Abstract
M.Ing.
The modelling of water quality in distribution systems is a relatively new field. It has become necessary to attend to the problem of water quality at the consumer's tap because water quality after treatment is not a guarantee for the water that reaches the consumer. It is increasingly important to design and manage distribution systems carefully to ensure safe drinking water up to the consumer's tap. This report gives the results and a discussion thereof on the chlorine decay in laboratory batch tests as well as on pipeline systems. Significant contributions were made to the different objectives of the study. Standardised test procedures for both free chlorine and monochloramine were developed. It was found that the n th-order, 1 parameter model described the decay of chlorine significantly better than did the generally assumed first order mathematical model. It was also found that the reaction orders are different for different water sources, and even different for the same water source at different times. It was also clear that the rate constants are dependent on temperature - the higher the temperature, the higher the decay rate constants. The reaction order for free chlorine varied from 0.36 to 1.22 with no apparent underlying pattern. For monochloramine, the reaction order fell into two distinctively different categories. For monochloramine decay up to 24 hours, the reaction order varied from 0.02 to 0.05. For monochloramine decay from 24 hours onwards, the reaction order varied from 0.92 to 0.95. As was expected, the free chlorine decay rates obtained in pipelines were higher than in parallel bulk decay rate tests. On the assumption that the bulk decay reaction order is equal to one, the mass transfer coefficient between the bulk flow and the pipe wall varied between 0.0065 m/h and 0.0457 m/h. Further research is highly recommended on the effect of secondary and tertiary chlorination as well as on more direct methods of estimating pipe wall-related chlorine reaction constants.