Every day, North Santiam River water flows into the Geren Island Water Treatment Facility where it is filtered through slow sand filters and disinfected with chlorine.  Fluoride is added as well as Soda Ash (to reduce the leaching of lead from household plumbing).  Per the Federal Safe Drinking Water Act, the City of Salem routinely monitors conditions in the watershed collects water quality samples from the treated water as it leaves Geren Island and from within the distribution system (in-town). 

During algae season, from April to October each year, additional monitoring and water quality sampling occurs within the North Santiam River watershed.

Plant Improvements at Salem's Geren Island Water Treatment Facility

Slow sand filtration

The slow sand filtration system at Geren Island is one of the largest in the country and has provided effective treatment of Salem's drinking water for more than 80 years. Salem has been able to use this system due to the consistent high quality water we receive from the North Santiam River.

Raw water enters Geren Island through an intake screening structure on the North Santiam River. From there it flows down a constructed channel to one of six slow sand filter ponds. As water passes through the first 1/4″ of the filter, small organisms that occur naturally in the river begin treating and filtering the raw river water. Organisms, such as amoeba and vorticella, “eat” algae and masses of small debris at the surface layer of the filter. This biological surface layer is called the Schmutzdecke layer. When the water reaches 8″ down in the sand filter, E. coli bacteria is 99.5 percent filtered out of the water, and by 14″, 98% of viruses and other bacteria are filtered out. Filters are 3 feet when newly sanded. Once the filtered water reaches the perforated pipe at the bottom of the slow sand filter, the water is piped to the west side of the facility via gravity where it is then treated with chlorine to disinfect any bacteria or pathogens that may have passed through the filter. In general, processing water at Salem’s drinking water facility requires little chemical and electricity or energy.

In May 2018, for the first time, cyanotoxins from the die off of a certain type of blue-green algae made their way through the slow sand filters and were detected at Aldersgate, the entrance to our distribution system. As the algal season progressed, the City began to add Acetic Acid (vinegar) to the water as it flowed into the slow sand filters. This helped the biological surface layer to grow, and boosted the ability of the Schmutzdecke to remove cyanotoxins from the water. As a result, and in preparation for the coming algal season, the City will begin to add acetic acid to the slow sand filters in April.

We have added treatment steps—including ozone treatment, increasing chlorine dosage and using powdered activated carbon to control cyanotoxins. These additional barriers, along with slow sand filtration, reduce and eliminate cyanotoxins.

The Environmental Protection Agency requires that chlorine be added to drinking water treatment processes for disinfection when surface waters (streams, rivers, lakes, etc.) are the source.  For this reason, chlorine is always used at Geren Island as final step in the treatment process.

If cyanotoxins are present, the level of chlorine used in our water treatment will be slightly increased, enough to destroy the cyanotoxins. Then, using a type of salt (sodium bisulfite), chlorine levels are reduced to the normal levels as the treated water leaves the plant.

We anticipate the changes to how the water tastes and smells due to the additional treatment steps will not be noticed by most residential customers. Nor will these changes impact our industrial or commercial users. We will be conducting water quality and tastye studies to ensure our customers continue to receive the highest quality water and consistent aesthetics.

 What is the long-term solution for removing cyanotoxins?

We are designing and installing a state-of-the-art drinking water treatment system to remove cyanotoxins. Ozone is one of the strongest disinfectants used to treat water. This treatment produces no taste or odor and no ozone is left in the water after treatment. The project will be completed in spring 2021.

What are the benefits of ozone?

The use of ozone treatment offers many benefits:

Is capable of managing cyanotoxins and other pathogens.