What to breathe? Steering microbial electron acceptors for sustainable onsite wastewater treatment

Abstract

Onsite wastewater treatment systems, which collectively manage ∼12% of global wastewater, remain a preferred choice in rural and low-density communities. However, conventional systems such as septic tanks often exhibit limited treatment efficiency for nutrients and emerging contaminants and are flood prone, raising concerns about their long-term sustainability and environmental impacts. Recent advances in microbial ecology have uncovered alternative nitrogen removal processes with nitrite, iron, manganese, or sulfate as terminal electron acceptors. In this perspective, we collectively describe these processes as robust electron acceptors for nitrogen elimination from wastewater (RENEW) and evaluate their potential application in onsite wastewater treatment from thermodynamic, kinetic, and engineering standpoints. Thermodynamic calculations indicate that processes like conventional nitrification and denitrification, Anammox and Mnammox are favorable at neutral pH, while Sulfammox is unfavorable under typical wastewater conditions. Kinetic evidence highlights that Mnammox (8.15 gNH₄⁺-N gVSS⁻¹ d⁻¹) and Feammox (7.16 gNH₄⁺-N gVSS⁻¹ d⁻¹) can achieve a substantial nitrogen removal rate, with lower oxygen demand nearly 20-fold compared to traditional nitrification–denitrification. Meanwhile, microalgal photosynthesis with a higher yield (15.84 gVSS gNH₄⁺-N⁻¹) have a lower nitrogen removal rate (0.050 gNH₄⁺-N gVSS⁻¹ d⁻¹). We propose that RENEW processes could offer advantages in system footprint, climate-resilience, greenhouse gas emissions, and biosolid production. Novel onsite treatment systems can be further developed by integrating RENEW processes with artificial intelligence.