We take for granted the water that comes out of the taps in our home when we turn them on – but a lot of work goes into getting it there. Chemistry, too, has a hand in making sure that the water is safe to drink. Here, we take a look at the water treatment process, and in particular the chemicals used to get clean drinking water to your tap.
The water that ends up in our homes can begin in a number of places. Much of it is groundwater – water beneath the Earth’s surface, trapped between the pores and cracks in rocks. This water can actually be relatively pure, due to natural filtration through chalk and similar rock types, and as such can sometimes skip through some of the treatment steps we’ll detail here, as it contains little by way of debris and organic material.
Water from reservoirs or rivers, on the other hand, will need these to be removed before further chemical treatment can begin. The very first step in this case is removing the larger debris from the water. This is a simple mechanical process, by which the water is passed through a grille which traps this debris. Removing it first prevents it from causing blockages in the treatment works later in the process.
Screening may remove larger bits of waste from the water, but it doesn’t help remove smaller pieces, or dissolved substances in the water. This is where chemistry makes its first appearance. In order to remove organic matter, chemicals referred to as coagulants are added – commonly aluminium sulfate or iron (II) chloride. Coagulants work because they help neutralise the negative charge on the small particles of organic matter in the water, stopping them from repelling each other and allowing them to clump together. Flocculation is the name given to the mixing process that increases the size of the clumped particles, forming what’s known as a floc.
The sludge formed by the coagulation and flocculation processes is allowed to settle in the subsequent stage, so that it can be removed from the water, treated, and disposed of. This sludge is partly in the form of metal hydroxides, formed during the coagulation process, as well as the organic materials removed. Some of its potential uses include being added to fields on which crops will be grown. Failing this, it must be sent to landfill or incinerated to dispose of it.
After the sludge has been removed from the water, it moves to the filtration stage. Here, it is passed through beds of material to help remove any organic matter and particles not previously removed by coagulation. The material used is commonly a bed of sand on top of a bed of gravel, though carbon in the form of charcoal can also be used, helping to remove compounds that can make water taste and smell bad.
Once the water has been filtered, its acidity can sometimes be adjusted. This is because very acidic water can lead to pipe corrosion, which lead to discolouration of the water. It can also result in toxic metals such as lead getting into the supply. To reduce acidity, the water is passed through filters containing crushed limestone, which is primarily composed of calcium carbonate. The calcium carbonate helps to raise the pH of the water, making it less acidic. Conversely, acids can also be added if the pH of the water is too high (too alkaline), though this is less common. Water softening can also be carried out at this stage in some treatment plants (for more on water hardness, see this post on limescale).
In some treatment plants, it’s also necessary to add anti-corrosion agents at this point, to prevent metals from pipes making it into the water. Chemicals called orthophosphates, such as phosphoric acid, are added for this purpose. These orthophosphates prevent lead in particular from making it into the water supply, by forming insoluble lead phosphate complexes on the inner surface go the pipes. This is exactly the measure that wasn’t taken in Flint, Michigan, one of the factors leading to the well-publicised high levels of lead in the city’s water.
Though at this point almost all of the solids and dissolved substances in the water have been removed, pathogens such as bacteria and viruses can still be present. As such, the water must be disinfected before it travels through the network of water pipes to peoples’ homes. This is often accomplished by adding small amounts of chlorine to the water. Chlorine is a strong oxidising agent, disintegrating cell walls and inactivating enzymes and proteins of pathogens.
Chlorination of water isn’t entirely problem-free, however. The chlorine can react with residual levels of organic compounds remaining in the water to form compounds called trihalomethanes (THMs). These compounds are potentially carcinogenic; there is inadequate evidence of their increasing the risk of cancer in humans, but they have been shown to be carcinogenic in animals. Despite this, the benefits of water chlorination, preventing the spread of waterborne diseases such as cholera and typhoid, greatly outweigh the possible minute increase in cancer risk due to THMs.
THM levels in water are closely monitored and are not allowed to exceed permitted levels. In addition, since it is the presence of organic compounds in the water that facilitates the formation of THMs, it is for this reason that chlorination usually takes place after organic matter has been mostly removed by filtration. Some facilities are bypassing this problem by using ozone to disinfect water instead of chlorine; however, the advantage of chlorine is that a residual amount of it remains in the water after leaving the treatment facility, helping to keep water pathogen-free until it arrives in your home.
Finally in some areas, where fluoride concentration in water is naturally low, fluoride can also be added before the water leaves the treatment facility. The purpose of this is to try and help prevent tooth decay. There’s more on the science behind water fluoridation in this previous post. After all of this, the water is finally free to leave and make its way to you – something to think about the next time you turn on the tap!
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References & Further Reading