Activated Carbon for DBP Control in Drinking Water: GAC Selection, Pretreatment and Performance

Safe drinking water requires disinfection, but natural organic matter (NOM) can be contaminated by chlorine and other disinfectants to produce disinfection byproducts (DBPs). THMs and HAAs are frequently utilized as DBP regulators.. (Cl)
DBPs can be controlled by granular activated carbon (GAC) through two methods: firstly by eliminating organic precursors before disinfection; secondly, by simply absorbing some of the formed DBPs. The most effective method is dependent on the raw water, disinfection process, carbon properties, empty bed contact time (EBCT), and the location of the GAC filter in the treatment train.
GAC is seldom sufficient for drinking water plants to select by iodine number alone. A more dependable selection takes into account pore structure, NOM characteristics, hardness, ash, electrical conductivity (ECO), breakthrough behavior, drinking-water certification, and pilot-test results.

Why do we get DBPs in water?

A disinfectant can create DBPs by reacting with either organic or inorganic water.’
A simplified pathway is:
Natural Organic matter and disinfectant combined with the byproducts of infection.
Surface water and other groundwater sources, as well as decaying vegetation such as algae and wastewater impacts, are all potential sources of NOM. The makeup of it differs greatly depending on the water source.ea.

The most familiar DBP groups include:

• Trihalomethanes (THMs)
• Haloacetic acids (HAAs)
• Haloacetonitriles (HANs)
• Haloaldehydes (HALs)
• Brominated and iodinated DBPs

This matters when selecting activated carbon because DBPs are not a single contaminant. Different compounds have different molecular properties and adsorption behavior.

The practical objective is therefore not simply to find a carbon that “removes DBPs.” The treatment strategy should address where the DBPs are coming from and where GAC is positioned in the process.

How Does Activated Carbon Control DBPs?

GAC can contribute to DBP control through two main pathways.

1. Remove DBP Precursors Before Disinfection

For many drinking water systems, preventing DBPs from forming is more useful than trying to remove them afterward.

A GAC filter installed before chlorination can adsorb part of the NOM and dissolved organic carbon (DOC) that would otherwise react with chlorine.

Raw water → Pretreatment → GAC → Disinfection

With fewer reactive organic precursors entering the disinfection step, the potential for THM and HAA formation can decrease.

The Australian Drinking Water Guidelines specifically describe GAC used before disinfection as a method for reducing DBP formation by removing organic precursors.

Full-scale research supports this approach. A study covering three drinking water treatment plants found that GAC reduced DBP precursors, DBP formation, and total organic halogen across different GAC service lives.

2. Adsorb Some Preformed DBPs

GAC can also be installed after chlorination or used to treat water in which DBPs have already formed.

In this configuration, adsorption targets the contaminants already present rather than their precursors.

However, this should not be interpreted as “GAC removes every DBP.” Adsorption performance varies among DBP species and depends on water chemistry, carbon properties, and carbon service age.

In the three-plant full-scale study, GAC removed a range of preformed DBPs, including some haloacetonitriles and iodoacetic acids. At the same time, several DBPs, particularly some brominated and nitrogenous compounds, showed more complicated behavior after GAC treatment.

That is why DBP control is a more useful engineering concept than simply DBP removal.

Should You Target DBPs or Their Precursors?

The answer depends on the treatment objective.

For a water source with high NOM and high DBP formation potential, precursor removal is often the more logical starting point.

For water that already contains elevated THMs or other preformed DBPs, direct adsorption may also need to be evaluated.

In some systems, both objectives can be relevant.

How to Select GAC for DBP Control

There is no universal “DBP removal carbon.” GAC selection should start with the water chemistry and treatment conditions rather than a single specification on a product datasheet.

1. Pore Size Distribution

Pore structure determines which organic molecules can access the internal surface of activated carbon.
A useful distinction is:

• Micropores: generally associated with adsorption of smaller molecules
• Mesopores: important for larger organic molecules and transport into the carbon structure
• Macropores: provide transport pathways into smaller pores

For NOM and DBP precursor removal, the relationship between pore structure and performance can be more useful than total surface area alone.

A 2020 Water Research study evaluating multiple GAC media found that DOC removal and DBP reduction correlated particularly well with the prevalence of pores in the 5–10 nm range in the tested waters. The study also found that GAC did not selectively remove precursors of individual regulated DBPs; DBP formation potential was strongly related to residual DOC.

