The City of Lake Oswego treats water by pumping raw (river) water 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.
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.