Preliminary treatment of water for slow sand filters
Many waters require but little clarification in order to make them acceptable for domestic purposes, although from a sanitary standpoint constant purification by means of filtration is essential.
For this class of waters filtration through sand beds of large area at a comparatively low rate of flow may be practised for the larger part of the year with excellent results. However, at certain periods, such as follow a heavy rainfall and subsequent flooding of the streams, these waters may become quite turbid with finely divided suspended matter. Slow sand filters will not continuously clarify such waters, and in consequence preparatory treatment has become quite general.
This preliminary treatment may consist of the coagulation of the suspended matter with chemicals and settling out as much of it as possible in reservoirs before passing the water to the filters. In so far as this method of procedure is followed, it differs in no way from that described under coagulation and sedimentation in a preceding post.
The efficiency of this type of preliminary treatment fr the preparation of turbid waters for filtration through slow sand filters, is strikingly shown in the case of the Washington D.C filter plant.
The clarification of the water was effected, however, by the use of sulphate of alumina , which produced coagulation of minute clay particles in suspension. The coagulated water was next settled in sedimentation basins and then filtered through a slow water filter.
To avoid the use of coagulating chemicals preliminary filters have been designed and installed in different plants for purpose of removing a portion of the suspended matter, so that the slow sand filters may not become too quickly overloaded. There filters are designed with the idea of mechanically straining out as rapidly as possible the coarse gravel and sand, and are operated at relatively high rates of flow