What Is Biological Activated Carbon and How Does It Form?
Biological activated carbon (BAC) is an operational state that develops in GAC beds that are not chlorinated or otherwise biocidally maintained. When ozonated or otherwise pre-treated water passes through a GAC contactor, dissolved organic compounds adsorb onto the carbon surface and provide a nutrient substrate for microbial colonization. Over time — typically weeks to months of continuous operation — a biofilm establishes itself on the carbon surface and within the pore structure. This biofilm metabolizes biodegradable organic compounds including some natural organic matter fractions and certain organic micropollutants, providing the biological treatment component that distinguishes BAC from strictly physical adsorption on GAC.
BAC is widely used in drinking water treatment in Europe and increasingly in North America as a means of improving biodegradable organic carbon removal, reducing disinfection byproduct formation potential, and extending GAC service life by biodegrading some of the NOM that would otherwise compete with target contaminants for adsorption sites. The question of whether these benefits extend to PFAS removal — and specifically whether biofilm formation helps, hinders, or is neutral for PFAS capture — is a technically important question for utilities considering BAC as part of a PFAS treatment strategy.
PFAS Biodegradation Potential: What the Science Shows
PFAS are among the most chemically persistent organic compounds known to exist in the environment. The C-F bond is one of the strongest single bonds in organic chemistry (bond energy approximately 544 kJ/mol), and the electronegativity of fluorine makes PFAS highly resistant to oxidative, reductive, hydrolytic, and photolytic degradation under environmental and drinking water treatment conditions. Complete mineralization of PFAS — breaking down the perfluorocarbon chain to CO2 and fluoride — requires high-temperature processes (pyrolysis, electrochemical oxidation) or high-energy UV conditions not present in conventional water treatment.
Under the relatively mild biological conditions of a BAC biofilm operating at ambient temperature, PFAS mineralization does not occur to any practically significant extent. However, some research has demonstrated partial transformation of certain PFAS precursor compounds — fluorotelomer alcohols (FTOHs), N-substituted perfluoroalkyl sulfonamides — by biofilm microorganisms under specific conditions. These transformations can convert precursors into terminal PFAS like PFOA and PFOS, which is potentially counterproductive from a treatment perspective. The net biological effect on the sum of regulated PFAS in a BAC system treating water with a complex PFAS mixture (including precursors) is thus not straightforward and may in some cases increase regulated PFAS concentrations relative to a non-biological GAC system.
Effects of Biofilm on Physical Adsorption of Ionic PFAS
For the ionic PFAS compounds regulated under the 2024 EPA rule — PFOA, PFOS, PFNA, PFHxS, and related compounds — biological degradation is essentially negligible in BAC systems at drinking water treatment conditions. The relevant question is whether the biofilm affects the physical adsorption of these compounds on the carbon surface. Research findings on this question are mixed but suggest a modest negative effect: biofilm formation on GAC surfaces reduces available adsorption sites for PFAS by physically occupying surface area and pore entrances, and biofilm metabolic byproducts (extracellular polymeric substances, secondary metabolites) may compete with PFAS for remaining adsorption sites in a manner analogous to NOM competition.
A meta-analysis of published BAC studies comparing PFAS removal in BAC versus non-biological GAC systems found that BAC systems showed PFAS removal efficiency 5-15% lower than equivalent GAC systems without established biofilms, when tested under otherwise equivalent hydraulic and influent concentration conditions. This performance reduction was more pronounced for short-chain PFAS than for long-chain PFAS, consistent with the general principle that compounds with lower intrinsic adsorption affinity are disproportionately affected by any additional competition or site blockage.
Design Recommendations for BAC Systems with PFAS Compliance Requirements
For water utilities operating BAC systems and subject to the 2024 EPA PFAS MCLs, several design and operational considerations apply. First, treat PFAS removal in BAC systems as primarily physical adsorption, not biological transformation, and size the system accordingly using adsorption-based design parameters (EBCT, Freundlich isotherms, bed volumes to breakthrough). Do not apply biological treatment credits to PFAS removal calculations.
Second, recognize that established biofilm in BAC systems may reduce PFAS adsorption capacity by 5-15% relative to fresh, non-biologically colonized GAC. Incorporate this reduction as a safety factor in bed life calculations. Third, be attentive to the potential for PFAS precursor transformation in BAC systems; if source water contains fluorotelomer-based PFAS precursors (FTOHs, sulfonamides), monitor for increases in regulated PFAS concentrations across the BAC bed, not just decreases. Fourth, for systems where short-chain PFAS are a regulatory concern, the combination of reduced adsorption capacity (biofilm effect), NOM competition, low intrinsic affinity, and temperature sensitivity creates a compounding performance challenge that may require supplementary treatment — ion exchange downstream of BAC — to achieve reliable compliance with the hazard index requirement.