Non-Pressable Sludge

October 4, 1994
Rev. Jul 1997

Very frequently we are asked if some material can be dewatered in a Vincent press.  There is a very simple fist test that indicates if a screw press will work: First, put a small mount of material in the palm of your hand.  Next, close your fingers gently around the mass of material.  Work the material with your palm and fingers so that something is squeezed out between your fingers.  If a liquid comes out between your fingers and if, in the end, there is some solid material left in your palm, then a screw press might succeed.

For example, if you ran this test with mashed potatoes in your fist, you would see that it cannot be pressed.  On the other hand, if you shredded some paper, mixed it with water, and worked that material in the palm of your hand, you would see that paper pulp can be pressed successfully.

Digested organic material is the most important non-pressable sludge.  This includes sludge from sewage treatment, slaughterhouse, and cooked food plants. At these treatment facilities, the fine cloud in the wastewater is agglomerated with polymer flocculent.  In a DAF (Dissolved Air Flotation) system this sludge floats to the surface and is skimmed over the edge of a tank.  In other systems it is allowed to decant to the bottom of a clarifier tank, from which it is pumped out as underflow.

When sludge is skimmed off it will have 80% to 95% moisture; clarifier underflow is even wetter.  It is very expensive to dispose of because it can represent a huge tonnage going to landfill.  Generally, belt presses can be used to filter out some more of the water. But the end result cake is still likely to have more than 80% moisture.

Other sludges that will not press are finely ground inorganic materials.  These include settled materials such as pond dregs, tank bottoms, and clarifier silt.  Use the fist test if you are in doubt.

Because of the big potential payoff, we have run sludge tests using variety of press aids.  We also tried heating sludge to 200°F before pressing it.  Another effort involved adding bleach to break down the polymer.  Not one of these efforts has come close to working.

We have seen conflicting results with the use of polymer. We had a case where water was readily squeezed from a belt press cake with a bare hand; yet, the Vincent screw press could remove nothing.  In fact, after passing the sludge through our screw press, the fist test was no longer successful. At the same time there are several paper mill installations where the Vincent screw press works only when polymer is used on the waste stream.  The fist test is not a fail-proof determinant.

Issue 16