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East Bay Municipal Utility District operates a high-purity oxygen activated sludge wastewater treatment plant in Oakland, CA. During 1997 and 1998, the District was required to over-feed bisulfite chemical by 10 to 15 mg/L in order to maintain a bisulfite residual of 3 mg/L in the outfall. This paper reports on research to define the magnitude and kinetics of bisulfite consumption in the District's facilities.

Theoretically, dissolved oxygen can oxidize bisulfite residuals, but this reaction was not able to be reproduced with the District's effluent, indicating that some other causative mechanism must be producing the high bisulfite consumption.

The excessive bisulfite consumption occurred suddenly after the disinfection/dechlorination systems underwent major modifications in 1996. Based on the historical record, it was determined that the excess bisulfite consumption began immediately after a sharp drop in chlorine dosage. Based on this finding, it was tentatively concluded that the excess bisulfite consumption was associated with the very low chlorine/hypochlorite dosages begun in July, 1996. This conclusion was subsequently confirmed by observing bisulfite consumption declines with increasing hypochlorite dosage and subsequent increases with decreasing hypochlorite dosage. This phenomenon is not normally observed at treatment plants. Distinguishing characteristics of the EBMUD system include abnormally low chlorine residual (0.1–0.2 mg/L) and abnormally long bisulfite contact time prior to monitoring for effluent bisulfite residual.

This paper provides additional discussion, including a postulation that the bisulfite consumption is caused by biological activity in the slime layers inside the outfall pipeline. Such activity may be inhibited by slowly-dechlorinated chlorine residuals when the residual exceeds 2.5–3.0 mg/L.
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Document Type: Research Article

Publication date: 01 January 2000

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