There are three night-soil treatment plants in Seoul and each plant has its own service area based on 25 Gu(districts). Recently Seonam plant, one of three plants, is planed to double it’s existing design capacity, 2,000 tons per day. On the contrary, Nanji plant is supposed to cut down volume of treated night-soil, because the amount of nitrogen or phosphorus contained in night-soil is getting closer to the permission level, 10% of nitrogen or phosphorus originated from sewage. Also, it is not certain if existing service areas are reasonable. Hence it is necessary to reevaluate the existing service area of three night-soil treatment plants. This study aims to develop a methodology for zoning three plants’ service areas, satisfying both lower greenhouse gas emission and cost. Another major purpose is to recommend alternative service areas, reflecting various circumstances.
Seoul has about 600 thousand septic tanks for flush toilet, and they are legally demanded to clean up more than once a year, resulting in 10,817 tons septage in 2011. With septage growing year by year, three treatment plants have already faced capacity shortages. The shortage is barely managed by overuse of Seonam plant. In methodology, design capacity of night-soil treatment plant, acceptability of sewage treatment plant, greenhouse gas emission and cleaning cost were chosen as evaluation indices. Service area can be drawn up through the evaluation process, which include defining calculation elements, setting up scenarios and their goals, calculating and comparing scenario goal with final draft.
Considering the present, near future or ideal situation, five scenarios were set up. The ideal service area turns out impractical because it inevitably leads the septage volume and amount of nitrogen or phosphorus in Joongnang plant to exceed regulated level. Existing service area is so reasonable that the shift of service plant for Jung-gu from the Nanji plant to the Joongnang is only beneficial and other changes aggravate the problems. The enlargement of the Seonam is a crucial way to solve the problems.
When the enlargement of the Seonam is complete, Seonam’s service area would increase from 6 Gu to 9 Gu and in the other hand the Nanji’s would lessen from 8 Gu to 6 Gu. In case without the enlargement of the Seonam, it would be recommendable to lower the load on the Nanji by reducing existing 8 Gu to 7 Gu. All change of service area would necessarily demand change of user charge for cleaning septic tanks, and results from this study, might be helpful to assess the costs related to labor and vehicle requirement.