June 28, 2001 ISSUE #M17
Engineers designing manure handling systems frequently ask about capture rate. They need to know the percent of the solids in the manure that will be captured in a manure separator.
The captured solids are those that come out in the press cake. The solids not captured are those that flow in the liquid stream draining from the screen of the separator. The capture rate is calculated by dividing the solids in the press cake by the total solids pumped to the screw press.
Late last year capture rate tests were run at the University of Tennessee Dairy Experiment Station in Lewisburg, Tennessee. These tests confirmed a very significant difference in capture rate between flush and scrape barn systems.
The data show that when manure with 7% solids, typical of a scrape barn, are pumped to a screw press, the capture rate runs about 50%. In contrast manure with 2% solids, typical of a flush barn, had a capture rate of only 25%. (These figures are from a graph prepared by Dr. Robert Burns.)
The problem with a very dilute inbound flow can be explained as follows: The freeness tends to be high and a very large amount of water rushes through the screen. It goes through the screen so fast that what fiber is present tends to wash through the perforations or slots of the screen. Thus the capture rate is low with very dilute flows.
With thicker flows the longer particles tangle together, acting as a press aid. This forms a mat that entraps the finer particles. Thus the capture rate is higher with pre-thickened feed to a screw press.
It is noteworthy that solids in manure can be classified as either suspended solids or dissolved solids. The dissolved solids are like sugar in water: they cannot be captured with a mechanical filter. Since most of the manure flow to a screw press leaves as liquid, most of the dissolved solids pass through the press with this liquid. This places a definite limit on capture rate.