​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.

In addition, the City is now investing close to $50 million dollars for the design and construction of the ozone treatment facility on Geren Island. This new facility is anticipated to be completed in the Spring of 2021.

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.



Additional Drinking Water Treatment in Algal Season (April to October)

Additional monitoring and water quality sampling occurs above Geren Island, in the North Santiam River watershed (Salem's drinking water source) during algal season.  Based on information learned in 2018, the City can now adjust drinking water treatment processes based on watershed conditions and the water quality data received.  Additional treatment processes used to remove cyanotoxins and to ensure safe drinking water include:

  • Acetic Acid (added to Slow Sand Filtration)
  • Increased chlorine dosage
  • Powdered Activated Carbon (PAC)

In addition, the City is now investing close to $50 million dollars for the design and construction of an ozone treatment facility on Geren Island.  This new facility is anticipated to be completed in the Spring of 2021.

Preparation for algae season

Acetic acid

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.

Added treatment steps to manage cyanotoxins

We have added treatment steps​—increasing chlorine dosage and using powdered activated carbon to control cyanotoxins. These additional barriers, along with slow sand filtration, reduce and eliminate cyanotoxins if they enter the water treatment plant. Until ozone treatment is available, increased levels of chlorine coupled with powdered activated carbon are the most effective way for us to protect our community from cyanotoxins. These treatment steps will be used until the long-term solution​—ozone treatment​—is in place in spring 2021.

Increased chlorine dose

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.

​Powdered Activated Carbon (PAC)

How PAC works

When toxins are present, powdered activated carbon is added to the water, right at the intake to the treatment plant. Cyanotoxins stick to powdered activated carbon in a process called adsorption. The powdered activated carbon settles out of the water, along with the cyanotoxins, in a roughing filter. The clean water then goes through normal slow sand filtration treatment, which provides another barrier for cyanotoxins. 


​Step 1

Water from the North Santiam River flows into the Geren Island Water Treatment Facility near Stayton. The raw water enters the facility through a quarter-mile-long intake channel.

Step 2

Powdered activated carbon (PAC) may be added to the water as it passes through the intake channel.

​Step 3

In the intake channel, the small particles of PAC are mixed and kept suspended in the water through constant agitation using four large mixers called “Gridbees©.”

​Step 4

If the water contains harmful cyanotoxins caused by the blue-green algae, the toxins will stick to the powdered activated carbon in a process called adsorption.

​Step 5

The bottom layer, or settled floc, is eventually removed, but the clean water at the top flows off to the next step, which consists of a large pond with a layer of sand called a roughing filter. The roughing filter helps to remove any remaining carbon particles before water is further filtered in Salem's normal slow sand filtration system. 

 ​Step 6

After the water passes through the roughing filter, a small amount of acetic acid is added to provide a source "food" for the good microorganisms that help process the water in the next treatment step —slow sand filters. Slow sand filtration is one of the oldest and most reliable surface water treatment technologies in the world, and works best when used to treat water from pristine watersheds like the North Santiam River. In slow sand filtration, the good microorganisms form the schmutzdecke to remove the contaminants of concern, including pathogens. The addition of acetic acid is a temporary measure, and is necessary because the added PAC not only removes the algal toxins, but also a portion of the food in the water that the good microorganisms need to stay healthy. 

​Step 7

As water passes through slow sand filters, the good bacteria, consisting of harmless bacteria, fungi, protozoa, and other components, remove particles, organic material, and other contaminants. After passing through the Schmutzdecke, the water infiltrates through the supporting sand layer. The filtration process is relatively slow compared to other surface water filtration processes, thus the name slow sand filtration.

​Step 8

After passing through the slow sand filters, as part of our normal drinking water treatment routine, chlorine is used to further treat the drinking water, disinfecting the final product and providing a chlorine residual that maintains the quality water between the water treatment facility and your home. Soda ash compound is also added, as necessary, to maintain the proper pH balance, successfully minimizing the presence of contaminants such as lead and copper. 

​Step 9

Finished drinking water travels through our transmission lines to our residents and drinking water customers. Throughout the process, water quality samples are taken to ensure safe drinking water is delivered to our customers.


Some words explained

Acetic acid

The main compound of vinegar apart from water.


The adhesion of atoms, ions, or molecules from a gas, liquid, or dissolved solid to a surface.


Chlorine (Sodium Hypochlorite) is a chemical element with strong oxidizing qualities and is used to disinfect drinking water.

Finished Water

The water that has passed through a water treatment plant and is ready to deliver to customers.


The process by which fine particles are caused to clump together into a floc. The floc may float to the top of the liquid, settle to the bottom of the liquid, or be readily filtered from the liquid.

Raw Water

Water found in the environment that has not been treated and does not have any of its minerals, ions, particles, bacteria, or parasites removed.

Roughing Filter

Filters often used to pretreat water by removing suspended solids from the water that could rapidly clog a slow sand filter.

Roughing Filter

Filters often used to pretreat water by removing suspended solids from the water that could rapidly clog a slow sand filter.

Schmutzdecke (biological layer)

Hypogeal biological layer formed on the surface of a slow sand filter. The schmutzdecke is the layer that provides the effective purification in potable water treatment, the underlying sand providing the support for this treatment layer.

Slow Sand Filtration

This process percolates untreated water slowly through a bed of porous sand, with the influent water introduced over the surface of the filter, and then drained to the bottom.

Soda Ash

Also known as sodium carbonate, is used in the softening of water by neutralizing the pH in the water. 

​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. 
  • Increases flexibility to handle changes in source water quality.
  • Consistently produces water that is pleasant tasting, year-round.
  • Reduces the amount of chlorine needed for disinfection.
  • Reduces the formation of harmful disinfection byproducts.
  • Complements the existing Geren Island Water Treatment Facility by enhancing the existing biological filtration process.
  • Represents proven technology, with ozone installations increasing in Oregon and across the U.S. due to its ability to provide multiple water quality benefits

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