Skip to Main Content
City of Lake Oswego

Treatment Process

City of Lake Oswego Oregon Official Website

As part of the Lake Oswego Tigard Water Partnership project, Lake Oswego’s water system has been undergoing many changes and upgrades, including the water treatment process. The current treatment process, placed online in spring 2016, is conventional treatment using ballasted flocculation and granular activated carbon filtration.

When raw (untreated) water reaches the plant from the Clackamas River, state licensed drinking water operators take the water through a purification process that makes it safe to drink.  Alum (aluminum sulfate) is added to make the fine particles of silts, clays and other naturally occurring organic matter clump together to form larger particles called "floc".  These larger particles then settle out of the water during the next treatment process called ballasted flocculation.

Ballasted flocculation uses micro-sand and alum to rapidly settle dirt, sediment and contaminants out of the water before filtration. The sand helps the floc to form and settle to the bottom of the settling tank. The settled (or clarified) water is then filtered through a bed of granular activated carbon and silica sand, to remove any remaining tiny particles, microbes and contaminants from the water.

In comparison to the previous direct filtration process that was used from 1968-2015, with the addition of the ballasted flocculation process, the new filters operate for longer periods between cleanings and better remove any remaining particles and contaminants. This is particularly effective during winter months when water from the river contains more sediment.

After the water has been filtered, it must be made non-corrosive to materials that commonly make up home plumbing systems, such as older plumbing that may contain lead.  Sodium hydroxide is added to increase the pH of the water to between 7.7 and 7.9. It has been determined that this is our optimal level for corrosion control city-wide.

Also at this point, a small amount of chlorine is added to disinfect the water.  This kills harmful pathogens such as bacteria and viruses, and keeps these pathogens from growing out in the 200+ miles of pipes that distribute water through the City.  The required contact time is obtained in a clearwell (underground water storage reservoir) before the water is then pumped through a large transmission pipeline to a network of storage reservoirs and distribution lines, before arriving at customers’ taps.

In early 2017, the third and final phase of construction at the treatment plant will introduce a state-of-the-art ozone disinfection process to our water treatment system. Ozone, a powerful oxidant, destroys taste and odor causing compounds and removes more impurities from the water supply. It is one of the best technologies for improving taste and odor, and it provides an additional treatment barrier to protect public health.

Prior to the plant upgrade, water was treated using direct filtration, which uses a chemical coagulant, such as iron or aluminum that is added to the water. It then passed through dual media anthracite filters. Direct filtration does not perform as well as conventional filtration in the purification of highly turbid (dirty) water.

The new plant upgrades consist of a much more effective and dependable water treatment process that will provide decades of excellent quality drinking water to Lake Oswego and Tigard customers.