Section 611.533  Filtered PWSs

A supplier using a surface water source or a groundwater source under the direct influence of surface water and providing filtration treatment must monitor in accordance with this Section.

a)         The supplier must perform turbidity measurements Section 611.250 requires on representative samples of the PWS's filtered water every four hours (or more frequently) when the supplier serves water to the public.  A supplier may substitute continuous turbidity monitoring for grab sample monitoring if it validates the continuous measurement for accuracy on a regular basis using a protocol the Agency approved in a SEP.  For a supplier using slow sand filtration or filtration treatment other than conventional treatment, direct filtration, or diatomaceous earth filtration, the Agency must reduce the sampling frequency to once per day in a SEP if the Agency determines that less frequent monitoring is sufficient to indicate effective filtration performance.  For a supplier serving 500 or fewer persons, the Agency must reduce the turbidity sampling frequency to once per day in a SEP if the Agency determines that less frequent monitoring is sufficient to indicate effective filtration performance regardless of the type of filtration treatment used.

b)         RDC Entering Distribution System

1)         Suppliers Serving More Than 3300 Persons.  The supplier must continuously monitor the RDC of the water entering the distribution system, and the supplier must record the lowest value each day, except that the supplier may conduct grab sampling every four hours in lieu of continuous monitoring if there is a failure in the continuous monitoring equipment, but not for more than five working days following the equipment failure.

2)         Suppliers Serving 3,300 or Fewer Persons.  The supplier may take grab samples in lieu of providing continuous monitoring on an ongoing basis at the frequencies each day Table C prescribes. If at any time the RDC falls below 0.2 mg/L in a system using grab sampling in lieu of continuous monitoring, the supplier must take a grab sample every four hours until RDC is equal to or greater than 0.2 mg/L.

c)         Points of Measurement

1)         The supplier must measure the RDC at least at the same points in the distribution system and at the same time as sampling total coliforms, as Sections 611.1054 through 611.1058 specify. The Agency must allow a supplier using both a surface water source and a groundwater source or a groundwater source under direct influence of surface water and a groundwater source to take RDC samples at points other than the total coliform sampling points if the Agency determines that such points are more representative of treated (disinfected) water quality within the distribution system.  The supplier may measure HPC, as Section 611.531(a) specifies, in lieu of RDC.

2)         Subsection (c)(1) does not apply if the Agency determines under Section 611.213(c) that a system has no means for having a certified laboratory analyze a sample for PHC under the requisite time and temperature conditions Section 611.531(a) specifies and the supplier provides adequate disinfection in its distribution system.

BOARD NOTE:  This Section derives from 40 CFR 141.74(c).

(Source:  Amended at 47 Ill. Reg. 16486, effective November 2, 2023)