Swimming Pool Shock Treatment
Recreational water-borne infectious disease is an important problem
both in the United States and elsewhere. For 1999 and 2000, the Centers
for Disease Control and Prevention (CDC) reported 59 outbreaks of
infectious disease associated with recreational water use in the United
States, affecting almost 2,100 people and resulting in four deaths (CDC,
2002). A wide variety of microorganisms are responsible for
recreational water-borne disease, including Cryptosporidium, Shigella,
E. coli O157:H7, and norovirus (CDC, 2000, 2002) (noroviruses are named
after the original strain, Norwalk virus, which caused an outbreak in a
school in Norwalk, Ohio, in 1968). The occurrence of outbreaks in
ostensibly treated water suggests that current water treatment practices
are not effective for pathogen control. Most of the reported outbreaks
were associated with disinfection failures under routine conditions
rather than nonroutine incidents such as accidental fecal releases
(AFRs). The root causes of inadequate disinfection include human error,
mechanical or electrical failure, unanticipated increases in bather
load, unobserved incidents that introduce microorganisms, and improperly
maintained filtration equipment. Microbiological contamination of pools
and spas is widespread. Sandel (1990) surveyed 15 residential spas and
126 pools in the United States. That study found freehalogen residuals
of less than 1 milligram per liter (mg/L) in eight of the 15 spas,
including in three with nondetectable levels of halogen. The average
total plate count in pools was 20.1 colony-forming units per milliliter
(CFUs/mL), with a range from 0 to "too numerous to count." Martins and
co-authors (1995) surveyed 60 swimming pools and found similar low
levels of chlorination (0-2.5 mg/L), high levels of microorganisms
(heterotrophic bacteria in 70.4 percent of the samples), and a
significant negative correlation between chlorine and total and fecal
coliform bacteria, heterotrophic plate count, and fecal streptococci. It
should be a simple matter to maintain an efficacious
free-available-chlorine residual of 3 mg/L throughout a pool; however,
the data suggest that current treatment protocols are inadequate, as
evidenced by the number and severity of outbreaks. What is needed is a
new protocol that includes shock treatment to minimize both the
opportunity for human error and the consequences of an accident. The
term "shock treatment" as applied to recreational waters has a variety
of meanings; however, it is often used synonymously with
"superchlorination." We investigated the regulatory perspective on shock
treatment by contacting public health officials in 10 states that note
shock information in state guidelines or laws.
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| Public health officials from
all 10 states stated that protection of public health was one of the
most important reasons they would recommend shock treatment of pools or
spas; however, three recommended shock treatment only in response to an
AFR. Many environmental health authorities recommend the use of shock
treatment in response to AFRs. For example, the Maryland Department of
Health and Mental Hygiene (MD-DHMH) (2001) has developed a comprehensive
protocol for the use of shock treatment as an intervention following a
pool accident that may have released pathogens. This protocol, based on
CDC guidelines (CDC, 2001), provides detailed directions for responses
to AFRs and other accidents. Some agencies also recommend shock on a
routine basis, including the New South Wales Department of Health
(NSWDH) in Australia, CDC (1997), and some states. NSWDH has a "strong
recommendation" for superchlorination in public pools and spas (NSWDH
1996, 1999) and advocates weekly or fortnightly superchlorination that
maintains 10 mg/L of free-chlorine residual for a minimum of eight
hours. CDC's guidelines for spas aboard cruise ships recommend
maintenance of disinfection levels at 3 to 10 mg/L for chlorine or 4 to
10 mg/L for bromine and daily shock treatment. Texas and Georgia both
recommend shock treatment daily for spas. In addition to its application
for AFRs, shock treatment may be used as a prophylactic for pathogen
control. Moreover, a protocol that emphasizes the routine use of
prophylactic shock may help in achieving failsafe disinfection.
Definitions that could be used in such a protocol for "shock treatment,"
"routine shock," and "intervention shock" are provided below: o Shock
treatment is defined as the addition of an immediately available
oxidizing disinfectant, routinely or as an intervention. A shock
chemical must be effective against a wide variety of pathogens and
nuisance microorganisms with a reasonable CT factor. (The CT factor is
the product of the residual disinfectant concentration [C] in mg/L and
time [T] in minutes that the disinfectant is in contact with the water.)
In addition, a shock chemical should be capable of destroying organic
contaminants and nitrogen compounds with a reasonable reaction time.
Routine shock involves raising free-chlorine levels to a minimum of 10
mg/L for one to four hours weekly for pools and daily for spas. |
