COLUMNISTS

Struvite capture:  A potential tool for managing manure phosphorus

Jamie Patton
Midwest Manure Summit is completely virtual this year.

Ice is on the lakes, snow is on the ground, and for many of us, we are deep into the 2021 nutrient management planning season. As we plan for the efficient use of our on-farm nutrient sources, we know transportation costs often make long-distance manure hauling uneconomical. But what if we could change that? What if we could capture manure-derived phosphorus in a concentrated solid, thereby reducing transportation costs and increasing the ease of handling?

It is estimated approximately 27 percent of the phosphorus consumed by a dairy cow is excreted in her milk. A very small amount of the remaining phosphorus is used to build her own tissues, with a majority ending up in the manure stream. Based on first-year nutrient availability, dairy manure slurries (4.1 to 11 percent solids) are estimated to contain 6 pounds of P2O5 per 1,000 gallons. While liquid dairy manures (4 percent or less solids) have a first-year nutrient book value of 3 pounds of P2O5 per 1,000 gallons.

The low concentration of phosphorus and other nutrients in dairy manure is an economic challenge for transport, often resulting in inefficient applications to fields closer to the barn. Additionally, the ratio of major nutrients - nitrogen, phosphorus, and potassium - in dairy manures often does not match the nutrient ratios required by plants, resulting in an overapplication of phosphorus when applying manures to meet crop nitrogen needs.

Off-site movement of soil phosphorus, via erosion and runoff, can result in water quality concerns. Fresh waters, such as rivers, lakes, and streams, typically contain a limited amount of available phosphorus. Therefore, when excess phosphorus is introduced into these waters, overgrowth of algae can occur, potentially resulting in poor water quality, low dissolved oxygen levels and fish kills, degraded aquatic habitats, and/or the production of deadly cyanotoxins, if high populations of blue-green algae are present.

To address these issues, Washington State University researchers have developed a mobile extraction system to concentrate and solidify manure-derived phosphorus. The system utilizes fluidized bed technologies to extract phosphorus and convert it into crystalline (solid) struvite (NH4MgPO4·6(H2O)).

In simple terms, the technology involves the pumping of anaerobically digested manure through an inverted cone containing a starter amount of struvite. Sulfuric acid and injected ammonia gas are used to manipulate the solution’s pH, promoting the formation of struvite from the flowing liquid. According to the Washington State University website, phosphorus extraction efficiencies of anaerobically digested manures range from 60 to 80 percent. 

The struvite solids produced by this system are approximately the size of sand, low in moisture, and have a fertilizer analysis of 6-29-0 (+16 Mg). When applied to the soil, the material acts as a slow-release fertilizer and has been shown to perform similarly to conventional phosphorus fertilizers in triticale, corn silage and alfalfa field trials.

By concentrating and solidifying the manure phosphorus, this system creates a product that is not only easier to handle than liquid manure, but can also be exported greater distances, helping farms utilize nutrients more efficiently and economically across fields covering a larger geographic area.

If you are interested in learning more about enhanced manure handling and processing techniques, such as struvite capture, I encourage you to check out this year’s Midwest Manure Summit. This free, online event will be held February 24th from 9:00 am to 3:15 pm via Zoom. Pre-registration is required by 5 pm, February 22, 2021 at http://midwestmanure.org. The first 100 registrants will be mailed a free “goodie bag” filled with resources and materials to complement the webinar format and topics.

The 2021 Midwest Manure Summit is sponsored by the UW-Madison, Division of Extension and College of Agriculture and Life Sciences. Questions regarding the  Summit can be directed to Heather Schlesser, Extension Marathon County at heather.schlesser@wisc.edu or 715-261-1230, ext 2.

Jamie Patton

Patton is a senior Outreach Specialist for the UW-Nutrient and Pest Management Program