Micropollutant Mass Transfer Zones in Granular Activated Carbon Contactors

Abstract

A “minicolumn” test approach was recently developed to estimate the performance of preloaded granular activated carbon (GAC) in water treatment plants for micropollutant removal. One of the requirements of this test is that the GAC bed must have a broad mass transfer zone (MTZ) for the contaminant of interest, but this has never been verified. In this study, GAC of different ages harvested from three different bed depths was subjected to lab-scale minicolumn and isotherm tests to measure adsorption kinetics and thermodynamic properties, respectively. The GAC was challenged with micropollutants with a range of adsorption characteristics. The GAC was found to exhibit approximately uniform removal efficiencies for all of the selected micropollutants across the depth, indicating a broad MTZ. Additionally, the MTZ for 2-methylisoborneol and geosmin in a newly installed GAC filter was observed to broaden to beyond the depth of the filter within 7 weeks of installation. This work helps to verify the minicolumn test protocol and supports a novel approach by eliminating the dependence of GAC performance on bed location. With such an approach, utilities may more easily conduct the minicolumn test to establish an accurate GAC replacement schedule and to optimize GAC operation for maximum contaminant removal.

Is In-Service Granular Activated Carbon Biologically Active? An Evaluation of Alternative Experimental Methods to Distinguish Adsorption and Biodegradation in GAC

“…Dissolved organic carbon (DOC) removal was used as one indicator to evaluate the adsorption characteristics of GAC. In addition, six representative micropollutants of different chemical groups were selected to cover a broad range of affinity for adsorption: geosmin and MIB are prevalent taste and odor contaminants in drinking water, and exhibit medium GAC adsorbability, with MIB being less well adsorbed; , carbamazepine is among the most frequently detected pharmaceuticals in drinking water, and it has a high adsorption affinity to GAC; , caffeine, the most widely consumed stimulant, was selected since it is typically used as an indicator of wastewater contamination in drinking water sources and is relatively poorly adsorbed; and PFOA and PFOS were selected as representative compounds of the per- and polyfluoroalkyl substances (PFAS) group. It has been reported that both chemicals can be effectively adsorbed by virgin GAC but poorly adsorbed by preloaded GAC in drinking water treatment plants .…”