From Liquid Manure to Drinkable Water

June 9, 2014 07:01 PM
From Liquid Manure to Drinkable Water

For every gallon of milk shipped from the front door of a dairy farm, approximately two gallons 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 that manure—sand, fiber, nitrogen, phosphorus and potassium and water—and re-use them to their best value.

Some companies have already tried—and failed. McLanahan Agricultural Systems has now entered the arena, offering a four-step process that takes raw manure and separates its constituents down to those six components: Sand, fiber, nitrogen, phosphorus and potassium and water.

Fully processed, up to 50% of the water in that manure is drinkable, potable water that could be legally discharged back to public waters or re-used on the dairy for everything up to and including watering cattle.

It’s not a simple process, 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?"

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 individual needs, says Wedel. Some farms might only want to produce tea water, separating out phosphorus. Others might go the full route, fully separating out all of the nutrient components.

On June 5 and 6, McLanahan hosted an open house on the Chad and Evelyn Minnis’ Car-Min-Vu Farms near Webberville, Mich. Car-Min-Vu Farms are the site of the McLanahan pilot project of the Nutrient Separation System. The farm was selected because it is located just 20 minutes southeast of Michigan’s State Capitol. The demonstration project is partially funded by a grant from the Michigan Department of Agriculture and Rural Development.

The Minnises milk approximately 800 cows (950 total with dry cows). 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 not unlike 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 fully scaled 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 unltrafiltration tubes—large straws, if you will—that allows water and dissolved ammonia and potassium to filter through the membranes. Particulate matter, containing phosphorus, is captured—capturing up to 95% of the phosphorus in 30% of the volume. That concentrates the phosphorus 3X, and can be produced in either liquid or solid form and then used 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 filler 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 removed about 15% of the volume as concentrated potassium.

What is left is water—clean enough to discharge into public waters or for re-use on the dairy. Taken through the whole process, this clean, dischargeable water represents 50% of 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,’" says Wedel.

McLanahan says the system might be most applicable to large, commercial dairies with 3,500 cows. At that scale, Wedel says the capital cost would range from $350 to $500 per cow, depending on how much processing is needed by the dairy. (And these costs are after sand and solid separation is completed.) Operating costs—labor, maintenance, chemicals and power—will be about 1¢/gallon of manure.

For more information on the system, go to:

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