“Having a reliable supply of drinking water is paramount for a household, and it is not as easy as recovering rainwater or pumping from a nearby river.”
CAWST Biosand Filter
Nearly 800 million people in the developing world don’t have access to clean drinking water, and climate change is threatening to worsen this situation.
You can live three weeks without food but only three days without water. Having a reliable supply of drinking water is paramount for a household, and it is not as easy as recovering rainwater or pumping from a nearby river. Constructing a biosand filter can allow your family to survive for a long time by having needed drinking water, and it can be done without electric tools or exotic materials.
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There is no way you will be able to manufacture a simple and cheap filter that can keep up with the average U.S. family’s regular consummation of water, which is around 80 gallons per person per day. Lowering the needs of water must be the first goal to achieve if you want to be able to supply your household in a survival or emergency scenario.
Slow Sand Filters
Over the years, the World Health Organization (WHO) and organizations such as the Centre For Affordable Water and Sanitation Technology (CAWST) have gained a tremendous amount of experience in how to filter river, rain and well water into drinking water with the recycled materials you can find laying around in most country villages. Cost is very low, availability is high and performance is good enough to avoid contamination from typical bacteria and protozoa.
First, we need to know the type of water we will have available, as this will dictate the minimum needs in terms of filtering procedures. It will not be the same to filter rainwater collected from your roof than to filter water from a stream contaminated by cattle.
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The most widespread, and traditional, filtering system for household use is the slow sand filter. This filter uses gravity, not pressure, to force water though the filtering layers, and consists of layers of gravel, sand and other materials to filter solids, clarify water and, if using the upgraded biosand version, even bacteria, protozoa and viruses.
The kind of contamination you face will dictate the amount of filtering layers, its composition, the sand’s fineness and therefore the speed in which it is capable to filter water through it. The amount of filtered water will be small but enough to supply drinking water at around 1/10 gallon minute (400ml/min). A slow sand filter will also need a way to deactivate bacteria and other pathogens present in the water supply.
The biosand filter is a household water treatment system designed as a refinement of traditional slow sand filters. The biosand filter removes pathogens and suspended solids from your supply of drinking water using a combination of physical filtering and biological processes in a container filled by layers of sand and gravel and covered by a biofilm. It also helps reduce the bad taste, odor and discoloration of water and radically reduces the occurrence of diarrheal diseases that threaten most consumers of non-treated water.
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As of 2013, more than 650,000 biosand filters have been implemented in 55+ countries around the world, benefitting over 4 million people. They are also the perfect solution to provide drinking water to a household in case the grid goes down.
A biosand filter is smaller than the slow sand filter (about 40 inches tall, 11 inches wide) and does not flow continuously, making it suitable for use in homes. The filter container can be made of concrete or plastic. It is filled with layers of specially selected and prepared sand and gravel. A biological community of bacteria and other microorganisms grows in the top 2 centimeters of the sand. This is called the biolayer. The microorganisms in the biolayer eat many of the pathogens in the water, improving the water treatment.
“You can use any kind of water in the biosand filter—well water, borehole water, pond or river water, tap water or rainwater.”
You can use any kind of water in the biosand filter—well water, borehole water, pond or river water, tap water or rainwater. The water must not have been chlorinated though, or the chlorine will kill the biolayer. The water should also not contain any dangerous chemicals, because the biosand filter cannot remove most chemicals from water.
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Contaminated water is poured into the top of the biosand filter at least once per day (but not continuously). The water poured into the top of the filter slowly drips through the holes in the diffuser and flows through the sand and gravel. Treated water flows out of the outlet tube. One hour will produce three to four gallons of filtered drinking water.
Pathogens and suspended solids are removed through biological and physical processes that take place in the sand, such as mechanical trapping, predation, adsorption and natural death.
Ideally, the sand should consist of crushed rock and sand types of varying coarseness levels that you can dry and sieve. if this is not available, materials from quarry or pit sand will do. Try to avoid river and beach sand as they are full of organic matter and are difficult to clean.
Construction & Results
The first step to layering is filling the top reservoir with around 12 liters of untreated water (one bucket). Since the reservoir water is higher than the outlet tube, gravity will force the untreated water though the first 22-inch-thick filtering section of sand. The sand in this section should be finer than 0.03 inches. The water will slowly pass through it and then cross the 2-inch-thick separation section bellow it, filled with 0.04- to 0.25-inch gravel. Reaching the bottom, it will cross the 2-inch-thick drainage section, with the coarsest 0.25- to 0.5-inch gravel, and up the pickup tube to the outlet.
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The biosand filter has been shown to remove all of the helminthes parasites, which are the largest in size, as well as protozoa like cryptosporidium and giardia, and most bacteria, such as salmonella and E. coli. It will also be fairly effective against many viruses, too.
The filter can also remove up to 95 percent of turbidity (dirt and cloudiness) and up to 95 percent of iron (which people often don’t like because it turns water, laundry and food red!). Like other filters, the biosand filter cannot remove dissolved contaminants or chemicals, such as salt, arsenic or fluoride, which will need boiling. There is also an adaptation of the biosand filter using rusty nails, called the Kanchan Filter, which can remove arsenic from water.
The biosand filter is best used as one step in a multi-barrier approach to safe drinking water. Although the water may look clear after filtration, there may still be some bacteria and viruses in the water. It is necessary to also disinfect the filtered water to ensure the safest drinking water possible. The most common low-cost methods used are chlorine treatment, solar disinfection and boiling.
For more information, including free training and educational materials, visit http://www.cawst.org.
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This article was originally published in the SURVIVOR’S EDGE TM Fall 2014 issue. Subscription is available in print and digital editions here.
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