Author(s): Q. S. Qiao; S. N. Chan; K. W. Choi; J. H. W. Lee
Keywords: Dense jets; Chlorine demand; Disinfection; Residual chlorine
Abstract: In the Hong Kong Harbour Area Treatment Scheme (HATS), concentrated (10%) sodium hypochlorite (NaOCl) solution is used as the disinfectant for sewage that receives Chemically Enhanced Primary Treatment (CEPT). Dense NaOCl solution is injected into the treated sewage flow in the form of multiple dense jets (specific gravity of 1.2). Field observations have revealed large variations of chlorine consumption in the treatment system. In view of the importance of the disinfection system to the environmental impact and operational cost of HATS, the mixing of a chlorine jet with the CEPT sewage is studied in a small-flume experiment using sewage and chlorine solution obtained from the sewage treatment work. NaOCl solution of different concentrations is injected into the sewage flow as a single horizontal dense jet. Measurement of total residual chlorine (TRC) concentration at the flume outflow shows that the reaction of chlorine solution and sewage is extremely rapid; about 50% of the chlorine mass injected is lost under a few seconds of contact (travel) time.