| 摘要 |
<p>PROBLEM TO BE SOLVED: To provide a pretreatment apparatus for slow filtration capable of efficiently removing pollutants from raw water before supply of heavily polluted raw water to a slow filtration water tank. SOLUTION: The pretreatment apparatus for slow filtration for removing pollutants from raw water to be supplied to a slow filtration water tank is composed by successively layering a cobblestone layer 2, a rubble layer 3, a pebble layer 4 in this order from the lower end in the inside of a pretreatment tank 1, forming filtration layers 5-14 of sand in a plurality of stages on the pebble layer 4 in a manner that the grain size of the sand is smaller than that of the pebble layer 4 and becomes smaller in the upper stage side. The grain size of the filtration layer 14 in the uppermost stage is smaller than 0.3 mm and the thickness of the filtration layer 14 in the uppermost stage is made thicker than that of any other filtration layer 5-13. A raw water inlet 15 is formed in the upper end of the pretreatment tank 1 and a treated water outlet 17 is formed in the lower end of the pretreatment tank 1. It is made possible to supply back washing water through the treated water outlet in the state that the raw water supply through the raw water inlet 15 is stopped.</p> |
