Studies on the optimization of chlorine disinfection dosage control at the Stonecutters Island sewage treatment works, Hong Kong

Abstract

At the Stonecutters Island Sewage Treatment Works (SCISTW) of Hong Kong Harbour Area Treatment Scheme (HATS), chlorination is applied to disinfect the Chemically Enhanced Primary Treated (CEPT) sewage before discharge to protect the marine environment and surrounding beaches. A high concentration chlorine solution (in sodium hypochlorite form; ≈ 10⁵ mg/L) is injected into the CEPT effluent through multiple dense jets. Field observations have revealed unexpected large amount of chlorine consumption (around 70 - 80%) by contaminants in CEPT sewage before inactivating pathogenic microorganism. In view of the importance of sewage disinfection to the environment, it is essential to study the optimization strategies of chlorine disinfection dosage at SCISTW. In this study, several important aspects of chlorine dosage optimization at SCISTW are investigated. Firstly, the chlorine demand by dominating chlorine consumption constituents (ammonia nitrogen and organic nitrogen) in the CEPT sewage has been systematically studied in chlorine jet mixing with ambient sewage coflow (ammonia nitrogen, organic nitrogen and CEPT sewage) in a laboratory experimental flume. This is the first time that the chlorine demand by (i) ammonia nitrogen; (ii) organic nitrogen and (iii) real CEPT sewage in a reacting chlorine jet mixing with ambient sewage coflow have been studied. A total number of 128 runs have been performed. The chlorine demand by ammonia nitrogen and organic nitrogen highly depends on the source chlorine concentration and the jet mixing conditions in laboratory flume tests. The largest chlorine consumption by breakpoint chlorination is approximately 11% of dosed chlorine at a source chlorine concentration of 9,500 to 12,500 mg/L in laboratory experimental flume. The largest chlorine consumption by dissolved organic nitrogen is approximately 27% of dosed chlorine at a source chlorine concentration of 10,000 mg/L in laboratory experimental flume. The chlorine demand measurements in jet mixing with ammonia nitrogen coflow in laboratory flume can be explained by jet dilution as predicted by a well-established jet model - VISJET. To overcome the time lag of chlorine dosing system and improve the treatment efficiency for chlorine disinfection dosage control, an autoregressive (AR₍₄₎) model to forecast sewage flow (with 30 mins lead time) in SCISTW has been developed. It is found that the deviation of the predicted flow from the actual flow is in general less than 5% or 1.5 m³ /s. The potential of automatic enzymatic measurements (as a surrogate) of E.coli concentration using a ColiMinder has also been studied.

Date of Award	2022
Original language	English
Awarding Institution	The Hong Kong University of Science and Technology
Supervisor	Xiangru ZHANG (Supervisor) & Hun Wei Joseph LEE (Supervisor)