Advanced Waterworks Mathematics, 2019a

Pin It! Misconception Alert
Sometimes logs (logarithms) can seem scary! Think of the Log as the
power to which a number has to be raised to get another number.
Cryptosporidium will not be discussed in this class due to the complexities in the 2003 update to
the SWTR known as the Long Term 2 Enhanced Surface Water Treatment Rule. Instead, we will
focus on Giardia and viruses.
Table 7.2 (below) depicts the log requirements that are required for both Giardia and viruses.
Notice in the second column that Giardia must be disinfected to 3 Log (99.9%) and viruses to 4
Log (99.99%). Various treatment processes (shown in the first column) account for some of the
inactivation or removal of pathogens from the raw water. Therefore, the SWTR provides
“credits” toward the inactivation of Giardia and viruses (shown in the third column). Credits
are subtracted from the inactivation requirements to determine the level of disinfection still
required after water goes through treatment.
Table 7.2 - Treatment Credits and Log Inactivation Requirements
Treatment
Required Log
Log Inactivation
Removal Credit Logs Inactivation from
Requirements
Disinfection
Giardia Viruses Giardia Viruses Giardia Viruses
Conventional 3 4 2.5 2 0.5 2
Direct Filtration,
DE, or Slow
Sand
3 4 2 1 1 3
For example, the CT requirement for viruses is 4 Log. This requirement can be satisfied through
disinfection, treatment, or a combination of the two.
If raw water is treated using direct filtration, it would receive 1 Log credit. After water leaves
the treatment plant, the disinfection requirement remaining for viruses would be 3 Log, as
shown in the fourth column of Table 7.2 (complete calculation illustrated below the table). The
remaining 3 Logs will need to be inactivated by disinfecting the water using the appropriate
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