After hundreds of years of development, Philadelphia has lost much of the organic matter, minerals, sand and clay that make up natural soil. Instead, underneath our feet is lots of "rubble, debris and the history of the city," said Daniel Flinchbaugh, assistant landscape planner at the University of Pennsylvania.
A pilot project from Penn, Bennett Compost and Remark Glass is creating a solution with a new resource: sand made from recycled glass. In the fall, the partners created an engineered soil mixture for planting 13 new trees on the school's campus, hoping it would help keep the trees healthy and prolong their lives.
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Remark accepts about 10,000 to 15,000 pounds of glass per month through community drop-off programs and a small collections route for reuse into glass design products like cups and vases. In 2021, the company began collaborating with architecture firm OLIN to create a waste-based mixture that could be made with local materials. For this project, the blend was added to Bennett's compost, which is made from food waste at Penn, and a type of carbon-rich charcoal known as biochar. OLIN, the Morris Arboretum Urban Forestry, Craul Land Scientists and BrightView Landscape Contractors contributed to research for the soil mixture.
To create the sand, the glass is crushed through a grinder, churned and turned into particles that are soft to the touch. Co-founder Rebecca Davis said it's comparable to 3 millimeter construction sand and can be colorful if made from green, blue or red bottles.
After digging a hole, the soil is set in layers: first the biochar, then the glass sand and topsoil and then compost. This simulates more of a natural environment, Flinchbaugh said, with smaller pores at the top and larger ones at the bottom for aeration and drainage.
The glass helps with avoiding compaction, which prevents water from penetrating the tree roots, and is better for drainage while also holding onto moisture during drought periods. Davies said it has very few contaminants because the glass is originally made to hold food or drinks.
Scaling this process up for larger projects is exciting to Davies because demand for sand keeps increasing, especially for use in construction and concrete. But extracting it from places like the New Jersey Pine Barrens can damage those environments. The World Economic Forum reported sand as the second most exploited natural resource after water.
"Sand is an exploited resource so having an alternative to that and developing it over the next few years together is really important," Davies said.
The resistance to compaction is particularly important in cities with all the walking, driving and development on urban streets, said Brent Lewis, university landscape architect at Penn. City trees have a life expectancy of 19 to 28 years, only a fraction of the hundreds of years rural trees can live. Resisting compaction can add five to 10 more years of life, he said.
Still, it will be a while until the experiment can be replicated on a mass scale — Penn has over 7,000 trees on campus and replaces as many at 100 each year. Unlike smaller plants, it can take a few years to determine the full impact of soil and other environmental changes on trees, so the team will continue monitoring these 13.
"We're really hitting multiple levels of sustainability," Lewis said. "Street trees have a finite life and so anything that we can do to get more life out of that means a longer period of carbon sequestration and more oxygen and better overall benefits for our entire community here."
