October 11, 1994
Rev. July 1997

Probably the most frequent inquiry received at Vincent involves using our screw press to dewater sludge. The reason for this is that screw presses have been promoted as being able to press sludge even though there are many cases where they cannot actually do so. At the same time, there is a great demand from plant operators. They know a screw press operates relatively unattended and requires a minimum of maintenance, while belt presses do not share this reputation.

There are many of different types of material referred to as sludge, and they possess a wide range of characteristics. The types of sludge where our screw press has proven most successful are in the plastics and paper recycling industries.

In recycling post-consumer plastics, mostly bottles, the first steps are to grind (shred) the material and to wash it. The washing is done in stages (cold, then hot) and sludge accumulates in the wash tanks. This sludge consists mostly of paper and film labels, along with glue, dirt and residue from the original containers. The key characteristics are that the sludge is not biologically active and it contains a significant amount of fiber.

In recycling paper, the first operation is to convert the waste paper into a slurry by stirring it in water. The slurry is screened in hydraclones and pressure screens, along with static and drum screens. The reject material can be pressed if it not too heavy in clay, ash, glue, and dirt. Once again, the key characteristics for success are rejects (sludge) that (1) are not biologically active and (2) contain a significant amount of fiber.

Just because a material can be pressed with a belt press does not mean that it will work in a screw press. Some do and some do not, depending largely on the fiber content.

The sludges that cannot be pressed in a screw press have fine organic particles, clay paper coating and short (if any) fibers. They come from waste water treatment plants. The most common are from municipal sewage plants; slaughterhouse and cooked food wastewater plants have similar operations. Our next issue of Pressing News will discuss these non-pressable sludges in more detail.

Issue 15