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How Does the City Treat the Water?
Raw (river) water is pumped from our Clackamas River intake at Gladstone, under
the Willamette River to the Treatment Plant in West Linn. When
the water reaches the plant it receives doses of chemicals that
help to make it safe. Chlorine is added to begin the process
of disinfecting the water. Alum (aluminum sulphate) is
the primary coagulant. A coagulant is used to make the
fine particles of silts, clays and other naturally occurring
organic matter clump together to form larger particles called "floc". The
larger floc particles then settle out of the water during the
next treatment process called "sedimentation". The
settled water is then dosed with a filter aid polymer, the type
and amount of which varies by water temperature. The water
flows into six dual media filters. The dual media is approximately
3 feet of anthracite coal and one foot of silica sand deep. It
will last more than 30 years before the media needs replacement, but only 30 hours
of use is possible before the filter needs to be washed.
To wash a filter, filtered water is pumped in reverse of the
regular flow direction and accumulated dirt is washed out of
the filter into sludge lagoons (not pictured). Washing
a filter requires at least 50,000 gallons of water, which sounds
like a lot, but considering that the filter provides several
million gallons before it needs washing, it's literally a drop
in the bucket.
Click on this water treatment schematic thumbnail
to see it in larger format.
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After the water has been filtered, it must be made non-corrosive
to materials that commonly make up home plumbing systems. At
this point an addition of calcium (hydrated lime) is added to
increase the pH of the water to between 7.2 and 7.6. It has been
determined that this is our optimal level for corrosion control
city-wide.
Also at this point a final addition of chlorine is made to
ensure that the farthest areas of town have a minimum free chlorine
residual of 0.2 ppm (parts per million). As the water is
being pumped to town, a final measurement of pH is taken and
a dose of carbon dioxide injected in order to ensure that the
pH is within a very close tolerance of the required level.
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