Linking nitrous oxide emissions to population dynamics of nitrifying and denitrifying prokaryotes in four full-scale wastewater treatment plants.

Ammonia-oxidizing bacteria (AOB), ammonia oxidizing archaea (AOA) and NO-reducing denitrifiers were measured by quantitative real-time PCR (qPCR) in activated sludge samples from four full-scale wastewater treatment plants (WWTPs) in South Spain, and their abundances were linked to the generation of NO in the samples using multivariate analysis (Non-metric multidimensional scaling, MDS, and BIO-ENV). The average abundances of AOA remained in similar orders of magnitude in all WWTPs (10 copies amoA/L activated sludge mixed liquor), while significant differences were detected for AOB (10-10copies amoA/L) and NO-reducers (10-10copies nosZ/L). Average NO emissions measured in activated sludge samples ranged from 0.10 ± 0.05 to 6.49 ± 8.89 mg NO-N/h/L activated sludge, and were strongly correlated with increased abundances of AOB and lower counts of NO-reducers. A significant contribution of AOA to NO generation was unlikely, since their abundance correlated negatively to NO emissions. AOB abundance was favoured by higher NO and NOconcentrations in the activated sludge.