How Swimming Pools Work

­Conceptually, swimming pools are pretty simple -- they're just big basins of water. But on a hot summer day, a swimming pool can seem like the greatest invention known to man. And as it turns out, there really is a lot of cool technology at work in your average pool -- much more than you might expect.

In this article, we'll find out how pools are built, and we'll take a look at the plumbing system that keeps the water clean and chemically balanced.

Pool Basics

Swimming pools come in all shapes and sizes, but nearly all of them, from the backyard personal pool to the water park wave pool, work in the same basic way. They use a combination of filtration and chemical treatment to continually clean a large volume of water.

An apartment complex pool, mid-construction: It looks like a big hole in the ground, but it's really much more.
An apartment complex pool, mid-construction: It looks like a big hole in the ground, but it's really much more.

A typical swimming pool needs seven major components:

  • A basin
  • A motorized pump
  • A water filter
  • A chemical feeder
  • Drains
  • Returns
  • PVC plastic plumbing connecting all of these elements

The basic idea is to pump water in a continual cycle, from the pool through the filtering and chemical treatment systems and back to the pool again. In this way, the pumping system keeps the water in the pool relatively free of dirt, debris and bacteria. Some pools also include heaters in the mix, in order to keep the water at a certain temperature.

In the next section, we'll look at the different types of pools.


Types of Pools

The main difference between different types of pools is how the basin is constructed. There are several different pool styles, each with its own advantages and disadvantages.

  • Above-ground pools are the cheapest construction option, as well as the easiest to build. Most above-ground pools are made from prefabricated kits, which even an amateur can put together (though most people go with professional installers). First, the installers level off the ground to form a flat building surface. Then they assemble a perimeter track, which supports the outer wall (made of metal, plastic or wood). Next, they spread sand in the pool area and lay the plumbing. Finally, they secure the vinyl liner over the pool walls, fill the pool with water, smooth the liner and fasten it into place. As soon as they hook up the pump and filtering system, the pool is ready to go. The main disadvantage of this sort of pool is that it's less durable than other designs, and generally less attractive. It's also less permanent, which can be a good thing -- it's relatively easy to disassemble the pool and move it to a new location.
  • Fiberglass pools are made from fiberglass-reinforced plastic, which has been molded into a basin shape. To install the pool, a construction crew digs an appropriately sized hole, lays the necessary plumbing, adds some sand filler and lowers the preformed pool structure into the hole. Then they level the pool, hook up all the plumbing and backfill in the area around the pool. Usually, the pool is surrounded by a concrete deck structure.
  • Vinyl-lined in-ground pools are a lot like above-ground pools, structurally, but they look more like conventional in-ground designs. The construction crew digs a hole and assembles a metal, plastic or wood frame wall around the hole's perimeter. As in an above-ground pool, the crew lays sand along the bottom of the hole and secures the vinyl lining to the structural wall. These pools are a lot cheaper than other in-ground designs, but not as durable. Typically, the liner needs to be replaced every 10 years or so.
  • Gunite pools are the most popular design in much of the United States. To build one of these pools, the construction crew digs a hole, puts the plumbing in place and assembles a framework grid with 3/8-inch steel reinforcing rods (rebar). The rebar rods are spaced about 10 inches apart, and secured together with wire. When the grid is in place, the crew sprays a heavy coating of gunite, a mixture of cement and sand, around the rebar. The sprayer unit combines dry gunite mix with water just before spraying -- this produces the wet concrete material. The crew trowels the gunite smooth and lets it sit for a week or so before applying a smooth finish to the rough surface. The most popular finish is called plaster (actually a mixture of cement and marble sand), but a lot of people finish their pools with special concrete paint. Gunite pools can also have tile, exposed aggregate or even fiberglass finishes. Gunite pools (and their cousins, shotcrete pools) are highly durable, and they can be built in any shape or size.
  • Poured-concrete pools are similar to gunite pools, but they're a lot harder to build. Instead of spraying concrete material around a rebar framework, concrete is actually poured into conventional wooden forms. With the rise of gunite methods, poured-concrete pool construction has mostly fallen by the wayside. In masonry block pools, the walls are constructed with concrete blocks.

While these pool designs are quite different, they all rely on the same basic plumbing and filtering systems. In the next few sections, we'll see how these components actually keep the pool going.

Pool Drain Systems

We've already seen that the water in a swimming pool needs to circulate through a filtering system, to remove dirt and debris. During normal operation, water flows to the filtering system through two or more main drains at the bottom of the pool and multiple skimmer drains around the top of the pool.

The main drains are usually located on the lowest point in the pool, so the entire pool surface slants toward them. Most of the dirt and debris that sinks exits the pool through these drains. To keep people from getting their hair or limbs caught in the plumbing, the drains are almost always covered with grates or antivortex covers (a cover that diverts the flow of water to prevent a dangerous vortex from forming).

