Étape 5 - Frequently Asked Questions
Answers from the following document: https://static1.squarespace.com/static/5afb5f6225bf020305339c10/t/5b4c1df8758d462d957b8a49/1531715067103/french-intro-to-bsfs_ohorizons_Introduction_au_filtre_biosable.pdf
How much water can be filtered per day?
A biosand filter made from OHorizons wooden mold will filter 11 liters each use, this means that it will filter 11 liters each time it is refilled. It is recommended to fill the filter a maximum of four times per day and a minimum of once a day. This means that the owner of the filter can obtain anywhere from 11 to 44 liters of water or about 3-12 gallons each day. A maximum of four refills per day is recommended to allow for long enough rest periods between each operation of the filter.
How much does a biosand filter cost?
The cost varies depending on the place where the filter is made and the local cost of materials and labor. In general, the cost varies between 25 and 65 dollars USD per filter.
How much does a filter weigh?
An installed filter (with filter media) can weigh up to 350lb or 160kg. Once installed, the filters should never be moved. The stabilization of the sand helps to eliminate the pathogens and the sand can be disturbed in moving the filter. The concrete filter box weighs about 150lb or 70kg. If the filter must be moved, the sand and the gravel must be taken out, washed and reinstalled in the new filter location.
How much time does it take to develop the biological layer?
After approximately 30 days of use, the biological layer will be entirely developed and the filter will function at its optimal rate in eliminating pathogens. Over the course of the first 30 days, the filter removes around 70% or more of the pathogens and approximately 100% of chlorine resistant pathogens. During this period, the filtered water can be consumed, but we recommend that users also boil the water or use chlorine to ensure that the water is completely drinkable. OHorizons recommends this multi-barrier approach even beyond the initial 30 day period. Although the filter functions in an optimal way after the first month, different factors can affect its efficiency over time, like inappropriate use by the user or changes in the level of contamination of the water source. The use of a multiple-barrier approach throughout the life of the filter guarantees that users will always drink the water that is as purified as possible.
Why doesn’t the sand go down the PVC tube or pipe?
The sand is only the top layer. The bottom two lower layers of the filter are comprised of small and large gravel. The small gravel prevents the sand from flowing out and the large gravel prevents the small gravel from escaping or from obstructing the exit tube.
Other than refilling the filter with water, how is the filter maintained? How is it cleaned?
The biosand filter needs very little cleaning. Once the concrete box is completely set, it must be cleaned well with soap and water to eliminate all the residual sand or dirt. Then, it can be safely filled with sand, gravel and water in the user’s home. It is recommended to regularly wipe down the exterior, the cover and the diffuser plaque.
The filter is very simple to use, just like caring for a houseplant. The most important part of maintenance is to ensure that the biological layer stays healthy by feeding it one to four times a day with contaminated water. Once fed, the biological layer must digest and recover, so there must be at least one hour between each use. Just like a house plant, the biological layer cannot survive if there is too much or too little water. While the filter is not being used, a 5cm layer of water covers the top of the sand. This layer must be maintained or the living microorganisms may die. If cloudy or visibly dirty water is poured through the filter, the sand will accumulate dirt which will slow the flow of water. To correct this, a non invasive method that does not disturb the biological layer called stir and dump is used to clean the top of the sand and improve the flow.
Who is responsible for maintaining biosand filters?
The owner of the filter is responsible for its maintenance. The owners are usually trained in maintenance before or during the installation of a filter. OHorizons ensures that all of our partners have appropriate training in matters of maintenance and installation. These partners are available to households to answer their questions, make repairs, reeducate and provide additional assistance, as needed.
How long does a filter last? How often should the sand be replaced?
Barring exceptional circumstances that would cause a leak in the concrete housing, there should be no reason to replace the filter. If it is correctly maintained and installed, a filter can last a lifetime and the sand does not need to be replaced. If the water used is particularly cloudy or visibly dirty, the owner of the filter will regularly use the spin and dump technique to get rid of the dirt. This method removes a small quantity of sand and, over time, the filter will need more filtration sand added.
Does the filter need a pump, electricity or a mechanical system to work?
No, the biosand filter works due to the force of gravity. Gravity pulls the water down through the sand and the pipe due to a natural siphon effect. It does not need electricity or pumps. This means it is an excellent solution for areas that are not connected to an electrical grid.
Are specific types of sand needed?
Yes, the best sand is crushed rock which can be obtained from most local quarries. Crushed rock does not contain the organic matter or salt that is often found in sand from the river or the beach. The crushed rock sand is also more varied in size and shape which makes the filter more effective. The sand must be washed and sifted before being put in the filter so that only the grains of a certain size range are used and nothing harmful is introduced in the filter. The gravel must also be of varied shapes and must be washed before being placed in the filter. Please see our appendix for more information on the best type of sand to use for filtration.
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