Updated: July 2002

1. Purpose

It is since the first half of the 1960's that water has been recycled as water used in our daily life in such places where the water supply is insufficient to the demand, specially in the National Capital region, Kyoto-Osaka area and Kitakyushu area. The wastewater reuse system in the individual building and specific areas has been developed and spread in the recent years. It treats the wastewater or rain water and recycles it for reuse as miscellaneous service water to the individual buildings or specific areas. These are used as toilet flushing water, sprinkling water, car washing water, cleaning water or the like whose required quality is less strict than needed for city water. It is expected that the water demand continues to grow with a rise in the standard of living and economic growth. On the other hand, it is becoming more difficult to newly develop a water source. As a result, the demand for the water reuse system must continue to expand for the individual buildings and the specific areas specially in large cities.

2. Outline

1) Classification of the water reuse system
The water reuse system is classified into the following three types:
Individual reuse system : Wastewater discharged from the individual building is reused within the building.
Area reuse system : Wastewater discharged from communities such as district dwelling houses is reused in the area.
Wide area reuse system : Wastewater discharged from public sewage is reused in the wide area on a large scale.
2) Source of building water
Source of building water generally includes miscellaneous wastewater (from washstand, kitchen or bath room), toilet flushing water, refrigerating water and cooling water. It is advisable to select the source of water from these wastewater considering the water quality, water volume, its fluctuation and stability, applications, operating process, etc.
3) Quality of the water to be reused
While quality of the reused water is determined according to the usage, the tentative standard of water for toilet is reported by the Building and Repairs Section, Secretariat of the old Minister of Construction. A part of the report is given below for reference.
BOD : < 20 mg/l (less than 15 mg/l in case of individual reuse system)
COD : < 30 mg/l (in case of membrane process)
Coliform count : < 10m/l
Smell/appearance : Not unpleasant
4) Standard flows
The Building and Repairs Section, Secretariat of the old Minister of Construction also made a report on the standard flows.
Standard flows are divided into two main treatment flows: the biological treatment and treatment using membranes. The choice of treatment flow depends on the type of source of water, usage of recycled water, treatment scale, installation space required and so on.

3. Example of experiences

The following is an example of the building reuse system we installed in a certain building in Tokyo. With this installation, an ultra filtration system is used to separate sludge form the biologically treated water. Application of this type of individual water reuse system using the ultra filtration systems is steadily increasing.
Volume of the water used in the building: 130m³/d
Volume of the recycled water: 170m³/d
Type of the source of water: Wastewater from kitchen, miscellaneous wastewater
Usage of the recycled water: For flushing toilets
Quality of the recycled water: BOD; < 10 mg/l
SS; < 5 mg/l

Water recycling system