Can waste glass be repurposed as a planting medium for green infrastructure?

By Timothy A. Schuler 

It is easy to paint landscape architecture as an inherent “greener” of communities, particularly when it comes to green infrastructure and the profession’s more recent emphasis on creating and sustaining urban ecologies. But every project has an environmental footprint, including, in some cases, the destruction of wilderness areas hundreds of miles from the project site through sand mining and soil removal, which provide the raw material for landscape soil blends. “We put ourselves out there as purveyors of sustainability, but meanwhile we’re kind of like these crazy organ harvesters, borrowing healthy soil and transplanting it somewhere else,” says Richard Roark, ASLA, a partner at OLIN in Philadelphia. “I was like, can we stop that?”

That is exactly what OLIN is attempting to do through a multidisciplinary research project known as Soil-less Soil. Led by the firm’s research division, OLIN Labs, the landscape architects and their partners are studying the feasibility of repurposing postconsumer waste glass as planting media for green infrastructure projects. It isn’t the first time recycled glass has been tested as a replacement for sand. In 2013, a team of researchers planted panic grass and sea oats in an artificial dune made of cullet, crushed glass fines that cannot be recycled, and found that both species outperformed the control group. Similarly, a 2017 study found that using recycled glass in a sand drain improved permeability and drainage when compared to traditional specifications.

Roark and Rebecca Popowsky, ASLA, a research associate at OLIN Labs, are working with university researchers and industry professionals to test a variety of glass-based soil blends to evaluate the feasibility of cullet as a planting substrate in urban landscapes. If viable—and preliminary results are promising, Roark says—the alternative would address a trifecta of environmental concerns: the extraction of sand and other natural resources, the landfilling of huge amounts of glass (which has increased dramatically in recent years owing to shifts in the global recycling market), and uncertainty in the performance of green infrastructure (because of soil specifications not always being followed). By creating a specific biofiltration mix made from a local waste product—the composition of which OLIN plans to make public—Roark sees a possibility to increase consistency at a decreased cost.

So far, the OLIN team has evaluated the safety, permeability, and other characteristics of glass cullet, and a current bench trial at the University of Pennsylvania will study its effects on plant growth and mortality. Popowsky says some of the initial findings have surprised them. “The biggest problem area so far is that glass comes in with a high pH,” she says. Roark says this is because most glass contains sodium carbonate. As the material is crushed into finer and finer particles, its pH level increases. But the pH can be lowered to acceptable levels through the addition of ferrous sulfate, Roark says.

With plans to pilot the Soil-less Soil concept at Penn in a closed-loop system, the project is an example of how landscape architects can apply their practical knowledge to intervene in large-scale infrastructural processes and design not just spaces but systems. There is little doubt that recycled glass has a smaller carbon footprint than the alternative; a life cycle assessment conducted by a Penn graduate student showed that the use of recycled glass cullet would produce 67 percent fewer greenhouse gas emissions and use 48 percent less water than sand. The key to adoption, however, will be to alter as few variables as possible in terms of sourcing and blending soil mixes, Popowsky says. “The closer we can parallel that existing process, the more luck we’re going to have.”

Timothy A. Schuler, editor of Now, can be reached at timothyaschuler@gmail.com and on Twitter @Timothy_Schuler.