Preventing trihalomethane formation: the ability of granular activated carbon and brimac media to remove colour and total organic carbons, from drinking water in the north west of Scotland

Background: Granular activated carbon (GAC) and Brimac media has been used for many years in water treatment works to reduce and remove colour and total organic carbon (TOCs). This reduces the formation of trihalomethanes (THMs) formed by disinfection with chlorine. The TOCs arise from humic and fulvic acids found in source waters for many, small, scattered remote communities in Scotland. A number of different configurations and empty bed contact times (EBCTs) of GAC and Brimac media were studied at Scottish Waters Acharacle water treatment works using a purpose built pilot plant of flexible design. This was used to establish the absolute design threshold for colour and TOC to meet Scottish Water's water specifications for their removal. Results: The 50:50 combination of Brimac and GAC in series reduced raw colour of up to 39°Hazen to below 10°Hazen and raw TOCs from up to 5.7 mg/l to <2.0 mg/l producing the lowest levels of THMs after chlorine addition of <10 μg/l. An EBCT of 5.4 hours produced an average colour removal of 79.2% and average TOC removal of 79.1%. Conclusions: 50:50 Brimac then GAC in series with an EBCT of 5.4 hours was the best configuration for the removal of colour and TOCs and gave rise to the lowest formation of THMs. The trials have successfully proved how the THM precursors can be removed by the GAC once the colour has been removed by the Brimac. This has not only provided a design solution for Acharacle WTW but the “way forward” for Scottish Water with regard to media change out on their numerous existing GAC plants which are faced with similar problems.