Rapid granular filters are the most commonly used filters in drinking water treatment plants and are the focus of this PhD study. They are usually constructed with sand, anthracite, activated carbon, garnet sand, and ilmenite and have filtration rates ranging from 3 to 15 m/h. Filters are often the last barrier against disinfection resistant protozoan pathogens and this has led to increased regulation of the filtration process. To be able to produce high-quality filtrate in a constant and reliable manner while meeting stricter drinking water guideline values, it is important to be able to optimize the design and operation of filters. However, the operation of the filtration process is considered to be easy and the design and control of filters are still based on empirical values, rules of thumb, simple guidelines, or past experience To optimize the use of granular filters, it is necessary to be able to observe the physical state of the filter. The aim of this PhD study is to contribute to the understanding and optimization of the granular media filtration process. The focus of the work is to develop methodologies and diagnostic tools to analyze the physical state of rapid filters and improve their use and performance. A review of diagnostic tools for rapid granular filters has uncovered both conventional tools, state of the art tools and tools currently in the development or conception stage. The use of the tools for investigating a filter is described. Simple diagnostic tools can be used in a preliminary investigation to observe the symptoms of filter failure. These observations can then motivate a preliminary diagnosis and then appropriate diagnostic tools can be selected and a thorough analysis conducted. From the information obtained, the preliminary diagnosis can be revised and mitigation options prescribed. The diagnostic tools are then used again to verify the efficiency of the solution applied. If the problem is not solved the whole process starts again. These tools are of significant interest for the development of the Water Safety Plans recommended by WHO to monitor filters in a proactive manner. They can also be used to optimize the filtration process. However, further research is necessary to relate the information obtained through the tools to specific causes. New tools such as the total dissolved gas probe, salt tracers and ammonium profiles are presented. Potential tools from the soil and groundwater field such as the hand penetrometer, time domain reflectometry and ground penetrating radar are suggested. The heterogeneity of rapid filters has not been previously studied at full scale. Filter heterogeneity is not desirable because it makes it difficult to achieve constant and reliable filter performance, and water quality compliance. A salt tracer tool is developed to be used in full-scale filters to investigate the heterogeneity of the filter bed. The tool allows the pore velocity to be estimated in different locations of the filter bed during the duration of a filter run. Similarly, despite the importance of nitrification in groundwater treatment, the removal of ammonium and the determination of the kinetics of nitrification have been insufficiently researched in full-scale filters. A tool is developed to describe nitrification quantitatively on full-scale filters under real conditions with varying inlet flow and concentrations. Experiments conducted in full-scale filters demonstrate that rapid granular filters cannot be considered homogeneous. The estimated pore velocities were shown to be variable in both space and time. A model was used to demonstrate that filter heterogeneity can result in higher filter outlet contaminant concentrations. An experiment also showed that nitrification in full-scale filters is heterogeneous. The ammonium profiles exhibited variation in time and in space, vertically and laterally within the filter. The nitrification rate constants varied randomly in time and it was not possible to determine a clear nitrification reaction order. The cause of the observed nitrification heterogeneity was discussed. Clogging in the top layer of the porous media in a pilot-scale filter was shown to be a possible explanation for the unexpected zero-order nitrification rate. It was also observed that nitrification in the studied full-scale filter was mass transfer limited because the local first-order nitrification rate constants were linearly related to the local pore velocity. By introducing the use of diagnostic tools to Water Safety Plans, new monitoring measures with specific critical limits can be established that can provide an early warning of deteriorating filter performance. However, research on the use of diagnostic tools has to be conducted to document and standardize each procedure, and to relate the information provided by the tools to guideline values or design criteria, and to specific filter failures. Moreover, further research is necessary to develop promising tools such as the hand penetrometer, time domain reflectometry and ground penetrating radar.