Gravity tank is low-cost way to separtate manure, control odor
By Leanne C. Lucas, U of I College of ACES
UrbanaUsing 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.
The gravity tank is the cheapest way to separate liquids from solids, said Ted Funk, University of Illinois Extension specialist in environmental engineering. But its not just cheap. Its effective, according to U of I research.
Three factors are essential to optimum separation, said Funk. The tank bottoms 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 dont 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 thats 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, its 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 youll 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 manurenitrogen, phosphorus and potassium.
Oftentimes, the acreage thats 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 shouldnt 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 that will go with the liquid when separated.
As a result, the farmer has nitrogen to irrigate in liquid form, said Funk. Its 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.
Funks 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.