New Purification Method Uses Sand to Clean Chemical-ridden StormwaterSiobhan Treacy | August 30, 2018
The University of California Berkeley has developed a new method to purify stormwater to create a clean source of water for water-stressed communities. The method uses mineral coated sand that reacts with and destroys organic pollutants.
When it rains, stormwater travels over roofs, lawns and streets, gathering many chemicals and contaminants that make the water undrinkable. This excess water can overflow sewer systems, flooding streets and basements. The team wanted to find a way to use this excess water to store for future drinking water during dry seasons.
"The way we treat stormwater, especially in California, is broken. We think of it as a pollutant, but we should be thinking about it as a solution," said Joseph Charbonnet, a graduate student in civil and environmental engineering at UC Berkeley. "We have developed a technology that can remove contamination before we put it in our drinking water in a passive, low-cost, non-invasive way using naturally occurring minerals."
The researchers directed stormwater through the sand into underground aquifers that could help gather water in areas that have long dry spells followed by big rainstorms, like Los Angeles. The collected water can be stored for later use during future dry spells.
"Before we built the buildings, roads and parking lots that comprise our cities, rainwater would percolate into the ground and recharge groundwater aquifers," said David Sedlak, professor of civil and environmental engineering at UC Berkeley and co-director of the Berkeley Water Center. "As utilities in water-stressed regions try to figure out how to get urban stormwater back into the ground, the issue of water quality has become a major concern. Our coated sands represent an inexpensive, new approach that can remove many of the contaminants that pose risks to groundwater systems where stormwater is being infiltrated."
The new coated sand is plain sand that is mixed with two forms of manganese that create manganese oxide. Manganese oxide binds to organic chemicals then breaks them down into smaller, less toxic and more biodegradable chemicals.
"Manganese oxides are something that soil scientists identified 30 or 40 years ago as having these really interesting properties, but we are one of the first groups to use it in engineered ways to help unlock this water source," Charbonnet said.
The coated sand removes most, but not all contaminants and chemicals. For this reason, it would need to be used with an additional filtration service.
The sand was tested by percolating stormwater through the material. During the first tests, the sand successfully removed all the BPA in the water, but over time it became less effective. Manganese oxide can be recharged by bathing the sand in a solution that has a very low concentration of chloride. It takes two days to fully recharge a half nm layer of sand in a solution that had a 25 ppm mix of chlorine in the water.
"If you have to come in every year or two and dig up this sand and replace it, that is incredibly labor intensive, so in order to make this useful for community stakeholders it's really important that this stuff can be regenerated in place," Charbonnet said.
The paper on the new sand was published in Environmental Science and Technology.