Water filter

Lawrence Berkeley scientist Ashok Gadgil is developing a cheap way to provide safe drinking water to people in Bangladesh who live under the spectre of arsenic poisoning.

For reasons not entirely understood, the shallow tube wells that the people there depend on for water have dangerous concentrations of the toxic substance which, if ingested over long periods of time, leads to debilitating lesions, cancer, and death.

Gadgil’s solution is to remove the arsenic from the water by creating arsenic filters from coal ash – the fine grey powder that piles up at the bottom of furnaces at all coal-fired power stations, waiting to be discarded.

The coal ash itself is composed of particles that measure between one and 10 microns in diameter. This means that even a small volume of the powder boasts a lot of surface area, maximizing the opportunity for surface reactions to snare arsenic. To make it work as a filter, Gadgil coats the ash particles with ferric hydroxide, a chemical that reacts with arsenic and forces the element to precipitate onto the particle.

Initial tests indicate this specially treated coal ash makes a very powerful filter. After spiking lab water with so much arsenic that its concentration soared to an extremely toxic 2400 parts per billion (ppb), the filter lowered the water’s arsenic concentration to 10 ppb. The Bangladeshi standard for safe drinking water is 50 ppb.

Gadgil estimates that five grams of this material could render about three gallons of Bangladeshi well water – with an average arsenic concentration of 400 ppb – safe to drink.

Put another way, a filter the size of a teabag filled with the material could provide drinking water for a family of six for one day. Gadgil estimates the technique will cost about 30 cents per person per year. The next best option is a filter developed by a Bangladeshi engineer, and backed by the nonprofit organization IDE-International, that uses pulverized brick instead of ash. It would cost $9.70 per person per year.

In the US, the California Energy Commission’s Public Interest Energy Research program recently awarded Gadgil $250,000 to explore whether a variation of the technique could help the state comply with an Environmental Protection Agency rule effective in 2006 that tightens the US arsenic drinking water standard from 50 ppb to 10 ppb.

Currently, 600,000 California residents consume water with concentrations above 10 ppb. Gadgil will determine whether ash derived from US coal can be developed into a filtration system, and whether such a system can work at small municipal water treatment facilities.

Initial results appear promising. Currently, the cost of arsenic removal at small municipal water systems ranges from $58 to $327 per household per year. Gadgil estimates his method would cost less than $1 per household per year, not including the one-time cost of the reactor for removing the arsenic from water.