BioCycle May 2005, Vol. 46, No. 5, p. 55
The most common screening units in use at composting sites are deck / flat, disc, grizzly, orbital, star and trommel screens. More recently, facilities are adding an air separation step to remove film plastic.
George Savage, Luis Diaz and Nora Goldstein
WHEN composting various types of organic residues derived from agriculture, yard trimmings, wood wastes, and segregated collection of MSW, the finished product almost always contains a certain amount of contamination, such as small plastic film particles, rocks, and other materials that must be removed to improve the quality of the compost.
Since separation of contaminants generally is more difficult on a material with a high moisture content (i.e., corresponding to the material at the beginning of the process), it is preferable to carry out the segregation at the end of the composting process. The machines typically used for this operation usually are screens and air classifiers.
This article, based on a soon-to-be released book titled Modern Composting Technologies, focuses on physical separation of compost materials for two very general purposes: 1) Removal of contaminants; and 2) Sizing of material for product marketing. While just about every screen on the market can be used for both of these purposes, air classification systems are primarily used for contaminant removal and in particular, the very pesky category of film plastic. Other equipment, such as destoners, is used at some facilities to remove heavier contaminants, e.g., small rocks, hard plastics and glass shards. Most composting facilities that market their end product have a screen on-site to process material to market specifications. Some facilities, especially those receiving and processing land clearing debris or a mix of yard trimmings, use a screen ahead of grinding to remove dirt and fines.
Over time, we have found that facility operators favor a certain screen design for a host of reasons (oftentimes different designs for the same reason!), ranging from its ability to process high moisture content compost and ease of changing screen sizes to the rate of throughput for a particular kind of compost and multiple separation with a single screening unit. Larger facilities often purchase several types of screen, e.g. a workhorse unit for initial separation of particle sizes and/or contaminants, and a different style of screen for sizing finished product. This article reviews the range of screen and air classifier designs that are available in the marketplace.
Screening is a unit operation which involves the segregation of a mixture of materials with different particle size distributions into two or more fractions (each fraction having relatively similar particle size distribution) by means of one or more sets of screen mesh openings (or screening surfaces). The material that remains on the screening surface generally is known as the oversize (or plus) material. The material that passes through the screen apertures commonly is referred to as the undersize (or minus) material. The screening surface may consist of plastic, cloth, wire mesh, perforated plate or bars.
Screening can be carried out when the material is wet or dry. There are several types of screens; the most common units applied to the composting of solid wastes are: deck (flat), disc, grizzly, orbital, star and trommel screens. A brief description of each follows.
Deck screens (also known as flat screens) consist of a rectangular frame that contains a perforated plate or a wire cloth (mesh). The frame is set at a slight angle and can be suspended by rods or cables or supported from a base by flexible flat springs. The size of the mesh openings determines the particle size of the unders. Nearly all deck screens vibrate or oscillate in some manner. The vibration bounces material along the deck, exposing it to the screen surface. The overs move across the flat screen due to the vibration and the incline of the deck. Vibration can be induced mechanically or electrically. Flat screens can achieve a relatively high capacity and high efficiency.
One advantage of this design is that several decks of screen panels can be stacked vertically to sort particles into more than two size fractions. The panel with the largest openings sits on top. The unders that fall through land on a second panel with smaller openings for a second level of separation. Two panels yield three size fractions; three panels yield four. Different conveyors capture and isolate the separated fractions.
Traditionally, flat screens are not very well suited to processing higher moisture content materials because the mesh tends to blind. This situation can be addressed by using a more aggressive vibration stroke, or by having hole clearing devices like ball decks that bounce against the screen as it vibrates. Some manufacturers vary the oscillating motion of the screen to minimize blinding and reduce spearing. Mixtures that contain materials that are thin and relatively large in the other two dimensions (e.g., cardboard, newsprint) may prevent small uni-dimensional particles from contacting and passing through the mesh.
Disc and star screens are similar in design. The distinction between them is in the geometrical slope of the elements that are employed to transport material over the screen, namely disc-like elements or star-like elements. This type of equipment basically is composed of: a loading hopper; a screening surface formed by a series of shafts on which the discs are mounted; and two telescopic conveyor belts for transporting the fine and coarse materials. The shafts rotate in the same direction so that the coarse material is moved forward on the surface of the screen, while the fine fraction passes through the openings. In this manner, two different streams are obtained and two different piles are formed, by means of conveyor belts.
