Clean water is one of humanity’s most fundamental needs, and those of us who live in urban areas usually get our water from some kind of centralized public system. Operating a water system is a major responsibility that has implications for public health and safety. In dense urban areas, a clean and abundant supply of water is an absolute necessity, not just for drinking, but also for sanitation and firefighting. And it’s not just something we need every so often; water is a constant necessity both day and night, on weekends, holidays, and any time in between. So, the job of finding enough water, making it safe to use, and then reliably distributing it to the system customers with almost no downtime is a monumental task that requires a lot of infrastructure. And probably the most visible component of a public water system is the elevated storage tank, also known as a water tower. I’m Grady and this is Public Works, my video series on infrastructure and the humanmade world around us.
Let’s say you’re the owner of a public water system. You’ve found a source of water of sufficient quantity for your customers, you’ve found a way to clean that water so it’s safe for them to use, and now it’s time to send the water on its way. There are a few ways we can get water from one place to another. One of them is just to carry it there. Whether it’s on the back of an animal, on a truck, or a bottle in your backpack, we still physically carry water all the time. But it’s usually not the most efficient way. The first infrastructure dedicated to water conveyance was the open channel. Whether in a ditch, canal, or aqueduct, the water is carried by gravity, sometimes over very long distances. We still use open channels to carry water for irrigation and drainage, but they have some disadvantages as well. The water is exposed to pollution and contamination, channels bisect the land, making it difficult to get across, and the water can only flow to areas of lower elevation than where it started. And that last one is a big disadvantage, especially if you’re trying to deliver water to an area with hills or mountains. So most public water systems today rely on pipes for distribution.
Simply putting a top on an open channel allows us to take advantage of pressure to move fluids where we want them to go. Just like electrons in a wire flow from high to low voltage, a fluid in a pipe will flow from high to lower pressure. So, if you raise the pressure at one end of a pipe, you can send your clean water to anywhere you want it to go. And how do you raise the pressure of water? With a pump. A pump is a device that moves fluids. In some cases a pump literally lifts the fluid to a higher elevation, but in most cases a pump imparts energy to a fluid by raising its pressure. And pumps, especially the size of pumps that serve entire cities, are expensive. So if you’re tasked with choosing the size of the pump you need for your public water system, what do you do? Maybe you measure the amount of water that the city uses in a given day and select a pump that can match that flow rate. Let’s see how that would work.
It’s midnight in your city and most of your water customers are asleep. Besides the industrial customers that run 24/7, water demands are minimal, and your pump is having no trouble meeting them. But around 5 am, automated sprinkler systems start kicking in. Around 6 am, people start waking up, taking showers, brushing their teeth, cooking breakfast, all things that require water. It doesn’t take long before the water demand exceeds the capacity of your pump. Almost right off the bat, your new pump can’t meet your system demand, because it was only sized for the average. Water demand in large urban areas can vary significantly over the course of a normal day, with the peak hourly demand (usually in the morning or evening) sometimes being as much as 5 times the average daily demand. So, if you are trying to meet your customer’s water needs using just pumps, instead of just one, you might need as many as five pumps (or one huge pump that can do the work of 5). And not only that, you’ll have to be constantly cycling the pumps on and off to meet the variable demand, increasing the wear and tear on your equipment. And here is where storage comes in. Let’s add a water tower to the system and try this experiment again.
It’s midnight and demand is low, but your pump is running full wide open. Instead of water flowing customers, it’s flowing into your water tower, filling the tank slowly but surely. As morning comes and demand starts to increase, your pump continues running. It’s not able to meet the demand on its own, but the stored water in the tank is making up the difference. All your customers are getting the water they need. As people start their day, demand again drops below average. But, the pump keeps running and the extra flow goes into the tank. Demand again begins to spike as the residents of the city start cooking dinner, taking baths, and watering the plants. All this extra water use drains the tank again before most people go to bed and the cycle starts again.
