A predictive model for N2O production in anammox-granular sludge reactors: Combined effects of nitrite/ammonium ratio and organic matter concentration

Once the use of anammox reactors has been increasing on a global scale, it is important to understand the mechanisms of N2O emissions and how to minimise the emissions by optimising the operating conditions. In this study, the influence of chemical oxygen demand (COD) (from 0 mgO(2) L-1 to 100 mgO(2) L-1) and nitrite/ammonium ratio from 0.79 to 2.21 (maintaining ammonium at 100 mgN L-1 and varying nitrite from 79 mgN L-1 to 221 mgN L-1) in the N2O emissions from anammox-granular sludge reactor was investigated in two steps. Step 1 consisted of batch tests, using central composite design, and Step 2, long-term operation of a 6.5 L continuous up-flow reactor. The results showed that the N2O emissions were minimized by controlling, in the influent, the NO2--N/NH4+-N ratio from 1.1 to 1.3 and maintaining the COD concentration below 100 mgO(2) L-1. TN removal efficiencies were higher than 70% in all conditions tested".