Exfiltration from sewers - is it a serious problem?

This paper contends that estimates of exfiltration leakage from sewers, and the problems arising from it may be too high due to an underestimation of the “self-repairing” action of sewage and sewage sediments in raw sewage. Two aspects of a continuing programme of research are reported; (i) the measurement of exfiltration rates from a range of defects in a sewer pipe with pipe bedding effects simulated by a dry gravel trench surround to the pipe, and (ii) an assessment of the persistence of pathogens in the gravel and soil beneath the test rig pipe, using coliforms as a biological indicator. The results show that the introduction of a gravel surround resulted in more rapid and effective sealing across the entire range of defects tested compared to previously performed experiments with the defects discharging to air. Complete sealing eventually occurred on every run for a 10 mm x 10 mm defect situated at the invert of the sewer and the lowest recorded levels in each experiment for a 10 mm wide radial defect were of the order 10-3 to 10-4 l s-1. These results have been scaled-up to estimate leakage rates in lengths of sewers and sub-catchments and levels significantly lower than previously estimated are indicated. Additionally, the pilot experiment to investigate the fate of biological contaminants in the exfiltrate suggests rapid reduction in microorganisms levels beneath the sewer pipe.