It’s pretty easy to see how storage makes your water system more efficient. It smooths out the peaks and valleys of water demand not just on your pumps but all your upstream infrastructure, including your water treatment plant and raw water supply. Without storage, all those facilities would need to be sized for peak demand, increasing their cost. With enough storage, pumps and other infrastructure can be sized for average demands, saving not only cost, but also complexity, because you don’t have to predict changes in demand and respond accordingly. Sometimes those peaks and valleys are predictable, but sometimes they’re not. Some of the biggest water demands in urban areas are from fires. Without a firefighting force and enough water to supply them, fires can burn out of control in dense urban areas. In fact, many of the deadliest disasters in history were fires in cities before modern water systems. Now most municipalities and building codes have minimum requirements for the amount of water that must be available to firefighters. And having water stored and ready, like in a water tower, goes a long way to being able to respond to an emergency.
You may thinking, c’mon Grady. This is nothing new. Storage is the age-old solution to any situation where the supply doesn’t match the demand. And, yeah, it might not be anything remarkable to store water in a big tank. But water towers aren’t just big tanks, they’re big tanks elevated above the ground. And that’s because water towers aren’t just storing water; they’re also storing energy. Water distribution systems rely on pressure to get the water where it’s going. If you’ve ever taken a shower with low water pressure, you know how frustrating it can be, because you just can’t get enough water out of tap. Pressurizing a water system is also important for public health. Without enough pressure in the pipes, contaminants could make their way into the system through taps or small leaks. Most water systems get their pressure from pumps, and it takes a lot of energy to maintain this pressure. So, having the ability to store not only the water itself, but also the energy that has been imparted to it by the pumps is important. In some areas, where electricity costs vary based on demand, you can run the pumps at night when electricity is cheap to fill up your water tower. Then, leave the pumps off during the day when electricity is more expensive, allowing just the tower to pressurize the system and serve your customers. Storing energy this way is also carried out at a larger scale to help with electrical grid reliability, but that’s a topic for another video. Elevated storage is also beneficial during a power outage, by keeping the system pressurized even when pumps are out of service.
But how elevated do they need to be? You might know that the pressure within a body of water is related to the depth. The deeper you go, the greater the pressure. Just like in a pool or the ocean, a water distribution system has the same relationship between depth and pressure. It just happens to be confined within a series of pipes. So, you can imagine a water distribution system as a virtual ocean under which we all live, and the water surface in elevated storage tanks represents the surface of the virtual ocean. Imagining a water system this way makes it easy to see the challenge of delivering water to customers at the right pressure. If our cities were flat, this would be pretty simple. All the buildings would sit at the same depth in the virtual ocean. But most areas have at least some amount of topographic relief. Customers at low elevations are at the bottom of the virtual ocean, where pressures can be too high. You might think this is a good thing, but plumbing pipes and appliances are only rated to certain pressures, so exceeding those ratings can cause serious damage. Sometimes buildings at low elevations will be equipped with special valves to reduce the pressure. Customers at high elevations will be near the surface of the virtual ocean, having very low water pressure. As I mentioned, this can be not only frustrating, but also lead to contamination of the system. To solve this challenge, many large cities maintain separate distribution systems called pressure zones, each with their own water tower, to serve customers at different elevations within the city.
But, what happens if you need to serve customers at different elevations in the same location? Tall buildings, like skyscrapers, can have adequate water pressure on the lower floors, while the higher floors can go up near the surface or even above the virtual ocean in the water distribution system. So, instead of relying on city water pressure, most tall buildings use their own pumps to provide water to the upper floors. And some cities, like New York, even require that each building have its own elevated storage tank.
Not every city uses water towers. Some have their entire water supply at a higher elevation, minimizing the need to add pressure to the system. And, sometimes it just makes more sense to rely on pumps alone to keep the system up and running. After all, water towers aren’t cheap, they take up quite a bit of space, and they can allow water to stagnate if it isn’t circulated enough. But, with public water supplies, reliability is key. And, it’s been a long time since gravity was knocked offline from a thunderstorm, so elevated storage tanks (in some form or fashion) are definitely here to stay. Thank you for watching, and let me know what you think!