"SUSTAINABLE URBAN WATER RESOURCES IN THE 21ST CENTURY",
September, 1997, Malmo, SWEDEN
Restoration of Polluted Urban Watercourses
in Tokyo for Community Use
Shoichi Fujita
Director, Technical Division, Regional Sewerage Center,
Tokyo Metropolitan Government
Nishikicho 1-7-26, Tachikawa, Tokyo, 190 Japan
Abstract
Along with urbanization, the quantity of water in the streams of Tokyo has
decreased drastically. People in urban areas gradually came to need a clean
water flow for their daily relaxation. In this report, some actual examples of
restoration of polluted or dried urban watercourses are introduced.
Introduction
Waterfront areas provide urban citizens with comfort and joy as a place to
observe nature and to enjoy landscapes. However, along with urbanization,
water is decreasing or drying up in many watercourses. With the rise of living
standards, people tend to seek spiritual affluence as well as material
affluence and have come to reconsider waterfronts as part of familiar nature
and places for recreation. Thus, there has been a growing demand for
restoration of polluted urban watercourses. Some examples of having
succeeded in restoring waterfront areas are shown below that can be
enjoyed by urban inhabitants through various measures such as advanced
treatment, combined sewer overflow measures and infiltration of stormwater.
There is a small history of development.
(1) Recovering the clean water flow in a dried irrigation channel by advanced
treated wastewater.
(2) Introducing treated wastewater into a dried stream.
(3) Constructing an artificial stream above underground sewers.
(4) Introduction of groundwater into a dried stream.
1. Recovering clean water flow in old dried channels
(Example of Tamagawa Josui Water Channel)
a) Dried-up irrigation channel
The Nobidome irrigation channel was constructed in 1655, having an
extension of 9.6 km and a width of 1.8 m. It was built to get drinking water
and irrigation water from the Tama River, running in the southwestern part of
Tokyo. However, urbanization of Tokyo resulted in the disappearance of
agricultural lands, putting this channel almost into disuse. After 1973, there
was no water flowing in this channel which turned into a so-called "empty
canal. "
On the other hand, the Tamagawa Josui Water Channel was built in 1654 to
lead drinking water from the Tama River to the center of Tokyo, having an
extension of 18.0 km and a width of 2.0 m. However, it went into disuse with
the development of the water supply system in Tokyo, and water was not
flowing after 1970.
Similarly, the Sengawa Josui Water Channel was constructed in 1696 to lead
water from Tama River to the northern part of Tokyo, having an extension of
5.0 km and a width of 1.5 m. After 1971, water was not flowing.
There had been a growing demand among the inhabitants to seek for revival
of clean water flows in such old water channels. Thus, the Tokyo Metropolitan
Government (TMG) launched projects leading water treated in wastewater
treatment plants into these old channels.
b) Treated wastewater to dried-up channels
The water of 0.38 m3/s filtrated by sand followed by ozone treatment at
Tamagawa Joryu Wastewater Treatment Plant is pumped for a distance of
18 km to be discharged at an upstream point in the Tamagawa Josui Water
Channel as shown in Figure 1.
Thus, clear streams came back in three channels;
(1) the Nobidome irrigation channel with an amount of 10,000 m3/day on
August 21, 1984,
(2) the Tamagawa Josui Water Channel with 13,200 m3/day on August 27,
1986, and
(3) the Sengawa Josui Water Channel with 10,000 m3/day on March 29, 1989.
The targeted quality of the water running through these channels is 8 mg/L
or less in terms of T-BOD5.
People have been enjoying these channels with tall trees and promenades on
both sides more than ever before since clear streams came back, as a place
of recreation and relaxation.
Furthermore, the old trees on both sides suffering from lack of water restored
their vigor with flowing water, and became more and more luxuriant.
Figure 1. Restoration of clean stream
Total expenditures required for these projects account for 6.7 billion yen, or 67
million US dollars, and annual maintenance costs are about 300 million yen, or 3
million US dollars, (20 yen/m3, or 20 cent/m3).
