Charcoal has been a part of water treatment for at least 4000 years. Continuing the tradition, farmers in northern Thailand who are concerned about agricultural runoff in their drinking water have begun to use a form of charcoal called biochar as a filter. Research suggests that biochar could, in fact, remove pesticides and other agro-chemicals from water, says Josh Kearns, who studied the issue in his role as science director at Aqueous Solutions, a non-profit water, sanitation and hygiene development organization.
But a lot depends on how you make the char, Kearns says. Here’s a tutorial: How to Make a Biochar Gasifier.
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.
The Thai communities make their charcoal in traditional kilns that, when burning well, heat the material to 350°-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.
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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). And the video below is a construction guide, provided by Aqueous Solutions.