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Drinking water: From the lakes and rivers to your home

Welcome back to MJ Aquablog! We learned together a lot about the uniqueness of water and how is it with water on Earth in general. Now, it’s time to learn how we clean water.

People usually don't think much about where the tap water in their homes comes from, but most likely it came through a municipal water treatment plant. Plus, there are usually two types of water treatment plants; one for drinking water and one for wastewater. The plants treating wastewater do not send it back to your homes but this water ends up in streams or rivers. We’ll focus more on the plants that produce drinking water sending it through the city's pipe network distribution system to all their residents.

To put it simply, water from rivers and lakes goes through various processes involving chemicals and filters, so water can be removed from toxins and hazards and become potable again.

Simplified diagram of water treatment plant. (source: Wikipedia)

The process of cleaning water starts with the basic removing of the large stuff like rocks, pieces of wood or even human garbage as water enters the plant through a grate. Then comes the process of removing all the settleable and dissolved solids suspended in the water. Normally, that would be a long process so to speed it up, chemicals called coagulants like aluminium sulphate are added to the water. There are more chemicals that can be used as coagulants like polyamines or ferric sulphate, it varies country to country but even plant to plant.

Essentially this chemical has the opposite charge from the suspended solids, like clays or silts, which then neutralizes the charge and allows for the particles to stick together and precipitate out of the water. The subsequent mixture is slowly mixed in a flocculation basin in order to continue to form what are called floc particles. These floc particles then settle on the bottom in a sedimentation basin, and cleaner water flows overtop a weir.

This is only the first step, which mainly removes larger particles in the water. Some smaller particles may remain, as well as chemicals and, of course, bacteria. Following sedimentation, the next step is typically filtration through a sand filter. Seems too simple, right? Nevertheless, sand filters have been used since the very beginning of water treatment, and they are even required in most water treatment plants to assure a standard level of clarity.

In theory, coagulation and flocculation might be skipped and we could only use sand filters but that has a catch. The sand filter would need to be cleaned too often, reducing the efficiency of the treatment plant, which would end up not being really economic and that’s always a big issue in current world. Sand filters can be set up in two ways, either the water flows in from the bottom and exits the top, or the water flows in from the top and exits the bottom. The typical set-up is inflow at the base and outflow at the top because it is overall more efficient.

After passing through the sand filter, the water should have a level of clarity set by the law. So, the water is nice and clear, but we cannot forget about all the bacteria still floating inside.

Thus, the final step is the disinfection, which usually includes chemical compounds with a lot of chlorine like chloramines (organic compounds with -Cl and -NH2 groups) or even chlorine gas (Cl2) itself. The other options are using ozone (O3), metals like copper or silver, kalium permanganate (KMnO4), alcohols and many other chemicals. However, chlorine-based compounds are used most often.

When these chemicals are added, they kill microorganisms, but they also react with any organic material left in the water. The reason you would add chlorine at the last step is that its reaction with organic matter can create disinfection by-products, which can result in carcinogens or other harmful chemicals being present in the final water product. Chlorine is used mainly because of how it kills pathogens. Chlorine concentrations are actively present in the resulting drinking water, keeping pathogens from entering the water from pipes or other contamination sources. Most cities will have codes as to what the maximum and minimum chlorine levels must be at service points throughout a water network.

Sometimes, the chemical disinfection is combined with the physical one by using UV light which also effectively kills bacteria. UV light is simply shined through the water, which makes changes in the the bacteria's DNA. UV light doesn’t directly kill them, but it makes them sterile so they cannot reproduce and are harmless if ingested.

Now that the water has been filtered and disinfected, it is ready to be pumped into the distribution system. Constant pressures of 40 psi must be kept in the system to keep water from inflowing into the pipes at certain high elevation points. If water drops below certain pressures, it has to be flushed, at risk of contamination.

One of the coolest things about the water treatment process is the freedom it gives the civil engineer behind the process. As long as the end result is clean water, cities and governing authorities tend to not care about the processes you are using to treat the water. This article mainly focused on drinking water treatment, and while wastewater treatment is similar, it often involves more intensive processes and different additives.

Hopefully, by now, you have some understanding of how the water coming from your tap got there, and how it got clean. There is incredible amount of people and technologies behind this whole process so why not safe some water in your home. Next, we’ll discuss exactly that - what is the water usage in homes all around the world and give you some advice how to safe as much water as possible. See you then.

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