Mainstream Attached Growth Partial Nitritation and Anammox: Design and Optimization
Creators: name: Ikem, Juliet Ogochukwu
Publisher: name: Université d'Ottawa / University of Ottawa
Year: 2023
Subjects: Anammox
MBBR
Nitrification
Municipal Wastewater

description: There is a significant need to remove ammonia from municipal wastewater to meet increasingly stringent regulations set by Canada, US, and Europe. Although existing conventional biological wastewater treatment technologies are shown to achieve effective ammonia treatment, they are substantially limited by increased operational intensity and cost. Due to these limitations, other cost-effective biological treatment technologies, such as partial nitritation/anammox (PN/A), have become a more attractive solution for nitrogen removal at wastewater resource recovery facilities (WRRF). A moving bed biofilm reactor system (MBBR) operating under a novel design strategy using elevated total ammonia nitrogen (TAN) loading rate has shown promise to achieve robust partial nitritation and the oxidation of TAN with limited oxidation of nitrite without the need for intense operational measures. However, the novel and promising design strategy using elevated TAN loading rate was applied at higher influent TAN concentrations that are typically greater than concentrations in mainstream municipal wastewater. Therefore, the objective of this dissertation is to investigate and optimize the design and performance of a promising elevated loaded partial nitritation MBBR technology for mainstream, municipal wastewater treatment followed by downstream anammox to complete the design of a robust, stable, energy-efficient, and low operational cost total nitrogen removal PN/A system for mainstream wastewaters. The first specific objective of the dissertation is to isolate the optimal design parameter of a mainstream elevated loaded partial nitritation MBBR system. The results identifies optimal distinct elevated surface area loading rate (SALR), hydraulic retention time (HRT), and airflow rate that achieve stable partial nitritation performance (i.e., optimum total ammonia nitrogen (TAN) removal kinetics and percent NOₓ as nitrite) in a mainstream elevated loaded partial nitritation MBBR system. The study shows that TAN SALR, HRT, and airflow rate significantly affect TAN surface area removal rates (SARR) and percent NOₓ as nitrite and, as such, identifies the optimal design parameters (TAN SALR, HRT and airflow rate) of a mainstream elevated loaded partial nitritation MBBR system. A TAN SALR of 5 g TAN/m²∙d, HRT of 2h and airflow rate of 1.5 L/min are identified to provide stable partial nitritation performance with a TAN SARR of 2.3 ± 0.3 g TAN/m²∙d and a percent of NOx as nitrite of 84.8 ± 1.2% in the mainstream elevated loaded partial nitritation MBBR system. The second specific objective further identifies a new design configuration and the mechanism of nitrite oxidation suppression of the mainstream elevated loaded partial nitritation MBBR technology. The results identifies a unique design strategy using an elevated TAN SALR of 5 g TAN/m²∙d to achieve cost-effective, stable, and elevated rates of partial nitritation in an MBBR system under mainstream conditions. The elevated loaded partial nitritation MBBR system achieves a TAN SARR of 2.01 ± 0.1 g TAN/m²∙d and NO₂⁻-N:NH₄⁺-N stoichiometric ratio of 1.15:1, which is appropriate for downstream anammox treatment. The elevated TAN SALR design strategy promotes nitrite-oxidizing bacteria (NOB) activity suppression rather than a reduction in NOB population as the reason for the suppression of nitrite oxidation in the mainstream elevated loaded partial nitritation MBBR system. NOB activity is limited at an elevated TAN SALR, likely due to thick biofilm embedding the NOB population and competition for dissolved oxygen (DO) with ammonia-oxidizing bacteria for TAN oxidation to nitrite within the biofilm structure, which ultimately limits the uptake of DO by NOB in the system. The third specific objective of this research characterizes the effects of distinct mixing and aeration strategies on the performance of the mainstream elevated loaded partial nitritation MBBR technology. This is addressed through a study investigating and comparing the kinetics, biofilm characteristics, and embedded biomass of three distinct mixing and aeration strategies employed to operate the mainstream elevated loaded partial nitritation MBBR system. The study compares the conventional mixing and aeration condition, continuous aeration with mechanical paddle &amp; aeration, and recirculation pump &amp; aeration utilized to optimize the partial nitritation MBBR system to achieve low DO effluent concentrations for optimal downstream anammox treatment. The results show that maintaining mixing and aeration in the elevated loaded partial nitritation MBBR system with recirculation pump &amp; reduced aeration achieves lower effluent DO concentration and stable partial nitritation with appropriate NO₂⁻-N:NH₄⁺-N stoichiometry ratio of 1.09:1 for subsequent anammox treatment compared to operation with continuous aeration or mechanical paddle &amp; aeration. The fourth specific objective of this research investigates the promising elevated loaded PN/A configured system for nitrogen removal under mainstream conditions. This is achieved through the operation of the elevated loaded partial nitritation MBBR system following the anammox unit as a combined two-stage system for nitrogen removal at mainstream municipal concentration. The elevated loaded partial nitritation MBBR system provides optimal NH₄⁺-N:NO₂⁻-N stoichiometric effluent ratio of 1:1.17, resulting in the successful operation of a downstream anammox unit with a total nitrogen removal rate at 0.22 ± 0.2 g N/m²/d and total nitrogen removal efficiency at 74.1 ± 0.7%. The average NO₂⁻-N to NH₄⁺-N molar removal ratio is 1.05 ± 0.1 from the anammox unit. Also, the anammox bacteria (AnAOB) gene copies are at 3.28 ± 0.7 × 10⁸, a value significantly higher than the AOB and NOB gene copies at 9.17 ± 1.1 × 10⁴ and 6.23 ± 1.0, respectively. This confirms that anammox activity is established and nitrogen removal is primarily through the anammox process. The results and overall system performance demonstrate that the combined two-stage mainstream elevated loaded partial nitritation/anammox MBBR system has shown promise and offers great insights for further advancement of the anammox process at mainstream municipal wastewaters. Finally, the economic evaluation and cost comparative analyses conducted show that compared to the conventional biological nitrification/denitrification process for nitrogen removal, the two-stage elevated loaded PN/A system offers a 57.6% savings on energy cost, 100% savings on chemical cost, and 68.7% savings on the cost of sludge disposal. Therefore, the two-stage elevated loaded PN/A system, in addition to high nitrogen removal efficiency, reduced footprint, and ease of operation, is also economically favorable and reduces the overall operational cost of wastewater treatment system by 61.6%, thus saving up to an average of 3.7 million CAD every year.