Impact of fluctuations in gaseous H2S concentrations on sulfide uptake by sewer concrete: The effect of high H2S loads

Abstract The acid production from the oxidation of hydrogen sulfide (H 2 S) in sewer air results in serious corrosion of exposed concrete surfaces in sewers. Large fluctuations of gaseous H 2 S concentrations occur in sewers due to the diurnal profiles of sewage flow and retention times and the necessity of intermittent pumping of sewage from pressure pipes into gravity pipes. How the high concentrations of H 2 S due to these events may affect H 2 S uptake and subsequent corrosion by concrete sewers is largely unknown. This study determined the effect of short- and long-term increases in H 2 S levels on the sulfide uptake rate (SUR) of concrete surfaces with an active corrosion layer. The results showed that during the high load situation the SUR increased significantly but then decreased (compared to the baseline SUR) by about 7–14% and 41–50% immediately after short- and long-term H 2 S high-load periods, respectively. For both exposure conditions, the SUR gradually (over several hours) recovered to approximately 90% of the baseline SUR. Further tests suggest multiple factors may contribute to the observed decrease of SUR directly after the high H 2 S load. This includes the temporary storage of elemental sulfur in the corrosion layer and inhibition of sulfide oxidizing bacteria (SOB) due to high H 2 S level and temporary acid surge. Additionally, the delay of the corrosion layer to fully recover the SUR after the high H 2 S load suggests that there is a longer-term inhibitive effect of the high H 2 S levels on the activity of the SOB in the corrosion layer. Due to the observed activity reductions, concrete exposed to occasional short-term high H 2 S load periods had an overall lower H 2 S uptake compared to concrete exposed to constant H 2 S levels at the same average concentration. To accurately predict H 2 S uptake by sewer concrete and hence the likely maximum corrosion rates, a correction factor should be adopted for the H 2 S fluctuations when average H 2 S levels are used in the prediction.