March 11, 2010 ISSUE # 220
As a general rule, if you want a screw press to squeeze more juice out of an organic material, it pays to shred the material before putting it into the screw press. This is
especially true in the case of produce waste such as cabbage and lettuce.
There is always an exception to the rule.
In recent months we have worked with PetroAlgae to improve the pressing of duckweed. Duckweed is an aquatic plant with very tiny leaves. It does a remarkably good job of
cleaning up wastewater in which it grows. PetroAlgae's work is aimed at producing high protein animal feed and possibly a biofuel by-product from duckweed.
At PetroAlgae the process is to pump the duckweed from ponds to a static dewatering screen. From there the thickened material (96% moisture) goes to a Langsenkamp knife
mill where it is macerated into a puree. This crushed material is fed through a belt press which gets the moisture content down to 90%. The cake from the belt press then goes to a screw press.
We had trouble getting the moisture out of the duckweed. Series CP and KP presses, and even one Series TSP Twin Screw, were tried with generally poor results. We kept
changing screws, screens and cones, while varying cone pressure and screw rpm.
Fortunately Sunrise Ridge Algae brought samples of duckweed to Tampa for testing. (We tried for better performance using steam injection, but that did not work out.)
The important thing was that Sunrise insisted on running their duckweed without any shredding ahead of the screw press. The press cake came out remarkably dry, at 64%
Previously at PetroAlgae our duckweed press cake had come out at 83% moisture content. On returning the PetroAlgae we by-passed the Langsenkamp and went directly
to the press. The cake came out at 74% moisture content, a major improvement.
Further testing is underway at PetroAlgae. One thing we will definitely keep in mind: there are exceptions to the rule that shredding a material will improve its dewatering
characteristics in a screw press.
[It is worthy of note that duckweed grows a lot slower in cooler water than in warm summer water.
Duckweed has a low level of dissolved solids, probably about 1 Brix. Press liquor has low solids content, usually around 2%, but reaches as high as 5%.]