Disinfection - Public Water Systems - page 663

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High
AOC?

Yes No

Biological
Treatment
BAF or GAC

Consult Primacy Agency When
Changing Treatment Process

High
DBPFP?

No Yes

Extended
Distribution
Time?

Extended
Distribution
Time?

Yes No No Yes

Chlorine
Chlorine Dioxide
Monochloramine

Chlorine
Monochloramine

Chlorine Dioxide
Monochloramine

Monochloramine

Fig. 19-21. Flow diagram 4 to select a new secondary disinfectant ( Source: Reference 6. Re-
printed from Proceedings of the 1998 Water Quality Technology Conference, by permission. Copy-
right 1998, American Water Works Association.)

mg / L. In these cases, additional biological or GAC treatment should be considered to
stabilize the finished water and prevent regrowth in the distribution system.
• DBP Formation Potential (DBPFP). The DBPFP serves as an indication of the
amount of organic by-products that could be expected to form in the distribution
system if chlorine is used. Because DBP formation continues in the distribution system,
the DBP content at the plant effluent should be limited. A high DBPFP is defined as
a water meeting one of the following criteria:
• TTHM seven-day formation exceeds the MCL (0.08 mg / L under Stage 1
D / DBP Rule)
• HAA5 seven-day formation exceeds the MCL (0.06 mg / L under the Stage 1
D / DBP Rule)
• Distribution System Retention Time. In a large distribution system, booster
stations may be required to maintain the disinfection residual. Since chlorine dioxide
has an upper limit for application, its usage may not be feasible if relatively high doses
are required to maintain a residual in the distribution system. A distribution system
retention time is considered high if it exceeds 48 hours.