December 13, 2012

An Ancient Filtration Material Removes Pesticides from Drinking Water

Charcoal has been a part of water treatment for at least 4000 years, but can it remove modern synthetic pesticides from drinking water? Farmers in northern in Thailand, concerned about agricultural runoff, put the question to Josh Kearns, an environmental engineering doctoral candidate at the University of Colorado in Boulder and the science director at Aqueous Solutions, a non-profit water, sanitation and hygiene development organization.

“I didn’t know the answer, and searching the scientific literature revealed that, in fact, no one knows the answer,” Kearns told E4C.

The answer, Kearns discovered through his own tests, is that it can. But a lot depends on how you make the char.

See our Solutions Library for more clean drinking water solutions.

Gasifiers and char

Charcoal removes impurities from water by a process called adsorption, meaning that the contaminants adhere to the charcoal’s surface. Because it is porous, however, water can flow through and permeate the charcoal. That permeation is the better-known process of absorption. Dropping the prefixes gives the word “sorption,” which covers both processes.

300 million tons of synthetic organic compounds are produced annually, 5 million tons of which are pesticides.

The Thai communities make their charcoal in traditional kilns that, when burning well, heat the material to 350 to 500 C. In contrast, simple gasifiers burn at 900 C. At that temperature, the wood and agricultural waste that they burn converts more completely into char. The biomass releases gases as it heats, and those are burned as fuel. The release of gases and combustible material leaves behind char that is highly porous with a greatly increased surface area.

“Gasifier char may therefore be an optimal choice for sorption of pesticides, industrial and fuel compounds, human and livestock pharmaceuticals, and other synthetic organic compounds of increasing concern to water quality,” Kearns says.

Design principles

One problem with introducing new technology is that people might not use it if it doesn’t feel comfortable to them.

“Don’t fight culture; if people cook by stirring their stews, they’re not going to use a solar oven, no matter what you do to market it,” says Ethan Zuckerman, founder of Global Voices. Zuckerman’s advice and the maxim “consider context” is the second of E4C’s Design Principles, found in Career Resources on this site.

Biochar increases crop yields, carbon sequestration and water filtration

Gasifiers seem like a match. They operate in a way that is similar enough to kilns, and after a demonstration, they proved to be superior. Kearns has built gasifiers with the help of people in the community.

“In regions around the world where charcoal filtration is still a known practice, there would presumably not be strong cultural barriers to uptake,” Kearns says. “However, for many traditional charcoalers, the design and operation of a gasifier for char production are not immediately intuitive.” To help, Kearns spent six months on a farm in Thailand over the winter of 2011 developing the 55-gallon (200L) gasifier drum featured in this handbook.

“After seeing several batches of char made this way, they were won over because, one, it was less work to make gasifier char than traditional kiln char; two, it only took two hours for a burn as opposed to all day for a similar sized traditional kiln; and three, there was no smoke to beleaguer the operator,” Kearns says.

Water treatment with local materials

Working with the farmers, Kearns built a water treatment system that includes 35 cubic feet (one cubic meter) of biochar. It’s hard to overstate the importance of using locally available materials, and the water treatment systems Kearns and the farmers are developing are entirely local. One proven system links four containers in a series, the first three filled with filtering materials such as sand, char and stone, and the final container holds the pure water. This handbook explains the build: Constructing a multi-barrier water treatment system (pdf).

Sanskrit and nerve gasses

Charcoal may have been used to treat water as early as 4000 years ago. A line translated from Sanskrit in a collection of Indian medical documents called the Ousruta Sanghita reportedly reads, “It is good to keep water in copper vessels, to expose it to sunlight, and filter through charcoal.” And other documents offer similar glimpses into the use of charcoal in the region.

Biochar has been known to remove sediment and even chlorine from water, but, as Kearns discovered, its usefulness on pesticides was less clear.

Pesticides in agriculture may predate even charcoal filters, but synthetic chemicals are relatively new. Their discovery stemmed from research into nerve gas weapons in World War II, when scientists realized that the compounds also kill insects. The proliferation of agents like DDT in the 1930s and 1940s reduced malaria and improved crop yields. The damage they inflicted on insects, however, was not limited to insects.

Toxic trouble

Today, “pesticide pollution” makes two appearances in the top 10 ofThe World’s Worst Toxic Pollution Problems Report 2011 by the Blacksmith Institute.

Kearns calls drinking water contamination with pesticides and similar chemicals a “major worldwide problem.” He cites a paper in the journal Science that points out two alarming facts. The first is that 5 million tons of pesticides and 300 million tons of synthetic organic compounds in general are produced every year and they are major contaminants. The second is that farmers in developing countries often lack training in the use of agricultural chemicals. They tend to overuse them, and they sometimes buy from a “black market” of illegal agents.

“In South and Southeast Asia where my colleagues and I work, for example, around 75 percent of the pesticides used are banned or heavily restricted in the West due to deleterious ecological and human health effects,” Kearns says.

From organ damage to birth defects

Agricultural chemicals lurk in drinking water all over the world, including in developed countries. The US Environmental Protection Agency regulates chemicals and establishes maximum amounts of each that can run from the tap. Those limits are based on estimates of a potentially harmful dosage, and those doses are estimated from evidence gathered in epidemiological studies and from laboratory animals and other research (EPA’s health risk assessments for many pesticides). Legal wrangling, new evidence and shifting political pressures can lower and raise those limits. In developing countries, scarce resources can prevent adequate regulatory control. That’s where we see some of the most dire consequences of long-term chemical exposure.

Drinking water laced with pesticides and other chemicals on a regular basis is linked to cancer, organ damage and reproductive defects in lab animals. And pesticide exposure in the workplace, through breast milk and in food and water is linked to sperm and fertility defects, spontaneous abortion, fetal growth retardation, developmental problems and possibly childhood leukemia.

A local solution

In the absence of water treatment infrastructure in many rural communities, the Blacksmith Institute recommends small-scale filtration in people’s homes. Kearns agrees. He and Aqueous Solutions have created a series of handbooks in English, Thai and Burmese to explain gasifier and water filter construction.

Comments from the Community

1 Comment

  1. Kamalakar P says:

    Nice article. Very helpful. Could you please tell us how often should we change the saNd , gravel and charcoal. If one uses about 100 liters of water a day. and could u please let us know the final tds Value of the end filtered water. Thank you.

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