Abstract
When the bed resistance of slow sand filter increases, the schmutzdeck on the surface bed must be removed by scraping. The indication of the decision of this work is the increase of the head loss. However the head loss changes with the fil tering rate. The calculated head loss at the normal flow rate of 20cm/h is introduced as the normalized head loss which is popular in England. The normalized head loss at Someya water works in Ueda city, Nagano quickly increases in winter when the water temperature is low. In the growing season of filamentous algae, the normalized head loss does not increase.The algal mat is like as a blanket herb and it prevent the clogging of filter. The filter operations at several water works are compared with the data redrawn in normalized head loss.
