This study investigated the influence of filter media characteristics on manganese (Mn) removal in groundwater biofilters during the start-up phase. Six pilot scale biofilters containing three different granular activated carbons (GAC), two anthracite, and one sand media were run for 133 days to examine their Mn removal performance at a drinking water utility, while also monitoring ATP and bacterial growth as indicators of biological activity. Key findings demonstrate the critical role of media characteristics, especially for GAC media. Initial Mn adsorption on GAC, with its higher surface area, higher macropore volume, and surface charge, promoted physicochemical and biological oxidation, thus contributing to the early onset of Mn removal during the start-up of the GAC biofilters. In contrast, biological processes dominated Mn removal on anthracite and sand biofilters during start-up. As expected, the presence of Mn-oxidizing bacteria was detected in biofilters, and ATP levels were correlated to Mn removal until the biofilters were acclimated, showing the potential of ATP as an acclimation monitoring metric. Once acclimated, all biofilters consistently reduced Mn levels from 60.9 +/- 4.5 mu g/L to below 5 mu g/L (>90 % removal), while concurrently removing iron, thereby highlighting the biofilter's effectiveness at low water temperatures (<15 C-degrees). This study demonstrates the advantage of GAC media for Mn removal in biofilters during start-up in regions with lower water temperatures.
