You probably don't think much about where the water in your tap comes from, but odds are that it has come through a municipal water treatment plant. There are two main types of treatment plants: drinking water and wastewater. Both serve the purpose of cleaning the water, but in general, the output of wastewater plants are streams or rivers, and the output of drinking water plants are your city's pipe network distribution system. So, how exactly does a treatment plant take dirty river water and turn it into clean water? Well, through processes involving chemicals and filters, water can be removed of most toxins and hazards and become potable again.
All drinking water will start off at the water source, which is generally a freshwater lake, river, well, or sometimes even a stream. The first step of treatment is to remove the settleable and dissolved solids suspended in the water. In order to speed the settling and removal process up, chemicals called coagulants are added to the water. The most common coagulant is aluminum sulfate, but this varies by water treatment 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. Now that the solids in the water can begin sticking together, the mixture is slowly mixed in a flocculation basin in order to continue to form what are called floc particles. These floc particles then settle out of the mixture in a sedimentation basin and cleaner water flows overtop a weir.
This process is only the first step, and it has mainly removed larger particles in the water, but some smaller particles may still remain, as well as chemicals and bacteria. Following sedimentation, the next step is typically filtration through a sand filter. Sand filters have been used since the beginning of water treatment, and they are required most everywhere to be included in the treatment process to assure a standard level of clarity. A sand filter is essentially exactly what it sounds like, a basin of fine to course sand that filters water. It would be possible to completely remove all solids from water using only sand filters, skipping over coagulation and flocculation. However, this would mean the sand filter would need to be cleaned more often, reducing efficiency of the treatment plant. 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. Each presents their unique problems, but the typical set-up is inflow at the base, and outflow at the top for reasons in regards to cleaning efficiency.
After passing through the sand filter, the water should have a clarity (turbidity) of around less than .3 Nephelometric Turbidity Units (NTU), or whatever the local code is for water clarity. The water is clear, but bacteria are still present.
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The final step in the process is disinfection. There are two main ways to disinfect water, each with their pros and cons. In the US, the main method is by adding chloramines, or chlorine based compounds. 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 because its reaction with organic matter can create disinfection byproducts 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.
Aside from chlorine, the other most common method is ultraviolet radiation, however, Ozone can also be used. UV light is shined through the water, which scrambles the bacteria's DNA. This does not kill them, but it makes it impossible for them to reproduce, rendering them harmless if ingested. The only downside to this method is it is a one-time treatment, so if bacteria enter the water system after the treatment plant, there is no way to mitigate that risk.
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. This is one of the reasons why you may see fire hydrants randomly running, or you get a boil water notice, but more on that in a later article.
[Image Source: Wikimedia]
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. Stay tuned to Interesting Engineering to learn more about sewage and wastewater treatment. Hopefully by now, you have some understanding of how the water coming from your tap got there, and how it got clean. Don't forget about the thankless water treatment plant operators that keep treatment plants running 24/7 so you can always have fresh water. A lot of work goes into making sure you can have that nice cold glass of water.