Biogenic sulfide corrosion

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Biogenic sulfide corrosion is a bacterially mediated process of forming hydrogen sulfide gas and the subsequent conversion to sulfuric acid that attacks concrete and steel within wastewater environments. The hydrogen sulfide gas is biochemically oxidized in the presence of moisture to form sulfuric acid. The effect of sulfuric acid on concrete and steel surfaces exposed to severe wastewater environments can be devastating.[1]

Environment[edit]

Corrosion may occur where stale sewage generates hydrogen sulfide gas into an atmosphere containing oxygen gas and high relative humidity. There must be an underlying anaerobic aquatic habitat containing sulfates and an overlying aerobic aquatic habitat separated by a gas phase containing both oxygen and hydrogen sulfide at concentrations in excess of 2 ppm.[2]

Conversion of sulfate SO42- to hydrogen sulfide H2S[edit]

Fresh domestic sewage entering a wastewater collection system contains proteins including organic sulfur compounds oxidizable to sulfates and may contain inorganic sulfates.[3] Dissolved oxygen is depleted as bacteria begin to catabolize organic material in sewage. In the absence of dissolved oxygen and nitrates, sulfates are reduced to hydrogen sulfide as an alternative source of oxygen for catabolizing organic waste by sulfate reducing bacteria (SRB), identified primarily from the obligate anaerobic species Desulfovibrio.[2]

Conversion of hydrogen sulfide to sulfuric acid H2SO4[edit]

Some hydrogen sulfide gas diffuses into the headspace environment above the wastewater. Moisture evaporated from warm sewage may condense on unsubmerged walls of sewers, and is likely to hang in partially formed droplets from the horizontal crown of the sewer. As a portion of the hydrogen sulfide gas and oxygen gas from the air above the sewage dissolves into these stationary droplets, they become a habitat for sulfur oxidizing bacteria (SOB), of the genus Acidithiobacillus. Colonies of these aerobic bacteria metabolize the hydrogen sulfide gas to sulfuric acid.[2]

Corrosion[edit]

See also: Sulfidation

Sulfuric acid converts the calcium hydroxide in concrete to calcium sulfate. This change simultaneously destroys the polymeric nature of calcium hydroxide and substitutes a larger molecule into the matrix causing pressure and spalling of the adjacent concrete and aggregate particles.[4] The weakened crown may then collapse under heavy overburden loads.[5]

Prevention[edit]

Sewage flows more rapidly through steeper gradient sewers reducing time available for hydrogen sulfide generation. Providing good ventilation of sewers can reduce atmospheric concentrations of hydrogen sulfide gas and may dry exposed sewer crowns. Acid resistant materials like PVC or vitrified clay pipe may be substituted for concrete or steel sewers.[6]

See also[edit]

Sources[edit]

  • Hammer, Mark J. Water and Waste-Water Technology John Wiley & Sons (1975) ISBN 0-471-34726-4
  • Metcalf & Eddy Wastewater Engineering McGraw-Hill (1972)
  • Pomeroy, R.D., 1976, "The problem of hydrogen sulphide in sewers". Published by the Clay Pipes Development Association
  • Sawyer, Clair N. & McCarty, Perry L. Chemistry for Sanitary Engineers (2nd edition) McGraw-Hill (1967) ISBN 0-07-054970-2
  • United States Department of the Interior (USDI) Concrete Manual (8th edition) United States Government Printing Office (1975)
  • Weismann, D. & Lohse, M. (Hrsg.): "Sulfid-Praxishandbuch der Abwassertechnik; Geruch, Gefahr, Korrosion verhindern und Kosten beherrschen!" 1. Auflage, VULKAN-Verlag, 2007, ISBN 978-3-8027-2845-7

Notes[edit]

  1. ^ O’Dea, Vaughn, “Understanding Biogenic Sulfide Corrosion,”MP (November 2007), pp. 36-39.
  2. ^ a b c Sawyer&McCarty p.461&462
  3. ^ Metcalf & Eddy p.259
  4. ^ USDI pp.9&10
  5. ^ Hammer p.58
  6. ^ Hammer pp.58,326&327

Pomeroy's report contains errors in the equation: the pipeline slope (S, p. 8) is quoted as m/100m, but should be m/m. This introduces a factor of 10 underestimate in the calculation of the 'Z factor', used to indicate if there is a risk of sulphide-induced corrosion, if the published units are used. The web link is to the revised 1992 edition, which contains the units error - the 1976 edition has the correct units.