What if every town could afford to filter stormwater?
Recycled material could protect water quality—cheaply and simply
What’s at stake
Extreme rain events don’t just mean flooding—they mean water pollution. Stormwater sweeps toxins such as heavy metals and bacteria from streets, parking lots, and roofs into storm drains, often emptying directly into local waterways. Stormwater contaminants enter surface water, where they affect plants and wildlife.
Heavy metals, common in stormwater runoff, are particularly hazardous to the environment. Once introduced, they don’t go away on their own. They are highly toxic to animals even at low levels and become more concentrated as they move up the food chain. High levels of heavy metals can reduce fish and other aquatic animal populations. Fish exposed to heavy metals may even lose their ability to communicate with their surroundings.
Municipalities could protect water quality by filtering heavy metals from stormwater as it passes through storm drains, but not all towns can afford filters. New, low-cost materials could make filtration more accessible to U.S. towns and other resource-constrained settings that wish to protect the environment.
Capturing pollutants
Dibyendu “Dibs” Sarkar of the Stevens Institute of Technology teaches his engineering students to recycle and repurpose materials whenever possible. Creating whole new systems or structures is unnecessary—existing systems can be retrofitted, saving money while making progress for the environment.
Some cities and towns fill their stormwater filters with sand. Sand can remove debris but doesn’t capture metals or other chemical contaminants in runoff. Sarkar aimed to remove heavy metals from stormwater by replacing sand with a more effective medium. His team met the challenge by recycling various waste products obtained at little or no cost.
Aluminum-based water treatment residuals (WTR) are non-hazardous materials left over from water treatment plants. WTR would otherwise end up in landfills, so it is available for free. And it’s plentiful; two million tons of WTR are produced in the U.S. annually. WTR acts as a powerful adsorbent, meaning pollutants cling to it like a magnet.
“Anytime I think about something holistically, recycling comes to mind; repurposing comes to mind. Those are the low-hanging fruits.”
- Dibs Sarkar
To establish the right balance of filter and flow, Sarkar’s team tested many combinations of materials in laboratory column experiments, followed by a field study. They selected a mixture of WTR beads, sand, and carbon material to fill storm drain filters near campus parking lots. The mixture removed 27 percent of copper, 36 percent of lead, 69 percent of zinc, and 46 percent of hydrocarbons from stormwater in the drains.
Sarkar notes that many cities already filter their stormwater. Those municipalities don’t have to create new systems to improve water quality. All they need to do is substitute one filter material for another. That makes change easier.
“Development comes at a cost,” says Sarkar. “And you cannot do sustainability at any scale unless you think holistically. Anytime I think about something holistically, recycling comes to mind, repurposing comes to mind. Those are the low-hanging fruits.”
Beware the “complexity burden”
Susan Amrose is a research scientist and water quality expert at MIT. She cautions that looking at hidden complexities is essential before declaring WTR a universal low-cost solution. There’s a tradeoff between cost and complexity, she says. “Often, to operate at low cost, you add complexity.”
For example, transportation is not a problem in the U.S., but it could be a limitation in developing countries. “Waste materials are great. They’re very low cost, but there’s added complexity to gathering those materials. You have to be able to handle that complexity burden,” says Amrose.
Amrose says further research is needed to determine how widely WTR could be scaled to filter out heavy metals. The more conditions in which researchers test WTR, the better.
It’s also possible WTR varies by source in its ability to adsorb metals. If it varied widely, each municipality using WTR would need to test the materials as they obtained them, adding complexity and cost. That complexity burden would be significantly reduced if, as Sarkar envisions, municipalities did not have to compile adsorbent materials but could receive them from a central processor.
Another consideration is waste disposal. WTR would end up in a landfill anyway. But once used to filter stormwater, it brings heavy metals with it. Municipalities should weigh the long-term impact of having heavy metals in landfills against having them in aquatic environments.
Small steps to create impact
Over the past twenty years, researchers in the U.S., South Africa, and China have studied various low-cost adsorbents for stormwater treatment. Sarkar says he hasn’t found a low-cost adsorbent that meets the sustainability challenge as effectively as WTR.
“You do not have to create a sea change; you can take one little step at a time. Nice small, incremental changes,” Sarkar says. “We are facing a tough time in terms of climate change; there is no question about it. But there are small remedies, little footsteps. They will accumulate, and it will create an impact down the road.”