Leveraging Full-Scale Continuous Flow Densification of Activated Sludge at Robert W. Hite Treatment Facility for Meeting Long Term Treatment and Capacity Needs

Introduction
Amidst progressively tightening effluent nutrient limits over the next 20 years (Figure 1), Metro Water Recovery (Metro) is exploring secondary treatment technologies at the Robert W. Hite Treatment Facility (RWHTF) to examine potential opportunities to leverage the existing infrastructure to increase capacity, provide flexibility to meet future effluent nutrient limits, improve reliability while minimizing footprint, reduce chemicals and energy, and reduce complexity of operation. The RWHTF, located in Denver, Colorado, is comprised of two independent liquid stream processes (North Treatment Complex and South Treatment Complex) where preliminary, primary, secondary, and disinfection treatment occurs. The North Secondary Complex (NSEC) is rated for 106 million gallons per day (MGD) while the South Secondary Complex (SSEC) is rated for 114 MGD, for a total treatment capacity of 220 MGD. The RWHTF currently achieves annual median effluent limits of total inorganic nitrogen (TIN) < 15 mg/L and total phosphorus (TP) < 1 mg/L. Secondary treatment at the RWHTF currently utilizes a combination of biological, chemical, and physical processes to remove solids, carbon, nitrogen, and phosphorus from primary effluent. Both NSEC and SSEC employ a Modified Ludzack-Ettinger (MLE) process for nitrogen removal with flexibility to operate in an enhanced biological phosphorus removal (EBPR) mode using a three-stage A2O (anaerobic, anoxic, oxic) configuration. The existing NSEC capacity is effectively limited by the secondary clarifiers, while the SSEC capacity is limited by aeration basin capacity. Both systems are currently designed with aggressive minimum aerobic SRTs (3 to 5 days), which presents challenges for meeting future effluent ammonia and TIN standards. Additionally, settleability issues limit the ability to reliably operate at MLSS values greater than 3,000 mg/L, particularly in the NSEC where the clarifiers are notably shallow (10-foot sidewater depth, compared to a 14-foot sidewater depth in the SSEC). Although limited, both NSEC and SSEC have available space for expansion; however, the facility is landlocked and quickly reaching build-out. For NSEC, there is space to build two additional aeration basins and two additional 130-ft clarifiers. Similarly, for SSEC, there is space to build two additional aeration basins and two additional 140-ft clarifiers. Yet, substantial capital investment would be required for such expansions: $80M for NSEC and $67M for SSEC. Moving into the future, technologies that allow Metro to cost effectively increase aerobic solids retention time while improving sludge settleability would significantly benefit secondary treatment capacity at the RWHTF without the need for additional aeration and clarification infrastructure and the associated capital expenditure. More specifically, Metro identified the need to increase aerobic solids retention (aSRT) to between 6 and 8 days to meet nitrogen removal targets at future flow and load conditions. Densified activated sludge (DAS) was identified as a promising technology that may support intensification and allow Metro to meet aSRT requirements while minimizing, if not eliminating, the need for additional aeration basins and secondary clarifiers. Metro commissioned a full-scale continuous flow DAS demonstration train in 2018 to: i) assess the feasibility of achieving densification, ii) inform treatment capacity rating and expected performance of full-scale DAS, and iii) gain insights into long term DAS operations. This work documents how findings from the full-scale demonstration are being leveraged to maximize treatment capacity of the existing infrastructure.
Methods
The full-scale DAS demonstration (denoted as 'test') facility consisted of an isolated, paired aeration basin (2.05 million gallons (MG)) and secondary clarifier (13,270 ft2) that was equipped with a dedicated primary effluent pumping station designed to mimic full-scale diurnal flow patterns. The test facility was equipped with a hydrocyclone skid comprised of eight 10 m3/h hydrocyclones that was used for physical selective wasting. The test facility has been in operation since May 2018 and been used to test eight (8) distinct metabolic and kinetic selector configurations. Metro has also operated 'control' trains that do not have similar metabolic, kinetic, and physical selection capabilities.
Sampling and Data Collection
- Over the course of the demonstration operation, field sampling/analysis was performed and operational data was collected to characterize performance and inform subsequent process modeling. Modeling and Conceptual Design - Data from testing were used to calibrate and validate whole plant process models and computational fluid dynamics (CFD) models of the RWHTF secondary clarifiers. These calibrated models were used to determine the capacity of the existing infrastructure under different settling and aSRT conditions as well as assess performance and risk mitigation strategies for addressing treatment capacity needs. A conceptual design of full-scale DAS was developed and will be discussed in the paper and presentation.
Results and Discussion
The test train had significantly improved settling characteristics versus the control train (Table 1). Results from stress testing confirmed that the test train could be loaded at surface overflow rates (SORs) > 700 gpd/sf and solids loading rates (SLRs) > 60 pounds per day per square foot (ppd/sf) and still maintain reliable blanket control and stable effluent TSS (Figure 2). The control train was observed to experience failure if SLR exceeded 35 ppd/sf. Results from the whole plant evaluation indicated the following: - The capacity of the existing infrastructure is projected to be exceeded between 2030 and 2032 under historical settling conditions (non-DAS condition). Up to 4 new treatment trains would be required to meet the projected 2040 flow and load conditions while supporting increasingly stringent nutrient limits. - Operating in the highly densified mode (i.e.,75th percentile SVI < 80 mL/g) reflective of test train performance is expected to provide the necessary capacity within the existing infrastructure through 2040 without the need for any additional aeration basins or secondary clarifiers. Under this condition, Metro can operate at an 8 day aSRT and maintain one aeration basin and secondary clarifier out of service. - Sensitivity analyses indicated for a 75th percentile SVI of 100 mL/g, the 2040 flow/load and nutrient removal requirements can be achieved by either operating at a 7 day aSRT with one basin and clarifier out of service OR operating a 8 day aSRT with all units in service. - Additional risk mitigation strategies available to Metro, in the event that full-scale DAS 75th percentile SVI exceeds 100 mL/g include increasing metabolic and kinetic selective pressure through modifications to reactor basins and implementing technologies to selectively increase aSRT. Cumulatively this work illustrates that the combination of metabolic, kinetic, and physical selection can be used to successfully achieve continuous flow densification. Additionally, by leveraging DAS, Metro will be able to offset secondary treatment expansion costs associated with up to 4 aeration basins and secondary clarifiers (estimated at $147M). Full details regarding the model calibration and utilization as well as the design basis for the full-scale facility will be provided in the paper and presentation.