Activated Carbon Filters Explained: Granular vs Carbon Block

If your tap water tastes of swimming pool or carries a musty smell, activated carbon is almost certainly the right tool. It is one of the most widely used filtration media in the world, and for good reason: it is effective, relatively affordable, and well understood. But the term ‘activated carbon filter’ covers two quite different products — granular activated carbon (GAC) and carbon block — and the distinction matters more than most suppliers let on.

This guide explains how each type works, what each can and cannot remove, and how to decide which suits your home. It also covers where activated carbon fits within a broader water filtration system, because carbon is rarely a standalone solution.

How activated carbon actually works

Activated carbon is carbon — usually derived from coconut shell, coal or wood — that has been treated with high heat and steam to open up millions of tiny pores. This creates an enormous internal surface area: a single gram can have a surface area measured in the hundreds of square metres. Contaminants in the water are drawn onto and held by this surface in a process called adsorption (not absorption — the pollutants stick to the surface rather than soaking in).

The main categories activated carbon handles well:

• Chlorine and chloramines — the disinfectants municipalities add, which cause that characteristic chemical taste and smell
• Trihalomethanes (THMs) and other disinfection by-products
• Volatile organic compounds (VOCs) — solvents, pesticides, herbicides at low concentrations
• General taste and odour compounds — hydrogen sulphide (‘rotten egg’ smell), geosmin (earthy smell)

What activated carbon does not remove:

• Dissolved minerals, including hardness (calcium and magnesium)
• Nitrates, fluoride or heavy metals such as lead — unless the carbon has been specifically impregnated with a secondary media
• Bacteria, viruses or other pathogens
• Total dissolved solids (TDS)

Because chlorine removal is the headline job, it is worth reading more about chlorine in tap water to understand what you are actually filtering out and why it is added in the first place.

Granular activated carbon (GAC): the case for it

GAC filters hold loose carbon granules through which water percolates. Because the granules are not bound together, there are larger voids between them, and water moves through relatively quickly.

Advantages of GAC:

• Higher flow rate — less pressure drop across the cartridge
• Lower cost per cartridge
• Suitable for whole-house pre-filtration or for treating large volumes where taste and odour correction is the primary goal
• Straightforward to source and replace

Limitations of GAC:

• Channelling: water can find a path of least resistance through the loose granules, reducing contact time and effectiveness
• Less effective against smaller molecules and some synthetic chemicals that require longer contact time
• The loose bed can shift during installation or high-flow periods

GAC is commonly used as a first or second stage in multi-stage under-sink systems, or in larger whole-house housings upstream of finer filtration.

Carbon block filters: when you need more contact time

Carbon block filters compress activated carbon particles together with a binder into a solid, uniform block. Water is forced through the entire matrix rather than finding shortcuts.

Advantages of carbon block:

• Much greater contact time between water and carbon, improving adsorption of difficult compounds
• More consistent pore structure means they also act as a mechanical filter, typically rated at 0.5–5 microns (confirm the rating on the specific cartridge you buy)
• Better performance against chloramines, which are harder to remove than free chlorine
• Lower risk of channelling

Limitations of carbon block:

• Slower flow rate — you will notice reduced pressure, especially in a point-of-use drinking filter
• Generally costs more per cartridge than an equivalent GAC cartridge
• The mechanical filtration function means sediment loads them up faster; a sediment pre-filter upstream is strongly advisable

For drinking water at the tap, a carbon block stage — often preceded by a sediment filter and compared alongside other media types — is usually the better choice.

Comparing GAC and carbon block at a glance

Feature	Granular (GAC)	Carbon Block
Flow rate	Higher	Lower
Contact time	Shorter	Longer
Chlorine removal	Good	Very good
Chloramine removal	Moderate	Better
Mechanical filtration	No	Yes (0.5–5 µm typical)
Sediment pre-filter needed	Advisable	Essential
Relative cartridge cost	Lower	Higher
Best use	Whole-house taste/odour, pre-stage	Under-sink drinking water, final stage

Where each type fits in a South African home

Most South African municipal supplies are treated to SANS 241 (2024 edition), which sets drinking-water quality standards. However, the water reaching your tap can pick up taste compounds, disinfection by-products and occasional sediment along the way — particularly in older reticulation networks or during periods of high demand. Carbon filtration addresses the taste-and-odour dimension well; it does not replace disinfection.

A practical setup for a Johannesburg townhouse or suburban home often looks like this:

1. Whole-house sediment filter (20–50 micron) at the mains entry
2. GAC whole-house filter for chlorine reduction throughout the house
3. Carbon block under-sink unit (sometimes combined with reverse osmosis) for drinking and cooking water at the kitchen tap

If your concern is purely drinking-water taste, a standalone under-sink carbon block unit is often enough. If you want chlorine-free water for showers, a GAC whole-house stage makes sense.

Not sure which filter type suits your water and your plumbing? Compare 3 free quotes — vetted installers, no obligation.

Carbon filter lifespan and when to replace

Activated carbon exhausts over time. Once the adsorption sites are saturated, the filter passes contaminants straight through — and you will often not notice, because the water may still look clear. Replacement intervals depend on cartridge size, the volume of water processed and your incoming water quality.

As a rough guide, under-sink carbon block cartridges are often rated at 6–12 months or a set volume of litres — whichever comes first. GAC whole-house cartridges may last a similar period but at much higher flow, so volume is the more relevant metric. For a detailed breakdown of what drives these intervals, see how often to replace water filters.

Do not rely on taste alone as your replacement signal. By the time you can taste the difference, the carbon has likely been exhausted for some time.

What activated carbon cannot do alone

It is worth being direct: activated carbon is a single tool, not a complete water-treatment solution. It will not address:

• Hardness — if you have scale on your kettle and geysers, you need a water softener or a scale-inhibition device
• Biological contamination — if your supply is from a borehole or a rainwater tank, carbon alone is insufficient; you need UV sterilisation or chemical dosing after filtration
• High TDS or nitrates — these require reverse osmosis or ion exchange

For homes on borehole water, carbon block is often a useful final polishing stage, but it must sit downstream of more robust treatment. If you are matching treatment to a specific water quality problem, getting your water tested first — and then getting quotes from installers who can interpret the results — will save you money and frustration.

Ready to match the right filter setup to your actual water? Get installer quotes tailored to your situation — no cost, no commitment.

Choosing between GAC and carbon block

If you are primarily correcting municipal chlorine taste and odour across the whole house, GAC is cost-effective and fit for purpose. If you want the best possible drinking water at the tap — particularly in an area where chloramines are used or where VOC exposure is a concern — a carbon block stage is the stronger choice.

In most cases, the two types complement each other rather than compete. A GAC pre-stage reduces the load on a downstream carbon block, extending its life and improving overall performance. That layered approach, matched to your water quality and household volume, is what a good installer will help you design.