Of all the resources on Earth, water is one of the most important to humans. Think about it, water is used for cleaning, cooking, transportation, cooling, and of course drinking, and that’s just to name a few. The major sources of water are rivers, creeks, and water from underground aquifers. However, it’s not normally a good idea to drink directly from those sources because it’s never pure water. Instead, river and creek water is a solution of many different types of materials. For many people how the water gets from rivers and streams to their homes is a mystery. All they see is the end result: clean water magically flowing from the faucets in their home. The process connecting the two can be complicated, but it really just comes down to filtering out all the harmful and unwanted things. With this filtering comes a lot of interesting Chemistry. The website Compound Interest publishes articles relating Chemistry to a broad range of topics. One such topic is the Chemistry of purifying water, discussed in the article, “The Chemistry Behind Your Home’s Water Supply”.
So how does dirty river water become clean tap water? Step 1 involves removing the biggest materials via large filters. Trash and other waste is removed, but organic compounds, dissolved chemicals, and small particles can get through. The first bit of Chemistry shows up in Step 2 when coagulates are added. These are added in an attempt to remove the organic compounds. Typical coagulants include aluminum sulfate and iron (II) oxide.With coagulants in the water, the charges on the ions of the organic compounds are neutralized so that they will begin to clump together because of the lack of repulsive forces. Once added, the mixture is called “sludge” and contains many metal hydroxides which can be used on farms in the fields. The clumps in the sludge can be removed by allowing them to settle at the bottom of the water tank. Step 3 uses classic filtering techniques using sand, gravel, and carbon (charcoal) to remove more smaller particles. The second bit of chemistry shows up in Step 4, which addresses the acidity of the water. Depending on what remains in the water after steps 1 – 3, the pH levels can be too acidic (low). Acidic water will break down pipes over time which can contaminate the water with unwanted compounds. Pushing water through calcium carbonate (limestone) raises the pH levels, thus making the water more neutral. Step 5 deals with other corrosion-causing substances in the water. For this, orthophosphates are added which creates insoluble phosphates when combined with materials like lead. After all of these steps, chlorine is added to kill off bacteria, and lastly fluorine can be added to help with dental strength.
With so many steps needed to purify the water, it is no wonder that the process is not common knowledge. However, there is controversy surrounding one step of the purification process: the optional fluoride step at the end. As discussed in the article, “Fluoride and Water Fluoridation-An Undeserved Reputation” because of rumors about fluoride amounts in water, many people are against adding it. Studies on fluoride levels in water convey ominous messages of lowered IQs and dental fluorosis ( enamel loss). The studies which publish negative findings, though, use huge amounts of fluoride in their tests, amounts much higher than those found in tap water. The World Health Organization recommends 1 ppm of fluoride in tap water, which is a very small amount. In general, the fluoride is not harmful unless in very large doses.
So why is fluoride useful? The short answer is that it helps keep teeth strong. Tooth enamel is made up of a compound called hydroxyapatite which contains calcium, phosphate, and hydroxide. It is strong, but over time acidic sources can cause the enamel to decay. Fluoride helps to prevent this decay by reacting with the hydroxides in the hydroxyapatite to create fluorapatite.The new compound more strongly resists decay from acids and helps to prevent cavities. Thus, putting small amounts in water is beneficial.
The two articles discussed in the previous paragraphs show how chemistry is used in a way the impacts everyone. Water is vital to every human on the planet, and what we treat it with is important. It also highlights public concern surrounding chemical use, which can be amplified to unreasonable levels (see my blog, Chemical Free?). With these two subjects in mind, a third connected subject also crossed my mind. Most of the Earth is covered by water, but a lot of it is salt water. Fresh water is preferred by humans, but converting saltwater to freshwater can be difficult because the salt is dissolved in the solution. Not only that, there are other impurities in the oceans which may not be accounted for in the river water cleaning process. Overall, if an easy and efficient way of converting large amounts of saltwater to freshwater could be found, then many global concerns could be relieved. Luckily, we have a lot of smart Chemists and scientists in the world to solve this problem. -WH