How is My Water Treated?

State-of-the-Art Water Treatment 

Conventional Filtration Plus Ozone

Completed in 2017, the water treatment plant utilizes a state-of-the-art water treatment process, known as conventional filtration plus ozone. 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 then is treated with ozone.

In the ozone disinfection process, 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 – capable of meeting emerging concerns for pathogens, algal toxins, disinfection by-products, pharmaceuticals, and personal care products.

Following ozone disinfection, the water is filtered through a bed of granular activated carbon and silica sand, to remove any remaining tiny particles, microbes and contaminants from the water.

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 (caustic soda) 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 throughout the cities.

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 hundreds of miles of pipes that distribute water through Lake Oswego and Tigard. 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.

Take a Tour!

In celebration of National Drinking Water Week (May 5-11) and National Public Works Week (May 19-24) 2024, please register to attend a behind-the-scenes tour at the Lake Oswego-Tigard Water Treatment Plant, located in West Linn!

Tours will take approximately 1 ½ hours and are limited to 15 people per tour. Click here to sign up! 

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