Ocean discharge of sewage sludge, which started in the year of 1993 in Korea, was stopped in the beginning of 2012, Landfill of sewage sludge was also prohibited in 2003 owing to public acceptance of the lanfill sites, and partially reopened in 2007 due to the necessity of organic component in sludge to produce methane gas. Sludge recycle ratio will increase in sewage sludge treatment by volume reduction and drying, and then recycle of the thermal energy as fuel in power plant and also material as raw material of cement. Several drying processes are compared with reasoning and an appropriate drying sequence is suggested.

This study has cross checked the change of internal sludge-recycle in Anaerobic-Digestion, and researched about not only the improvement of Bio-gas production from the digested sludge but also the efficient method of sludge minimization. Ultimate object of the study is to reduce the amount of sludge by the improved efficiency of contact with the organic-matter and the microbes in Anaerobic-Digestion. The sludge-recycle fluidized sludge layer and raised the activity of the sludge, the optimal sludge-recycle ratio, VS and COD removal ratio were 1,000%, 28.2% and 27.7%, respectively. Through these results of this study, it may be of use to treat waste sludge by the sludge-recycle ratio in terms of minimization and circulation of resources.