Sewage Sludge Melting Process: Preliminary System Design and Full-Scale Plant Study

Abstract

A sewage sludge melting process has been developed and some full-scale planls have been installed or are now under construction in Japan. Sludge melting process has its main advantages in that most sorts of hazardous materials, such as heavy metals, are tightly fixed in solid phase and the slag produced by this process can be used as construction materials. This article analyzes sludge treatment and disposal costs of the popular sludge treatment alternatives with emphasis on thermal processes, especially the sludge melting process. From preliminary system designs on a common design basis, relative energy requirements and total treatment & disposal costs were compared. As a result of cost analysis in terms of the annual treatment cost, it was revealed that the lime conditioning systems cost some 50% more than the polymer conditioning, that in a comparison between the incineration and the melting systems the treatment cost differed when the difference in energy cost was significant, and that in the anaerobic digestion systems the superiority in energy balance did not contribute to treatment cost reduction. Even if as one of the cost conditions to introduce the melting system in place of the incineration system, the ash disposal price is set as low as 5,000 yen/t, the coke melting system is still advantageous in the lime conditioning. Further, it turned out that in the polymer conditioning systems it was possible to establish cost conditions to introduce the melting system when the incinerated ash disposal price, the melting process coke ratio and the coke price took certain values.