Gravity Tank Low-Cost Way to Separate Manure, Control Odor

URBANA - Using an inexpensive, low-tech gravity tank in a swine manure liquid-solid separation process will help producers control swine odor and keep phosphorus and potassium levels in the soil under control.
calendar icon 22 May 2003
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"The gravity tank is the cheapest way to separate liquids from solids," said Ted Funk, University of Illinois Extension specialist in environmental engineering. But it's not just cheap. It's effective, according to U of I research.

Three factors are essential to optimum separation, said Funk. The tank bottom's angle of slope, the retention time of liquid in the tank and the percentage of solids content in the liquid all work together to determine the performance of the settling tank.

"We found that the angle of repose of manure is about 30 degrees," said Funk. "At 30 degrees, we don't have any accumulation in the bottom of the tank that keeps bulking up and running over the outflow."

When the tank is at a 30-degree angle, the manure solids continually slide to an area where removal is enhanced by a slow-moving auger.

Retention time is another important factor 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," said Funk.

Finally, the lower the solids content going into the tank, the better the separation. "If you have high solids content," said Funk, "it's not as easy to get settling and separation to occur."

An initial solids content of 1 to 3 percent improves the separation efficiency, he said. Swine manure usually runs at 5 or 6 percent.

If you combine clean water with swine manure, as with a flush system, the percentage of solids content is reduced. "Then those solids will settle out and you'll have pretty clean water coming out the other side," said Funk.

Proper liquid-solid separation is vital because it helps in managing the nutrients found in manure--nitrogen, phosphorus and potassium.

"Oftentimes, the acreage that's around a swine production facility has been manured for years," said Funk, "and the soil tests are so high in phosphorus and potassium that you really shouldn't be putting any more on those fields for a long time."

The goal of manure liquid-solid separation is that much of the phosphorus and potassium is diverted into the solid fraction. Then these solids can then be transported off site to facilities that do not raise livestock, but raise crops and need fertilizer.

Nitrogen, on the other hand, must be applied yearly. About half of the nitrogen in the mixture of manure is in a soluble fraction which will go with the liquid when separated.

"As a result, the farmer has nitrogen to irrigate in liquid form," said 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."

Funk believes this research will give producers a much better idea of how gravity liquid-solid separation can be used in a modern manure-handling system.

"We know the slope, the retention time and the solids content information," he said. "This tank is something a farmer can build."

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

Source: ACES News - 22nd May 2003

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