July 9, 1996
Traditionally we think of the water in a material as being either free or bound. This is a very useful concept in analyzing the operation of a screw press. We expect to drain out the free (bulk or loose) water with ease, while we know that it gets progressively more difficult to remove the bound water.
An article by P. Aarne Vesilind, Sludge Dewatering: Why Water Wins, that appeared in the March/April issue of Industrial Wastewater has proven helpful in analyzing screw press performance. The article expands on the concept of four different kinds of water.
Vesilind describes four identifiable types of water, only two of which can be removed by mechanical means. His work was done with sludge, but the concepts apply readily to the usual screw press materials such as screen rejects in a paper mill, manure, fruits, and vegetable waste.
Vesilind starts with a shredded material and adds water to it until the suspended solids in the sample are widely dispersed. Specifically, the sample is continually diluted under low shear conditions until the solids define their own sizes and structures.
Next the sample is dried while carefully measuring the rate of evaporation. The rate of evaporation is measured as grams of water evaporated per minute when a thin layer of sample is dried at ambient temperature.
It was found that the rate of evaporation is constant as the Free Water is removed. (This Free Water is the first kind of water.) This evaporation starts to decelerate at a steady rate once the Free Water is evaporated.
The steady rate of deceleration continues for a period during which the second kind of water is removed. The author calls this Interstitial Water. It is water trapped in the crevices and interstitial spaces of the suspended solids.
Once the Interstitial Water is evaporated a further, distinct, change in the deceleration rate occurs. The rate of evaporation becomes noticeably slower. It is during this period that the third kind of water, Vicinal Water, is removed.
Vicinal Water is defined as layers of water molecules held tightly to the particle surface by hydrogen bonding. This type of water can also be contained in cells, as long as it is associated with a solid surface. Material with a preponderance of fine solids will have more surface and thus more Vicinal Water.
Vesilind points out that mechanically removing Vicinal Water is quite difficult. In the case of sludges, it is necessary to condition the material with flocculent chemicals before a screw press will be capable of removing the water.
Once the Vicinal Water evaporates, there is no further evaporation of water from the sample. Yet there remains a fourth kind of water, Water of Hydration, in the material. This is defined as water that is chemically bound to the particles. It is removable only by the expenditure of thermal energy, as in a dryer.
This explains why organic materials can be dewatered only to a certain level in a screw press. For example, orange peel can be pressed to an absolute minimum of about 58% moisture. Beyond this it takes the addition of heat to reduce the moisture to a bone dry condition.
The concept of four kinds of water is useful in evaluating the level of dewatering that can be achieved with a screw press.