This　study　investigated　the　disinfection　efficiency　of　a　photoreactor　equipped　with　a　helical　water　flow　channel　and　ultraviolet-C　(UV-C)　light　emitting　diodes　(LEDs).　Theoretical　simulations　and　biodosimetry　tests　were　conducted　to　investigate　the　effects　of　coil　diameter　and　flow　rate　on　the　reactor’s　performance　in　inactivating　Escherichia　coli.　The　interplay　between　hydrodynamics　and　UV　radiation　was　analyzed　to　determine　the　UV　fluence　absorbed　by　the　microbes.　The　simulations　revealed　that,　primarily　due　to　the　specific　radiation　pattern　of　the　UV　LEDs,　the　coil　diameter　strongly　influenced　the　distribution　of　irradiance　in　the　water　and　the　UV　fluence　received　by　microbes.　The　experimental　results　indicated　that　the　photoreactor　achieved　the　highest　inactivation　value　of　2.8　log　when　the　coil　diameter　was　48　mm　for　a　flow　rate　of　40　mL/min;　this　log　value　was　superior　to　those　for　coil　diameters　of　16,　32,　64,　and　80　mm　by　approximately　1.9,　0.4,　0.5,　and　0.7　log　units,　respectively.　This　optimal　coil　diameter　leading　to　the　maximal　UV　irradiance　and　the　highest　degree　of　irradiance　uniformity　along　the　flow　channel.　This　study　offers　design　guidelines　for　constructing　a　high-efficiency　water　disinfection　reactor　with　a　helical　flow　channel　configuration.　©　2024　Informa　UK　Limited,　trading　as　Taylor　&　Francis　Group.