In urban and suburban areas where people are packed closer together and where there is a lot more wastewater to treat, the community will construct a sewer system that collects wastewater and takes it to a wastewater treatment facility.

Photo courtesy photos.innersource.com Water treatment plant in Libertyville, IL

Why are manhole covers round? Because it avoids accidents. Since manhole covers are round, it is impossible for a cover to fall down the manhole. If they were square or rectangular, they could.

In the ideal case, a sewer system is completely gravity-powered, like a septic system. Pipes from each house or building flow to a sewer main that runs, for example, down the middle of the street. The sewer main might be 3 to 5 feet (1 to 1.5 m) in diameter. Periodically, a vertical pipe will run up from the main to the surface, where it is covered by a manhole cover. Manholes allow access to the main for maintenance purposes.

The sewer mains flow into progressively larger pipes until they reach the wastewater treatment plant. In order to help gravity do its job, the wastewater treatment plant is usually located in a low-lying area, and sewer mains will often follow creekbeds and streambeds (which flow naturally downhill) to the plant.

Normally, the lay of the land will not completely cooperate, and gravity cannot do all the work. In these cases, the sewer system will include a grinder-pump or a lift station to move the wastewater up over a hill.

Photo courtesy photos.innersource.com Screw pumps

Once the water reaches the wastewater treatment plant, it goes through one, two or three stages of treatment (depending on the sophistication of the plant). Here's what each stage does:

• The first stage, known as primary treatment, does the same thing a septic tank does. It allows the solids to settle out of the water and the scum to rise. The system then collects the solids for disposal (either in a landfill or an incinerator).

  Primary treatment is very simple -- it involves a screen followed by a set of pools or ponds that let the water sit so that the solids can settle out.

  Photo courtesy photos.innersource.com Primary screen

  Photo courtesy photos.innersource.com Primary clarifiers

  Primary treatment might remove half of the solids, organic materials and bacteria from the water. If the plant does no more than primary treatment, then the water is chlorinated to kill the remaining bacteria and discharged.

• The second stage, known as secondary treatment, removes organic materials and nutrients. This is done with the help of bacteria -- the water flows to large, aerated tanks where bacteria consume everything they can.

  Photo courtesy photos.innersource.com Aeration tank

  The wastewater then flows to settling tanks where the bacteria settle out. Secondary treatment might remove 90 percent of all solids and organic materials from the wastewater.

  Photos courtesy photos.innersource.com Secondary clarifier

• The third stage, known as tertiary treatment, varies depending on the community and the composition of the wastewater. Typically, the third stage will use chemicals to remove phosphorous and nitrogen from the water, but may also include filter beds and other types of treatment. Chlorine added to the water kills any remaining bacteria, and the water is discharged.

  Photo courtesy photos.innersource.com Final clarifier

  Photo courtesy photos.innersource.com Chlorination tank