What is the function of an activated carbon filter in a purified water system?

Activated carbon filters are a commonly used pretreatment process in ultrapure water systems. They employ activated carbon with numerous micropores and a large specific surface area as the filtration medium, giving it strong physical adsorption capacity. Their primary functions are to adsorb residual chlorine and organic compounds, while also effectively removing odors, colloids, and colorants from the water. So, what is the role of an activated carbon filter in an ultrapure water system?

What is the function of an activated carbon filter in a purified water system?

1. Adsorption of Organic Compounds

(1) Water contains a large amount of organic matter, which can be broadly classified into non-synthetic and synthetic organic compounds. Non-synthetic organic matter refers to naturally occurring organic substances, such as aquatic organisms and their secretions, as well as humic substances. Contamination of organic anion-exchange resins can be categorized into physical and chemical effects. Organic contaminants adhere to the resin surface, penetrate deep into the resin’s network structure, coat internal functional groups, and block the resin’s micropores, thereby impeding or reducing ion exchange and increasing flow resistance through the resin bed. Organic-induced failures in reverse-osmosis systems account for 60–80% of all system malfunctions. When organic fouling occurs, the system not only exhibits a decline in water flux but also experiences clogging of the feed-water distribution network, leading to excessive pressure loss. In severe cases, this can cause mechanical damage to the membrane elements.

(2) A balanced surface concentration is established on the surface of the activated carbon particles, after which organic impurities are adsorbed into the particles, resulting in very high adsorption efficiency during the initial stage of use. However, over time, the adsorption capacity of the activated carbon diminishes to varying degrees, leading to a decline in adsorption performance. Therefore, activated carbon should be periodically cleaned, regenerated, or replaced.

(3) The adsorption capacity of activated carbon for organic compounds in water is influenced by multiple factors and typically does not achieve complete removal, with removal efficiencies ranging from approximately 20% to 90%, depending on the specific conditions. Activated carbon is most effective at adsorbing organic compounds with molecular weights between 500 and 3,000; adsorption capacity generally decreases as molecular weight increases. Organic compounds whose molecules are larger than the pore size of the activated carbon are difficult to adsorb. When the diameter of an organic molecule is comparable to the pore size of the activated carbon, it may cause pore blockage, leading to irreversible adsorption. Therefore, the appropriate type of activated carbon should be selected based on the molecular diameter of the organic compounds present in the water and the micropore size distribution of the activated carbon.

2. Adsorption of residual chlorine

(1) Residual chlorine can be classified into combined residual chlorine, free residual chlorine, and total residual chlorine. In tap water, the term “residual chlorine” refers to free residual chlorine. Free residual chlorine exists in forms such as OCl⁻, HOCl, and Cl₂. In water, it exhibits rapid bactericidal action and strong disinfecting efficacy, but it also dissipates quickly; hence it is referred to as free residual chlorine.

(2) Residual chlorine in tap water is the primary cause of degradation of ion-exchange resins and polyamide reverse-osmosis membranes in pure-water systems. Oxidation of the ion-exchange resin results in a whitish appearance, increased transparency, and swelling and fragmentation, which reduce the resin’s volumetric exchange capacity, increase pressure drop across the resin bed, and lower the purity and pH of the permeate.

(3) Activated carbon adsorption of residual chlorine is more economical than the addition of reducing agents, because activated carbon can treat water containing active chlorine at a volume approximately 100,000 times its own. Moreover, activated carbon can remain effective for about 1 to 1.5 years, making it a cost-effective solution. Activated carbon exhibits preferential adsorption for residual chlorine over organic compounds, with an adsorption efficiency approaching 100%. However, residual chlorine can cause significant damage to the micropores of activated carbon, and a common issue is that the carbon particles tend to fracture easily, generating fine debris.