Reducing Breakage in the Collection of Glass Containers for Recycling Material: Recycled Glass Issue: A major barrier to achieving high recovery rates in recycling programs for container glass is the degradation in quality that occurs during collection. When glass is broken during collection, it is more difficult to identify and remove contaminants and to color-sort. Impact forces which occur during collection handling, combined with the weight of the containers, can cause a significant fraction of the containers to be crushed, creating an unsortable residual. This residual "break" glass must be removed in processing and handled for disposal or channeled to lower value applications. Municipal recycling programs collecting commingled recyclables have found it difficult to achieve marketable glass recovery rates of better than 50% to 70% of collected glass. These low rates represent a significant threat to the economic viability of recycling programs. Best Practice: Several techniques for controlling breakage have been tested in curbside collection programs across the country. The techniques focus on implementing design modifications to reduce the impact forces in the loading and unloading of containers. Curbside programs generally collect glass containers of commingled colors, and, in many cases, with other recyclables. The most effective way to control breakage and to insure the overall quality of collected post-consumer glass is to manually sort glass at the curbside. This type of handling insures that only glass of accepted colors is collected because the containers are manually placed into separate color compartments on the truck. Curbside sorting can boost glass recovery rates to over 90%, but the higher cost of truckdriver's wages necessary for handling can be difficult to justify at recycled glass market prices. Three modifications in collection vehicle design have been suggested by different studies as strategies for reducing glass breakage. The designs include the use of interior baffles to deflect the impact of glass containers, a net suspended within the truck body, and wires strung through truck compartments. The first modification involves adding an interior center deflection baffle to reduce breakage in truck loading. A solid baffle, built of durable synthetic canvas, is suspended inside the truck along the full length of the mixed recyclables or glass compartment, half-way between the roof and the floor. The baffle, sloped-off to the side at a 30 degree downward angle, is designed to catch glass containers as they drop and let them roll off into the bottom of the truck. In effect, the baffle reduces the distance the glass containers fall from a drop height of eight feet onto a steel floor, to a drop of four feet and a roll down the baffle. The study found that the installation of a baffle was a practical method for reducing glass breakage during collection. Expressed as a weighted average, the percentage breakage was 37.3%, a 21.4% reduction from the baseline of 58.7% breakage. Another design modification calls for a net suspended above the base
of the truck body. This modification involves the design and installation
of a webbed grid, suspended horizontally, approximately 3 feet above
the base of the mixed recyclables compartment of the vehicle. The
grid is designed to retain most plastic containers due to size, while
allowing glass, aluminum, and tin containers to pass. The plastic
containers can provide a cushion for the initial loading fall of glass.
The net itself also restricts the free fall of glass containers during
loading and can be attached to the sidewalls with springs which will
further A third approach involves wire cables strung through a truck's recycling compartments. The wire cables break the glass containers' fall in the truck. Wire cables strung throughout recycling compartments represents a relatively simple design modification. Before the wire system modification was made, the baseline recovery rate was 70.5%. After the wire modifications, the rate during the study was recorded at 79.4%, an improvement of 8.9%. Implementation: Effective design modifications require a broader consideration of other elements in collection, including the materials to be handled and how they will be loaded. Baffles, nets, or cables are most likely to be useful in preventing breakage where materials are dumped in bulk. It is also necessary to consider whether design modifications will interfere with unloading. For instance, in a configuration using a suspended net, it may be necessary to detach and clip back one side of the net to allow a load of paper to properly flow from the dump body. Any of these configurations will also reduce the load capacity. The maintenance needs and durability of any design alternative need to be considered. The use of industrial strength synthetic materials in baffle or net construction will help to extend the life of those mechanisms. The wire system is attractive because of simplicity and durability. Ultimately, the amount of breakage in a given truck load of collected glass is a function of how the vehicle is loaded and unloaded, the size and configuration of the truck, and the size of the load. Benefits: The 2" minus broken glass that is not recovered in commingled curbside collection is costly for recycling programs for several reasons, including disposal fees, lost revenue from the portion that cannot be recycled, premature equipment degradation from the broken glass abrasion, reduced processing productivity, and cross-contamination of materials. Design modifications can result in decreased production of a residual glass fraction, improving the revenue potential and economic viability of glass recycling programs. Application Sites: Curbside collection programs. Contact: for more information about this Best Practice, contact CWC mailto:info@cwc.org. References: |
