You flush the toilet and, with a giant gurgle, the contents simply disappear. It's a miracle. Where the water goes, what happens to it when it gets there, and where it ends up may never cross your mind.
You could go through life, as most people do, without ever worrying your pretty little head about such distasteful subjects as sewage treatment. But if you care about clean water, or even just about being able to flush your toilet forever, it is helpful to know the basics. After all, more than 28 billion gallons of raw sewage are treated in the United States every year. What exactly does "treated" mean? And what can you do to help the process along? Stand by for a guided tour of your typical treatment plant.
You make a deposit. You flush the toilet. Goodbye!
The sewage joins used bath and sink water and drains from your house into a large pipe under the street. This pipe drains into another, and so on, until the sewage eventually makes its way to a treatment plant. Some communities combine storm drains and sewers, sending all the water to the treatment plant. Others have dTC separate systems, where storm runoff drains directly into lakes or streams.
First stop at a typical plant is a bar screen. The screen filters out large objects such as sticks, tampons and big wads of paper. This non-toxic stuff is trucked to a landfill or an incinerator.
The next stop is a grit chamber, where sand and gravel settle to the bottom.
Next are primary sedimentation tanks, where the water just sits for about three hours. Here, the bulk of the solids -- i.e., feces -- settles to the bottom, forming a sediment called sludge. The sludge is piped to a digester, a closed tank in which it is heated and broken down by bacteria.
This is primary sewage treatment. For many communities, this is as far as it goes. The water, still substantially dirtier than when it lay waiting in your toilet tank, is now disinfected with chlorine and released to a nearby river, bay or ocean. We then rely on the natural ability of water to cleanse itself to further purify our wastewater.
Thanks to the Clean Water Act, most of America's 15,000 or so municipal treatment plants now go one step further, to secondary treatment. Here, air is bubbled through the water, which is seeded with bacteria. The bacteria feed happily on the sewage for a few hours; the water then flows to another settling tank. Sludge-filled bacteria are bundled off to the digester. The water is disinfected and discharged.
The fate of the sludge depends on how polluted it is. High-grade sludge, low in heavy metals and other pollutants, can be used as fertilizer. Most sludge is too toxic to qualify and meets the sad fate of ordinary household garbage.
Though the water, or effluent, from a treatment plant isn't anything you'd want to drink, the process does take out about 90 percent of the solids from the raw sewage we send to it. Engineers are tinkering with superior systems that use living wetland plants -- a sort of controlled swamp -- to treat water, and some day we may all be lucky enough to have such systems operating in our communities.
In the meantime, there are a few things each of us can do to make sewage treatment more effective. The general idea is not to use your toilet or sink as a garbage can.
Don't dispose of leftover chemicals such as paint thinner, photography supplies or degreasers by dumping them in your toilet or sink. Try to avoid using pesticides and minimize chemical fertilizers; they will run off your lawn into storm drains. Replace toxic household products such as oven cleaner and drain opener with harmless substitutes. Use detergent with low or no phosphates. Recycle motor oil.
Other no-nos for toilet disposal: bits of plastic such as bandages, cigar tips or tampon applicators. Condoms. Baby wipes. Diaper tabs.
Though "bits of plastic" don't sound terribly dire, in fact this debris can really foul up the system. At best, the trash is screened out and the treatment plant has to pay to haul it away. At worst, the plastic can snarl up the works of a tank, forcing the plant manager to have the machinery drained, cleaned and repaired.