The City of Lewiston’s $18 million wastewater treatment plant upgrade is 90% complete and on track for full operational status by July, city engineers reported Wednesday. The project, which has been under construction since 2024, replaces aging treatment infrastructure, doubles the plant’s processing capacity, and brings the facility into compliance with increasingly stringent environmental discharge standards set by the Idaho Department of Environmental Quality and the EPA.
The upgraded plant will have the capacity to treat 4.5 million gallons of wastewater per day, up from the current 2.8 million gallons — headroom that city planners say is essential to accommodate the residential and commercial growth projected for Lewiston and surrounding areas over the next 20 years. Without the expansion, the city would have been forced to impose moratoriums on new water and sewer connections within 3-5 years.
Why the Upgrade Was Necessary
The existing treatment plant was built in the 1990s and designed for a community roughly half the size of current Lewiston. As Nez Perce County’s population has grown, the plant has operated at or near its maximum capacity, leaving virtually no margin for the surges in flow that occur during heavy rain events or spring snowmelt. Operating at or above capacity increases the risk of partially treated effluent being discharged into Clearwater River — a violation of the city’s discharge permit that carries significant fines.
The new treatment processes also produce a cleaner effluent with lower levels of nitrogen and phosphorus, nutrients that can cause harmful algal blooms and degrade water quality in receiving waters. The upgraded plant uses biological nutrient removal technology that achieves treatment levels well below the permitted limits.
Funding the Upgrade
The $18 million project is funded through a combination of a low-interest State Revolving Fund loan ($12 million), connection fees from new development ($4 million), and the city’s sewer utility reserves ($2 million). The loan will be repaid over 20 years through sewer service charges, which increased approximately $8 per month for residential customers in 2025 to cover debt service costs.
What Comes Next
The final phase of construction includes commissioning and testing of the new treatment processes, which will occur over a 60-day period beginning in May. The plant will transition to full operation by July 1. Residents may notice temporary chlorine odors from the plant during the startup period, which are normal and will dissipate once operations stabilize. Questions about sewer service can be directed to the Lewiston Public Works Department.