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  • 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.

  • [This video is sponsored by Bombas.

  • More on that later.]

  • 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