When a municipality is trying to meet state recycling goals, all the glass in the world won't do it any good if it's broken or contaminated.
At the Solid Waste Authority of Palm Beach County, Fla. (SWAPBC), program performance suffered from excessive glass breakage during collection and processing. The authority reported a breakage rate as high as 80 percent (see chart).
Most of SWAPBC's broken glass passed through the MRF unsorted, resulting in large stockpiles of mixed-color glass. This was a problem since prevailing glass markets purchase only color-separated glass and broken glass re-quires more labor to manually sort fragments by color.
In addition, without any mixed cullet buyers, the material was landfilled or reused in ways which did not generate any revenue and the unmarketed mixed cullet could not be counted as "true" re-cycling.
In order to determine the causes of breakage, SCS Engineers, Long Beach, Calif., evaluated the au-thority's commingled glass collection and recycling program.
To rank the causes of breakage, the engineers sorted and weighed broken and unbroken glass at each stage of the collection and re-covery process (curbside set-out; separating and loading into various collection vehicles; glass discharge at consolidation points and at the MRF; and processing the glass into a marketable feedstock).
The glass-handling activities were ranked as follows:
MRF processing broke approximately 37 percent of all glass collected. Almost all of this breakage occurred at a shaker table, where the broken glass and garbage are separated from other recyclable materials.
When materials sorted curbside were automatically dumped into the body of the collection truck, an estimated 23 percent of all glass was broken.
Collection vehicle discharges (when glass loads were dumped on the MRF or transfer station tipping floor) broke an estimated 4 percent.
Loading and discharging the transfer trailers broke an estimated 8 percent of materials collected (combined). Most of this breakage occurred while using a hopper to load the recyclables from the tipping floor to the transfer trailers waiting in a bay one story below.
Residents or drivers sorting the curbside glass broke an estimated 1 percent of all glass collected.
SWAPBC and most non-SWAPBC trucks use automated overhead loaders; manually-loaded fleets reportedly break less glass. Auto-mated trucks, for example, broke approximately 50 percent of the glass during loading while manually-operated trucks reported a 7 percent breakage rate.
As a result of the study, the consultants recommended to alter:
* The MRF process line. Cover the steel fingers on the shaker with rubber or plastic sleeves; rubber or plastic bumpers could be installed on the sides of the shaker. Also, reduce the shaker table's area (or length) to minimize losing smaller containers and large glass pieces.
* Collection vehicle discharge practice. Minimize the commingled recyclables discharge speed. Begin dumping with the body in a horizontal position and use a gentle tilt to make the glass slide from the truck. Lessen or reduce the height from which commingled recyclables are discharged. On most trucks, begin dumping with the internal door released prior to tilting.
* Collection practices. The fall from the overhead dumping position (approximately 8 to 10 feet) ends with a hard steel floor. Cover the vehicle's floor with a shock-absorbent covering such as a rubber sponge or plastic matting. Al-ternatively, use netting or baffles inside the truck to break the fall of collected glass. While this technique had only limited success at SWAPBC, the costs are relatively low on a per-truck basis.
Since floor coverings inside a collection vehicle may cause secondary problems, this strategy was given the lowest priority for potential implementation. The mats, for example, may absorb rain water and other liquids which collect in recyclable containers.
Armed with insight into the causes of broken glass, SWAPBC has begun to explore design chan-ges at the MRF. Drivers also are being retrained with better procedures to collect curbside glass.
To date, SWAPBC has:
* Modified loading and unloading practices at transfer points and the MRF and minimized glass handling.
* Changed the shaker table's design and screen size to reduce glass breakage and ease color sorting.
* Established financial penalties against contract MRF operators for excessive residue tonnages. Mixed cullet now counts towards the res-idue tonnage total for the MRF (rather than a recovered tonnage), giving the contractor incentive to cooperate in controlling glass breakage.
Funding for the study was partially provided by the Southeast Glass Recycling Program of the Glass Packaging Institute.