supply

Domestic hot water is provided by means of water heater appliances, or through district heating. The hot water from these units is then piped to the various fixtures and appliances that require hot water, such as lavatories, sinks, bathtubs, showers, washing machines, and dishwashers..

Fixtures and appliances

Everything in a building that uses water falls under one of two categories; Fixture or Appliance. As the consumption points above perform their function, most produce waste/sewage components that will require removal by the waste/sewage side of the system.

Fixtures are devices that use water without an additional source of power.

Appliances are devices that use water coupled with an additional source of power. connection to these appliances incorporates a backflow prevention principle of some form -- the minimum is an air gap. See cross connection control & backflow prevention for an overview of backflow prevention methods and devices currently in use, both through the use of mechanical and physical principles.

Pipe materials

In old construction, lead plumbing was common. It was generally eclipsed toward the end of the 1800s by galvanized iron water pipes which were attached with threaded pipe fittings. Higher durability, and cost, systems were made with brass pipe and fittings. Copper with soldered fittings became popular around 1950, though it had been used as early as 1900. Plastic supply pipes have become increasingly common since about 1970, with a variety of materials and fittings employed, however plastic water pipes do not keep water as clean as copper and brass piping does. Copper pipe plumbing is bacteriostatic. This means that bacteria can't grow in the copper pipes. Plumbing codes define which materials may be used, and all materials must be proven by ASTM, UL, and/or NFPA testing.


Galvanized steel supply pipes are commonly found with interior diameters from 1/2" to 2", though most single family homes' systems won't require any supply pipes larger than 3/4". Pipes have National Pipe Thread (NPT) standard male threads, which connect with female threads on elbows, tees, couplers, valves, and other fittings. Galvanized steel (often known simply as "galv" or "iron" in the plumbing trade) is relatively expensive, difficult to work with due to weight and requirement of a pipe threader, and suffers from a tendency to obstruction due to mineral deposits forming on the inside of the pipe. It remains common for repair of existing "galv" systems and to satisfy building code non-combustibility requirements typically found in hotels, apartment buildings and other commercial applications. It is also extremely durable. Black lacquered steel pipe is the most widely used pipe material for fire sprinklers.


Tubing made of copper was introduced in about 1900, but didn't become popular until approximately 1950, depending on local building code adoption.

Copper Tubing Sizes (CTS) for Plumbing


Outside diameter (OD)


Inside diameter (ID)


Type K

Type L

Type M

3/8 1/2 0.402 0.430 0.450

1/2 5/8 0.528 0.545 0.569

5/8 3/4 0.652 0.668 0.690

3/4 7/8 0.745 0.785 0.811

1 1-1/8 0.995 1.025 1.055

1-¼ 1-3/8 1.245 1.265 1.291

1-½ 1-5/8 1.481 1.505 1.527

2 2-1/8 1.959 1.985 2.009

2-½ 2-5/8 2.435 2.465 2.495

3 3-1/8 2.907 2.945 2.981


Common wall-thicknesses of copper tubing are "Type K", "Type L" and "Type M":

  • Type K has the thickest wall section of the three types of pressure rated tubing and is commonly used for deep underground burial such as under sidewalks and streets, with a suitable corrosion protection coating or continuous polyethylene sleeve as required by code.

  • Type L has a thinner pipe wall section, and is used in residential and commercial water supply and pressure applications.

  • Type M has the thinnest wall section, and is generally suitable for condensate and other drains, but sometimes illegal for pressure applications, depending on local codes.

Types K and L are generally available in both hard drawn "sticks" and in rolls of soft annealed tubing, whereas type M is usually only available in hard drawn "sticks".

Thin-walled types used to be relatively inexpensive, but since 2002 copper prices have risen considerably due to rising global demand and a stagnant supply.

In the plumbing trade the size of copper tubing is measured by its nominal diameter (average inside diameter). Some trades, heating and cooling technicians for instance, use the outside diameter (OD) to designate copper tube sizes. The HVAC tradesman also use this different measurement to try and not confuse water pipe with copper pipe used for the HVAC trade, as pipe used in the Air-conditioning trade uses copper pipe that is made at the factory without processing oils that would be incapatable with the oils used to lubricate the compressors in the AC system. The OD of copper tube is always 1/8th inch larger than its nominal size. Therefore, 1" nominal copper tube and 1-1/8th" inch ACR tube are exactly the same tube with different size designations. The wall thickness of the tube, as mentioned above, never affects the sizing of the tube. Type K 1/2" nominal tube, is the same size as Type L 1/2" nominal tube (5/8" ACR).

