Gravity Based Solid Liquid Manure Separation Offers Low Cost Phosphorus Management Option

An engineering study undertaken on behalf of the Manitoba Rural Adaptation Council shows gravity offers a potentially low cost solution to managing the phosphorus in liquid hog manure.

A research team working in partnership with the Niverville, Manitoba based Puratone Corporation conducted three trials during the summer of 2008 to determine whether the solid and liquid factions of hog manure could be separated by simply pumping the manure into a storage tank and allowing the solids to settle.

The team was headed by Larry Slevinsky, a Winnipeg based Professional Agrologist, and included Gary Plohman a business development specialist/livestock engineer with Manitoba Agriculture and Rural Initiatives (MAFRI) based in Beausejour and Dr. Geza Racz, a professor emeritus with the University of Manitoba's Department of Soil Science.

"The three of us were interested in looking at whether or not we could separate the solids, particularly phosphorus, from manures merely by pumping them into a tank and measuring how rapidly the phosphorus would settle out," recalls Dr. Racz.


7,000 Gallon Capacity Research Tank Constructed

To prepare, Puratone staff constructed an approximately 40 foot long by eight foot wide by four foot deep steel "gravity settling tank" on the site of one of the company's 6,800 pig capacity grower finisher barns. At one end, the tank was fitted with sampling ports at various depths. The tank was filled with liquid swine manure and daily samples were collected as the solids settled, over four weeks in the first trial and over two weeks in two subsequent trials, and analyzed for solid content and phosphorus content.


Greatest Amount of Settling Occurs Within 48 Hours

Dr. Racz reports, "We found that within about one or two hours the phosphorus in the upper top layers actually started to settle out. Within a period of about 48 hours most of the phosphorus in the top three to three and a half feet of manure had settled to the bottom of the tank."

Plohman adds, in some cases, phosphorus levels in the liquid fraction that had entered the tank at levels as high as eight pounds per thousand gallons were reduced to two pounds per thousand gallons over the period of the test. As well, solids levels in the range of five to seven percent fell to about one percent in two of the three trials.

He observes the greatest amount of setting occurred over a period of about two days when  the larger particles settled out and leaving the manure longer allowed the finer particles time to settle out.


New Phosphorus Application Limits Expected to Create Interest

Plohman expects the study to be of particular interest to farmers compelled to reduce the application of phosphorus on the land they use for manure application. He views the gravity setting approach as one option to effectively and possibly economically remove the phosphorus and solids from liquid swine manure and transport it to other locations for growing crops.

"This will reduce their need to purchase more acres and allow them to be able to meet the requirements of the manure regulations in the province," he says.

Dr. Racz adds some livestock producers will have difficulty meeting the new regulations because they do not have sufficient land on which to recycle all of the phosphorus they have in manures. They are going to have to transport some of this phosphorus off farm and in some cases, considerable distances.

He suggests, "If we can separate the phosphorus from the rest of the liquid manure then we need to transport only a smaller portion of that manure a considerable distance and thereby save costs. The remaining liquid, which would be high in nitrogen, could be utilized (by producers) on land such as they do today."


Solid Liquid Separation Results in Greater Consistency

Slevinsky continues, separating the manure into two streams also reduces the variability of the nutrient contents in both streams resulting in a liquid fertilizer that is low in phosphorus and a solid component which is rich in nutrients that can also be used as a fertilizer.


Additional Research Still Necessary

"The research is not complete yet," Dr. Racz stresses.

This may have a place in solid liquid manure separation and managing phosphorus, for particularly the smaller livestock producer, he agrees. But, he acknowledges there are still a few other issues to be addressed. For example, what is the best way to handle the solids that settle out?

"In our trials we composted it and we had success composting it but there may be other things that might be just as feasible or more economic."

Other options might include simply moving the solids in the state in which they settle out or they could be further processed before being transported.

As well, Dr. Racz notes some manures, for example those that were very high in solids, didn't settle out as well as would have been liked which suggests adding a chemical flocculate such as iron might provide better separation.

However, he stresses, "If you don't need to add chemicals, I would prefer not to."

For example, he cautions, the addition of iron could make the phosphorus less soluble and less available to the plant.


Researchers Examine Continuous Flow Option

Plohman notes testing has also been done using the same research tank in a continuous flow system  rather than a batch system.

He explains, "Running it as a continuous system more closely resembles what producers will be doing in terms of emptying the barn out and pumping manure on a regular basis into their manure storage."

Running continuously, he says, will allow producers to add a smaller amount of manure on a regular basis and allow it to settle out over time.

"We were able to find out that the setting tank worked very well to reduce the solids and phosphorus in the manure when we were pumping in approximately one eighth of the total volume of the tank on a daily basis. Over that period of time manure was pumped in and, in the end, the solids content was reduced from five to six percent solids down to about two percent solids."


Gravity Based Separation Offers Potential

Slevinsky believes this is a step in the right direction.

"These results are very encouraging and positive." He suspects gravity based solid liquid separation may serve as an alternative to the two cell storage systems currently used by a majority of hog operations.

Plohman agrees, "We think that there is great potential for utilizing this type of approach for manure separation and helping producers do a lower cost separation than some of the other technologies that are out there."