One very challenging application has been onion waste. Onion processors will easily generate 10,000 to 100,000 pounds per day of waste. This waste comes from two flows: cull onions (spoiled, bruised, crushed), and the skins (parchment with outer layers of onion, with top and bottom trims).
When cull onions were run in a screw press, a paltry 20% was converted to drain liquor. With this poor a volume reduction, there was no justification for pressing the waste. The results pressing skins were even worse. Some improvement was achieved by pressing the waste in a twin screw press; however, the cost of these presses is excessive for the application.
In order to achieve significant volume reduction, it is necessary to shred the material before it is placed in the screw press. This is done with a shredder, or pre-breaker, that is mounted over the inlet to the screw press. This means, essentially, that two machines instead of one must be purchased. There is a good payback on this investment: waste can be reduced by 50% to 70%.
