Transforming nutrient-rich residues to resources through a circular economy

The approach

To begin building a circular bionutrient economy, we embarked on a pilot project to introduce the first pyrolysis system (pyrolysis is heating under the exclusion of air) to create biochar from dairy manure on a commercial dairy farm, Spruce Haven Farm, in Upstate New York in 2023. We explored whether biochar could be produced from digested dairy manure cost-effectively and whether the energy generated by pyrolysis was sufficient to dry the dairy manure, allowing us to convert dairy manure into biochar without the use of fossil fuel energy. We were also interested in the overall nutrient balance to understand how nutrient use could be maximized through manure processing. A commercially-available pyrolysis system was installed at Spruce Haven Farm and adapted to local manure handling needs, including a screw press system that separates liquid from the solids in manure. The team developed a heat recovery system that uses the heat generated by pyrolysis to dry manure solids.

The findings

We found that the pyrolysis process generates sufficient energy from the dry manure solids (221 MJ/hr) to dry the wet solids (34 MJ/hr) and cover all electrical power needs to operate the plant (8 MJ/hr). A weight reduction by 96% and volume reduction by 81% allows more efficient storage and transportation of the manure solids. 

Nitrogen and phosphorus (as phosphates) are key nutrients to consider. Over 93% of the nitrogen is contained in the separated liquids, which is usable as a fertilizer, but is ideally recovered as a dry material, because the nitrogen is so diluted in the liquids that it cannot be efficiently stored and transported. When the manure solids are pyrolyzed, the nitrogen is no longer available as a fertilizer; however, this is only a very small proportion of the total nitrogen. A greater proportion (about one third) of the phosphate remains in the separated solids. After pyrolysis, phosphate in the biochar is 70% more concentrated and 13% better available to plants than untreated dairy manure solids. A 179 MJ/hr excess of energy that is not needed in running the pyrolysis operation may be utilized for other farm needs. One example could be heating dairy manure as required in the process of anaerobic digestion to produce methane as “green energy”. Fossil heating fuels pose a large financial cost to the farm and reduce net green energy credits. 

The implications of this work for the neighboring Owasco and Seneca Lake Watersheds are reduced needs for managing dairy manure as a waste.  Moreover, the separation of phosphate and nitrogen in the biochar facilitates phosphate application when needed by the crop without consideration of nitrogen. Application of the nutrients can also take place at a time when negative environmental impacts such as those associated with excessive runoff can be minimized.

Next steps

In an effort to introduce this promising technology to farmers, policy makers, and agricultural stakeholders in New York State, we have hosted monthly outreach events. We will continue to hold these Open House events to explore the possibility of installing more and larger pyrolysis units and to develop and demonstrate a nutrient recovery system that allows nitrogen and phosphate removal from the separated liquids for use as biochar-based fertilizers. We intend to prototype a full energy recovery for heating water to wash cows before milking or heating dairy manure for anaerobic digestion. We are also interested in prototyping the approach in wastewater treatment and source-separating toilets at household and community scales in New York State as well as globally.  We also intend to examine the potential for this technology to mitigate harms from nutrient pollution and compounds including PFAS and microplastics, in the context of human excreta or sewage sludge. 

Society needs effective regional and global policies to support the development, adoption and diffusion of technologies such as pyrolysis that reverse the wasteful and environmentally damaging effects of current practices for managing organic residues. Moreover, the global fertilizer industry must reinvent itself from a mining industry dependent on fossil fuels to one based on nutrient recycling.