A new mixing–oxygenating technology for water quality improvement of urban water source and its implication in a reservoir

Abstract

The Water-lifting Aerator, which is a retrofit of the hydraulic gun, was developed to oxygenate lower-layer water and mix the water on the upper and lower layers in stratified reservoirs where pollutants increase from sediments because of anoxic condition in the lower-layer water. The Fenhe reservoir began to supply raw water to the drinking water treatment plant of the City of Taiyuan, China in 2004. The surface of the reservoir froze in winter, and the lower-layer water above the reservoir bed became anoxic because of oxygen consumption by the sediments. Hence, ammonia–nitrogen released from the sediments and trapped in the lower-layer water. After the ice surface thawed in spring, the ammonia–nitrogen in the deepwater was brought up owing to the wind–wave mixing processes. Thus the concentration of ammonia–nitrogen in the outlet water exceeded the permitted level, and the water supply from the Fenhe reservoir had to be cut off. To solve the problem, the Water-lifting Aerator system was installed in the reservoir in the winter of 2005, the dissolved oxygen concentration in the lower-layer water has been maintained at more than 3 mg/L and the ammonia–nitrogen concentration has been reduced to less than 0.1 mg/L. The ammonia–nitrogen concentration in the outlet water is a 95% reduction compared to the same period of last year before the system installation. Since then the water supply from the Fenhe reservoir has no longer been interrupted. Compared with traditional water treatment technology to remove ammonia–nitrogen, the new technology saves energy for 77%.