Fluid mechanics of chlorine disinfection for the Hong Kong Harbour Area Treatment Scheme

Joseph Lee, S.N. Chan, David Choi, Q.S. Qiao

Wednesday 1 july 2015

8:45 - 9:00h at Antarctica (level 0)

Themes: (T) Special session, (ST) Marine outfall system

Parallel session: 8D: Special Session: Marine Outfall System

To protect the water quality of nearby bathing beaches, chlorination disinfection has been provided to the treated sewage in a major sewage treatment works in Hong Kong. A ten-percent sodium hypochlorite solution (specific gravity 1.2) is injected in the form of multiple dense jets (from dosing pipes at two vertical levels) into the sewage cross flow in a flow distribution chamber (FDC). Bench scale experiments show that higher chlorine concentrations will lead to significantly higher chlorine demand. Thus, the mixing and transport of chlorine in the FDC plays an important role in the disinfection efficiency and determining the effluent quality. It is essential to achieve rapid mixing of chlorine with the sewage in order to achieve maximum disinfection efficiency in the chlorine contact culverts before discharge to the receiving water via a tunnelled sea outfall. The diurnal variation in the sewage inflow (8 - 22 m3/s) and tide level results in variation between pressurized and free surface flow in the hydraulic system. Furthermore, to maintain a minimum water depth, an oblique weir is installed across the middle section of the chamber – leading to complex 3D flow patterns and uneven distribution of flow and chlorine concentration downstream of the FDC. Theoretical and experimental modelling of the internal hydraulics of the dosing unit revealed a curious distribution of dense jet flows. Due to the large relative density difference between chlorine solution and sewage, the relative dosing flow between the lower and upper pipes may undergo a “flow switching” – leading to sewage intrusion into the upper pipe when the dosage flow is low. The hypochlorite solution (100,000 mg/L) needs to be diluted to around 10 – 20 mg/L for complete mixing. Using the integral jet model VISJET, it is found that with the present two-level dosing unit design, a dilution of only 100 to 1000 is achieved within the FDC at a distance of 10m. A physical hydraulic model and CFD are employed to study the complex interaction between the multiple dense jets and the non-uniform ambient flow through the FDC. An improved dosing unit design with pre-dilution is suggested and tested to give optimal initial mixing and higher residual chlorine concentration – resulting in significant savings of operation cost and energy.