June 26, 2006 | General

Screening Strategies To Optimize Compost Quality

BioCycle June 2006, Vol. 47, No. 6, p. 44
Compost and mulch producers mix and match screening equipment to meet multiple markets and finished compost variability.

The Lehigh County, Pennsylvania yard trimmings composting facility’s experience with screening finished compost is not atypical of many sites that have been operating for a number of years. When first selecting a screen for compost and mulch, many programs opt for a trommel, a versatile piece of equipment that can handle a wide variety of materials. Then, as time goes by, markets for the compost and end product challenges such as variable moisture content become evident, and facility managers and owners may diversify their screening options, adding star or vibrating deck screens, for example.
“We started out with a Rawson trommel that is now 11 to 12 years old,” says Tim Bollinger, operations manager at Lehigh County’s Organics Recycling Facility. “Then, because of the higher moisture content of our compost, we purchased an Erin Star screen. Finally, about a year-and-a-half ago, we were in the market for a new trommel screen and we visited a facility that had a CEC multiple deck screen. Given our end markets and some challenges we were having with the star screen, we ended up buying a CEC unit.”
The Lehigh County site processes between 85,000 and 100,000 cubic yards of leaves, grass and brush per year. Its Valley Green product line includes straight compost and topsoil blends, as well as mulch. The trommel is used for topsoil blending and some compost screening. Facility staff rebuilds components as needed to keep the machine in operation. The star screen is used primarily for mulch, which seems to be a better application for the county’s purposes. “We had a lot of challenges with the stars getting clogged,” explains Bollinger. “Even with the coating on the stars, a lot of material was sticking to them. But it works great for mulch or drier material that doesn’t stick to the stars.”
The vibrating deck screen is used to produce a three-eighth-inch compost, but can screen to one-quarter inch if needed. The 2-inch overs fraction, primarily contaminated rejects, is landfilled. The county is looking for customers for the middle-sized material fraction. Overall, he adds, “there is minimal spiking with the screen and on average, the throughput is 100 cubic yards/hour. But we’ve found throughput is really more a matter of what the operator can put out versus the screen itself.”
Much has been learned over the years about screening compost and mulch to yield high quality, high value products. Some key considerations to maximize the effectiveness of final product refinement include:
Adequate Decomposition: Try to achieve adequate decomposition of organic feedstocks, especially cellulosic material, prior to product refinement. Filamentous cellulosic strands can behave like film plastic in the final screening system, so maximizing their decomposition improves product quality. This also benefits separation of organics from inerts such as glass and stones.
Optimum Moisture Content: The literature cites an optimal compost moisture content between 30 and 40 percent prior to final product refinement. Material that is too moist, usually greater than 40 to 45 percent, does not separate as cleanly as dry material. (If material is too dry, however, complications result from dust.) This results in compost carry-over into the residue, or too much film plastic in the final compost. If space allows, storing compost prior to screening under a open-side structure, or spreading it out to allow moisture to escape (when rain isn’t predicted) are several options for hitting the 30 to 40 percent moisture range. At aerated composting facilities, windrows of finished material can be run on positive air for a brief period to reduce the moisture content.
Vibrating Action: If possible, maximize the “presentation” of material as it enters the screen. This may include vibratory conveyors to break materials apart (e.g., shaking plastic loose from compost), wider screen decks to spread the material out, and finding the optimal rate of feed. Some facilities prefer using vibrating deck screens to achieve a similar effect.
