Field Experiments of Chlorine Demand in Disinfection of Treated Primary Effluent

In the Hong Kong Harbour Area Treatment Scheme (HATS), a concentrated 10% chlorine solution (in sodium hypochlorite with a specific gravity of 1.2) is used directly to disinfect sewage that receives a chemically enhanced primary treatment (CEPT). The dense chlorine solution is injected into the treated sewage flow in the form of multiple dense jets. 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 effluent and its associated chlorine consumption are studied for the first time using a $1:2$-scale sectional physical model located inside the sewage treatment plant, using sewage and a chlorine solution obtained on-site. The total residual chlorine (TRC) concentration distribution is measured at different cross sections of the chlorine mixing chamber. The overall disinfection efficiency is assessed by detailed measurement of the water quality of the outflow from the chamber, which receives only a few seconds of chlorine exposure. At a targeted dosing rate of $10–20\mathrm{mg}/\mathrm{L}$, only approximately 60–80% of the sewage flow is exposed to chlorine upon leaving the chamber, and approximately 70–80% of the chlorine mass flux is consumed within a very short distance (0.5–1 m, or a matter of several seconds) from the chlorine dosing unit. Overall, there is a 1-log Escherichia coli (E. coli) kill within the chlorine mixing chamber, with effective E. coli kill associated with a TRC concentration above $15\mathrm{mg}/\mathrm{L}$. The detailed field-scale model tests help to unravel the causes of the observed large TRC fluctuations in the plant and provide a basis for optimization of the chlorine disinfection operations (e.g., use of lower source chlorine concentration).