Updated: July 2002

1. Purpose

At the end of 1997, sewage treatment had spread to 56% of households. This figure was as high as 90% in large cities, while in cities with a population of under 50,000, it was only 20%. More than 40% of towns and villages in Japan have no sewerage system. Accordingly, to increase the above figure of 56%, it is necessary that sewerage systems be introduced to towns and villages.
In order to promote the rapid development of sewage systems, but at the same time to preserve the local environment of cities, towns and villages to which they have not yet spread, small-scale sewage treatment facilities are being planned and constructed.

2. Outline

Small-scale sewage treatment facilities are plants that treat diversion wastewater, which is chiefly composed of household drainage, at a rate of 600 to 18,000 m³/d (planned maximum daily amount).
The quality of treated water is similar to that of urban wastewater, though in some areas it is regulated in terms of nitrogen and phosphorus content.
The facilities have been designed in particular to cope with large fluctuations in volume of inflowing water and pollution load, with regional characteristics, such as whether the place is cold, is a sightseeing spot, or it has topographical limitations, taken into consideration.

3. Points to be Bear in Mind about Facilities, and Trends in Treatment Systems

Points to bear in mind about small-scale sewage treatment facilities are regional characteristics, easiness of operation, safety, reliability of operational management, and compactness of facilities. These days, improvement in working conditions is eagerly desired. In particular, the working conditions associated with pretreatment equipment have been said to be bad, so new types of pretreatment equipment have been developed and used. For example, the rake rotating-type fine-mesh screen (which includes a dehydrating mechanism) is a night soil treatment machine that has recently attracted attention. This machine selects, washes, conveys and dehydrates screenings, and satisfies the demands of small-scale sewerage.
For sewage treatment; the oxidation ditch process, the batch-type activated sludge system, and the standard reservoir balancing process have been used.
The batch-type activated sludge system, which has recently been used in many places, can cope with sudden changes in water quantity and pollution load, and means the first and final setting tanks are not needed.
Therefore, it, along with the oxidation ditch method, will be increasingly applied to small-scale sewage treatment facilities, since it simplifies facilities and makes their maintenance and management easier.

Small-Scale Sewage Treatment Facility
(Batch-Type Activated Sludge System)