The Stage 2 Disinfectant/Disinfection Byproducts Rule (Stage 2 D/DBP Rule), enacted by the Environmental Protection Agency (EPA) in December 15, 2005, changes the way systems must account for levels of byproducts in their water and presents new challenges to water quality management. Most systems currently use free chlorine, chlorine dioxide, ozone, or ultraviolet light (UV) as the primary disinfectant. Primary disinfection occurs at the treatment plant as part of the treatment process to comply with the requirements of the Surface Water Treatment Rule (SWTR). Secondary disinfection, achieved by the addition of free chlorine or chloramines, provides a residual disinfectant in the finished water that is transmitted to the distribution system and aids in maintaining safe water.

The Stage 2 D/DBP Rule places new demands on systems. Specifically, Stage 2 D/DBP changes the way systems account for levels of byproducts in their water and presents new challenges to water quality management. The rule requires Initial Distribution System Evaluations (IDSE). These require the system—whether rural water system or municipality—to analyze existing water quality data, obtain additional water quality data at different monitoring locations, or create hydraulic models that predict where the highest levels of byproducts occur. The results of the IDSE will influence the location of the Stage 2 D/DBP compliance monitoring sites.  Stage 2 D/DBP requires running annual averages at each individual site to be below the Maximum Contaminant Levels (MCLs) as stipulated in the State 2 D/DBP.
 

Specific Stage 2 DBPR rules apply to consecutive systems—those which receive some or all of its finished water from one or more wholesale systems. According to the EPA, consecutive systems are part of combined distribution systems (CDSs), consisting of wholesale systems and the consecutive systems that receive the finished water. Wholesale systems are public water systems that treat source water and deliver some or all of it to another public water system.
 

For CDSs—the wholesale producer and all consecutive systems—the system with the largest population determines the IDSE compliance schedule for sampling for all systems within the CDS, though there is an exemption for emergency connections.  The number of samples and frequency of sampling for each system within the CDS for the IDSE and Stage 2 D/DBP compliance monitoring are based on the individual system’s population, not the system with the largest population.
 

There is a Very Small System Waiver (VSS Waiver) available for systems serving fewer than 500 people and that meet other criteria. This is the first of four options for meeting IDSE requirements. The second option is for systems meeting specific eligibility requirements may apply for 40/30 Certification which provides a waiver from additional IDSE activities. EPA or the state can, however, require the system conduct Standard Monitoring (SM) or System Specific Study (SSS) even if the system is eligible for the VSS or 40/30 Waivers. Note that VSS Waiver and 40/30 Certification are only a waiver from additional IDSE activities.  Stage 2 D/DBP compliance monitoring is required for all systems.
 

Standard Monitoring (SM) is the third option for compliance with IDSE requirements. Most systems that need to conduct IDSE monitoring will use this option. This option’s steps include:

1. Select Standard Monitoring Locations
2. Prepare and Submit Standard Monitoring Plan
3. Conduct Standard Monitoring
4. Select Stage 2 DBP Rule Compliance Monitoring Locations
5. Prepare and Submit IDSE Report

Systems that don’t have a VSS waiver, or qualify for the 40/30 certification, may be able to use the System Specific Study (SSS) instead of the Standard Monitoring for IDSE. This is the fourth IDSE option. The EPA will have a national review team for these, so they won’t be reviewed by regulatory agency staff. Hydraulic modeling and justification of any grandfathered data will likely be required.

Remember, in each of the four options, the system with the largest population within a combined distribution system determines the schedule for sampling for all systems.  The table below indicates the milestone requirements associated with the IDSE.
 

It is important for consecutive systems to communicate, share data and other pertinent information with the other systems in their combined system. The rule requires consecutive and wholesale systems to conduct the IDSE at the same time as the largest system in the combined distribution system. This may not always be the wholesaler.  It is not the responsibility of the wholesale system to contact the consecutive systems that it serves.  Consecutive systems are encouraged to reach out to their wholesale system to make initial contact as it pertains to the IDSE.