Tackling arsenic contamination in India through mathematical modelling and engineering.

Oxford mathematicians collaboration with IIT Kharagpur seeks to address one of the world’s most pressing water quality challenges.

Arsenic is a naturally occurring element found in the rocks and earth’s crust, and is among one of the most hazardous contaminants in drinking water sources. Arsenic is embedded in geological sources and enters into the water supply as the rocks are eroded by the flow of water from rivers and rainwater. Further, use of manure containing phosphate, and other agricultural activities break down different rocks and release arsenic into natural water. Consequently, if not properly managed, the level of arsenic contamination will continue to rise because of human activity and agricultural growth. At present, more than 200 million people and over 70 countries in the world are affected by arsenic contamination. The WHO has declared 10μg/L as the safe limit of arsenic in drinking water in 2000.

To address this issue it is necessary to have a water-purification technology that: (i) meets the safe drinking water criteria; (ii) requires minimal energy; (iii) offers high throughput; (iv) is easy to scale up to cater for large populations; and (v) generates minimal waste. A novel strategy that achieves all five of these criteria has recently been discovered, using readily available laterite soil. Laterite is iron-rich and is able to remove arsenic through adsorption. Raw laterite can be treated chemically to enhance the surface area and increase adsorption capacity by several factors. Refining the granular laterite to produce laterite dust increases the surface area and improves adsorption capacity even further. Filters that use this laterite soil are currently providing potable water to more than 5000 people.

However, as with any other adsorption technique, such filters have a certain lifespan, beyond which the filter media becomes saturated with contaminant and the filtrate no longer meets the safe limit for drinking water. For example, a household Brita water filter should be replaced after around two months. Thus, for the design and successful implementation of an adsorption technology, it is vital to be able to predict the long-term behaviour of the filter. Such performance is influenced by the operating configuration, specifically, the input rate of the contaminated water, the mass of the adsorbent and the contaminant concentration level. To understand and predict this warrants the need of a suitable mathematical model to understand and characterise the operation and to predict the adsorption behaviour and filtration performance. Researchers Dr Ian Griffiths, at the University of Oxford’s Mathematical Institute, Dr Raka Mondal (formerly Oxford), Dr Sourav Mondal (formerly Oxford, now IIT Kharagpur) and Professor Sirshendu De (IIT Kharagpur) recently teamed up to tackle this challenge.

Together, they derived a mathematical model that characterises arsenic removal and circumvents the need for time-consuming experiments. The model couples fluid flow in a porous medium with the convective, diffusive and adsorption dynamics of the arsenic within the water as it passes through the filter medium. Using asymptotic analysis, they reduced the model to a system that may be described by a single dimensionless parameter, which they termed the filter rating, that encapsulates the entire filter behaviour. The resulting model was validated using laboratory-scale experiments conducted at IIK Kharagpur with contaminated water from the community before being used to make predictions on the lifetime of this filter in a specified role, such as on a domestic or community scale.

In October 2017 the results of the work were presented at a workshop in IIT Kharagpur to government officials and UNICEF representatives. 40 community scale filters are now planned to be deployed in 2018. The ultimate outcome of the model analysis is the generation of a performance–lifetime relationship for field implementation that provides a protocol for ensuring the sustainable operation of such filters.

The results of this work led to an MPLS Impact Award for the contribution made to the development of a maintenance protocol for water filters, enabling cost-effective deployment in India. The researchers are now developing their ideas for the removal of other contaminants from water with an ultimate objective of providing potable water for the world.