In response to the cry of the people demanding revival of aquatic life, TMG has
made efforts to recover the ecosystem by changing the form of the revetment
and the bottom, creating meandering water flows, and forming rapids and deep
pools. As a result, many types of living creatures came to inhabit these areas.
The movement of the people demanding recovery of the clear stream heightens
the awareness regarding the relationship between urbanization and nature, and
gave a severe impact to other movements. TMG has plans to introduce treated
water into 30 water channels.
c) Responding more to the demand of citizens
--- Elimination of offensive odors
Wastewater treated by the conventional activated sludge process at the
wastewater treatment plant was provided with high-speed sand filtration by
PAC addition. When water flow was restored, there were a lot of complaints
from citizens that the water smelled bad. Therefore, TMG installed ozone
treatment facilities to remove offensive odors. After that, there have been
almost no complaints from the citizens.
d) Groundwater recharge effect
When water began flowing again in these channels which had been dried up
for a long time, almost all the water from the treatment plant was infiltrated
into the ground, and a water flow was not formed. These channels were
constructed by open excavation, and the bottom and both sides on the banks
were formed on natural ground. Judging from the permeability of the soil, this
phenomenon was quite natural. When water was fed at the rate of 0.15 m3/s
in the channel having a width of 2 meters, the leading end of the flow
proceeded only 100 meters a day.
Even today, many parts of the water in these channels have been recharging
the ground water. As illustrated in Figure 2, part of the groundwater must be
flowing out into the Nogawa Stream. Restoration of clear streams has a
favorable impact to trees, grasses, flowers, insects and birds inhabiting both
sides of the channels through the flowing water. Furthermore, the flow turns
into groundwater to give positive effects to the environments of other regions.
Figure 2. Supply of groundwater to the Nogawa Stream
2. Introducing treated wastewater into a dried
watercourse (Example of Meguro Stream)
a) Watercourse to discharge stormwater
Meguro Stream has a length of 18.8 km and a basin area of 45.8 km2 with a
population of 700,000 therein. It used to be a brook flowing through the
southwestern area of Tokyo. The upper part of Meguro Stream was made
into underground culverts in the 1970s, and was used as trunk sewers for the
combined sewer system. Accordingly, on fine days, the wastewater is diverted
into a sewage treatment plant. Since the lower part from the diverting point
of Meguro Stream is an open channel, wastewater does not flow through
there on fine days. On rainy days, however, it acts as a "stormwater drain"
receiving the overflows from the combined sewer system. Meguro Stream is
located downstream from the underground trunk sewers, shown in Figure 3, so
it used to be "a dried watercourse"on fine days.
Figure 3. Meguro Stream as an outlet for CSO
In this case as well, the groundwater level was lowered by a decrease of
stormwater infiltration due to urbanization, and there was almost no natural
flow of water in Meguro Stream on fine days.
b) Treated wastewater into the dried up watercourse
To revive a water flow in this stream, TMG introduced 0.35 m3/sec of the
water treated by advanced methods from the sewage treatment plant 17 km
away, into the lower portion of Meguro Stream. Since March 1995, Meguro
Stream has had clear water at all times. Table 2 shows the water quality
treated by advanced technology.
However, the coliform group number in this table exceeds the level of water
for landscaping (10 coliform groups/mL) stipulated by the Ministry of
Construction. To solve this problem, ultraviolet radiation facilities were
installed at the outlets. Ultraviolet radiation facilities were introduced for the
following reasons: 1) There was no space to ensure a sufficient time of contact
when chlorine and ozone were dosed, and 2) the discharged water through UV
should have little impact on the living creatures in the stream.
The quantity of water to be supplied to the stream was determined with
consideration of landscape, self-purification capacity, and living conditions for
the living creatures. Living conditions for carp and crucians are said to be
satisfied by T-BOD5 of 8 mg/L, a water depth of 10 cm, and the velocity of
0.1 m/sec. Meguro Stream has a depth of 20 cm and a velocity of 0.30 m/sec.
