Towards Unifying Densification and Low TN / TP Operation at the South Durham Water Reclamation Facility

Introduction
Densified activated sludge (DAS) is a process intensification technology that can allow water resource recovery facilities (WRRFs) to utilize a combination of biological (metabolic and kinetic) and physical selection to improve the settleability of the sludge. Intensification via DAS can allow WRRFs to operate at elevated MLSS concentrations (>5,000 mg/L) and solids loading rates (>40 lb/ft2-day) to clarifiers. Beyond capacity benefits, there is great interest in leveraging DAS to reliably achieve low TN/TP standards (e.g., TN < 5 mg/L and TP < 0.2 mg/L) while minimizing aeration and supplemental carbon demand.

This paper will provide insights from a successful multi-year effort to combine DAS with low DO operation at the 20 mgd South Durham Water Reclamation Facility (SDWRF). The focus of the optimization has been to enhance post-anoxic denitrification WITHOUT supplemental carbon addition to achieve TN <5 mg/L, while maintaining DAS properties.

Facility Description
The SDWRF is an advanced biological nutrient removal (BNR) treatment facility, owned and operated by the City of Durham, NC. The SDWRF consists of influent screenings and grit removal, primary clarification, 5-stage BNR, secondary clarification, tertiary filtration, and UV disinfection (Figure 1).

Kinetic and metabolic selection facilitated DAS at the SDWRF
Settling properties (SVIs, settling velocity and compaction coefficients) are summarized in Table 1. Median SVIs have remained below 90 L/mg and the facility has largely been able to minimize settling property variability, as illustrated with 95/50th SVI ratios consistently below 1.25. These results indicate that SDWRF has successfully achieved DAS with biological selection only; physical selection was not necessary. Achieving and maintaining DAS at this facility is critical as it allows the plant to minimize the number of aeration basins and clarifiers in service, reliably treat peak flows (PF of 5) while maintaining solids inventory, reducing operating costs and deferring the need for costly repairs of aging infrastructure.

50% reduction in effluent TN was facilitated by stimulating endogenous denitrification through low DO/post-anoxic zone operation
Effluent nutrient, operating SRT and primary effluent BOD/TKN ratios are summarized in Table 2. Effluent TN has decreased from 6 mg/L in 2019 to less than 4 mg/L in 2024. This decrease in effluent TN was accomplished WITHOUT supplemental carbon addition to the post-anoxic zones. To accomplish this, SDWRF staff reconfigured bioreactors to: i) reduce the pre-anoxic volume, ii) extend post-anoxic volume by operating lower DO through aerobic zones (< 0.5 mg/L), and iii) increase MLSS concentration/total SRT in the basins (Table 2).

Nutrient profiling data (Figure 2) confirms that basin and operational modifications reduced nitrate concentrations throughout the aerobic and post-anoxic zones. This reduction is likely due to increased endogenous denitrification that occurs because of the lengthened post-anoxic operation/low DO operation.

Ex-situ denitrification batch testing indicated that specific denitrification rates observed at SDWRF have changed since process configuration changes. Prior to 2022, batch testing indicated that SDWRF possessed a distinct fast and slow kinetic profile that that has been previously seen in systems hypothesized to have internal carbon driven denitrification. Testing in 2024 indicated that SDWRF continues to show a fast/slow kinetic profile; however, the 'fast' endogenous denitrification rate is significantly lower than that observed in 2021 (Table 3).

Given the magnitude of TN reduction observed since 2022, it is hypothesized that the process changes at SDWRF are allowing the biomass to effectively consume the fraction of biodegradable carbon that could previously stimulate 'fast' endogenous denitrification. This reduction of the 'fast' carbon bank results in lower nitrate in-situ but effectively reduces the specific denitrification rate observed during ex-situ testing.

Further investigation in the source of the electron donors for denitrification observed at SDWRF is underway to more fully understand endogenous denitrification trends.

<B>Optimization of TN reduction must be balanced with impacts to settling properties
While SDWRF has maintained settling qualities typical of DAS, the initial settling velocity (Vo) has decreased since 2022, when changes were made to improve TN performance. EPS characterization has suggested an increase in the loosely bound fraction since operational changes have been made. This increase in EPS fraction appears to correlate with the increase in SVI (Figure 3).

Cumulatively, these results suggest that it is possible to leverage low DO and extended post-anoxic zone operation to reduce TN while maintaining low variability in sludge settling characteristics. However, there may be some tradeoff on initial settling velocity properties. Further work is underway to determine how to best leverage kinetic selective pressure to increase settling velocity to prior levels while maintaining enhanced endogenous denitrification observed recently.

We envision that the lessons from this work will provide insights on how to design and operate WRRFs to consistently achieving low SVIs while also achieving low effluent TN and TP limits with no supplemental carbon addition.