NOVELTY - A system for improving water quality by perchlorate reduction comprises a bioreactor; a media comprising granules of electron donor material and pieces of calcium carbonate material having at least 90 wt.% of calcium carbonate; and an exit for the liquid. The media is positioned in a bioreactor cavity. The pieces of calcium carbonate material are in fluid communication with the granules of electron donor material. The system has an entry passage for delivery of a perchlorate-containing liquid into the bioreactor cavity such that the liquid contacts the media.    USE - For improving water quality by perchlorate reduction (claimed) in the waste-water treatment. Also useful in the treatment systems for individual households, commercial establishments, industrial facilities, military bases, small communities, municipal wastewater treatment systems, farms, aquaculture systems, storm water treatment systems, surface water treatment systems, bioreactor systems for the treatment of perchlorate, in situ permeable reactive barrier walls for treatment of urban and agricultural runoff, and/or nitrate-contaminated and/or perchlorate-contaminated groundwater and in situ permeable reactive barrier walls for treatment of perchlorate-contaminated groundwater. In situ reactive barriers for stormwater treatment, contaminated groundwater, combined sewer overflows (CSOs), surface water treatment, and aquaculture systems.    ADVANTAGE - The system achieves increased perchlorate reduction using the electron donor and offers other substantial and significant advances over conventional autotrophic, perchlorate reduction systems. The system is provided as a passive system (e.g. neither electric power nor daily dose of chemicals are required), although use of electric power and regular replenishment of chemicals are also consistent thus making the system advantageous for wastewater treatment applications where round-the-clock supervision is impractical or impossible, such as on-site systems, in situ reactive barriers for stormwater treatment, contaminated groundwater, CSOs, surface water treatment, and aquaculture systems.    DETAILED DESCRIPTION - INDEPENDENT CLAIMS are included for the following:    (1) a method (M1) for treating liquid from a source to reduce perchlorate concentration in the liquid involving supplying inlet liquid from a liquid source; providing the bioreactor having a cavity and at least one inlet and at least one outlet opening connecting to the cavity; positioning the media in the bioreactor cavity such that a liquid passing through the bioreactor cavity makes fluid contact with the media and such that the pieces of calcium carbonate material are in fluid communication with the granules of electron donor material, and seeding the media with a sludge containing bacteria, delivering the inlet liquid having a first perchlorate concentration into the bioreactor to come into fluid contact with the media and thereby form a treated liquid having a second perchlorate concentration less than the first, and passing the treated liquid out of the bioreactor outlet opening. The liquid has a first perchlorate concentration;    (2) a method (M2) of reducing perchlorate concentration in water from a water source, involving supplying inlet water having a perchlorate concentration from a water source; providing an ion-exchange perchlorate-reduction system having at least one inlet and at least one outlet; providing a bioreactor having an interior cavity and at least one inlet and at least one outlet; providing the media comprising several granules of electron donor material and pieces of calcium carbonate material and seeding the media with the sludge containing sulfur-utilizing, perchlorate-reducing bacteria; providing a brine source; configuring the ion exchange system, the brine source, the bioreactor and each of the inlets and outlets to enable connecting the brine source to the primary ion-exchange, perchlorate-reduction system and subsequently to allow the brine to pass into the bioreactor cavity in such manner that the brine will make fluid contact with the media and subsequently exit from the bioreactor; and operating the combined ion exchange, brine and bioreactor system by the following sequential steps of passing inlet water through the primary system to exchange perchlorate from the inlet water into the resin, creating perchlorate enriched resin, flowing the brine through the primary system and exchanging perchlorate into the brine to form perchlorate-enriched brine and simultaneously regenerating the ion exchange resin to a relatively lower perchlorate concentration, passing perchlorate-enriched brine into the bioreactor to come into fluid contact with the media and forming the treat brine having relatively reduced perchlorate concentration, and passing treated brine out of the bioreactor outlet opening as regenerated brine for reuse in a subsequent cycle through the primary system.