White Paper: Odor & Corrosion Control Microbiology; Metabolic Pathways Of Odor & Corrosion Potential

Odor and corrosion control within the wastewater treatment system is critical for quality of life and economic reasons. Odor and corrosion are primarily associated with the production of hydrogen sulfide gas. Hydrogen sulfide (H₂S) is produced only by sulfate-reducing bacteria (SRB) under anaerobic conditions using sulfate as the final electron acceptor. Several of these types of bacteria are commonly found in the human colon (Desulfovibrio, Desulfotomaculum, Desulfobulbus, Desulfomonas, and Desulfobacter) (Gibson et al, 1991). Corrosion of concrete sewers is associated with the biological oxidation of H₂S to sulfuric acid by bacteria of the genus Thiobacillus (Padival et al, 1995).

The SRB are a unique physiological group of bacteria in that they are capable of using sulfate as the final electron acceptor in respiration (Barton, 1995). Bacteria outside of this group cannot use sulfate as the final electron acceptor in respiration and do not produce H₂S external to the cell, ever. The SRBs are a very hardy group of anaerobic bacteria in that they are capable of limited oxygen metabolism, have a broad range of metabolic capabilities, and many are motile. Most of the SRB species are mesophilic, capable of growth between 20-42°C and can be found growing in pH ranges of 4.0-9.5. Desulfovibrio sp. are SRBs that are incapable of forming spores, motile through single polar flagella, and have a generation time of around 180 minutes. Desulfotomaculum sp. are SRBs that are capable of forming spores, motile through peritrichous flagella, and have a generation time of around 540 minutes.

The principal end products of SRB respiration using sulfate are H₂S and CO₂ at a ratio of 1:2, respectively, in cultures where acetate accumulates from lactate catabolism. Most SRBs are also capable of using many other compounds as a final electron acceptor. Some Desulfovibrio and Desulfobulbus species are able to utilize nitrate as an electron acceptor producing ammonia and water as end products. Depending on the species, in the presence of both electron acceptors either sulfate (Widdel and Pfennig, 1982) or nitrate (Seitz and Cypionka, 1986) may be the preferred electron acceptor, or both may be reduced at the same time (McCready et al., 1983). The SRB's have considerable capability and diversity in using various compounds for electron donors. In the absence of sulfate, certain strains of SRB can use a single carbon compound as both the electron donor and an electron acceptor by a process called dismutation. The SRB's are generally thought to be obligate anaerobes but recently it has been determined that they can tolerate oxygen for short periods of time and even proliferate in the presence of oxygen (Cypionka, 2000). This gives SRBs a slight advantage over more oxygen-sensitive anaerobes. However, oxygen respiration has not been found to be long-term, in fact only one doubling has been observed in homogenously aerated systems (Marschall et al, 1993).

Thiobacillus sp. are typically found in high numbers in severely and moderately corroded sewers but not in minimally corroded sewers. Thiobacillus thiooxidans occupies a constrained ecological niche that is characterized by extremely low pH and low-organic conditions. It assimilates carbon as carbon dioxide and obtains energy exclusively from aerobic oxidation of reduced sulfur forms such as H₂S (Padival et al, 1995; Vincke et al, 2001). Other Thiobacillus sp. that are capable of growing at neutral pH allow the local pH to be lowered to a point that T. thiooxidans can tolerate. Once colonization occurs by T. thiooxidans and the pH is lowered to conditions not tolerated by microbial competitors, growth is unchallenged and corrosion is unchecked. These organisms will not proliferate if H₂S is not present and they are unable to modify the environment to their preferred niche. Slow growth rates place the thiobacilli at a competitive disadvantage in a neutral pH environment.

Typical domestic wastewater contains high sulfate concentrations (ca. 100 to 1000 ppm) relative to organic carbon concentrations, but almost no nitrate or nitrite (Ito et al., 2002). Thus, SRBs thrive in this environment while other anaerobic bacteria struggle with the low oxygen and nitrate levels and Thiobacillus sp. become dominant in aerobic areas such as the pipe crown and the sewer headspace. The SRBs become the dominant bacteria in the anaerobic parts of the wastewater system and begin to generate H₂S in quantities sufficient to create malodorous conditions and Thiobacillus sp. become dominant in aerobic parts of the system and create a corrosive environment.