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City of Gresham: Upgrading a Wastewater Treatment Plant to be Energy Net Zero

An Interview with Senior Wastewater Treatment Plant Engineer, Alan Johnston

Through thoughtful planning and follow-through, the City of Gresham, Oregon was the first city in the Pacific Northwest to operate an Energy Net Zero Waste Water Treatment Plant. This plant generates enough energy on site to meet its demand for electricity to run the plant. In fact, it often generates more energy than it needs. The City of Gresham’s Business Sustainability Coordinator, Gregg Hayward, interviewed Alan Johnston, Senior Engineer and driving force behind the journey to Net Zero, in August 2018.

Left: One of Gresham’s two 400kw co-gen engines running off biogas produced from the wastewater and FOG mixture. Top-right: Engineer, Alan Johnston receiving a Gresham Green Business Award from Gregg Hayward. Bottom-right: The LEED-certified tank meeting room.

By Gregg Hayward, City of Gresham, OR


The inside of a former wastewater treatment tank is an interesting place to meet for an interview. The old concrete walls have been scrubbed and sealed and large windows above cast warm daylight into the space. A projector hangs from beautiful stained beams and a whiteboard has been pressed into a side of the cylindrical wall. Sounds carry easily, but the space is quiet between sentences. This old, out of service tank has become the central meeting space of the Gresham Wastewater Treatment Plant. Surprisingly, the repurposed tank is now a LEED-certified building. It was in that space that I chatted with Alan Johnston, the calm and humble 25-year veteran of the City who helped dream up this innovative movement.

Achieving Net Zero

The City of Gresham’s Mayor, Shane Bemis, signed the US Council of Mayors Climate Action Agreement in 2007. Growing out of that declaration, the City began envisioning new ways to reduce greenhouse gas emissions and increase operational efficiency to save money. Upgrading all street lights to LED, which has saved the City $700,000 per year, and upgrading the Wastewater Treatment Plant to Energy Net Zero are some of the big wins.

“We’ve had a lot of fun figuring out how to put this project together,” the affable Johnston explains. “Back when I started with the City in 1991, we actually had a co-generation engine here at the Wastewater Treatment Plant (WWTP). It was old technology, and only helped power about ¼ of the plant. We wound up turning it off in 2002 when it burned out. When I came on board, I wasn’t really thinking of how to reengineer this whole thing, I was just interested and absorbed in figuring out how to run the facility. But over time, as I came to better understand the system, and learned about new technology, I became interested in trying to find ways to make it more efficient, while still doing its job. We often become absorbed in merely thinking about operating these facilities to take care of our water ecosystems. This continues to be our priority, but these plants consume massive amounts of electricity (the WWTP is one of the largest energy users in the city), and I became increasingly interested in finding ways to change this.”


The Net Zero journey began when the WWTP received a grant from the Oregon Economic Development Commission to study ways to increase the environmental and operational efficiency of the treatment plant. An outcome of that was a study on the benefits of accepting Fats, Oils and Grease (FOG) to remove them from the waste stream and instead use them to generate electricity. The study delved into the potential return on investment (ROI) of the project. The conclusion was that it would be cost effective with an ROI of 7 years with a FOG tip fee (fee paid by haulers of FOG when they deliver FOG to the facility).

“This document was what we passed around to get interest from City management and elected officials. They came on board with it, and it became part of the capital improvement plan for the treatment plant,” says Johnston. Investment began in the 2010 capital improvement cycle where net zero became a real goal. The aim was to make improvements cost-effective and bring the plant into net zero energy use range. A formal Energy Management Team was created, and they established a goal of achieving Energy Net Zero at the WWTP within 5 years (by 2015).  The Energy Management Team met monthly to assign appropriate tasks to staff and ensure the energy net-zero goal was met. In February 2015, on schedule and on budget, the very first energy net zero month occurred. The WWTP generated more electrical energy on site from renewable biogas cogeneration and solar power than the plant consumed. The WWTP is now on its 4th year of operation as a net zero WWTP.


Several consultants were hired to help do studies to prove the feasibility of the plant’s net zero goal, along with design work and construction. Johnston oversaw the project, and currently oversees the management of the WWTP Program. “Once we had the energy net zero path forward figured out in 2010 with support from management and staff, we phased in the capital upgrades to the WWTP with four major capital projects over a five-year span.


Phase 1 (2005):  Installation of the first 400 kw cogeneration engine with a modern biogas scrubbing system.

Phase 2 (2010): Installation of a 420-kW solar system.

Phase 3 (2011): Power conservation project that reduced plant-wide power consumption by 17%.

Phase 4 (2012): Installation of a 10,000-gallon FOG receiving station

Phase 5 (2014): Expansion of the FOG receiving station to 30,000 gallons.

