The Drains
We saw in the last section 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 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 this system, 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.

The return port

Most swimming pools also have a couple 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.

A typical vacuum port

A pool vacuum cleaner

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.

Light it Up
These days, most swimming pools are built with underground lights, partially for aesthetic appeal but mainly to let night swimmers see what they're doing. In one common underwater lighting design, an incandescent light bulb is sealed in a water-tight fixture, which sits inside a niche embedded in the pool wall. The insulated electrical wire runs into the fixture through a special seal, keeping water away from the conductive elements. The wire runs back to the house (or wherever the power source is) through a long tube, which is filled with water most of the way. There is enough extra wire in the tube that you can pull the entire fixture out of the niche and up above the water level when you want to change the bulb.

Some people use fiber-optics to light their pools, instead of embedded incandescent fixtures. In this system, the actual light source doesn't have to be underwater, so you can skip the whole bulb-changing, water-proof electrical-component issue entirely.