In the event of heavy rainfall, the valves at the discharge outlet of Meguro
Stream are remote-controlled from the wastewater treatment plant through
the optical fiber cable installed in the water pipe, thereby adjusting the amount
of water to be supplied.
c) Citizens and a concrete-made watercourse
As shown in Photo 1, Meguro Stream is a so-called "three sides concrete
watercourse". The side walls and the base are consolidated by concrete; it is
an "artificial watercourse" far different from a natural watercourse.
Even if clear water is flowing, waterweeds, fishes and insects could not easily
inhabit there.
However, people living along the watercourse have come to take notice of this
tiny flow. What is most important in the restoration of the flow of stream is to
create a "connection between people and water."
TMG is planning to modify the river walls and bed for the easy entry of ground
water, to create river beds with rapids and pools for the fish to live in.
Furthermore, it intends to plant trees along promenades on both sides of the
watercourse with colorful tiles. The clean water flow of Meguro Stream has
been loved and enjoyed by the people.
d) Project for restoration of Meguro Stream
To ensure clean water flow in the Meguro Stream, the following measures have
been taken: 1) To improve water quality, sand filtration in addition to the
conventional activated sludge process was adopted at the wastewater
treatment plant; furthermore, 2) Immediately before discharging into Meguro
Stream, the water is subjected to ultraviolet disinfection. In addition, 3) the
capacity of the intercepting sewer was increased in order to reduce the CSOs,
as illustrated in Figure 3. and 4) A massive tunnel storage basin, with a diameter
of 2.2 - 6.5 meters, extension of 7,310 meters, and a capacity of 150,000 cubic
meters was built to reduce the first flush of the CSOs.
3. Reviving the water flow
above underground watercourses
(Example of Kitazawa Stream)
a) From a watercourse to a trunk sewer
Kitazawa Stream has an extension of 5.7 km and a basin area of 10.5 km2 with
a population of 150,000 therein. It used to be a brook flowing through the
western urban area of Tokyo. Rapid urbanization of Tokyo is illustrated in
Figure 4 which resulted in a decrease of stormwater infiltration, accompanied
by a decrease of groundwater. In the early 1960s, as a result, there was almost
no natural flow of water in Kitazawa Stream during fine weather.
In those days, sewage facilities were not yet provided in this area, and
miscellaneous wastewater from households was led into this watercourse.
Figure 4. Land use in Tokyo
Thus, Kitazawa Stream turned into a stinking contaminated watercourse.
However, once there was a rainfall, stormwater in the basin suddenly came
into the watercourse, often causing floods.
Then, TMG turned this watercourse into an underground culvert where
offensive odors were enclosed and contamination was made invisible.
In this way, the watercourse became a trunk sewer of the combined sewer
system, and the ground above the watercourse was turned into a promenade,
as shown in the photo 2. Trees and flowers were planted, and the contaminated
watercourse was transformed into a comfortable green promenade for urban
inhabitants.
b) Restoring a brook along the Promenade
However, 20 years after the construction of the promenade, people came to
seek a brook with flowing water as there used to be. People of the urban
community tend to aspire for nature in their neighborhoods. There is an
increasing number of city inhabitants desiring to have greenery and water in
the congested urban area of Tokyo. Time has come when they attach greater
importance to leisure and quality of life.
To meet the needs of the people, TMG launched a project in the late 1980s to
recover clean water flow in the watercourses. Wastewater highly treated in
wastewater treatment plants is utilized in such water channels, and the number
of such examples is ever increasing in recent years.
In Kitazawa Stream, 0.02 m3/sec of the treated water is led from a sewage
treatment plant 11 km away, and a new "rivulet" was born along the promenade.
This is called a "two-storied watercourse," a new water environment system of
the urban area.
