A Journey of Upgrades and Innovations to Achieve Capacity Improvements at Metro Water Services' Central WRF

Introduction The Central WRF has undergone significant upgrades to improve its treatment efficiency and capacity. With a rated capacity of 125 MGD and peak secondary flow of 350 MGD, the facility's treatment process includes preliminary treatment, PCs, aeration tanks (ATs), SCs, and UV disinfection. A key upgrade was the conversion of the aeration tanks from draft tube aerators to fine bubble diffused aeration and from two-pass, complete-mix reactors, to a plug-flow A/O process. This upgrade came with several unique challenges due to the sidewater depth of the tanks exceeding 30 ft. The draft tube aerators were inefficient at mixing resulting in pockets of low DO or anoxic conditions and thus reduced nitrification efficiency and promoted low DO filamentous bulking and foaming. The deep tanks were also followed by SCs with a sidewater depth of 16-ft. These conditions would result in nitrogen gas dissolving into the process water in the deep ATs that would evolve in the SCs causing poor settling and floating sludge blankets. The design criteria for diffused aeration at such depths were also not available from manufacturers or in the limited literature. The deepest diffuser submergence that manufactures could factory test their diffusers at was 23 ft. Therefore, assumptions of SOTE had to be made during design that accounted for potentially lower SOTE values than expected due to the air-side oxygen depletion during bubble transit. The assumed values were validated after startup and commissioning by performing off-gas testing. Methods Relevant tank improvements in each of the 8 ATs included anaerobic selector zones, baffle walls, a new aeration system, surface wasting stations, and degassing zones. The new aeration system included blowers, control valves and airflow meters, instrumentation, and fine bubble diffusers. The efficiency of fine-pore diffusers was calculated in clean water according to the ASCE (2006) standard and in wastewater via off-gas testing (ASCE, 2018). Plug-flow was achieved by installing RFP baffle walls in a serpentine manner and divided into 5 aeration zones as shown in Fig. 1. Since the PC effluent and RAS are added separately to the anaerobic zones, an influent chimney was used to direct the influent to the bottom of the anaerobic zone where it would come into close contact with the incoming RAS flow as shown in Fig. 2. To degas the mixed liquor before the SCs, a novel solution was implemented that included mid-depth coarse bubble aeration at the end of the aerobic zones as shown in Fig. 2. This system is critical at peak flows because the degassing that occurs in the aerated effluent channels between the ATs and SCs is significantly reduced at such short retention times. To help control and reduce nuisance foaming, surface wasting stations were installed in each AT as shown in Fig. 3. As the mixed liquor flows past the wasting station, a surface baffle directs any foam to the wasting station and is wasted from the system. Results and Conclusions The anaerobic selector zone has been instrumental in improving sludge settleability. Selector zones create an environment with a high F/M to out-select filamentous organisms, favoring floc-formers with high substrate uptake rates. The first two upgraded ATs were put in service in Jun 2022, resulting in an immediate drop in SVI as demonstrated in Fig. 4. The SVI continued to stabilize as additional pairs of upgraded ATs were put in service in Sep and Nov 2022. SVI averaged 235 +/- 92 mL/g for Oct 2021 — May 2022, and subsequently averaged 91 +/- 41 mL/g for June 2022 — Dec 2022. The SVI has also started to track with changes in F/M: when F/M is greater than 1, SVI averages 69 +/- 18 mL/g. This suggests that selector zone operation is critical for SVI stability. The new surface wasting stations also contributed to improved settleability by removing nuisance organisms accumulated in surface scum that cause foaming and bulking. Rapid reductions in the scum layer were observed after the first upgraded ATs were put in service. A comparison of the surface scum of the SCs before and after upgrades are provided in Fig. 5. As shown in Fig. 6, the average effluent ammonia concentration was approximately 2.5 mg /L (Jan 2021 — May 2022) and after the initiation of the upgrades fell to consistently below the detection limit of 0.4 mg/L (>95% of samples). As shown in the plot, the effluent ammonia has not peaked above 2 mg/L even though the plant operates at an aggressive SRT of 6-8 days year-round. Off gas testing of three ATs was carried out to assess the change in performance with different diffuser age (2, 6, and 10 months of operation). The off-gas testing results are compared to the design assumptions and clean water testing in Table 1. The assumed alpha and fouling factors used to correct the clean water performance for field conditions aligned well with the values measured in the AT with 2-month-old diffusers. The fouling factors decreased significantly for the 6- and 10-month-old diffusers. This information supported the recommendation that the diffusers should be inspected and cleaned at least annually to maintain adequate efficiency. In conclusion, the upgrades at the Central WRF, including the replacement of the draft tube aerators with a fine bubble aeration system, have significantly improved plant performance. The full paper and presentation will contain expanded discussions and all technical detail