Influences of different fractions of extracellular polymeric substances on the co-slurrying properties of sewage sludge with coal and petroleum coke

Coal-sludge-slurry (CSS) and petroleum coke-sludge-slurry (PCSS) can be used as liquid fuel or gasification material, thus providing a new approach for recycling sewage sludge without pre-drying. Given that slurrying properties directly influence the industrial application of CSS and PCSS, this study investigated the influences of different fractions of extracellular polymeric substance (EPS) on the slurrying viscosity (η100, average viscosity at a shear rate of 100 s−1), maximum solid concentration (SCmax), and rheological behaviors of CSS and PCSS. Sodium methylene bis-naphthalene sulfonate (NNO) and sodium lignosulfonate (SLS) were used as the dispersants during CSS and PCSS preparation. At the two dispersants conditions, when raw sludge was mixed with coal slurry at a sludge-to-coal ratio of 17:100, SCmax decreased from 52.40% (coal slurry without sludge) to 42.99% and 42.32%, respectively. Similarly, when raw sludge was mixed with petroleum coke slurry with NNO and SLS as the dispersants, SCmax decreased from 72.25% to 61.59% and 61.28%, respectively. The high amounts of trapped water in the sludge and the strong linkage between sludge flocs decreased the SCmax of the slurry. After loosely and tightly bound EPS (LB-EPS and TB-EPS, respectively) were removed, the SCmax of CSS and PCSS increased again because dead water, which increased the water content of the slurry system but did not act as a lubricant or buffer, was removed with the removal of EPS. LB-EPS had more influence on SCmax than TB-EPS because LB-EPS trapped more water than TB-EPS. Thus, more trapped water was removed with the removal of LB-EPS. Shear stress versus shear rate curves were fitted using the Herschel-Bulkley model. The rheological parameters of CSS and PCSS with a η100 of 1000 mPa s were obtained by linear interpolation method. The rheological behaviors of CSS negligibly changed after the removal of EPS. The yield stress and pseudoplasticity of PCSS, however, increased and decreased, respectively. The rheological behaviors of PCSS likely changed after the removal of EPS because sludge particles could no longer link easily to form a spatial structure. Moreover, hydrophobic agglomerations between petroleum coke particles increased in the PCSS system.