A simple, inexpensive gravity tank in a swine manure liquid-solid separation process will help producers control hog odors. It will also keep phosphorus and potassium levels in the soil in balance.

"The gravity tank is the cheapest way to separate liquids from solids," says Ted Funk, University of Illinois Extension specialist in agricultural engineering. And the settling tank is also effective, according to Funk’s research.

Three factors are required for proper manure separation, says Funk. The tank bottom angle of slope, the retention time of liquid in the tank and the percentage of solids content in the liquid all determine the performance of the settling tank.

"We found that the angle of repose of manure is about 30 degrees," he explains. "At 30 degrees, we don’t have any accumulation in the bottom of the tank that keep bulking up and running over the overflow." At the 30-degree angle, manure solids move to an area where removal is supported by a slow-moving auger.

Proper retention time in the tank is vital in settling. "Thirty minutes is all you need to leave the liquid in the tank to get most of the settling that’s going to happen," says Funk.

Last, the lower the percentage of solids flowing into the gravity tank, the better the separation. "If you have high solids content, it’s not as easy to get settling and separation to occur," he says.

Swine manure usually runs at 5% or 6% solids content. Combining clean water with manure, as with a flushing system, reduces solids content. "Then those solids will settle out and you’ll have pretty clean water coming out the other side."

Proper liquid-solid separation is important for managing nutrients in manure – nitrogen, phosphorus and potassium. "Oftentimes, the acreage that’s around a swine production facility has been ‘manured’ for years," Funk states. "The soil tests are so high in phosphorus and potassium that you really shouldn’t be putting any more manure on those fields for a long time."

The goal of manure separation is that much of the phosphorus and potassium gets diverted into the solids. Solids can then be moved to cropping areas that need fertilizer.

Nitrogen, however, must be applied yearly. About half of the nitrogen in manure is in soluble form and will separate out with the liquid.

As a result, the farmer has nitrogen to irrigate in liquid form," suggests Funk. "It’s cheaper than knifing in a sludge and he can use it close to the facility, so he can raise a corn crop year after year without building up the soil tests."

With all of the information described, a gravity tank is something that a farmer can build, he says.

Funk’s research is part of the Swine Odor and Waste Management Strategic Research Initiative and is funded by the Illinois Council on Food and Agricultural Research.