Water Supply 101 – Demand and the Water Tower

Water towr in Estherwood, Louisiana courtesy of Flickr user Bill Herndon

This post builds on the previous one about hydrostatic pressure.  If you haven’t read it yet, why not?

To me, water towers are an iconic symbol of small towns.  I’m not sure why I associate them with small towns, they’re common in larger cities also.  It must have something to do with all the small towns I visited in my younger years and my childhood fascination with big things.  My dad used to get annoyed if the shorties in the back seat asked too many questions so water towers were a good way to keep track of where we were.  They were always tarted up with the local high school mascot  and town.  Advertising uses aside, water towers have a major role in making sure everyone has enough to drink.

Of all the things that enable modern civilization, drinkable water is one of the most important.  Modern, reliable water delivery systems are what enable people to live close together and form towns and cities.  It’s something few people consider, but it’s a minor marvel that water can travel in underground pipes and somehow flow up into your house and out your faucets.  The answer, of course, is pressurization.

Every water utility of any size uses electric pumps to pressurize their distribution system, but supplying enough pressure can be tricky.  Every time someone turns on the water, that faucet acts like a leak just like letting the air out of a balloon a little at a time.  One or two leaks is no big deal, but when lots of people start using water all at once the balloon can’t blow up fast enough and people don’t get their water.  This mostly happens during certain times of the day when lots of people are using water at the same time.  This is a common problem first thing in the morning when people are showering before work, and in the early evening when everyone is fixing dinner and washing dishes, or for people who live near factories which use a lot of water.

Idealized with morning and evening peaks. Found here.

The dilemma is between the level of service to your customers and cost effectiveness.  If you buy your pumps for high demand you end up with a lot of expensive pumping capacity that sits idle 20 hours of the day.  If you pick your pumps for average demand then you save money, but it might be tough for people to get all the water they need at certain times of the day.  And we haven’t even considered the high pressure needed for fighting fires or the pumps dependency on electricity.  The system doesn’t need high volume often, but when it does need it, it REALLY needs it.

The answer to this dilemma….. the common, garden variety water tower.  Water towers are storage which sits idle until demand is higher than pump capacity.  The tower is connected to the water distribution system and gravity feeds water in as necessary.  It’s a very elegant solution as gravity pulls in as much or as little water as you need for free.  Typically the water level starts high and slowly lowers throughout the day.  Then a small dedicated pump fills the tower back up during the wee hours when most of the world is asleep and not using water.

The water tower adds a lot of flexibility to a water distribution system.  They are able to pick up the slack if one of your pumps needs to be turned off for maintenance, or if you lose electricity to a pump station.  And due to the way hydrostatic pressure works, you can create higher water pressure by building your tower higher without requiring more volume.  If you find a hill 50 ft higher than the surrounding countryside then build a tower on 50 ft legs and make it 30 ft deep you’re adding 50+50+30 = 130 ft of pressure to your system.

River to treatment to water tower to customer.

If you’re lucky, you can take advantage of existing features to build your water tower.  If you have a sufficiently hilly area you don’t even need the tower.  In 1889 Nashville built a large reservoir on a hill overlooking 8^th Avenue.  New York City has taken advantage of its many tall buildings by putting water tanks on top of many of them.  Most really tall buildings provide water and pressure to their higher floors by building a water tank (or tanks) into their floor plans near the top.  This way they can provide everyone in the building water without depriving neighboring buildings.

Water towers are key features to a smoothly functioning water system and provide extra water storage and pressure in most municipal systems.  They were an important part of making sure Nashville had plenty of water when the May 2009 flood knocked out one of the water treatment plants and cut the water entering the system in half.