There are challenges to creating biosand water filters in developing countries. Many lack the professional capability of constructing the metal forms to pour the concrete into. Finding proper mesh sizes to sift the sand layers may also be absent. In Nicaragua you may find metal workers capable of welding rebar for home construction, however, you will not find sheet metal bending equipment to create the metal molds. Sand is not sold in hardware stores like in the United States. It is most likely purchased by the pickup load from streambeds or pits and the only mesh available is 1/4 inch which is too large. When traveling to a third world country it might be best to take with you the proper mesh screens.