Sewer versus On-Site System

In Japan, extension of sewer network is measured by the indicator of sewer service accessibility ratio (SSAR).
Its denominator is the number of registered citizens in the total area of governments, such as nation, region, prefecture, and municipality.
Its numerator is the number of registered citizens in the serviceable area.
There is not sanitary or combined sewer network without a treatment plant.

The current discussion is what the final goal of SSAR should be.
The National SSAR reached 72.7% by the end of fiscal year 2008, that is March 31, 2009.
The SSAR excludes on-site sewage treatment system and small sewer system for farmers’ community.
If they are included, the ratio is 84.8%.
The inclusive ratio is called sewage treatment accessibility ratio.

Do we have to continue to install sewer?
Population is on the decrease.
The remaining area without sewer is suburb.
Therefore, installation of sewer becomes inefficient.
Governments are heavily indebted.
Stop costly sewer expansion projects by public.
Instead spread on-site systems by subsidizing home owners.
This is a critical opinion which a municipality with sewer expansion projects faces.

The refutation is as follows.
On-site system is difficult to maintain to produce high quality effluent especially if nutrient removal is required.
It needs space to install, one car park at minimum.
Some houses are too small to install on-site system.
It needs drainage channel from individual property to public waters.
It needs periodical desludge by vacuum truck for good.
It needs treatment facility to dispose of the sludge.
Therefore, comparison of only installation cost of sewer versus on-site system is misleading.