Evaluation of thermal hydrolysis efficiency of mechanically dewatered sewage sludge via rheological measurement

In this study, laboratory tests of both low temperature (60–90 °C) and high temperature (120–180 °C) thermal hydrolysis (LTHP and HTHP) were performed on mechanically dewatered high-solid sludges (at total solid of 14.2 wt% and 18.2 wt%) to evaluate the extent of organic solubilization through rheological measurements. The effects of treatment temperature and duration on organic solubilization and viscoelastic behavior of the sludge were comprehensively investigated. The results indicated that the organic solubilization contents including soluble chemical oxygen demand, soluble protein, and soluble polysaccharides increased logarithmically with the treatment time. Protein solubilized considerably faster than polysaccharides during thermal hydrolysis. The rheological curves exhibited the Payne effect in the amplitude sweep oscillation test. The elastic modulus in linear viscoelastic regime decreased logarithmically with treatment time. The viscoelastic behavior of sludge was well modeled by the Kaye–Bernstein–Kearsly–Zapas (KBKZ) model with paralleled Maxwell elements to describe the frequency dependence of elastic modulus and viscous modulus. With respect to the relaxation spectrum, the relaxation modulus first decreased with relaxation time and then increased. The relaxation modulus in each Maxwell element decreased with the treatment temperature and duration. Furthermore, in the HTHP, the influence of treatment temperature on enhancing organic solubilization and decreasing viscoelasticity exceeded the influence of treatment duration. In contrast, the treatment duration played a more important role than temperature in the LTHP. The content of organic matters was linearly related and logarithmically related to the elastic modulus in the LTHP and in the HTHP, respectively. The rheology analyses demonstrated that viscoelastic properties could be used as indicators to estimate the extent of organic matter solubilization in thermal hydrolysis process. The developed viscoelastic model provided insights for future research in numerically simulating the fluid dynamics of sludge