How a Water Treatment Plant Works?
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.
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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 the 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 coarse 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 the 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.
The final step in the process is disinfection. There are two main ways to disinfect water, each with its 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 that 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.