Many new technologies have promised to remove arsenic from drinking water but little has changed on the ground, finds T. V. Padma.
Razia Begum’s family, like many thousands of others in Bangladesh, was given an ‘alcan’ filter — a simple unit containing activated alumina that absorbs arsenic from water — under a UN project in 2006. Two years later, the filter stopped working as it became clogged up and needed specialist attention that was no longer available.
Straightforward solutions to the arsenic problem that affects hundreds of millions of people have, so far, been hard to come by.
“I am not aware of any research that has led to a widespread application for providing arsenic-safe water to people in the affected areas,” says Mohammad Yunus, senior scientist at the International Centre for Diarrhoeal Disease Research, Bangladesh, which is based in Dhaka and works in Nagda and neighbouring villages.
This is despite the fact that scientists have made great progress in understanding how, where and why arsenic ends up in soil and water, and have designed promising tests and filters. But for such inventions to survive, they must overcome basic, yet hard-to-resolve issues that lie far beyond the laboratories.
Several mitigation options — filters, sinking deep tube-wells, rainwater harvesting and purifying surface water by simple filtration — have been tried but they have had limited success, says Yunus.
Techniques that attempt to remove arsenic rely on a few basic principles: adding oxygen to ‘free’ arsenic from water; adding aluminium or iron salts to precipitate a solid salt that can then be filtered out; or passing water through a membrane that allows certain chemicals through according to their concentration.
Mixed results
The last two options — filters and membranes — currently have technical limitations, says Bhaskar Sengupta, a civil engineer at Queen’s University, Northern Ireland.
An arsenic mitigation project that ran from 2006–07 by UNICEF and the Bangladesh Department of Public Health Engineering supplied filters suitable for household and community use at 10–20 per cent of the actual cost.
In 2009, an assessment of the project found that 70–80 per cent of households were using the filters but 5–15 per cent of the filtered water nevertheless contained arsenic at more than 50 micrograms per litre — above recommended levels. The study concluded that the filters could be useful as one tool in an array of options for arsenic-safe water — but only if deployed and explained properly to local people, not an easy process with a poor and often illiterate population.
And the project did not factor in maintenance and refills, or building local technical capacity, points out Yunus.
Membranes don’t clog up but have their own problems. They produce two streams — one of pure water that is stripped of not just arsenic but every other mineral, many needed by the body; the other the arsenic-rich waste stream which needs safe disposal.
The WHO agrees with Yunus that there are no proven technologies for the removal of arsenic at water collection points such as pumps, tube wells and springs.
Even measuring arsenic levels is far from simple. The WHO notes that it needs sophisticated, expensive laboratory techniques as well as trained staff. Field test kits for detecting the low arsenic concentrations of concern for human health are unreliable.
Oxygen gets publicity
Scientists are hopeful that recent work will prove more successful. There is excitement now about a chemical-free method to remove arsenic from ground water, developed by a team of European and Indian scientists under a project funded by the European Union (see Indian arsenic clean-up ‘working well’) [doi:10.1016/j.envpol.2009.09.014].
The method involves pumping air into aquifers. The oxygen frees the arsenic, and water with most of the arsenic removed is pumped into storage tanks and piped to households, says Sengupta, leader of the research team.
The technique cuts arsenic levels of as high as 70 micrograms per litre to just two micrograms, well within the WHO guideline of ten micrograms per litre, as well as killing most diarrhoea-causing bacteria.
Six plants are now operating in West Bengal state in India, after they were set up with aid from a two year World Bank project that ended in 2008. And on 28 October 2009, the Blacksmith Institute in the United States rated it among ten recent ‘revolutionary’ technologies.
Sengupta says setting up a treatment plant costs about US$2,200 and most of the parts can be bought and installed locally.
Nanontechology — and other advances
Elsewhere, scientists are examining the potential of nanotechnology. In May 2009, researchers at the US-based Rice University reported field tests in Mexico of ‘nanorust’ — tiny particles of iron oxide less than a billionth of a metre in size — that remove arsenic from water.
The team plans to coat the sand in sand filters with nanorust and run water through them to remove both arsenic and diarrhoea-causing viruses.
Meanwhile, an intriguing discovery in the Ganges-Brahmaputra-Meghna delta of Bangladesh and India offers yet another potential solution. Geologists from Dhaka University in Bangladesh and from the United States found that levels of arsenic are higher in deeper sediments in the river delta than in surface sediments.
Reporting their findings in Proceedings of the National Academy of Sciences (5 September 2009), they say complex chemical reactions in the soil cause the arsenic to stick to iron particles in the sediments. This forms an impenetrable ‘iron curtain’ or barrier that prevents arsenic from being discharged into the ocean.
Yan Zheng, of Queens College, City University of New York, United States, says it might be possible to engineer a natural reactive barrier in a well , which needs to be just deep enough to intercept the water table, typically 7–8 metres. The resulting, arsenic-free water could be pumped away for irrigation, he says.
“There is very little maintenance as the barrier will operate for decades if not longer. It will be very cost effective because holes are dug all the time,” says Zheng.
As for a better test for arsenic , scientists at the US-based University of Massachusetts reported in April 2009 “the first accurate test for arsenic compounds in soil”.
The researchers first extracted arsenic compounds by adding an acid and an alkali to the water. They next separated the mixture by running it along a column where different substances in the mixture percolate down at different rates depending on how heavy or light they are. An electric arc is passed through the samples and the substances are measured by the intensity of light they emit.
Bridging the ‘know–do’ gap
But will these exciting results ever become a workable solution for Razia?
Yunus says that translating arsenic research into practice depends on multiple factors, many of which are absent in affected countries. These include political commitment and priorities; planning processes; commitment by international donors such as UNICEF and the WHO to support projects; and the availability of multidisciplinary teams comprising public health experts and clinicians, engineers, hydrologists, geologists, social and behavioural scientists and communication experts.
“There is sufficient information about the various mitigation options to provide arsenic-safe water to people in affected areas. The missing link is the ‘know–do’ gap,” says Yunus. “Bridging this gap should now be the priority for action.”
The scientists’ role ends with publishing research findings, says Sengupta. He agrees that demonstrating feasibility on the ground usually requires the intervention of donor agencies, after which it is up to national and local governments to prioritise a technology.
Of the Queen’s University invention, for example, where oxygen is pumped into aquifers, Sengupta says India’s Rajiv Gandhi National Drinking Water Mission, which aims to provide safe drinking water to rural areas, is happy to adopt the technology. But the supply of drinking water is a ‘state’ subject in federal India, meaning it is up to each state government to take the final decision. And so the ball now lies in the court of the West Bengal government, which is yet to back the technology.
And ultimately, observes Sengupta, the real test lies in whether local people can understand and use a technique.
Source: T. V. Padma, SciDev.net, 24 Nov 2009