Phase 6 (2015):  Expansion of the cogeneration system from 400 kW to 800 kw (Two, 400 kW cogeneration engines)

Integrating with Your Utility

Passing through the gate at the WWTP today, one is met by a large array of solar panels, a very visible note that sustainability is an important value here. While phasing in this solar array, Johnston worked hand in hand with Portland General Electric, the utility provider for the area. They, along with the Energy Trust of Oregon and the Oregon Department of Energy helped fund about 40% of the overall cost of the energy related capital projects. The Energy Trust of Oregon’s model is a vital piece of the puzzle; in short, energy users across the state, both residential and commercial, pay a small additional fee on each utility bill. That fee is added to a fund that supports the Energy Trust, who then takes the money and uses it to provide monetary incentives for commercial, residential and city projects that increase energy efficiency or produce sustainable power. Residents can order free energy conservation toolkits from the Trust. Commercial buildings can get rebates to help with LED upgrades. Restaurants can get incentives to upgrade their cooking equipment. These incentives help push Oregon forward towards energy efficiency, helping lower costs for the utilities while also helping the state work to meet its energy and climate goals.

Johnston worked with PGE and the Energy Trust to work out the details of incorporating an electricity-generating system into the WWTP’s grid. The plant generates the energy it needs most of the time, more than it needs some of the time, and sometimes relies on power for the grid. Overall, the WWTP generates about 10% more electricity than it needs. Because the electricity produced at the WWTP is made with renewable biogas, the energy produced has Environmental Attributes that can be used to lower the carbon footprint of the City.  “In a sense, think of it like avoiding the need to use unsustainable carbon-based fuel sources to produce electricity,” says Johnston.


Aerating ponds with solar array in the distance

From Stinky Grease to Renewable Energy

Every day, several pump trucks drive around Gresham and the Portland metro area picking up waste fats, oils and grease (FOG) from restaurants and commercial kitchens. These pump trucks then make their way to the treatment plant where they offload into large FOG storage containers. This FOG is then slowly injected into the existing anaerobic digesters at the WWTP. “FOG has a lot of energy stored in it, about 12 cubic feet of biogas produced for every gallon injected into the digesters,” says Johnston.  Injecting the FOG into the digesters greatly increases the methane gas generated as microbes eat the solids. This biogas is then pumped through pipes into two massive yellow 400 kW Caterpillar engines.  Turbo V8 engines hum at a loud pitch as the biogas is combusted in them, turning the electrical generators that produce the electricity. It’s pretty amazing that these two co-generators produce the 6 million kwh per year needed to operate the entire WWTP. This energy runs multiple processes throughout the WWTP that aerate, mix and treat wastewater, eventually discharging the clean water to the nearby Columbia River.


Changing City, Changing Thought


Johnston related, “Back when I started in 1991, Gresham was growing like crazy… we did $30 million in growth projects in two years in the late 1990s, which greatly increased the overall capacity of the plant from 15 million gallons per day to 20 million gallons per day. Back then we were just starting to hear more about the possibility of wastewater treatment plants producing more power than they consume. Sustainability wasn't a word used much in the 1990s. Early in the 2000’s we started hearing more, and as we learned and saw how this could be implemented, instead of just talking about it, we did it. We did it because it made financial sense and it was the right thing to do.  Gresham was really concerned with

project payback and we made sure that whatever we did it helped with rates. Other cities may look at a triple bottom line a bit more than Gresham, but here it really needed to make financial sense as well.  We couldn’t recommend a 10-20-year payback on an energy project. When the numbers finally came together, and we saw that the energy net-zero goal could have a seven-year payback, our Energy Management Team and our leadership at the City were on board. It’s been an awesome journey and has been really cool to see us hit the Energy Net Zero target on time and on budget. It makes financial and environmental sense.”

The fog tanks and receiving station.

The numbers really do tell the story. “We had a $50,000 a month energy bill from PGE. Now we have a $4,000 a month bill but are also generating $350,000 in revenue per year from FOG tipping fees, hence the relatively quick payback.” The numbers are also equally impressive in terms of greenhouse gas reductions. Being in the hydropower-rich Pacific Northwest, it’s possible to achieve a light greenhouse gas power mix, but looking at it on a national average, this project reduces our overall carbon impact significantly. Purchased power has seen a reduction of 3,100,800 kWh since 2007 i.e. 2,308 MT CO2e of greenhouse gas emissions (Fig 1). This means that a net zero energy WWTP keeps 494 passenger vehicles driven off the road for one year or can power 346 homes’ electricity use for one year or carbon sequestered by 2718 acres of U.S. forests in one year. (Source: EPA’s GHG equivalencies calculator). It’s commendable that in switching to 100% renewable energy, they have saved about $4.9 million and avoided $2.1 million in utility costs since 2006 (Fig 2). This financial and greenhouse gas win came alongside the LED streetlight switch and helped frame the conversation for further work both internally and externally at the City.


