Wastewater Treatment Plant Generates Clean Energy, Has Room for Improvement

Last week, I had the opportunity to take a tour of the Columbia Boulevard Wastewater Treatment Plant in Portland, OR and learn firsthand about how Oregon’s largest sewage treatment plant is cutting energy costs and greenhouse gas emissions thanks to a cogeneration system running off the plant’s abundant biogas. While the plant should definitely be lauded for its commitment to reducing greenhouse gas emissions, I can't help but feel like there are still valuable opportunities to reap even greater energy savings.

Travis Leipzig, River Network’s water-energy intern, joined me for this adventure. Our tour guide for the day was Mike Stebbins, who was seasoned and knowledgeable and possessed of incredible patience for my incessant questioning about all things energy around the facility.

The Columbia Boulevard Wastewater Treatment Plant serves just over 600,000 people in the Portland metro region, treating an average of 73 million gallons of wastewater per day with a peak flow capacity of about 280 million gallons daily. It is located in North Portland, not far from where the Willamette and Columbia Rivers meet:

The reason this treatment plant has been built to handle nearly four times its average daily flow is because a large share of Portland operates on a combined sewer system which sends stormwater to the treatment plant. Prior to a recent expansion, the combined sewer system would overflow regularly, dumping raw sewage and dirty stormwater directly into the Willamette River.

While Portland has received a great deal of attention for reducing combined sewer overflows through its downspout disconnection and green streets programs, the folks at the wastewater treatment plant have also been implementing their own fair share of green innovations.

As most of you know, treating wastewater requires a lot of energy to pump the sewage through the treatment plant, operate all of the plant machinery, offices and control systems, and heat the sludge to around body temperature so that anaerobic bacteria can efficiently break the waste down.

Here's a picture of one of the plant's massive, energy guzzling pumps that can move up to 75 million gallons of water per day.

The organic matter in wastewater (i.e. poo) is relatively energy-dense, and much of the latent energy in sewage is converted into a biogas during the anaerobic digestion phase of the treatment process. About 1 million cubic feet of biogas consisting of about 55% of methane is produced each day by the Columbia Boulevard Wastewater Treatment Plant .

Wastewater treatment plants typically flare or burn their biogas (methane is a potent greenhouse gas), but a growing number of treatment plants - including the Columbia Boulevard Plant - are coming to view this waste as a resource and generate energy from the abundant biogas.

It should be noted that while biogas is widely considered a clean fuel, it is not carbon free. Rather, the biogas is carbon neutral because it contains, and releases once burned, the carbon that naturally exists in the sludge. Despite the carbon emissions released by burning biogas, harnessing energy from biogas is still beneficial as it offsets the CO2 that would have been emitted by the electric utility providing power to the treatment plant, and the emissions would have been released into the atmosphere anyways when the gas is flared.

At the Columbia Boulevard plant, about half of the biogas is consumed by two 850 kilowatt internal combustion generators which can create about 1.7 megawatts of electricity combined. The system is called cogeneration or a combined heat and power system because the waste heat released by the electricity generators is captured and used to heat buildings and warm the sludge heading into the facility’s anaerobic digesters. Additional biogas is consumed at the Columbia Boulevard plant by 4 microturbines, while some is sold off through a pipeline connected to a nearby roofing company.

Below on the left you can see a heat exchanger that harnesses waste heat and on the right are pipes sending biogas to one of the electric generators.

In total, the cogeneration system generates just over 12 million kilowatt hours of electricity per year, providing about 40% of the treatment plant’s daily power demand. The system results in more than $60,000 worth of energy savings and revenue from selling excess biogas, and with a price on carbon and higher energy rates likely in the future, these savings should continue to grow.

Yet despite the clear financial and environmental benefits that can be accrued by harnessing the energy from biogas, the plant only uses about 70% of what’s available and flares off the rest, as evidenced by the huge flame we spotted in the middle of our tour (sadly we forgot to take a picture of the flame!).

I had to ask our tour guide Mike why they’d waste such a valuable fuel by burning the biogas into thin air. I’m still not sure if I completely understand the logic, but here’s what I believe Mike explained to me(while noting that energy and biogas were not his expertise):

Due to technical issues with turning the system on and off, the cogeneration units have to operate continuously, therefore engineers have designed its capacity to match the least amount of biogas available on a given day. In other words, it is like the inverse of how wastewater treatment plants are designed to handle peak flows on days of heavy rain. Rather than build the system to handle the maximum load, as in the case of treatment plants, cogeneration systems have to be designed to run off of the lowest possible flow of biogas into the system.

Although I’m still not clear what the technical issues are with turning the cogeneration system on and off, or throttling it depending on the amount of biogas available, I do know there is a lot of biogas that isn’t being utilized. I spent some time barraging Mike with questions about this issue but ultimately let the issue rest because what was supposed to be a 1 hour tour had somehow ballooned into a 3 hours due to my relentless interruptions.

It seems to me that there are still a few things which could be done to utilize more biogas and further reduce greenhouse gas emissions at the Columbia Boulevard treatment plant. While I’m sure the folks at the treatment plant have already been scratching their chin over of many of these issues, I think that they are worth pointing out.

First, increase biogas storage and add more capacity to the 1.7 megawatt cogeneration system. If biogas storage capacity is expanded, biogas reserves could be built up and the supply of biogas available to the cogeneration system could remain more or less constant if large enough reserves could be built. This could possibly allow a larger system to be designed to run off the average amount of biogas produced by the digesters as opposed to the current design which runs on the minimum amount available.

