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PCWS Water Quality Monitoring - Disinfection By-Products (DBPs)

Disinfection By-Products

This page was designed specifically to offer a brief introduction to disinfection by-products, the Federal Rules regulating these, and to provide you with what you will need to complete a Stage 2 DBP Compliance Monitoring Plan. As either the administrative contact or the operator of an NTNC, or Community Public Water System that either chlorinates drinking water or purchases drinking water already chlorinated, you likely collected DBP data under the Stage 1 Disinfection By-product Rule (DBPR)(can we link to this document? is it a separate document?). Stage 1 DBP monitoring requirements sunset September 30, 2013. Beginning October 1, 2013 systems will become responsible for Stage 2 Compliance Monitoring.

All systems affected by the Stage 2 Disinfection By-product Rule (DBPR) are required to submit Stage 2 DBP Compliance Monitoring Plans (can we link to a PDF or sample plan?) These plans must include recommendations on a compliance monitoring schedule and sampling locations in accordance with the schedule, below:


Source Water Type

Population Size Category

Monitoring Frequency1

Distribution System Monitoring Location

Total per monitoring period

Highest TTHM Locations

Highest HAA5 Locations


Subpart H (Surface Water)






500 – 3,300

Every 90 days




3.301 – 9,999

Every 90 days





Ground Water






500 – 9,999






1 All systems must take at least one dual sample set during the month of highest DBP concentrations or warmest water temperature.
2 System is required to take individual TTHM and HAA5 samples (instead of dual sample set) at the locations with the highest TTHM and HAA5 concentrations, respectively. Only one location with a dual sample set per monitoring period is needed if highest TTHM and HAA5 concentrations occur at the same location.


  • The Science of Disinfection By-products
  • The Regulation of Disinfection By-products
  • Transitioning from Stage 1 DBP Compliance Monitoring to Stage 2 DBP Compliance Monitoring (FAQs) (see below)
  • Directions for Electronic Submission of your Stage 2 DBP Compliance Monitoring Plan (see below)
  • Additional DBP Resources (see below)


The Science of Disinfection By-products (DBPs)

Disinfection byproducts (DBPs) form when water that contains natural organic matter (NOM) is mixed with certain forms of chlorine. NOM, referred to as a DBP precursor, is most commonly found in surface water where organic matter frequently enters the water body from runoff and aquatic organisms. All public water systems using surface water must disinfect the water prior to delivery to customers. DBPs are harmful to human health and are regulated under the Safe Drinking Water Act (SDWA). In 2001, USEPA published the STAGE 1 DISINFECTANTS AND DISINFECTION BYPRODUCT RULE (Stage 1) to address DBPs in treated water (surface water or otherwise). There are many USEPA GUIDANCE DOCUMENTS on Stage 1 available. Stage 1 created Maximum Contaminant Levels (MCL) for DBPs, monitoring requirements and DBP precursor removal requirements.

The most common DBPs are Trihalomethanes, Haloacetic Acids, Bromate and Chlorite (plants using chlorine dioxide). The Maximum Contaminant Level (MCL) for Trihalomethanes is for Total Trihalomethanes (TTHM). The MCL for Haloacetic Acids is for the five most common Haloacetic Acids, referred to as HAA5. The MCLs for TTHM, HAA5 and Bromate is based on the Running Annual Average (RAA) of the all the samples collected over a twelve month period. The MCLs for these DBPs are as follows:






80 ug/l

Running Annual Average


60 ug/l

Running Annual Average


10 ug/l

Running Annual Average


1 mg/l

Daily at plant


Trihalomethanes typically continue to form in the distribution system over time so the highest concentrations of these are most often found at the outermost edges of a distribution system (furthest from the water treatment facility) where the "oldest" water is found. Thus, this is where sampling locations for TTHMs are most appropriately located (the Maximum Retention Time or MRT Site). Haloacetic Acids, on the other hand, tend to degrade in the distribution system after treatment and thus are often found at their highest levels more centrally within the distribution system. Other factors influencing DBP formation include pH (higher pH favors THM production, lower pH favors HAA production) and temperature (higher temperature favors DBP production so summer time often sees higher DBP concentrations).

The Regulation of Disinfection By-products (DBPs)

In 2006, USEPA published the STAGE 2 DISINFECTANTS AND DISINFECTION BYPRODUCTS RULE (Stage 2). This rule built on and strengthened Stage 1. One big change is that the MCL was and will be based on the local running annual average (LRAA) which means that concentrations are averaged at individual sampling locations rather than over the entire distribution system as in Stage 1.

