Production of Nitrous Oxide From Anaerobic Digester Centrate and Its Use as a Co-oxidant of Biogas to Enhance Energy Recovery

Coupled Aerobic-anoxic Nitrous Decomposition Operation (CANDO) is a new process for wastewater treatment that removes nitrogen from wastewater and recovers energy from the nitrogen in three steps: (1) NH₄⁺ oxidation to NO₂^–; (2) NO₂^– reduction to N₂O gas; and (3) N₂O conversion to N₂ with energy production. In this work, we optimize Steps 1 and 2 for anaerobic digester centrate, and we evaluate Step 3 for a full-scale biogas-fed internal combustion engine. Using a continuous stirred reactor coupled to a bench-scale sequencing batch reactor, we observed sustained partial oxidation of NH₄⁺ to NO₂^– and sustained (3 months) partial reduction of NO₂^– to N₂O (75–80% conversion, mass basis), with >95% nitrogen removal (Step 2). Alternating pulses of acetate and NO₂^– selected for Comamonas (38%), Ciceribacter (16%), and Clostridium (11%). Some species stored polyhydroxybutyrate (PHB) and coupled oxidation of PHB to reduction of NO₂^– to N₂O. Some species also stored phosphorus as polyphosphate granules. Injections of N₂O into a biogas-fed engine at flow rates simulating a full-scale system increased power output by 5.7–7.3%. The results underscore the need for more detailed assessment of bioreactor community ecology and justify pilot- and full-scale testing.