Wastewater Treatment and Biofuels, Part 1: Poplar Trees for Treatment and Resource Recovery in the Pacific Northwest

This post is based on my lab group's recent Journal of Environmental Engineering publication entitled: Populus Species Mitigate Nitrogen Breakthrough in Wastewater Infiltration Systems for Year-Round Treatment and Recovery. This is the first of three publications based on my PhD research and dissertation.

The global economy in the 21st century is predominantly linear in nature. Raw materials are mined, used for their purpose, and thrown away. Depending on the nature of the item, there may be some attempts at recycling or reuse first, but the ultimate destination for many products and materials is the landfill.

The circular economy challenges this linear narrative by asking how we can design products to stay in circulation for as long as possible. Goals include the creation of products that can be repurposed multiple times throughout their lifecycle without losing value, the incorporation of easily recyclable materials, and the design of manufacturing processes that inherently create less waste (USEPA, 2021).

In short, a circular economy considers the entire lifecycle of a product from manufacturing through disposal, and tries to put off that disposal for as long as possible. Rather than an ending, the final life stage of a circular product is seen as a new beginning for the materials it contains. While society is nowhere near ready for a large-scale circular economy that includes all or even most products, individual industries have made great strides in designing products and manufacturing processes that move towards the circular model. One example is the biofuels industry, which is of growing importance in the Pacific Northwest.

Biofuels in the Circular Economy

Biofuels are ethanol (and diesel) fuels made from renewable plant resources. First-generation biofuels were produced mainly from corn and sugarcane. Advances in the technology for processing lignin, a tough biomolecule produced by woody plants, have allowed us to utilize other, non-food sources for biofuels. These include certain species of trees, switchgrass, and byproducts like corn stover from harvested corn and bagasse from processed sugarcane. Biofuels from these sources are called lignocellulosic biofuels because the lignin has been processed in addition to the cellulose.

Biofuels, particularly those made from non-food crops, have many benefits. Lignocellulosic biofuel crops can be grown on low-quality land that cannot support food crops. Communities that choose to start growing biomass experience job creation and other economic benefits. And attempts to increase the circularity of the biofuel industry have led to the discovery of new products that can be made from lignin and other byproducts, allowing us to create additional value from waste.

Another benefit is that biofuels can be easily slotted into existing infrastructure, and they have a lower carbon footprint than traditional fuels. Most vehicles can run on ethanol mixtures containing biofuels, and there are no major infrastructure changes required to add biofuels to current gasoline mixtures. This makes biofuels a convenient solution that pairs waste reduction with positive climate impacts.

The University of Washington's School of Environmental and Forest Sciences has a small but dedicated group of researchers studying biofuels. One large-scale project undertaken by the group just before I began my own PhD research was an economic and logistical exploration of constructing a modest-sized biorefinery in central Washington state (Chowyuk et al., 2021). While the biorefinery has yet to become a reality, the analysis provided several new research directions that the group has pursued in the years since. One is exploring a method for on-site wastewater treatment using the same biofuel crop that the facility is designed to process: hybrid poplar trees.

Land Application for Wastewater Treatment: A Nature-Based Solution

For better or for worse, circular economies cannot be created without the proper financial incentives and systems in place. A major goal of the circular economy, then, is to valorize waste, to make it profitable to recover waste materials rather than sending them for disposal. This means making useful products out of materials otherwise destined for the landfill.

Expensive wastewater treatment technologies exist which can help with resource recovery. They can pull nutrients from the wastewater and process them into something usable--at very high energy and financial costs. For more economically feasible treatment options, we can turn to a nature-based solutions for more direct recycling of waste into something useful.

Included in the extensive plan for biorefinery development was a proposal for on-site wastewater treatment via land application enhanced with poplar trees. In land application, wastewater is applied to soil at or below the surface. Natural processes in the soil, including microbial activity and filtration by the soil matrix (de Bustamante 1990), break down pollutants as the water moves through the system. Land treatment is employed around the country in a variety of contexts, including rural communities with small populations where advanced treatment technologies may be cost-prohibitive.

The effectiveness of land application is enhanced when trees or other vegetation are planted on the site, sometimes called a vegetation filter. Vegetation provides a direct route for nutrient removal through uptake and incorporation into plant tissues. Plants also enhance microbial activity in the soil, because the root structure of the plant can support a larger and more diverse microbial population than soil alone (de Miguel et al., 2014).

At the proposed biorefinery, a poplar vegetation filter would be established beside the facility. Wastewater would be partially treated on site and then run through the vegetation filter to remove trace nutrients. The poplars grown on site would be used by the biorefinery and, at the scale proposed, could meet over one-third of the biorefinery's yearly feedstock needs.

