Sewer Systems

Why do we need a sewer system?

Each time you flush the toilet or you wash something down the sink's drain, you create sewage (also known in polite society as wastewater).
One question that many people might ask is, "Why not simply dump this wastewater onto the ground outside the house, or into a nearby stream?"

There are three main things about wastewater that make it something you don't want to release into the environment:

1. It stinks. If you release wastewater directly into the environment, things get very smelly very fast.
2. It contains harmful bacteria. Human waste naturally contains coliform bacteria (for example, E. coli) and other bacteria that can cause disease. Once water becomes infected with these bacteria, it becomes a health hazard.
3. It contains suspended solids and chemicals that affect the environment.

For example:
Wastewater contains nitrogen and phosphates that, being fertilizers, encourage the growth of algae. Excessive algae growth can block sunlight and foul the water. Wastewater contains organic material that bacteria in the environment will start decomposing. When they do, these bacteria consume oxygen in the water. The resulting lack of oxygen kills fish. The suspended solids in wastewater make the water look murky and can affect the ability of many fish to breathe and see. The increased algae, reduced oxygen and murkiness destroy the ability of a stream or lake to support wildlife, and all of the fish, frogs and other life forms quickly die.

No one wants to live in an place that stinks, is full of deadly bacteria and cannot support aquatic life. That's why communities build wastewater treatment plants and enforce laws against the release of raw sewage into the environment.

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.

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.

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.

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.