Leveraging Glycerol-Driven and Primary-Effluent-Driven PdNA within Advanced Wastewater Treatment Facility

Introduction The Los Angeles County Sanitation Districts (LACSD) and Metropolitan Water District of Southern California (MWD) have collaborated to evaluate the development of the Pure Water Southern California (PWSC) Program. The PWSC Program would implement a full-scale advanced water treatment facility (AWTF) to produce up to 150 million gallons per day (mgd) for indirect and potentially direct potable reuse. One of the AWTF options considered would include a new biological nutrient removal (BNR) facility followed by RO/UV/AOP. The new BNR facility would be constructed at the existing A.K. Warren Water Resource Facility (Warren Facility), which is a 400 mgd capacity high purity oxygen activated sludge facility. The AWTF and BNR upgrades are intended to achieve a nitrate <19 mg/L, prior to the RO/UV/AOP facility. Partial nitrification/denitrification/anammox (PANDA/PdNA) was identified by the PWSC team as a promising approach to intensify BNR, by facilitating reduction of supplemental carbon needs and potentially increasing TIN removal capacity, without increasing tank volume. This work summarizes findings from an ongoing multi-year pilot effort focused on validating the PANDA/PdNA concept within a large-scale reuse program. Specifically, the objectives were to: i) demonstrate the ability to achieve nutrient targets reliably by intensifying a tertiary MBBR and IFAS configuration with PANDA/PdNA; ii) validate carbon savings that can be achieved by implementing PANDA/PdNA; and iii) investigate the feasibility of incorporating primary effluent driven partial denitrification to further reduce projected operating costs associated with the PWSC Program. Methods Tertiary MBBR: A tertiary MBBR was configured as a step-feed BNR process, consisting of alternated aerobic and anoxic cells, with zone configuration and influent feed split (Figure 1). Both primary effluent (PE) and a glycerol-based supplemental carbon source (MicroC 2000) were used for supporting denitrification in anoxic zones. This flexible configuration allowed for performing both conventional BNR and PANDA/PdNA. Tertiary IFAS: The MBBR system was modified to include clarifiers to retain flocculant biomass. Additionally, a Solitax probe (Hach) was added in the last cell of the system to monitor the mixed liquor suspended solid concentration in the system. The first aerobic zone of Tertiary MBBR was converted to anoxic because the RAS feed was able to recycle sufficient NOx load into the that zone, negating the need for nitrification in that zone. Monitoring: Composite samples (24-hr) were used to monitor influent and effluent solids, organics and nutrients. Profile samples were grabbed from different cells across the system periodically to analyze for nitrogen species and organics (Hach TNT). Operational Modes: Three different operational modes were tested: 1) Hybrid conventional BNR mode in MBBR, during which both PE and glycerol were supplied to the system to perform full denitrification; 2) PANDA/PdNA mode in MBBR, during which both PE and glycerol were supplied to the system to perform partial denitrification and anoxic ammonia removal via anammox; and 3) PANDA/PdNA mode in IFAS (on-going), during which suspended biomass is retained in the system by using a clarifier. Results and Discussion Hybrid BNR Mode: The system operated in this mode from July 2022 to April 2023. Findings were reported in Sun et al., 2023. Briefly, both the ammonia target of 1 mg/L and the NOx target of 16 mg/L were achieved during hybrid BNR mode operation, confirming that a conventional BNR option could reliably achieve nutrient targets. MBBR PANDA Mode: The MBBR system was transitioned to PANDA/PdNA mode in May 2023 and operated until October 2023. Key findings are as follows:

*The system was able to reliably achieve the effluent ammonia and NOx targets of 1 mg/L and 16 mg/L, respectively (Figure 2). It was also possible to further decrease effluent TIN as low as ~ 9 mg/L, without increasing total system volume (Figure 2).

*Carbon usage was tracked during different phases of operation (Table 1). During the hybrid BNR and PANDA phases, approximately 7 to 9% of the total flow was comprised of PE. The COD addition/N removal ratio reduced from ~3.9 lb sCOD/lb NOx during hybrid BNR phase to ~1.2 lb sCOD/lb NOx during the PANDA/PdNA phase. This reduction was due to enhanced anammox activity and represents greater than 70% savings in supplemental carbon needs.

*Investigation into PdNA activity within the system revealed that partial denitrification was likely driven by both PE (first anoxic) and glycerol (second anoxic), with PE driven PdNA accounting for 3 to 7 mgN/L anoxic ammonia removal (Figure 3) and glycerol driven PdNA accounting for up to 9.4 mgN/L anoxic ammonia removal (Table 1). Batch tests confirmed that PE and internally stored carbon could be used to successfully drive partial denitrification if sufficient control of carbon, nitrate and ammonia is provided (Figure 4). PANDA/PdNA in IFAS Mode: In November 2023, the system was transitioned to operate in IFAS PANDA/PdNA mode. The IFAS phase of operation will be completed in mid-2024, the results of which and lessons learned will be presented during the conference.