The return port A typical vacuum port A pool vacuum cleaner
The return port A typical vacuum port A pool vacuum cleaner
The return port A typical vacuum port A pool vacuum cleaner
The return port A typical vacuum port A pool vacuum cleaner
The return port A typical vacuum port A pool vacuum cleaner
The return port A typical vacuum port A pool vacuum cleaner
The return port A typical vacuum port A pool vacuum cleaner
The return port A typical vacuum port A pool vacuum cleaner

The skimmers draw water the same way as the main drains, but they suck only from the very top of the pool (the top eighth of an inch, typically). Any debris that floats -- leaves, suntan oil, hair -- leaves the pool through these drains. The diagram below shows a common system.

In the system described here, the floating weir, the door at the inlet passageway, swings in and out to let a very small volume of water in at a time. To catch debris effectively, the goal is to skim just the surface level. The water flows through the strainer basket, which catches any larger debris, such as twigs and leaves. In addition to the main inlet, the skimmer system has a secondary equalizer line leading to a drain below the surface level. This line keeps the skimmer from drawing air into the pump system if the water level drops below the level of the main inlet.

The water is pumped through the filtering system and back out to returns, inlet valves around the side of the pool. This system involves a lot of suction, but if the pool is built and operated correctly, there is virtually no risk of suction holding somebody against one of the drains. The only way the plumbing system could apply this sort of suction is if there were only one open drain. In a safe pool, there are always multiple main drains as well as several skimmer drains, so if somebody or something blocks one drain, the pumping system will pull water from one of the other drains. This eliminates the suction on the blocked drain.

Most swimming pools also have a couple of vacuum ports, which are only used in pool cleaning. These ports attach to pool vacuum cleaners, which work something like ordinary vacuum cleaners except that they suck water instead of air. The vacuum ports may have their own pumping system, but in most pools they are driven by the main pump.

After making its way into the various drains, the water flows on to the filtering stage. In the next section, we'll find out what the pumping and filtering systems are all about.

Pool Pump Systems

Pipes from the main drain, skimmer and vacuum ports lead to the pumping system.
Pipes from the main drain, skimmer and vacuum ports lead to the pumping system.

To most of us, a pool is, more or less, a big hole in the ground. We don't see most of the expensive machinery in a pool system, because it's usually tucked away in nearby pump room. But this is the stuff that really makes the pool work.

The heart of the pool system is the water pump. In a typical pump system, an electric motor spins an impeller inside the pump housing. The impeller drives the water from the various drains through the filter and back out to the water inlets.

Just before it flows into the pump, the water passes through a metal strainer basket that catches leaves and other large debris that might clog up the pump.

The strainer basket sits right in front of the pump.
The strainer basket sits right in front of the pump.
The strainer basket, removed for cleaning
The strainer basket, removed for cleaning

Next, the water flows into the filter (or, in this setup, one of two filters). In the next section, you'll find out what happens at this point in the system.

Pool Filters

Dual sand filters Specially made filter sand
Dual sand filters Specially made filter sand
Dual sand filters Specially made filter sand
Dual sand filters Specially made filter sand

The filters in this system are high-rate sand filters. Sand filters consist of a large tank, made of fiberglass, concrete or metal, containing a thick bed of special-grade sand, which has a squarish shape.

During filtering operation, dirty water from the pool comes in through the filter's inlet pipe, which leads to the water distribution head inside the tank. While gravity pulls the water down through the sand, the tiny sand particles catch any dirt and debris. At the bottom of the tank, the filtered water flows through the pick-up unit and out the outlet pipe.

Over time, the collected dirt and debris in the sand slows down the water flow. Pressure gauges at the filter inlet and outlet give the pool custodian an idea of the blockage level inside. If gauges show much greater pressure on the inlet pipe than the outlet pipe, the custodian knows there's a lot of collected debris in the sand. This means it's time to backwash the filter. To backwash, the custodian adjusts a number of valves to redirect the water flow. He or she closes the return pipe leading to the pool and opens the drainage pipe, which lead to the sewer system. He or she adjusts a valve at the filter to connect the pipe from the pump to the outlet pipe and connect the drainage pipe to the inlet pipe. With this arrangement, water from the pump pushes up through the sand, dislodging the dirt and debris. At the top of the filter tank, the dirty water flows out through the inlet pipe and into the sewer.

To redirect the water flow for backwash, the custodian turns large handles to adjust plumbing valves.
To redirect the water flow for backwash, the custodian turns large handles to adjust plumbing valves.

In place of a sand filter, some pool systems use a diatomaceous earth filter or a cartridge filter. In a diatomaceous earth filter, water from the pool passes through filter grids coated with diatomaceous earth, a fine powder made from the chemically inert, fossilized remains of sea organisms called diatoms. In a cartridge filter, dirty water passes through a filter made out of polyester cloth or corrugated paper. Instead of backwashing, you simply remove the filter and hose it off. After a few years (or as many as eight years), it's time to discard the old filter and put in a new one.

