University of Manitoba researchers are hoping to kill two birds with one kidney stone by mining manure
Struvite is a naturally occurring mineral that forms kidney stones, says Dr. Don Flaten, soil scientist. The mineral, which can be made from hog manure, is also a phosphate source which is less water-soluble than commercial phosphate fertilizers.
Regular phosphorus fertilizers dissolve quickly, leaving high salt concentrations, which can kill seedlings if too much fertilizer is applied in the seedrow of sensitive crops such as canola.
Right now a farmer should apply no more than 20 pounds of phosphate per acre with canola seed. “And yet the crop usually needs more than that,” says Dr. Francis Zvomuya, research leader for the project.
Struvite releases phosphate much more slowly than fertilizers such as ammonium phosphate, says Zvomuya, and so less salt accumulates in the seed row. That might make struvite a useful in-row fertilizer for salt sensitive crops such as canola, corn and soybeans.
“There may be some potential to use this product in a very convenient fashion and still maintain our soil phospohurs concentrations. But the big question is, how does the product perform agronomically as a source of nutrients,” Flaten says.
In a study supported by the Canola Council of Canada, Manitoba Pork Council, and Agrium, Zvomuya and his colleagues found that uptake for the commercial and struvite fertilizer was the same.
But biomass yield, or total dry matter, at the flowering stage was lower for the struvite fertilizer. Yield differences were likely due to alkaline soil and the low solubility of the struvite fertilizer, which might not release phosphate fast enough to meet crop demand.
Zvomuya says moderate phosphate levels in the soil may have affected results as well. Researchers are now looking at how struvite performs in phosphate-deficient soils. They’re also using a canola-wheat rotation to mimic conditions on many Prairie farms, and examining how weather may affect phosphate toxicity, while applying several different treatments to plants in a greenhouse.
“Once we identify in the greenhouse what treatments are performing the best, then we take those out in the field to test them under field conditions,” says Zvomuya.
Researchers will also measure grain yield and quality, as well as other parameters, during field trials, Zvomuya says.
Dr. Nazim Cicek, a biosystems engineer at the University of Manitoba, devised the phosphate-extraction process. Cicek adapted the technology municipalities use for waste water to treat hog manure.
Cicek says the struvite extraction technology is not meant to replace traditional fertilizers, but instead solve an environmental problem while producing a high-value, niche product. “It would help livestock farmers with their environmental pressures and hopefully help overall sustainability in terms of replacing fossil-fuel based fertilizer with a green fertilizer.”
Inorganic struvite can be created in the lab, but the cost is prohibitive for most crops. Cicek’s method is cheaper and produces organic struvite, which benefits soil structure and other soil properties, Zvomuya says.
Cicek and his colleagues ran a pilot on a hog farm in Manitoba. Before extracting the struvite, settling tanks separated the solids from the liquids. Though most of the phosphorus is associated with the solids, which is typically used as fertilizer, 20 to 40 per cent of the phosphorus is left in the liquid.
The liquid is put into the struvite reactor, where it’s aerated. Aerating increases the pH levels and crystallizes the phosphorus, along with magnesium and ammonium.
“It’s just like having too much sugar in your tea. When you drop the temperature, some of that will come out. Or when you look at sugar in a jar of jam — when you cool it down, some of the sugar crystallizes out,” Cicek says.
The crystallized phosphorus sinks to the bottom of the reactor, and comes out as a white or greyish powder, or struvite.
The amount of phosphate extracted depends on a number of factors, including how the pigs were fed, the age of the pigs, whether the manure was stored or digested, and how much water is used for flushing.
“We’re looking at kilograms out of tons of manure,” says Cicek.
Cicek and his colleagues are looking at setting up the struvite reactor on the university’s research farm to fine-tune the process. The university farm has swine, beef and dairy operations. Beyond that, they’re considering installing it at a commercial dairy operation.
“They have some interest in bedding recovery and solid separation of their manure, which allows you to access the liquid fraction where the struvite can be made. So dovetailing to that kind of activity in that sector might help this particular technology,” says Cicek.
Liquid dairy manure lends itself to the process once it goes through an anaerobic digestion system, but Cicek would need to adapt the process to deal with higher calcium levels in dairy manure, which scavenges the phosphorus.
The extraction process could be set up on individual farms. But Cicek says selling struvite would not necessarily pay for the system installation and operation.
“On balance I’d suggest it is lower cost than alternative struvite extraction systems because it does not require you to dose in chemicals, which is a cost both of purchasing the chemical and storing. And adding it, so there’s labour issues involved in making sure that works,” says Cicek.
Cicek says they’ve looked at the economics of struvite extraction with the hog farm they initially worked with. The actual cost depends on several elements, including the farm’s size, the manure type, electricity cost, labour, and the struvite market. Scaling up and improving the technology would improve margins, but a market that paid a premium for struvite would have the biggest impact on the viability of the technology.
“We would like to test this particular pilot system on a number of platforms to see if it makes sense and go from there. I think it’s a little bit early to say that at the current energy and phosphorus prices this might be viable,” says Cicek.
“But with future trends, the way it looks I think we would have to be more careful about sourcing phosphorus and reusing and recovering it from a number of waste streams.” †