March 2009, Vol. 21, No.3


Debating “Facultative”

“Pattern Recognition” (January) contains a serious scientific error. The author states, “Facultative bacteria prefer free or ‘dissolved’ oxygen, but dissolved oxygen is scarce in wastewater streams and is consumed rapidly. However, sulfate — another sulfur compound found in most wastewater streams — contains oxygen atoms. The facultative bacteria attach to the sulfate compound and strip away the oxygen, thereby creating sulfides.”

 It has been well understood that the primary sources of hydrogen sulfide in wastewater are sulfate-reducing bacteria (SRB). Desulfovibrio are the primary SRB species, and these are strict anaerobes, not facultative bacteria.  SRBs are the overwhelming source of hydrogen sulfide; moreover, such bacteria will preferentially reduce nitrates in the presence of sulfates when nitrates are in abundance over sulfate. For this reason, nitrate salts of sodium or calcium are added to control hydrogen sulfide formation in wastewater.  Some facultative bacteria compete directly with SRB and are effective in reducing the their populations and activity, as referenced by E. Korenblum, et al., in Journal of Applied Microbiology 2005, 98, 667–675, among other articles.

It has been proven that sewer biofilm can be modified through the use of facultative bacteria to inhibit and suppress the formation of hydrogen sulfide. This process delivers numerous, ancillary cost savings and is applicable to the entire sewer system as opposed to the localized, conventional techniques he describes.

J. Rodney Dickerson
 Dickerson Consultants
Homer, LA


Author Bryan Haan responds: The use of the term “facultative” bacteria may have been an oversimplification of a sewer’s complex microbial metabolic environment; however, it implies that organisms can utilize alternative electron acceptors in their metabolic pathways. While it is true that SRBs are primarily strict anaerobes, meaning they cannot utilize molecular oxygen to generate energy, many SRB strains also be can considered “facultative” as they are capable of using nitrate or organic compounds in respiratory or fermentative reactions.

While the Korenblum article describes the ability of selected Bacillus species to produce biocides that may be active against the corrosive activity of certain strains of SRBs in closed-loop environments such as cooling towers and storage tanks, it is difficult to understand how a commercially prepared product of general facultative bacteria can be economically added in sufficient quantities to effectively overwhelm the diverse native flora found within a sewer, especially when this native population is being continuously replenished with every flush. It should be noted, however, that the article did not discuss or attempt to disprove that sewer biofilm can be modified through the use of facultative bacteria to inhibit and suppress the formation of hydrogen sulfide in sewers.

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