Biogas-driven sidestream nitrogen removal: A perspective on replacing partial nitritation with aerobic methane...

Sidestream reject water from anaerobic digesters imposes a disproportionately high nitrogen load (typically ∼15-25% of the total plant nitrogen load despite comprising <5% of total influent volume) on wastewater treatment plants. However, existing biological removal technologies, including partial nitritation/anammox and anaerobic methane oxidation-based processes, are structurally disconnected from onsite biogas valorization. In this review, we evaluated the feasibility of replacing the conventional partial nitritation stage with aerobic methane oxidation coupled with denitrification (AME-D) in an integrated two-stage system toward carbon-neutral sidestream nitrogen removal. We synthesized the current knowledge on methane-oxidizing microbial ecology, interguild carbon transfer networks, and the enabling roles of reject water composition and biogas-derived CO₂ in sustaining integrated process performance. The key findings indicate that AME-D can serve as a multifunctional upstream stage, concurrently supplying nitrite, reducing ammonium load, and providing carbon intermediates to the downstream nitrate/nitrite-dependent anaerobic methane oxidation (n-DAMO) consortium. This enables biogas-driven complete nitrogen removal without external carbon input. The principal challenges involved in this process include greenhouse gas emission management, membrane fouling under high-strength sidestream conditions, and constraints on micronutrient bioavailability, which may compromise long-term microbial stability. This review proposes an integrated conceptual framework for AME-D/n-DAMO process design, identifies research priorities in pilot-scale validation and multi-guild community engineering, and articulates a pathway toward circular bioeconomy implementation in urban water resource recovery.