Naoko Nakagawa, Norio Sugiurab, Motoo Utsumic, Kazuya Shimizud, Tomonao Miyashiroe
Sewage sludge is treated by various technologies before disposal to avoid environmental risks that are caused by pathogens, hazardous chemicals and heavy metals. However, many of conventional treatment methods are costly, using significant levels of energy. To solve this problem, a subcritical water treatment (SCWT) technology was developed. The characteristics of the SCWT technology is that it can process any kinds of wastes with a wide range of water content by a liquid mixed reaction using steam at a high temperature and pressure. In this study, demonstration experiments with sewage sludge and livestock manure were undertaken using the SCWT reactor by varying temperature, pressure, and time. As for the results, the appropriate conditions regarding treating sewage sludge were found to be at a temperature of 100 °C - 200 °C and a pressure of 1.0 MPa - 2.0 MPa. In addition, the content analysis of the treated samples gave the excellent results relevant for a fertilizer. For example, the content of the treated sewage sludge were: water content 60.97 %, pH 4.16, organic content 87.33 %, T-N 0.69 %, P2O5 0.705 %, K 3,150 mg/kg , Ca 70,500 mg/kg , Mg 3,090 mg/kg , and Na 1,150 mg/kg . These results show good elements for a general organic fertilizer, and it was verified to be pathogen free, as well as hazardous chemicals with heavy metals low enough to satisfy standards for continuous applications on agricultural lands.
fertilizer; organic wastes; recycling system; subcritical water