Disinfectants & Disinfection Byproducts Rules

Disinfection is essential for controlling disease-causing microorganisms in drinking water, but disinfectants can react with naturally occurring materials in water and form disinfection byproducts, commonly called DBPs.

The federal Stage 1 and Stage 2 Disinfectants and Disinfection Byproducts Rules, commonly called the DBPRs, are designed to limit exposure to regulated disinfectants and DBPs while preserving adequate microbial protection.

For operators, this creates an important operating principle: control DBPs without weakening disinfection to the point that microbial safety is compromised.

Why Disinfection Byproducts Form

Disinfectants can react with substances naturally present in source water.

Important DBP precursor materials include:

• natural organic matter;
• total organic carbon;
• bromide;
• other oxidizable compounds.

The type and amount of DBPs formed depend on source-water chemistry, disinfectant type, dose, contact conditions, temperature, pH, and water age.

Major Regulated Disinfection Byproducts

The federal DBPR framework regulates several important groups or individual DBPs:

• total trihalomethanes, or TTHM;
• five haloacetic acids, or HAA5;
• bromate;
• chlorite.

Total Trihalomethanes

TTHM is the sum of four regulated trihalomethanes:

• chloroform;
• bromodichloromethane;
• dibromochloromethane;
• bromoform.

The federal TTHM MCL is:

0.080 mg/L

This is equivalent to:

80 micrograms per liter, or 80 ppb.

Haloacetic Acids

HAA5 refers to the sum of five regulated haloacetic acids.

The federal HAA5 MCL is:

0.060 mg/L

This is equivalent to:

60 micrograms per liter, or 60 ppb.

Bromate

Bromate is a regulated disinfection byproduct associated primarily with ozonation when bromide is present in the source water.

The federal bromate MCL is:

0.010 mg/L

or:

10 ppb.

Chlorite

Chlorite is associated with chlorine dioxide treatment.

The federal chlorite MCL is:

1.0 mg/L.

Disinfectants Are Also Regulated

The DBPR framework does not regulate only byproducts.

It also establishes Maximum Residual Disinfectant Levels, or MRDLs, for important drinking-water disinfectants.

The regulated disinfectants include:

• chlorine;
• chloramines;
• chlorine dioxide.

Chlorine MRDL

The federal MRDL for chlorine is:

4.0 mg/L as Cl2.

Chloramine MRDL

The federal MRDL for chloramines is:

4.0 mg/L as Cl2.

Chlorine Dioxide MRDL

The federal MRDL for chlorine dioxide is:

0.8 mg/L.

MRDL Versus MRDLG

A Maximum Residual Disinfectant Level, or MRDL, is an enforceable regulatory limit.

A Maximum Residual Disinfectant Level Goal, or MRDLG, is a non-enforceable health goal.

For exam purposes:

• MRDL = enforceable;
• MRDLG = non-enforceable health goal.

Stage 1 DBPR

The Stage 1 DBPR established or strengthened federal limits for:

• TTHM;
• HAA5;
• bromate;
• chlorite;
• chlorine residual;
• chloramine residual;
• chlorine dioxide residual.

The rule also includes requirements related to precursor removal for certain surface-water treatment systems.

Stage 2 DBPR

The Stage 2 DBPR strengthened control of TTHM and HAA5 exposure in distribution systems.

A major change was the move toward compliance based on individual monitoring locations rather than allowing high concentrations at one site to be masked by lower concentrations elsewhere in the distribution system.

Running Annual Average

Under the earlier Stage 1 framework, TTHM and HAA5 compliance was generally evaluated using a running annual average, or RAA, across applicable monitoring results.

A running annual average considers results over a moving four-quarter period.

Locational Running Annual Average

Stage 2 introduced the Locational Running Annual Average, or LRAA, for TTHM and HAA5 compliance.

The LRAA is calculated separately for each compliance monitoring location.

This means:

each monitoring location must independently meet the applicable TTHM and HAA5 MCL.

Why LRAA Matters

DBPs can increase as treated water travels through the distribution system.

Locations with long water age can have higher DBP concentrations than locations close to the treatment plant.

Using a system-wide average could hide a persistent high-DBP location.

The LRAA framework reduces that problem by evaluating each monitoring location individually.

Example of an LRAA

Suppose quarterly TTHM results at one monitoring location are:

• Quarter 1: 0.060 mg/L;
• Quarter 2: 0.070 mg/L;
• Quarter 3: 0.090 mg/L;
• Quarter 4: 0.080 mg/L.

The four-quarter LRAA is:

(0.060 + 0.070 + 0.090 + 0.080) ÷ 4

= 0.075 mg/L

This LRAA is below the TTHM MCL of 0.080 mg/L.

A Single Result Above the MCL

A single TTHM or HAA5 sample above its numerical MCL does not automatically mean the LRAA exceeds the MCL.

Compliance is determined using the applicable LRAA framework.

However, a high individual result can be an important warning that future LRAA compliance is threatened.

