Sidestream Treatment with Energy Recovery from Nitrogen Waste: The Coupled Aerobic-anoxic Nitrous Decomposition Operation (CANDO)

The Coupled Aerobic-anoxic Nitrous Decomposition Operation (CANDO) is a new wastewater treatment process that removes and recovers renewable energy from reactive nitrogen (NH4+). The process consists of three principal steps: (1) NH4+ oxidation to NO2- (2) NO2- reduction to N2O, and (3) N2O conversion to N2 with energy recovery by using N2O as an oxidant in biogas combustion. Stanford University and Delta Diablo (Antioch, CA) partnered in 2012 to demonstrate CANDO at the bench- and pilot- scales. A bench-scale, the CANDO system treated anaerobic digester centrate where 75-80% of the influent NH4+ converted to N2O with >95% nitrogen removal. Some organisms stored phosphorus as polyphosphate granules, possibly enabling simultaneous biological phosphorus removal. Operation of a pilot-scale CANDO system (∼1/200th of full-scale) is under way. Preliminary results show N2O production over repeating cycles, but the operation is not yet optimized. We demonstrated power gains in a fullscale biogas-fed internal combustion engine subjected to injections of N2O at flows that are expected from a full-scale CANDO system. Increases in power output ranged from 5.7-7.3%, depending on the ratio of biogas to N2O. We also describe the academic-utility partnership as a successful model for development of innovative wastewater technology. If successfully scaled up, CANDO can be an attractive option for nitrogen removal by decreasing oxygen requirements, reducing biosolids production, enabling greater diversion of organic matter to methane production, and recovering energy from nitrogen.