A N S W E R:   It's a dirty question, but somebody has to answer it. After all, of the many technological innovations, engineering marvels, and sheer feats of logistics, manpower, and capital investment that make modern human society workable, what is the most important? Transportation, communication, energy, modern agriculture are all vital stuff, I grant you.

But what about our ability to deal with waste water and trash we generate? But without our extensive networks of sewage pipes and massive sewage treatment plants, we might well drown in a stinking sea of our own filth. So a good argument could be made that a good sewer system is really the foundation of civilization.

Yucky Water

So, Maxine, not to be indelicate, but the real question you're getting at here is, "what happens after you flush?" For the sake of simplicity and brevity, we're going to limit the scope of the answer to urban sewage systems for residential users. That leaves out the septic tanks used by people who live in the rural areas, as well as industrial systems used by big manufacturers.

On the other hand, we will cover the basic workings of the 16,000 or so waste water systems that handle the 100 gallons a day of yucky water that the average American citizen flushes, pours, and rinses down the drain from toilets, sinks, showers, dishwashers, and washing machines each and every day.

OK. You flush. Where does it go? Into a pipe that leads out of your house and ties into the local sewer system. The pipe leaving my house is four inches in diameter and it is connected to a six-inch pipe that also serves two of my neighbors. That pipe leads out towards the street where it connects to a larger pipe that serves the neighborhood.

Pipes are most commonly made of clay tile, concrete, and plastic. Sewage systems are designed so that gravity does most of the work of getting waste water from your house to the treatment plant. Where the terrain doesn't cooperate, pumps are used.

Your Taxes at Work

How much pipe does it take? The system that serves the Boston area — which includes some 2 million people — uses more than 54,000 miles of sewer ranging in size from eight inches to 11 feet. That, folks, is an example of your tax dollars at work.

Once your 100 gallons has made its way through all that pipe, it reaches the waste water treatment plant. (If you live in Boston, your 100 gallons is part of a daily deluge that averages close to 400 million gallons.) First, all that water passes through fairly coarse metals screens that filter out bigger chunks of debris such as branches, paper, and rags. The water then passes into a grit chamber where sand, dirt, and inorganic solids have a chance to settle. The chunks and gritty stuff collected so far are normally shipped off to the local landfill.

From the grit chamber, the waste water moves very slowly through what are usually called the primary clarification tanks. Here gravity does most of the work. Some of the heavier gunk sinks to bottom. That's sludge. Some of the light stuff floats to the top. It's called scum.

During the couple hours that your 100 gallons spends in the primary clarification tank, 50 percent to 70 percent of the suspended solids and toxic materials will separate out. Machines scrape or skim the results. Typically the sludge and scum is concentrated and sent to digestor tanks where microorganisms feed on it, breaking it down into methane gas, carbon dioxide, water, and a much smaller volume of sludge. These days, one of three things happens to that leftover sludge: It either goes to a landfill, to an incinerator, or it is turned into fertilizer.

Food for Bacteria

After passing through the primary clarification tanks, waste water is ready for secondary treatment. The most common method is called the "activated sludge process." First the water flows into aeration tanks where oxygen is added, which promotes the growth of bacteria that feeds on the organic waste still in the water. Over the course of two to eight hours, masses of what is called "floc" is generated as the microorganisms multiply like mad, thanks to the ideal combination of food and oxygen.

The water then passes into clarifying tanks where the floc is removed, some of which is returned to the aeration tanks to keep the activated sludge process moving along as new wastewater flows into the system and some of which goes to the digestor tanks for processing. By the time this secondary treatment phase is complete, about 90 percent of the original load of pollution has been removed.

The third step — called, cleverly enough, "tertiary treatment" — includes the addition of chemicals which remove some of the phosphorous and help settle most of the remaining floc. Chlorine is also added to kill harmful bacteria. The water then passes through filters, usually activated charcoal or charcoal and sand, where all but a tiny amount of the remaining organic and inorganic matter is removed.

Good Enough to Drink?

The result? Water that's good enough to drink, or at least, water deemed clean enough to discharge into the nearest large body of water.

Sewer systems have been around for a long time. The Romans built one in the sixth century B.C. to drain the area around the Forum. The first sewage system in Paris was constructed at least 500 years ago. Simple sewage treatment dates back at least to the middle of the 19th century. During the last 30 or 40 years, significant improvements have been made both in the systems that collect waste water and the technologies that go into treating that sewage.

However, according to the Water Environment Federation (WEF), a nonprofit that focuses on water quality issues, the future of waste water treatment here in the United States isn't looking so rosy. A recent report estimates that we'll need to spend $2 trillion over the next two decades to guarantee the cleanliness and safety of the water we use every day.

According to the WEF, we'd need to spend $23 billion more than we're already spending each year just to repair and replace out-of-date systems and comply with federal water quality standards. Failure to do so, says the report, threatens one of the things we take for granted the most in this country — the ability to turn on the tap, fill a glass of water, and chug it down without worrying about getting sick.

Todd Campbell is a writer and Internet consultant living in Seattle. The Answer Geek appears weekly, usually on Thursdays.