Mixing and rapid chlorine demand of a high concentration chlorine jet in stagnant primary treated sewage

Abstract

In the Hong Kong Harbour Area Treatment Scheme (HATS), the Chemically Enhanced Primary Treated (CEPT) sewage is routinely disinfected by chlorination at the Stonecutters Island Sewage TreatmentWorks (SCISTW) in order to meet beach water quality standards. A high concentration chlorine solution (in sodium hypochlorite form; ~10⁵ mg/L) with a relative density of 1.2 is injected in the form of multiple dense jets into the primary treated sewage effluent. Field observations have revealed unexpected large amount of chlorine consumption; significant part of the dosed chlorine was not used for inactivating pathogenic microorganism. In view of the importance of sewage disinfection to the environment, it is essential to understand the mixing and reaction of a high concentration chlorine jet with CEPT effluent. In previous studies, the chlorine demand of CEPT sewage was determined by bench-scale beaker tests at the fully mixed chlorine concentration. Very recently, field scale model experiments of dense chlorine jets in a coflow have been performed. However, the detailed structure of mixing and chemical reaction in a chlorine jet remains largely unknown. A small scale model of a high concentration dense chlorine jet discharging in otherwise stagnant CEPT sewage has been designed and constructed in the chemical laboratory at SCISTW. The prototype high concentration sodium hypochlorite solution (~10⁵ mg/L) and primary treated sewage effluent are used. Four series of experiments with different source chlorine concentrations (C₀), flow rates (Q₀) and types of ambient flow (CEPT sewage, ammonia nitrogen solution and tap water) have been conducted. Cross-sectional profiles of Total Residual Chlorine (TRC) and density within the dense chlorine jet have been measured in a total number of 154 experiments. The chlorine consumption and buoyancy change of chlorine jet in CEPT sewage have been studied. The mechanism of the high chlorine consumption near the jet nozzle under the dynamic mixing process has been revealed. The chlorine consumption by ammonia nitrogen in primary treated sewage has been studied. An Eulerian integral model accounting for a second order reaction kinetics between chlorine and CEPT sewage and nonlinear relationship between density and TRC concentration is developed. The buoyant jet spreading rate is db/dz = 0.122. Cross-sectional TRC concentration and buoyancy force of dense chlorine jet discharging in CEPT effluent can be well-described by Gaussian distributions. The ratio of jet concentration to velocity half-width is around 1.07. The chlorine demand (△TRC) depends on its local TRC concentrations: △C_g ~ 600 mg/L for chlorine jet of C₀ = 10.0%, △C_g ~ 500 mg/L for C₀ = 7.5%, △C_g ~ 400 mg/L for C₀ = 5.0% and △C_g ~ 250 mg/L for C₀ ≤ 2.5%. Two main transitions exist for the chlorine consumption pattern; The first one at TRC = 350 mg/L, corresponding to the “breakpoint” reaction of ammonia nitrogen in CEPT sewage. When TRC < 350 mg/L, the chlorine consumption increases as an increase of TRC concentration. When 350 mg/L < TRC < 2000 mg/L, the chlorine consumption almost remains the same indicating an exhaustion of simple organic matters and inorganic impurities. When TRC > 2000 mg/L, the chlorine consumption starts to rise with an increase of TRC concentration, which is the second transition reflecting the oxidization of organic debris. Ammonia nitrogen is the key constituent with a large amount of chlorine demand. It is found that at low TRC concentrations (TRC < 3000 mg/L), the chlorine demand of CEPT sewage is mainly due to ammonia nitrogen (> 50%). At high TRC concentrations (TRC > 3000 mg/L), around 2/3 of chlorine is consumed by complex organic debris. Predicted cross-sectional, centerline and ambient TRC concentration agree well with experimental measurements.

Date of Award	2017
Original language	English
Awarding Institution	The Hong Kong University of Science and Technology