Catalytic Carbon Filter: Chloramine and H2S Removal | Western Carbon

Standard granular activated carbon removes free chlorine well, but it struggles with two common problems: chloramine, which many water suppliers use as a long-lasting disinfectant, and hydrogen sulphide, the rotten-egg gas found in many well and groundwater supplies. For these, a catalytic carbon filter is often the better tool.

This guide explains what catalytic carbon is, how it works on chloramine and hydrogen sulphide, how to size and operate a filter, and where the limits are. Western Carbon supplies granular activated carbon and specialised media, and the broader context of how carbon is used for water is in activated carbon in water treatment. If you want the basics of what carbon removes in general, start with our guide to what a carbon water filter removes.

1. What Is Catalytic Carbon?

Catalytic carbon is activated carbon whose surface has been modified, usually during manufacture, so that it accelerates certain chemical reactions. Ordinary carbon works mainly by physical adsorption. Catalytic carbon still adsorbs, but its surface also acts as a catalyst, promoting reactions such as the reduction of chloramine and the oxidation of hydrogen sulphide.

The product is generally granular and looks like standard carbon, often in mesh sizes such as 8×30 or 12×40. The difference is in the surface chemistry, and in how the carbon behaves in service. Because it relies on reactions rather than only on capacity, catalytic carbon can achieve removal in much shorter contact time than ordinary carbon for these specific contaminants.

It is not a universal improvement. For many organic chemicals, a standard high-capacity carbon remains the better choice, and catalytic carbon should be selected for the contaminants where its chemistry helps.

2. Why Chloramine and Hydrogen Sulphide Are Hard Problems

Chloramine

Chloramine, formed from chlorine and ammonia, persists in distribution systems longer than free chlorine, which is why many suppliers use it. The US EPA explanation of chloramines in drinking water describes its purpose and why some users, such as dialysis centres and aquarium keepers, need to remove it. Standard carbon removes it slowly, so conventional beds would need to be impractically large.

Hydrogen sulphide

Hydrogen sulphide is a gas dissolved in water, often from groundwater in contact with sulphur-bearing minerals or from sulphate-reducing bacteria. It gives a rotten-egg smell at very low concentrations; estimates of the odour threshold in water are around the order of a tenth of a milligram per litre or less. It also tarnishes metals and stains fixtures. In air it is a toxic hazard, and the NIOSH pocket guide entry for hydrogen sulfide and OSHA guidance describe the exposure risks, which is relevant when sulphide-laden water is aerated or vented.

It helps to remember that catalytic carbon is still a carbon, and it still adsorbs. The difference is that its surface also drives a reaction, which means two mechanisms are working at once. In real water the adsorption side can become saturated by organics over time while the catalytic side keeps working, which is why a bed may continue to treat chloramine long after its adsorptive capacity for taste and odour compounds has been spent.

3. How Catalytic Carbon Removes Chloramine

Chloramine is removed by a catalytic reduction reaction at the carbon surface. In simplified terms, the chloramine reacts with water and the carbon surface to produce chloride and ammonia. The carbon surface is slowly oxidised in the process, which is part of why the carbon eventually loses activity.

Standard carbon can also do this, but its catalytic activity towards chloramine is lower, so it needs far longer contact time. Catalytic carbon has a surface that makes the reaction faster, so a shorter empty bed contact time achieves the same result. In practice, designers commonly allow several times the contact time for chloramine that they would use for free chlorine, and catalytic carbon reduces the multiple.

The detailed rate depends on temperature, pH and the carbon, so supplier data or a pilot test is the reliable basis for design.

4. How Catalytic Carbon Removes Hydrogen Sulphide

For hydrogen sulphide, the catalytic surface promotes oxidation by dissolved oxygen in the water. The sulphide is oxidised first to elemental sulphur, which can deposit on the carbon, and in part to sulphate, which passes through in the treated water. The reaction therefore needs oxygen. If the water is oxygen-poor, as many sulphide-bearing groundwaters are, a source of oxygen such as aeration upstream may be needed.

Because some of the product is solid sulphur that builds up on the carbon, the bed loses capacity gradually. Backwash helps flush loosely held deposits, and eventually the carbon is replaced, or in some cases treated with caustic to dissolve sulphur, depending on the grade and supplier guidance.

Water chemistry sets the practical limits. Chloramine removal rate depends on pH, temperature and the ratio of chlorine to ammonia in the water, so a design that works on one supply may need more contact time on another. Cold water slows the reaction, and designers serving cold climates usually allow a deeper bed or a longer contact time to hold the same outlet residual through winter.

5. Catalytic Carbon vs Standard GAC and Other Media

Choosing catalytic carbon makes sense where chloramine or sulphide is the main target. Where the main targets are chlorine, taste and organics, standard carbon, perhaps coconut shell or coal-based, is generally more economical. The two types can also be combined, with a catalytic stage first for chloramine and sulphide, followed by a high-capacity stage for organics.

