In this research, we present a new methodology to indirectly assess the concentration levels of the biochemical oxygen demand (BOD) and total nitrogen (T-N) of dewatered cake by evaluating the amounts of the volatile solids (VS) and total solids (TS). Information regarding the BOD and T-N concentrations of dewatered cake could then be used to estimate the Greenhouse Gas (GHG) emissions caused by sewage treatment. To this end, regression equations were derived by considering the relationship between the product of two solid terms: (TS × VS) and BOD/T-N levels of digested sludge. The optimal regression equations for BOD and T-N were computed as y = 152425x (R2= 0.969, p < 0.05) and y = 128378x (R2= 0.970, p < 0.05), respectively. For the purposes of verification, the applicability of the regression equations was tested using the data for other periods not considered in the regression analysis. Accordingly, the differences between the measured and estimated concentration data (derived using the regression equations) were within the standard deviation of the measured concentrations. However, the concentrations estimated by regression equations were quite different from those obtained by conventional methods. Nonetheless, such differences did not significantly change GHG emissions, thus we conclude that the plant specific regression equations can be derived from the methods presented in this study, although more efforts are needed for its validation in various respects.