Effects of different operating conditions on total nitrogen removal routes and nitrous oxide emissions in a lab-scale activated sludge system

Abstract

This study sought to determine the effects of different operating conditions, such as variable organic loading, different sludge retention times (SRTs) and airflow rates, limited dissolved oxygen (DO) concentrations and ammonium (NH₄⁺) shock loading on total nitrogen (TN) removal routes and nitrous oxide (N₂O) emissions in a lab-scale activated sludge system. Short SRT (5 days) combined with very low DO levels (0.5 mg L^-1) were responsible for lower TKN oxidation efficiencies and, consequently, negligible NO₂^- accumulation rates. These results suggest that nitrification efficiency was hampered by the oxidation of organic matter, with a large part of TN removed by sludge waste process. As the SRT increased (from 5 to 10 days) and DO was set to 1.0 mg L^-1, TKN oxidation rates and NO₂^- accumulation reached their maxima, which are thought to be the optimal conditions for both organic matter oxidation and partial nitrification. Under these conditions, gas transfer to the atmosphere became the preferential route for TN removal instead of incorporation into the sludge waste. However, N₂O contribution is estimated as less than 5.6% (with respect to TN in the influent). Insufficient aeration and stress conditions (such as NH₄⁺ shock loading) can cause limited DO conditions and NO₂^- accumulation, leading to higher amounts of emitted N₂O. Therefore, the adequate control of DO concentrations is a key factor to avoid NO₂^- accumulation and consequently high N₂O emissions.