The disinfection of pathogenic microbes in
drinking water has been largely successful over the last century due
to the use of chlorination. However, research conducted in the 1970's
revealed that by-products formed during the chlorination process are
potentially carcinogenic and that there is a direct correlation
between the concentration of chlorination by-products and the
probability of certain cancers and other health problems. Following
these discoveries, drinking water regulators have struggled to find a
balance between the benefits of chlorination and the harmful side
effects caused by chlorination, within the confines of technological
and economic limitations.
In the U.S.A., the Surface Water Treatment Rule (SWTR) of 1989
mandates inactivation levels for giardia cysts and enteric viruses,
and also sets treatment standards for Trihalomethanes (THMs). The SWTR
provides guidance to drinking water facilities through "CT" tables
that prescribe the inactivation efficacy of various processes under
varying water quality conditions. By following this guidance, most
water treatment plants were able to provide an adequate degree of
disinfection while not compromising their Disinfection By-Product
(DBP) limits and without requiring major changes to their
plants. However, continuing DBP health effect research indicated that
even the DBP standards required in the 1989 SWTR produced an
unacceptable level of risk and the SWTR was amended in 1996 to further
lower DBP standards. In addition, a major outbreak of
cryptosporidiosis in Milwaukee in 1993, and other minor
cryptosporidiosis outbreaks caused regulators to create a removal
requirement for cryptosporidium oocysts in the 1998 Interim Enhanced
Surface Water Treatment Rule (IESWTR) and most likely a disinfection
requirement in the final ESWTR (LT2ESWTR). The new DBP standards have
caused many plants to fall out of compliance, requiring either
extensive plant modifications or new disinfection strategies. The
LT2ESWTR will include a cryptosporidium disinfection requirement and
many surface water plants will fall out of compliance due to the very
poor efficacy of chlorination for cryptosporidium. Therefore, due to
these apparently conflicting conditions, a void was created for a
water treatment technology that is effective for protozoa and viruses,
does not create DBPs, and is economically feasible.
UV technologies have long been known to be effective for viruses and
bacteria in drinking water and guidelines for the disinfection of
viruses exist in the Alternative Disinfectants and Oxidants Guidance
Manual. However, UV was widely considered to be ineffective for
encysted protozoa as it was thought that the UV light would not
penetrate the cyst membrane, and since giardia is the controlling
microbe for chlorine dose determinations, no reductions in chlorine
usage could be gained by using UV. Therefore, UV Disinfection was not
used for surface waters in North America.
New breakthrough research conducted by Calgon Carbon Corporation in
1998 however proved that UV disinfection is, in fact, very effective
for inactivating cryptosporidium and giardia at low UV
doses. Subsequent to Calgon Carbon's research, the USEPA created a UV
subworkgroup to report to the Federal Advisory Committee (FACA) on
issues and costs related to UV disinfection. In advance of new
guidance manuals for UV disinfection, many utilities have begun to
consider UV disinfection in their plants either as an additional
barrier for protozoa disinfection or to get "CT" credits for UV for
giardia so that chlorine doses can be lowered to meet the 1998 DBP
standards.
Authors: Daniel Brooks, Gary Van Stone, and
Wayne Lem, Calgon Carbon Corporation, Pittsburgh, Pennsylvania,
USA.
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