How do water treatment plants work? is a question few would ask before turning on their taps. For almost everyone around the world, turning on the tap and getting fresh clean water is just a way of life.
While this might seem to be a simple fact of modern civilization, it’s a relatively new innovation in the timeline of human development.
Access to fresh water is one of the largest drivers of creating a modern society, but what allows everyone access to seemingly endless supplies of potable water? It depends on where you live, but chances are behind your tap there’s a municipal water treatment plant.
There are two main types of water treatment plants: drinking water and wastewater plants. We’ll be focusing in on drinking water plants here.
Both types of treatment plants serve the purpose of cleaning water, but in general, wastewater treatment plants will output water, or effluent, into streams or rivers and drinking water plants, or potable water treatment plants, will output their treated water into a city’s pipe distribution network.
The question still remains, how exactly does one treatment plant take dirty river or well water and turn it into water that is safe to drink? It involves a lot of processing using chemicals, filters, and removing all of the toxins and hazards from a given water source. The entire process starts with something called Coagulation and Flocculation.
Coagulation and Flocculation
All drinking water has to come from somewhere, usually being freshwater lakes, rivers, wells, or streams. In this water there is going to be some amount of sediment as well as organic materials in the water. These particles of sand, bacteria, dirt, wood, etc. all have to be removed before the water can be sent out to city residents. To do this, chemicals called coagulants are added to the water. These chemicals, such as aluminum sulfate or ferric chloride work, work to help the particles in the water congeal together. Essentially, these chemicals have the opposite charges of suspended solids, like clays or silts. When combined, the charges of the particles are neutralized allowing for them to stick together.
Coagulants are often added to the water right at the inlet of a drinking water plant, where the water then goes into mixing basins called flocculation basins. In these basins, the solution of water and coagulant will be slowly mixed together to form what are called floc particles. After mixing for a set amount of time determined by water quality in the flocculation basins, the water is moved to sedimentation basins. It’s in these holding tanks where the water sits for the floc particles to begin settling out at a faster and faster rate, which is the intended goal of this process.
By simply adding and mixing these chemicals into the water and letting that water sit, a large portion of the sediment will settle out to the bottom as sludge, which can be taken away to a landfill or holding facility. The floc particles are ultimately removed from the bottom of the basins and the clean water, being the cleanest at the top due to settling particles, flows over weirs to the next stage in the process.
In some plants, engineers used dissolved air flotation tanks in place of sedimentation basins. In these cases, air is pumped into the bottom of the tanks creating a cascade of tiny air bubbles. As these bubbles float to the surface, they take the floc particles with them, forming a film of floc on the surface of the tanks. Then, a sweeper arm pushes that floc into a collection basin where it is taken to a handling facility. In dissolved air flotation tanks, the clean water is taken from the bottom, where there is the least amount of floc.
When it comes to designing wastewater treatment plant, engineers will vary techniques based on the water source and how they want their final water to taste, feel, or otherwise be treated. The only regulation is the quality of water being pumped to the city, in most cases, it doesn’t matter how plants get the water to that point – which allows civil engineers a lot of creative freedom in the process.
After the majority of solids are removed from the water during coagulation and flocculation, the water flows over the weirs to the next step: Filtration.
The water at this point will start looking clear, but there will still be bacteria and very small suspended solids in the mix. The filtration process will seek to remove the remainder of the suspended solids and bacteria to further bring the water up to snuff.
Nearly every potable water treatment plant will use a sand filter for this process. Even in plants where engineers may not think it’s needed, often sand filters will be added simply for extra measures to ensure clean water.
Sand filters are exactly what they sound like, a basin of fine to coarse sand that filters water. It would theoretically be possible to treat drinking water with only sand filters, but that would result in frequently clogged sand filters resulting in lower efficiency and more frequent cleaning.
These sand filters can be set up in essentially 2 different ways: either the water flows from the bottom and exits the top, or the water flows in from the top and exits the bottom. Each layout presents it’s own set of unique problems, but the most common technique is bottom up flows, with the outflow being at the top.
This is done mainly for cleaning reasons, as clean water can be used to rapidly clean the tank of sediment and as the cleaning sand particles settle, they will do so in the correct order, largest on bottom and smallest on top. In basins that flow from top down, the largest particles are on top, meaning that engineers have to get creative in either altering the material type or cleaning in other ways.