Other methods address sulphide as well. Oxidation by aeration, chlorination or oxidising filter media is common at higher concentrations. Media such as manganese dioxide are used for iron, manganese and sometimes sulphide duty. Very high sulphide levels are usually better handled by an oxidation step ahead of carbon, since sulphur deposition would otherwise exhaust the carbon quickly.

6. Sizing a Catalytic Carbon Filter

Sizing follows the same principle as any carbon bed. Choose an empty bed contact time for the target, then carbon volume equals peak flow multiplied by that time. The difference lies in the contact time chosen. Free chlorine needs only a short contact time. Chloramine needs longer, and catalytic carbon reduces the multiple compared with standard carbon. Sulphide duty depends on concentration and oxygen availability.

Design inputs

• Chloramine or sulphide concentration, and the target outlet level.
• Peak and average flow.
• Temperature and pH, since rates fall at low temperature.
• Dissolved oxygen, for sulphide duty.
• Iron and manganese, which foul the bed.
• Sediment load, which calls for pre-filtration.

Surface loading and bed depth limits are the same as for any bed, and the supplier’s data sheet gives the recommended loading limits for each grade. Use the EPA drinking water treatability database for contaminant-level information, and the WHO drinking water quality guidelines for the international context of the contaminants involved.

Sulphide control has its own practical details. The reaction needs dissolved oxygen, and where the water has little or none, aeration ahead of the bed or a small dose of oxidant can restore the reaction. Without it, the carbon may simply adsorb sulphide until it is full, and the bed then fails far sooner than expected, often with a sudden return of the rotten egg smell.

7. Operating Catalytic Carbon: Backwash, Oxygen and Fouling

Operating practice has a large effect on life. Backwash regularly to remove fines, sediment and loosely held sulphur, and to keep the bed from compacting. Ensure backwash flow is enough to expand the bed without losing carbon, as discussed in granular carbon mesh size.

Protect the bed from fouling. Iron and manganese precipitate on carbon and reduce its activity. A pre-filter or oxidation stage reduces the burden. Sediment also blocks the surface, so a sediment filter ahead of the tank pays for itself.

Oxygen for sulphide

If dissolved oxygen is too low, sulphide removal falls. Aeration or air injection ahead of the bed supplies oxygen, though care is needed because venting hydrogen sulphide gas creates an odour and safety issue; see the OSHA and NIOSH guidance noted above.

8. Critical Applications: Dialysis, Aquaculture and Beverage

Chloramine removal is especially important where it harms the end use. In dialysis water treatment, chloramine can injure patients if it reaches the dialysate, so carbon beds are used with long contact times, commonly in series, and testing is required between the beds. In aquaculture and aquariums, chloramine is toxic to fish and is removed from make-up water, and the ammonia it releases must also be considered.

In beverage and food production, chloramine and sulphide affect taste and can interfere with processes. In reverse osmosis systems, chloramine can damage membranes, and carbon pre-treatment protects them as described in activated carbon in RO systems. For drinking water more generally, the National Primary Drinking Water Regulations set the context for disinfectant residuals.

Backwashing is essential for catalytic carbon, because it keeps the bed open and lets trapped solids and elemental sulphur move out rather than building up. Operators should back wash on a regular schedule and after any event that brings in sediment, and should expand the bed enough to separate the grains without losing carbon to the drain.

9. Limits, Troubleshooting and Replacement

Catalytic carbon has limits. It does not remove ammonia, hardness, nitrate or microbes. It can lose activity as its surface is oxidised or fouled. Common signs of trouble are returning sulphur smell, detectable chloramine in the outlet, and rising pressure drop.

• Chloramine breaking through: bed too small, flow too high, cold water or exhausted carbon.
• Sulphur smell returning: low oxygen, bed fouled with sulphur or iron, or carbon exhausted.
• Rising pressure drop: sediment or sulphur build-up, or compaction needing backwash.
• Ammonia in the outlet: normal by-product of chloramine reduction, not removed by carbon.

Plan for replacement by testing the outlet regularly and replacing on measured breakthrough. Our guide to when to replace an activated carbon filter provides practical triggers. Check guidance from the American Water Works Association for plant-scale practice.

10. Who Uses Catalytic Carbon Filters?

Catalytic carbon is used wherever chloramine or sulphide must be controlled, and Western Carbon supports each of these buyers.

• Dialysis and healthcare water treatment systems.
• Homes and buildings on chloraminated or sulphur-bearing supplies.
• Aquaculture, aquarium and hatchery water systems.
• Beverage, food and industrial plants and reverse osmosis pre-treatment.

Water-focused lines such as WestAqua and WestFiltrA and the applications overview show how carbon is applied across sectors.

11. Related Reading

12. Frequently Asked Questions