Individual and Combined Effects of Freeze-Thaw and Ferrate(VI) Oxidation for the Treatment and Dewatering of Wastewater Sludges

The study examined the individual and combined effects of potassium ferrate(VI) additions and freeze-thaw conditioning for the treatment and dewatering of sludge samples. The first part of the experiments, using primary sludge, compared potassium ferrate(VI) additions prior to freeze-thaw treatment (pretreatment) versus potassium ferrate(VI) additions following freeze-thaw treatment (posttreatment). A low dose (LD) of 1.0 g/L and a high dose (HD) of 10.0 g/L of potassium ferrate(VI) were evaluated along with a freezing temperature of −20 °C and freezing periods of 1, 8 and 15 days. Following the designated freezing period, the samples were removed from the freezer and thawed at room temperature for 12 h. The second part of the study, using anaerobically digested sludge, evaluated the effects of potassium ferrate(VI) pretreatment, using LD = 0.5 g/L and HD = 5.0 g/L, and used simulated drainage beds to separate meltwater from the sludge cake during the thawing period. The study demonstrated that stand-alone freeze-thaw can reduce faecal coliform by >3-log after being frozen for only 1 day, and pretreatment with potassium ferrate(VI) can be used to improve the effects of freeze-thaw on faecal coliform inactivation in sludge. Furthermore, the drainability of the sludge following freeze-thaw was not significantly deteriorated when potassium ferrate(VI) was added to the sludge prior to freezing, despite greater than fourfold increases in the concentrations of soluble proteins and soluble carbohydrates. The meltwater collected during the sludge thawing was approximately 85 % of the initial sludge volume. When 5 g/L of potassium ferrate(VI) was added to the sludge prior to freezing, the meltwater collected had <0.28 MPN/mL faecal coliform, the turbidity was <10 NTU and the pH was 9.1. Pretreatment with potassium ferrate(VI) also reduced the concentration of faecal coliform in the sludge cake, suggesting that freeze-thaw coupled with potassium ferrate(VI) additions can be used to stabilise sludge and reduce sludge volume.