Modern compost screening presents multiple challenges to produce high quality saleable product at volume. As competition increases, requiring finer high quality screening, several screening solutions may provide the key to this sticky problem.
By Shawn Gibson
Mechanical screening is essential for producing a usable end product; however, variables like high moisture, high volume and low bulk density make compost a tough customer when it comes to screening. The daily variability of moisture content can make the selection of screening media and opening size a real challenge, especially when using trommel screens or other conventional vibrating screen shakers. These screens tend to be very inefficient and blind easily in wet, sticky material. Cleaning the screens is often a time-consuming nightmare, as anyone who has tried to clear a compost-plugged trommel can attest. Brushes and other automatic cleaners that work reasonably well with other materials are useless in fighting this sticky plugging material.
Materials are categorized by different attributes. Common factors such as moisture, composition and flowability help to determine the preferred screening method. Free flowing materials such as aggregates and stone products are typically processed with dry screening methods. A properly designed dry screening operation produces maximum capacity at the minimum cost per ton. As the material characteristics conspire to make the screening more difficult, conventional dry screen methods reach their limits. In aggregate and mining applications, the solution to difficult materials is often a transition to a wet screening process where water is used to keep the screen media clean. For compost and many other materials, this is not an option.
Methods for Compost Screening
Over the past 25 years, several screening methods have been tested for compost screening and many of them failed. Most compost facilities attempt to produce a -1/4″ finished compost and had to settle on -3/8″ or larger product size because of blinding and plugging. Compost material differs greatly from aggregates, stone and sand materials.
Trommel Screens
Trommel screens, a form of dry screening, have limited success in processing compost. Higher moisture level of the compost will negatively affect the performance of the trommel. The rotating drum of the trommel fitted with screen cloth is able to separate -1″ compost if the screens are continuously cleaned by rotating brushes. A well maintained trommel screen will produce a -3/8″ product at a very low production rate. As the desired product size decreases from -1″ to -3/8″, the operating costs increase and the product rate decreases. The brushes wear at a faster rate with the -3/8″ product and require frequent cleaning, adjustments and replacement. Many compost facilities operate on a limited budget and are unable to purchase new equipment. Trommel screens offer lower production and higher operating costs than conventional dry screening equipment.
Flip-Flow Screens
A better solution is the flip-flow type screen that dramatically reduces blinding and pegging when screening wet, sticky materials. The rapid tensioning and relaxing of urethane screening media introduces the material to forces of up to 50Gs, which helps to both liberate clumpy material and keep the screen panels clean and free of buildup. Rather than sliding down the screen, the compost is thrown into the air from one screen panel to the next, allowing for rapid stratification of the material bed. This reduces wear on the panels and allows for a much more efficient separation than with conventional screens, as the fine material quickly migrates to the bottom of the bed and through the screen openings. Flip-flow screens are able to screen compost as fine as 1/8″ depending on the application.
These screens are used in a wide variety of operations for screening finished compost, from municipal yard waste and food scraps to sewage sludge. Moisture levels in composted sludge routinely exceed 60 percent, making them a natural fit for the flip-flow screen. This equipment is screening compost across the U.S., from Florida to California, typically making separations between 1/4″ and 3/8″.
Star Screens
For larger separations during the intermediate phases of the composting, some operations have turned to star screens to reduce the plugging. These separators consist of spinning shafts lined with star- or hook-shaped discs, often made of rubber, which propel the material down the screen. Keeping the compost moving does indeed reduce blinding, but at the expense of efficiency, as a significant amount of undersized material is propelled across the top of the screen deck and ends up in the oversized material pile. Because of the driven nature of the screen shafts, star screens are excellent at handling the high volume nature of the material but are generally limited to sizing above 3/8″. For high volume operations where product quality is not as stringent, this may be the proper selection.
Some other commonly encountered problems with star screens involve the passage of longer splinters into the product and wrapping issues involved with plastic present in the material stream. The hooks or stars also have a tendency to grab and fling material, allowing long pieces of un-composted wood to become vertically oriented and either go through the openings with the fine material or to get stuck and plug the opening. The result is either product contamination of the fine compost or a shutdown and cleaning of the screen. Long pieces of stringy material (paper, plastic) get snagged on the stars and loop around the shaft, quickly jamming the machine.
Disc Screens
New innovations in disc screening technology provide the high capacity benefits of the conventional star screen while addressing the cleaning and maintenance short falls, including eliminating the hooks on the discs, which become worn and ineffective very quickly anyway, and replacing them with other shapes that do not snag material. Instead of rubber, use of other materials with excellent wear characteristics provides superior performance. The shaft must be isolated so that long stringy material cannot wrap and jam the machine. With these changes, you will end up with a disc screen that can handle the wettest compost at high volumes without plugging, and do it efficiently.
Producing High Quality Product
Modern compost screening presents multiple challenges to produce high quality saleable product at volume. As competition increases, requiring finer high quality screening, flip-flow screens and disc screens provide solutions to this sticky problem.
Shawn Gibson is an Applications Engineer with Aggregates Equipment, Inc. (Leola, PA), specializing in difficult screening applications. Shawn has more than 25 years of experience in handling of difficult materials. He can be reached at [email protected].