Although chlorine is the primary disinfectant of choice in
water treatment practice, many waterborne pathogens are resistant to chlorine
and are often found in finished water. These chlorine-resistant pathogens
include viruses, parasites and bacteria that can cause hepatitis,
gastroenteritis, cryptosporidiosis and Legionnaires’ disease.
In the past decade, some water treatment advancements have improved
disinfection efficiency. Enhanced coagulation process and rapid sand
filtration have been used to effectively remove a significant percent age of
Cryptosporidium and Cyclospora. Post-treatment or on-site disinfection are
also available to enhance biological safety of drinking water. For example,
the Pittsburgh Water and Sewer Authority studied post-treatment options in an
uncovered reservoir to remove Giardia cysts and Cryptosporidium oocysts.
Pilot testing compared ozonation and membrane filtration. All
tested membrane filter systems acted as ab solute barriers to Giardia cysts (5
log removal) and Cryptosporidium 00- cysts (6 log removal); whereas ozone
inactivated O.1-to-O.5 log units using doses of 2.9 to 6.6 mg/L of ozone. The
current Surface Water Treatment Rule sets Maximum Contaminant Level Goals (MCLGs)
for Legionella, Giardia and viruses to zero because any exposure to these
conminants represents some health risk. However, MCLGs—unlike maximum
contaminant levels (MCLs)—aren’t enforceable and on- site treatment may be
required to remove these pathogens. For ex ample, many hospitals have used on-
site treatment with copper-silver ionization, chlorination or chlorine di
oxide to control Legionella from the water distribution systems. An epi
demiological study also suggested that fewer Legionnaires’ disease out breaks
were reported in municipalities using monochioramine as a residual
disinfectant vs. free chlorine. However, the effectiveness of chioramines as
microbiocidal agents is still somewhat controversial.