This is one reason why simply choosing the GAC with the highest iodine number can lead to the wrong conclusion.

2. Iodine Number

Iodine number is useful as a general indicator of adsorption capacity for relatively small molecules, but it should not be treated as a direct measurement of DBP removal performance.

Two carbons with similar iodine numbers can behave differently in real drinking water because of differences in pore-size distribution, surface chemistry, raw material, activation process, and competing organic matter.

Use iodine number as one selection parameter, not the entire selection method.

3. Methylene Blue and Other Larger-Molecule Indicators

Methylene blue or similar tests can provide additional information about adsorption behavior toward larger molecules.

These values can help when comparing carbon grades, but they still do not replace actual water testing.

The relevant question is not:
Which carbon has the highest laboratory number?

It is:
Which carbon provides the required adsorption capacity and service life under the actual water and operating conditions?

4. Hardness and Abrasion Resistance

GAC in a fixed-bed water treatment system is exposed to hydraulic loading, backwashing, transport, and repeated handling.
Low mechanical strength can generate fines, which may:

• Increase pressure drop
• Escape the carbon bed
• Create downstream filtration problems
• Increase carbon losses during backwashing

For this reason, hardness and abrasion resistance deserve attention alongside adsorption capacity.

5. Ash and Drinking-Water Quality

Ash represents inorganic material remaining in the carbon.

For drinking water applications, buyers should also consider extractables, metals, pH effects, and the relevant drinking-water contact requirements.

Do not assume that an industrial-grade carbon is automatically appropriate for potable water.

Depending on the target market, the project may require certification or compliance with applicable drinking-water standards and regulations.

6. EBCT: The Specification That Cannot Be Ignored

Empty Bed Contact Time (EBCT) is one of the most important operating parameters for a GAC system.
A simplified calculation is:
EBCT = GAC bed volume ÷ water flow rate

A higher EBCT generally provides more contact time between the water and carbon, but it also means a larger carbon bed and potentially higher capital cost.

A 2022 drinking-water case study evaluated different GACs and EBCTs for DBP precursor removal. The researchers found a trade-off between GAC selection, EBCT, capital cost, and operating cost.

This is why a supplier should not recommend a GAC grade based only on iodine number or carbon price.

Coconut Shell vs Coal-Based GAC for DBP Control

Coconut shell and coal-based GAC can both be used in drinking water treatment. The right choice depends on the carbon properties and the treatment objective.

The key point is that raw material alone does not determine DBP performance.

A 2022 full-scale case study compared coconut shell and bituminous coal GAC and found that both could achieve more than 90% NOM removal under the tested conditions. However, the tested carbons showed different adsorption and column performance, and GAC/EBCT selection affected the economics of the system.

Therefore, “coconut shell is always better” or “coal is always better” is not a useful engineering rule.

Why GAC Sometimes Fails to Control DBPs

A GAC system can underperform even when the carbon specification looks impressive on paper.

Failure 1: Selecting Carbon by Iodine Number Alone

High iodine number does not automatically mean high NOM or DBP precursor removal capacity.

Failure 2: Treating All DBPs as One Contaminant

THMs, HAAs, HANs and other DBPs do not necessarily behave the same way during adsorption.

Failure 3: Ignoring NOM Characteristics

Source water can contain different fractions of organic matter. A carbon that performs well in one water source may perform differently in another.

Failure 4: Insufficient EBCT

A high-capacity carbon operated at an unsuitable flow rate may not deliver the expected removal.

Failure 5: Ignoring Carbon Service Life

Fresh GAC and exhausted or biologically active GAC do not necessarily behave in the same way.

Failure 6: Ignoring Bromide and Iodide

GAC removes organic compounds, but it is not a universal solution for inorganic DBP precursors such as bromide.

Recent drinking-water guidance also identifies EBCT, water temperature, GAC configuration, carbon type, backwashing, DBP species, source-water quality, and pretreatment as factors affecting GAC performance.

Where Should GAC Be Installed in a Drinking Water Treatment Train?

The correct position depends on the treatment objective.

For DBP Precursor Control

Raw Water → Coagulation/Clarification → Filtration → GAC → Disinfection → Distribution
Here, the primary objective is to reduce NOM and other organic precursors before chlorine or another disinfectant is applied.

For Dechlorination and RO Protection

Pretreatment → GAC → Cartridge Filter → RO
GAC can remove residual chlorine and selected organic compounds before the RO membrane.

