In order to comply with the Stage 2 Disinfectant/Disinfection Byproduct (D/DBP) Rule, an
evaluation or study of disinfection byproduct (DBP) formation in the distribution system is
required. This paper discusses two options, the first being an evaluation of DBP formation in the distribution system,
called an Initial Distribution System Evaluation (IDSE), that requires that one year of paired
total trihalomethane (TTHM) and the five haloacetic acid species (HAA5) samples be
collected every other month at eight locations within the distribution system. Compliance
sample sites are then selected based on the DBP results. The other option is to
complete a System-Specific-Study (SSS). An SSS uses historical data, distribution
system models or other methods to assess the distribution system and select
representative sample locations. The IDSE or SSS is due two years after rule promulgation.
IDSE requirements apply to all public water systems serving a population of >10,000 and
using surface water or groundwater under the influence (GWUI) of surface water that are
subject to the requirements of the Surface Water Treatment Rule. The IDSE
requirements also apply to groundwater systems that use a primary disinfectant other
than ultraviolet (UV) or add a residual disinfectant to their water, or systems that deliver such water.
The US Environmental Protection Agency (USEPA) has made the Draft Stage 2 Disinfectants and Disinfection Byproducts Rule
Initial Distribution System Evaluation Guidance Manual (USEPA 2001) available to the
public for review. While still in draft form, the manual covers IDSE requirements for all
system types and sizes. Details of the IDSE requirements summary that follows in this
report can be found in the draft guidance manual.
Specific requirements have been developed to meet the requirements for a SSS using a
hydraulic distribution system model. These guidelines address only hydraulic modeling
and do not address distribution system water quality models. An integrated hydraulic
and water quality distribution system model may provide even better information about
DBP formation in distribution system environments. As with all SSSs, an integrated
hydraulic and water quality distribution system model must provide equivalent or
superior information for site selection than a system monitoring plan (SMP). To meet the requirements of the SSS, both the hydraulic and water quality components
of the integrated distribution system must be detailed, comprehensive, and well
calibrated. It is important to understand how each of these components function
individually to meet the goal of understanding DBP formation in a distribution system
environment. Hydraulic models simulate water movement in a distribution system to
understand water age, demand patterns, and pressure. Water quality models estimate
changes in source and treated water quality based on plant treatment processes or
distribution system conditions. Specifically related to the IDSE, water quality models can
characterize disinfection residual and disinfection byproduct concentrations.
The main objective of a SSS is to allow utilities to gain a better understanding of the
hydraulics and water quality in their distribution system. It is a tool that can be used to
identify trouble spots within the distribution system. In addition, the resulting model is a
powerful planning tool that can be used to predict DBP formation in the distribution
system for proposed modifications. Finally, the model allows the utility to refine its
operations and thoroughly evaluated alternatives on a site-by-site basis potentially
avoiding costly system-wide capital changes such as the implementation of
chloramination in order to decrease the formation of DBPs in the distribution system.
For these reasons, the City of Golden elected to pursue the SSS compliance approach.
This paper focuses on the steps taken to perform a full-scale SSS, specifically
| Edition : | Vol. - No. |
| File Size : | 1
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| Note : | This product is unavailable in Ukraine, Russia, Belarus |
| Number of Pages : | 12 |
| Published : | 11/01/2002 |
