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Resumen de Operational strategies towards nitritation-anammox implementation for mainstream municipal wastewater treatment

Tiago Rogerio Vitor Akaboci

  • The need for moving the current paradigm to sustainable and energy-neutral wastewater treatment plants (WWTP) is pushing for the development of novel approaches to obtaining efficient processes with less energy and chemical inputs. In this context, nitrogen removal from wastewater through anoxic ammonium oxidation (anammox)-based processes has pointed up due to their cost effectiveness compared to the conventional nitrification-denitrification processes. Anammox process has been commonly applied for the treatment of reject water, in the sidestream line of the urban WWTP. However, the mainstream line accounts for the majority of nitrogen in the WWTP and anammox implementation has been focused on dealing with sewage. The downsides of anammox at mainstream conditions are (i) the low growth rate, mainly during winter season when temperature can drop far below the optimum temperature for anammox growth, and (ii) robust operational strategy to control nitrite-oxidizing bacteria (NOB) growth, once they can compete for oxygen and nitrite, reducing nitrogen removal efficiencies. In this Thesis, operational strategies to achieve stable nitrogen removal by anammox process at mainstream conditions were studied.

    An operational strategy that consisted in controlling the oxygen transfer according to the ammonium loading rate was evaluated in a one-stage partial nitritation-anammox (PNA) carried out in a sequencing batch reactor (SBR). This strategy resulted in extremely low bulk-liquid dissolved oxygen (DO) concentration. Both sidestream (high N strength- around 600 mg N·L-1, and temperature of 25 oC) and mainstream conditions (low N strength- around 75 mg N·L-1, temperatures of 25 and 15 oC) were assessed. At moderate temperature, average nitrogen removal rates (NRR) of 0.34 ± 0.05 and 0.37 ± 0.07 kg N·m-3·d-1 at sidestream and mainstream, respectively, were observed. NOB activity was completely suppressed at sidestream conditions due to a low DO availability and assisted by free ammonia (FA) inhibition. However, the results obtained demonstrate that uniquely low DO is not sufficient for complete NOB repression at mainstream conditions in one-stage PNA, although the possibility of maintaining NOB activity limited at long term is highlighted. At 15 oC, anammox activity deteriorated and led to increased nitratation rates. Hence, removal rates were limited by lower anammox activity and higher oxidation of nitrite by NOB. Results showed that NOB suppression strongly relies on anammox process performance and activity imbalance with nitritation.

    How the operational strategies applied to the one-stage PNA-SBR affected the microbial community was also explored with high-throughput techniques (16S rRNA sequencing and quantitative real-time polymerase chain reaction, qPCR). Sequencing analysis revealed the dominant bacterial groups in the microbial community that clustered within the phyla Planctomycetes, Proteobacteria, Chloroflexi, and Bacteroidetes. Aerobic ammonium-oxidizing (AOB) and anammox bacteria (AnAOB) were affiliated to the genera Nitrosomonas and Candidatus Kuenenia, respectively, and furthermore, they remained unaltered despite lowering N and temperature values. Nitrospira was the main NOB obtained when the SBR was switched from sidestream to mainstream conditions. Studies were also conducted aiming to obtain one-stage PNA in a plug-flow reactor with an aerobic and anaerobic compartment. The temperature in this reactor was maintained at room temperature (between 16 and 27 oC), and DO lower than 0.20 mg O2·L-1 most of the experimental time. Nitrogen removal rate lower than 20 mg N·L-1·d-1 was achieved, because AnAOB growth was limited by NOB activity.

    Finally, the effect of different inorganic carbon (IC) availability on nitrifying activities in a SBR operated at mainstream conditions (temperature of 15 oC) was also investigated. An influent NH4+:IC of 0.73 ± 0.03 allowed nitrite build-up in the SBR, and therefore, alongside DO control at lower level, a ratio of 80% for nitrite accumulation was reached and maintained for several weeks. Activity tests have also shown that the maximum ammonium conversion rates increased throughout the reactor operation, while the maximum nitrite oxidation dropped after IC limitation imposition.

    In summary, this PhD Thesis brings insights into the strategies for anammox application at mainstream conditions, limitations which can be found at long-term operation and alternative parameters which have the potential to enhance process robustness.


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