Ru　supported　on　activated　carbon　(C),　Al2O3,　CeO2,　MgO,　TiO2,　and　ZrO2　were　prepared　and　employed　to　boost　supercritical　water　gasification　(SCWG)　of　microalgae　hydrothermal　liquefaction　(HTL)　downstream　water　at　600　°C　for　1　h.　Effects　of　catalysts,　water　density,　and　Ru/C　loading　on　total　organic　carbon　(TOC)　reduction,　carbon　gasification　efficiency　(CGE),　and　gas　yield　were　examined.　All　the　Ru　catalysts　showed　a　positive　effect　on　TOC　reduction　and　converted　organics　in　the　HTL　downstream　water　to　fuel　gas,　with　H2　and　CH4　as　the　two　most　abundant　component.　With　the　addition　of　Ru/C,　almost　complete　removal　of　TOC,　the　highest　CGE,　and　H2　yield　were　achieved　due　to　its　largest　metal　dispersion.　With　the　addition　of　Ru/C,　increasing　water　density　and　catalyst　loading　favored　TOC　reduction　and　CGE　increase　as　well　as　the　total　gas　yield.　The　highest　H2　content　of　50%　and　yield　of　2.62　mmol/g(aq.)　were　obtained　at　the　highest　catalyst　loading　of　12.0　mg(Ru/C)/mL(aq.).　This　study　suggested　that　SCWG　with　the　addition　of　Ru/C　was　a　promising　strategy　to　produce　fuel　gas　rich　in　H2,　from　the　organics　in　the　microalgae　HTL　downstream　water.　©　2021　Elsevier　Ltd