Report No FR0431

APRIL 1994



A better understanding will be gained of the microbiological implications of using granular activated carbon as a replacement for sand in rapid gravity filters.


To assess the impact on the microbial quality of water using granular activated carbon as a filtration medium in water treatment.


Granular activated carbon (GAC) is widely used in the treatment of drinking water to remove undesirable tastes and odours and trace organic contaminants. It is also used as part of treatment strategies to minimise the formation of disinfection by-products. Apart from its ability to adsorb a wide range of organic compounds, GAC also encourages the proliferation of micro-organisms. The attached microbial population derive their nutrients from both the soluble fraction in the water and those adsorbed to the carbon particles. Although successful in reducing the organic content of water, the impact of the increased microbial loadings which GAC filtration can create, on the final stage of treatment, is not well understood.


  1. The microbial quality of the water from a sand filter was better than that from a GAC filter at a site using prechlorination. In the case of the sand filter, persistence of a chlorine residual controlled the numbers of bacteria on the medium.

  2. Following a backwash cycle, the effluent water from a GAC filter contained higher numbers of thermotolerant coliforms than a filter in service for several hours. This effect was observed up to an hour after the backwashed filter was brought back on line.

  3. GAC filtration always produced an increase in heterotrophic plate count organisms (22C). Because bacteria colonise GAC the heterotrophic plate count (22C) of water from GAC filters is always greater than that from equivalent sand filters where prechlorination is practised.

  4. In the summer months thermotolerant coliform organisms (presumptive E. coli) and increased numbers of heterotrophic plate count organisms (37C) were detected in the water from a GAC filter and from the bed material.

  5. Thermotolerant coliforms were associated with coagulant flocs in the clarifier but were not detected in the effluent water from this process.

  6. The source of these thermotolerant coliform organisms remains unclear. Although they could not be detected in the water from the clarifier supplying the filters, they were present on the surface of the GAC beds.

  7. As thermotolerant coliforms were present on the GAC bed, it was concluded that the procedures for recovering E. coli, injured by disinfectant, failed to detect these organisms in clarified effluents.

  8. Particles of GAC, some colonised by bacteria, were capable of penetrating final disinfection.

  9. Consideration should be given to the location of processes containing GAC within the overall treatment train, to minimise their impact on the microbial quality of water whilst at the same time maximising their ability to remove organic compounds.


This report describes studies conducted as part of the Foundation for Water Research contract "Microbiological Efficiency of Water Treatment" (F-1401) and focuses on the microbiological implications of using granular activated carbon during water treatment. In support of this study, previous work undertaken for the Department of the Environment has been included to provide a complete assessment of GAC in water treatment. During this study, the performance of GAC was evaluated at a full scale treatment works. Investigations were conducted to examine aspects of the operation of the filters and determine their impact on the microbial quality of the effluent water. Consideration was given to an examination of release of GAC particles from the filter bed and their passage into the distribution system.

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