Standard sediment filters are fantastic at removing physical dirt, dust, and rust from your rainwater. However, if your stored rainwater has a yellow tint, smells faintly of sulfur or decaying leaves, or just tastes "off," a sediment filter won't help. These issues are caused by dissolved organic compounds and chemicals that are smaller than any physical mesh. To fix the taste and smell of your water, you need the magic of Activated Carbon.
The Science of Adsorption
Notice the spelling: adsorption, not absorption. When a sponge soaks up water, it absorbs it into its volume. Adsorption is a chemical process where gas or liquid molecules adhere tightly to the surface of a solid. Activated carbon is nature's ultimate adsorption material.
Carbon (usually derived from coconut shells, wood, or coal) is "activated" by superheating it in an oxygen-free environment and then exposing it to steam. This process causes the carbon to fracture at a microscopic level, creating an incredibly vast network of internal pores. A single teaspoon of high-quality activated carbon has the internal surface area of an entire football field! As water flows through this highly porous media, organic molecules, chlorine, and volatile organic compounds (VOCs) are chemically trapped on the vast surface area.
Tackling Tannins
The most common taste and color issue in rainwater harvesting is tannins. If leaves sit in your gutters or sneak past your first flush diverter, they steep in your tank like a giant tea bag. Tannins give water a yellowish/brown color and a bitter, astringent taste. Activated carbon is highly effective at adsorbing tannins, returning your rainwater to a crystal-clear, tasteless state.
Carbon Block vs. Granular Activated Carbon (GAC)
When purchasing a carbon filter, you will generally choose between two styles:
- Granular Activated Carbon (GAC): These cartridges are filled with loose, sand-like grains of carbon. They allow for an excellent flow rate with minimal pressure drop. However, "channeling" can occur, where water finds the path of least resistance through the grains, bypassing some of the carbon. GAC filters are great for whole-house applications where high flow is needed.
- Carbon Block: These are solid blocks of compressed carbon dust. Because the water must force its way through the dense block, there is no channeling, resulting in much higher filtration efficiency and longer contact time. The trade-off is a significantly lower flow rate and a larger drop in water pressure. They are ideal for dedicated under-sink drinking water taps.
Placement in the Filtration Array
Carbon filters clog very easily if exposed to physical dirt. Therefore, a carbon filter should always be placed after your primary sediment filters (e.g., after the 5-micron or 1-micron spun poly cartridge). The water entering the carbon housing should already be physically clean.
Conclusion
If you're using your rainwater purely for irrigation, carbon filtration is an unnecessary expense. But if the goal is delicious, crystal-clear drinking water, an activated carbon block or GAC cartridge is an absolute necessity for that final polish.