• Waste management in Japan
• Circular economy in Japan
• Waste management in Asia
• Disaster waste management

Johkasou: on-site domestic wastewater treatment system

Johkasou, like sewerage, is a system for domestic wastewater treatment. While sewerage is a centralized system that collects wastewater from tens of thousands of households and treat it at a plant, Johkasou is a decentralized system. It treats wastewater per premise, ranging from a house to a housing complex, by using a facility installed in a hole underground at a parking lot or other open space within the premise (for more information on Johkasou, see our article in the March 5, 2007 issue [in Japanese]). Since Johkasou provides a basic service necessary to ensure our healthy living environment, just like a service of collection and incineration of solid waste, it needs to be disaster-resistant and with features that enable quick restoration when damaged (see another relevant article on disaster-resilient waste treatment in the July 2015 issue).

When an earthquake strikes…

If an earthquake strikes Johkasou, how would it be affected? Surveys on Johkasou damage conducted after major earthquakes in Japan with lower-6 or higher intensity (on the Japanese seismic scale) from 2003 to 2007 revealed that most Johkasou facilities remained functional even after these events. The results show that in most cases, only single-digit percentages (except for a few cases of just over 10% at most), of Johkasou facilities in the earthquake-stricken municipalities were affected.¹ Because this is the case in the worst-affected municipalities, we can say that Johkasou as a whole is not easily affected by earthquakes.

The March 2011 Great East Japan Earthquake of magnitude 9 on the Richter scale, the largest ever recorded in Japan, caused the most severe human and property damage in Japan's post-war period. The Ministry of the Environment of Japan (MOEJ) conducted a survey targeting municipalities (excluding the areas where houses and buildings were completely washed away by the Tsunami) that experienced the quake of upper-5 or higher intensity. The results show that 24.6% of the Johkasou facilities in the surveyed areas became operational again after some quick repairs and that only 3.8% of them required full replacement.² Our team visited the affected areas immediately after the earthquake and several times afterward, and conducted various activities such as an assessment of the damage caused, presenting response measures, and compiling and providing scientific knowledge.³

Earthquake's impact differs between Johkasou types⁴

Johkasou facilities vary between different manufacturers in terms of their capacity and structures. In these results of the above-mentioned survey after the Great East Japan Earthquake, we further examined in detail the damage to the 22 different types of Johkasou at 684 locations in total. We found that some types were more easily pushed up to the surface by liquefaction than other types and that some construction materials were more resistant to earthquakes than other materials. As mentioned in the previous section, Johkasou appears to be generally resistant to earthquakes. Clarification of reasons for their strength or damage is crucial to make Johkasou more disaster-resistant and more resilient to (meaning "can quickly recover from") damage in future.

Earthquake-resistant Johkasou designs and construction technique⁵

Johkasou is usually buried underground to receive wastewater from drainage outlets of a house/building under the floor. To construct a Johkasou facility, in brief, the following steps are taken: (1) excavation of a hole larger than the Johkasou's size, (2) installation of the Johkasou unit in the hole, (3) connection of the unit with inlet and outlet pipes, and (4) backfilling of the hole with earth and sand. We conducted a simulation experiment of liquefaction by using a shake table and a model-scale Johkasou. In the simulation, vibration made the backfilled sand get under the Johkasou unit and made the unit float. Out experiments with different types of Johkasou show that, in brief, a Johkasou type having its center of gravity at a low level and with a flat and smooth bottom larger than top area was less likely to float. The result also shows that proper compaction of sand was generally effective for preventing the Johkasou unit from floating. Since excessive compaction may also damage the unit and produce an undesirable result, it is crucial to apply appropriate design, strength, and installation method (construction technique).

Efficient transportation of sludge⁶

Johkasou operation generates accumulation of microbial flocs called "sludge" (see our article in the February 2013 issue [in Japanese]). The sludge needs to be removed periodically. However, not all municipalities have sludge treatment facilities, or night soil treatment plants, and these facilities themselves might be damaged by disaster events and become unusable. It is therefore important that neighboring municipalities jointly prepare a plan on sludge transportation and treatment in emergency situations before a disaster occurs.

To find the most effective way for municipalities to collaborate with each other, we selected a prefecture and simulated its total sludge transportation load in different disaster scenarios by calculating the amount of sludge and night soil as well as distances to their treatment facilities. The results show that there is a case with a 3.4 times increase in total load (transported amount x distance) compared to pre-disaster time. The simulation also revealed that the increase can be minimized if the sludge is transported in two-process steps, namely, first to transfer stations and then to the treatment facilities. To use as transfer stations, storage tanks can be installed at some treatment facilities. We conducted similar simulations using different damage scenarios and identified the treatment facilities that can serve as important transfer stations. Either increasing treatment capacity of these facilities or constructing storage tanks there can ensure efficient sludge transportation and continuous use of Johkasou if any damage is caused to other treatment facilities.

Based on these results, we believe that prefectural governments can play a leading role in preparing a disaster response plan beyond municipal boundaries and strengthen collaboration among municipalities. Making such a pre-disaster plan can minimize the negative impact of disasters and lead to more disaster-resilient wastewater treatment systems.

Conclusion

The achievements of this study are presented in MOEJ's Technical Information Document (No.1-28) for the revised 2014 Guidelines for Disaster Waste Management (in Japanese).⁷ Based on these results, we have started another study on evaluation and standardization of Johkasou with the aim of developing appropriate earthquake resistance standards. Generally, there are many cases of earthquake resistance evaluations of houses/buildings as well as their basement structures that are connected by continuous columns with aboveground structures. However, no such evaluation was conducted for Johkasou in the past. The reason for this situation is that, although Johkasou is also an underground structure, it does not have any solid connection to the aboveground structure. Developing earthquake resistance standards for such detached structures is not easy, but we will continue our study with careful and steady approach.