1. How can rainwater be used?
	Untreated rainwater can be used for the following: -flushing toilets -watering plants -washing cars The only 'treatment' rainwater used to flush toilets in buildings in Sumida receives is sedimentation where loose particles in the water sink to the bottom of the tank and water is drawn from the top of the tank so as not to disturb the sediment.

2. Where is rainwater collected from?	From the roofs of buildings. However, it is important for the quality of the water that the collecting surface is as clean as possible.
3. Doesn't rainwater become fetid over time?
	No, in most cases, rainwater remains pure even if left for many years. However, algae may appear in the water if organic material is mixed in and if the tank is in the sun.
4. What about acid rain?
	Rainwater is said to be 'acid' if it has a pH of 5.6 or below. It is caused by air pollutants such as sulphuric acid and nitric acid. The acidity is higher in the first flush of rainwater, so using mechanisms to cut the first flush (prevent it from going into the tank) are effective in ensuring acid rain is not a problem.

5. How do you exclude first flush rainwater?

The 200 liter 'Tensuison' raintanks have a pipe going down the outside of the tank. Rainwater initially flows from the downpipe into this pipe. Once this pipe overflows, rainwater then goes into the tank (see fig. 1). When it stops raining, the valve on this pipe must be opened and the first flush water drained. In Osnabruck, Germany, a funnel-shaped insert is incorporated into the downpipe in order to cut the first flush (see fig. 2). When it first starts to rain, dirty first flush rainwater runs down along the outside of the insert into the sewer and not into the funnel. After a while, when the volume of rainwater running down the downpipe increases, begins to flow into the storage tank through the funnel.

図１	図２

6. What volume of rainwater should be excluded as 'first flush'?

According to research done in March - April 1980 on rainwater collected from the roof of an office building in Chiyoda Ward in central Tokyo, about 0.5 - 1.0 mm of rain needed to fall to wash the surface of the roof. Other research, done in August - September 1986 in Minato Ward, also in central Tokyo, measuring the degree of change in water quality in rainwater collected from a roof after a period of 261 hours (about 11 days) of no rain, showed that water quality is generally stable after 1.5 -2.0 mm of rain had fallen. In other words, even after a dry spell, only about the first 1.5 mm of rainwater that fall is affected by pollutants accumulated on the rain-collecting surface.

7. What do you do if rainfall is low?	＜ TOP >
	When the rainwater runs out, there should be a mechanism to supplement it with tap water. It is important not to allow rain water to backflow into tap water supply pipes as this would contaminate the city water. In order to avoid this, pipes should be clearly identifiable, for example by making the rainwater pipe a different color.
8. What happens if the rainwater tank freezes?
	It is not usually the rainwater in the tank that freezes, but stagnant water that is left in the supply pipes. It is necessary, therefore, to make sure the pipes have a thermal insulating structure. For people using tanks in the colder northern areas of Japan, we recommend that the tank is placed indoors and is kept at about 5 degees Celsius. If tank water is not to be used for an extended period, we recommend that it be emptied.

9. How about maintenance?

No difficult maintenance is necessary. However, leaves etc. need to be cleared away from filtering screens and the sediment which builds up on the bottom of tanks should be cleaned regularly. Also, when cooling towers etc. on the roofs of buildings are cleaned, be careful the water used for cleaning does not go into rainwater tanks.

10. How do you prevent insects from breeding in the tanks?

Insect (especially mosquito) breeding can be prevented in rainwater storage tanks installed in large structures by installing insect proof nets or other insect proof devices in vent routes ( drain pipes, overflow pipes etc.) Home use tank water is consumed and replenished at a relatively fast rate, so rainwater does not stay in storage tanks for a long time. Therefore, even if insect eggs are mixed into tank water or a mosquito enters into the tank and lays eggs, there is virtually no likelihood of mosquito proliferation.

11. Is there a sewage charge if rainwater is flushed into the sewage system after use?

Sewage charge is calculated according to how much waste water goes into the sewage system so utilizing rainwater should reduce this cost. When rainwater is used, for example to flush toilets or to do laundry, and is then discharged into the sewer, the sewage charge is calculated in much the same way as for well-water. That is, a water meter is installed to measure the amount of rainwater used and residents have to pay the sewage cost equivalent to this amount. However, when rainwater is used for watering plants, etc. and does not drain into sewers, sewage cost will not be charged. Storage or infiltration of rainwater contributes to preventing river water pollution induced by discharging raw sewage from sewage pumping stations, and to controlling urban floods in heavy rain. Therefore, drainage of used rainwater should be free or the cost reduced when a rainwater utilization system has been installed as it also serves as a sewage system.

12. Is it possible to convert septic tanks which are no longer used into rainwater tanks?

In Koshigaya, Saitama Prefecture we convert disused septic tanks into rainwater tanks by firstly getting cleaning experts to empty the tank and then wash it out with water. In some cases, simple chemical disinfection is carried out. If tank water is changed many times for several months before the start of use, the tank water will become clean.

13. How big should the rainwater tank be?

＜ TOP >

Depends on the use. For example, to use rainwater for flushing toilets in an office-type building, a tank one tenth the size of the collecting surface is usually sufficient (eg: if the collecting surface is 100 square meters, a 10 ton tank is sufficient. For individual residences, approximately 80% of water required for watering plants etc. in a 4 person family can be supplied with a 10 ton tank. For 2 adults, a 2 ton tank will supply enough water to flush the toilet.

14. What sort of subsidies are available for rainwater users in Japan?

As of April 1998, the following cities and prefectures in Japan provided some sort of subsidy to install rain water utilization equipment: Tokyo: Sumida Ward, Taito Ward, Katsushika Ward, Tama City, Chofu City, Saitama Pref: Kawaguchi City, Tokorozawa City, Shiki City, Yoshikawa City, Miyashiro Town Kanagawa Pref: Kamakura City Shikoku: Takamatsu City Kyushu: Kagoshima City Okinawa Prefecture Okegawa City Gamagouri City Local Governments which provide subsidies to convert septic tanks which are no longer used into rainwater tanks: Saitama Pref: Kawaguchi City, Koshigaya City, Yoshikawa City, Miyashiro Town Kanagawa Pref: Kamakura City, Fujisawa City Tohoku area: Yamagata City Aichi Pref: Toyoda City Okegawa City Minami Ashikaga City Organizations that have a system of special financing for installation of rainwater utilization facilities: Residential Finance Corporation Development Bank of Japan