Quantification and verification of the QUEST method as a tool for rapid assessment of exfiltration from gravity sewers

Exfiltration from sewers poses negative environmental effects and health risks. Quantification of exfiltration is the first step towards solving these detrimental effects. QUEST/QUEST-C (Quantification of Exfiltration from Sewers with Artificial Tracers) methods were tested in this research. Two different tracers are used by these methods, in order to calculate mass balances rather than flow measurements. The tracers are injected in two different locations, and measured in one common downstream point. QUEST is based on pulse injection whereas injection in QUEST-C is continuous. Both methods are thought to be insensitive to errors in flow measurements. Nevertheless, they were developed theoretically and implemented straight away in real sewers without prior examination under controlled conditions. This was the goal of the current work, namely, to assess both methods under controlled conditions in a pilot-scale sewer rig. Under steady flow both methods were found accurate (average error ≤2%). However, out of the two, only QUEST-C was found to be a feasible, reliable and practical method for quantifying exfiltration from sewers under unsteady flow conditions (average error ±20%). In order to minimise errors, QUEST-C tests should be run for several hours at a time. QUEST, in contrast, did not present good reliability and accuracy and thus is not recommended for implementation in actual sewers, characterised by unsteady flow.