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.
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
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
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
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