Enhanced nitrogen removal under low-temperature and high-load conditions by optimization of the operating modes and control parameters in the CAST system for municipal wastewater

Sequencing batch reactor (SBR) technology has become one of the commonly used treatment systems for municipal and industrial wastewater in the past decades. Cyclic activated sludge technology is one of the newly developed variations of SBR process. In this study, because the water quality situation of effluent under low-temperature and high-load conditions was unstable in Wang-hill Wastewater treatment plant, and the phenomena of exceeding discharge standard about ammonia-nitrogen (NH3-N) and total nitrogen (TN) were serious in winter, the different operation modes (operation cycle and aeration time) and control parameters were selected and optimized to improve nitrogen removal performance, respectively. The results indicated that the national first level B criteria of the effluent quality could be reached and the stable discharge could be achieved under the operation conditions of the C mode with 6 h per cycle [influent 2 h, aeration 2.5 h (aeration starting from 1.5 h after influent), settling 1 h, draw/idle 1 h], the average removal rates of chemical oxygen demand (CODcr), NH3-N, and TN in the reactor were 91.8, 75.6, and 62.9%, respectively. Furthermore, the dissolved oxygen (DO) level and mixed liquor suspended solids (MLSS) content in the aeration phase were both optimized on the basis of the preferred C mode. The best ranges of DO and MLSS concentration during the aeration phase as high as 2.0-3.0mg/L and 5,000-6,000mg/L are essential for the best nitrogen removal performance. The average removal rates of CODcr, NH3-N, and TN in the reactor were 90.3, 83.2, and 69.1%, respectively. Obviously, alternating operation mode combined with parameter optimization was the optimal nitrogen removal strategy for low-temperature and high-load wastewater. These data are of great practical significance for the scientific control and management of the wastewater treatment plant and for references of water engineering professionals.