Understanding sewer corrosion

Everyone knows that as far as sewers go they are usually ‘out of sight and out of mind’. But as anyone who has been personally involved in a sewer malfunction it is a mortifying and scary experience. It shows us how thin is the membrane that protects us from reverting to medieval levels of public health. For the UK water industry, responsible for maintaining the country’s largely Victorian sewer infrastructure, it’s also clear that this is one of the most neglected and costly of their responsibilities since being out of sight also means it is difficult to ascertain the local condition of these vital conduits often until it’s too late.

How exciting is it therefore that work done by Mark T. Hernandez of the University of Colorado, Boulder, reported in Environmental Science & Technology and described in a recent issue of the Economist (14 June 2014) applied the latest bioscience understanding and sensor technology to described a methodology to warn when and where sewers may potentially fail.

Prof. Hernandez has been looking at the gases associated with concrete corrosion in sewage pipes. He has focused on two in particular, hydrogen sulphide, and carbon dioxide. Although hydrogen sulphide contributes strongly to the odour in sewage it does not appear to directly affect pipe decay. However the presence of particular bacteria will under certain conditions convert it into sulphuric acid which does damage pipes!

Photo: Graphic from the article by Hernandez and his colleagues in Environmental Science & Technology.
Photo: Graphic from the article by Hernandez and his colleagues in Environmental Science & Technology.

Studies showed that sewage above areas of pipe corrosion were, as would be expected, highly acidic (pH 1). The biodiversity of the bacteria in that region was also much lower than above areas where there was no damage. In this impoverished ecosystem a bug that turns hydrogen sulphide into sulphuric acid predominated showing how the corrosion process becomes increasingly aggressive. Importantly in the area of the corroded pipe, gas measurements showed not only high hydrogen sulphide concentrations (above 100 ppm) but also very high carbon dioxide levels (10,000 ppm).

Analysis based on these results should lead to an extremely accurate diagnostic test as to where damage is occurring. We hope water companies will use this scientific result to monitor their assets rather than wait until there is a catastrophic structural collapse when there is no recourse other than to digging holes.

Roger Ford

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