A downstream cartridge or safety filter may also be used to capture carbon fines before sensitive equipment.

For Preformed DBP Removal

Disinfection → GAC → Further Treatment/Distribution
This configuration requires careful evaluation because the target DBP species, carbon properties, and contact conditions determine whether direct adsorption will be effective.

What Should Be Tested Before Buying GAC?

A laboratory or pilot evaluation is often more useful than choosing carbon from a generic product datasheet.
Useful water-quality information includes:

• TOC or DOC
• UV254
• Existing THM and HAA concentrations
• DBP formation potential
• Chlorine or chloramine residual
• Bromide and iodide where relevant
• pH
• Temperature
• Turbidity
• Pretreatment conditions
• Flow rate

For GAC selection, consider testing:

1. Several candidate carbon grades
2. Adsorption capacity
3. Adsorption kinetics
4. Column performance
5. Breakthrough behavior
6. EBCT requirements
7. Carbon service life
8. Final treated-water quality

Pilot or rapid small-scale column testing is particularly useful when the project involves a large GAC bed or long-term municipal operation.

What to Ask an Activated Carbon Supplier

Before purchasing GAC for DBP control, ask the supplier more than:
“What is your iodine number?”
A useful technical request should include:

• What is the carbon’s pore-size distribution?
• What are the iodine number and methylene blue value?
• What is the hardness or abrasion resistance?
• What is the ash content?
• Is the product suitable and certified for the intended drinking-water market?
• What EBCT has been used in comparable applications?
• What raw-water conditions were used in performance testing?
• Can the supplier provide a pilot sample?
• Can the supplier evaluate the customer’s actual water?
• What data are available for breakthrough or service-life evaluation?

A serious supplier should be able to discuss carbon properties together with operating conditions, rather than selling one laboratory number as the answer to every water-treatment problem.

Frequently Asked Questions

Does activated carbon remove DBPs from drinking water?

Yes. GAC can adsorb some preformed DBPs, but its role is often more valuable in removing organic precursors before disinfection and reducing subsequent DBP formation. Performance depends on the DBP species, water chemistry, carbon properties and operating conditions.

Is GAC better for DBP removal before or after chlorination?

There is no universal answer. Before chlorination, GAC can remove NOM and DBP precursors. After chlorination, it can be evaluated for direct removal of some preformed DBPs and residual chlorine.

Is coconut shell activated carbon good for THM removal?

It can be, particularly when the carbon’s pore structure and operating conditions are suitable. However, THM performance should be evaluated using the actual water rather than assumed from the raw material alone.

Is coal-based GAC suitable for DBP precursor removal?

Yes. Coal-based GAC is widely used for NOM and DBP precursor control, but performance varies among grades. Pore structure, adsorption capacity, EBCT and source-water characteristics should be considered together.

Does a higher iodine number mean better DBP removal?

Not necessarily. Iodine number is useful for comparing certain adsorption characteristics, but it does not directly predict performance against every DBP or NOM fraction.

How important is EBCT for GAC DBP control?

Very important. EBCT affects the amount of contact time available for adsorption and therefore influences removal and breakthrough behavior. It should be considered together with carbon properties and raw-water quality.

Can GAC remove bromide from drinking water?

GAC is not a universal treatment for inorganic bromide. If bromide is contributing to brominated DBP formation, the treatment strategy should address the bromide source and the overall DBP formation pathway.

Should GAC for DBP control be pilot tested?

For significant municipal or industrial drinking-water projects, pilot or rapid small-scale column testing can provide much more useful information than relying only on standard carbon specifications.

Conclusion

Activated carbon can play two different roles in DBP control: removing organic precursors before disinfection and adsorbing selected DBPs after they have formed.
The best GAC cannot be selected from iodine number alone. Pore-size distribution, NOM characteristics, EBCT, carbon service life, hardness, ash, water chemistry and treatment configuration all influence the final result.
For a new DBP-control project, start with the water rather than the carbon catalogue. Define the target DBPs, measure the organic precursor load, determine where GAC should be positioned, and test candidate carbons under realistic operating conditions.
That approach gives engineers a much more reliable basis for selecting GAC—and gives the treatment plant a better chance of achieving stable DBP control over the life of the carbon bed.
If you are designing a DBP control system, our technical team can review your water data and recommend candidate GAC grades for pilot testing.