Project Report PR-95-02
Extending the effective life of the GAC used to treat well water: Phase I of evaluative study at Mililani
Delwyn S. Oki, Gordon L. Dugan, Roger S. Fujioka, Henry K. Gee, L. Stephen Lau, and Gerald S. Takei
July 1994
ABSTRACT
During 1986 and 1987, the City and County of Honolulu Board of Water Supply (BWS) placed granular activated carbon (GAC) water treatment plants in service at Mililani, Waipahu, and Kunia on the island of Oahu, Hawaii. The GAC treatment plants were designed to remove target organic chemicals DBCP, EDB, and TCP from well water in central Oahu. Original operational cost estimates for the GAC treatment plants were based on carbon requirements determined through laboratory minicolumn experiments. Actual carbon usage rates, however, have proved to be several times higher than originally estimated. This study, which represents the first phase of a two-phase study to determine the potential for extending the effective life of GAC used to remove target organic chemicals from groundwater in central Oahu, addresses the basic information necessary to understand the problem. Levels of target organic compounds in groundwater in the Pearl Harbor aquifer of central Oahu do not appear to be decreasing significantly. In fact, based on the analysis of spent GAC samples from contactors at the Waipahu treatment facility, DBCP, which was previously undetected at the Waipahu wells, now appears to be occurring at low levels of a few nanograms per liter in groundwater near Waipahu. Analyses of spent carbon samples also indicate that the adsorptive capacity of the GAC for a particular compound is directly related to the concentration of that compound in the influent water. Total organic carbon (TOC) levels in groundwater samples from the Pearl Harbor basaltic aquifer are typically a few tenths of a milligram per liter. This concentration of background organics is two to three orders of magnitude greater than TCP, which is the target organic compound typically found at the highest concentrations in groundwater in the study area. TOC and infrared analyses indicate the presence of background organic compounds in groundwater, a factor which could shorten the service life of GAC contactors. Results of this study seem to indicate that inorganic cations and anions are not significantly adsorbed by the GAC. In addition to chemical effects, bacterial growth may also play a role in the adsorption of target organic compounds. Although attached bacteria were recovered from spent GAC samples, subsequent scanning electron microscope analysis did not reveal a significant biological slime layer which may inhibit adsorption.