Comparative evaluation of six different granular activated carbon for TCP removal using rapid small scale column test

Abstract

A Constant Diffusivity-Rapid Small-Scale Column Test (CD-RSSCT) was performed to estimate the Granular Activated Carbon (GAC) usage rates of six different activated carbons for 1,2,3-Trichloropropane (TCP) removal. TCP is a synthetic chemical compound contained in industrial solvents and soil fumigants and is believed to be a human carcinogen. The State of Hawaii's current maximum contaminant level (MCL) for TCP is 600 parts per trillion (ppt), but consideration is being given to lower this MCL to 5 ppt. GAC adsorption is an efficient water treatment technique used to remove natural and synthetic organic compounds, chlorine, and some metals. The GAC is housed in contact columns at water treatment facilities. For this research, three separate Board of Water Supply (BWS) water treatment facilities on Oahu were selected as water sources for GAC adsorption tests. This project was conducted in collaboration with AECOM, a multinational consulting engineering firm with a branch office in Honolulu, Hawaii. The objectives of this study were to determine whether any GAC could meet the possible new 5 ppt MCL for TCP, and if so, which type of GAC would be the most effective for TCP removal. In addition, as a preliminary step, it was necessary to create a method to quantify TCP at the 1 ppt level in order to conduct the study. This was accomplished by modifying an EPA method; this is the first such achievement in Hawaii. The GAC type that is able to treat the most bed-volumes at the selected breakthrough points was determined and recommended to replace the currently GAC used in Hawaii's water treatment facilities. From the RSSCT results, numerous key findings were made. (1) Any of the 6 GACs tested can meet the new possible MCL of 5 ppt for TCP, (2) GAC C, the currently GAC used, was by far the least effective GAC in each of the three water well sites, (3) there was no one GAC that was the most effective for all three well sites; therefore, it is necessary to find a most effective GAC type specific to the well site, (4) the most effective GAC for Kunia I was GAC A/B and D, for Waipahu III was GAC E, and for Mililani I was GAC A/B, (5) the smaller the GAC particle size, the less effective the GAC was, (6) the water matrix affected TCP adsorption, and (7) along with TCP, EDB and DBCP removal will also be taken into consideration for the GAC selection. The findings from this RSSCT study are of great value and significance. The fact that current GAC was the least effective is very insightful because it means that the selection of any of the other GACs would improve the GAC process performance in the field. Replacing the current GAC unit with a more effective one means the GAC units would need to be changed less frequently, thus saving on maintenance and operating costs.