The product size is determined by the dimensions of the openings, which can range from a half-inch to 6-inches (15 to 150 mm). The dimensions of the openings can be varied by changing the distance between the discs mounted on the shafts. The shafts rotate at speeds in the range of 30 to 120 rpm.
Disc screens can be powered by diesel engines with outputs of 45 to 115 hp, or with electric motors with outputs from 57 to 90 kW. Both of these units can be equipped with a remote control system. The material is loaded on a hopper equipped with a conveyor belt and a horizontal shaft to convey it to the screening surface; the oversized material is transported by the screening devices to the discharge belt.
This type of equipment allows efficient treatment of materials with a high moisture content or that is rich in fiber without clogging. One of its advantages is the high screening efficiency obtained from the screening surface, which varies in size from about 3 m2 for the smallest machine to 11 m2 for the largest units. The throughput of these screens varies from 10 to 80 lbs (5 to 35 kg) of treated material per second. Some types of star-screens can also work on steep terrain, with slopes of up to 15 degrees.
Grizzly screens consist of a series of parallel bars kept apart by spacers positioned at a predetermined distance. The bars can be vibrating or stationary. Grizzly screens commonly are used in the composting industry to remove fines prior to size reduction or additional screening.
Orbital screens are mainly small-size equipment that separate coarse from fine materials by combining the effects of gravity, centrifugal force and the presence of a screening surface. The material generally is loaded continuously by means of a conveyor belt or with a loader; the rotation of the hopper screen causes the coarse materials to roll over the fine materials. The fine materials pass through the screen. The unit has two belts to discharge the two different fractions. The size of the openings varies from 0.25 to 2-inches (6 to 50 mm).
Trommel screens consist of a cylindrical frame, covered by a perforated plate or wire cloth. The cylinder is open at both ends, rotates at relatively low speeds (15 to 20 rpm), and is set at a slight incline. The material to be processed is introduced at the upper end; the undersize fraction falls through the screen, and the oversized fraction exits at the lower end. Some trommel screens are divided into sections covered by different sizes of mesh. Trommels are very popular in the waste management industry in applications that require segregation of fines from mixtures that include large, thin pieces of materials (such as plastic film or paper). The tumbling action does not allow the large pieces of materials to settle to the bottom of the screen, where they could obstruct (blind) the mesh. In composting, the trommels generally are used to process the finished compost such that large pieces of biomass are removed and a finished product of uniform size is produced.
A typical trommel screen system is composed of a screening sub-system (generally a grizzly) to separate fine materials (e.g. rocks or dirt); a rotating cylindrical sieving drum; a rotating brush system to clean the drum; and two conveyor belts: the first one to unload compost of the desired size; the second to form a heap with oversize materials.
The prescreening system for removing coarse materials consists of a mesh with openings of 4-inches (100 mm) or more. The drum of the trommel screen essentially is a cylinder manufactured from steel, with openings in the drum surface sized to produce a product that meets the required specifications. For a high quality product, about 0.25-inch (5 to 6 mm) openings are used; for a coarse material, openings of roughly three-eighths to a half-inch (10 to 15 mm) would be sufficient.
Trommels also can be equipped with a cover over the drum to reduce the emission of dust. This solution is more effective when the equipment is located inside buildings, where dust transport due to wind is very limited. Small trommel screens also are available for plants processing relatively small quantities of materials.
AIR SEPARATION PROCESS
Air classification or air separation probably is the least familiar separation technology to some composting facility operators. Although routinely used in materials recovery and other industries, few composting facilities have adapted air separation machinery to use in their compost production systems. Most appear to be waiting for equipment specifically designed for compost. Such equipment has now emerged in the North American market and is beginning to find its niche.
Air separation uses an air current to separate materials primarily according to their density. The size and shape of the particles also are very important factors. The concept is well illustrated by a conventional vertical air separation system, commonly called air classification. Mixed materials are fed into a chute with an upward-flowing stream of air generated by a blower. The light materials are carried with the air; the heavy materials fall down into a bin or onto a conveyor belt. As the air stream continues with the light particles entrained, it enters into a cyclone, where the air velocity slows, causing the particles to settle out.
There are several variations of air separation. For example, it is not an absolute requirement to include a cyclone to collect the light particles. Cyclones are particularly efficient at collecting particles, particularly small ones like powders and dust, prior to discharging the air into the atmosphere. Other devices that slow the air velocity and thus capture the light particles also are used.
