The method for selecting the appropriate treatment system in low-density areas was studied by statistical data analyses and questionnaire surveys. The main results obtained are as follows.(l) The efficiency of sewerage service has been decreasing. With decrease of the population density in served areas, treatment cost is increasing from 7,000 yen/household in 1961 to near 40,000 in 1989.(2) By considering the external cost of treated water quality, the marginal house density beyond which collective systems become advantageous, was estimated to be 9.5 houses/ha compared to the original value of 13 houses/ha.(3) Improvement of water quality, saving energy, and efficient uses of sludge are considered to be important for the environmental impact of sewerage systems. Suitable allocation of optional treatment systems, energy saving technology and prevention of sludge contamination must be the important issues for the sewerage service on a small scale.

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