Normal Organic Matter (NOM) is measured in the form of Total Organic Carbon (TOC). Individual treatment plants are required to achieve a specified percent removal of influent TOC between the raw water sampling point and the treated water TOC monitoring location. The percentage reduction depends on the influent TOC and the alkalinity (the percent reductions recognize that TOC removal is more difficult with increasing alkalinity). The TOC reduction requirements are found in the following table: 


Source Water TOC (mg/l)

Source Water Alkalinity (mg/L as CaCO3)

0 to 60

>60 to 120


>2.0 - 4.0




>4.0 - 8.00




> 8.0





Transitioning to Stage 2 DBP from Stage 1 DBP Compliance Monitoring

The Stage 2 Disinfectants and Disinfection Byproducts Rule (Stage 2 Rule) sets new monitoring requirements for total trihalomethanes (TTHM) and haloacetic acids (HAA5). USEPA published the Stage 2 Rule in January 2006. The Stage 2 Rule applies to all community and non-transient, non-community (NTNC) public water systems that deliver water continuously treated with a primary or residual disinfectant other than ultraviolet light (including those that purchase treated drinking water). The Stage 2 Rule does not apply to water systems that only use these disinfectant chemicals temporarily, such as after facility cleaning or repair.

The Stage 2 DBPR requires systems to conduct operational evaluations where operational evaluation levels (OELs) -- calculated using Stage 2 DBPR compliance monitoring data – exceeded MCL values for TTHM and/or for HAA5. For resources helpful in understanding and navigating the requirements of OELs, please utilize the following links:

Stage 2 Disinfectants and Disinfection Byproducts Rule (DBPR) Guidance: Operational Evaluation Levels (OELs), Operational Evaluations & the Operational Evaluation Report  MSWord
Guidance Sheet/Interactive Form: Conducting an Operational Evaluation & Preparing an Operational Evaluation Report
Guidance Sheet/Non-Interactive Form: Conducting an Operational Evaluation & Preparing an Operational Evaluation Report
Interactive Spreadsheet for Calculating OELs

Additional Resources about OELs:

USEPA – Stage 2 Disinfectants & Disinfection Byproducts Rule: Operational Evaluation Guidance Manual
USEPA Distribution System Evaluation Checklist  MsWord
USEPA Operational Evaluation Reporting Form   MsWord
USEPA Source Water Evaluation Checklist    MsWord
USEPA – Stage 2 Disinfectants & Disinfection Byproducts Rule: Consecutive Systems Guidance Manual
DBP Control Strategies for Consecutive Systems and Other Problem Areas in Distribution Systems
Drinking Water Disinfection Byproducts: Review and Approach to Toxicity Evaluation
Preparing for the Next Frontier – Distribution System Optimization Plans


How do I know which Stage 2 Rule due dates apply to my system?
Water system size (population served) determines which “Schedule” water systems must follow to comply with Stage 2 Rule requirements. If your water system has a non-emergency interconnection with another water system, the population that determines your “Schedule” is based on the population of the largest water system in your combined distribution system.

Schedule Number

Public Water System Population1

Submit IDSE document2 or receive a 40/30 Certification of Very Small System Waiver (VSS)

Complete IDSE monitoring or study

Submit IDSE Report

Begin routine monitoring according to Stage 2 Rule (Stage 2 Compliance Monitoring)

Due Dates


≥ 100,000

October 1, 2006

September 30, 2008

January 1, 2009

April 1, 2012


50,000 – 99,999

April 1, 2007

March 31, 2009

July 1, 2009

October 1, 2012


10,000 – 49,999

October 1, 2007

September 30, 2009

January 1, 2010

October 1, 2013


< 10,000

April 1, 2008

March 31, 2010

July 1, 2010

October 1, 20133

October 1, 20144

1 Public water systems that have non-emergency interties with other water systems must comply with the deadlines for the largest water system in their combined distribution system.

2 An Initial Distribution System Evaluation (IDSE) document is a standard monitoring plan or system-specific study plan. NTNC water systems serving < 10,000 people are exempt.

3 Ground water systems and surface water systems that did not have to do Cryptosporidium monitoring under the Long Term 2 Enhanced Surface Water Treatment Rule (LT2ESWTR).

