Utilization of treated sewage

The increase in the volume of sewage is proportional to the increase in water demand. The rate of coverage of the sewerage system in Japan passed 50 per cent in 1995. Japan is still a developing country as far as sewage is concerned. However, if one considers just the Ward Areas in Tokyo, the rate is now 100 per cent.

Treated sewage is beginning to be utilized for a variety of purposes, including toilet flushing (but not for drinking water). Treated sewage has also been utilized to cope with the exhaustion, even in normal conditions, of the Tamagawa Canal and the Nobidome Canal, which were constructed in the Edo period. A daily volume of about 43, 000 m³ has been transferred from the sewage treatment plant in the upper basin of the Tama River into the Nobidome Canal, starting in 1985, and into the Tamagawa Canal, from 1986. Since 1984, a maximum daily volume of 8, 000 m³ of treated sewage from the Ochiai Sewage Treatment Plant has been supplied to the Shinjuku Subcentre Area, where the Yodobashi Purification Plant used to be located.

Furthermore, since 1995, treated sewage has been utilized to supply water, at a rate of 1 m³/sec, to the Shibuya River, the Meguro River, and the Nomi River, where the volume of flow in normal conditions has shown a marked decrease.

The biggest cause of the decrease in flow in these streams, at normal times, has been the spread of the sewerage system. The sewerage system in Tokyo is designed to cope with heavy rain of 50 mm/hour. Consequently, most rainwater is drained through the sewerage system, and drainage by small streams has become unnecessary, even at times of heavy rain. In other words, most of the surface flow has been transformed into underground flows.

It is ironic that the treated sewage from sewage treatment plants has been put back into these streams in order to maintain their environmental balance, when discharges have decreased markedly because of the spread of the sewerage system. In some cities in Europe, small streams have been revived by discontinuing the use of sewerage systems. Sewerage systems have been a symbol of civilization, but this is no longer the case. One's view of twenty-first-century civilization is now affected by the flow of treated sewage into streams that have lost much of their original water volume.