November 2009, Vol. 21, No.11

Letters to the Editor

Letters to the Editor - Delving Into Activated Sludge

In the June issue, we asked what subtopics of the activated sludge process you’d like to see us cover. In response, Benji Kojo Addo, a project engineer in the Public Works Facilities Group at Pate Engineers Inc. (Houston) asked if we had received any submissions on the topic of recycling mixed liquor into force mains upstream of the plant headworks.

Addo indicated that this practice was used primarily to control odor. He writes, “This concept is pretty amazing to me, and I wonder if any submittals have been made from a process standpoint — how this actually works.”

To date, neither WE&T nor Operations Forum has received any submissions on this topic, but we agree that the topic is one we’d like to learn more about.

Readers, do you have experience with using this technique for odor control? Let us know by sending an e-mail to 

Quiz Correction

Dick Apgar, a supervisor in the King County, Wash., Wastewater Treatment Division, caught an error in the first question of the Certification Quiz from the October issue. The question asks, "What bacterium is responsible for hydrogen sulfide gas production in sewer lines?"

Apgar writes, “The answer to question No. 1 is given as Thiobacillus bacteria. But Manual of Practice 25 (Control of Odors and Emissions from Wastewater Treatment Plants) from the Water Environment Federation indicates that it should be Desulfovibrio. Thiobacillus does not produce sulfide; it actually oxidizes hydrogen sulfide to sulfuric acid, causing corrosion problems.”

We regret the error.