Improved sludge dewaterability by tannic acid conditioning: Temperature, thermodynamics and mechanism studies

Waste activated sludge (WAS), containing biological pathogens, chemical elements and high moisture (>99%), was generated continually from municipal sewage treatment plants. Sludge dewatering could diminish the volume of WAS and control the diffusion of environmental pollution efficiently. In this study, tannic acid (TA), as a plant-derived phenolic compound, was investigated for improving WAS dewaterability at different temperatures. Apparently, the WAS dewaterability was enhanced by TA conditioning in the range of 25-55 °C, but further increase in temperature did not significantly affect the dewatering. With the TA addition of 0.15 mmol/gTS (total solid) at 55 °C, the WAS dewaterability was improved by 84.5% decrease in capillary suction time (CST), 96.5% decrease in specific resistance of filtration (SRF), and 19.9% decrease in water content (Wc) of dewatered sludge cake. TA facilitated removing supernatant viscosity and protein of sludge EPS (extracellular polymeric substances), specifically with 88.9% and 75.0% protein removal of slime EPS (S-EPS) and loosely bound EPS (LB-EPS). Thermodynamics modeling revealed that the improved dewaterability was dominantly attributed to the hydrophobic bonding between TA and EPS proteins, which was strengthened with the increase in temperature. However, when the conditioning temperature exceeded 55 °C, thermal effect took place and accelerated the release of biopolymers into EPS and hence, counteracted the beneficial effect of TA conditioning to further improve WAS dewatering. The results offered not only the dewatering effectiveness and mechanism of TA conditioning, but also a potential approach of applying plant waste to treat WAS for the high dewaterability.