Abstract: The invention relates to a system and method to reduce perchlorate in wastewater utilizing perchlorate-reducing bacteria, sulfur as an electron donor and mollusk shells as alkalinity agent. Embodiments of the invention include a perchlorate-reduction system comprising a bioreactor unit having perchlorate-reducing media comprising elemental sulfur, oyster shells and a microbial community, and can further include an optional pretreatment unit, wastewater-catchment and/or post-treatment components. Embodiments of the method include multiple steps utilizing the system and additional process steps that achieve enhanced perchlorate reduction.