Nitrogen recovery from reject water for improved sustainability of wastewater treatment

Flows of reactive nitrogen (Nr) and greenhouse gas emissions from society are exceeding planetary boundaries, posing a serious risk to the stability of living conditions on Earth. Wastewater contains the largest flows of Nr in urban society, so recycling Nr from wastewater treatment plants (WWTPs) could reduce the climate impact and the need for new Nr. The reject water from dewatering anaerobically digested sludge contains high concentrations of Nr and recovery of this Nr would decrease the load on biological nitrogen removal processes, and thus nitrous oxide emissions. Simultaneously, the need for external carbon sources and energy for aeration will decrease. In a case study at Rya WWTP in Gothenburg, Sweden, three Nr recovery technologies were investigated: (1) conventional ammonia stripping to ammonium sulphate; (2) thermal stripping to ammonium sulphate and (3) distillation of ammonia from reject water to ammonia water. All three technologies were found to decrease the climate impact compared with the removal processes currently used at Rya WWTP for the removal of Nr. Recovery by distillation to ammonia water had the lowest climate impact, while conventional stripping minimised the energy requirement. HIGHLIGHTS Recovery of reactive nitrogen (Nr) from reject water in WWTP lowered GHG emissions. Recovery of Nr lowers GHG emissions also when fossil fuels have been phased out. The climate impact of recovery depended on emission factors from electricity and heat use. Recovery of Nr increased truck transport at the WWTP and in society. For Rya WWTP, recovery of Nr as ammonia water gave the lowest climate impact and WWTP transport.