Second, consider partnering with solid waste managers to augment the flow of biosolids (essentially fuel) into the anaerobic digesters. If food waste or other organic waste are collected, processed and added to the wastewater stream entering the treatment plant, more biogas could be produced in the digesters as the bacteria would have a larger food source.. Plus, there would be the added benefit of dealing with both solid and liquid waste streams at once.

East Bay Municipal Utility District (EBMUD) in the San Francisco Bay area is already doing this exact thing by collecting food scraps from local restarant’s and grocery stores and delivering them to a nearby wastewater treatment plant. Although their program is still being studied to determine the actual costs and benefits over time, the results thus far look extremely promising:

Anaerobic digestion of food waste has approximately three times the methane production potential by volume than municipal wastewater solids. The study found that the methane production potential of biosolids was 120 m3 gas/ton and food waste around 367 m3 gas/ton. Additionally, anaerobically digesting 100 tons of food waste per day, five days a week, provides sufficient power for approximately 1,000 homes.

Food waste is more readily digestible than wastewater sludge, thus a shorter residence time is needed to process the waste. The shorter residence time means that food waste can be processed in smaller digesters than municipal solids, resulting in lower capital costs. Additionally, a digester can accept much more food waste at one time than wastewater solids without adverse impacts

As the EBMUD study shows, food scraps are actually a more efficient feedstock for biogas digesters than wastewater, and it appears that utilities would reap nothing but benefits from adding solid waste to their anaerobic digesters. By adding food scraps, the wastewater treatment plant would have better control over what and how much they feed into the biogas digesters. Thus, the Columbia Boulevard plant could expand the capacity of their cogeneration system knowing that they could always produce more biogas by simply adding food scraps to the digesters on the days that the wastewater stream doesn’t have sufficient biosolids.

It also seems that Portland could easily begin to collect food scraps from local restaurants or markets to be sent to the treatment plant. The only real costs involved would be figuring out the logistics of processing and delivering the food scraps to the plant, but these costs would likely be small since the food waste would otherwise be delivered somewhere else and not likely reused. Many restaurants already separate their food scraps for composting, and I’d be willing to bet that the majority of restaurant owners in eco-friendly Portland would be willing volunteers if they knew that their food wastes were being used to create clean energy.

My limited understanding of biogas digesters is that they can be fickle mechanisims, so there is a chance that food scraps would have to be added to separate digesters to maintain optimum efficiency. But as the EBMUD study explains, “The shorter residence time means that food waste can be processed in smaller digesters than municipal solids, resulting in lower capital costs,” and, “an anaerobic digester can accept much more food waste at one time than municipal wastewater solids without adverse process impacts.” So all and all, even if new digesters had to be installed they wouldn’t cost much and are significantly more efficient at producing biogas than wastewater solids, implying a relatively rapid return on investment.

Third, if increasing biogas storage and adding food scraps to the waste stream aren’t feasible, the city or utility should consider creating incentives to find another consumer of their biogas (remember, the Columbia Boulevard plant already sells some biogas to a nearby roofing company). As reducing GHG emissions becomes more important – or, hopefully, legally mandated – the value of the biogas will inevitably rise so getting rid of it shouldn't be a tough sell.

The city could encourage gas using companies to relocate nearby the wastewater treatment plant in order to take advantage of the carbon neutral, locally produced renewable energy supply. Or, the utility could give the biogas away for free to nearby companies, potentially receiving credit for carbon offsets. To be honest, I’m not sure what policy levers are possible, but it seems like something could be done to encourage companies to utilize the 300,000 cubic feet of biogas that is wasted each day at the wastewater treatment plant.

In addition to finding a consumer for the plant’s biogas, the city could also try find someone to take advantage of any waste heat that isn't being utilized. I’m honestly not sure if all of the heat produced from the cogeneration system is used onsite, but if it is not, significant savings could be achieved by creating a district heating system that sends excess heat from the wastewater treatment plant to nearby homes or businesses.

As I mention these things that the Columbia Boulevard plant could do to be more sustainable, it’s worth noting that they are already doing a lot. One exciting project they have planned is a 500 kilowatt solar photovoltaic system which will generate electricity onsite and reduce the amount of electricity purchased from the grid.

The Columbia Boulevard plant’s biosolids program is also environmentally friendly:

Biosolids are a natural, nutrient-rich by-product of municipal wastewater treatment. Since 1990, Portland has hauled its dewatered biosolids to eastern Oregon for application to crop and pasture land. Biosolids add organic matter and nutrients to increase soil productivity and decrease erosion. Cattle grazing on pasture treated with Portland biosolids produce eight to 12 times more beef than cattle that graze on similar pasture where no biosolids have been applied.

The city’s biosolids land application program is sustainable and has environmental benefits, such as reducing greenhouse gasses, but a truck hauling biosolids to the application site and returning to Portland travels more than 400 miles. To possibly save fuel costs and reduce vehicle emissions, the city recently issued a request for proposals to consider alternative biosolids strategies to supplement the existing program.

In conclusion, the Columbia Boulevard Wastewater Treatment Plant has done a number of innovative things over the years. Most of them involve treatment processes, odor elimination and other aspects of wastewater treatment. Quite a few exciting strategies have already been implemented to make the Columbia plant more energy efficient and sustainable, but if the city of Portland wants to continue burnishing its green credentials, wastewater treatment plants should be turned into mini clean energy power plants by, utilizing every last molecule of biogas, augmenting liquid waste with solid waste, and increasing cogeneration capacity.