最終処分場延命と地球温暖化のトレードオフを考慮した下水汚泥溶融スラグの有効性の評価

Abstract

Objective. Although slag produced by the sewage sludge melting process offers the advantage of reducing the final volume that must be landfilled, such recycling also presents an important disadvantage due to emission of abundant greenhouse gases. A trade-off exists in the waste management sector between the life extension of landfill sites and global warming, and appropriate solutions are required to find an appropriate balance between these factors. This study aims to evaluate the environmental impact of slag produced by the sewage sludge melting process and to investigate conditions to promote slag production under trade-off condition.Results and Discussion. This study evaluates two cases: (1) slag is produced from sewage sludge and recycled for use in construction materials, and (2) sewage sludge is incinerated and the incineration ash is landfilled. Case studies at a particular treatment plant were conducted for comparative evaluation, and environmental loads of CO2 and final disposal were targeted. CO2 emissions of 1,400 and 500 kg-CO2/tonne were determined for cases (1) and (2), respectively. Then, a comprehensive evaluation of the trade-off between the life extension of the landfill site and global warming was conducted using the distance-to-target method. The indicator value was 89% lower for case (1) than for case (2) when the reduction target of final disposal from FY2011 to FY2030 was 67%. However, this result supposes that all of the slag produced was recycled. Assuming effective utilisation of slag (68%), the difference in the final volume between the two cases would drop to 45%. A sensitivity analysis using distance-to-target method was conducted to investigate the percentage of the final disposal reduction or greenhouse gas target that would be required to revive the results of cases (1) and (2). The results indicated effective utilisation of slag has advantageous comparing with case (2).Conclusions. The present study indicates that slag produced by the sewage sludge melting process is important to address the trade-off between life extension of landfill sites and global warming. Moreover, effective utilization of slag is a key factor in determining the effectiveness of slag recycling. Therefore, finding ways to increase the effective utilization should be made a priority.