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Demonstration of robustness of partial denitrification – anammox
144 Results and discussion
Robustness under winter conditions
The filter pilots achieved TIN removal rates of 707 g N/m3/d and 755 g N/m3/d with TIN effluent quality below 3 mg N/L when upstream aeration control was optimized14. During the summer time, maximum AnAOB activity tests under non-limited substrate conditions were 3 times higher than the pilot activity driven by the loading applied which showed clear buildup of AnAOB and excess mass (Figure 1A).
Going into winter (14°C), excess AnAOB activity decreased, but was maintained at 1.5 times higher than the operational rates (Figure 1B). No impact of temperature was observed in the pilot due to the excess AnAOB mass. This was in contrast with previous studies that showed an impact of temperature on AnAOB activity. 10,15,16 The stable production of nitrite through the PdN pathway and good AnAOB retention in filters allowed this application to overcome changes in kinetic rates during the winter through excess mass buildup. This increases the reliability and robustness of full-scale PdNA filters.
Figure 1. Maximum AnAOB activity tests vs operational removal rates in A. summer and B. winter.
Robustness under extreme backwash events
The reported PdNA performance rates (Figure 1) were achieved under runtimes of 2-4 days for both filters. Additional stress testing was performed to evaluate the risk of AnAOB mass washout after repeated backwashes to simulate response to wet weather events in practice or more extreme backwash conditions to simulate technical issues. Figure 2 shows a summary of the testing performed and the direct AnAOB rates measured under non-limiting substrates after the backwash test. After 3 series of three backwashes in a row, an increase in AnAOB activity in the methanol filter was observed compared to the baseline, suggesting improved substrate diffusion to AnAOB happened after washing out some of the denitrifiers. This was observed before in smaller scale17 testing as well as pilot-scale testing18. This behaviour was not observed in the glycerol filter although we expected thicker biofilms and potentially increased heterotrophic mass due to higher yield (Figure 2). This showed that repeated backwashes under similar backwash settings as normal operation did not impact AnAOB retention nor did it impact performance as excess AnAOB was maintained (1.5-4 times, Figure 2).
No impact on AnAOB activity was observed in either filter when a backwash occurred with a longer air scour (4 hours). Only when the concurrent and final rinse times were increased, a decrease in AnAOB activity by 1/3 was observed in both filters. However, it should be noted that activity recovered within two days, suggesting that mass might not have been fully lost, but potentially regrowth of heterotrophs might have been needed to protect AnAOB mass from oxygen or carbon to restore full activity. Filter performance was at no point impacted by the shear testing or loss of activity (extreme event). However, it was observed that the MeOH filter seemed to be more robust and showed less risk
145 of AnAOB mass loss. This could be attributed to higher density of the biofilm in the methanol filter compared to the glycerol filter as carbon sources with lower growth yield tend to form denser biofilm.19 It should be noted that the backwash conditions applied were extreme and rarely happen in practice. It was a true challenge to find backwash conditions that resulted in AnAOB loss. These results thus indicated the robustness of PdNA filters as AnAOB could not be washed out of the system under the extreme stresses applied to the filters.
Application standpoint
The demonstrated reliable performance of PdNA filters through seasonal changes and under extreme shear will advance the application of PdNA confidently in practice. This study showed that AnAOB are not the limiting factor in the PdNA filters application and risk of application is most likely driven by the need for upstream aeration control rather than AnAOB retention or activity.
Figure 2. Impact of different backwash procedures on AnAOB activity. The green line indicates the daily ammonium loading to the filters
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Presenting Author
Rahil Fofana
DC Water / Howard University
Is the presenting author an IWA Young Water Professional? Y
Bio: Rahil Fofana is doing her PhD at Howard University where she studies Environmental Engineering. She has a bachelor’s degree in Chemistry and a master’s in Water Resources. Her research focuses on the application of partial denitrification – anammox (PdNA) in polishing filters with regards to media types and carbon sources selection.
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