Water Treatment Process

Raw Water Pumping

Raw water for the treatment plant comes from the Skagit River. Pumps at the intake station on the west side draw the water, send it through traveling screens and into a 42-inch pipe underneath the river to the plant.

Ballasted Sedimentation

The raw water line splits into two 36-inch pipes, sending the water to an Actiflo ballasted sedimentation system including two separate treatment trains. Aluminum sulfate, sodium hydroxide, and sodium hypochlorite are added to aid in coagulation flocculation of suspended matter, alkalinity adjustment, pH adjustment, and disinfection. The chemicals are thoroughly dispersed with a rapid mixer.

The raw water enters a coagulation tank at the start of the Actiflo system where small particles agglomerate into larger particles or floc. Next, the water flows into an injection tank where micro-sand and polymer is slowly added. The water, coagulant, micro-sand, and polymer are mixed in a maturation tank allowing the floc to increase in size and weight. The flocculated water enters a settling tank where the particles settle to the bottom and a circular scraper rakes them into a sump in the center of the basin. From the sump, the material goes into hydrocyclones, which separate the sand from the solids or sludge. The sand recycles to the injection tank and the solids flow by gravity to lagoons. Water from the settling tank flows up through tube settlers, over weir troughs, and into a settled water mixing chamber, where filter-aid (aluminum sulfate or polymer) is injected as needed.

The Actiflo Process


From the settled water mixing chamber, the water flows through a filter influent channel onto dual media filters comprised of granular activated carbon and sand. The filters remove all remaining particles.


The filtered water is injected with sodium hypochlorite between a low-lift pump station and a clearwell water tank used to provide chlorine contact time for disinfection. A high-service pump station pumps the treated water from the clearwell to the distribution system.

Residuals Handling

Sludge from the Actiflo system, along with solids back-washed from the filters, are pumped to the lagoons and settle to the bottom while the water is removed with floating decanters.

1 Hand with 3 Water Samples

The Decant Pump Station sends the settled water to the river. Settled solids in the lagoons are removed and recycled. 

The entire treatment process is continuously and closely monitored. The plant is staffed 24 hours per day, 365 days per year, by certified water treatment plant operators. Water samples from each phase of the process are tested according to a strict daily schedule at the plant’s laboratory and independent laboratories conduct additional tests.