In early July 2003, Carollo Engineers initiated a six-month pilot study at the Castaic Lake Water Agency (Santa Clarita, California). The ultimate goal of this project was to demonstrate the efficacy of fixed-bed biological treatment for perchlorate removal from Saugus aquifer water (i.e., show consistent removal of perchlorate to below the method detection limit of 4 mg/L). Specific objectives of the project were to: verify that sufficient perchlorate-reducing biological activity can be developed in the pilot-scale reactors using microorganisms indigenous to the Saugus aquifer; develop full-scale design criteria for empty-bed contact time (EBCT), backwashing procedures, electron donor (acetic acid) addition, and nutrient addition; evaluate the robustness of the fixed-bed biological process with respect to system upsets, such as electron donor feed failure, process shut-downs, and spikes in the feedwater oxidant concentrations; and, determine post-treatment requirements for the fixed- and fluidized-bed perchlorate removal processes (i.e., characterize process effluent quality). Results demonstrated that a perchlorate-reducing, fixed-bed biological reactor can be acclimated using organisms indigenous to the Saugus aquifer, that perchlorate can be removed to below detection using reasonable contact times, and that effluent water from these processes is of fairly high quality. Ultimately, the results of this project will be used to evaluate the feasibility and costs associated with treating perchlorate-laden Saugus aquifer water using both fixed- and fluidized-bed biological reactors.