Water Treatment
Water Purification by Municipalities
In most areas, water that comes from your faucet is cleaned by the city from a water source near your home. Reservoirs are built by damming up rivers and their natural flow. This water is then cleaned by the city or municipality and sent to your home. The diagram below shows a typical cleaning process for a city. Each individual step is then described below.
In most areas, water that comes from your faucet is cleaned by the city from a water source near your home. Reservoirs are built by damming up rivers and their natural flow. This water is then cleaned by the city or municipality and sent to your home. The diagram below shows a typical cleaning process for a city. Each individual step is then described below.
Screening: When water is taken from the reservoir, the water is first put through a screen. This screen is similar to the screens on the windows of a house, just made of tougher metal. The object of this step is to clean out any big objects from the reservoir, like rocks, twigs, fish, plant material, etc. When this is completed, the water is still heavy with dirt and microorganisms that travel through the screen.
Pre-Chlorination: This step adds chlorine to the water to kill any living organisms in the water. Large amounts of chlorine are used, but most of this chlorine is removed by further processes.
Flocculation: The water is now treated with lime. This lime, a derivative of limestone rock, attaches itself to organic material and dirt. The lime is then heavy enough to sink to the bottom of the water.
Settling: The water with the lime is put into a settling tank where the lime material sinks to the bottom. The cleaner water is then pumped off the top of the tank, once settling is complete.
Sand Filtration: The water is then run through a high density sand column. The sand in this column is packed so close, the idea is that water only is small enough to get through. This cleans out any lime or other solid material that doesn’t settle in the settling tank. The flocculation and sand filtration removes most of the chlorine added in the pre-chlorination step.
Post Chlorination: In this step, the now clean water has a little bit of chlorine added to it. This chlorine is designed to kill any microorganisms that may get in the water any point down the line (in the water tower or the pipes up to your house). This small amount is the chlorine that can be tasted in tap water.
Optional Treatments:
Aeration - running the water over a water fall or through a fountain to increase the air in the water to make it taste better.
pH Adjustment - if the pH is too far from 7 (neutral) after this process, the appropriate amounts of neutralization material is added to bring the pH to 7.
Fluoridation - Sodium fluoride is added to the water. This material is added solely to improve the health of the consumers teeth.
Activated Charcoal - running the water through activated charcoal can remove all organic matter, living or dead, from the water.
Ozone - alternative to post-chlorination. This will kill the bacteria and not retain any chlorine taste.
Pre-Chlorination: This step adds chlorine to the water to kill any living organisms in the water. Large amounts of chlorine are used, but most of this chlorine is removed by further processes.
Flocculation: The water is now treated with lime. This lime, a derivative of limestone rock, attaches itself to organic material and dirt. The lime is then heavy enough to sink to the bottom of the water.
Settling: The water with the lime is put into a settling tank where the lime material sinks to the bottom. The cleaner water is then pumped off the top of the tank, once settling is complete.
Sand Filtration: The water is then run through a high density sand column. The sand in this column is packed so close, the idea is that water only is small enough to get through. This cleans out any lime or other solid material that doesn’t settle in the settling tank. The flocculation and sand filtration removes most of the chlorine added in the pre-chlorination step.
Post Chlorination: In this step, the now clean water has a little bit of chlorine added to it. This chlorine is designed to kill any microorganisms that may get in the water any point down the line (in the water tower or the pipes up to your house). This small amount is the chlorine that can be tasted in tap water.
Optional Treatments:
Aeration - running the water over a water fall or through a fountain to increase the air in the water to make it taste better.
pH Adjustment - if the pH is too far from 7 (neutral) after this process, the appropriate amounts of neutralization material is added to bring the pH to 7.
Fluoridation - Sodium fluoride is added to the water. This material is added solely to improve the health of the consumers teeth.
Activated Charcoal - running the water through activated charcoal can remove all organic matter, living or dead, from the water.
Ozone - alternative to post-chlorination. This will kill the bacteria and not retain any chlorine taste.
Dangers of Chlorination
Trihalomethanes (THMs) can be made by mixing organic material and chlorine. When chlorine is mixed with dead organic material, as in the situation where the chlorine kills microorganisms in the pre-chlorination step of water treatment, a new compound called THM’s can be formed. The simplest trihalomethane is CH3CCl3. THMs are carcinogenic, or cancer causing. The following solutions have been proposed to attempt to prevent THM formation in city water.
Solutions:
Activated Charcoal - removes the organic material, but is very expensive. Only done when absolutely necessary.
Ozone alternative - Ozone can be used in place of chlorine in both pre- and post-chlorination to kill microorganisms. However, the ozone does not stay in the water once the water leaves the treatment plant, which can allow for bacteria growth in the water tower or faulty pipes near your home.
Eliminate Pre-Chlorination - Instead of adding chlorine twice, just add chlorine at post-chlorination, thereby reducing the amount of THM’s that can be made. The disadvantage is that the post-chlorination may not be large enough to kill all the microorganisms.
The key problem here is balancing the benefits of disease prevention by using chlorine as a microorganism killer to the side effects of THMs made by using chlorine in the presence of organic material.
Activated Charcoal - removes the organic material, but is very expensive. Only done when absolutely necessary.
Ozone alternative - Ozone can be used in place of chlorine in both pre- and post-chlorination to kill microorganisms. However, the ozone does not stay in the water once the water leaves the treatment plant, which can allow for bacteria growth in the water tower or faulty pipes near your home.
Eliminate Pre-Chlorination - Instead of adding chlorine twice, just add chlorine at post-chlorination, thereby reducing the amount of THM’s that can be made. The disadvantage is that the post-chlorination may not be large enough to kill all the microorganisms.
The key problem here is balancing the benefits of disease prevention by using chlorine as a microorganism killer to the side effects of THMs made by using chlorine in the presence of organic material.
Hard Water
Hard water is water containing an excess of calcium (Ca+2), magnesium (Mg+2) and iron (Fe+3) ions. Many parts of the country have hard water, as shown by the following diagram:
These ions, when present in water, will combine with soap to form insoluble compounds. These insoluble compounds do not rinse away easily, and can leave soap deposits on dishes and on your skin. This is not really harmful, but there are incoveniences: Hard water ions will cause fading of colors on clothing, will cause a white film to appear on dishes and can dry out your skin. The hard water can also cause calcium carbonate deposits to form in pipes and in water heaters, which reduces efficiency and flow through the pipes.
To remedy the inconveniences of hard water, many households where hard water is present will have a water softener, which transforms hard water into soft water.
To remedy the inconveniences of hard water, many households where hard water is present will have a water softener, which transforms hard water into soft water.
Operation of a Water Softener
The water enters your house, and the pipe goes directly to the water softener. As the hard water flows into the water softener, the hard water comes in contact with an ion exchange resin. The resin absorbs the hard water ions of calcium, magnesium and iron. As the hard water ions are absorbed, ions of sodium are released into the water. This soft water ions does not affect the cleaning abilities of soap, and is harmless. As the water runs, the ions are replaced throughout the day. The soft water goes up through the center of the softener and then into your house faucets.
At night, when water is not being used, the valves on the left in the figure above close, closing off the water coming into the house. Salt water is then pumped into the water softener. The salt water flows into the ion exchange resin, and the sodium in the salt replaces the hard water ions that have been absorbed into the resin. The resin becomes saturated with sodium ions, and the hard water ions are rinsed out of the water softener to the waste. This process regenerates the ion exchange resin. The recharged resin can the soften more water on the next use. |