Water supply
From Wikipedia, the free encyclopedia
- "Waterworks" redirects here. For other uses, see Waterworks (disambiguation).
A water supply system provides water to the locations that need it. This term has several contexts:
- Municipal water supply network for household and commercial use;
- Fire fighting water supplies;
- Any water stored for later use.
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[edit] Global overview
Water supply is the process of providing drinking water to residential, public, commercial and industrial users, usually through piped systems.
In 2004 about 3.5 billion people worldwide (54% of the global population) had access to piped water supply through house connections. Another 1.3 billion (29%) had access to safe water supply through other means than house conncetions, including standpipes. Finally, more than 1 billion people (10%) did not have access to safe water, meaning that they have to revert to unprotected wells or springs, canals, lakes or rivers to fetch water.[1] An example of the challenges posed by inadequate access to water supply can be found in the entry on water and sanitation in Latin America.
[edit] Service quality
Many of the 3.5 billion people having access to piped water receive a poor or very poor quality of service, especially in developing countries. Water supply service quality has many dimensions: continuity; micro-biological water quality; physico-chemical water quality; pressure; and the degree of responsiveness of service providers to customer complaints.
[edit] Continuity of supply
Continuity of water supply is taken for granted in most developed countries, but is a severe problem in many developing countries, where sometimes water is only provided for a few hours every day or a few days a week. It is estimated that about half of the population of developing countries receives water on an intermittent basis.
[edit] Water quality
Drinking water quality has a micro-biological and a physico-chemical dimension. Important parameters of micro-biological water quality are the presence of fecal coliform bacteria and e.coli. There are thousands of parameters of physico-chemical water quality, of which salinity, turbidity, heavy metals and PH are only some.
[edit] Water pressure
Pressure in the water supply system is necessary to keep the water flowing through the system. Pressure in an urban water system is typically maintained either by a pressurized water tank serving an urban area, by pumping the water up into a tower and relying on gravity to maintain a constant pressure in the system or solely by pumps at the water treatment plant and repeater pumping stations.
Water pressure due to gravity is approximately 42 pounds per square inch (psi) (290 kPa) per 100 feet (30.5 m)of elevation. Thus, if the level of water in a water tower is 100 feet (30.5 m) above ground level, the pressure in the system would be 42 psi (290 kPa); a 150-foot (45.7 m) tower would produce pressure of 63 psi (434 kPa). A more intuitive way of expressing the hydrostatic pressure is in metres water column. Water pressures vary in different locations and water mains below the street may operate at higher pressures, with a pressure reducer located at each point where the water enters a building or a house.
[edit] Institutional responsibility
Water supply is a natural monopoly, which implies that it is most economically provided by a single provider.
A great variety of institutions are responsible for providing water supply services around the world. In most countries responsibility for urban water supply systems rests with municipalities or municipal utilities. In some countries smaller municipalities associate in regional inter-municipal utilities to benefit from economies of scale. In the United States these can take the form of special-purpose districts with independent taxing authority. In rural areas, especially in developing countries, community-based organizations often manage water supply systems. Some countries or states in federal countries, again especially in developed countries, have also established national water utilities. Such national utilities are especially prevalent in West Africa and Central America, but also exist, for example, in Tunisia und Uruguay.
Water supply providers can be either public, private, mixed or cooperative. Most water supply services around the world are provided by public providers. Private or mixed companies provide services under concessions, leases or management contracts with the public entity that is legally responsible for service provision.
[edit] Tariffs
Almost all service providers in the world charge tariffs to recover part of all their costs. These tariffs can take many different forms. Where meters are installed, tariffs are typically volumetric. In their absence, flat rates independent of consumption are being charged. In developing countries, industrial and commercial users are often charged higher tariffs than public or residential users. Also, metered users are charged higher tariffs for higher levels of consumption (increasing-block tariffs). In both cases, the rationale is to cross-subsidize residential users and low-volume users that are assumed to be poor. However, these cross-subsidies do not always reach their objective. Given the overall low level of water tariffs in developing countries even for higher levels of consumption, most consumption subsidies benefit the wealthier segments of society.
In developed countries, tariffs are usually the same for different categories of users and for different levels of consumption. Overall, water tariffs in developed countries come closer to covering the costs of supply than in developing countries.
In the United States, many water bills are divided into two parts: per month and per usage. A typical U.S. family of 4 in a suburban city will pay between $15 and $50 per month for water and sewer services.(source missing)
[edit] Metering
Metering of water supply is usually motivated by one or several of four objectives: First, it provides an incentive to conserve water which protects water resources (environmental objective). Second, it can postpone costly system expansion and saves energy and chemical costs (economic objective). Third, it allows a utility to better locate distribution losses (technical objective). Fourth, it allows to charge for water based on use, which is perceived by many as the fairest way to allocate the costs of water supply to users. Metering is considered good practice in water supply and is widespread in developed countries, except for the United Kingdom. In developed countries it is estimated that half of all urban water supply systems are metered and the tendency is increasing.
