PURPOSE: To secure processing and to eliminate secondary pollution by processing sewage by a combination of purification, separation and classification based upon its grain size and properties.

CONSTITUTION: Sewage, containing mixed faces, garbage, etc., deposited in a sewer, etc., is carried to a sewage disposal plant by a conveyor, etc., and screened firstly to remove large driftwood, masses of concrete, etc., of large diameters which can not be processed. Thus, waste (sewage) of less than about 50mm is supplied to purifying process 1 and purified in an exposure purification pond, etc., so that mixed sludgy organic matter will float and be removed by flowing out together with purifying water. Next, the sewage is supplied to separating process 2 and separated into organic matter and inorganic matter by a wet mill, etc., and the former is dehydrated, dried and burned or disposed at a reclaimed land, etc. The latter is sent to the next classifying stage 3 and, after being classified by a liquid cyclone, etc., disposed or utilized as the aggregate.