Stonecutters Island Sewage Treatment Works is one of the largest sewage treatment plants in the world and consists mainly of a chemically enhanced primary treatment (CEPT) unit and a disinfection unit. It was realized a long time ago that most of the dosed chlorine (15 mg/L) is lost at the beginning part of the disinfection unit (i.e., flow distribution chamber) during the disinfection of the CEPT effluent. Lab-scale tests were therefore conducted in this study to determine the cause. Because ferric chloride is used in CEPT, ferrous iron in the CEPT effluent (from the reduction of ferric iron) was initially thought to be the main chlorine consumer. However, the chlorine consumption by ferrous iron was found to be 1.2 mg/L at most. Suspended solids also contribute insignificantly to the...[ Read more ]

Stonecutters Island Sewage Treatment Works is one of the largest sewage treatment plants in the world and consists mainly of a chemically enhanced primary treatment (CEPT) unit and a disinfection unit. It was realized a long time ago that most of the dosed chlorine (15 mg/L) is lost at the beginning part of the disinfection unit (i.e., flow distribution chamber) during the disinfection of the CEPT effluent. Lab-scale tests were therefore conducted in this study to determine the cause. Because ferric chloride is used in CEPT, ferrous iron in the CEPT effluent (from the reduction of ferric iron) was initially thought to be the main chlorine consumer. However, the chlorine consumption by ferrous iron was found to be 1.2 mg/L at most. Suspended solids also contribute insignificantly to the chlorine consumption. Batch tests were conducted to evaluate the effects of mixing condition and chlorine stock solution concentration on the chlorine consumption. Less chlorine was consumed upon increased mixing. Using a high-concentration (25,000 mg/L) chlorine stock solution resulted in a 4-times-higher chlorine consumption than using a low-concentration (2,500 mg/L) chlorine stock solution in absence of mixing. By correlating the losses of ammonia and total nitrogen with the chlorine consumption, we propose that the use of a high-concentration chlorine stock solution under poor mixing leads to a localized high ratio of chlorine to ammonia, resulting in breakpoint chlorination and an unusual excessive chlorine consumption. A novel apparatus was developed to quantify the nitrogen gas generated during the chlorination of a simulated wastewater and the mass balance of nitrogen-containing species (i.e., ammonia, nitrogen gas, nitrite and nitrate) during chlorination was inspected. The good fit between the measured chlorine consumption and that back-calculated from nitrogen-containing species verifies our hypothesis. Moreover, considering that the newly developed apparatus is effective on the measurement of gas formation, it was used for the further study of breakpoint chlorination. Although several researchers have studied the kinetics of breakpoint chlorination, all the previous studies were based on the measurement of chlorine species (free chlorine and chloramines) during the reaction period, which were incomprehensive. In this study, kinetics of conventional breakpoint chlorination was modified and the bromide effect during breakpoint chlorination was evaluated from the perspective of final products (nitrogen gas and nitrate) quantification by using the new apparatus. Rate constants of five redox reactions involved in breakpoint chlorination system were amended. The formation of nitrogen gas was found to be accelerated during breakpoint chlorination with the presence of bromide in the first 15 seconds. However, as the concentration of bromide increased, the extent of enhancement first increased and decreased subsequently. Such trend was mainly due to the discrepancy of intermediate products formation (e.g, NHBrCl, NHBr₂, etc.), which was attributed to the presence of different concentrations of bromide.