A Fungus Among Us: Improving on Composted Digestate
Posted by Embrey Bronstad | November 30, 2020
This is part of a series of posts highlighting work by Washington State University researchers through the Applied Bioenergy Research Program of the Agricultural Research Center at Washington State University, College of Agricultural, Human, and Natural Resource Sciences.
I really didn’t like this fungus joke at first. But it’s growing on me.
And thank goodness for that, because fungus may just be a key component to addressing another issue ripe for CSANR blogpost humor: cow poop.
Washington is home to almost a quarter million dairy cows, and as we all know, they make manure as well as milk. That manure is full of nitrogen and phosphorus, two essential ingredients for soil and crop health. While some of the manure can be land applied as fertilizer, agronomic application rates of these nutrients prevent complete utilization of this impressive resource. There is either insufficient acreage on which to distribute the nutrients, the nutrient loads would be too high for the crop in question, or the hauling of the manure to nutrient-scarce areas is too costly. There is therefore an opportunity, some might say a need, to explore other ways to utilize nutrients generated by dairy farms that could transform the manure into a more saleable product that benefits dairy farmers and Washington state both economically and environmentally.
Composting manure is a practical option for generating a stabilized and sanitized end product. Composting anaerobically digested manure is even better, as you get the added benefits of further waste stabilization and pathogen reduction while generating biogas and, therefore, energy. But digesters have a hard time breaking down lignocellulosic material, which makes up a good part of the dairy fiber that makes its way into the digester (Figure 1). Depending on the fiber load into the digester, the digestate, or what comes out of the digester, can vary in quality. And quality variations impact downstream uses.
Dairy fiber is notoriously recalcitrant: the hard plant cell walls are extremely resistant to degradation. Several industrial chemical and physical pretreatments exist to break open the plant cell wall and release energy-rich sugars. These treatments, however, are not realistic options for a dairy: they typically involve hazardous chemicals and solvents (hydrochloric acid and sodium hydroxide, to name a few), generate toxic by-products that would have to be specially disposed, or are energy intensive. A few physical pretreatments, such as milling, may require fewer inputs, but it is still a piece of equipment that requires capital and operational expenditure. If there were just a way to safely and economically treat the fiber…
There is! Researchers in Dr. Shulin Chen’s lab developed a biological solution. The idea was to enhance dairy fiber degradation through a white-rot fungal pretreatment. Why? Fungi are legendary lignocellulose crumblers (Figure 2), and fungal degradation of plant matter also generates humic acids. Humic acids, in turn, provide substantial benefits to soil and plant health, which would make fertilizer derived from this process more environmentally and ecologically valuable. The team inoculated carbon-rich lignocellulosic biomass (poplar and wheat straw) and nitrogen-rich manure with the fungus Ceriporiopsis subvermispora in a lab-scale composting process to mimic a common digestate management method. Not only was the resulting compost rich in humic acids, but the process was also successful at temperatures typically found in compost maturation stages (~100°F) rather than the higher thermophilic compost temperatures (~125°F-150°F). Lower temperatures are easier to maintain, and lower temperatures also reduce the amount of nitrogen lost as volatilized ammonia. Thus, the digested and composted manure contains not only beneficial humic acids but retains more of the macronutrient nitrogen in the end as well.
An optimized treatment scheme for pairing these two biologically-driven, waste-to-resource processes is still being researched and developed. But by starting with white-rot fungi, we can maintain mild conditions with little energy input, no wastewater production, and no generation of toxic compounds that affect subsequent processes. White-rot fungal composting of residues from anaerobically digested manure could be a treatment scheme that produces both renewable energy through biogas generation and a compost richer in humic acids for a more consistent and saleable fertilizer end product. This means that the dairy farmer, once limited by the availability and nutrient limitations of surrounding cropland for application of manure, can potentially generate a saleable product that is capable of wider distribution. Not only does this help with nutrient sharing between rich and deficient areas, but the farmer also derives an economic benefit.
Xiao, Fu, 2019: Manganese Assisted Fungal Treatment of Lignocellulosic Biomass for Biofuels Production, PhD Dissertation, Washington State University
Lipczynska-Kochany, Ewa, 2018: Humic substances, their microbial interactions and effects on biological transformations of organic pollutants in water and soil: A review. Chemosphere, 202, 420-437, https://doi.org/10.1016/j.chemosphere.2018.03.104.