Design and operation of a slow sand filter

The design experience at Empire, Colo., and the postdesign evaluation have provided knowledge about the efficiency of the slow sand filtration process at full-scale and the basis for recommendations concerning slow sand filtration practice. Operation of the filter required daily inspection plus measurement of water depth and effluent turbidity, along with monthly scraping. Scraping required only two workhours, however, and only 0.2 in. (0.5 cm) of surface was removed. The projected life of the sand bed at this rate of removal is 15 years, with a residual depth of 1 ft (0.3 m). Giardia cysts were found in the raw water on five occasions, but none were found in the finished water. Microscopic organisms and particles were found in the raw water but either zero or rare numbers were found in the finished water.