In open filters, which media arrangement provides superior removal of organics?

• A. GAC outperforms Anthracite
• B. Anthracite outperforms GAC
• C. They perform equally
• D. Neither removes organics

Answer: (A)

The main idea is that removing organics in open filters relies on adsorption, not just physical filtration. Granular activated carbon has a very large surface area and a highly porous structure, so as water passes through, dissolved organic molecules—like natural organic matter, tastes, and odors—are attracted to and held on the carbon surfaces. This makes GAC much more effective at removing organics than anthracite, which mainly traps and filters out suspended solids and offers limited adsorption for dissolved organics. So, arranging media to include GAC yields better organics removal than using anthracite alone. In practice, placing GAC where the water first contacts the bed (often as a top layer) maximizes organic adsorption, with anthracite providing subsequent physical filtration. The other options aren’t as accurate because organics removal depends on adsorption capacity, which is stronger for GAC than for anthracite, and it isn’t equal or nonexistent.