Click here to read the complete Hollow Fiber Membrane vs. BioSand Filter position paper
Hollow Fiber Membranes vs. Biosand Filters
Sand filters employ a technology that has been proven over centuries of time. When constructed properly they are very reliable, but they are very slow. Sand filter are constructed by forming layers of aggregate increasing in size from the top to the bottom. The top layer being very fine sand and the bottom layer being very course large gravel. The container needs an exit port on the bottom. Water on the top layer will form a bio-layer where micro-organisms will “eat” the pathogens as they pass through them. After time this bio-layer will grow to the point it will not allow any water to pass through it. At this point the layer needs to be removed. Two common practices are to either dry out the layer and then remove it or to stir it up and remove the dirty water from the top. The stirring method is quicker but greatly reduces the time span between cleanings. The optimum depth of a sand filter is 2 meters. Diameter is determined by volume of water needed. If constructed properly they should produce an average flow rate of 0.1 meter/hour. A family of four needs 16 liters per day, which would require a .5 meter diameter tank.
Compare this to a hollow fiber filter mounted to a standard 5 gallon/20 liter bucket. Water can be filtered at a rate of 1 liter per minute. Total filtration time will take about 16 minutes compared to 24 hours with the sand filter. This will enable water to be filtered as needed. The result will be less chance of the filtered water becoming contaminated during storage. Additionaly, the hollow fiber filter’s quick cleaning cycle eliminates the need to store water compared to the long cleaning cycle of the sand filter.
Another advantage of the hollow fiber filter is that it a true barrier filter. If water is passing through it, it is safe. The sand filter has a questionable period after cleaning as to when the water is safe.