MUNICIPAL MULCH
PROCESSING 45,000 TONS OF YARD TRIMMINGS AND WOOD
St. Petersburg, Florida turns vegetative and woody materials into mulch products for residents, bagging companies and in highway beautification projects.
Located on the northern extreme of the subtropic zone, the city of St. Petersburg, Florida has a nine-month growing season. That makes yard trimmings processing a priority for Jim Ragsdale, mulch program coordinator for the city’s Sanitation Department, which accepts yard trimmings and wood residuals from residents and its own parks department. Landscapers can drop off materials if they are accompanied by their residential customer. The city processes about 45,000 tons/year of material into mulch and a small fraction as boiler fuel.
Yard trimmings are collected at six dropoff sites. They are then delivered to the processing area, where a contractor runs them through a Mighty Giant hammermill tub grinder. The contract requires that no more than five percent of the ground product be over four inches long. Depending on market demand, some of the mulch may be piled up for up to a year. Most is moved well before then to an adjacent site, where it is placed in windrows about 24 feet wide, ten feet high and 150 feet long. “We like to keep the yard waste there for a minimum of six weeks,” explains Ragsdale. “More often than not, it stays for three months, and it is turned once before it goes out.”
Most of the yard trimmings mulch is given away for free to residents. “We tell them that it should be used only as a topdressing,” continues Ragsdale. “Every once in a while, someone will bury a plant in it, but we don’t recommend that because it is high in lignin.”
About 2,000 tons/year of mulch are put through a Re-Tech Eliminator trommel with a three-eighths-inch screen and sold as a “soil builder” for $6 to $8/cubic yard (cy). “Customers use the screened product in their plant beds or as a topdressing on for lawn improvement,” says Ragsdale. “We promote it for its physical qualities — ability to retain moisture, hold fertilizer and lower bulk density. We always instruct customers that there needs to be pH modification and supplemental nitrogen provided with it. The soil here has a high sand content from our close proximity to Gulf of Mexico. Condos and hotels on the beach use our soil builder product, and we sold 1,500 yards to a company that’s doing bioremediation using plants to pull salts out of soil.”
About 5,000 tons/year of tree cuttings without green vegetation are dropped off at one of six municipal sites. “Many homeowners accompany the tree trimmers who serve them,” says Ragsdale. “Residents also stop by with trees they have cut down themselves.” The logs must be no longer than five feet and stumps cannot weigh more than 300 lbs. They are transferred to the central processing site in a 100-cy live bottom transfer trailer or a 75-cy open top push-blade trailer.
As enough wood accumulates and dries out, the grinding contractor is brought in for a processing job that lasts from three days to two weeks. Grinding is scheduled to take place only a few weeks ahead of orders because customers prefer a light color similar to that of the competing cyprus mulch. The ground mulch stays on site for about 30 days if possible, and usually is gone within a few months.
At $4 to $7/cy, the recycled log mulch is about half the price of cyprus mulch. “Ours is shredded as opposed to being chipped with a blade, so the particles are a little bigger and tend not to ‘float’ as much,” notes Ragsdale. Most sales are made in the spring and fall. The biggest market is highway beautification projects because government agencies have specified recycled content. For example, 25 truckloads of log mulch were ordered recently for application on an expressway. Bagging companies provide another important outlet for sales. When the mulch market is down, some product is marketed as boiler fuel. Everything generated in a given year is distributed within 18 months. — D.B.