Optimizing Partial-Denitrification/Anammox (PdNA) Startup in IFAS system with Low TIN Discharge Targets

Introduction
To advance PdNA implementation, it is important to better understand PdNA startup strategies and timelines. Previous PdNA startups showed detection of anammox activity within three months for both methanol and glycerol as carbon sources, and MBBR, filters and IFAS configurations (Justin et al., 2022; Bachmann et al., 2024). At Blue Plains, PdNA integration is intended within the nitrification reactors, leaving a full denitrification polishing zone with MeOH downstream of the PdNA IFAS zone. This also means that the mixed liquor will maintain a full denitrification methylotroph population while PdN is targeted within the PdN zone. Previous work showed reduced PdN efficiencies under those conditions (Ladipo-Obasa et al., 2022) compared to only dosing MeOH with PdNA zone (Bachmann et al., 2024; Fofana et al., 2023) and also showed the importance of AnAOB presence to increase PdN efficiency by competing for nitrite (Ladipo-Obasa et al., 2022). The question remains however how one can startup a PdNA IFAS system when the MLSS maintains a healthy full denitrification methylotroph population and no initial AnAOB mass is present to compete for nitrite. The latter conditions are tested in this paper and the results of an IFAS PdNA system with MeOH for PdNA and downstream full denitrification is discussed.

Materials and Methods
A mainstream nitrogen removal pilot was operated at Blue Plains Advanced Wastewater Treatment Plant. The schematic for the pilot is shown in Fig.1. The pilot was initially inoculated with nitrification-denitrification sludge from the full-scale biological nutrient removal system.

Results and Discussions

1. Permit Compliance and Nitrogen Removal
As shown in Table.1, the effluent TIN concentration averaged 2.59 ± 1.2mg N/L, successfully complying with the discharge permit requirements throughout the entire operational period (< 3 mg N/L).

2 PdN selection
PdN was successfully established within seven days and gradually improved until day 42, reaching an PdN efficiency of 20%, and confirmed by activity tests (Fig.2). This result aligns with expectation of observing lower PdN efficiencies with MeOH in the absence of AnAOB (Ladipo-Obasa et al., 2022). However, PdN efficiency declined to below 10% between days 70 and 112, which coincided with suboptimal methanol dosing. Operational constraints, such as understaffing, also hindered the maintenance of conditions essential for optimizing PdN selection during this period. Fluctuations in PdN efficiency persisted throughout the first 100 operational days due to limitation in nitrate removal rates not allowing for observable nitrite accumulation (Fig.2). This limitation was corrected on day 115, by increasing nitrate removal rates. Also, SRT was reduced from 28±0.4 days to 21±4 days, minimizing endogenous decay. Those actions led to establishing PdN efficiencies back to 20% (Fig. 2A). An improvement in PdN efficiency was observed after the enrichment of AnAOB up to PdN efficiencies of 44.03±10.01%, providing results close the previously reported PdN efficiencies (Ladipo-Obasa et al., 2022). This reinforced the critical role of AnAOB in enhancing PdN performance with methanol.

3 AnAOB enrichment
The first measurable AnAOB activity was detected after 90 days (Fig.2D). However, operational inefficiencies delayed significant progress until corrective actions were implemented after day 115 (discussed above) to allow for nitrite levels of 0.82 ± 0.51 mg N/L to be maintained (Fig.2C). To ensure sufficient substrate availability, ammonium concentrations were maintained at ≥2.3 mg N/L (Fig.2C). After establishing nitrite levels > 0.5 mg N/L from day 131 onwards it took until day 153 to see consistent ammonium removal of at least 0.55±0.11 mg N/L or 22.38±10.71 mg N/m2/d (Fig 2D). For that point on, ammonium removal increased exponentially and reached 2.23±0.57 mg N/L or 151.48±48.8 mg N/m2/d at the end of the experiment (Fig.2D).

The observed anammox growth rate was calculated as 0.036 ± 0.004 d-1, which aligns with those reported in polishing filters and MBBR systems using glycerol as a carbon source (Fofana et al., 2022; Schoepflin et al., 2022) but is lower than growth rates in suspended PdNA systems (Le et al., 2021). This discrepancy may be attributed to diffusion limitations in biofilm systems or competition with heterotrophic denitrifiers for nitrite (Stewart Philip, 2003). Additionally, this study confirms that it is important to generate a limited amount of nitrite accumulation together with ammonium availability to enrich AnAOB.

The timing of the first measurable AnAOB activity (day 90) aligns with prior findings (Bachmann et al., 2024; Kocamemi et al., 2018; Verma et al., 2021), further validating the feasibility of these approaches under mainstream PdNA conditions. we believe earlier optimization of nitrate removal to maintain nitrite could have reduced startup time. During this experiment about 60 days were lost.

Conclusion
This study successfully demonstrated AnAOB enrichment in PdNA IFAS system with downstream MeOH based polishing.