Bromate,　a　carcinogenic　contaminant　generated　in　water　disinfection,　presents　a　pressing　environmental　concern.　While　biological　bromate　reduction　is　an　effective　remediation　approach,　its　implementation　often　necessitates　the　addition　of　organics,　incurring　high　operational　costs.　This　study　demonstrated　the　efficient　biological　bromate　reduction　using　H2/CO2　mixture　as　the　feedstock.　A　membrane　biofilm　reactor　(MBfR)　was　used　for　the　efficient　delivery　of　gases.　Long-term　reactor　operation　showed　a　high-level　bromate　removal　efficiency　of　above　95　%,　yielding　harmless　bromide　as　the　final　product.　Corresponding　to　the　short　hydraulic　retention　time　of　0.25　d,　a　high　bromate　removal　rate　of　4　mg　Br/L/d　was　achieved.　During　the　long-term　operation,　in　situ　production　of　volatile　fatty　acids　(VFAs)　by　gas　fermentation　was　observed,　which　can　be　regulated　by　controlling　the　gas　flow.　Three　sets　of　in　situ　batch　tests　and　two　groups　of　ex　situ　batch　tests　jointly　unravelled　the　mechanisms　underpinning　the　efficient　bromate　removal,　showing　that　the　microbial　bromate　reduction　was　primarily　driven　by　the　VFAs　produced　from　in　situ　gas　fermentation.　Microbial　community　analysis　showed　an　increased　abundance　of　Bacteroidota　group　from　4.0　%　to　18.5　%,　which　is　capable　of　performing　syngas　fermentation,　and　the　presence　of　heterotrophic　denitrifiers　(e.g.,　Thauera　and　Brachymonas),　which　are　known　to　perform　bromate　reduction.　Together　these　results　for　the　first　time　demonstrated　the　feasibility　of　using　H2/CO2　mixture　for　bromate　removal　coupled　with　in　situ　VFAs　production.　The　findings　can　facilitate　the　development　of　cost-effective　strategies　for　groundwater　and　drinking　water　remediation.