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Membrane Filtration Regulations and Determination of Log Removal Value 139
developed by the US EPA for the use of ultraviolet (UV) disinfection for LT2ESWTR
compliance (6). Guidance for the use of all other toolbox options is given in the Toolbox
Guidance Manual for the Long Term 2 Enhanced Surface Water Treatment Rule (7).
3.2. Stage 2 Disinfectants and Disinfection Byproducts Rule
The Stage 2 DBPR is designed to reduce DBP occurrence peaks in the distribution system
by changing compliance monitoring requirements. The requirements of the Stage 2 DBPR
apply to all community water systems (CWSs) and nontransient noncommunity water
systems (NTNCWSs) – both ground and surface water systems – that either add a chemical
disinfectant (i.e., a disinfectant other than UV light) or deliver water that has been treated
with a chemical disinfectant. The primary components of the rule are summarized as
follows (3).
3.2.1. Initial Distribution System Evaluations
Under the Stage 2 DBPR, systems are required to conduct an initial distribution system
evaluation (IDSE) to identify compliance monitoring locations with high total trihalomethane
(TTHM) and haloacetic acid (HAA5) levels. The IDSE consists of either a standard monitoring
program (SMP) or a system-specific study (SSS). NTNCWSs serving fewer than 10,000
people are not required to conduct an IDSE, and other systems may be eligible to receive
waivers from the IDSE requirement.
3.2.2. Compliance Determination and Schedule
The Stage 2 DBPR changes the way DBP sampling results are averaged to determine
compliance, implementing a methodology based on a locational running annual average
(LRAA) rather than the system-wide running annual average (RAA) used under the Stage 1
DBPR. This new methodology introduced under the Stage 2 DBPR is introduced in two
phases: Stage 2A and Stage 2B. Under Stage 2A, all systems must comply with TTHM and
HAA5 MCLs of 120 and 100 mg/L, respectively, measured as LRAAs at each Stage 1 DBPR
monitoring site, while continuing to comply with the respective TTHM and HAA5 Stage 1
DBPR MCLs of 80 and 60 mg/L, measured as RAAs. Subsequently, under Stage 2B, systems
must comply with respective TTHM and HAA5 MCLs of 80 and 60 mg/L at the sampling
locations identified under the IDSE.
3.2.3. Compliance Monitoring
Stage 2B compliance monitoring requirements (in terms of number of sites and frequency
of sampling) are expected to be similar to the Stage 1 DBPR requirements for most, but not
all, systems. Some small systems will have to add an additional monitoring location if the
highest TTHM and highest HAA5 site do not occur at the same location.
3.2.4. Significant Excursion Evaluations
Because Stage 2 DBPR MCL compliance is based on an annual average of DBP measurements,
a system could from time to time have DBP levels significantly higher than the MCL
(referred to as a significant excursion) while still maintaining compliance. If a significant