Lead leaching

Generally, copper tubes are soldered directly into copper or brass fittings, although compression, crimp, or flare fittings are also used. Formerly, concerns with copper supply tubes included the lead used in the solder at joints (50% tin and 50% lead). Some studies have shown significant "leaching" of the lead into the potable water stream, particularly after long periods of low usage, followed by peak demand periods. In hard water applications, shortly after installation, the interior of the pipes will be coated with the deposited minerals that had been dissolved in the water, and therefore the vast majority of exposed lead is prevented from entering the potable water. Building codes now require lead-free solder. Building Codes throughout the U.S. require the use of virtually "lead-free" (<.2% lead)="" solder="" or="" filler metals in plumbing fittings and appliances as well.


Copper water tubes are susceptible to cold water pitting, bad plumbing ground pinholes, and erosion corrosion.

Bad Plumbing Ground Pinholes

Pinhole leaks occur anytime copper piping is improperly grounded. Typically not found in new homes, pinholing due to bad grounding occurs in homes where the original plumbing has been modified. Homeowners may find a new water filtration device has interrupted the ground when they start seeing water leaks after a recent install. It occurs very rapidly, usually being seen about six months after the ground interruption. Correctly installed appliances will have a copper jumper cable connecting the interrupted pipe sections. Non-copper (i.e. Pex) installs, do not have this problem.

The effect is known as galvanic erosion or electrolytic pinholing. It occurs because the water is forced to act as an electrical conduit across the jumpered section, resulting in ionization of materials in the water. When the water conducts the electrical potential back to the copper on the other side of the gap, the ionized minerals bind with the copper creating copper salts. Eventually pin hole leaks form, and where there is one, there are usually more. If you call a plumber out for two pin hole leaks, be sure to examine the grounding. It is very aggressive.

Detecting and eliminating bad grounding is relatively straightforward. Detection is accomplished by use of a simple voltmeter set to DC with the leads placed in various places in the plumbing. Typically, a probe on a hot pipe and a probe on a cold pipe will tell you if there is improper grounding. Anything beyond a few millivolts is important, potentials of 200 mV are common. A bad ground will show up best in the area of the gap, as potential disperses as the water runs. Since the bad ground is usually seen near the water source, as filtration and treatment equipment are added, pinhole leaks can occur anywhere downstream. It is usually the cold water pipe, as this is the one that gets the treatment devices.

Fixing the problem is a simple matter of either purchasing a copper jump kit, composed of a stranded copper cable at least 5mm in diameter and two clamps for affixing it the plumbing. Simpler fixes are possible by taking a length of electrical wire, stripping it on both ends, and affixing it to both sides of the gap. Thin wire, such as household electrical wire, will cure the ground problem, but if there is a surge from a lightning strike, it may break the thin wire, which is why store bought kits are thicker wire -- to survive electrical surges.

A similar jumper wire can also be seen crossing gas meters, but for a different reason.

Note, if homeowners are experiencing shocks or sparks from plumbing fixtures or pipes, it is more than a bad ground, it is likely an electrical wire bridging to the plumbing, but the result is the same, galvanic corrosion.

Pinhole leaks from galvanic corrosion can result in 1000's of dollars in plumbing bills, and sometimes necessitating the replacement of the entire affected line.


Plastic piping placed for a sink
Plastic pipe is in wide use for domestic water supply and drainage, waste, and vent (DWV) pipe. For example, polyvinyl chloride (PVC), chlorinated polyvinyl chloride (CPVC), polypropylene (PP), polybutlyene (PB), and polyethylene (PE) may be allowed by code for certain uses. Some examples of plastics in water supply systems are:

  • PVC/CPVC - rigid plastic pipes similar to PVC drain pipes but with thicker walls to deal with municipal water pressure, introduced around 1970. PVC should be used for cold water only, or venting. CPVC can be used for hot and cold potable water supply. Connections are made with primers and solvent cements as required by code.

  • PBT - flexible (usually gray or black) plastic pipe which is attached to barbed fittings and secured in place with a copper crimp ring. The primary manufacturer of PBT tubing and fittings was driven into bankruptcy by a class-action lawsuit over failures of this system. However, PB and PBT tubing has returned to the market and codes, typically first for 'exposed locations' such as risers.

  • PEX - cross linked polyethylene system with mechanically joined fittings employing barbs and crimped steel or copper fittings.

  • Polytanks - plastic polyethylene cisterns, underground water tanks, above ground water tanks, are made of linear polyethylene suitable as a potable water storage tank, provided in white, black or green, approved by NSF and made of FDA approved materials.

  • Aqua - known as PEX-Al-PEX, for its PEX/aluminum sandwich - aluminum pipe sandwiched between layers of PEX and connected with brass compression fittings. In 2005, a large number of their fittings were recalled.

Fittings and valves

Potable water supply systems require not only pipe, but also many fittings and valves which add considerably to their functionality as well as cost. The Piping and plumbing fittings and Valves articles discuss them further.

