Can Sand Clean Polluted Storm Water?

With the increasingly visible ubiquity of climate change, cities, towns and villages across the globe are being subject to more extreme weather events. On the one hand, this can result in water scarcity, as droughts and extended periods of dry weather contribute to a lack of available drinking water. On the other, it can exacerbate pollution, as storm water can pick up unwanted contaminants and then flood lakes, reservoirs and other bodies of water.

Now, scientists from the University of Berkeley in California are seeking to address both problems with the same solution, having published their findings in the journal Environmental Science and Technology late last month. Through the use of mineral-treated sand, the researchers believe they are able to effectively filter most unwanted contaminants out of storm water, thus reducing pollution and generating fresh drinking water for water-deprived communities in one fell swoop.

Problem becomes solution

As the intensive rainwater of tropical storms washes over cities and the countryside, it picks up a wide array of different pollutants on its way. Petrol and diesel spills from cars, herbicides and pesticides from crop fields, slurry and other animal waste and a cocktail of toxic chemicals can all find themselves being swept up in the torrent, which all too often finds its way into lakes and reservoirs.

Traditionally, this has resulted in the difficult conundrum of how best to clean up these flooded bodies of water, but University of Berkeley scientists believe they may have struck upon an ingenious solution using manganese-coated sand.

“The way we treat storm water, especially in California, is broken. We think of it as a pollutant, but we should be thinking about it as a solution,” explained Joseph Charbonnet, lead author on the paper. “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.”

Passive, low-cost solution

Sand filtration systems have been widely used in wastewater treatment plants around the world for several years, including at several Thames Water treatment plants in the UK, though these systems have yet to harness the power of manganese. When harmful organic chemicals like bisphenol-A (BPA), herbicides and pesticides come into contact with manganese, they bind to the element and break down into more easily biodegradable pieces.

Scientists have known about this process for some 30 or 40 years but have yet to discover many beneficial, practical applications for it. Charbonnet and his team are attempting to do just that with their study, which has seen the manganese-coated sand filter almost all BPAs from simulated storm water. Though some contaminants remain, these can be purged by using the new technology in conjunction with existing wastewater treatment methods.

It is true that the manganese does lose its effectiveness over time – but thankfully, this can be simply rebooted by soaking the sand in a chlorine solution for 48 hours. “If you have to come in every year or two and dig up this sand and replace it, that is incredibly labour-intensive, so in order to make this useful for community stakeholders it's really important that this stuff can be regenerated in place,” he said.