Washington Administrative Code
Title 246 - Health, Department of
WATER SYSTEMS
Chapter 246-290 - Group A public water supplies
Part 6 - SURFACE WATER TREATMENT
Subpart B - Requirements for Filtered Systems
Section 246-290-662 - Disinfection for filtered systems
Universal Citation: WA Admin Code 246-290-662
Current through Register Vol. 24-18, September 15, 2024
(1) General requirements.
(a) The purveyor shall provide continuous
disinfection to ensure that filtration and disinfection together achieve, at
all times the system serves water to the public, at least the following:
(i) 99.9 percent (3-log) inactivation and
removal of Giar-dia lamblia cysts; and
(ii) 99.99 percent (4-log) inactivation
and/or removal of viruses.
(b) Where sources receive sewage discharges
and/or agricultural runoff, purveyors may be required to provide greater levels
of removal and inactivation of Giardia lamblia cysts and
viruses to protect the health of consumers served by the system.
(c) Regardless of the removal credit granted
for filtration, purveyors shall, at a minimum, provide continuous disinfection
to achieve at least 68 percent (0.5-log) inactivation of Giardia
lamblia cysts and 99 percent (2-log) inactivation of
viruses.
(2) Establishing the level of inactivation.
(a)
The department shall establish the level of disinfection (log inactivation) to
be provided by the purveyor.
(b)
The required level of inactivation shall be based on source quality and
expected levels of Giardia lamblia cyst and virus removal
achieved by the system's filtration process.
(c) Based on periodic reviews, the department
may adjust, as necessary, the level of disinfection the purveyor shall provide
to protect the health of consumers served by the system.
(d) Systems granted no Giardia
lamblia cyst removal credit and no Cryptosporidium
oocyst removal credit shall:
(i) Unless
directed otherwise by the department, provide interim disinfection to:
(A) Ensure compliance with the monthly
coliform MCL under WAC
246-290-310;
(B) Achieve at least 99.9 percent (3-log)
inactivation of Giardia lamblia cysts; and
(C) Maintain a detectable residual
disinfectant concentration, or an HPC level less than 500 organisms/ml, within
the distribution system in accordance with subsection (6) of this section. The
department may approve a written request to use a lower value. At a minimum,
the request to use a lower value must identify the instrument used to measure
the residual disinfectant concentration and include the manufacturer's
documentation on the instrument's accuracy to measure the lower
value.
(ii) Comply with
the interim disinfection requirements until the system can demonstrate to the
department's satisfaction that it complies with the operating requirements and
turbidity performance requirements under WAC
246-290-654 and
246-290-660(1),
respectively.
(3) Determining the level of inactivation.
(a) Unless the department has approved a
reduced CT monitoring schedule for the system, each day the system serves water
to the public, the purveyor, using procedures and CT values acceptable to the
department such as those presented in department guidance of surface water
treatment, shall determine:
(i) CTcalc values
using the system's treatment parameters and calculate the total inactivation
ratio achieved by disinfection; and
(ii) Whether the system's disinfection
process is achieving the minimum levels of inactivation of Giardia
lamblia cysts and viruses required by the department.
(b) The department may allow a
purveyor to determine the level of inactivation using lower CT values than
those specified in (a) of this subsection, provided the purveyor demonstrates
to the department's satisfaction that the required levels of inactivation of
Giardia lamblia cysts and viruses can be achieved.
(4) Determining compliance with the required level of inactivation.
(a) A
purveyor shall be considered in compliance with the inactivation requirement
when a total inactivation ratio equal to or greater than 1.0 is
achieved.
(b) Failure to provide
the required level of inactivation on more than one day in any calendar month
shall be considered a treatment technique violation.
(5) Residual disinfectant concentration entering the distribution system.
(a) The
purveyor shall ensure that all water entering the distribution system contains
a residual disinfectant concentration, measured as free or combined chlorine,
of at least 0.2 mg/L at all times the system serves water to the public;
and
(b) Failure to provide a 0.2
mg/L residual at entry to distribution for more than four hours on any day
shall be considered a treatment technique violation.
(6) Residual disinfectant concentration within the distribution system.
(a) The
purveyor shall ensure that the residual disinfectant concentration in the
distribution system, measured as total chlorine, free chlorine, combined
chlorine, or chlorine dioxide, is detectable in at least ninety-five percent of
the samples taken each calendar month.
(b) Water in the distribution system with an
HPC less than or equal to 500 organisms/ml is considered to have a detectable
residual disinfectant concentration for the purposes of compliance with WAC
246-290-662(6)(a).
Statutory Authority: RCW 43.20.050(2) and (3) and 70.119A.080. 03-08-037, § 246-290-662, filed 3/27/03, effective 4/27/03. Statutory Authority: RCW 43.02.050 [43.20.050]. 99-07-021, § 246-290-662, filed 3/9/99, effective 4/9/99. Statutory Authority: RCW 43.20.050. 94-14-001, § 246-290-662, filed 6/22/94, effective 7/23/94; 93-08-011 (Order 352B), § 246-290-662, filed 3/25/93, effective 4/25/93.
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