Intermittent aeration enhances biological nitrogen removal from low C/N synthetic domestic wastewater

This work reports on the application of intermittent aeration (IA) in continuous-flow systems for biological nitrogen removal from low C/N wastewater (3.5 gCOD/gNH₄-N). In similar scenarios, low organic matter concentrations hamper the denitrification step due to e-donor limitation of ordinary heterotrophic organisms (OHO) (e-donor need: 2.86/(1-Y_obs) gCOD/gNO₃-N). A 25 L completely stirred bench-scale bioreactor equipped with on-line probes (pH, DO, ORP, NH₄-N, NO₃-N) was operated under IA at a moderate sludge retention time (SRT close to 25 ± 5 days) for 160 days of operation. The experimental period was structured in three main steps: continuous aeration (CA) (STEP 1); time-controlled IA (STEP 2); probe-controlled IA (STEP 3). The organic matter removal efficiency was not markedly affected by IA with removal efficiency increasing from 90% (STEP 1) to 98% (STEP 3) after a slight decrease down to 82% (STEP 2), likely due to a transitional phase during microbial adaptation. After the application of the IA process, a decrease in Y_obs was observed from 0.43 to 0.26 gCOD_biomass/gCOD_rem for STEP 1 and STEP 3, respectively. This resulted in about 40% less of sludge production, and disposal costs saving. Because a lower Yobs reduces the e-donor demand during the denitrification step, the biological nitrogen removal efficiency increased from 46.7% (STEP_2,a) to 78.2% (STEP_3,a). STEP 3 (probe-controlled IA) proved to be the best optimized operational condition compared to STEP 2 (time-controlled IA), due to the probe-controlled logic that avoids excessive aerated or undesirable anaerobic periods especially in cases of low influent organic and/or nitrogen loads. These remarkable results highlight that the IA process, operated at moderately high SRT and optimized using probe-controlled logic, is an attractive strategy for biological nitrogen removal from low C/N wastewater.