Effect of Time and Mixing in Thermal Pretreatment on Faecal Indicator Bacteria Inactivation

Faecal indicator bacteria of faecal coliform, Salmonella spp., and faecal Streptococcus are present at high levels in faecal sludge and affect human health. Mesophilic anaerobic digestion cannot reduce faecal indicator bacteria to meet the standards for biosolids; therefore, the thermal pretreatment of faecal sludge is essential. The main objectives of this research were to evaluate the effect of thermal (70 °C) pretreatment time (20, 40, 60, 80, 100, and 120 min) and mixing velocity gradient (no mixing, 133, 191, and 238 s(−1)) on faecal indicator bacteria inactivation and determine the kinetics of the inactivation. The results showed that mixing has a more positive effect on pretreatment; thermal pretreatment with mixing was able to completely inactivate faecal indicator bacteria within 80 min, and inactivation followed first-order kinetics. In addition, under optimal mixing at a velocity gradient 191 s(−1), the thermal pretreatment with mixing had a positive effect on the sludge solubilisation. Soluble chemical oxygen demand (SCOD, 71,430 mg L(−1)) and soluble protein (7.96 g L(−1)) were higher than the values obtained with thermal pretreatment without mixing, which were a SCOD value of 63,600 mg L(−1) and soluble protein of 6.78 g L(−1).