## From 'Black Box' to 'Precision Solutions': The Era of Functional Microbial Communities In environmental engineering, it was often remarked that wastewater treatment plants operated as 'black boxes.' Operators relied on a heterogeneous mixture of microorganisms known as **activated sludge**, adjusting operational parameters in the hope that indigenous populations would function effectively. This paradigm has persisted for over a century. However, this passive reliance on native consortia is increasingly inadequate given today's stringent discharge standards and 'dual carbon' targets. Global wastewater treatment energy consumption is substantial, accounting for approximately 1-3% of total societal electricity usage, with over half dedicated to aeration. To treat wastewater with low carbon-to-nitrogen ($C/N$) ratios, many facilities must supplement with expensive external carbon sources such as methanol. This incurs significant operational costs and indirectly increases carbon footprints. Furthermore, the release of nitrous oxide ($N_2O$)—a greenhouse gas with 298 times the warming potential of $CO_2$—during treatment creates an unsustainable cycle. The future of this field lies in the screening, construction, and intelligent regulation of functional microbial communities, transitioning from empirical management to precision engineering. ## 1. Anammox and PNA Nitrogen Removal Processes The discovery of Anaerobic Ammonium Oxidation (Anammox) in the 1990s fundamentally reshaped our understanding of the nitrogen cycle and introduced a disruptive autotrophic pathway for nitrogen removal. ### 1.1 Metabolic Principles and Substrate Chemistry Anammox bacteria (e.g., _Candidatus_ Brocadia), belonging to the phylum Planctomycetota, utilize nitrite ($NO_2^-$) as an electron acceptor to oxidize ammonia ($NH_4^+$) directly into nitrogen gas ($N_2$) under strictly anaerobic or anoxic conditions, producing minimal nitrate and limited biomass. - $NH_4^+ + 1.32 NO_2^- + 0.066 HCO_3^- + 0.13 H^+ \rightarrow 1.02 N_2 + 0.26 NO_3^- + 0.066 CH_2O_{0.5}N_{0.15} + 2.03 H_2O$ Since most wastewater primarily contains ammonia rather than nitrite, Anammox is typically integrated with Partial Nitritation (PN) to form the Partial Nitritation-Anammox (PNA) process, often termed deammonification. In PNA, ammonia-oxidizing bacteria (AOB) are precisely regulated to oxidize approximately 50% of ammonia to nitrite, providing the ideal substrate ratio for subsequent Anammox. ### 1.2 Technical Advantages and Engineering Bottlenecks The theoretical advantages of Anammox are significant: it eliminates the need for external organic carbon, reduces aeration energy consumption by 60%, and drastically cuts sludge disposal costs due to low cell yield. Economic analyses suggest that deammonification costs in sidestream treatment are only one-third of those associated with traditional biological nitrogen removal. However, implementing Anammox in mainstream municipal wastewater faces major hurdles: 1. **Slow Microbial Growth**: Anammox bacteria have doubling times of 10 to 20 days, making them prone to washout. This necessitates advanced biomass retention technologies such as Moving Bed Biofilm Reactors (MBBR), granular sludge, or Integrated Fixed-film Activated Sludge (IFAS) systems. 2. **Out-selection of Nitrite-Oxidizing Bacteria (NOB)**: This remains the primary challenge in mainstream PNA. In high-temperature sidestreams (e.g., sludge digester liquor), high free ammonia (FA) naturally inhibits NOB. In mainstream municipal wastewater, lower temperatures and ammonia concentrations allow NOB (particularly _Nitrospira_) to outcompete AOB for oxygen, oxidizing nitrite into nitrate and disrupting the substrate chain. Emerging solutions include intermittent aeration, low dissolved oxygen (DO) control, and online bioaugmentation using sidestream-generated free nitrous acid (FNA). 3. **Greenhouse Gas Emissions**: While Anammox bacteria do not produce $N_2O$, AOB in the PN stage may release $N_2O$ under stress from low DO and nitrite accumulation, with emission rates in full-scale reactors ranging from 0.2% to 1.7%. ## 2. Ecological and Engineering Potential of Comammox Bacteria Until 2015, it was a textbook axiom that nitrification required the synergy of separate AOB and NOB groups. The discovery of Complete Ammonia Oxidation (Comammox) bacteria overturned this century-old rule. ### 2.1 Metabolic Mechanisms and Ecological Niches Comammox bacteria, identified within the _Nitrospira_ genus (e.g., _Ca. Nitrospira inopinata_), encode all necessary enzymes to oxidize ammonia directly to nitrate within a single cell. These organisms were long overlooked in metagenomic surveys because traditional _amoA_ (ammonia monooxygenase) primers failed to amplify their sequences. Ecologically, Comammox are typical K-strategists.