Water meters are a key element of metering. Water meters are read by one of several methods:
- the water customer writes down the meter reading and mails in a postcard with this info to the water department;
- the water customer logs in to the website of the water supply company, enters the address, meter ID and meter readings [2]
- a meter reader comes to the premise and enters the meter reading into a handheld computer;
- the meter reading is echoed on a display unit mounted to the outside of the premise, where a meter reader records them;
- a small radio is hooked up to the meter to automatically transmit readings to corresponding receivers in handheld computers, utility vehicles or distributed collectors
- a small computer is hooked up to the meter that can either dial out or receive automated phone calls that give the reading to a central computer system.
Most cities are increasingly installing Automatic Meter Reading (AMR) systems to prevent fraud, to lower ever-increasing labor and liability costs and to improve customer service and satisfaction.
[edit] Costs and Financing
The cost of supplying water consists to a very large extent of fixed costs (capital costs and personnel costs) and only to a small extent of variable costs that depend on the amount of water consumed (mainly energy and chemicals). The full cost of supplying water in urban areas in developed countries excluding sanitation is about US$1-2 per cubic meter depending on local conditions and is often about half as high in low-income countries. However, only part of these costs is usually billed to consumers, the remainder being financed through direct or indirect subsidies from local, regional or national governments.
Besides subsidies water supply investments are financed through internally generated revenues as well as through debt, which can take the form of credits from commercial Banks, credits from international financial institutions in the case of developing countries, and bonds in the case of some developed countries and some upper middle-income countries.
[edit] Technical aspects of supply systems
Water is vital to everyday life, and throughout history people have devised systems to make getting and using it more convenient. Early Rome had indoor plumbing, meaning a system of aqueducts and pipes that terminated in homes and at public wells and fountains for people to use.
Modern water supply systems get water from a variety of locations, including aquifers, lakes, rivers, wells, desalinated seawater, and other sources. The water is then purified.
The intake from these water sources usually is through a large cage-like box designed to screen out large particulate matter before it enters the system. After it is sucked in by a pumping station or allowed in by a gravity-feed system, it is usually filtered further, chlorinated, fluoridated, and then pumped either to holding locations like water towers or reservoirs, or fed directly into the user's spigot or tap.
Once water is used, it has to go somewhere. Typically wastewater is piped away in a sewer system and treated in a wastewater treatment plant before discharge, which is again almost always a service provided by the same authority as the water supply, since use of one system implies use of the other.
Sometimes, due to actual or suspected contamination by pathogens (often indicated by high counts of indicator organisms) a boil water advisory, known as as a Boiling water order in the UK, may be invoked.
[edit] Water and public health
An adequate supply of conveniently supplied water is needed for basic hygiene. For example trachoma, an infectious disease of the eye, which results in many cases of blindness in developing countries, is associated with both poor sanitation and failure to adequately process human excrement.
[edit] Examples of water supply systems in the United States
In the city of Highland Park, Illinois, the water source is Lake Michigan. A large pipe extends out into the lake to a large concrete and metal structure on the lake bottom which acts as the intake filter. This filter screens out only the largest of debris, including wood or large rocks. A set of large electric submersible pumps in a pumphouse building on the shoreline sucks the water into a system of sand and other filters to screen out large and small debris, including fish, leaves, or gravel. The water then is pumped through a set of pipes where it is chlorinated to kill bacteria and any small multicelled organisms that manage to make it through the filters. The water is then fluoridated to benefit public dental health. Finally, the water is tested by various sensors to determine if it is safe to drink, and pumped to higher locations. In this case, the higher locations are water towers situated around the town. These water towers provide water pressure to prevent leaks in the system from allowing untreated and unsanitary groundwater from entering the system. This also has the benefit of providing a constant water pressure to the town, achieved by the height. Water pipes lead to almost every house and business in town, as well as to fire hydrants. Water pipes enter each building and (once inside to prevent damage from freezing weather) there is a water meter to track water usage. From there, it is piped through the building to the sinks, water heater(s), toilets, showers, garden hoses, and other usage points. Water costs about $1 per 100 cubic feet (2.8 m³).
In the city of Lawrence, Kansas, the water source is the Kansas River. A small dam in town provides electricity and has a water intake for the water treatment plant. Like Highland Park, above, the water is filtered, chlorinated, fluoridated, pumped to water towers, and used.
In the city of Cottage Grove, Oregon, the water source is a large reservoir, a specially designed, built, and maintained lake, on a mountainside above the town which provides water pressure. The water is filtered, chlorinated, fluoridated, and delivered through town.
As late as the 1960's, the water supply pipes buried through town were made of wood, which is the source of the name "Trunk Main" used when describing larger bulk-water transfer mains, due to the large timber industry and ready availability of wood as a construction material. These were later replaced with concrete, steel, and PVC pipes. This work was delayed because it takes a lot of water to equal the cost of digging up a leaky pipe with expensive construction equipment and replace it with better pipes.
