The Daily Churn

May 30, 2021 | by Ashley Hanson

Recycling phosphorous in cow manure to make sustainable fertilizers

River landscape stock photo

Can’t live without it

Phosphorous is best enjoyed in doses. We can’t live without it, but too much exposure can suck the oxygen out of life. It’s also in chronically short supply, with around 72 percent of phosphate reserves held by Morocco according to the 2016 United States Geological Survey. A phosphorous (P) recycling pilot at Washington State University (WSU) shows early promise of addressing these concerns by converting cow manure into a slow-release P fertilizer, or struvite.

One of five dairy-related projects across the state to share $4 million in grants from the Washington State Conservation Commission (WSCC), the project responds to three driving factors: “1) diminishing supply of non-renewable P supplies, 2) the need for limiting the build-up and subsequent loss of P from soil, and 3) the high cost of fertilizers, according to their final report published on WSCC’s website.

Joe Harrison, the WSU nutrient management specialist spearheading the research, is working with 30 dairy farms across Washington to finesse a process for recycling the P in manure so that farmers can restore just the right amount of nutrients back to the soil, where it feeds the next round of crops. Low in moisture and more like sand, the resulting struvite is also easier to handle and transport than cow manure.

Balancing act

“If applied in excess, if the crops can’t take it up or if Mother Nature doesn’t cooperate, that extra nutrient has the potential to run off the field and enter our surface waters,” says Nichole Embertson, a nutrient management specialist at Whatcom Conservation District in Washington. “We’re trying to use technology to help us custom-balance these products to minimize overloading and any type of a loss pathway to our waterway.”

Embertson, who works closely with dairies on various aspects of their nutrient management programs, says phosphorous-recycling technology can go a long way to preventing problems. And Harrison’s research appears to bear that out.

“To extract the phosphorous in the form of struvite from manure, we need three key nutrients at the right concentrations,” says Harrison. “Its compound is a combination of magnesium, ammonium and phosphate.” 

After removing solids from cow manure, the liquid manure is mixed with sulfuric acid to bring the pH below 6, freeing calcium from the phosphorous. Up to 800 gallons of this mixture is then pumped into a giant 15-foot cone. 

“Inside that cone, we have a bed of the struvite, which looks like sand,” says Harrison. “You pump the liquid manure up through this bed and as it flows up through that sand-like material, it keeps the bed fluidized.” 

Ammonia water is mixed into the fluidized bed, resulting in the formation of more struvite crystals, ready to be used as a fertilizer. The liquid manure, now stripped of most of its phosphorous, overflows out the top of the cone and is later applied to crops. 

Who wants struvite

Harrison says costs for running the system vary. Some dairies may only need to run the recycling system for a month, while others may need four months. This makes it challenging to estimate costs, according to Harrison, though he says a 2,500-cow dairy could expect to pay approximately $0.44 – $0.45 a day per cow. 

This investment only makes sense for dairy farmers if there is a market for the struvite, which Harrison suggests there is. For starters, the low supply of phosphate has driven up the cost of fertilizers. From 2002 to 2011, the dollar value of imported fertilizer to the United States increased 466.2 percent according to a 2013 Bureau of Labor Statistics report (PDF).

A local supply of struvite could offset costs and transportation while also creating a positive feedback loop on farms.

With a moderate release, accompanying magnesium and nitrogen and ease of transportation, says Harrison, it’s a desirable product for other agricultural producers — including people who grow alfalfa, corn, triticale, potatoes or greenhouse crops.

Plus, his research shows that the recycled phosphorous is on par with other fertilizers in terms of productivity.

“In short, what we’ve learned so far from the field demonstrations with the supporting data of the plot work,” according to the findings, “is that Struvite can provide enough P in the first year after fertilization to reach comparable yields and P uptake as MAP.”

Early adopters

“We were early adopters,” says Andy Werkhoven, one of the farmers who worked with WSU. He and his brother Jim joined their parents’ dairy in Monroe, Washington after graduating college in 1984. It was clear to them from the start that urbanization pressures would require them to get innovative with their manure management. 

“You have to be really kosher with everything you do with manure,” he says. “If we can make manure better, we need to do that.” 

Currently the Werkhovens aren’t planning to sell off any of their nutrients (or cow manure). Their farm plan — designed to support several families — involves purchasing more ground to raise more crops. But Andy says they are preparing themselves for any eventuality and have teamed with WSU to make the most of their on-farm nutrients. 

“We have to maximize those things,” Werkhoven said. “So, we’re going to spend the money. We’re going to get it done and we’re going to move forward.” 

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