October 22, 2003 ISSUE #143
When vegetable material is squeezed in a screw press, there will be particles of suspended solids in the press liquor. Commonly it is desirable to filter these solids from the liquid. Static screens, rotary drum screens, Fiber Filters, and a variety of other filter machines are employed.
Another devise for separating suspended solids is a decanter tank. This is a simple tank into which the press liquor is allowed to flow. Usually the tank is rectangular, with the flow being admitted to one end. The clarified liquid flows out an overflow connection at the top of the far end of the tank.
Because of the relative size of the tank, flow velocity is so low that solid particles settle to the bottom of the tank. Sometimes labyrinth panels are placed in the tank, sending the flow in a circuitous path. Alternatively, a series of three tanks are used, connected by overflow weirs, one flowing into the other, each at a slightly lower elevation than the previous.
Decanter tanks work best where the suspended solids are significantly heavier than water. A good example is to be seen at a Starbuck's soluble coffee plant. The spent grounds are dewatered in a Model CP-10 screw press. The press liquor goes to a decanter tank where fine particles settle out. This clarifies the press liquor to where it can be sewered. Once a week an operator shovels the solids out of the tank.
Other applications for decanter tanks include separating starch from the press liquor that results with potato peel is dewatered, and separating fines from the press liquor that results from dewatering chili pepper that has been dried, ground, and re-hydrated. Another use of decanter tanks is to allow oil to float to the top when whole limes are crushed and pressed in a screw press.
Dairy farmers that use sand bedding for their cows use a variation of the decanter tank. In order to dewater the manure in a screw press, it is necessary to first separate the sand from the manure. This is done in huge concrete troughs. Flow velocities are set so that the sand falls out of the flowing stream, while the manure particles remain suspended in the liquid. This liquid is pumped to the screw press. Periodically a front-end loader is used to remove the sand that fills the trough.