The turbulent spots in your wastewater collection system are directly linked to the rate at which your system will decay and the amount of money you will have to spend on rehabilitation. That is because turbulence plays a role in generating hydrogen sulfide (H2S) — a precursor of sulfuric acid that can slowly destroy concrete pipes.
Corrosion is caused by the displacement of oxygen (O2), the oxidation of sulfur, and the creation of sulfuric acid, both atmospherically and biologically. You can take affirmative steps to reduce corrosion by reducing H2S formation.
Effects of turbulence
In an abstract, Air water transfer of hydrogen sulfide: an approach for application in sewer networks1 the authors have studied the effects of turbulence in wastewater and concluded that the level of turbulence and the pH of the water directly affect the amount of H2S released into the atmosphere.
This means that septic water at lower pH will cause more H2S release than normal. On lift station force main discharge lines and flat, slow-moving trunk lines, this is especially prevalent. Rough manhole inverts, uneven pipeline bottoms, debris in pipes, and unrestrained internal drops in manholes are also large sources of H2S.
As H2S levels rise, oxygen is displaced. This causes the perfect atmosphere for the growth of anaerobic bacteria. The common perception is that the H2S actually attacks concrete. In part that is true, but H2S still needs scum from anaerobic bacteria to due so.
In an article titled, A Brief Primer on Microorganisms,2 the author describes a sticky substance, polysaccharide, secreted by anaerobic bacteria, that traps H2S as the bacteria grow. At this point, the H2S needs an oxidizer to turn into sulfuric acid. This is supplied by fermentation of biomaterial in the manhole in the form of nitrogen dioxide (NO2). This is one way that sulfuric acid is produced in sewers, but it is very slow.
Role of bugs
Another more rapid way of producing sulfuric acid in sewer atmospheres is through direct sulfur-oxidizing bacteria, Thiobacillus. This bacterium thrives in H2S-rich environments and is the key to sewer system deterioration.
In an article, The Importance of Hydrogen Sulfide, Thiosulfate, and Methylmercaptan for the Growth of Thiobacilli during Simulation of Concrete Corrosion,3 the author studied the effects of three corrosives: H2S, thiosulfate (S2O3), and methylmercaptan (CH3SH). The concrete test specimens were subjected to Thiobacillus bacteria, then to the three corrosives.
In the nine- to 12-month period that the specimens were exposed to a continuous atmosphere of the three corrosives, those subjected to H2S deteriorated severely. This was due to the action of the sulfur-oxidizing Thiobacillus and its ability to create sulfuric acid. This finding directly correlates to the effect of sulfur-oxidizing bacteria in sewer systems with high H2S levels and the presence of sulfuric acid.
Taking concrete measures
All the information provided in these reports falls in line with 20 years of observation during a career in wastewater system rehabilitation. These facts indicate three basic ways in which to combat corrosion in sewer systems:
1. Reduce turbulence by building internal drops in manholes to lessen aeration of sewage and lower the release of H2S, releasing force main discharges into or under flow levels on gravity sewer trunks, repairing offsets in sewer pipes, and smoothing manhole inverts to produce an even transition for flows through the manhole.
2. Minimize bacteria breeding grounds by jet-cleaning debris from sewer lines, removing deteriorated concrete from manhole walls by high-pressure water washdown, and having combination truck/jetter crews use heavily chlorinated water when washing manholes and jetting lines.
3. Lining pipelines with acid-resistant cured-in-place liners, using bacteria-reducing underlayments to rebuild manholes, and using acid-proof trowel-down coatings to line manholes.
Joseph A. Nuciforo Jr. is manhole rehabilitation supervisor with J.P.C.I. Services, a manufacturer of manhole and sewer pipe rehabilitation products in Mesa, Ariz. He can be reached at jnuciforo@jpciservices.com.
Sources
1 Yongsiri, C., Vollertsen, J., Rasmussen, M., Hvitvedjacobsen, T.: Air-water transfer of hydrogen sulfide: an approach for application in sewer networks; Water Environment Research, 2004; Vol.76 No.1, p. 81-88.
2 Owens, Sam R., A Brief Primer on Microorganisms: Ethanol Producer Magazine, November 2006.
3 Sand, Wolfgang; Importance of Hydrogen Sulfide, Thiosulfate, and Methylmercaptan for Growth of Thiobacilli during Simulation of Concrete Corrosion: Applied and Environmental Micro-biology, July 1987, p. 1645-1648, ©1987, American Society for Microbiology.
