Effect of Thermal Hydrolysis Pretreatment on the Friability of Thermally-Dried Digested Biosolid Pellets

Louisville MSD plans to implement a new biosolids processing system consisting of a thermal hydrolysis pretreatment (THP)-enhanced anaerobic digestion (AD) process and rotary drum dryers at its 120-MGD Morris Forman Water Quality Treatment Center (MFWQTC), the largest wastewater treatment facility in Kentucky, USA. The new biosolids processing system will receive thickened primary sludge (TPS) and thickened waste activated sludge (TWAS) from the MFWQTC, as well as imported wasted activated sludge (IWAS) cake from other regional treatment facilities. Friability refers to the tendency of a material to fracture and breaking into smaller fragments under external pressure. In the case of biosolids, such external pressure could result from agitations during rotary drum drying, conveying during processes, and transportation, which ultimately leads to dryer inefficiency, dust generation, and poor product quality for land application. It has been reported that some full-scale installations experienced challenges with drying THP-AD sludge due to the high friability and dust issues within the dryer (Spalding & Smoot, 2019). However, the friability of dried biosolids and the causes have never been quantitatively measured and investigated. Therefore, this study aims to experimentally evaluate the impact of THP on the sludge drying property (in terms of friability) following anaerobic digestion of TPS, TWAS, IWAS, and blended sludge. To understand the THP effect, the experiment was executed in two parallel batches with Batch 1 as a control with AD only and Batch 2 as an experiment by adding a THP upstream of the AD (Figure 1). Samples of TPS and TWAS collected from MFWQTC were pre-dewatered using a pilot-scale screw press thickening unit and then blended with samples of dewatered IWAS collected from another Louisville MSD-operated WQTC at a dry mass ratio of TPS, TWAS, and IWAS was 2.4 : 2.2 : 1. The pre-dewatered sludge were diluted to 16% total solids (TS) followed by processing through a pilot-scale Cambi THP unit operated at 165 SMƒ for 30 minutes. The pretreated sludge was further diluted to 10% TS and then inoculated with AD sludge collected from DC Water Blue Plains WWTP followed by anaerobic incubation at 36.5 SMƒ until the cumulative biogas production reached a plateau. The digested sludge dewatering method was modified from the lab-scale dewatering protocol developed by Bucknell University detailed in a previous study (Higgins et al., 2014). To the best of our knowledge, there is no established protocol to evaluate the friability of dried biosolids. Thus, a lab-scale vortex mixer-based friability test method was developed by VT-CAWRI for this study. The dewatered sludge samples were uniformly pelletized using a 1-cm semi-sphere silicone mold. The wet pellets were then dried using a moisture balance (Figure 2) at 105 SMƒ to dryness of 90%, 95% (for blended groups only), and 100%, respectively. Four dried biosolids pellets were accurately weighed and placed in a 50 mL centrifuge tube. The tube containing the pellets was then constantly agitated by a digital vortex mixer at 3000 RPM for 10 minutes (Figure 3). After the test, any particles retained on a 1-millimeter (mm) stainless steel mesh sieve screen, i.e., particles with diameters greater than 1 mm, were weighed again, and the friability of the pellets was quantified as the percent weight loss through the sieve as defined in Eq. 1. In summary, three variables were investigated in this study: sludge types (i.e., TPS, TWAS, IWAS, and blended sludge), with and without THP before AD, and pellet drying dryness (90%, 95%, and 100%). The results in Figure 4 showed that the pellets produced from THP-AD sludge generally exhibit higher friability than those without THP. It was observed that the friability of digested TPS pellets was the lowest and remained essentially unchanged after applying THP. However, WAS-containing pellets tended to have the higher friability regardless of the use of THP. Moreover, pellet friability always increased as the pellet dryness increased from 90% to 100% for each sludge type (Figure 4). Statistical analysis showed that such increases in friability were significant (Student's t-test, p < 0.05), indicating over-drying of biosolids could lead to higher friability. Results in Figures 5a and 5b showed that friability tended to monotonously decrease with cake dryness and increase with CST. This observation suggests that the pellet friability is inversely related to the sludge dewaterability. It is our hypothesis that biosolids with poor dewaterability tended to trap more water and thus formed more porous structures in heat-dried pellets after water evaporated. It was this porous structure that led to a higher tendency to crumble under mechanical agitation. In summary, over-drying led to increased friability of biosolids pellets, and both types of WAS pellets became more prone to exhibiting friability at higher levels of dryness when THP was introduced, thereby potentially compromising the efficiency of dryers due to continuous recycling of undersized pellets or dust. In full-scale operation, our data suggest that these two obstacles may be addressed by drying the biosolids to lower dryness levels (~90-92%) and blending WAS with more PS prior to THP (if applicable) or AD.