Testing for cleanliness

Testing is currently underway at a composting facility in Warngau, Germany. The DUT - Device Under Test - is a new Multistar S3, and the objective is to get cleanly screened compost.

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It’s a small but neat site about 20 kilometres south of Munich. The company’s business is regional waste avoidance, information and reclamation. One of the reclamation jobs is the composting of about 15,000 tonnes of organic waste from surrounding towns. The finished compost goes to a soil producer, farms and increasingly to private individuals. Klaus Rohm is the director of the composting plant, and thus responsible for the machinery used as well as for the quality of the compost product. “It has always been important to us to deliver the highest quality compost to our customers. We are in control of the biological and chemical parameters. But getting rid of the foreign matter, especially plastic, is a real challenge,” says Rohm, speaking from experience.

Always the plastic

The stricter quality criteria with the new total surface index has given the contaminant problem a new urgency. Contamination of compost by foreign matter is no longer measured just by weight, but also optically. Trash bags and plastic packaging have an especially negative effect on evaluations. Since no immediate improvement in waste separation accuracy can be hoped for, plant operators are forced to gear up. Accordingly, VIVO is looking for a suitable screen with wind sifting. 

Wind sifting versus the plastic cycle

Rohm is convinced he can break the “plastic cycle” by means of wind sifting. “We run a dry fermentation process followed by hygienization and drying. Then we screen the compost to about 15 millimetres. But the screen overflow still has clumped fines, so we run it back through a high-speed shredder and screen it again. And that’s where the problem comes in. If I don’t get the plastic out first, it gets shredded along with the rest. Then the plastic stays in the undersize fraction despite fine screening, and gets into the product. To solve this problem, Rohm has his eye on a star screen with integrated wind sifter, but the expenditure has to be in relation to the amount of use the machine will get. “Up until now our volumes were too low for most star screens. The Multistar S3 is the exception. It’s just right for our size.”

Knowing what matters

With over 20 years experience, Klaus Rohm is part of the plant “inventory” as he puts it. So who better to decide whether the new Multistar S3 should also be taken into inventory? The S3 scores its first big points with its powerful wind sifters, which to Rohm’s way of thinking only really do their job when they work in combination with star screening. “The motion of the stars bumps the material up and down on the deck. The wind sifter is right above it and can grab the light material much better.” Adjusting the screening cut is something he’s familiar with, since the old screen system also let him do that. “We get plenty of compost out of the shredded screen overflow. To make sure no plastic bits get into the finished product, we reduce the screen cut to about ten millimetres. That’s easy to do and very precise on Komptech star screens.”

Quick popularity

His colleagues who load the machine by wheel loader appreciate the low loading height of just two and a half metres. “We can handle that easily with our existing machine park - we don’t even need a high tipping shovel. The compact dimensions made it easy to find the ideal place to put the machine. It gives us short transportation runs, which saves costs and aggravation for the drivers,” says Rohm of another practical characteristic of Komptech’s smallest star screen. Although the Multistar S3 has no wheels, it’s mobile enough for Rohm. Since the company also has another composting site, sometimes the machine needs to be moved. “No problem,” says Rohm. “It has a hook lift frame. Since we collect waste ourselves, we have trucks that can carry it, and there are also plenty of transport companies that can do it. No chassis means the loading height is low, plus it costs less so it’s easier for us to afford - it all works out.”