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I have seen a number of projects using home made sand and gravel filters at household level in several countries and the one common feature they have is that they have all failed. The problem being that while they are relatively easy to construct, they have a significant management requirement which is very commonly not there at household level. While at commercial scale either gravity fed sand filters (including a level of biological treatment) or pressurised sand filters are commonly used for urban water supplies, there is trained staff to manage the systems and carry out the back-washing and surface skimming as required. Also with small systems there is a danger that water will take the easiest route down the edge of the tank, avoiding the filter medium entirely. For individual hosehold use you cannot beat a simple ceramic candle filter through which water passes from one bucket to a lower bucket. These are relatively easy to maintain and effective against bacteria if handled correctly. I have seen them in use in Kenya so they are probably available to buy there. Also they start working immediately whereas a sand filter requires time for the establishment of the "schmutzdecker" - the top biologically active layer which performs part of the treatment.