●Research Trends and Results

Water utilization effects of rainwater andrecycled water
TORII Kenichi (PhD.), HeadTOYODA Tadahiro, Researcher
River Department, Water Management and Dam Division(Key words) Use of rainwater, closed system circulation method, impact during droughts
1. Introduction
Rainwater and recycled water can be positioned aswater sources which are part of the water cycle alongwith rivers. A quantitative evaluation of the effect ofwater use was performed to estimate future water
usage in order to ensure the balance of regional watersupply and demand, taking into account the fall ofdrainage caused by the decline of rainfall and watersaving measures taken during a drought
2. Estimates for recycled water and rainwater
The study was conducted in Fukuoka City, where
the use of rainwater and recycled water is promoted.
Rainwater reservoirs are installed in private homes
and there are systems installed in buildings and publicfacilities. Recycled water is used in three ways. They
are, individual circulation, district circulation, andregional circulation. Individual circulation is a methodof reusing gray water by installing a recycling system
independently in buildings or other individual
facilities,for this study, we shall assume that it is usedin conjunction with rainwater. District circulation andregional circulation are methods to use reclaimed
water which was treated by sewage treatment facilities.
The Difference between them is the difference
between the uses of scale. In Fukuoka City, housing
rainwater is used along with individual circulationaccording to regional circulation. This study, and thequantity of water used is calculated considering thewater saving rate during droughts with reference tostatistical data.
3. Results of the estimation
It is hypothesized that circulation is the individual
method and the source is wastewater reused in thebuilding, but when a drought lowers the watersupplied to the building, also reducing wastewater, theamount of recycled water used will also fall. In thisstudy, processing of recycled water is assumed to bestably maintained as long as the water supply is notdepleted even during a drought, and its relationshipwith the amount of water usage was calculated. The
quantity of recycled water used for individual
circulation was estimated to be about 1.1% of all watersupplied in Fukuoka City. Figure 1 shows one
example of the calculation result. As a result of

utilizing individual circulation, the amount of thewater supply is usually less under normal
circumstances. Even during a drought, for part of therecycled water to supplement the decline in watersupply, high resistance to drought is a decrease inwater use within the facility will be gradual ascompared to normal use. In the future, it will benecessary to also increase the cost of facility
improvement, maintenance etc., so it will also benecessary to consider the cost balance of droughtdamage and facility cost etc., but it is assumed that
increasing the quantity of recycled water used could
increase resistance to droughts.
Total quantity used inside the3facility (m /day)
Quantity of recycledwater used
Quantity supplied to thewater system

No recycled
Recycled
water
water
100% supplied to the
public system

No recycled
water

Recycled
water

70% supplied to the
public system

No recycled
Recycled
water
water
30% supplied to the
public system

Figure 1. Example of Estimation of Quantity ofRecycled Water Used by Individual Circulating
In addition, the future quantity used was also
estimated based on rainwater use per detached houseand on regional circulation and future numbers offacilities1)2).
4. Conclusion
It was estimated that the effectiveness of water use
as a total quantity for all of the methods of use is only
a few percent. This evaluation is an evaluation fromthe perspective of water use, and multi-faceted
evaluations also considering flood control, theenvironment, etc., are expected to be even more
effective. And in light of the fact that new water
resource development is difficult, it is important to try
and expand from the existing form of water supply to
discover new utilization possibilities.
[Reference]
1) S. Mitsuishi et. al.: Study of water utilization
effectiveness of general purpose service water in●Research Trends and Results
Fukuoka City, Collected Papers on HydraulicEngineering, No. 65, March 2012.
2) K. Torii: Water utilization effects based on rainwaterand recycled water etc., Base statistical documents,
Construction Industry Survey Association, March 2012.