Charcoal filter / Activated carbon
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Charcoal can be quite effective at removing some tastes, odours, and colour. Ordinary charcoal available locally could be used, but activated carbon is more effective. An example of such a filter is the UNICEF upflow sand filter. However, if the charcoal is not regularly renewed or if the filter is left unused for some time, there is evidence that it can become the breeding ground for harmful bacteria.
Activated carbon filters have a porous surface which traps microscopic particles and large organic molecules. The activated surface areas cling to (adsorb) smaller organic molecules. There are two basic types of activated carbon filters: granular and solid block. Granular activated carbon (GAC) is most commonly used in household drinking water filtration products. These are easy to install and inexpensive. They significantly reduce bad taste and odor caused by gases and chemicals like chlorine and its byproducts, while leaving behind natural minerals that are essential to good health. Depending on micron size, they may also remove some microorganisms. Solid block carbon filters have a solid honeycombed structure. These are usually whole-house filters, installed directly into the house water supply. They are more expensive than granular activated carbon filters but require less frequent installation, and they reduce more chemicals including pesticides.
Carbon sources and effectiveness
Carbon can be obtained from a variety of sources such as coconut shell, wood or coal, and all of which are readily available practically everywhere in the world. The activation process is also quite simple and can be done with an industrial oven. Although carbon blocks have a higher contaminate removal ratio, granulated activated carbon are more commonly used in home filter systems. Activated carbon bits cannot be reused and need to be replaced after filtering about 150L of water. Activated carbon prepared from local agroforestry waste residues such as macadamia nut shells, baobab shells and marula fruit stones is effective in the removal of heavy metal ions from aqueous solution. Mostly, charcoal and activated carbon are good at removing contamination such as pesticides, industrial solvents, polychlorinated biphenyls (PCBs), and polycyclic aromatic hydrocarbons (PAHs). These filters generally do not remove dissolved solids, heavy metals (except the agroforestry-derived kind), lead, or coliforms, though they are able to remove arsenic, chromium, and mercury found in organic complexes.
Higher water turbidity and flow rate also decrease removal efficiency. If these filters become saturated, the trapped contaminants can be released back into the filtered water. Also, the particles that accumulate within the filters may serve as food for bacteria, resulting in high concentrations of bacteria within the filter that can eventually be released into the treated water.
Costs
Charcoal filters have an annual estimated cost of US$10 to 100 for a family that uses 25 liters of water per day. Countries with local access to charcoal, or materials such as coconut husks from which charcoal media can be derived will face significantly lower costs that those that have to import the charcoal filters. Activated carbon filtration systems have an estimated average annual cost of over US$100 for a family that uses 25 liters a day. These systems use GAC filters that need to be replaced every 9 to 12 months. Pour-through GAC filtration units cost about US$10 and use replacement filters that cost about US$2 each and need to be replaced after 40 gallons of water. Labor and maintenance costs are limited to purchasing and replacing the filters. SBAC (larger) units begin at about US$330, but generally cost about US$2500. Replacement filters are needed about once a year. SBAC replacement filters cost about US$0.07 per gallon of water treated.
Manuals, videos and links
- Low-Tech Coconut Shell Activated Carbon Filter. Blue News, 5/17/2011.
- Best Water Purifiers are Cheap Graphite Coated Sand Grain Filters.
- Potential of activated carbon for manganese and iron removal. Abuja, Nigeria, 2003.
Acknowledgements
- Emergency treatment of drinking water at point-of-use or (alternative link). WHO.
- A Simple Guide to Water Filtration. Filtersfast.com
- Chan, Louis; Chan, Marcus; Wang, Jingwen. Design of Water Filter for Third World Countries. March 26th, 2009.
- Misihairabgwia, Jane, Ntulia, Victor, et al. Application of locally produced activated carbons in water treatment. Department of Biochemistry, University of Zimbabwe.
- Overview and comparison of conventional water treatment technologies with nano-based treatment technologies. Meridian Institute’s Global Dialogue on Nanotechnology and the Poor: Opportunities and Risks (GDNP).
