Domestic and urban use of water is a substantial fraction of total water use. Use by large cities, particularly mega cities in arid regions such as Beijing or Los Angeles, impacts surrounding territory profoundly.
The urban water cycleEdit
Nearly all water comes from a very limited number of sources for the urban environment. We refer to them as ground water and surface water. Ground water as you might have expected is water that is removed from the ground usually by well but sometimes by artesian pressure. The second source, surface water can be from either bodies of water such as natural and man made lakes or from free flowing bodies of water such as streams and rivers. Also seas and oceans depending on where you are located can be sources of water.
In nearly all cases the first step of getting water to your urban environment would be some sort of pump system. Usually these pumps will have a very rudimentary screening system to remove branches and logs. Of course this is dependent on the source for some are pristine and others are very filthy.
Nearly all water at least in the industrialized nations is treated to varying degrees. I am going to concentrate on common treatment practices in the Pacific Northwest of North America. After the water has entered the pumping system it will usually depending on the turbidity and density of such pass through a slower channel or basin where the bulk of the turbidity (dirt) will simply drop out from gravity. After this usually a chemical of some sort is added to help with coagulation such as alum or ferric chloride. This is added in a step called rapid mix where the water is usually pushed past a rapidly spinning propeller which while the chemical is added to get thorough mixing with all the water. After the rapid mix you would arrive at a flocculation basin where the chemically treated water is once again slowed down and mixed slowly with large paddles or other mixing devices to give the turbidity time to bind with the chemical addition. This mechanical system is commonly named a septic tank or sewage treatment plant
After the flocculation stage typically the water will move into a very large chamber called a settling basin that has a large amount of holding time. Here the water moves very slowly allowing the now large particles of turbidity that have formed floc to settle out to the bottom where they are removed with some sort of sludge removal system. From the flocculation basin and sometimes simply at the end of the basin would be a area called the clarifier where the very surface water is allowed to flow into weirs and then to the actual filtration stage.
The most common type of filtration would be sand and among the different types of sand would be rapid sand. This is a mixed bed of first larger media gravel up to fist size and then on to smaller and smaller media till typically on top you have silica sand or some form of coal. These different media are all stratified by density. Hence the larger particles will have a higher specific gravity then the next higher layer keeping all the particles of similar size together.
The water from the clarifier will flow on top of these filter beds and rapidly flow through them leaving the majority of their particulate mater behind. The clean water that hopefully nearly meets your local authorities regulations for potable then goes to a clearwell where usually a final pH adjustment is made for pipe corrosion control and also a chemical is added to disinfect such as CL2 gas although the industry seems to be moving to techniques such as ultra violet radiation and away from chlorine to help reduce disinfection byproducts.
This water now free of particulates and viable organisms is now pumped under pressure usually 90psi depending on elevations and condition of the system from the treatment facilities and into our urban environment. Trickling down through ever smaller pipes (36 inch pressure mains are common in my experience) till they finally reach the pipes of your home so when you turn the tap at the sink you can have cool refreshing water for your drinking enjoyment. Or at least it will be safe to digest.
From the take the water will take a turn down into the drain entering the collection system for the used water to be disposed of either to a treatment facility or if your in a more backwoods location simply to your local creek or body or water.
(this is where I hope to pick up next time I try to share my knowledge with WaterWiki as we head back to our source water through a treatment process.)
- ↑ "It takes a village – and much more – to quench megacity water demands" MSU Center for Systems Integration and Sustainability (CSIS) December 3, 2015
- "Safe Drinking Water Is Essential" "(www.drinking-water.org). A tool to provide international decision makers with peer-reviewed scientific and technical information about the options available to enhance the safety and availability of drinking water supplies around the world." In English, French, Spanish, Chinese, and Arabic.
- "Septic tank Information" - Experience and renowned resources from Cotterill Civils..
- "Sewage Treatment Plant Information" - Experience and renowned resources from Cotterill Civils..