Residuals processed at these facilities are as follows: Intervale, Manure, yard trimmings, commercial food and food processing residuals; Bluestem, Yard trimmings, commercial food residuals, paper mill solids, and industrial organics (e.g. pharmaceutical); Seacoast, Yard trimmings and commercial food and food processing residuals; MES/Dickerson, Yard trimmings; Albuquerque, Biosolids and yard trimmings; and Palm Beach, Biosolids and yard trimmings.
The first step was to get basic screening information, including function (e.g. prescreening to remove contaminants, postscreening to size for markets) and type of screen(s) used. The Intervale has a trommel (Powerscreen 620) that it uses for postscreening. Bluestem has a trommel (Royer Powerscreen 830) used exclusively for postscreening. Seacoast uses its screen (a Wildcat RHC 5-140 trommel) to ready its finished compost for bulk, wholesale and retail sales. Dickerson has two trommels that process materials after the composting cycle to remove oversize particles and contaminants. The City of Albuquerque screens its finished compost with an older model Wildcat trommel. Palm Beach County has two screening operations. The first is adjacent to the in-vessel composting site, where yard trimmings are processed through a star screen (Erin) prior to being mixed with biosolids. After composting, the facility uses a deck screen (Bivitech).
MOISTURE CONTENT, HOLE SIZE AND THROUGHPUT
BioCycle articles about screening compost discuss the impact that moisture content has on screen productivity. As such, operators were asked about the moisture content they shoot for prior to screening compost. We followed up with a question about what happens if the compost isn’t in that moisture range but is screened anyway. The final question in that section asked about the screen hole size(s) and throughput (roughly in cubic yards/hour).
Intervale: “I shoot for 35 percent moisture content, but it is usually 42 to 45 percent,” says Adam Sherman. “We significantly lose production efficiency and most of the product rolls out the end of the barrel if we don’t hit our target.” The trommel has three-eighths inch holes and throughput is between 35 and 60 cy/hour. “It should be 100 cy,” adds Sherman. Overall, the screen is operated about 30 hours/week.
Bluestem: While Dave Hogan said any exact moisture content percentage would be a guess, they try to only screen compost that is friable, blendable, flowable and not wet. “Material stored under cover is used when outside conditions are not right,” says Dave Hogan. “It can be blended or used exclusively until more desirable moisture conditions are present.” Screen size starts at a half-inch and reduces to three-eighths inch for a net three-eighths inch minus total effect. The screen is operated roughly 30 hours/week.
Seacoast: The targeted moisture content is less than 50 percent if possible, but if the compost is high in moisture, “we don’t have a lot of options, especially in February and March prior to our busy season,” says Bob Kelly. “This just slows production.” The facility uses a screen size of three-eighths inch and one-quarter inch for a fine blend. “We achieve 40 to 50 cy/hour throughput with this,” adds Kelly. From February to June, the screen is operated 25 to 35 hours/week. From July through October, it drops to 20 hours/week.
MES/Dickerson: “Best case for us is a moisture content of 20 percent or less,” says Nanci Koerting. “If not, we end up with compromised production!” Screen size is three-eighths inch and throughput ranges due to compost maturity. “The average is about 150 cy/hour and up,” notes Koerting. “When a new batch is started, production is much slower. The older the batch, the faster it screens.” She estimates that the screens are operated 700 to 1,000 hours each on an annual basis, prefering to give a yearly average because screening is weather and market dependent.
Albuquerque: The targeted moisture content is 40 percent or less. “If compost is too wet, it clogs the screen mesh; when it’s too dry, it gets dusty,” says Steve Glass. Material is screened at three-eighths inch mesh; throughput is 40 cy/hour. It operates about 60 hours/week.
Palm Beach County: While a moisture content of 45 to 50 percent is optimal, the deck screen is capable of functioning at either a higher throughput (dry) or lower throughput (wet), says Pat Byers. Yard trimmings are screened prior to composting to one- and one-quarter-inch minus, with a throughput rate of about 60 tons/hour. Compost is screened to one-half inch and one-quarter-inch minus, with throughput of about 80 tons/hour. The star screen runs about 100 hours/month while the deck screen is used about 80 hours/month.
