GAC Contactor Design for NOM Removal: Implications of EBCT and Blending

Abstract

Many drinking water utilities may choose to use granular activated carbon (GAC) to remove disinfection by-product precursors from their water to comply with proposed U.S. Environmental Protection Agency rules. Activated carbon usage rates can be minimized by blending effluents from multiple parallel GAC columns. An important issue for the preliminary design of GAC systems is to select the best column size (empty-bed contact time) and number of parallel columns. This study provides an example of the trade-offs between the activated carbon usage rate and frequency of regeneration, for one water source. The variables examined included empty-bed contact time and the number of parallel columns. The following two important trends were observed that apply to all utilities that use GAC: (1) The most dramatic improvement in the activated carbon usage rate from blending effluent occurs when going from one to two parallel columns, whereas the incremental improvement for adding more than two parallel columns decreases rapidly; and (2) a significant disadvantage of utilizing a large number of parallel columns is the need for frequent GAC replacement.