Using mathematics to realise the potential of a novel soil-based filter.

The Ganges-Brahmaputra Delta is a global hotspot for arsenic groundwater contamination. Naturally occurring arsenic concentrates in water drawn from deep tube wells, creating a major public health issue in West Bengal and Bangladesh, described as the “largest mass poisoning of a population in history”.

Arsenic mitigation technologies mostly rely on allowing the contaminated fluid to run through a porous network onto which the contaminant adsorbs. However, traditional adsorbents suffer inherent limitations, including cost unfeasibility and problems associated with regeneration and disposal of the spent adsorbent. Researchers at the Department of Chemical Engineering at the Indian Institute of Technology (IIT) Kharagpur, led by Prof Sirshendu De, have recently developed a novel technology that uses a laterite soil bed to filter arsenic. The set-up is simple: the contaminated water trickles through the soil, onto which the arsenic adsorbs. The abundance of laterite means that this filter can easily be scaled up, and has already been piloted in three communities in India, serving more than 5000 people.

However, while the spread of single contaminants through porous media is well-known, transport, when multiple contaminants interact through a soil bed, is poorly understood. In the areas in which the filter is being piloted, pesticides, fertilisers, and heavy metals are all present in the environment and it is unclear how their interaction impacts the effectiveness of the filter.

To develop a better understanding of these processes, IIT Kharagpur have begun a collaboration with Oxford mathematician Ian Griffiths and his postdoctoral researchers Sourav Mondal and Raka Mondal. The Oxford team are combining homogenization theory and asymptotic analysis with computational fluid dynamics techniques to derive a series of mathematical theories that predict the chemical transport and fluid dynamics processes taking place within the filter. The models will be combined with experiments and field data gathered by IIT Kharagpur during the collaboration to improve the prediction of how these chemicals spread. The results will offer insight into the action of the new filter and its expected lifetime before regeneration, and ultimately determine how this emerging technology can realise its full potential to remove contaminants from the soil and groundwater.

The researchers hope that this collaboration will instigate future UK–India research and development co-operation and boost science and technological endeavours aimed at providing solutions to environmental problems, particularly those faced by low-income countries.