Process takes liquid manure to drinkable water.
For every gallon of milk shipped from the front door of a dairy farm, around 2 gal. of liquid manure is produced and must be managed out the back door.
The Holy Grail, of course, is a manure processing system that could fully separate all the constituents of manure—sand, fiber, nitrogen, phosphorus, potassium and water—and reuse them to their best value.
Some companies have already tried—and failed. But McLanahan Agricultural Systems has entered the arena with its four-step process that takes raw manure and separates its constituents down to those six components: sand, fiber, nitrogen, phosphorus, potassium and water.
Fully processed, up to 50% of the water in manure is drinkable, potable water that could be legally discharged back to public waters or reused on the dairy for everything including watering cattle.
It’s not a simple process. That’s because it involves a pre-treatment vessel similar to a methane digester,
ultrafiltration, low-pressure air stripper towers, and reverse osmosis technology. “Our corporate values are to make things safe, simple and smart, but complicated problems sometimes take complicated solutions,” says Andrew Wedel, general manager of McLanahan Ag Systems.
“But what if you didn’t have to haul half of your manure and still had all the nutrients? Would that have some cost benefit?” he adds.
Ultrafiltration allows water and dissolved constituents, such as ammonia and potassium, to filter through a membrane while excluding particulates containing phosphorus.
The beauty of the system is that it is not a one-size-fits-all, take-it-or-leave-it technology. Instead, it can be adapted by individual farms to meet their individual needs, Wedel says. Some farms might only want to
produce tea water, separating out the phosphorus. Others might go the full route, fully separating out all of the nutrient components.
This past summer, McLanahan hosted an open house on the Chad and Evelyn Minnis’ Car-Min-Vu Farms near Webberville, Mich. to show the Nutrient Separation System technology. Car-Min-Vu Farms is the site of the McLanahan pilot project because it is located just 20 minutes southeast of Michigan’s state capitol. The project is partially funded by a grant from the Michigan Department of Agriculture and Rural Development.
The Minnises milk 800 cows (950 total with dry cows). At any moment, the manure from about 100 cows is flowing through the pilot project, after sand and manure solids have been separated.
The separated slurry flows first to a pre-treatment tank similar to a vertical methane digester. Here, the material is homogenized to produce a consistent liquid. Microbes in the tank also produce biogas.
Because the volume of gas is limited, it is simply flared off. In a full-scale version, biogas production would be sufficient to produce enough electricity to power the entire system, says Jim Wallace, the McLanahan engineer in charge of the pilot project.
From the pre-treatment tank, the slurry is forced through a series of ultrafiltration tubes—resembling long straws—that allow water and dissolved ammonia and potassium to filter through the membranes. Particulate matter, which contains phosphorus, is captured for further processing into fertilizer. That concentrates the phosphorus 3X, with up to 95% of the phosphorus in 30% of the volume. From there, it can be produced in either liquid or solid form for use as fertilizer.
The remaining tea water, containing the dissolved ammonia and potassium, is then blown at high speed through two, 16' tall air stripper stacks filled with baseball-sized, whiffle-like balls that provide surface area to capture and concentrate the ammonia. The system uses sulfuric acid to capture the ammonia. The stripper stacks take out just 1% of the volume, but create very concentrated ammonium sulfate fertilizer.
The remaining feedstock is then pumped at high pressure through a reverse osmosis (RO) system that removes the remaining potassium in a concentrated stream. The RO process removes about 15% of the volume as concentrated potassium.
The reverse osmosis process separates the feedstock based on molecular size, producing a concentrated potassium stream and clean, potable water.
What is left is pure water—clean enough to discharge into public waters or to reuse on the dairy. After it has made its way through the entire process, this clean, dischargeable water represents about half the original volume. “Fifty percent of the manure going into the system is converted to clean, dischargeable water—that is 50% of your manure volume that can ‘go away,’ ” Wedel says.
McLanahan says the system might be most applicable to large, commercial dairies with at least 3,500 cows.
At that scale, Wedel adds, the capital cost would range from $350 to $500 per cow, not including sand and solid separation. The cost also depends on how much processing is needed by the dairy. Operating costs of implementing the system, including labor, maintenance, chemicals and power, will be about 1¢ per gallon of manure.
Illustration shows four-step process to converting 50% manure liquids to drinkable water.