Operational Evaluation Level

The Stage 2 DBPR includes an Operational Evaluation Level, commonly abbreviated OEL.

The OEL is designed to provide an early warning that TTHM or HAA5 concentrations at a monitoring location could soon produce an LRAA exceedance.

Operational Evaluation

When an applicable OEL is exceeded, the system must investigate the operational factors contributing to elevated DBPs.

The evaluation can examine:

• source-water conditions;
• treatment performance;
• disinfectant dose;
• precursor removal;
• storage operation;
• distribution-system water age;
• tank turnover;
• booster disinfection;
• seasonal conditions.

The OEL Is an Early Warning Tool

An OEL exceedance is not the same thing as an MCL violation.

Its purpose is to identify conditions that could lead to a future TTHM or HAA5 violation if the system does not respond.

Why TTHM and HAA5 Often Increase in the Distribution System

DBP formation can continue after water leaves the treatment plant.

Factors that can increase TTHM or HAA5 formation include:

• high natural organic matter;
• higher disinfectant dose;
• long contact time;
• high water age;
• warm water;
• certain pH conditions;
• poor tank turnover.

Temperature

DBP formation often increases as water temperature increases.

As a result, many systems experience their highest TTHM or HAA5 concentrations during warm seasons.

Water Age

Water age is one of the most important distribution-system variables affecting DBP formation.

Long water age gives disinfectants and organic precursor material more time to react.

Locations with chronic low demand, oversized storage, dead ends, or poor turnover can therefore experience elevated DBPs.

Storage Tanks

Poor tank turnover can increase water age.

Operational approaches that can improve turnover include:

• adjusting operating levels;
• changing fill-and-draw patterns;
• improving mixing;
• balancing pressure zones;
• reducing unnecessary storage residence time.

Natural Organic Matter

Natural organic matter is an important precursor for TTHM and HAA5 formation.

Removing precursor material before disinfectant addition can reduce DBP formation potential.

Total Organic Carbon

Total organic carbon, or TOC, is commonly used as an indicator of organic precursor material.

Certain surface-water systems using conventional treatment have regulatory requirements related to enhanced coagulation or enhanced softening to improve organic precursor removal.

Enhanced Coagulation

Enhanced coagulation uses coagulation conditions designed not only for turbidity removal but also for increased removal of organic DBP precursors.

Operational variables can include:

• coagulant dose;
• pH;
• alkalinity;
• mixing;
• flocculation conditions.

DBP Control Begins Before Disinfection

A common mistake is trying to control DBPs only by reducing chlorine dose.

A better treatment strategy can include reducing precursor material before disinfection.

This can allow strong microbial treatment while reducing DBP formation potential.

Chlorine Dose

Increasing chlorine dose can increase DBP formation when sufficient precursor material is present.

However, chlorine dose cannot simply be reduced without considering microbial requirements.

Contact Time

Longer disinfectant contact can increase formation of some DBPs.

At the same time, sufficient contact time is essential for microbial inactivation.

Operators must therefore manage both:

• required microbial CT;
• unnecessary extended contact with DBP precursors.

Point of Disinfectant Application

Changing the point at which disinfectant is applied can influence DBP formation.

For example, delaying chlorination until after substantial precursor removal can reduce DBP formation in some treatment configurations.

Any such change must preserve required microbial treatment.

Chloramination

Some systems use chloramines as a secondary disinfectant because chloramination can reduce formation of certain chlorinated DBPs compared with free chlorine.

However, changing to chloramines creates other operational concerns.

These can include:

• nitrification;
• ammonia control;
• chloramine residual management;
• biofilm response;
• different disinfection effectiveness.

Do Not Treat Chloramination as a Universal DBP Solution

Chloramines can reduce some DBPs but create a different distribution-system chemistry.

A system converting disinfectants must evaluate the entire treatment and distribution system.

Ozone and Bromate

Ozone is a strong disinfectant and oxidant.

When bromide is present in source water, ozonation can form bromate.

Bromate control can therefore involve managing:

• source-water bromide;
• ozone dose;
• pH;
• contact conditions;
• treatment configuration.

Chlorine Dioxide and Chlorite

Chlorine dioxide can produce chlorite as a disinfection byproduct.

Systems using chlorine dioxide must monitor and control both chlorine dioxide residual and chlorite according to applicable requirements.

Microbial Risk Versus DBP Risk

The DBPRs are part of a broader microbial and disinfection-byproduct regulatory framework.

Operators must protect customers from both:

• short-term microbial risk;
• long-term excessive DBP exposure.

Do Not Sacrifice Microbial Protection

Reducing disinfectant residual or contact time can reduce some DBP formation.

But if the change causes inadequate microbial inactivation, the system has traded one public-health risk for another.

That is not acceptable treatment optimization.

Simultaneous Compliance

Simultaneous compliance means operating treatment so that the system continues to meet both microbial protection requirements and DBP limits.

This can require coordinated control of:

• coagulation;
• precursor removal;
• filtration;
• disinfection dose;
• CT;
• pH;
• distribution residual;
• water age;
• storage turnover.

