Economic evaluation of combined treatment for sludge from drinking water and sewage treatment plants in Japan

In Japan, water supply utilities need to renovate their sludge treatment facilities. This study therefore evaluated treatment of the sludge from drinking water treatment plants (DWTP) by two approaches: an individual system and a combined system. In the individual treatment scenario, the DWTP treats the sludge using its own facilities, whereas in the combined treatment scenario, DWTP sludge is transported to a sewerage system. The construction cost is much lower for combined treatment than individual treatment, while the operation cost is higher for combined treatment owing to the sewer charge. However, the additional sewer charge is less than the increase in cost for constructing an individual sludge treatment system. With the combined treatment system, sludge must be transported from the DWTP to a sewage sludge treatment plant (SSTP). A cost analysis reveals that sludge transportation by road is less beneficial than by pipeline in the sewerage system unless the plants are nearby. In economic terms, combined treatment and a sludge liquor pipeline is the most advantageous of all cases. The total cost depends on the given conditions and assumed unit costs. Sensitive parameters are the sewer charge and the distance between the DWTP and sewer pipeline.