Internally, the City continues to strive to operate in a financially responsible manner for all ratepayers and taxpayers, while also doing its part to take care of the environment. The City has since made upgrades to lighting around City facilities, purchased low-emission, fuel efficient vehicles, and employs waste reduction and sustainability outreach staff that work with the community to help make businesses and citizens more sustainable. Shannon Martin, Recycling and Solid Waste Manager, states, “We look at the WWTP story as a huge win for the city, and an inspiring model of how other businesses can think in the short and long term about how they can become both more financially and environmentally sustainable. We work with businesses to help them strategize and implement changes through our Gresham Green Businesses Program. We also bring them together at monthly coffee hours to see and talk with other businesses that have implemented changes and to motivate one another.”


Energy consumption has increased over the years (Fig. 1), owing to Gresham’s growth. However, WWTP’s on-site renewable energy production has kept pace with its growing consumption. Purchased power has seen a reduction of 3,100,800 kWh since 2007 i.e. 2,308 MT CO2e of greenhouse gas emissions. This means that a net zero energy WWTP keeps 494 passenger vehicles driven off the road for one year or can power 346 homes’ electricity use for one year or carbon sequestered by 2718 acres of U.S. forests in one year. (Source: EPA’s GHG equivalencies calculator). It’s commendable that in switching to 100% renewable energy, they have saved about $4.9 million and avoided $2.1 million in utility costs since 2006 (Fig 2). Note: 4.9 million including FOG Revenues, $3.4 million with just Cogen1, Cogen 2 and Solar.


Over the years generation of power from renewable sources has continued to increase over the years. Major scaling took place from 2010 onward to help attain Energy Net Zero status (Fig. 3). For the past three years the WWTP has been a net exporter to the grid.

Energy Net Zero status has been met by a combination of methods (Fig. 4). The plant has worked to reduce its overall consumption of energy, installed an expansive solar array and installed two cogeneration systems with FOG injection beginning in 2015.

Consumption of energy as the Gresham WWTP has been reduced by installing several efficient systems. It also may have been impacted by a reduction in flow during the Great Recession (2008-2012). Production of energy was greatly increased by adding cogeneration system 2 and injecting FOG starting in 2015.

Replicating Elsewhere

Concepts that have been learned and applied in Gresham can be used elsewhere, especially at medium to large wastewater treatment plants. Johnston remarks, "The main function of what is done in our Energy Net Zero Wastewater Treatment Plant can be done in many municipalities around the country. Adding FOG to a municipal WWTP makes sense if you have some extra capacity in your existing anaerobic digesters or are looking at expanding your digesters. Being in a central, easy to reach location for haulers helps. These are all things to consider." Johnston adds, "If you're thinking of doing something similar, study your area, show your ROI. Figure out what your tip rate would be to make the system pencil out and see if that rate makes sense in your local market.” Gresham staff looked at and projected future electrical utility rates and FOG tipping fees and were able to create a financial model. Management and staff were a bit skeptical of the cost effectiveness of almost $10 million dollars in projects over five years, but because staff had a detailed accounting of the payback, costs, grant opportunities, avoided utility costs and FOG tipping fee revenues, the capital projects were approved, and the rest is history.


Alan showing the power mix to a Gresham Green Business Program tour group.

Alan showing the power mix to a Gresham Green Business Program tour group.

Johnston added, "We estimated at the beginning that almost 40% of our capital projects could be paid for with grants from the Energy Trust of Oregon and the Oregon Department of Energy.  In the end, we got 38% or $3.5 million in grants for the projects. We wrote all our own grant applications. You don't get money if you don't apply. You need to have a good engineer, reliable cost analysis, and support from the top. There is excellent guidance on cothickening, codigestion and FOG via the EPA. There are tons of resources there. EPA even tracks co-thickening plants and maintain a database. You can also get information from DEQ (Oregon Department of Environmental Quality) and the Oregon Department of Energy and peruse databases on biogas from around the country though resources like BioCycle Magazine. There are several biogas professional organizations."

Johnston’s concluding advice is, “Always, first and foremost, save energy. It’s the biggest bang for your buck - low energy turbo blowers, fine bubble diffusers in aeration basins, linear motion mixers in digesters - we did 3 projects that dropped the plant’s overall energy usage by 17% before we started the FOG receiving and cogeneration projects. Always set goals that are better than where you are now, and good things will happen. Work with your green team and other partners. Some states still don't allow export of power within the grid. Some years ago, Oregon changed the net meter rules that allowed us to export to the grid. Continue to work with your local and state partners to modernize the regulations to allow for innovation and efficiency. It's an ongoing process; you're never really done, but there is a lot of enjoyment, satisfaction and much good that comes with working towards becoming energy net zero."

To reach Alan Johnston for more information or to schedule a tour of the Gresham Wastewater Treatment Plant, contact him at Alan.Johnston@GreshamOregon.gov, or 503-618-3454.


#biogas #cogen #renewables #solar #FOG #Gresham

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