High Rate of Microbial Production of N2O for Energy Generation

Stanford researchers have developed a method for converting ammonia in wastewater into nitrogen gas while simultaneously generating power in a bioreactor system. This method produces energy from carbon and nitrogen waste and provides significant cost and energy savings over current options. Using bacteria in carefully controlled aerobic and anoxic reaction phases the organisms in the bioreactor convert ammonia into nitrite in a first stage at an extremely low dissolved oxygen level and subsequently convert nitrite into nitrous oxide gas (N2O) in a second anoxic stage through either biotic or abiotic mechanisms. The nitrous oxide can then be decomposed exothermically into nitrogen and oxygen by passing it over a heated catalyst bed or burned with methane resulting in direct energy generation from the waste nitrogen. The bioreactor outputs clean nearly nitrogen-free water and heat that can be used to produce power. This process greatly lowers costs in the removal of nitrogen from wastewater by reducing the oxygen demand for nitrification by at least 38% compared to conventional methods (aeration is often 50% of a wastewater plant’s operating costs). It also lowers costs by significantly reducing the amount of waste biomass produced (this is often the 2nd highest operational cost of wastewater treatment plants). This process can also double or triple the methane produced from organic matter by significantly reducing the amount of organic reducing power needed in denitrification. In conventional denitrification nitrate is converted to nitrogen gas a process that consumes 5 electron equivalents that comes from organic matter. In this process nitrite is converted to nitrous oxide a process that consumes 2 electron equivalents. Converting nitrite to nitrous oxide requires 60% less organic matter in denitrification and enables much more organic matter to be converted to methane (a renewable source of energy). Furthermore this process offers greater robustness and shorter start-up times as compared to alternative nitrogen short-circuit cycles by utilizing fast growing organisms and very fast abiotic reactions.