Pilot-Scale Anaerobic Digestion and Dewatering of Aerobic Granular Sludge

Charlotte Water (CLTWater) has 123 MGD of capacity between five major wastewater treatment plants (WWTPs) and is interested in sustainable and cost-effective technologies that may be selected for future WWTP expansions or for replacement of existing treatment infrastructure. Aerobic granular sludge (AGS) is an innovative secondary wastewater treatment technology that can achieve biological nutrient removal (BNR). Aqua-Aerobic Systems, Inc (AASI) is the vendor for AquaNereda, the patented AGS system that utilizes specific biological and hydraulic selection pressures to produce and maintain the AGS in a sequencing batch reactor (SBR). CLTWater elected to pilot AquaNereda to understand how it might be used to expand the capacity of our overall system, either at an existing plant or in place of another BNR technology at our new plant under design, the Stowe Regional Water Resource Recovery Facility (SRWRRF). An AGS pilot was performed at McDowell Creek WWTP from June 2019 to February 2020 to determine its viability as a treatment for CLTWater. Before starting the AquaNereda pilot, CLTWater developed a list of objectives to achieve during the piloting process with actionable and achievable goals defined for each objective. These objectives included 1. Evaluating simulated performance representing treatment with and without primary clarifiers 2. Observing development of granules from flocculant sludge 3. Evaluating solids characteristics 4. Evaluating process control and operability 5.Involving State regulatory agencies to facilitate potential future permitting processes To address the objective of evaluating solids characteristics, a separate solids study was conducted at Bucknell University to provide insight on solids handling and treatment impacts for potential full-scale Nereda implantation. CLTWater wanted to assess impacts to digestion performance, dewatering, and recycle streams when aerobic granular sludge (AGS) and conventional activated sludge (CAS) are treated concurrently. During the study, Bucknell operated four lab-scale digesters that were fed several solids blends (Table 1) including two 100% thickened waste activated sludge (WAS) digesters to demonstrate differences between AGS and CAS WAS and two 60% primary sludge (PS)/40% WAS digesters to simulate McDowell's current digestion process and the expected maximum contribution of AGS to any digestion process at CLTWater. Waste solids from the Nereda pilot reactor treating primary effluent were thickened to ‰¥5% using gravity belt thickening and shipped to Bucknell with thickened PS and CAS-WAS from McDowell's full scale processes. Bench-scale 20-day HRT mesophilic anaerobic digesters were fed daily. Biogas production was monitored throughout the trial, but other analyses were performed after 71 days (~3.5 SRTs) to ensure results were representative of steady-state conditions. Results include quantified metrics and other observations related to 1) digester performance VS destruction, biogas production; 2) solids handling sludge viscosity, polymer demand, cake solids, cake metals content and odor production; and 3) recycle streams digestate nutrient concentrations. For digester performance, results showed that volatile solids reduction (VSR) for AGS-WAS was comparable to CAS-WAS with slightly different methane yields between Digesters 3 and 4 (60% PS/40% WAS). Minor differences in solids handling parameters were observed including marginally higher viscosity and polymer demand for digested solids that included AGS, but these differences were more pronounced in the digestate from Digesters 1 and 2 (100% WAS). Recycle stream soluble TKN concentrations were similar regardless of WAS source; however, recycle streams from reactors including AGS-WAS were characterized by markedly lower soluble TP concentration. The finding of much lower soluble TP concentration for AGS-WAS was surprising given that the Nereda process includes enhanced biological phosphorus removal (EBPR) and the pilot reactor performed comparable EBPR as the full-scale mainstream conventional BNR process. Unfortunately, the pilot timeline did not allow for additional investigation into the cause of this observation, but further research may be warranted in this area. This presentation will discuss high-level results from each of the objectives targeted during the AquaNereda pilot but will focus mainly on the solids-related findings. We will present overall conclusions from the pilot as well as lessons learned related to implementing a pilot for a process that has broad reaching implications across a wastewater treatment plant. Finally, the future of AGS implementation at CLTWater will be discussed.