Review of diagnostic tools to investigate the physical state of rapid granular filters

This paper reviews diagnostic tools that can be used at waterworks to investigate the physical and operational state of rapid granular filters. Diagnostic tools can be of interest for the Water Safety Plans of WHO to monitor filters in a proactive manner. The review considers conventional and state of the art tools and tools currently in development or conception stage. The development status, usability and availability of each tool and procedure are discussed. Some conventional, simple and low-tech tools are available which can rapidly provide both qualitative and quantitative information. However, it is difficult to relate this information to guideline values or design criteria because the tools are often not documented and are most often used with past experience or rules of thumb. 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 paper discusses how the filtration process can be optimized once a malfunction is recognized by the diagnostic tools, and finally, research and development needs are identified.