The objective of this study was to provide a technique, based on the residence time distribution of a process, for modelling, assessing and improving flow in the processes of water and waste water treatment works. The technique should be accessible to the staff managing and operating the works. From a review of the literature, a preference was given for the experimental method used for determination of the tracer response, including choice of tracer and tracer addition and monitoring. Data analysis techniques were reviewed, and the method of time domain fitting was developed into a computer program, IMPULSE. IMPULSE provided a tool for analysis of residence time data, and removed the constraint of numerical complexity. Using the building blocks of IMPULSE, a realistic flow model can be constructed from tracer data and evaluated. IMPULSE allows a quantitative comparison of models proposed for a process, and provides the parameters of the models. These parameters quantify the non-idealities in a process. A knowledge of the non-idealities provides a basis for decision-making when modifying a process. The results of tracer experiments performed on some water and waste water treatment processes were analysed using IMPULSE. The results showed that collection of good experimental data was critical to the success of the analysis. It is proposed that a guide be produced which draws out the main points raised in the study, including collection of tracer data and use of IMPULSE. The guide should be accessible and easily understandable to the staff managing and operating water and waste water treatment works. / Thesis (M.Sc.Eng.)-University of Natal, Durban, 1995.