The history of Kitazawa Stream introduced above is shown in Figure 5.
Figure 5. The history of Kitazawa Stream
c) Water flow loved by citizens
The rivulet is planned with many types of plants and flowers so that various
kinds of living creatures can habitate in the watercourse. If left to nature, only
the strongest type of weed would grow. However, the verdure is not orderly
planted as an artificial flower bed. And the water channel is not straight but it
has the meandering flow with shallows and water puddles. Pebbles and rocks
are strewn so that water grasses, insects and fishes can live. At first glance,
it looks like a natural watercourse, but actually it is provided with artificial
considerations.
The living creatures that escaped from urban areas came back to the "artificial
watercourse" created along the artificial promenade and restarted their living.
In a tiny world of the restored nature, people find various aspects of nature with
seasonal changes. Here, people now have leisure and quality of life.
In the design of the water channel, views of the inhabitants are reflected, and
these people extend cooperation to the management of this water channel.
To protect the "creatures of nature" living in the "artificial watercourse, " an
organization of the community people has been set up, and a consensus has
been formed among the people to protect this watercourse by the efforts of all
the people.
d) Necessary project for the artificial watercourse
Springwater and river water can be a possible source to supply water to the
"artificial watercourse." However they were not adopted. Because the amount
of water flowing in adjacent watercourses was insufficient, and the water
quality was not good enough. Therefore, wastewater from a sewage treatment
plant was introduced after providing adequate treatment.
For this purpose, the following measures have been taken to improve the water
quality, 1) sand filtration in addition to the conventional activated sludge process;
furthermore, 2) the water is subjected to ultraviolet disinfection.
Then, 3) the water is treated by upflow filtration by addition of PAC
poly-aluminum chloride, and 4) the water is also treated by deodorization and
decolorization by ozone. Furthermore, 5) cobble stones of varied sizes are
arranged in the flowing water channel.
The targeted water quality is 3 mg/L or less in terms of T-BOD5.
4. Introduction of groundwater into dried up stream
(Example of the Nogawa)
a) Dried up stream by decrease of ground water
The Nogawa Stream has an extension of 18.2 kilometers and a basin area of 69.6
square kilometers with a population of 700,000. Nogawa is a watercourse flowing in
the hilly area in the suburbs of the western part of Tokyo.
On the left bank of the Nogawa, a 20-meter-high cliff rises steeply.
The Nogawa used to be filled with water coming from the spring of this cliff.
The Nogawa was supplied with the rich ground water of the Musashino Plateau
(loam layer with a high permeability). However, urbanization resulted in a
decrease of stormwater infiltration, and decrease of groundwater. This resulted
in a reduction of the amount of natural water flowing into the Nogawa.
On the other hand, urbanization of this basin was very rapid, and there was a
delay in the improvement of the sewerage system. Much of the wastewater had
been infiltrated into the ground. (Wastewater from toilets was separated and
carried by special vehicles.) In the early 1970s, 50 percent of the ground water
of this area is considered to have been recharged wastewater. Namely, in dry
weather, the Nogawa was supplied from three sources; (1) infiltrated stormwater,
(2) household wastewater infiltrated into the ground, and (3) wastewater
discharged directly from households.
When the sewerage system of this area was improved, the wastewater was all
drained to the treatment plant. As a result, there was a great decrease in the
amount of water in the Nogawa on fine days in the 1980s, as shown in Figure 7.
Figure 7. Quantity of stream flow in the Nogawa
and spread of the drainage works
b) Spring restoration measures
The spring of the cliff on the left bank of the Nogawa provided a place of
recreation and relaxation for the people of the area, and was a place of interest
to be distinguished from other places. The recharged area of each spring is as
small as 100 hectares, so the artificial ground water recharging is likely to be
effective.
The inhabitants of the community requested the TMG to find out a way to
recover the dried up springs, and the inhabitants themselves made efforts to
recover the springs.
