Method Development for the Quantification of Free, Interstitial, Surface, and Bound Water Content in Granular Sludge as a Predictor for Dewaterability

Abstract

The purpose of this study was to develop a method to understand the relationship between granular sludge and its distribution of water with respect to dewatering potential. Thermogravimetric analysis (TGA) was optimized to quantify free, interstitial, surface, and bound water. TGA tests were performed for both granular sludge sampled from pilot reactors (70L) and activated sludge fed with the same primary effluent wastewater. The impact of temperature, sample preparation, and sample type on the measured results were evaluated. Activated Sludge contained more free and interstitial water when compared to granular sludge regardless of preparation method; however, granular sludge contained higher amounts of surface and bound water. Additionally, mechanical dewatering of granular sludge had a higher impact on the decrease of interstitial water, when compared with activated sludge. This method was also used to compared granular and flocculent sludge sampled from the same pilot reactor, and the results mirrored those of granular sludge and activated sludge, by revealing higher percentages of free and interstitial water for flocculent sludge, and higher surface and bound water in granular sludge. Filterability was measured in terms of capillary suction time (CST) tests and then correlated with water distribution data obtained from TGA tests. Free and interstitial water within granular sludge showed a deterioration in the filterability. Moreover, granular sludge had improved filterability when compared with activated sludge; mostly due to the enhanced liquid solid separation established by tightly bound EPS (TB-EPS). Polymer dose testing was additionally performed to understand the difference between polymer demands for aerobic granular sludge and activated sludge. Granular sludge exhibited substantially less of a demand for polymer, while also producing a higher % cake after mechanical dewatering. Samples before and after conditioning were tested using the TGA method established in this study. It was determined that although water was removed after conditioning, which produced a higher % cake, activated sludge exhibited an increase in surface and bound water after polymer addition. This may result from increased surface area associated with sludge flocculation and excess chemical binding induced by the cationic polymer. Granular sludge required approximately ten times less polymer, and did not yield an observable increase with surface water. However, an increase in bound water was also established for granular sludge after polymer addition. Phase two of this study provided a fundamental understanding regarding some of the physico-chemical properties that affect the dewatering process of aerobic granular sludge. The findings indicated that higher tightly bound extracellular polymeric substances (TB-EPS) enhanced the overall sludge-water separation process. However, loosely bound EPS (LB-EPS) had a negative impact on filterability due to an increase of free and interstitial water after TGA evaluation. Particle sludge size provided another factor influencing this process, with increases in surface water associated with increased particle diameter. Nevertheless, an improvement in filterability was correlated with increasing particle diameter. Overall, the purposed TGA method used to quantify the composition of different water fractions within granular sludge gives researchers and utilities the ability to take a closer look at the complex relationship between water and sludge; which can provide additional insight regarding the selection of appropriate sludge handling methods for dewatering granular sludge.