Powdered Activated Carbon Use in the Pre-Treatment of Salem’s Drinking Water

​How PAC works

Before it is delivered to your tap, water from the City of Salem will go through the following treatment processes and quality procedures to ensure it is clean, clear, and safe to drink.

​On July 4, 2018, the City of Salem began pre-treating drinking water at the Geren Island Water Treatment Facility to minimize the presence of cyanotoxins. Cyanotoxins are caused by blue-green algae in Detroit Lake. This pre-treatment process uses powdered activated carbon (PAC) to reduce cyanotoxins that may still be present in the untreated water as it enters the water treatment facility. After the water passes through the new pre-treatment process, it continues through the normal filter processes.

​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) is 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 “Solarbees©.”

​Step 4

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

​Step 5

At the end of the intake channel, alum and cationic polymer, chemicals commonly used in Oregon’s other surface water treatment plants, are added to the water mixture. These help the fine carbon particles to clump together in a process called flocculation. 

​Step 6

At this point, agitation of the water mixture is stopped, allowing the flocculated particles to settle. These settled particles, or “floc” of carbon with the cyanotoxins stuck to them, sink and settle at the bottom of the settling pond, leaving an upper layer of clean, clear water.

​Step 7

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 8

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 9

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 biofilm, 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 10

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 11

Finished drinking water travels through our transmission lines to our residents and drinking water customers. Throughout the process, water quality samples are taken and tested using the City's ELISA system 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.

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.


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

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