Modelling water quality for water distribution systems

Maintaining water quality in distribution systems has become a prominent issue in the study of water networks. This thesis concentrates on disinfectant and particle counts as two important indicators of water quality. The models discussed in this work are based on data collected by the author. The experimental set-up and procedure are described and observations of particle counts, particle counter size distributions, monochloramine as disinfectant, temperature, heterotrophic plate counts and epi uorescence microscopy counts are reported. A model of the response of particle counts to an increase in ow is developed. This model is obtained from speci cation derived from the data and assumptions, and is validated by its interpretability and its t to data. A local shear-o density and an initial bio lm shedding pro le were introduced and thus a linear model for this part of the water quality dynamics could be obtained. A procedure for the identi cation of the parameters of the local shear-o function and for the determination of the bio lm shedding pro le is presented. This pro le can be used to provide information about the status of the distribution system in terms of shear-o from the bio lm on the pipe walls. Monochloramine decay dynamics are investigated. The chlorine meter data is preprocessed with the help of titration data to correct meter drift. The data is then used in calibrating two di erent possible chlorine models: a model with a single decay coe cient and a model with bulk decay coe cient and wall demand (as used in Epanet). Important di culties in identifying these parameters that come about because of the structure of the models are highlighted. Identi ed iii decay coe cients are compared and tested for ow, inlet chlorine and temperature dependence. The merits and limits of the approach to modelling taken in this work and a possible generalisation are discussed. The water industry perspective and an outlook are provided. iv Contribution to Knowledge The accompanying thesis is based on work carried out by the author at Brunel University between October 1995 and December 1998. All work and ideas in this thesis are original, unless otherwise acknowledged in the text by references. The work has not been submitted for another degree in this university, nor for the award of a degree or diploma at any other institution. In the following, a list of the contributions of this thesis to knowledge in the elds of water quality modelling for water distribution systems and system identi cation is provided. 1. A qualitative characterisation of data collected by the author shows the structure of the particle counts data and highlights relationships between particle counts and bacteriological data. 2. The preprocessing of pipe rig data, in particular the chlorine meter data, is addressed in some detail. It gives novel insight into the degree of uncertainty associated with chlorine data and presents an original procedure for the correction of these errors. 3. The approach to the development of the system structure from data that is taken in this thesis is novel. The modelling work of this thesis is based on data and is obtained with the help of speci cations and assumptions and validated by tting and physical or biological interpretation. 4. The particle counts model, resulting from the approach taken in this thesis, is another important new contribution. The concept of a local shear-o v function allows the development of a linear model that describes shear-o from bio lms that comes about due to a ow increase. 5. The particle counts model describes dynamic transient bio lm detachment in terms of particle counts, a process that previously found little attention and no mathematical description in the literature. 6. The determination of the bio lm shedding pro le opens a new way for testing distribution systems for critical areas in terms of particle shear-o . Thus, the state of the system can be investigated. 7. Finally, through the comparison of the parameter estimation of two possible monochloramine decay models, novel insights in the limitations and the usefulness of these models are presented. Parts of chapter 1, chapter 3, work related to section 8.2 and results from previous experiments were published at the Doctoral Research Conference, Department of Manufacturing and Engineering Systems, Brunel University [96]. Parts of chapter 4 were included in the Proceedings of the International Conference on Bio lms in Aquatic Systems [98]. Earlier results of the work presented in section 5.3 and chapter 7 were published at the International Conference on Computing and Control for the Water Industry [97]. vi