--- [log in to unmask] escreveu: > Dear Andrew and Blue:-
The material then> goes to a water purification plant which kills everything
> including bacteria before it is released into local rivers. How do
> snails survive this?
>     Q-Man

Ah. I can answer this one! I once visited the Denver (Colorado) metropolitan
sewage system during a course engagingly entitled Ecology for Engineers.
Sewage is treated in different ways in different places.

As some have mentioned, it may not be treated at all. This can happen even
in highly sophisticated systems, when stormwater exceeds a system's
capacity, or power outages prevent pumps from operating. Survivors of
hurricanes often have to deal with both problems at once.

At Denver in about 1983, sewage water was gathered in pipes of increasing
diameter and arrived at the treatment plant looking rather like dirty
dishwater, and smelling ammoniacal. The water was sent through a coarse
screen made of heavy rods, then finer screens. Very little was caught on the
coarsest screens, but we were told that branches were sometimes stopped
there. Then the water was filtered through a bed of sand, and after that, a
charcoal filter; between the two of these, much of the contaminants were
already removed by the time that the water was sent on to an open circular
tank with a rotating arm that skimmed the surface. The surficial material,
consisting largely of waxes, was recycled and purified for use in various
products -- I have never learned whether our tour guide was serious about
the products including chapstick. He was serious about everything else, and
I'm not sure that I want to know. I had to use chapstick often in Colorado's
dry climate.

After that, the sewage was allowed to "stew in its own juices" while being
aerated. Bacteria would eat up and render harmless most of the organics,
which could be skimmed off and sterilized. Some cities would dispose of the
result in landfills, others would sell it as fertilizer, and as I recall, it
wasn't uncommon for tomato seeds to retain their viability throughout this
process and sprout from the fertilizer. One problem with reusing this
material is that it tends to have an uncomfortably high content of heavy
metals, so I gather that the landfill option is pretty common these days.
The metals can be removed from sewage water, but it takes a more
sophisticated, "tertiary" process and of course that costs extra. Tertiary
treatment wasn't done in Denver.

But the treatment plant had just begun a new process that was rather
impressive. Some of the tanks were heated and oxygen was added to let the
bacteria grow faster. Oxygen was separated from air on-site cryogenically.
This process could clarify water far faster than the older method and they
were planning on extending it.

After that, the water was pretty clean-looking, and it was put into the
South Platte River, accounting for about one-fifth of its flow. Each
molecule of water in the river would be used and reused several times by
towns downstream in Colorado, Kansas, and so on down to the Mississippi
River and the Gulf of Mexico.

So, Art, you can see that mollusks might well survive a trip through the
sewers, bypassing the treatment plant with the help of an unusually hard
rain. The sand and charcoal filters would probably stop them physically and
the bacterial tanks would probably cook them, let alone the new heated
tanks. And this is nineteenth-century technology that would have been
installed in Cincinnati quite a long time ago.

I hope that you found this field trip as instructive as I did. It was a
great course. On another Ecology for Engineers field trip, we visited a
prairie dog town and I almost got bitten by small rattlesnakes -- but that's
another story.

Andrew K. Rindsberg
Geological Survey of Alabama