Unraveling the many benefits of DAS — Insights into nitrifier enrichment through SRT decoupling of flocs versus granules

Introduction:
Metro Water Recovery's (Metro) Robert W. Hite Treatment Facility (Hite) in Denver, CO serves 2.2 million people across an 805-square-mile service area. Hite is currently permitted for a maximum month (MM) flow of 220 mgd and has two parallel liquids treatment trains including the North Secondary (NSEC). The NSEC utilizes a modified Ludzack Ettinger (MLE) process with a sidestream anaerobic reactor (SAR) to achieve nutrient removal.

Metro's facility planning efforts identified a need to increase aerobic solids retention time (aSRT) to meet future potential total nitrogen (TN) and total phosphorus (TP) regulations (Colorado Regulation 31). Densified activated sludge (DAS) was identified as a means for improving sludge settleability to allow Metro to operate at aSRTs necessary for future potential nutrient removal targets. Metro has successfully demonstrated the ability to achieve DAS and operate at elevated aSRT and solids loading rates (Maltos et al., 2023).

Beyond settling benefits, DAS systems with a physical selector can provide the ability to decouple the SRT of floc versus granule fractions. It is hypothesized that aSRT decoupling may allow for operation at lower bulk aSRT while still allowing Metro to meet nutrient targets. This paper/presentation will provide insights from NAB2 investigations into SRT uncoupling effects. Specifically, testing was performed to inform the following questions:
i) What is the minimum bulk aSRT and associated granule fraction needed to sustain nitrification?
ii) At what bulk aSRT will preferential nitrifier retention on granules be observed and will there be a nitrifier enrichment factor that can be sustained?

Materials and Methods:
DAS facility: The full-scale DAS demonstration facility (denoted as NAB2) consisted of an isolated, paired aeration basin (7,700 m3 (2.05 MG) and secondary clarifier (1,200 m2 (13,270 ft2)) from the North Secondary (NSEC). Description of the facility is provided elsewhere (Avila et al., 2021). The DAS demonstration facility was equipped with eight (8) hydrocyclones (44 GPM each; HC) which were fed from the return activated sludge (RAS) line. The control reactor was operated in SAR-MLE mode without physical selection.

aSRT Operation: Starting in Q3 and continuing through the end of Q4 2024, Metro began to progressively decrease the bulk aSRT from ~5 day to a target of ~ 3 day. Effluent quality, settling properties and particle size distribution of MLSS, hydrocyclone UF, and hydrocyclone OF were monitored as described in Maltos et al., 2023. Activity testing of bulk mixed liquor (MLSS) and size fractioned MLSS was also performed.

Results and Discussion:
- The granule fraction (> 200 um) of NAB2 ranged from 18 to 38% (Figure 1) while the average SVI5 and 30 for NAB2 were 78 and 62 mL/g respectively. The Control granule floc fraction averaged less than 5% for the same period while the corresponding average SVI5 and 30 for the Control were 129 and 85 mL/g respectively.
- During this testing period, the average bulk aSRT for NAB2 was 3.6 day versus 6.9 day for the Control while the corresponding average effluent ammonia concentration were 1.1 mg NH3-N/L and 1.2 mg NH3-N/L respectively (Figure 2).
- NAB2 and Control reactors observed ammonia spikes at various points during testing.
o 10/18 event - Effluent ammonia for both systems increased to approximately 2 mg N/L but rapidly declined. Both reactors were operating at bulk aSRTs of approximately 4.5 day.
o 11/7 to 11/12 event — Effluent ammonia for the Control increased to 2.5 mg N/L while operating at a bulk aSRT of 6 day. Effluent ammonia for NAB2 increased to ~ 5 mg N/L while operating at a bulk and mass weighted bulk aSRT less than 3 day. Notably, NAB2 ammonia concentrations decreased despite continuing to operate at a bulk and mass weighted bulk aSRT of ~ 3 day.
o Cumulatively these results indicate that NAB2 did not completely lose nitrification even when operating at bulk aSRTs between 2.5 and 3 days. During these periods, the granule fraction was 25 to 30% of the MLSS while the corresponding granule aSRT was always greater than 5 days.
- Activity testing results are summarized in Figure 3.
o Measurable nitrifier activity (as quantified via nitrate production rate) was observed in all biomass fractions even when operating at low bulk aSRTs.
o Nitrifier activity in the <100 um and 200 to 600 um fractions was observed to increase as the bulk and granule aSRT increased (positive correlation).
o Nitrifier activity in the 100 to 200 um fraction was observed to decrease as the bulk and granule aSRT increased (negative correlation).
o Nitrifier activity in the > 600 um fraction did not show any correlation to bulk or granule aSRT.
o Notably nitrifier activity in all size fractions was observed to decrease as the granule composition of the MLSS increased.
o Complete activity results from denitrification and PAOs will be presented in the full-paper/presentation.

Significance of Work: This work provides evidence that it is possible to nitrify in DAS systems equipped with a physical selector at bulk aSRTs as low as 3 days provided that the granule fraction remains between 20 and 30% and the granule aSRT is greater than 5 days. Full discussion of nitrifier retention and enrichment factors will be provided in the presentation/paper.