In most regions, the law dictates that all the water in the pool (or more accurately, the equivalent volume) must pass through the filter in a certain amount of time -- typically between 30 minutes and six hours. For the apartment-complex pool pictured above, that means pumping 167,000 gallons (630,000 liters) of water through the filtering system every six hours!

The pump and filter system is also connected to a well or municipal water line so fresh water can be added to the pool. This is necessary to replace water lost to evaporation, backwashing and "splash-out" (water that splashes on the deck or is carried out on people's bodies and swim suits). When it's pretty hot out and there's heavy swimmer activity, this 167,000-gallon pool could lose 300 gallons (1,100 liters) or more in one day.

Next, we'll look at the chemicals at work in a typical swimming pool.

Pool Chemicals

An automated chlorine feeder hooked into the pump and filter system
An automated chlorine feeder hooked into the pump and filter system

A pool's filter system does the heavy lifting in keeping the water clean, but it takes chemistry to do the fine-tuning. It's important to carefully manipulate the chemical balance in pools for several reasons:

  • Dangerous pathogens, such as bacteria, thrive in water. A pool filled with untreated water would be a perfect place for disease-carrying microorganisms to move from one person to another.
  • Water with the wrong chemical balance can damage the various parts of the pool.
  • Improperly balanced water can irritate the skin and eyes.
  • Improperly balanced water can get very cloudy.

To take care of pathogens in the water, you have to introduce a disinfecting agent that will get rid of them. The most popular pool disinfectant is the element chlorine, in the form of a chemical compound such as calcium hypochlorite (a solid) or sodium hypochlorite (a liquid). When the compound is added to the water, the chlorine reacts with the water to form various chemicals, most notably hypochlorous acid. Hypochlorous acid kills bacteria and other pathogens by attacking the lipids in the cell walls and destroying the enzymes and structures inside the cell through an oxidation reaction. Alternative sanitizers, such as bromide, do basically the same thing with slightly different results.

Chlorine is typically prepared in liquid, powder or tablet form (though some professionals use gaseous chlorine), and it can be added to the water anywhere in the cycle. Pool experts generally recommend adding it just after the filtering process, using a chemical feeder. If it's added directly into the pool, using tablets in the skimmer boxes, for example, the chlorine tends to be too concentrated in those areas.

One problem with hypochlorous acid is that it's not particularly stable. It can degrade when exposed to ultraviolet light from the sun, and it may combine with other chemicals to form new compounds. Pool chlorinators often include a stabilizing agent, such as cyanuric acid, that reacts with the chlorine to form a more stable compound that does not degrade as easily when exposed to ultraviolet light.

Even with a stabilizing agent, hypochlorous acid may combine with other chemicals, forming compounds that are not very effective sanitizers. For example, hypochlorous acid may combine with ammonia, found in urine, among other things, to produce various chloramines. Not only are chloramines poor sanitizers, but they can actually irritate the skin and eyes and have an unpleasant odor. The distinctive smell and eye irritation associated with swimming pools are actually due to chloramines, not ordinary hypochlorous acid -- a strong smell usually means there is too little free chlorine (hypochlorous acid), rather than too much. To get rid of chloramines, pool custodians have to shock treat the pool -- add an unusually strong dose of chemicals to clear out organic matter and unhelpful chemical compounds.

Chloramine formation is related to the second major element in pool chemistry, maintaining the right pH in the pool.

Pool pH Levels

The water's pH is a measure of its total acid-alkalinity balance -- the relative proportion of acids and alkalis in the water (check out Chem4Kids: Acids & Bases for a description of acids and alkalis). Simply put, water that is either too acidic or too alkaline will cause undesirable chemical reactions. If the water is too acidic, it will corrode metal equipment, cause etching on the surface materials and cause skin irritation. If the water is too alkaline, it can cause scaling on the pool surface and plumbing equipment and can cloud the water. Additionally, both high acidity and high alkalinity alters the effectiveness of the chlorine. The chlorine won't destroy pathogens as well if the water is too alkaline, and it will dissipate much more quickly if the water is too acidic.

On the pH scale, zero indicates extreme acidity, 14 indicates extreme alkalinity and 7 indicates a neutral state. Most pool experts recommend a pool pH between 7.2 and 7.8. To raise or lower pH, a pool custodian simply adds acids or alkalis into the water. For example, adding sodium carbonate (soda ash) or sodium bicarbonate (baking soda) will generally raise the pH, and adding muriatic acid or sodium bisulfate will lower the pH.

Maintaining the proper balance of chemicals in the pool is a continual process, because any new element -- oils from a swimmer's body, a shot of chlorine, stuff that falls in the water -- shifts the water's total chemical makeup. In addition to pH, pool custodians also monitor total alkalinity, calcium hardness and total dissolved solids.

When you consider all the chemistry and machinery involved in swimming pools, it's clear that they really are remarkable pieces of technology. It takes a surprising amount of work and ingenuity to build and maintain these summertime staples.

For more information on swimming pools and related topics, check out the links on the next page.

Related HowStuffWorks Articles

More Great Links