Example: Reduce DBPs by Lowering Chlorine

A system has high TTHM concentrations and considers sharply reducing chlorine dose.

The operator must first determine whether lower chlorine will still provide:

• required primary disinfection;
• required CT;
• adequate distribution-system residual.

If microbial protection would be compromised, a different DBP-control strategy is needed.

Example: High DBPs at One Remote Site

A treatment plant has acceptable DBP concentrations near the plant, but one remote monitoring location repeatedly has high TTHM.

This pattern suggests the operator should investigate distribution conditions such as:

• long water age;
• poor storage turnover;
• low demand;
• excessive booster disinfectant;
• hydraulic configuration.

Example: Warm-Weather Increase

TTHM concentrations rise during summer even though chlorine dose is similar to winter operation.

The operator should recognize that higher temperature can accelerate DBP formation.

Seasonal operating strategies may therefore be needed.

Example: Bromate

A plant uses ozone and has measurable bromide in its source water.

The operator should recognize bromate as the DBP of particular concern.

TTHM is not the only DBP requiring attention.

Example: Chlorine Dioxide

A plant uses chlorine dioxide.

The operator must recognize both:

• chlorine dioxide MRDL requirements;
• chlorite MCL requirements.

Monitoring Plans

DBPR compliance sampling must be performed at approved locations and frequencies.

Monitoring locations are selected to characterize distribution-system DBP exposure, including locations that can experience relatively high TTHM or HAA5 concentrations.

Sampling Location Matters

A low DBP result at the treatment-plant outlet does not demonstrate that all distribution-system locations have low DBPs.

DBPs can continue forming as water moves through storage and distribution.

Operational Data to Review

When DBP concentrations increase, useful operational data can include:

• raw-water TOC;
• treated-water TOC;
• temperature;
• pH;
• coagulant dose;
• disinfectant dose;
• disinfectant residual;
• plant flow;
• storage levels;
• tank turnover;
• estimated water age;
• TTHM and HAA5 trends by monitoring location.

Trend Data Are More Useful Than One Number

A single compliant result does not prove that DBPs are well controlled throughout the year.

Operators should look for:

• seasonal trends;
• location-specific trends;
• changes after treatment adjustments;
• increasing LRAA values;
• OEL exceedances.

Common DBPR Exam Mistakes

• Confusing TTHM and HAA5 limits.
• Forgetting that bromate is associated with ozone.
• Forgetting that chlorite is associated with chlorine dioxide.
• Confusing an MRDL with an MCL.
• Confusing MRDL with MRDLG.
• Using a system-wide average when Stage 2 requires a locational running annual average.
• Assuming one sample above 0.080 mg/L automatically means a TTHM LRAA violation.
• Ignoring water age and storage turnover.
• Trying to solve all DBP problems by reducing disinfectant dose.
• Ignoring microbial CT while changing disinfection.
• Assuming chloramines eliminate every DBP problem.
• Ignoring bromide when using ozone.

A Practical DBP Investigation

1. Identify which DBP or disinfectant parameter is increasing.
2. Confirm the compliance monitoring location.
3. Review the current LRAA.
4. Determine whether an OEL has been exceeded.
5. Review seasonal source-water conditions.
6. Review TOC and precursor removal.
7. Review disinfectant dose and point of application.
8. Review microbial CT requirements.
9. Evaluate storage turnover and water age.
10. Evaluate distribution residuals and booster disinfection.
11. Implement corrective operational changes without sacrificing microbial protection.
12. Continue monitoring to verify improvement.

What to Remember for the Exam

• Disinfection byproducts form when disinfectants react with naturally occurring material in water.
• TTHM and HAA5 are major regulated groups of DBPs.
• The TTHM MCL is 0.080 mg/L, or 80 ppb.
• The HAA5 MCL is 0.060 mg/L, or 60 ppb.
• The bromate MCL is 0.010 mg/L.
• Bromate is especially associated with ozonation when bromide is present.
• The chlorite MCL is 1.0 mg/L.
• Chlorite is associated with chlorine dioxide treatment.
• The chlorine MRDL is 4.0 mg/L as Cl2.
• The chloramine MRDL is 4.0 mg/L as Cl2.
• The chlorine dioxide MRDL is 0.8 mg/L.
• MRDL is enforceable; MRDLG is a non-enforceable health goal.
• Stage 2 DBPR uses locational running annual averages for TTHM and HAA5 compliance.
• Each Stage 2 compliance monitoring location must independently meet the applicable LRAA requirement.
• An individual result above an MCL is not automatically the same as an LRAA violation.
• An OEL is an early-warning tool that can trigger an operational evaluation.
• Higher temperature and longer water age often increase DBP formation.
• Poor storage turnover can contribute to elevated distribution-system DBPs.
• Removing organic precursors before disinfection can reduce DBP formation.
• Lowering disinfectant dose is not acceptable if required microbial protection is lost.
• Operators must manage microbial protection and DBP control simultaneously.