AWWA WQTC60654

The Stafford Springs Water Treatment Plant (SSWTP) is a 1.0-MGD facility owned andoperated by the Connecticut Water Company (CWC). The plant utilizes Pulsator clarificationand filtration through four feet of mono-media granular activated carbon (GAC). The GAC filtermedia is changed out every three years for taste and odor control.A previous study done by Metcalf & Eddy (M&E) noted that prechlorination was formingsignificant levels of disinfection byproducts (DBPs) within the plant and recommended the useof chlorine dioxide in lieu of chlorine for preoxidation, predisinfection, and DBP control.Subsequently, M&E and CWC conducted a two-season, full-scale trial at the SSWTP in whichchlorine dioxide was utilized as a substitute for prechlorination.During the full scale trial, the effectiveness of chlorine dioxide for preoxidation was evaluated atdoses from 0.8 to 3.0 mg/L, and the distance that a free residual could be sustained through theplant was determined. Distribution system DBP levels and chlorite levels were also examined.It was possible to carry a chlorine dioxide residual through the clarifier, thereby providing asignificant disinfection barrier; however, a relatively high dose of chlorine dioxide was needed.Disinfection byproduct levels in the distribution system declined significantly as a result of thesubstitution of chlorine dioxide for pre-chlorine.This study was unique in that the chlorite removal ability of both newer and spent GAC filtermedia was evaluated side by side in a full-scale application. In order to examine the influenceof GAC age and natural organic matter (NOM) saturation on chlorite removal, the media intwo of their three GAC filters was replaced before the start of the full-scale trial. The 3-year oldmedia in the third filter was not replaced, thereby providing an opportunity to evaluate chloriteremoval through new GAC filter media with filter media close to NOM saturation. The filtercontaining the newer GAC media removed the majority of chlorite, while the filter containing 3year old spent media exhibited a limited capacity for chlorite removal which continued todecline throughout the study.The study concluded that chlorine dioxide could be a viable alternative to prechlorination;however, it was not considered favorable to carry a chlorine dioxide residual through the plantdue to the high doses required. The GAC filters were expected to control chlorite levels andcompliance with the 1.0 mg/L chlorite maximum contaminant level (MCL) in finished water;however, modification of the GAC filter media replacement schedule was recommended. Includes 7 references, tables, figures.

Product Details

Edition:

Vol. - No.

Published:

11/15/2004

Number of Pages:

20

File Size:

1 file , 680 KB