The phosphorus in liquid manure can be valuable as fertilizer, but only if it’s applied where it’s needed. In areas with concentrated livestock production, that can be a problem because so many fields already have high phosphorus levels and transporting large quantities of liquid manure isn’t practical.
One possible solution is to remove the phosphorus from the manure, according to Theresa Dirksen, Mercer County Ag Solutions coordinator. Dirksen has been working with livestock producers, in Mercer County, Ohio, to test methods of removing manure phosphorus.
“The question is whether they’re scalable and affordable,” she said.
The organization conducting the research, Mercer County Ag Solutions, was started in 2011 by a group of farmers in the Grand Lake St. Marys watershed. They joined together to seek out practical solutions to the water quality problems affecting the lake while also sustaining the vitality of the area’s farms.
In 2016, the Mercer County Commissioners provided funding for the group to hire Dirksen to help coordinate the group’s efforts. The manure phosphorus removal project was started in 2017 and in 2020 the group received a grant from the Ohio Water Development Authority to further the research.
Last summer, the research focused on deep pit swine manure. The researchers started by removing fibrous solids from the manure using a KDS rotating disc separator. The Japanese company Kendensha Co. manufactures the separators for use in the food processing industry and wastewater treatment, as well as in agriculture. When the manure goes through the separator, most of the phosphorus remains in the liquid coming through the machine, Dirksen explained.
The solid portion of manure could be applied to farmland but also has the potential to be a marketable product, she added. For instance, a composting company picked up a roll-off container of the solids and combined the solids with other materials to make compost.
To remove the phosphorus from the liquid portion of the manure, the researchers are testing a patented process developed by a North Carolina company, Renewable Nutrients. The “quick wash” process uses an acid solution added to the liquid manure portion. The acid breaks chemical bonds to convert manure nutrients to a liquid form.
Then, when lime is added to the liquid, it bonds to the phosphorus, forming calcium phosphate. To reduce the cost of the process, the Ag Solutions researchers tried skipping the acid wash step. They found they could recover nearly as much phosphorus without adding acid, Dirksen said. Another benefit of eliminating the acid is they don’t have to manage the foaming that tends to occur when acid is mixed with manure liquids.
After lime is added to the manure liquid, the next step is separating the calcium phosphate from the remaining liquid. Part of the research this year involved using a geotextile bag inside a settling vessel, to filter out the calcium phosphate.
That study proved geotextile bags are not a practical option, Dirksen said. For one thing, the bags are bulky and difficult to wrestle into position. They can be used only once, so waste is a concern as well, she said. The bags also require the use of polymers to flocculate the calcium phosphate.
The cost of the bags and polymers would add at least 15 cents per gallon to the manure processing cost, Dirksen added. “The geo bags are not attractive financially at all.”
Another dewatering method shows much more promise, Dirksen said. They tested a multi-disc screw dehydrator made by Ekoton, an international company that focuses on wastewater treatment systems. It requires less polymer than the geo bag system and has a low power demand. The dewatered calcium phosphate cake comes out of the dehydrator at 20-30% solids.
Dirksen is currently looking for fertilizer dealers or manufacturers interested in the recovered nutrients. She’s hoping sale of the phosphorus could help offset the processing costs and also make good use of the nutrients.
“That’s the goal: to get manure nutrients where they’re needed and out of the areas where they’re not needed,” she said.
Currently, the process being tested in Mercer County is able to remove more than 90% of the phosphorus from the manure stream at a cost of 2 to 4 cents per gallon of liquid manure treated. They’re hoping to get the cost down to 1 cent per gallon.
“It’s not ideal yet, but we’re getting closer economically to where we need to be,” Dirksen said.
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