In response to this request, TMG and the local municipalities of this area started
the project of restoring springs in the Nogawa basin.
c) Promotion of stormwater infiltration
To prevent the precious water of Nogawa from being dried up, efforts for
stormwater infiltration projects are being continued with the cooperation of the
citizens and the administration.
Mitaka City requested the owners of private houses in the related area to
install stormwater infiltration facilities in order to recover water for the spring
of Maruike, a representative spring of the city. During the period from 1992 to
1995, 4,000 soakaways were installed. The construction costs were entirely
borne by TMG and Mitaka City. Installation of two soakaways in each household
allows annual 120 cubic meters of stormwater (328 liters of stormwater per day)
to be recharged underground. Stormwater of 240,000 cubic meters can be
recharged by 4,000 soakaways every year.
Furthermore, Koganei City requests citizens to install stormwater infiltration
facilities within housing sites when they construct new houses or modify existing
houses. This effort was started in 1983. For the first ten years, the installation
costs were borne entirely by the inhabitants, because the people of Koganei
recognized the necessity and importance of stormwater infiltration. Since 1993
thereafter, the City has been paying part of such expenses. As a result, 26,522
soakaways and 10.4 km of infiltration trenches were installed in 5,733 houses by
the end of December 1996 in Koganei, having an area of 11.32 square kilo-meters
and a population of 102,000 with 24,740 houses. These efforts still continue at
present, resulting in ever increasing number of infiltration facilities.
On the other hand, in an effort to ensure the quantity of water in Masugatanoike,
a famous spring of the area, Kokubunji City worked out a program of installing 2,300
soakaways in the upstream area, and requested the cooperation of the inhabitants.
As a result, 2,018 soakaways were installed in 800 houses in an area of 2.38 square
kilometers during the period of four years from 1990 to 1993.
In Setagaya Ward located downstream of the Nogawa, 3,000 soakaways were
installed during the period of five years from 1992.
Though the effect of stormwater infiltration projects launched in various cities
within the basin area is not yet clear on a quantitative basis, it is reported that
there has been an increasing amount of water from the springs; it is considered
that there must have been some favorable effects.
Furthermore, to protect the cliff area along the Nogawa where the ground water
of the plateau is flowing out, TMG gradually bought up the land to own most of
this area. Efforts to preserve trees, grasses and flowers are carried out through
cooperation with the inhabitants.
d) Inhabitants of the Community and the Nogawa
The Nogawa flowing in the suburbs of Tokyo has long been loved by the people,
and has provided an important place of recreation. As shown in Photo 4, the place
is popular as a playground for children.
Mitaka City leads part of the water of the Nogawa into a nearby wetland, to
breed fireflies. Meetings of the people to enjoy the fireflies are held every year,
and this event is highly evaluated among the citizens.
In addition, various groups of citizens are taking part in many activities to enjoy
the nature of the Nogawa; insect watching, hearing of insect singing, "clean
tactics" (collection of wastes), water quality survey, vegetation analysis, nature
watching, barbecue parties, reed pipe concerts, camping, hiking, sketching,
photographing and many other activities. They publish small magazines for
communication on such activities.
5. Water resources in urban areas
a) Water resources for restoration of clear streams
Of the five water resources discussed in the following paragraph; (1) groundwater,
(2) treated wastewater, (3) rainwater, (4) water conservation, and (5) sea water;
treated wastewater and ground water are feasible water sources for restoration of
clear streams. In the future, the weight of treated wastewater and groundwater will
increase as new water resources.
To realize this, advanced treatment and groundwater recharging by stormwater
infiltration are of particular importance.
However, it is worthy of questioning if the use of sewage water treated by
advanced technology is the best way to restore clear streams in dried-up
watercourses. The quality and quantity of water required to restore clear
stream can be obtained by use of modern technology as shown in the examples
described above. However, there remains a question if this is the result of
forcible restoration of nature or not.