As part of my PhD research project, I demonstrated the feasibility of this on-site treatment and biofuel generation strategy. With the help of my advisor and many dedicated undergraduate research assistants, I tested poplar vegetation filters for wastewater treatment in the controlled setting of an outdoor greenhouse space.

The Project - Testing Land Application in the Pacific Northwest

The aim of our research was to quantify how well poplar trees can remove nitrogen and carbon pollution from wastewater. To do this, we established a set of test reactors at the University of Washington greenhouse. We started with tiny poplar saplings, generously donated by a Washington state organization focused on breeding region-specific poplar strains. The trees were grown in pots for two years, then planted in reactors made from large steel troughs, the kind that farmers use to feed and water livestock. We laid distribution tubing over the surface of the soil, punctuated with tiny holes to drip-irrigate the soil surface, and covered the reactors to prevent rainfall from affecting our experiments.

Then, we irrigated the reactors with partially-treated wastewater, similar to how it would be done at the theoretical biorefinery. Instead of real wastewater, we used a synthetic recipe that our lab developed to make large volumes of wastewater with specific nitrogen and carbon concentrations at a reasonable cost per gallon (Kargol et al., 2023).

We irrigated the reactors and collected the water from ports drilled into the bottom of the troughs. Using test kits designed specifically for water quality monitoring, we tracked how much carbon and nitrogen were in the water before and after it traveled through the system.

We also had two control groups, each testing different aspects of the poplar land application system. We kept some of the reactors as bare-soil controls, meaning no trees were planted. As I discussed in my post about microbial habitats, soil particles host microbial populations on their surface, but the concentration of microbes in soils with plants is up to 100x higher because the roots provide additional surface area and nutrients. We wanted to compare performance of the microbes alone against the performance of microbes and trees together.

We also designated a subset of the reactors with trees as tap water controls, meaning they were irrigated with tap water instead of the synthetic wastewater mixture. This was to track differences in tree growth when the trees received nutrient inputs via the wastewater. The final site set-up looked like this:

The shed in the back held a complicated set-up of pumps and controllers designed to dispense the correct volume and the right type of water (wastewater or tap water) to each reactor. We irrigated the reactors, collected samples, and tested performance for a total of 18 months.

We also monitored several other properties in the system, each of which we hoped would give us additional information about the nutrient removal in the reactors. We assessed the nutrient content in the leaves and the organic matter in the soil, to give us more information on where the nutrients from the wastewater might be going. Finally, we monitored soil moisture because it is an important factor for maintaining proper nitrogen cycling. Soil microorganisms require plentiful oxygen for the first part of the nitrogen cycle, and oxygen levels near zero, which often develop in soil water pockets, for the second part of the cycle. Monitoring soil moisture helps us understand whether both oxic (oxygen-rich) and anoxic (oxygen-free) environments are present in the soil.

So what happened to the nutrients in the wastewater? Were there differences between reactors with and without trees? How did the findings apply to what we're hoping to do with land application treatment at the biorefinery and beyond? Come back next week to find out! In part two of Wastewater Treatment and Biofuels, I will detail the results of our study and their implications for poplar land application treatment in the Pacific northwest.

Want to know more about how microbiology intersects with the biofuels industry? Reach out to AppliedMicrobio today to schedule a chat!

References

de Bustamante, I. 1990. "Land application: Its effectiveness in purification of urban and industrial wastewaters in La Mancha, Spain." Environmental Geology and Water Sciences 16: 179-185.

Chowyuk, A. N., El-Husseini, H., Gustafson, R. R., Parker, N., Bura, R. and Gough, H.L. 2021. "Economics of growing poplar for the dual purpose of biorefinery feedstock and wastewater treatment." Biomass and Bioenergy 153(9). 10.1016/j.biombioe.2021.106213.

Kargol, A. K., Burrell, S. R., Chakraborty, I. and Gough, H.L. 2023. "Synthetic wastewater prepared from readily available materials: Characteristics and economics." PLOS Water 2(9). 10.1371/ journal.pwat.0000178.

Kargol, A. K., Montes, D., Dahal, S. and Gough, H. L. 2025. "Populus Species Mitigate Nitrogen Breakthrough in Wastewater Infiltration Systems for Year-Round Treatment and Recovery." Journal of Environmental Engineering 151(12): 04025082. doi:10.1061/JOEEDU.EEENG-8180.

de Miguel, A., Meffe, R., Leal, M., Gonzalez-Naranjo, V., Martinez-Hernandez, V., Lillo, J., Martin, I., Salas, J. J. and de Bustamante, I. 2014. "Treating municipal wastewater through a vegetation filter with a short-rotation poplar species." Ecological Engineering 73: 560-568. 10.1016/ j.ecoleng.2014.09.059.

United States Environmental Protection Agency. “What Is a Circular Economy?” United States Environmental Protection Agency, 3 Nov. 2021, www.epa.gov/circulareconomy/what- circular-economy.