Not a drop to drink: The water in many parts of Bangladesh is laced with toxic levels of arsenic. Credit: Reuters/Rafiqur Rahman

Sacks of coal ash, a widely available waste product, promise to make arsenic-contaminated water safe to drink and provide relief to millions in South Asia.

A new filter dubbed ARUBA—for Arsenic Removal Using Bottom Ash—uses fine ash particles from coal-burning power plants in India. Coating the microscopic ash particles with a thin layer of ferric hydroxide and exposing them to air changes the fine gray dust into a rust-colored powder that traps arsenic on its surface.

Where a technology does not work, this is hardly ever the result of the technology. Jan-Willem Rosenboom, World Bank consultant

For nearly 30 years, villagers in Bangladesh and the neighboring Indian state of West Bengal have been drinking water laced with toxic levels of arsenic. Naturally present in the region's groundwater, the arsenic is pumped to the surface by tube-wells originally intended to provide a safe alternative to lakes and streams contaminated with bacteria. More than 4 million tube-wells provide the primary source of drinking water in the region, and an estimated 20% of those harbor unsafe levels of arsenic.

But residents there have few alternatives. “It's horrible to be forced to drink water that you know is going to kill you and have no choice,” says Ashok Gadgil, senior staff scientist at Lawrence Berkeley National Laboratory, who developed ARUBA.

The problem has inspired myriad solutions to trap the arsenic: from a primitive sand filter with a layer of rusty nails to activated aluminum or synthetic membranes. Several approved filters use adsorption and ion exchange to remove the arsenic. But most rely on imported materials, rendering them expensive, some are difficult to use or maintain and none work reliably for all water conditions. Gadgil estimates an initial cost of under $10 for ARUBA, and an annual maintenance cost of $2 per person to replace and dispose of the used filters.

What's more, Indian coal ash is comprised mostly of silica, which is both sterile and nontoxic. But the most important factor, Gadgil says, is the small size of the ash particles—less than one tenth the diameter of a human hair—and their smooth glass-like surface, which optimizes the filter's ability to snare arsenic.

But ARUBA has several hurdles to clear. For instance, there are significant differences in the water quality across the country or even in neighboring wells. Factors such as pH or variations in the concentrations of arsenic, iron or phosphate can dramatically alter a filter's performance. High iron or phosphate can quickly clog filters.

Improper use and maintenance can also cripple the technology, adds Jan-Willem Rosenboom, a consultant for the World Bank. “What we find time and again in Bangladesh and other countries is that where a technology does not work, this is hardly ever the result of the technology,” he says.

For instance, filters may need to be periodically changed or flushed out with chemicals to restore their effectiveness—and most users are not trained to do this. Pump valves could also jam or become choked with sand (Environ. Sci. Technol. 39, 4300–4306; 2005).

In the lab, ARUBA has been shown to reduce arsenic concentrations of 2,400 parts per billion (p.p.b.), more than twice the highest levels found in Bangladesh, to below the World Health Organization guideline of 10 p.p.b. and five times lower then the Bangladeshi standard of 50 p.p.b. Only 2 of 18 arsenic removal plants have consistently met the 50 p.p.b. standard, and none have met the World Health Organization guideline (Nature 436, 313; 2005). Coupled with a straightforward design that is tailored to fit the local user, says Gadgil, ARUBA might succeed where others have failed.