Disinfection By-products (DBPs)

Disinfection byproducts (DBPs) can form when naturally occurring organic carbon reacts with chemical disinfectants such as chlorine. These disinfectants are used to protect public health by controlling microorganisms but high DBPs can have health effects. This is particularly a concern with surface water sources where organic carbon can be high and water systems are required to disinfect. In Vermont, DBPs are often highest in summer or fall when organic carbon and temperature are highest.


Sampling for DBPs

Any Non-Transient Non-Community (NTNC) or Community public drinking water system that routinely and regularly applies a chemical disinfectant is required to monitor for DBPs and to submit a DBP Sampling Plan to be approved by the Division. Testing for two groups of DBPs is required, total trihalomethanes (TTHM) and five haloacetic acids (HAA5). Sampling for DBPs may be required quarterly, annually, or every three years depending on the water source, population size, and prior DBP results. A water system can check its monitoring schedule .  Water systems that have started chlorinating or changed their water source should contact the Division for guidance and an updated schedule.

DBP Sampling Form

TTHM sample sites should be located where previous sampling results were highest. TTHM tend to increase with time after chlorination, so where data is limited, sample sites can be located at distribution system endpoints far from the treatment plant (but not dead ends without use) or where previous sampling results were highest. 

HAA5 also tend to increase with time but may be degraded by biofilms growing in dead ends or areas with low chlorine residual. HAA5 sample sites should be based on previous high sampling results or may located near the end of small distribution systems or midpoint of large distribution systems.

Additional guidance on DBP Sampling Site Location  for small systems is discussed in this excerpt from USEPA publication # 815-B-06-001, 2006. 

For details on sample kits and sample collection procedures, consult a state-certified laboratory.


Contaminant Limits and Compliance

Total trihalomethanes (TTHMs) include chloroform, bromoform, bromodichloromethane, and dibromochloromethane. The Maximum Contaminant Level (MCL) for TTHM is 80µg/L (equal to 0.080mg/L). 

Haloacetic acids (HAA5) include many compounds; currently, five are included in the drinking water standard: monochloroacetic acid, dichloroacetic acid, trichloroacetic acid, monobromoacetic acid, and dibromoacetic acid. The MCL for HAA5 is 60µg/L (equal to 0.060mg/L). 

USEPA has more information on specific DBP contaminants. Vermont Department of Health has more information on health effects of DBPs and disinfection using monochloramine.

Under the Stage 2 DBP Rule, DBP level compliance is assessed based on the Locational Running Annual Average (LRAA) of each DBP group at each location where it is sampled compared to the MCL. The LRAA is the average concentration over the last four calendar quarters at a specific sample site. A LRAA over the MCL constitutes a violation. Systems with low DPB results may be eligible for reduced DBP monitoring (lower frequency and/or number of sampling sites).  A high DBP result may trigger increased monitoring (higher frequency and/or number of sampling sites), such as returning a system from reduced monitoring back to quarterly monitoring where the LRAA can be assessed.  The Division will contact a water system if its monitoring schedule changes.

USEPA has details on  DBP regulation under the Stage 2 DBP Rule.


Operational Evaluations

In addition to the LRAA described above, a locational Operational Evaluation Limit (OEL) for each DBP is calculated as a weighted average of the previous three quarters, with the most recent data weighted twice. In other words, the OEL is the sum of twice the current result, plus the previous quarter, plus the quarter before that, all divided by 4. The OEL is also compared to the MCL above, but an OEL over the MCL does not constitute a violation. OELs are assessed to identify increases in DBPs before violations occur. When an OEL is exceeded, the water system must complete an Operational Evaluation Report to identify possible causes of elevated DBPs and strategies to reduce DBPs.


Organic Carbon Removal

Some 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:

TOC Reduction Requirements
Source Water TOC (mg/l) Source Water Alkalinity (mg/L as CaCO3)
0 to 60 >60 to 120 >120
>2.0 - 4.0 35.0% 25.0% 15.0%
>4.0 - 8.00 45.0% 35.0% 25.0%
> 8.0 50.0% 40.0% 30.0%

For more information, contact the DBP rule manager, 802-585-4891.

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