Sustainable urban water use

The fresh water resource in the world is limited. And it is threatened by pollution over these decades. Thus, concern about the sustainable of urban water use is the strong motivation to develop the water management system in urbanized cities, especially in the field of water recycling and rainwater collection. Due to the sharply growth of urbanization, the problem of water shortage becomes a realistic issue which occurs in many big cities. Therefore, the achievement of sustainability is required to ensure the water supply. In this part, the city of Tokyo in Japan has a lot of useful information and excellent design which we can learn from it.

1. Water balance in Tokyo

Figure 1. Water balance and recycle in Tokyo (unit: mm/year)

The figure above shows the water balance in Tokyo (Tokyo Metropolitan Government, 1999). According to the data from this figure, the average annual rainfall is 1405 mm in Tokyo. And 29% of total rainfall is evapotranspiration which is 412 mm per year. But the serious problem is that there is 879 mm treated waste water run into the river system which is large flow. It will have significant impact on river environment. Because this situation, the government of Tokyo apply for advanced waste water treatment and try to reuse it as a water resource in urban area.

Figure 2. Proportion of treated sewage in urban river flow at the ordinary water level

2. The different water resources for sustainable water use in Tokyo

Firstly, it is the rainwater use. In Tokyo, the collected rainwater is usually used as toilet flushing or water-cooling. There are many rainwater storage facilities in Tokyo underground which is very large. The table below shows the capacity of each storage facilities in Tokyo.

Table 1

Secondly, the reclaimed wastewater also accounts a big part of total reuse water. By using the treated wastewater, it can reduce the wastewater discharge. And it also can make the water price cheaper compared with the distant water sources. There is an example of a sewage reuse system in Tokyo.

In this building, all the wastewater will be collected into an underground water tank. Then it goes to a sewage treatment plant. After the whole processes of treatment, the wastewater runs to a special tank for the step of disinfection. Finally, the reclaimed water go back to the reuse system for cleaning, cooling and dilution in many buildings and industrial plants. The most important thing is that the reclaimed water standard is also very high. The table below shows the standards.

Table 2

Finally, Tokyo government has built an experimental sewer system to collect infiltration.

Figure 3. Configuration of infiltration facilities in the Experimental sewer system

This is a complex system which can collect the infiltration. If this system works, it can collect almost 80% of total infiltration from rainfall. Because infiltration occupies about 26% of total rainfall, the amount of collected water by this system will be a large number.

To sum up, Tokyo has tried to several methods to meet the sharply increasing demand of water. And these methods really work well over these decades. As a result, rainwater and reclaimed sewage can be used in many aspects if the government develops the water management and the treatment technologies. Besides the city of Tokyo, many other urban cities should learn something from Tokyo and compare with the real situation they faced to build great water reuse and recycle system. I believe that if all the urban cities can improve their water-use efficiency, the city will be more sustainable and residents also will have a lot of benefits. 

Reference ;

1. Clements-Croome D. Intelligent buildings: design, construction and operation. London: Thomas Telford; 2004.

2. Jeronimidis G. Biomimetics: lessons from nature for engineering. The 35th John Player Memorial Lecture, The Institution of Mechanical Engineers, 22 March, London, 2000.