Will a slow sand filter / biosand filter purify water collected from a roof?
Yes, under certain conditions. If you intend to drink the water collected from a roof, I STRONGLY URGE YOU TO USE A “FIRST FLUSH DIVERTER”7; it will minimize the extent of dissolved chemicals from dust, pollution and roofing material in the water collected after a dry spell. Most asphalt 3 tab roofing material has a coating of inert mineral material attached to the asphalt base and may not leach significant amounts of petroleum based chemicals until it is near the end of its life and starts breaking down, or when it is very new.5 Although the water from a slow sand filter is of very high quality, I would strongly recommend the use of a UV filter followed by a ten stage carbon filter, or a “drip filter” to filter the water from the slow sand filter that you intend to drink on a regular basis. IMPORTANT! The roofing material on many houses in the U.S. contains poisons that kill moss. These poisons will be in the water collected from these roofs and cannot be completely removed by a biosa