FILTER AND METHOD OF FILTRATION

摘要

A filter with which a raw water can be treated at a high flow rate in a short time and bacteria can be removed from the raw water to a high level by a physical treatment only. Also provided is a method of filtration. The filter comprises a spring filter and, disposed upstream therefrom, a first precoat layer (A) constituted of a layer of a filter aid (41) and a second precoat layer (B) constituted of a layer of a filter aid (42). A raw water is caused to flow from the side of the second precoat layer (B) and filtered. This filter is characterized in that the filter aid (41) has a particle size distribution mainly constituted of coarse particles which aggregate at the gaps between the spring loops due to flows in water passing and part of which form bridges covering the gaps, while the filter aid (42) has a particle size distribution mainly constituted of fine particles which are smaller than the filter aid (41) and which themselves cannot form bridges over the gaps. The filter is further characterized in that the size of the gaps between the loops of the spring filter and the size of the particles of the second precoat layer (B) are governed by the first precoat layer (A), which is held by the spring filter due to the bridge phenomenon.