Regulation and compliance

Before a water supply system is constructed or modified, the designer and contractor need to consult the local plumbing code and obtain a building permits prior to construction. Even replacing an existing water heater may require a permit and inspection of the work. NSF 61 is the U.S. national standard for potable water piping guidelines. National and local fire codes should be integrated in the design phase of the water system too to prevent "failure comply with regulations" notices. Some areas of the United States require on-site water reserves of potable and fire water by law.

Waste water

The waste water from the various appliances, fixtures, and taps is transferred to the waste and sewage removal system via the sewage drain system. This system consists of larger diameter piping, water traps, and is well vented to prevent toxic gases from entering the living space. The plumbing drains and vents article discusses the topic further, and introduces sewage treatment.

Tap water vs bottled water

Both bottle and tap water contain minerals and nutrients that can improve health, as well as contaminants that can cause diseases. The quality and content of water varies greatly by location and bottle brand. Although the biggest benefit tap water has over bottle water is the addition of fluoride. Many water plants will add fluoride to their water through water fluoridation, compared to bottled water companies of which very few add fluorine. This nutrient addition helps prevent tooth decay and cavities.. A benefit bottled water has over tap, is the lower levels of copper and lead contamination from rusting plumbing lines, even though most contamination is within a safe level.

In 2007, it was found that some bottled water companies were selling water that was contaminated and less healthy for consumers than tap water. The Natural Resources Defense Council (NRDC) conducted a four year study on bottled water. The results of this study show that one-third of the bottled water tested contained levels of contamination which exceeds allowable limits under either state or bottled water industry standards or guidelines. In a study with 57 bottled water samples and tap water samples, all of the tap water samples had a bacterial content under 3 CFUs/mL and the bottled water samples' bacterial content ranged from 0.01-4900 CFUs/mL(colony-forming unit). Most of the water bottle samples were under 1 CFU/mL, though there were 15 water bottle samples containing 6-4900 CFUs/mL.

Many large corporations and some water companies and wholesalers, especially in the California Bay Area are now making a large effort to promote tap water over bottled water. Some of the Bay Area cities that promote tap over bottled water include San Francisco, Emeryville, Santa Clara, and Oakland. The Santa Clara Valley Water District in Santa Clara County launched its tap versus bottled water campaign, with the slogan, “Tap Water, the Clear Choice”, in 2007.

James Workman, author of the book "Heart of Dryness: How the Last Bushmen Can Help Us Endure the Coming Age of Permanent Drought" and co-founder of SmartMarkets says that he doesn't believe that "tap water is bad and bottled water is good." Rather he cites differences in quality regulations and standards."Bottled water is often tap water put through another filter and not held to the same quality regulations as public utility water is."

During the 2007 U.S Conference of Mayors, the mayors of San Francisco, Salt Lake City and Minneapolis signed a pledge to promote tap water over bottled water as part of the “Think Outside the Bottle” campaign.

For tap water in the U.S. chlorine is added as a disinfectant, which can create leave other products in the water like trihalomethanes and haloacetic acids. The level of chlorine found is small, 1L of chlorinated water gives 0.2mg of chlorine, which is too small to cause any health problems.

While most U.S. cities have what is considered safe tap water, contaminants ranging from bacteria to heavy metals are present in some tap water and violations of tap water standards have been well-publicized, such as the severe 1993 Cryptosporidium outbreak in Milwaukee, Wisconsinmarker, which led to several deaths and around 400,000 illnesses (see: Milwaukee Cryptosporidium outbreak). The University of Cincinnati recently completed a "Tap Water Quality Analysis" (funded by PUR) for major US cities.

There has been research showing pregnant women who drank tap water consistently during their first trimester, had higher rates of spontaneous abortion, compared to mothers who solely drank bottled water.

Dissolved Gases

Tap water can sometimes appear cloudy, and this is often mistaken for a mineral impurity in the water. Cloudy water, also known as white water, is actually caused by air bubbles coming out of solution in the water. Because cold water holds more air than warm water, small bubbles will appear in water with a high dissolved oxygen content that is heated or depressurized, because this reduces how much dissolved gas the water can hold. This condition is completely harmless, and the cloudiness of the water disappears quickly as the gas is released from the water.

See also


  1. The Real Cost of Bottled Water
  2. Denied:1up! Software
  3. Plumbing: the Arteries of Civilization, Modern Marvels video series, The History Channel, AAE-42223, A&E Television, 1996
  4. Copper Tube Handbook, the Copper Development Association, New York, USA, 2006
  5. Uniform Plumbing Code, IAPMO
  6. International Plumbing Code, ICC
  7. Pacific Institute - Bottled Water
  8. Tap v Bottled Site
  10. Massachusetts Water Resource Authority.
  • ASTM B75-02 Specification for Seamless Copper Tube
  • ASTM B42-02e1 Standard Specification for Seamless Copper Pipe, Standard Sizes
  • ASTM B88-03 Standard Specification for Seamless Copper Water Tube
AWWA Research Foundation, Residential End Uses of Water, ISBN 1-58321-016-4, 1999

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