Pretreatment and conditioning of municipal wastewater secondary sludge using freezing and a combination of ultrasound and freezing

Abstract

With the cost of sludge management on the rise, the volume reduction of municipal wastewater sludge is becoming an increasingly important issue for wastewater treatment plants. Current research is focused on pretreatment methods intended to increase the efficiency of anaerobic digestion. Unfortunately, many of the pretreatment methods studied show little to no improvements in dewaterability of the secondary sludge. This research was carried out to investigate the potential of freezing and combined ultrasound-­‐freezing methods for simultaneous sludge pretreatment and conditioning. Three methods of freezing were employed; conventional freeze-­‐thaw (FT), combined ultrasonic-­‐freezing (UF) and progressive ultrasonic freezing (PUF). The solubilisation of sludge organic matter, evaluated by measuring soluble chemical oxygen demand (sCOD), showed significant improvements for all freezing methods compared to the controls. The maximum increase in sCOD was 6.5 times the control for conventional freezing at -­‐30°C and 5 freeze-­‐thaw cycles, 5.3 times the control for combined ultrasonic freezing at 20% amplitude and 12 minutes of sonication and 7.7 times the control for the liquid portion of the progressive ultrasonic freezing samples with a three second sonication pulse for the duration of the freezing. The dewaterability of the freezing methods was also evaluated by measuring sludge volume index (SVI) and capillary suction time (CST). The three freezing methods showed significant improvements in dewaterability with CST ratios ranging from 0.12 ���� 0.21 for conventional freezing, 0.11-­‐ 0.21 for combined ultrasonic freezing and 0.15 ���� 0.26 for the solid portion of the progressive ultrasonic freezing samples. The freezing methods were compared to three commonly studied pretreatment methods (thermal, microwave and ultrasound) and showed equivalent or better abilities to solubilise sludge organic matter and improve dewaterability. Further tests revealed that the three freezing treatments also resulted in significantly higher concentrations of proteins as well as increased biodegradability and gas production. The gas production ratio over the control was greatest for conventional freezing (1.52), followed by combined ultrasonic freezing (1.17) and progressive ultrasonic freezing (1.13). The results suggest that freezing could be a very effective pretreatment method as it would be able to simultaneously improve both anaerobic digestion efficiency as well as dewaterability.