Civil Engineering Reference
In-Depth Information
These goals can be met using various treatment processes, primarily filtration and
disinfection.
Total Coliform Rule
To maintain biological stability in the distribution system, the Total Coliform Rule
requires that treated water contain a residual disinfectant of 0.2 mg / L when entering
the distribution system. A measurable disinfectant residual must be maintained in the
distribution system, or the utility must show through monitoring that the heterotrophic
plate count remains less than 500 per 100 mL sample.
FUTURE REGULATIONS
Regulations pertaining to drinking water standards are bound to change in the future.
This change is driven by several factors:
The objective is to produce a drinking water free from microbial contaminants,
thus requiring effective methods for the removal of microbes.
•
Disinfectants produce undesirable by-products because they are strong oxidants
and react readily with organic and inorganic compounds in the water to produce
these by-products. The formation of by-products needs to be reduced to a mini-
mum.
•
Organisms such as
Giardia, Cryptosporidium,
and other emerging pathogens may
be more resistant to traditional methods of disinfection and water treatment.
•
Safety concerns associated with handling and dosing hazardous chemicals are
requiring additional facilities and procedures to meet public safety concerns, and
are affecting the overall cost of water treatment facilities.
•
Recent advances in disinfection practices and new disinfectants are producing
highly effective inactivation to specific organisms at modest costs.
•
A need exists to maintain a disinfectant residual in the distribution system.
•
As of this writing, there is an intense regulatory effort to limit exposure to, and enhance
the removal of
Cryptosporidium
in water supplies. The Interim Enhanced Surface
Water Treatment Rule (IESWTR) of 1996 established an MCL (goal) of zero
Cryp-
tosporidium
oocysts in treated water. However, this and all future regulations will be
limited in effectiveness until a viable method of analyzing for
Cryptosporidium
is
developed. At present, the most promising approach is USEPA Method 1622. However,
even this improved technique is exceedingly time and labor intensive. It requires fil-
tering of large sample volumes, elution and pelletizing steps, separation of oocysts
using density gradients, staining of the oocysts with fluorescent immuno-antibodies,
and then physical enumeration under the microscope. After all this, the actual oocyst
recovery is only about 35 percent.
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New analytical techniques for both
Cryptosporidium
and
Giardia
are under devel-
opment. However, until a reliable and easily implementable technique is established,
it will be impossible to assess or even set an acceptable level of risk for these pathogens
in a drinking water system.
