Successful Full-Scale Continuous Flow Densification of Activated Sludge at Crooked Creek Water Reclamation Facility Without Physical Selection

Abstract: In this work, we demonstrate successful densification of activated sludge at the Crooked Creek Water Reclamation Facility, owned and operated by Gwinnett County Department of Water Resources, utilizing conventional bioreactors and secondary clarifiers. Densification was achieved through control of substrate utilization rates (kinetic selection) and use of anaerobic conditions (metabolic selection). Physical selection was not necessary to improve sludge settling characteristics. During densification, the CCWRF consistently achieved effluent NH3 and TP of less than 0.5 mg/L and 0.2 mg/L, respectively. Keywords: Intensification, Densified Activated Sludge, Granular Sludge
Introduction- Biological treatment for carbon and nutrient control is expected to remain a core process for water resource recovery facilities (WRRFs) of the future. Advancing biological technologies that allow for unlocking treatment capacity, while also minimizing capital and operating costs is therefore critical. Aerobic granular sludge (AGS) and densified activated sludge (DAS) are two approaches that seek to achieve intensification of BNR via improvements to sludge settling velocity and compaction properties, such that WRRFs can operate at elevated mixed liquor concentrations (> 5,000 mg/L) and solids loading rates (> 40 lb/ft2-day) to clarifiers. This work documents efforts to achieve full-scale densification at the Crooked Creek Water Reclamation Facility (CCWRF). Specifically, we will discuss, 1) Factors that have led to the development of DAS at CCWRF, 2) Data from start-up of new bioreactor basins and the process to achieve successful densification without physical selection and, 3) Improvement in water quality since transitioning from oxidation ditches to conventional biological reactors. The work documented in this paper will provide practitioners with insights into how to design and operate system to achieve continuous flow densification using conventional BNR technologies.
Background – The 16 mgd CCWRF was originally an Orbal Oxidation ditch that was converted in late 2014 into a plug flow A/O process with diffused air (Figure 1). After the conversion, the settling properties improved significantly with SVIs averaging 60 to 70 mL/g with 99th percentile SVI values of 80 to 85 mL/g year after year (Table 1). In 2018, we identified this plant as achieving DAS and investigated the plant performance to understand key drivers for this result. At the same time, design for new bioreactors using an A2O configuration to replace the old, converted oxidation ditches was being completed. Elements of the new design for the bioreactors were included to ensure that DAS was maintained. The new bioreactors were brought online (Figure 2) in March 2021 and successfully maintained DAS as discussed below. The A/O oxidation ditch plant was removed from service at the same time.
Methods - CCWRF is designed to treat 16 mgd using an A2O configuration. The facility does not practice anaerobic digestion and has achieved effluent NH3 and TP less than 0.5 mg/L and 0.2 mg/L, respectively. Field Sampling - Field sampling was performed at the facility to identify and characterize factors leading to densification of activated sludge. - Profile sampling was performed across the bioreactors to characterize nutrients and organic carbon, - Depth profile sampling in the selector zones was performed to characterize fate of phosphorus and carbon, - DAS size distribution analyses was performed to determine how particle size changes over time, - Sludge settling velocity and compaction rates were measured via settling column tests to develop quantitative metrics of densification for use in modeling, - Microscopic analyses were performed to qualitatively characterize filaments and distribution of phosphorus (PAOs) and glycogen accumulating organisms (GAOs) - DAS extracellular polymeric substance (EPS) content was characterized to determine changes over time. - Lastly, calibration of a secondary clarifier CFD model based on stress and column testing data was completed.
Data analytics- Historical data was examined using advanced data analytics techniques executed in Python. Daily operating data were interrogated to develop multi-day lookback averages (e.g., 3-day, 7-day, 15-day, and 30-day) to best elucidate long-term trends in performance. Supervised and un-supervised learning models were developed to illustrate relationships between environmental and operating parameters and sludge settling properties.
Results and Discussion – Prior to start-up of the new bioreactors (March 2021), the 50th and 99th percentile sludge volume indices at the plant were 62 and 84 mL/g respectively (2017 to March 2021). Since start-up of the new reactors the 50th and 99th percentile sludge volume indices have remained low at 60 and 80 mL/g respectively (2021; Figure 3). The current SVI 5/30 ratio of the system is 1.15. Between 30 and 50% of the biomass at CCWRF has a particle size greater than 212 um (Figure 4). Correspondingly, initial settling velocity at CCWRF was estimated to be ranging from 58 to 63 ft/hr. Floc settling parameter ranged from 0.20 to 0.35 L/g. These values indicate excellent floc characteristics, partial granulation, and excellent settling properties. Process sampling indicated that anaerobic F/M observed within the entire selector zone ranged from 0.5 to 1.0 lb sCOD/lb MLSS. The initial contact zone F/M was observed to range from 2.0 to 4.0 lb sCOD/lb MLSS. Cumulatively, these results indicate that it is possible to achieve stable densification in continuous flow applications through control of substrate utilization rates (kinetic selection) and use of anaerobic conditions (metabolic selection). Physical selection is not always necessary.
Capacity evaluations utilizing the calibrated clarifier CFD model and BioWin model have shown that it is possible to operate at MLSS concentrations up to 5,500 mg/L and peak solids loading rate in excess of 55 lb/ft2-day. Utilizing these parameters, the team has demonstrated the secondary process capacity of the facility can be increased beyond the design flow of 16 mgd without addition of new bioreactor basins. Achieving this additional treatment capacity using the existing infrastructure at the plant could facilitate significant savings if future expansion is required.