Field Dechlorination Tests at Tacoma Waters

The field study indicated that, when no chemical was added, free chlorine concentration in the water did not decrease significantly. Chlorine concentrations decreased from 1.2 to approx 1.0 mg/L after a travel of 450 ft (4 min, 10 s) on the semi-paved, asphalt road. When stoichiometric concentrations of dechlorination chemicals were added, most of the chemicals neutralized chlorine instantaneously. Samples analyzed 2 ft (reaction time approx 1 s) downstream of the diffuser contained less than 0.1 mg/L of chlorine. An exception to this trend was calcium thiosulfate. When calcium thiosulfate was added, chlorine concentrations decreased to 0.2 mg/L within 2 ft and residual chlorine was reduced to less than 0.1 mg/L after a travel of 200 ft (reaction time 1 min 40 s). 

When twice the stoichiometric concentration of chemicals was added, residual chlorine in all the tests (including calcium thiosulfate) decreased to below 0.1 mg/L immediately (approx 2 s). 

Test location Disinfectant nsed Chemicals tested Form of chemicals used Chemical feed conditions Initial chlorine (mg/L) pH Source of water Flow rate (gpm)  

Tacoma Waters, Tacoma, WA Free chlorine Sodinm metabisnlfite Sodinm snlfite Sodinm thiosnlfate Calcinm thiosnlfate Ascorbic acid Sodinm ascorbate 1% solution Stoichiometric concentrations and twice the stoichiometric concentrations needed for dechlorination 1.2 8.9 Surface water 300 

Bureau of Water Works, Portland, OR Combined chlorine Sodinm bisnlfite Sodium sulfite Sodium thiosulfate Calcium thiosulfate Ascorbic acid Sodium ascorbate 1% solution Stoichiometric concentrations needed for dechlorination 1.1 8.0 Surface water 300 

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