Ternary expression stage in biological sludge dewatering

The expression characteristics of original and freeze/thaw conditioned waste activated sludges were experimentally evaluated. Results in this study indicate that the first two stages of expression (primary and secondary consolidation) are similar to those of a particulate system, on which the model analysis by Shirato et al. (1974) is applicable. In the final phase of the expression of biological sludge, however, an unexpected constant-rate consolidation period, termed herein the ternary consolidation period, is observed. A modified combined Terzaghi-Voigt rheological model is proposed to describe the relation between void ratio and compressive pressure of the sludge cake. Whereas the resulting governing equation is solved analytically and the model parameters are found by regression analysis. The ternary consolidation is speculated to arise from the erosion of strongly bound moisture from the biological particles.