- Anaerobic conditions (redox <-200 mV for most sewer pH) creates conditions forming organic acids from fermentation of organics. If there is sulfate present (and there often is), anaerobic bacteria such as Desulvibrio sp. begin to used sulfate to degrade the organic acids in the sewage. The result is a sufide ion in the water (S=). As the pH drops with organic acid production, the S= is converted to H2S and escapes the water phase to enter the head space in the pipe.
- Above the water phase is sufficient oxygen for aerobic activity. With abundant hydrogen sulfide in the atmosphere and little organics, we see the growth of Thiobacillus sp. that grow by chemically converting reduced sulfides into sulfate. Unfortunately, this step has a final product of H2SO4 (sulfuric acid) and the Thiobacillus sp thrive at pH between 3 - 5 which causes corrosion. And, lowers water pH below releasing more H2S into the head space.
In Houston, summer temperatures are just around the corner and collection system water temperatures will be consistently above 20oC where sulfate reducing bacteria (SRB) will flourish in anaerobic sections of the collection system. The following shows the steps in sulfide formation and natural sulifde oxidation can further corrode concrete and metals by forming sulfuric acid.
Erik Rumbaugh has been involved in biological waste treatment for over 20 years. He has worked with industrial and municipal wastewater facilities to ensure optimal performance of their treatment systems. He is a founder of Aster Bio (www.asterbio.com) specializing in biological waste treatment.
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