Dear Andy and Blue;-
> (Is Blue your real name?)
  I once read that the waters of the Ohio river, from the time they leave Pittsburg, to the time they reach the Gulf, have been used over 19 times by drinking, washing, flushing, and lawn watering. Does that sound right?
   Art
> From: Andy Rindsberg <[log in to unmask]>
> Date: 2004/02/17 Tue PM 03:44:29 EST
> To: [log in to unmask]
> Subject: Re: Marisa and ramshorns
>
> --- [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
>

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