4 Surface water systems that also did Cryptosporidium monitoring under LT2ESWTR.

What are the differences between Stage 1 and Stage 2?
The Stage 2 Rule has three major changes to the Stage 1 Rule: 1) water systems choose Stage 2 monitoring locations based on monitoring data (historical Stage 1 data and/or data collected through the IDSE); 2) Stage 2 bases compliance on locational running annual average (LRAA) calculations at each individual monitoring location; and 3) the Stage 2 Rule now includes Consecutive Systems that do not provide their own treatment, but receive treated water from a supplying water system.

Which monitoring do I need to do – Stage 1 or Stage 2 – and when?
Water systems currently required to sample under the Stage 1 Rule must continue routine compliance monitoring for TTHM and HAA5 until the date for routine monitoring under the Stage 2 Rule begins. Stage 2 compliance monitoring requirements will replace the Stage 1 compliance monitoring requirements and take effect as noted in the table above. Before beginning the Stage 2 DBP compliance monitoring you should complete a compliance monitoring plan.

What changes are there for my bromate, chlorite, and disinfectant residual monitoring?
Monitoring for bromate, chlorite, and disinfectant residuals under the Stage 2 Rule will continue as required by the Stage 1 Rule. Water systems using ozone may qualify for reduced bromate monitoring if they use a laboratory certified to analyze bromate using one of the new USEPA-approved methods.

How do I determine my Stage 2 DBP Compliance Monitoring locations?
Stage 2 uses the data collected for both Stage 1 and the IDSE for choosing the best TTHM and HAA5 monitoring locations. The Stage 2 Rule gave very specific instructions for choosing the locations so it is important to follow the available guidance. You will use the selected monitoring locations for Stage 2 Rule compliance monitoring according to the schedule in the table above.

I completed the IDSE. What do I do next?
If you did not submit your IDSE directly to EPA, you must submit an IDSE Report to the Vermont Drinking Water & Groundwater Protection Division according to the schedule in the above table. The IDSE report may also be used as your monitoring plan for the Stage 2 compliance monitoring. If you intend to use the IDSE Report as your Stage 2 monitoring plan, it must include compliance calculation procedures for TTHM and HAA5 MCLs and disinfectant residual MRDLs.

I received a VSS Waiver, a 40/30 Certification, or my system is a NTNC – What do I do next?
There are three cases in which water systems are not required to conduct an IDSE: 1) the water system received a VSS Waiver (Very Small System Waiver), 2) the water system received a “40/30 Certification”, or 3) the water system is an NTNC water system serving less than 10,000 customers. If one of these situations applies to your system, you must use historic Stage 1 Rule monitoring results to select your routine Stage 2 Rule monitoring locations. Select sites and sampling times based on the locations and times of year when the TTHM and HAA5 results were highest. Then prepare a compliance monitoring plan using these selected locations. For guidance on preparing the plan please refer to the guidance document by following the link, below:

Stage 2 DBP Compliance Monitoring Plan Guidance (in Brief and Detailed) 

The Stage 2 compliance monitoring plan must be completed by the date routine Stage 2 DBP monitoring begins. Meeting this deadline will ensure you are ready to start the Stage 2 DBP compliance monitoring in 2012, 2013, or 2014, whichever applies to your water system.

Monochloramines and Chloramination:

For some systems, one treatment option for minimizing DBP formation is to avoid using chlorine as the disinfectant. Another disinfectant considered effective in minimizing regulated DBPs is monochloramine. The following resources should help in learning more about monochloramines and chloramination:


Vermont Water Supply Division Symposium on Drinking Water - Presentations


DBP Stage 2 - Additional Resources

The following reference and guidance documents may prove useful to you in better understanding and minimizing the formation of Disinfection Byproducts:

Resources for Maintaining Compliance with the DBPR:

Best Management Practices (BMP) for DBP Minimization:


Directions for Electronic Submission of your Stage 2 DBP Compliance Monitoring Plan

When you have completed your Stage 2 DBP Compliance Monitoring Plan- you can submit it electronically directly to the Vermont Drinking Water & Groundwater Protection Division. Save the completed digital plan form first to your computer’s hard drive (you may also wish to save a printed copy), then submit it as a file attachment e-mailed to the following with Stage 2 DBP Compliance Monitoring Plan in the subject line: doug.kievit-kylar@state.vt.us.


PLEASE attach also a distribution system schematic with ALL selected monitoring location(s) clearly labeled.


Upon review, systems will either be contacted with a request for additional information OR sent a letter of approval.




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