Controlled shear stress (CSS) test was used to study the effect of solid contents on the corresponding rheological parameters for sludge. Three types of sludge with or without conditioning, including activated sludge (AS), anaerobic digested sludge (ADS), and water treatment residuals (WTRs), were collected for the CSS test. Results showed that the yield stress and the cohesion energy of the sludge networks were improved with increased total suspending solid (TSS) contents in most cases. For the conditioned AS/ADS and the raw WTRs, exponential law was observed in the relationships between cohesion energy of material networks or yield stress and the TSS contents, whereas for the conditioned WTRs, only exponential law dependence was found between the parameters of shear modulus or critical strain and the TSS contents.
The changes in the physical characteristics of unconditioned and conditioned anaerobic digested sludge (ADS) biosolids, such as capillary suction time (CST), yield stress, average size and fractal dimensions, were investigated through a CST test, transient and dynamic rheological test and image analysis. The results showed that the optimum polymer dose range was observed when CST or its reciprocal value was employed as an indicator. There were good correlations between the yield stresses determined from both a controlled shear stress test and a strain amplitude sweep test. The yield stress and storage modulus (G ′ ) increased as the polymer dose increased in most cases. A frequency sweep test revealed that polymer conditioning could extend the frequency sweep ranges for their elastic behaviors over viscous behaviors as well as the gel-like structure in the linear viscoelastic range. These results implied that more deformation energy was stored in this rigid structure, and that elastic behavior became increasingly dominant with the addition of the polymer in most cases. In addition, both the average sizes and two-dimensional fractal dimensions for conditioned ADS biosolids presented a similar up-climax-down variation trend as the polymer doses increased, whereas the critical polymer doses at the highest average sizes or two-dimensional fractal dimensions, were different. Correlation analysis revealed that the conditioned ADS dewaterability was not correlated with the yield stresses, while the average sizes or the two-dimensional fractal dimensions for conditioned ADS biosolids could be taken as the indication parameters for ADS dewaterability.
