AUSTIN, Texas -- Civil engineers in the city of Austin’s public works department have been experimenting with viable uses for waste glass – as in the kind many assume gets recycled into new bottles.
For the most part, though, waste glass fails to meet the stringent criteria – clean and sorted by color – that manufactures require for making new bottles and jars. While some glass cullet fetches as much as $60 per ton, unsorted glass co-mingled with other waste is virtually worthless to manufacturers of new glass. But that doesn’t mean it can’t be removed from the waste stream to serve a useful purpose as an aggregate substitute in a variety of practical applications.
In Austin, where residents and businesses throw away 50,000 tons of glass annually, it’s no small issue. In one composite study, glass by weight made up a quarter of the city’s waste going to the landfill – higher than the U.S. average of 12 to 15 percent. That’s just one of the challenges the city faces with an ambitious plan to divert 90 percent of its waste from the landfill by 2040.
Keri Burchard-Juarez, Austin’s assistant director for engineering and capital project delivery, and Steven Penshorn, supervising engineer for quality and standards management, presented their findings at the American Public Works Association's recent International Public Works Congress and Exposition in Anaheim.
“Public works organizations have a great ability to further eliminate the landfilling of glass waste,” said Burchard-Juarez. “We have a lot of opportunity to use (glass as) alternate aggregate, and to experiment with it in some low-impact applications.”
The department has substituted ground glass cullet for aggregate in public works projects, using a glass crushing machine to produce consistencies as coarse as pebbles or as fine as sand, Burchard-Juarez said.
Austin’s public works department has opted for simpler applications, including landscape mulch for walking trails, drainage media for pervious pavements, backfill for a retaining wall, and bedding material under sidewalks and small-diameter water and service lines.
Penshorn said smaller projects with less challenging performance requirements, and applications that allow the cullet to be used “as is” without having to process it more than usual, provide opportunities that are much more viable and allow easy entry for public works organizations.
For a new sidewalk, workers substituted fine-grained glass cullet for the two-inch sand cushion layer they normally used.
“It was often as simple as arranging for our contractors to go out to the MRF (Materials Recovery Facility) to pick up the material instead of going out to the quarry,” said Penshorn.
For a vacant lot that had issues with drainage and standing water, public works created a hybrid grass and pervious pavement. Beneath the sod, they sandwiched re-purposed crushed concrete between layers of glass cullet.
“We had a problem with it draining so well it’s hard to keep the grass alive,” he said.
A chip seal project to resurface a parking lot proved to be more of a challenge. The glass cullet didn’t adhere properly to the pavement.
“Certainly the glass doesn’t have the porosity of a natural aggregate,” said Penshorn. “Some have used it successfully as chip seal, but it requires this extra process.” The sugar coating left over from beverages needs to be rinsed off.
Other advanced applications were considered but passed over, such as asphalt hot mix, or “glassphalt.” Not knowing the performance history and how it would support heavy loads was just one factor discussed within the department. The logistics of shipping the material to a hot mix plant was another. They also learned that the residual glass would likely find its way into the next batch of hot mix for the plant’s customers.
Public works agencies have experimented with glass cullet. Among those highlighted in the presentation were:
• City of Spokane, Wash.: Crews combined 1,500 tons of glass cullet with crushed rock. The resulting material was used to create bedding for the asphalt, a reconstruction project for Market Street spanning 1.25 miles.
• Washington State Department of Transportation: More than 1,200 tons of glass cullet has been used as bedding for large stormwater pipes.
• New York State Department of Transportation: Engineers used glass cullet to create a cost-effective filtration system for removing Total Suspended Solids from concrete slurry during hydro-demolition in 2005.
But if a state agency doesn’t allow for glass cullet projects, public works departments should focus on projects that don’t rely on state funding, said Burchard-Juarez.
“They may not be willing to use an alternate aggregate on a main highway system, but as a public works department, you may maintain a road within a park that you might be willing to take a chance and try something different.”
“If you can just find one or two good uses of alternate aggregate in your department, you can really have an impact on the amount of glass that's going to landfill in your community,” she said.
Mixed glass cullet, worth $5 to $6 a ton, can compete in price with natural aggregate. Considering the material is readily available, it doesn’t need to be mined or extracted, or excessively transported, reducing its carbon footprint.
But if left in a landfill, it will never biodegrade.
“You can save money if you send less of that to a landfill,” said Burchard-Juarez.
Texas has a pending bottle bill that would establish a refundable deposit for certain beverage containers. With a value placed on the containers, it would likely increase the number of glass bottles diverted from the landfill.
The EPA estimates the nationwide recovery rate of glass from the waste stream is about one-third. The glass recycling industry is exploring ways to increase its recovery rate through technology such as optical scanners.
But there will still be unmarketable mixed glass cullet that doesn’t reach recycling standards. And that means the alternative aggregate market could get bigger.