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  • Eduardo Abril de Fontcuberta -

Emergency diy water filters for your household.

Year 2 - Week 38

“When the well is dry, we learn the worth of water.” - Ben Franklin, Poor Richard’s Almanac 1733

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 rain water or pumping from a close up river. Constructing a bio sand filter can allow your family to survive for a long time by having the needed drinking water, and can be done without electric tools nor exotic materials.

First things first. There is no way in which you will be able to manufacture a simple and cheap filter so the average US family can keep up with their regular consummation of water, which is huge at around 80 gallons per person per day (p/p). 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. Comparing this waste of water with the African average of less than two gallons per person, is impressive, but you will argue they don’t have the same standards of living. Then compare a similar one, as the Dutch use only 27 gallons to perform the same tasks as US do.

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 even more even though they only need around 2-3 gallons per person per day. If we want to establish a baseline of what would be a sensible minimum consummation of water for drinking, hygiene and cooking, something around 6 gallons would be a number to keep in mind. On a family of four that number adds up to 24 gallons per day, which is a large amount of water, to purify and store, even though the World health Organization recommends a higher number. Once the SHTF, and only drinking water can be supplied that number can be lowered to around 1.0 gallons p/p, in a moderate climate.

The concept

Over the years the WHO and such non-governmental organizations such as the Center 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 rain water collected from you home roof than to filter water from a stream contaminated by cattle.

The most widespread, and traditional, filtering system for household use is what we can call slow sand filter. It is slow, as it uses gravity not pressure to force water though the filtering layers and it is sand because it consists of layers of gravel, sand and other materials to filter solids and clarify water and, if using the modern upgraded biosand version even bacteria, protozoa and viruses.

The problem

What kind of contamination you face will dictate the amount of filtering layers, its composition, the sand grain finesse and therefore the speed in which it is capable to filter water through it, which will be small but enough to supply drinking water at around 1/10 gallon minute (400ml/min) To add to this the filter will need a way to deactivate bacteria and other pathogens present on the water supply.

The solution

The biosand filter (BSF) 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. The BSF also helps reduce the bad taste, odor and discoloration of water and reduce radically the occurrence of episodes of diarrheal diseases that threaten most consumers of non-treated water.

It may sound like a new type of filter but it all started in the late 80s with a design from Dr. David Manz from the University of Calgary as an advancement of the 200 year old slow sand filter technology. He then founded the CAWST as a worldwide distributor of BSFs for developing countries with great success, as in 2012 more than 450.000 BSF units are already in use.

Biosand filters are not just for third World countries that have not access to treated water, it is also the perfect solution to provide drinking water to a household in case the grid goes down.

What is a Biosand Filter?

A biosand filter (BSF) is an adaptation of the traditional slow sand filter. It is smaller (about 1 m tall, 0.3 m wide on each side) and adapted so that it does not flow continuously, making it suitable for use in people’s homes. The filter container can be made of concrete or plastic. It is filled with layers of specially selected and prepared sand and gravel. The sand removes pathogens and suspended solids from contaminated drinking water. A biological community of bacteria and other micro-organisms grows in the top 2 cm of sand. This is called the biolayer. The micro-organisms in the biolayer eat many of the pathogens in the water, improving the water treatment.

How Does the Biosand Filter Work?

You can use any kind of water in the biosand filter – well water, borehole water, pond or river water, tap-stand water, or rainwater. This makes it very convenient for people because they can use whichever water source is closest to home, make it safe to drink. 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.

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 down through the sand and gravel. Treated water flows out of the outlet tube. No power is required - the filter works by gravity. It should take about 1 hour to get 3 -4 gallons of filtered drinking water.

Pathogens and suspended solids are removed through biological and physical processes that take place in the sand. These processes include: mechanical trapping, predation, adsorption, and natural death.

What can the biosand filter remove from water?

The BSF has been studied in the field and in labs. It has been shown to remove from contaminated water all of helminthes (worms) which are the biggest. Protozoa like cryptosporidium and giardia, next in size at 1 - 300 microns and also most bacteria (98, 5%) such as salmonella and E-coli, which measure 0.1 to 10 microns. It will be fairly effective (70-99%) against many Virus too, which are the smallest pathogens at 0.05 – 0.1 microns.

The filter can also remove up to 95% of turbidity (dirt and cloudiness), and up to 95% or 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 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. Different treatment methods remove different things from contaminated 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 around the world to disinfect drinking water are: Chlorine treatment, solar disinfection (SODIS) and boiling.

A lot of the information on this article and many more documents such as the 170 page BSF Construction manual comes from CAWST and are available free of charge for public use. All of CAWST's Training and Education Material is available free of charge under the Creative Commons Attribution 3.0 Unported license.

Facts from World Health Organization (WHO) on the World’s Water

  • The Earth has 1,386,000,000 km3 of water total but only 2.5 percent of that is fresh water (35,029,000 km3 or 9,254,661,800 billion gallons of fresh water).Less than 1 percent of the world’s fresh water (or 0.01 percent of all water) is usable in a renewable fashion.

  • The average person needs a minimum of 1.3 gallons (5 liters) of water per day to survive in a moderate climate at an average activity level. The minimum amount of water needed for drinking, cooking, bathing, and sanitation is 13 gallons (50 liters).

  • The average person in the United States uses between 65 to 78 gallons of water (250 to 300 liters) per day for drinking, cooking, bathing, and watering their yard.

  • The average person in the African nation of Gambia uses only 1.17 gallons (4.5 liters) of water per day.

Artículo publicado en la revista Survivors Edge, Otoño 2014

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