air diffusers in the aeration basins. The digester mixing improvements reduced plant energy 
consumption by 8.5 percent and the aeration improvements yielded a 6.5 percent reduction in plant 
energy consumption.  

Energy Generation and Recovery - With the completion of the Cogen Expansion Project on February 12, 
2015, the Gresham WWTP has achieved its seven year old goal of becoming energy net zero.  Using the 
biogas byproduct from anaerobic digesters to fuel cogeneration (cogen) engines the plant now 
generates more electricity than it uses on an annual basis and has turned the City’s biggest energy user 
into the City’s biggest energy producer. The Gresham WWTP is a shining example of the new way of 
looking at wastewater treatment and blazing a trail to be emulated nation-wide. It’s not just about 
treating waste, it’s about recovering resources - in this case generating renewable energy worth over 
$500,000 annually.  

A key element to this success was the installation of the Fats Oils and Grease (FOG) Station at the 
Gresham WWTP. Codigestion of FOG (a waste product from restaurant grease traps) with ordinary 
wastewater sludge in the anaerobic digesters has doubled biogas production, which in turn allowed 
electrical production to double. With a tip fee of 8 cents per gallon, receiving FOG also brings in 
approximately $300,000 annually.  

As the Pacific Northwest’s first energy net zero WWTP and one of only a handful in the U.S. the Gresham 
WWTP is getting national and international attention.  As such, it is helping to promote the use of this 
reproducible technology.    

The Gresham WWTP has a net metering agreement with the local electrical utility, Portland General  

Electric (PGE). The net metering agreement with PGE allows the City to put power on the grid during 
periods of high production or low consumption. The City receives credit for the power put on the grid, 
which in turn, can be used during periods of low production or high demand. The Gresham WWTP will 
only rely on grid power when one or both cogen units are down for maintenance. During those times, 
the plant will not be billed for grid power because the WWTP will be able to use credit for excess power 
that was previously exported to the grid. The annual net metering cycle restarts every March 1 – at that 
time, the net meter credit total (for excess power exported to the grid) is reset to zero.  

The payback period for the Cogen Expansion Project was calculated to be approximately seven years in 
several planning documents and grant applications. Now that it is operational, the City will continue to 
track its payback using actual maintenance cost data, tip fee revenue and electrical savings. Tracking this 
data helps to point out where cost savings could occur, to reduce payback time.  

This project will help the City meet two key goals of its sustainability policy implemented in 2009: the 
intention to have an 80 percent reduction in City greenhouse gas emissions by 2050, and the intention 
to use 100 percent renewable energy for City facilities by 2030.  




Proactive leadership that engages in both internal organizational and broader external 
community priorities