Civil Engineering Reference
In-Depth Information
Under certain conditions, regrowth of coliform bacteria can occur in water that
results in a false positive.
•
Aeromonas
can biochemically mimic coliform bacteria.
•
When coliforms are in the presence of high populations of other plate-count bac-
teria, false results can be obtained.
•
Several pathogens have been found to survive longer than coliform in natural
waters and / or through various treatment processes. For example, coliforms were
detected in only 55 percent of the protozoan outbreaks in 1993 and 1994, as
opposed to 92 percent for bacterial or unknown etiology. This suggests that the
use of coliforms as indicators of protozoan contamination is not reliable.
•
CURRENT REGULATIONS
With the passage of the Safe Drinking Water Act (SDWA) in 1974, the U.S. Environ-
mental Protection Agency was granted the authority to set standards for the drinking
water delivered by public water supplies. The SDWA contained a series of steps and
timetables for developing regulations. Since 1974, the SDWA has been amended to
include more restrictive criteria for pathogenic organisms, such as the Surface Water
Treatment Rule (SWTR), the Total Coliform Rule (TCR), and the Interim Enhanced
Surface Water Treatment Rule (IESWTR).
Surface Water Treatment Rule
The Surface Water Treatment Rule of amendments of 1996 reestablished goals of
microbial integrity shown in Table 4-4 for
Giardia
cysts and virus removal in surface
water supplies. To meet these goals, the SWTR established treatment requirements for
filtration and disinfection to meet certain removal levels of
Giardia
cysts and viruses.
TABLE 4-4. Log Removal / Inactivation Through Filtration and Disinfection Required
Under the SWTR*
Giardia
Cysts
Process
Virus
Total log removal / inactivation required
3.0
4.0
Conventional sedimentation / filtration credit
2.5
2.0
Disinfection inactivation required
0.5
2.0
Direct filtration credit
2.0
1.0
Disinfection inactivation required
1.0
3.0
Slow sand filtration credit
2.0
2.0
Disinfection inactivation required
1.0
2.0
Diatomaceous earth credit
2.0
1.0
Disinfection inactivation required
1.0
3.0
No filtration
0.0
0.0
Disinfection inactivation required
3.0
4.0
Source:
Reference 13.
* One log removal / inactivation
90%; two days
99%.
