(1) disinfectant residual maintenance, (2) prevention of biological regrowth, and (3) disinfection after open reservoirs. Utilities practicing booster disinfection ranged in size from 0.14 to 830 MGD, with an average of 55 MGD. Most (55 percent) of the booster stations operated with a constant delivery dose, although 35 percent used flow-pacing or residual pacing to adjust dose. A few stations used a time-dependent set-point regime. Fifty-seven (57) percent of the stations were controlled manually, 33 percent were automated, and ten percent were controlled remotely with the aid of SCADA. Half of the stations with automatic control also had remote alarms. Examples provided in the report show that incorporation of decay rate and THM formation data is fundamental to predicting whether there will be any net gain in maintenance of residual and formation of THMs when disinfectant application is changed from a single location to multiple locations. Important products of the study were the Booster Disinfection Design and Analysis software and network models, which aid in the placement and operation of booster disinfection systems. The software is capable of providing information such as (1) setting the dosing schedules given the locations are provided; (2) selecting of booster dose schedules and location; and (3) heuristic screening of potential booster locations.

Booster Chloramination

Approximately 12 percent of the respondents to the survey published in Uber (2003) practice chloramination, by one of two methods. Most reportedly use chlorine to bind excess ammonia—a useful approach if chloramine decay results in the excess ammonia or if sufficient ammonia remained during the initial formation of chloramine. Three booster stations were identified in the survey using a second method in which both chlorine and ammonia were applied at the same location. This approach is used when there is a need to increase the overall concentration of chloramine present. Wilczak et al. (2003) reviewed operating practices at utilities employing booster chloramination with the addition of free chlorine (Martin and Cummings, 1993; Cohen, 1998; Ireland and Knudson, 1998) or both chlorine and ammonia (Potts et al., 2001). Monitoring of chlorine and ammonia was practiced by all utilities, in the majority of cases with on-line combined chlorine analyzers. Nitrite, pH, and on-line free ammonia analyzers were also employed by some of the utilities. Process control options included manual dose control, dosage determined by the flow, dosage determined by the flow along with measurements of the chlorine residual, and dosage set by the flow and controlled by a desired feed level set point. In all cases, operators could manually alter the chemical doses depending on water quality results. The goal of all utilities was to maintain a total chlorine residual of at least 2.0 mg/L.

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