同©大学 Tongji University (2)Slow filtration This is the oldest form of filters,with fine sand loaded at low rate,0.05-0.2 m/hour. Slow sand filter treat the raw water by physical straining and biological degradation. There are 3-5 ft sand and form a layer of organic material and micro biofilm in the surface.Every few weeks or months,the top layer will be scrapped. This is a simple operation and no chemicals ones for small systems.Tongji University (2) Slow filtration This is the oldest form of filters, with fine sand loaded at low rate, 0.05-0.2 m/hour. Slow sand filter treat the raw water by physical straining and biological degradation. There are 3-5 ft sand and form a layer of organic material and micro biofilm in the surface. Every few weeks or months, the top layer will be scrapped. This is a simple operation and no chemicals ones for small systems