Disinfection - Public Water Systems - page 659

For simplicity, the decision tree is subdivided into four phases, as described below.

Evaluating the Current Primary Disinfection Practice Figure 19-18 shows the
decision tree used to determine whether the present disinfectant can meet disinfection
and by-product requirements. The decision points in Figure 19-18 include:

• Meeting microbial limits. Microbial limits are defined by the primary drinking
water standards. The regulated pathogens include Giardia lamblia , Legionella , HPC,
total coliform, and viruses. The requirements for these organisms and Cryptosporidium
are modified in the IESWTR, Long Term-1 ESWTR, and Long Term-2 ESWTR. The
disinfectant must be capable of meeting the inactivation requirements of disinfection.
If not, the plant must determine if the current disinfectant can meet the microbial
inactivation requirements solely by operational changes. A change may be to move
the application point, increase dose, increase contact time, or adjust pH. If not, a new
disinfectant is needed.
• Meeting DBP limits. The secondary limit imposed on disinfectant usage is the
ability to meet DBP requirements. At this decision point, the type of DBP is deter-

Start

Meet
Microbial
with Process
Modification?

Meet
Microbial?

No No

Change
Disinfectant

Yes Yes

Treatment
Optimized?

Meet
DBP?

OK

Yes No Yes

Consult Primacy Agency When
Changing Treatment Process

No

Optimize
Treatment

Fig. 19-18. Flow diagram 1 to evaluate current disinfection practice ( Source: Reference 6. Re-
printed from Proceedings of the 1998 Water Quality Technology Conference, by permission. Copy-
right 1998, American Water Works Association.)