In addition, the direction and mode of airflow can vary. The air can move as a result of a vacuum, with a fan above the feed, or by pressure, with the fan below the feed, or both. The air stream can be horizontal with the particles carried along a conveyor or screen. Additional equipment may be required such as air locks, chutes of various shapes, and a variety of conveyors. Several applications of conveyors include a vibrating conveyor to shake light particles to the surface so that the air can come in contact with them.
The division (split) between light and heavy materials depends on the air velocity. The velocity, in turn, depends upon the airflow rate and the size of the air channel. Higher velocities are needed to remove heavier particles. Different types of plastics, for instance PVC versus polyethylene film, require different air velocities for effective separation. If the air velocity is too low, it would reduce the separation efficiency. On the other hand, if the velocity is too high the air column would carry a high percentage of heavy particles along with the light fraction.
ADAPTATION TO COMPOSTING
With composting, air separation generally is used toward the end of the process, to remove contamination after size reduction, after composting, or after screening. It is usually one of several steps that remove plastics from compost. Hand sorting and screening still remain important components in producing compost that is almost free of plastics. In fact, screening generally is the unit process used to separate plastics from the finished compost. Unfortunately, air separation cannot reliably segregate plastics from finished compost products, at least with the techniques presently available. The density and aerodynamic properties of plastic particles and compost are too similar for practical separation. Therefore, air separation typically is applied to the overs from screening (the large fraction particles collected by the screen). The overs primarily are comprised of pieces of wood, which have substantially different densities than the plastics, at least the film plastics.
There are some advantages to removing the plastics from the screen overs. First, it reduces the amount of plastic that would be returned with the overs into the composting process and that might eventually contaminate a new batch of compost. Plastic particles become smaller with each pass and eventually will pass through the screen. Second, storage piles of overs often are so contaminated with plastics that they cannot be reused or even sold as mulch. In this case, air separation saves the costs associated with storing and landfilling the overs and at the same time recovers the organic particles for mulch or compost products.
The air classifiers achieve segregation of the materials as a function of the density, shape and size of the particles. Segregation is achieved as an air flux separates light materials, such as plastic film, from heavy particles. An air classifier with a capacity of 40 m3/h requires an input power of 20 kW. These machines generally are used in combination with trommel screens.
Air separation is just one alternative to manage contamination due to plastics. To substantially reduce the concentration of plastics in finished compost, other management practices must be considered, including: public education, source separation, the use of biodegradable bags and packaging, quality control in the tipping area, front-end sorting, proper size reduction, and screening. Producing a finished product that has a relatively small concentration of plastics is a major challenge.
George Savage and Luis Diaz are with CalRecovery, Inc. in Concord, California. Mr. Savage and Dr. Diaz are coauthors of Modern Composting Technologies, to be published in Summer 2005 by BioCycle/The JG Press, Inc.
SCREENS AND AIR SEPARATORS
861 Main St.
Hackensack, NJ 07601
1100 Holland Rd.
Suffolk, VA 23434
Construction Equipment Co.
18650 S.W. Pacific Hwy.
Tualatin, OR 97062
Continental Biomass Industries
22 Whittier St., Newton, NH 03858
Duratech Industries International, Inc.
P.O. Box 1940
Jamestown, ND 58402-1940
EarthSaver Equipment, Inc.
P.O. Box 7325
Kalispell, MT 59904
1 Avenue Premier
Riviere-du-Loup, PQ, Canada G5R 6C1
3460 Grant Drive
Lebanon, OH 45036
Finlay Hydrascreens (USA) Inc.
11001 Electron Dr.
Louisville, KY 40299-3849
Kuhn Knight Industrial Div.
1501 W. Seventh Ave.
Brodhead, WI 53520
McCloskey International Ltd.
1 McCloskey Rd.
Peterborough, ON Canada K9J 6X8
Norton Environmental Equipment
6200 Rockside Woods Blvd.
Independence, OH 44131
P.O. Box 40490
Eugene, OR 97404
Powerscreen, A Terex Company
11001 Electron Dr., Louisville, KY 40299
1772 Corn Rd., Smyrna, GA 30080
United Rotary Brush Corp.
20078 State Rt. 4, Marysville, OH 43040
West Salem Machinery Co.
P.O. Box 5288, Salem, OR 97304
Hwy. 81, Box 523
Freeman, SD 57029
May 24, 2005 | General
A COMPOST SCREENING PRIMER
BioCycle May 2005, Vol. 46, No. 5, p. 55