Regional Water Reclamation Plant

Client—City of Chehalis, Washington

Gibbs & Olson provided the planning, design and construction phase services for the Chehalis Regional Water Reclamation Plant. This facility serves the cities of Chehalis and Napavine along with Lewis County Water and Sewer District No. 4. The $40 million Regional Plant includes a sequencing batch reactor (SBR) with sand filtration treatment capable of treating up to 3.5 million gallons per day (mgd) of wastewater to Class A reclaimed water standards. When flow in the Chehalis River is less than 1,000 cfs the Class A reclaimed water is utilized for irrigation and groundwater augmentation at a 250 acre poplar tree plantation.

The original Chehalis wastewater treatment plant was built in 1949 and was designed to discharge treated wastewater directly into the “Centralia Reach” of the Chehalis River. The Centralia Reach is a slow moving lake-like stretch of river with a long history of poor oxygen levels during the summer months. Poor water quality in this stretch of river led to Ecology performing a Total Maximum Daily Load (TMDL) water quality study. As a result of the TMDL Study, Ecology mandated that the City of Chehalis remove the treated discharge from the river from May-October of each year. This directive presented the City with a serious challenge in that the ability to land apply effluent during May, June and October can be problematic in wet years and there was no potential user for the treated water.

Gibbs & Olson provided technical assistance to the City to negotiate an affordable alternative solution to meet the Ecology mandates. As a result of those negotiations, the TMDL was revised to allow the discharge to remain in the river based on river flow criteria rather than calendar months. This revised river flow-based criteria provided the City with a feasible solution. The resulting design is a new regional plant that allows operators to continue discharging secondary treated effluent to the river during wet weather (high river flow conditions) and to quickly change to Class A reclaimed water quality effluent for poplar tree irrigation and groundwater recharge during low river flows. This ground breaking project has helped redefine treatment plant design. By using sophisticated technology and treatment options, the City can maximize protection of the Chehalis River while giving the plant operators flexibility to modify operations under varying environmental conditions.

The Chehalis Regional Water Reclamation project received a Best in State Gold Award from ACEC for Exceeding Owner/Client Expectations.