Water to be loved and enjoyed by people should be obtained by leading natural
stormwater and groundwater in natural forms. Efforts to promote the technique
of assisting the working of nature such as storage and infiltration of stormwater
on-site are more desirable than the efforts for forcible treatment of sewage
water by advanced technologies.
In the future, it seems necessary to select water resources according to the
water use from the viewpoint of energy consumption, resources, load on the
environment, cost-benefit, risk, and a socially optimum water distribution system.
b) Sustainable water resources in Tokyo
When Japanese rivers are compared with those of other countries of the world,
they have steep slopes as shown in Figure 8; some foreign engineers are reported
to have said that Japanese rivers were very similar to cataracts.
This geographical feature allows the stormwater to be washed away immediately.
Flooding is likely to occur in rainy seasons, while drought is likely to visit in dry
seasons.
Figure 8. Gradient of Rivers in the World
Furthermore, due to urbanization, stormwater in urban areas is immediately
discharged into watercourses. This is likely to cause flooding. Reduction in
ground water levels results in a decrease in the amount of water supplied to
watercourses at normal times. Thus, many watercourses in urban area are
dried up.
Several dams are built as water sources for Tokyo, 150km away from Tokyo,
and water stored in these dams is sent to the city.
Tokyo discards the stormwater within its area and depends on other areas for
water resources. Tokyo is required to keep within the city the water source
available for the management of sustainable water resources. For this purpose,
it will be necessary to use water in many different ways according to the
quantity and quality. The current water circulation system in the city is based
on an one-dimensional tap water system. It is unnecessary to use potable
water for flushing, air-conditioning and for cleaning automobiles. However, it is
essential to improve such a simple system to create a "water circulation system".
Expansion of water resources has reached its limit. It is important to develop
potential water resources owned by the city and to find ways to make an
effective use of these resources.
c) Development of available water resources
1) Groundwater
Tokyo used to be rich in groundwater. However, subsidence of the ground level
was caused by excessive pumping up of groundwater, and substantial
restrictions are currently imposed on the use of groundwater.
Figure 9. Ground subsidence and groundwater pumping
Figure 9 shows the relationship between the sinkage of ground and the amount
of pumped water in Tokyo. The subsidence of ground is reducing as a result of
application of regulations on groundwater pumping.
Regarding the restrictions on groundwater pumping, a tap water system for
industrial use was started in 1964. This is a system to use water from treated
sewage as an alternative to groundwater.
Groundwater provides a precious water resource obtained within the urban area.
During the Great Earthquake in 1995, ground water made a significant
contribution as an emergency water source. In many places, however,
groundwater is contaminated by organic chlorine compounds (tri-chloro-ethylene,
tetra-chloro-ethylene, etc.) due to the wastewater discharged underground before
the construction of the sewerage system. It is essential to take more aggressive
steps to maintain the water quality of groundwater.
Currently, use of groundwater accounts for ten percent of the total water use in
Tokyo. It is a precious water resource. Efforts must be made to promote
groundwater recharge by stormwater infiltration so that groundwater use can be
implemented systematically. Stormwater infiltration is to store stormwater
underground temporarily. This may be one type of storage system.
The stormwater having infiltrated and stored underground comes out as
springwater, and then it flows into the watercourse to increase the amount of
water in the watercourse.
Furthermore, springs (653 springs in Tokyo, as of 1995) provide a place of
recreation and relaxation for the citizens. It provides an opportunity of getting
in touch with water in their daily life.
2) Treated wastewater
In addition to water supply for industrial use, the water from treated sewage is
used in Tokyo for the following purposes; wastewater reuse system,
miscellaneous use in buildings, restoration of clear streams, washing of cars on
the Shinkansen bullet train line, sprinkling of water to trees planted on avenues,
cleaning of the sewer, and fire fighting water, etc.
The daily amount of reused water in Tokyo is 400,000 cubic meters with respect
to the daily amount of treated wastewater 5,800,000 cubic meters.
