Water Treatment

Water Treatment Plant

The City of Anacortes is committed to providing quality water for all residents and making sure its citizens can rest assured of its safety and superior quality.

Water Treatment Plant

The City owns and operates a regional water treatment plant located near Mount Vernon, on the east bank of the Skagit River. In 2013, the City essentially replaced the previous water plant with a new plant on the same site on the Skagit River.

Construction included the installation of ballasted sedimentation for pretreatment, 8 new filters, a new above ground clearwell, and a new high service pumping station. The capacity of the new plant is 42 mgd, expandable to 55 mgd and serves around 56,000 residential, commercial, and industrial customers.

The Anacortes Water Treatment Plant uses a multi-barrier approach in turning the raw Skagit River water into tap water.

Raw Water Pumping & Conditioning

Raw water for the Anacortes Water Plant comes from the Skagit River. The intake is located across the river from the Treatment Plant, on the west side of the river. The raw water pumps draw water through traveling screens, and pump into a common 42 inch pipe that crosses the river to the plant. The raw water line splits into two 36 inch diameter pipes just before the Actiflo units (ballasted sedimentation). Alum, soda ash, caustic soda, and chlorine can be injected into the raw water for coagulation, alkalinity adjustment, pH adjustment, and disinfection prior to the Actiflo process. The chemicals are thoroughly dispersed into the raw water with a rapid mixer located in each raw water line.

Ballasted Sedimentation

Raw water then enters a coagulation tank where the coagulant promotes small particles to agglomerate into larger particles (floc). The water then flows into an injection tank where micro-sand and polymer is slowly mixed with the floc. The mixture of water, coagulant, polymer, and micro-sand is mixed in the maturation tank causing agglomerated particles and sand to flocculate together, in increasing size and weight. The flocculated water then enters a settling tank where the flocculated particles settle to the bottom of the tank and a circular scraper rakes them into a sump in the center of the basin. The solids are pumped from the sump into hydrocyclones, which separate the sand from the solids or sludge. The sand is recycled to the injection tank and the solids flow by gravity to the lagoons. Water from the settling tank flows up through tube settlers, over weir troughs, and into the settled water mixing chamber.

The Actiflo Process


Filter-aid (alum or polymer) is injected as needed into the settled water-mixing chamber. From the mixing chamber, the water flows to the filter influent channel. From the filter, influent channel the water flows onto dual media filters comprised of granular activated carbon (GAC) and sand. The filters remove all remaining particles.


The filtered water is injected with chlorine between the Low Lift Pumping Station and the clearwell. Between the clearwell and the High Service Pumping Station, the treated water can be injected with sodium hydroxide or chlorine for pH control or disinfection. The High Service Pumping Station then pumps the treated water into the distribution system.

Residuals Handling

Solids back-washed from the filters are pumped to the lagoons. Sludge from the Actiflo hydrocyclones flows by gravity to the lagoons. The solids settle to the bottom of the lagoons 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 infrequently removed and hauled to landfill for disposal.

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. Independent laboratories conduct additional tests.