Press aid is a term that is used to describe something that is added to material being fed into a screw press. It is added in order to improve the dewatering performance of the press. A search for the term on the Vincent web site reveals over a dozen applications.
Most commonly press aid is a fibrous material that is added to deciduous fruit so that, when pressed, a higher yield of better clarified juice is obtained. Some press aid is almost always used in the production of apple juice. We explain it this way: if apples are fed into a screw press, apple sauce comes through the screens. But if press aid is first added when crushing the fruit, apple juice will be produced.
Common press aids used in juice plants are rice hulls and cottonseed hulls. Rice hulls are popular because they are very clean, have little dust, and are free flowing. Cottonseed hulls work better because the hulls have hundreds of short fibers attached. They are more appropriate for waste streams.
An even better press aid is cellulose fiber in the form of ground wood or bleached market pulp, both products produced by paper mills. It is common for a juice plant to use a 50/50 mix of rice hulls and ground wood. Generally 3% press aid by weight is added to the crushed fruit.
Occasionally paper mills used press aid to improve the dewatering (pressability) of WWTP sludges. In the industry they say that "sweetener", in the form of reject fiber, is added to the sludge.
Press aid is also used in the sugar beet industry. Gypsum and alum, called press aids, are added to the sugar beet pulp after the sugar has been removed. These press aids allow greater moisture reduction by the screw presses which are used ahead of the pulp dryers. These aids are first dissolved in water. There is some correlation between their effectiveness and the amount of time during which they are in contact before pressing occurs. There is debate as to whether their effectiveness arises from a chemical reaction or by physically giving body to the material being pressed. Typically 2.5% press aid per ton of beets being processed is added.
Press aid is not an unfailing silver bullet. We see this when press aid, even in high proportion, is added to DAF sludge at a slaughterhouse. The action of the screw press is simply to expel the original sludge, unchanged, through the screen of the press, while the press cake produced is simply dirty press aid. No moisture is separated even though the inbound sludge may measure 97% moisture.
Press aid assists the operation of a screw press using two mechanisms. First, the fibers of the press aid retain insoluble solids, preventing them from being forced through the screen of the press. Secondly, the particles of the press aid tend to scour the inside surface of the screen, preventing the screen from being blinded or covered over.
All press aids have two things in common: they are cheap and they are edible. That is, they are inexpensive enough that their use is justified by the resultant improvement in press performance. And, since most of the materials being dewatered end up as animal feed, their presence in feedstuffs has long been accepted by agricultural experts.
A quick way to test the effectiveness of press aid is to take a 100 gram sample of material to be dewatered and add three to five grams of press aid (or combination of press aids). If lime, gypsum or alum is being used, massage the sample for five minutes to allow any chemical action to take place. Place the sample in a cotton cloth and twist it into a tight ball. Observe what comes through the cloth: this will give you a good idea of what a screw press will achieve. The mass remaining in the cloth can be placed in an oven in order to get an idea of press cake moisture content that can be expected.
(Several materials will not dewater properly in a screw press unless they are first reacted with hydrated lime,calcium hydroxide, Ca(OH)2. The lime is not thought of as a press aid because it works on the basis of initiating a chemical reaction which breaks down the cell walls. See Pressing News #159, Onions & Strawberries.)