Great East Japan Earthquake Strikes Sewerage System
Takeshi Yamauchi
1. Mechanism of Treatment of Domestic Sewage in Japan
Waste water from houses goes through pipes that lead to treatment plants wherein purification happened and then discharged into a river. This is general image of the treatment, but technically there are two types of systems: septic tank and sewerage.
1.1 Types of Sewage Systems
a. Septic Tank
Figure 1. Image of septic tank (1)
b. Sewerage System
In sewerage system, waste water from houses goes through drain pipes to be collected in one place. The collected waste water will flow down through drain pipes to purification plant by the force of gravity. Lastly, it will be discharged into the water bodies. Sewerage system basically collects waste water from one municipality. Some pipes may go long route, so if it simply rely on only force of gravity, pipes will go very deep before they get to purification plant. In that case, pumps are used to pump up waste water to flow down to purification plant again.
Figure 2.(a) Image of sewerage system(2)
Figure 2.(b) How waste water from a house is collected(2)
c. Percentage of people using sewerage system and septic tank
| Number of People utilizing | Percentage of people utilizing | |
|---|---|---|
| Iwate | ||
| Sewerage system | 589,692 | 43.80% |
| Septic tank | 147,002 | 10.90% |
| Others | 96,214 | 7.10% |
| Total | 832,908 | 61.90% |
| (Total population 1,340,852) | ||
| Miyagi | ||
| Sewerage system | 1,769,032 | 75.90% |
| Septic tank | 138,225 | 5.90% |
| Others | 92,668 | 3.98% |
| Total | 1,999,925 | 85.80% |
| (Total population 2,330,898) | ||
| Fukushima | ||
| Sewerage system | 982,127 | 48.10% |
| Septic tank | 371,616 | 18.20% |
| Others | 136,804 | 6.70% |
| Total | 1,492,587 | 73.10% |
| (Total population 2,041,844) |
1.2 Waste Water Treatment Process
There are several methods of sewage treatment. In this section, popular method used in Japan is presented.(4)
- Grit removal channel: Large solid waste and grit in wasted water from houses are removed in this channel, then sent to primary sedimentation channel.
- Primary sedimentation channel: run the waste water slowly to remove small sedimentary waste.
- Aeration tank: Add activated sludge to sewage and then introduce oxygen. Microorganisms in the activated sludge start to decompose organic matter in the sewage, which is the basis of sewage, and purify the sewage.
- Secondary sedimentation channel: Run the waste water slowly so that activated sludge will settle. Separate the top clear layer and the activated sludge.
- Chlorination channel: Chlorinate the cleaned water for disinfection and then discharge into the water bodies.