Evaluation of testing procedures for real-scale sewage pipes

Tests to determine the tightness of wastewater pipes can in some cases produce results that are worthy of discussion. Therefore, testing procedures for real-scale sewage pipes used for house connections were evaluated and data were statistically analysed. The results of the investigation showed that leaky pipes are detected with a very high degree of reliability by all leak test methods. The test methods are also robust against errors by expert testers and deviations from the test specifications. In contrast, tight pipelines can also be incorrectly classified as ‘leaking’ (test failed) to a significant extent during leak tests. Even for the more reliable test methods, i.e. air overpressure, air underpressure and water with low test pressure, a tight pipe is incorrectly classified as leaking (false positive) in one out of 10 cases (10%). The highest false-positive rate was 20% for water with high test pressure. In addition to the leak test methods, the quality of the visual inspection was also analysed. Here it was found that visual inspection is not sufficiently reliable for determining the tightness of pipelines above the groundwater level. Error rates of approximately 50% were found for the detection of tight and leaky pipelines.