Field Studies at EBMUD

EBMUD uses combined chlorine for disinfection. The purpose of the field study at EBMUD was to evaluate dechlorination when chemicals were placed either as tablets or as powder within the path of the chlorinated water. The following dechlorination chemicals were evaluated  

Several series of field tests were conducted under different conditions. Row was discharged from a hydrant on EBMUD s water distribution system, through a fire hose and onto a fairly level paved and curbed street close to the curb. The water flowed down the street, into a drop inlet 160 ft downstream. The drop inlet led to an onsite storm drain system at EBMUD s wastewater treatment plant that flows into the headworks of the plant. 

Chlorine residual concentrations of the water upstream and downstream of the dechloramination chemical feed points were measured using a Hach Chlorine Pocket Colorimeter. Temperature, dissolved oxygen (DO), pH, and oxidation-reduction potential (ORP) were also measured during the field study. 

The test procedure consisted of initially placing a single tablet of sodium sulfite, then 2, 4, 16, and 28 tablets of sodium sulfite in the water flow path 10-ft downstream of the flow control valve and meter. The flow rates ranged from 100 up to 500-1- gpm. Samples were collected 150 ft downstream of the tablets. As shown in Fig. 3, one tablet effectively reduced the chlorine residual of the flow it came in to contact with, to below 0.1 mg/L for 45 min at a flow rate of 100 gpm. The tablet was not fully consumed, but became ineffective after approx 1 h and the chlorine concentration rose again. 

When 12 tablets were placed across the flow of 100 gpm, the chlorine concentration decreased below the detection limit (0.1 mg/L) within 5 min. It remained below the detection limit even after 60 min. In the next test, initially a flow rate of 300 gpm was maintained and 16 tablets were placed across the flow. Within 5 min the chlorine concentration decreased to below detection limit. After 10 min, the flow rate was increased to 450 gpm. At this increased flow rate, the residual chlorine concentration increased to values of 0.6-0.8 mg/L, well above the detection limit of 0.1 mg/L (which is the allowable discharge hmit in many locations), within 25 min (Fig. 3). This indicates that the flow rate of chlorinated waters can significantly impact the efficiency of dechlorination operations. Higher flow rates may not provide sufficient contact time for dissolution of tablets into the stream. After approx 40 min, the number of tablets was increased to 20. This decreased the residual concentration to below detection limit within 5 min. The increase in the number of tablets probably provided an enhanced contact area and better dissolution of the tablets into the flow, resulting in a decrease in the residual chlorine concentrations. 

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