Effect of heating treatment on nitrogen mineralization from sewage sludge.

Heating of sewage sludge has multifunctional benefits for agricultural recycling of sludge, one of which is the heat-induced changes in N mineralization from the sludge. To enhance the understanding and practical use of this phenomenon, we subjected sewage sludge to heating treatments at 120°C or 180°C for 16 h with or without air-drying as a pretreatment. During the 84-day aerobic incubation period, N mineralization from the sludge mixed with samples of an Andosol, a Fluvisol, an Arenosol and an Acrisol was significantly accelerated by the heating of air-dried sludge at 120°C and was significantly retarded by heating at 180°C, regardless of the soil types and temperatures during the incubation period. More conventional heat-drying of moist sludge at 120°C or 180°C also exerted similar but less pronounced effects. These heat-induced changes were attributed to the transformation of sludge organic N, because volatilization of N during the heating treatments was negligible. Sequential extraction of sludge N enabled detection of the heat-induced N transformations accounting for some but not all of the incubation results, indicating that mineralization of N in the heated sludge materials was determined not only by chemical extractability but also by other factors. Our results suggest that heating of sewage sludge regulates the rate of N mineralization and presents a promising method for producing various organic N fertilizers from sewage sludge.