Assessment of the start-up of the passive aeration simultaneous nitrification and denitrification (PASND) process using activated sludge as a sole seed for low-energy C and N removal

Abstract

The conventional activated sludge-based wastewater treatment process is known to be energy intensive to operate due to the high energy requirement for compressed air supply into the bulk wastewater solution. The use of passive aeration exposes the biofilm directly to atmospheric air and enables operational cost savings associated with active aeration. This study evaluated the start-up and long-term performance of the passively aerated simultaneous nitrification and denitrification (PASND) wastewater treatment system using activated sludge as the sole seeding culture. The obtained results proved that the PASND bioreactor seeded with activated sludge successfully achieved sustained chemical oxygen demand (COD) and nitrogen removal (about 88% and 66%, respectively) after 18 and 30 days of operation at both 25 °C and 12 °C, respectively. Microbial community analysis revealed that Thauera (18.29%) and Candidatus competibacter (6.86%) were enriched, which were possible key microbes involved in anaerobic COD removal through intracellular poly-hydroxyalkanoates synthesis and degradation. The low C/N ratio of 2.5 was not favourable for Candidatus competibacter enrichment, thus resulting in a failure of PASND start-up. This study demonstrates the feasibility of starting an energy efficient PASND system using activated sludge as the sole seed, which offers promising energy saving and a lower sludge yield (both about 3-times less than the conventional activated sludge process).