UC Riverside

Excess nutrients, particularly nitrogen, from wastewater discharged into waterways lead to eutrophication, hypoxia, and oceanic red tide that threaten public and environmental health. Anaerobic Ammonium Oxidation or Anammox is a relatively new treatment approach used to remove nitrogen from wastewater. Compared to conventional multi-stage nitrification-denitrification nitrogen (N) removal systems, the Anammox process utilizes less energy and eliminates the need for organic carbon additives in wastewater treatment plants. However, the process has encountered a number of pragmatic challenges that has impeded its widespread adoption as a practical treatment alternative. Foremost among these challenges is the very slow growth rate of the Anammox bacteria, which necessitates long start-up times before high nitrogen removal efficiencies can be obtained on a steady-state basis. Attempts to reduce start-up times are challenged by inadequate seed biomass and bacteria washout during operation. As such, efforts are needed to promote faster bacterial growth and system retention. One approach is to amend bioreactors with materials that have beneficial surface and chemical properties that are capable of concentrating growth factors (e.g., ammonium, NH4+), while enabling selective colonization of Anammox bacteria on the materials’ surfaces. This strategy could reduce start-up time and minimize biomass effluent losses. With evidence of higher cation exchange capacity (CEC) and better ammonium ion exchange (IX) rates than natural clinoptilolites (zeolite, group 7), chabazite material (zeolite, group 4) is hypothesized to be a better choice for enhancing Anammox culture development. Therefore, it was proposed to amend Anammox SBR with chabazite particles to accelerate the startup period and to enhance culture development. Nonetheless, bacterial activity in the amended process risk inhibition from cations released during NH4+-IX. In relation, the study also sought to investigate the effect of chabazite sub-species (-Ca & -K), target cation (NH4+), and competing cation (i.e. K+) on the ammonium oxidation rate.

Anammox culture was developed from atypical thermophilic anaerobic digester (TAD) mixed-sludge added proportionately in three bench-scale reactors. All the reactors exhibited phenotypic Anammox activities at approximately 90% total nitrogen removal with varying start-up times. Increased amount of TAD mixed-sludge in the reactors led to fast start-up time, within 59 days, improved specific anammox activity (SAA) at high nitrogen loading rate, and increased Anammox bacteria population. The Anammox bacteria species in the reactors was identified as “Candidatus Brocadia sinica.” Enhanced culture development in terms of increased gene copy numbers was attributed to less competition among the bacteria involved in the process during start-up. In adding appropriate doses of chabazite to SBRs seeded with fresh return activated sludge (RAS), it was determined that chabazite particles had minimal influence on the Anammox process start-up time since both the amended (with chabazite) and non-amended reactors exhibited evidence of Anammox activity after 69 days. However, chabazite mitigated the effect of high feed concentration variability resulting in optimum feed concentration needed by the Anammox bacteria for development. The quick recovery from influent shock-loads exhibited by chabazite amended reactor was attributed to chabazite concentration of NH4+ on the surface leading to increased Anammox bacteria population. Different chabazite sub-species including the Bowie chabazite A ZUB (Na/Ca), AZLB-Na, and AZLB-Ca were studied to investigate the effective chabazite type for Anammox process amendment. In the order of NH4+ removal efficiencies, reactor amended with chabazite-Ca > Na/Ca > Na > non-amended (control) = sand (attached control).

For the two common chabazite forms in the bioreactors, the reactor containing chabazite-Na was impacted adversely to a greater extent at high feed concentrations compared to the chabazite-Ca reactor with ammonium removal efficiencies decreasing from >95% to 53.5% and >95% to 88%, respectively.

High ammonium removal efficiency observed in chabazite-Ca reactor was attributed to high NH4+ ion-exchange rate compared to chabazite-Na. The effect of chabazite form on ammonium removal depended on the extent to which NH4+ ion-exchange impacted the partial nitrification step of the Anammox process that is mediated by the ammonium oxidizing bacteria. Despite lower reactor activity than the chabazite-Ca reactor, chabazite-Na reactor total specific bacteria population was the highest among the various chabazite amended reactors. High bacteria population in the chabazite-Na reactor resulted from higher NH4+ ion-exchange rate that accelerated bacteria biofilm formation and growth over a prolonged period. The results imply that while chabazite-Ca greatly mitigates the effect of high feed variability on Anammox activities compared to other chabazite sub-species, chabazite-Na exhibits high bacteria population attributed to high NH4+ ion-exchange rate at start-up.

Lastly, sequencing batch reactors with active Anammox biomass were amended with appropriate doses of chabazite-Na, chabazite-Ca, and non-amended (control) to investigate the effect of competing cation (K+) on ammonium oxidation rate and to determine the effect of competing cation (K+) on feed substrates (NH4+ and NO2-) utilization rates in Anammox processes amended with different chabazite sub-species. Upon spiking the feed media with different K+ concentrations that ranged from 0 to 5.12 meq/L, it was determined that the NH4+ and NO2-utilization rates were higher in chabazite-Na reactor compared to chabazite-Ca reactor at high K+ concentration. Further, the effect of NH4+ ion-exchange on partial nitrification was mitigated at increased K+ concentration, which led to improved substrates utilization at increased Anammox bacteria population resulting in higher Anammox removal rates in chabazite-Na reactor than that in chabazite-Ca reactor