Geren Island Water Treatment Plant Improvements

The City of Salem owns and operates the Geren Island Water Treatment Plant (WTP) whose slow-sand filtration system, paired with the City’s robust water-quality monitoring of the North Santiam River, has been historically effective in removing cyanotoxins contained in their raw water. In recent years, however, wildfires, regional warming of surface waters, changes in river morphology, and modifications to the U.S. Army Corps of Engineers’ management of river flows have all contributed to an increase in harmful algal blooms within these local bodies of water.

After the Detroit Reservoir experienced an algal bloom in 2018 that led to a do-not-drink advisory that lasted almost 3 months—one of the longest in U.S. history—the City detected cylindrospermopsin and microcystin at 7.0 and 2.1 micrograms per liter (μg/L), respectively. These concentrations were much higher and earlier than what had been observed in previous years, thus significantly challenging the Geren Island WTP in reducing these levels to below recommended health advisory levels.

In need of an immediate response, the City contracted Carollo to develop near-term cyanotoxin-mitigation strategies to deploy in a matter of weeks. During this short period, Carollo tested the effectiveness of ozone, ultraviolet disinfection, powdered activated carbon (PAC), and granular activated carbon (GAC) and ultimately designed and installed a temporary treatment system with PAC adsorption, two-stage biological filtration enhanced with acetic acid, and chlorine oxidation. Several months’ worth of bench and pilot testing were fast-tracked into a matter of weeks, during which Carollo provided around-the-clock operational support.

As a result of these efforts, the plant was equipped with an effective temporary treatment system that successfully treated high levels of cylindrospermopsin and microcystin for the following two years, while the permanent, long-term solution was implemented. Furthermore, the City gained valuable insights into a sustainable long-term solution to build a resilient and safe water supply.

Today, Carollo continues to support the City as they implement resilience and treatment improvements at the Geren Island WTP. This multi-phased project includes the following major facility enhancements:

• Improve the process pump station to increase the overall firm capacity of the roughing filters.
• Construct a 75-million-gallons-per-day (mgd) intermediate ozonation facility to effectively treat cyanotoxins in the raw water.
• Enhance existing operations’ control systems and equipment.
• Expand the plant’s groundwater wells to augment the surface water supply.