May 2009, Vol. 21, No.5

Letters

Continuing the ‘Facultative’ Debate

J. Rodney Dickerson

The response [regarding facultative bacteria] offered by author Bryan Haan (Letters, March) continues the scientific inaccuracy.

Bergey’s Manual of Determinative Bacteriology defines “facultative” as follows: “An organism that can grow well both in the absence of oxygen and in the presence of a level of oxygen equivalent to that in an air atmosphere (21% oxygen). Some are capable of growing aerobically by respiring with oxygen and anaerobically by fermentation [anaerobic respiration is also possible]; others have a strictly fermentative type of metabolism and do not respire with oxygen.”

Calling obligate anaerobes facultative is not an “oversimplification.” Sulfate-reducing bacteria (SRB) are not primarily strict anaerobes; they are only strict anaerobes. Their ability to reduce sulfate or other oxygen containing compounds such as nitrate salts does not make them facultative.

Supplying nitrate salts to SRB as an alternative for sulfate in the metabolic pathway does nothing to reduce the presence of SRB. On the contrary, when SRB are supplied nitrates, they proliferate. This proliferation explains the increase in nitrate demand over time with continued application of nitrate salts. Moreover, if the nitrate demand is not met, the SRB revert to sulfate reduction when nitrates are depleted, and hydrogen sulfide production resumes at increased rates in proportion to the increase in SRB populations.

Chemical addition does not deal with the root cause of hydrogen sulfide production, namely the biofilm microbiology.

The author is quite correct that commercially prepared products sold on a unit basis for application by purchasers have not been proven effective at sustaining modification within sewer biofilm. There are, however, proprietary formulations prepared by proprietary means and sold as an engineered solution under a performance-based service agreement that have proven effective.

The holistic use of facultative, spore-forming bacteria in the sewer system in place of chemistry increases production of volatile fatty acids (VFAs) instead of destroying or preventing VFA formation. While it may be difficult to understand how facultative bacteria can accomplish suppression of hydrogen sulfide formation, the implication by the author’s misapplication of proper descriptions of the microbiology could lead many to believe otherwise and should be corrected.

J. Rodney Dickerson
Principal consultant
Dickerson Consultants, LLC
Homer, La.

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