MAINTENANCE AND REPAIRS
The rest of the questions on the mini-survey focused on maintenance and repairs. Responses were as follows:
Primary Maintenance Areas: Generally, it appears, the screens are pretty good workhorses and don’t require high levels of maintenance. At Bluestem, operators note there is high wear on the first two (inlet) screen sections, so those are replaced as needed. At the Dickerson site, hydraulic hoses are the number one maintenance area, followed by brushes and bearings. In Palm Beach County, the stars, bearings and chains are the primary parts to maintain, while the deck screen requires very little maintenance, says Byers.
Maintenance Schedule: The Intervale greases machine parts daily, and changes the oil and filters after every 100 hours of operation. Bluestem does a regular service after every 120 hours of operation that includes greasing and changing the oil and filters. The air filters and radiators are blown out every other day (or as needed), brushes are replaced every six months and the drum chain is changed annually. Seacoast operators grease the screen daily and review it weekly for wear points. “Brushes are key to good screening so these are cleaned weekly,” says Kelly. New brushes are installed annually, and new screens are put in every other year.
At the Dickerson site, the trommels are greased every ten hours, and the engines and hydraulics are serviced every 250 and 500 hours, respectively. “The brushes last about 400 to 500 hours depending on the maturity of the compost,” says Koerting. “The younger the batch, the tighter the screens have to be to the drum, accelerating wear.” In Albuquerque, general maintenance is done monthly on the trommel and brushes are replaced every six months “at $2,500 a pop!” says Glass.
Operators note that the conveyors feeding their screens have been mostly trouble-free. Bluestem’s conveyor still has its original belts after four years of operation; at Dickerson, the belts have been “virtually maintenance free” except for greasing, and that is after eight-plus years of use. Albuquerque’s experience has been similar. The site just replaced the belts and bearings (at a cost of $3,000) after eight years of operating the screen.
Screen Cleaning: Brushes are the primary tool used to clean the screens, although Palm Beach County uses compressed air and water. At the Seacoast facility, the brushes have a double rake mechanism that cleans them as well. In Albuquerque, efforts are underway to retrofit the screen with compressed air or a cheaper brush configuration, says Glass.
Maintenance Versus Repairs: We asked operators how much of their maintenance schedule is consumed by actual repairs. The answer, in short, was very little. “The only repair we have done in two-and-a-half years that was not maintenance is replacing a belt splice that broke in midscreening,” says Kelly. At Bluestem, two bearings were replaced for the first time in December. The site had a minor transmission shaft problem three years ago (under warranty), and has had no problem since. In Albuquerque, Glass notes that “very few repairs are needed.” At the Dickerson site, with its older trommel, repairs account for about 25 percent of maintenance, however repairs on the newer trommel are five percent or less of maintenance. The primary areas needing repairs (versus maintenance) are bearings and roller chains.
In terms of maintenance/servicing time to overall hours of operation, the following ratios were provided: Intervale, 1:8; Bluestem, 2.5 to 4:120; Seacoast, 1:8; Albuquerque, 2:60; and Palm Beach County, 15 minutes:10 (with both screens). Koerting at Dickerson notes that the ratio is dependent on the age of the compost being screened.
Repair and Maintenance Costs: Finally, the minisurvey asked operators about the annual cost of maintenance (labor and parts) as well as the annual cost of replacement parts. Bluestem spends about $6,000/year on maintenance, and about $3,000 to $4,000/year on replacement parts. Seacoast spends between $1,000 to $2,000/year on both, depending on whether they buy screen cloth or not. At Dickerson, replacement parts are about $3,000/year. Albuquerque spends about $1,500/year on maintenance and $5,000/year on parts. All sites own their screens excepting for the Intervale project, which leases its trommel.