Effect of influent carbon fractionation and reactor configuration on mainstream nitrogen removal and NOB out-selection†
An intermittently aerated, pilot scale biological nitrogen removal process was operated in modified Lüdzack Ettinger (MLE) and fully intermittent aeration (all reactors aerated) configurations. The process was fed both A-stage effluent (ASE), and primary clarifier effluent (PCE), which differ in chemical oxygen demand (COD) composition. The objective was to determine the effects of influent carbon fractionation and reactor configuration on nitrite oxidizing bacteria (NOB) out-selection and total inorganic nitrogen (TIN) removal during intermittent aeration. TIN removal was affected by both the type and amount of influent COD, with particulate COD (pCOD) having a stronger influence than soluble COD (sCOD). NOB out-selection was lowest in the MLE configuration, regardless of the feed, and highest in fully intermittent configuration with A-stage feed. During fully intermittent operation with A-stage effluent feed, nitrite accumulation correlated positively with influent particulate COD concentration, and correlated negatively with ex situ NOB activity rates. In addition, ex situ denitrification batch tests showed that nitrite was consumed faster than nitrate when NOB rates were low. These observations suggested that pCOD improved heterotrophic competition for nitrite, leading to ammonia oxidation rates higher than nitrite oxidation rates. Therefore, the influent COD fractions should be tailored to achieve the desired nitrogen removal goals downstream.