Air Separation: Several vendors market air separation units that are used after the primary screen to remove film plastic. Air classification generally has not been effective to remove plastic from the finer product, or unders, from the initial product screening. These systems can have difficulty differentiating light plastic from light compost. For example, a study on an air classification system was conducted at a yard waste composting site in the Toronto region operated by Miller Waste Systems. Trials were run on trommel screen overs, unders, and unscreened material straight from a windrow. With the unders fraction, the air separator removed both film plastic and fine compost particles as a result of their similar specific gravities. The fine compost particles burdened the effectiveness of the air separator and caused buildups on the ductwork and the fan assembly in the system tested. Conversely, the air separator was highly effective at removing film plastic from the overs fraction during this trial. Manufacturers of air classification equipment include the Hawker Corporation Air Lift Separator and the Komptech “Hurrikan.”
While the fundamental principles behind the designs of various screens (e.g., trommels, stars, deck) haven’t changed over the years, the innovations introduced by equipment manufacturers reflect the tailoring of machines to meet operational requirements. One critical requirement for many operators is portability of the screen, both for moving it around a facility, and transporting it from site to site. For example, Screen USA manufactures three models of portable star screens and two models of portable trommel screens. “For applications where high moisture is a problem, the star screen is definitely the solution,” says Rick Cohen of Screen USA. “In addition, particle sizes can be changed by simply turning a dial. When twine or banding material is abundant in the compost, we recommend our trommel screeners over the star. In drier applications, either screening technology works well.”
Recently, the company has focused on the smaller compost producers. Its Model TROM 406 portable trommel has a 4-foot diameter by 6-foot long screen. The machine, which can be towed behind a three-quarter ton pickup truck, is offered with a 42-blade pulverizer to break up “clumpy compost” before screening. “This is very important for the smaller producers who have not grown large enough to purchase a compost turner,” adds Cohen. The company also markets portable shredding/screening machines designed for smaller producers with a one to three cubic yard bucket loader.
Screen Machine also sells a portable shredding/screening trommel plant, marketed as the “Might.” It has a 1-cubic yard hopper with a grizzly, 22 free swinging blades in the shredder, and a 3-foot by 4-foot trommel with a self-cleaning brush.
Erin Systems introduced a Finger Trommel screen (1020 model) that combines both prescreening and fine screening technologies in one portable unit. The 5-foot wide by 10-foot long vibrating finger screen is designed to scalp out large debris, while a 6-foot by 20-foot trommel sizes the prescreened material. The machine can be used for rough topsoil, wood waste and construction and demolition debris.
McCloskey International Ltd. designs and manufactures trommel and vibrating screens and conveying systems. The company patented a 180 degree radial stacking conveyor on trommel screens to maximize stockpiling efficiency and lower operating costs. Its “Drum X-Change” system reduces time when changing trommel drums (less than 10 minutes, according to the McCloskey website). A more recent offering is a track-mounted screen to facilitate mobility around compost and mulch production sites.
Last month, Powerscreen introduced the Warrior Radial, a heavy-duty machine capable of three-way splitting and stockpiling compost, topsoil, mulch and other products. The unit has the capacity to screen 550 tons/hour. Its screen box can be raised and lowered hydraulically for maintenance. By removing the side conveyor, it can be converted into a two-way split machine with two products on the output conveyor. The fines conveyor can radial to 180 degrees. – N.G.
Allu Group
700 Huyler St.
Teterboro, NJ 07608
Amadas Industries
1100 Holland Rd.
Suffolk, VA 23434
Construction Equipment Company (CEC)
P.O. Box 1271
Lake Grove, OR 97035
DuraTech Industries
P.O. Box 1940
Jamestown, ND 58402
EarthSaver Equipment, Inc.
P.O. Box 7325
Kalispell, MT 59904
Erin Systems
1 Ave. Premier
Riviere-du-Loup, PQ,
Canada G5R 6C1
Komptech GmbH
Kuehau 37
Frohnleiten, Austria 8130
43 3126 5050
McCloskey International Ltd.
1 McCloskey Rd.
Peterborough, ON
Canada K9J 6X8
P.O. Box 40490
Eugene, OR 97404
11001 Electron Dr.
Louisville, KY 40299
Screen Machine Industries, Inc.
10685 Columbus Expressway Park East
Pataskala, OH 43062
Screen USA
1772 Corn Rd.
Smyrna, GA 30080
Terex Finlay Hydrascreens
11001 Electron Dr.
Louisville, KY 40299
United Rotary Brush Corp.
20078 State Rt. 4
Marysville, OH 43040
West Salem Machinery
P.O. Box 5288
Salem, OR 97304
Wildcat Manufacturing
420 South Highway 81
Freeman, SD 57029

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