The percentage of reuse is still only 6.9 %. Furthermore, the breakdown of the
reuse reveals that 83% is reused as miscellaneous water in the treatment plant,
and 9% is employed to restore clear streams, with 8% for private use.
Thus, the application is still limited, and must be expanded in the future.
Amid the growing insufficiency of water resources in Tokyo, the value of the water
from treated sewage is increasing. It is necessary to develop the treatment system
in conformity to the purpose of use and to set up the level of the quality of water.
The targeted quality of the reused water differs according to the particular
requirements. In Japan, the Ministry of Construction and the Ministry of Health
and Welfare set up the quality standards shown in Table 4.
There is an increasing number of examples where reuse of treated sewage is
essential despite higher treatment costs than normal water resources.
Meanwhile, in districts with heavy snowfall, disposal of snow poses a big problem.
There is an increasing number of cities which lead treated sewage into water
channels to melt the snow dumped by the inhabitants. Otherwise, treated water
is stored in an impounding pool and snow is carried there to be melted.
This is another form of utilizing treated wastewater.
3) rainwater
Use of rainwater is often practiced on islands where obtaining water resources
is extremely difficult. However, interest in use of rainwater is growing in cities
where there is a tight supply/demand relationship for water. There are many
examples where rainwater is stored in the basement of buildings. Expenses for
the tap water supply are generally less costly than that of rainwater storage
facilities. However, in view of stormwater runoff control, shortage of water
sources and water source for emergencies, there is an increasing use of
rainwater mainly in public facilities. In Sumida Ward of Tokyo, there are many
examples where citizens install tanks on the road to get water for daily
miscellaneous use or for emergency use. Sumida Ward grants a subsidy for such
storage facilities.
4) Water savings
There is a possibility of drought throughout the year in Tokyo; in summer there
is an increased demand for water, while in winter, there is a smaller amount of
rainfall. TMG launched a campaign to call for citizens to save water. Decreasing
the use of water in this way provides the same effect as finding new water
resources.
Water demand in Tokyo is expected to increase slightly but steadily.
At the same time, it is becoming more and more difficult to develop new water
resources. In 1973, TMG decided to formulate a water conservation policy, and
has been promoting voluntary water saving measures through campaigns and
through the development and commercialization of water saving equipment.
TMG requests makers to develop and produce water-saving fittings for taps,
toilets, washing machines, etc.
Underground leakage of tap water was 15% (261 million m3/year) in Tokyo in 1982.
As a result of such efforts as replacing 250 km of distribution piping annually into
using of stainless steel pipes, this was improved to the level of 9.3% (161 million
m3/year) in 1995. This is planned to be reduced to 7% (121 million m3/year) by 2000.
5) Desalination of sea water
Desalination of sea water is often carried out in small-scale projects in Japan.
However, very few cases are of practical use due to high costs.
Yokosuka City in Kanagawa Prefecture, however, is now ready to load
desalination equipment on a truck to provide a water source in the case of
emergency to ensure water supply at a required location. It should be noted
that Japan has advanced technology of desalination and has often extended
assistance to countries suffering from insufficient water resources.
CONCLUSION -- Recovery of effective water recirculation
It is important to recover favorable natural water recirculation, in addition to
restoration of clear stream discussed in this paper.
To achieve this, it is necessary to make efforts for maintenance and regeneration
of the overall water recirculation system in the entire basin area, through
(1) stormwater storage and infiltration, (2) preservation and recovery of springs,
(3) preservation of agricultural land, forests and cliffs, (4) protection of groundwater
against contamination, (5) advanced sewage treatment, (6) CSO measures,
(7) recovery of flow in watercourses, (8) water savings, (9) use of rainwater.
To realize measures discussed above, TMG is working out the "Water Circulation
Master Plan" to promote projects in an overall, systematic and effective way in
various sectors related to water, including tap water, wastewater, watercourses,
city planning, environment, agriculture, forestry and fishing.