## **From "Black Box" to "Precision Solutions": The Advent of Functional Microbial Communities** In the field of environmental engineering, we have historically referred to wastewater treatment plants (WWTPs) as colossal "black boxes." For over a century, the Conventional Activated Sludge (CAS) process has relied on a heterogeneous mixture of indigenous microorganisms known as "activated sludge," with operators attempting to achieve desired outcomes by modulating macro-level parameters. However, this passive reliance on native populations is increasingly inadequate in the face of stringent modern discharge standards and global carbon neutrality targets. Energy consumption in global wastewater treatment is substantial, accounting for approximately 1–3% of total societal electricity usage, with more than half of this energy dedicated to aeration. To treat wastewater with low carbon-to-nitrogen (C/N) ratios, many facilities must supplement expensive external carbon sources such as methanol. This not only incurs significant operational costs but also indirectly increases the carbon footprint. Furthermore, the emission of nitrous oxide ($N_2O$) during treatment—a greenhouse gas with 298 times the warming potential of $CO_2$—presents a major sustainability challenge. This "resource-intensive" model is fundamentally unsustainable. Since its inception, CAS has been the primary technology for global municipal wastewater treatment. To achieve biological nutrient removal (BNR), CAS systems typically evolved into multi-stage, spatially separated reaction configurations, such as the classic Anaerobic/Anoxic/Oxic (A2/O) or Bardenpho processes. In the standard A2/O process, nitrogen removal is strictly partitioned into distinct zones: 1. **Autotrophic Nitrification (Oxic Zone)**: Ammonia-oxidizing bacteria (AOB) and nitrite-oxidizing bacteria (NOB) oxidize ammonia ($NH_4^+$) sequentially under conditions of sufficient dissolved oxygen (DO) and alkalinity. AOB first oxidize $NH_4^+$ to nitrite ($NO_2^-$), which is subsequently oxidized by NOB to nitrate ($NO_3^-$). This process is highly oxygen- and alkalinity-demanding. 2. **Heterotrophic Denitrification (Anoxic Zone)**: Nitrate-rich mixed liquor is recycled via internal mixed liquor return (IMLR) to the anoxic zone. Heterotrophic denitrifiers utilize organic matter (COD) as electron donors to reduce $NO_3^-$ to nitrogen gas ($N_2$) through the pathway $NO_3^- \rightarrow NO_2^- \rightarrow NO \rightarrow N_2O \rightarrow N_2$. Research involving full-scale municipal WWTPs indicates that BNR processes combining CAS with trickling filters typically exhibit total nitrogen (TN) removal efficiencies between 50% and 61%, depending heavily on influent COD:N ratios and sludge retention time (SRT) control. Oehmen et al. (2007) provided a comprehensive review of A2/O and Enhanced Biological Phosphorus Removal (EBPR), analyzing the competitive dynamics between phosphorus-accumulating organisms (PAOs) and glycogen-accumulating organisms (GAOs) within the anaerobic-oxic cycle—a foundational guide for optimizing nitrogen and phosphorus removal. The future of this technology lies in the deliberate selection, construction, and intelligent regulation of functional microbial consortia. We must transition from stochastic reliance on indigenous populations to engineered precision solutions. ## 1\. Anaerobic Ammonium Oxidation (Anammox) and PNA Processes The discovery of Anaerobic Ammonium Oxidation (Anammox) in the 1990s fundamentally reshaped our understanding of the global nitrogen cycle and introduced a disruptive autotrophic nitrogen removal pathway. Strous et al. (1999), in their landmark Nature paper, identified the causative organisms as a novel group of Planctomycetes, overturning the century-old dogma that ammonia could only be oxidized under aerobic conditions. Subsequently, Kartal et al. (2011) elucidated the energy metabolism of Anammox, confirming that the highly reactive and toxic intermediate "hydrazine" is utilized to convert $NO_2^-$ and $NH_4^+$ into $N_2$, providing a proteomic explanation for its efficient nitrogen production. ### 1.1 Metabolic Principles and Substrate Chemistry Anammox bacteria (e.g., _Candidatus_ Brocadia) utilize $NO_2^-$ as an electron acceptor to directly oxidize $NH_4^+$ to $N_2$ gas under strictly anaerobic or anoxic conditions, producing minimal nitrate and extremely low biomass yield. - $NH_4^+ + 1.32 NO_2^- + 0.066 HCO_3^- + 0.13 H^+ \rightarrow 1.02 N_2 + 0.26 NO_3^- + 0.066 CH_2O_{0.5}N_{0.15} + 2.03 H_2O$ As most wastewater streams contain primarily ammonia rather than nitrite, Anammox must be integrated with a Partial Nitritation (PN) step to form the Partial Nitritation-Anammox (PNA) process, also known as deammonification. In PNA, AOB are precisely regulated to oxidize approximately 50% of the influent ammonia to nitrite, providing the optimal substrate ratio for the subsequent Anammox reaction. ### 1.
