This study investigated effects of the thermal hydrolysis pre-treatment on mechanical deep dewaterability of dewatered sludge to extend understanding of dewatering characteristics of thermally hydrolyzed sludge. Floc sizes of dewatered sludge were gradually reduced during the thermal hydrolysis pre-treatment at 170 °C and 185 °C with increasing retention time whereas longer retention time (>60 min) increased floc sizes of thermally hydrolyzed sludges at 200 °C due to formation of undesired refractory organic materials (ROMs), which might hinder the disintegration of dewatered sludge flocs. Similar trends were found for thermal hydrolytic solubilization of dewatered sludge. This demonstrated that the efficiency of the thermal hydrolysis pre-treatment at a higher temperature (200 °C) with longer retention time (≥60 min) could be strongly influenced by the formation of ROMs associated with changes of solid fractions and some free amino acids (i.e., β-aminobutyric acid, 4-hydroxyproline, and cysteine). Since the trade-off between the degradation of dewatered sludge and the formation of ROMs determined mechanical deep dewaterability of thermally hydrolyzed sludge, the lowest residual weight and moisture content were observed for thermally hydrolyzed sludges at 200 °C with retention time range of 60 min (residual weight = 0.165; moisture content = 55.38%) to 90 min (residual weight = 0.160; moisture content = 59.87%). These observations were intimately correlated to variations of extracellular polymeric substances during the thermal hydrolysis pre-treatment, but not in accordance with the change pattern of capillary suction time (CST) values. This is evident that the CST value was inadequate to estimate mechanical deep dewaterability of thermally hydrolyzed sludge.
Keywords: Dewaterability; Dewatered sludge; Moisture content; Residual weight; Solubilization; Thermal hydrolysis.
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