Impact of operating conditions on nitrogen removal using cyclic activated sludge technology (CAST)

Abstract

In this study, the nitrogen removal performance of a municipal wastewater treatment plant that employs cyclic activated sludge technology (CAST) was evaluated. The impact of key operational conditions such as temperature, dissolved oxygen (DO) concentrations and operating modes were examined in parallel. During summer, when the operating temperature ranged from 27–30°C, the NH4 +-N and total nitrogen (TN) removal efficiencies were 51 ± 7% and 42 ± 7%, respectively, and simultaneous nitrification and denitrification (SND) was considered to be the major nitrogen removal mechanism. In contrast, at a low operating temperature of 10–15°C, the DO concentration increased from 0.7 to 3.0 mg/L; however, the NH4 +-N and TN removals were both inefficient due to poor biomass activities, which was demonstrated by a lower specific oxygen uptake rate (SOUR) of 1.2 mg O2/g SS· h (15°C). Moreover, a 3h-mode for the CAST process was preferable during winter because the effluent wastewater quality was similar to that obtained when the 4h-mode was used. The extremely low organic loading was the primary reason for the poor bioactivity of the sludge in the CAST system, and this eventually led to deterioration of the nitrogen removal efficiencies.