Sand filters aren’t highly technical, water just passes through them and particles get caught as they run out of space to flow through. However, while not highly technical, sand filters are highly efficient.
Water flowing out of sand filters will need to have a clarity of around less than .3 Nephelometric Turbidity Units (NTU), or whatever the local code is for water clarity. This measurement is essentially just a way to determine the number of particles still remaining in the water, or turbidity.
Many plants will also pass water through activated carbon basins, which you can essentially think of as giant Brita filter tanks. Activated carbon particles have hundred of tiny pores on them that help remove the tiniest of sediment. This process isn’t wholly necessary, but it helps improve taste and odor, often a desired trait for residents.
At this point, the water should be mostly crystal clear but there still remains some residual bacteria. That leads us to the next step: Disinfection.
There are 3 main ways of disinfection in the water treatment process which can be used individually or in combination with one another. Chlorine/Chloramine treatment, Ozone treatment, or Ultraviolet Radiation treatment.
In the US, the main method of disinfection is by adding chloramines, or chlorine-based compounds. These compounds such as Chlorine Dioxide (ClO2)or Monochloramine (NH2Cl) work to kill microorganisms. One downside to this process though is that these chemicals can react with other organic material in the water to create Disinfection bi-products (DBPs). These DBPs are harmful to human health and have to be prevented or mitigated.
This extraneous reaction with other organic material is also why chlorine disinfection must come at the end of the treatment process. If added at the beginning, an abundance of DBPs would be created causes a plethora of issues.
Even with all this, chlorine is used mainly because of how it kills pathogens. It not only kills any pathogens in the water at the plant, but it residual chlorine levels also remain in the tap water killing any contaminants that might get introduced after leaving the plant. Most cities or states will have maximum or minimum chlorine levels in the treated water in order to keep both the water treated and city residents safe.
Ultraviolet Radiation Treatment
The other most common method of treatment is ultraviolet radiation. In this process, UV light is shined through the water which scrambles any bacteria’s DNA. This doesn’t kill the bacteria, but it does make it impossible for them to reproduce, thus making them harmless. The main downside to this disinfection process is that it isn’t residual, meaning any bacteria introduced to the water after this process won’t be treated.
Ozone treatment involves adding Ozone (O3) to the water. Called ozonation, the ozone kills the bacteria in the water as well as improving the taste and odor. In most cases, this treatment process also requires that a chemical be used to remove any extra ozone at the end, such as Sodium Bisulphate, as to not cause any harmful effects to a city’s population.
After the water is treated in any one of these three disinfection processes, it’s ready to be pumped into a city’s pipe network for delivery to your tap.
The water flowing out of the plant is tested for various levels of chemicals, particles, and other qualities all outlined in a plant’s permit. This permit regulates water quality and holds the treatment plant accountable in the treatment process. At this point all the water is fit for human consumption and the treatment plant is tasked with keeping it flowing through the pipes in your city.
Constant pressures of 40 psi must be kept at the outlets of the water in the system so that in areas of high elevation, the water retains residual pressure. 1 foot in water elevation is directly correlated to an increase or decrease of .47 psi, so as water is pumped throughout the city, it’s pressure at given points fluctuates.
Positive pressure always has to be achieved as to not let any external water in the ground flow in through pipe cracks or joints. This would contaminate the entire system and is one reason why you might experience a boil water notice.
Massive pumps at the treatment plant supply pressure to the system through constant operation. Some of this pressure can be stored as elevation in water towers, that can give pumps a backup in emergencies or fire flows. This combination of pumps and water towers allows your tap to remain flowing and otherwise allow you to live in the comfort of your home.
Civil engineers and treatment plant operators work around the clock to provide clean water through water treatment plants. At this point we’ve taken a look at the most common potable water treatment techniques, but there are tens to hundreds of deviations we haven’t discussed. This draws back to one of the coolest things about water treatment plant design, you’re able to use any number of creative solutions to clean the water and as long as it comes out clean, you’ve done your job well.
Don’t forget about the thankless job of being a water treatment plant operator, though. These men and women work around the clock to keep treatment plants running 24/7 so you can always have fresh water. A lot of time, money, and effort goes into making sure that you can have that nice glass of water on demand.