Granular activated carbon caps - A potential treatment barrier for drinking water cyanotoxins

Incorporation of granular activated carbon (GAC) in rapid media filtration has been shown to serve as an effective means to mitigate cyanotoxin exposure via extracellular adsorption. Control of organics, including dissolved organic carbon (DOC), may also improve via adsorption followed by biological treatment. Although beneficial, complete replacement of filter media with GAC is often associated with high capital costs. A seven-month pilot study conducted at a water treatment facility located in Eugene, Oregon examined the benefits of replacing the top 15 cm of anthracite media with GAC “caps” to enhance control of a cyanotoxin as well as DOC, disinfection by-product formation potential (DBP FP), and chlorine demand. Removal of cylindrospermopsin by GAC cap adsorption was assessed via simulated cyanotoxin events utilizing orange II dye as a surrogate over a 4-month period. A pilot-scale filter incorporating a GAC cap demonstrated up to 40% higher reduction throughout the study when compared to a control. This reduction however decreased to 9% as operation time increased. While installation of a GAC cap increased biomass density (ATP concentration), it did not improve DOC or DBP FP removal or chlorine demand at this facility, likely due to the low EBCT (1.2 min) in the GAC layer. This study presents evidence that incorporation of GAC caps may provide a cost-efficient, albeit low capacity, barrier for cylindrospermopsin exposure following several months of continuous operation.