A system (100; 200) for removing total organic carbon from ultrapure water comprising: a primary actinic radiation reactor (120, 218) comprising a vessel, and a first array of tubes in the vessel, the first array of tubes comprising a first set of parallel tubes (335a-c), wherein each tube has a longitudinal axis and comprises at least one ultraviolet lamp and a second array of tubes in the vessel, the second array of tubes comprising a second set of parallel tubes wherein each tube has a longitudinal axis and comprises at least one ultraviolet lamp, wherein the longitudinal axis of parallel tubes of the first array is orthogonal to a respective longitudinal axis of parallel tubes of the second array; a source of persulfate precursor compound fluidly connected to the primary actinic radiation reactor (120, 218) and disposed to introduce at least one persulfate precursor compound into the primary actinic radiation reactor; a TOC concentration sensor located upstream of the primary actinic radiation reactor; a persulfate concentration sensor (106; 207) located downstream of the primary actinic radiation reactor (120, 218), wherein the persulfate concentration sensor (106; 207) comprises at least one conductivity cell and a source of ultraviolet light, configured to measure a first conductivity, irradiate by ultraviolet light and measure a second conductivity; a source of a reducing agent disposed to introduce at least one reducing agent downstream from the primary actinic radiation reactor; a reducing agent concentration sensor located downstream of a point of addition of the at least one reducing agent; and a controller (105; 232) operatively coupled to receive at least one input signal from at least one of the TOC concentration sensor, the persulfate concentration sensor, and the reducing agent concentration sensor, and generate at least one control signal that regulates one of a rate at which the persulfate precursor compound is introduced into the primary actinic radiation reactor, an intensity of the actinic radiation in the primary actinic radiation reactor, and a rate at which the reducing agent is introduced to the system. a primary actinic radiation reactor (120, 218) comprising a vessel, and a first array of tubes in the vessel, the first array of tubes comprising a first set of parallel tubes (335a-c), wherein each tube has a longitudinal axis and comprises at least one ultraviolet lamp and a second array of tubes in the vessel, the second array of tubes comprising a second set of parallel tubes wherein each tube has a longitudinal axis and comprises at least one ultraviolet lamp, wherein the longitudinal axis of parallel tubes of the first array is orthogonal to a respective longitudinal axis of parallel tubes of the second array; a source of persulfate precursor compound fluidly connected to the primary actinic radiation reactor (120, 218) and disposed to introduce at least one persulfate precursor compound into the primary actinic radiation reactor; a TOC concentration sensor located upstream of the primary actinic radiation reactor; a persulfate concentration sensor (106; 207) located downstream of the primary actinic radiation reactor (120, 218), wherein the persulfate concentration sensor (106; 207) comprises at least one conductivity cell and a source of ultraviolet light, configured to measure a first conductivity, irradiate by ultraviolet light and measure a second conductivity; a source of a reducing agent disposed to introduce at least one reducing agent downstream from the primary actinic radiation reactor; a reducing agent concentration sensor located downstream of a point of addition of the at least one reducing agent; and a controller (105; 232) operatively coupled to receive at least one input signal from at least one of the TOC concentration sensor, the persulfate concentration sensor, and the reducing agent concentration sensor, and generate at least one control signal that regulates one of a rate at which the persulfate precursor compound is introduced into the primary actinic radiation reactor, an intensity of the actinic radiation in the primary actinic radiation reactor, and a rate at which the reducing agent is introduced to the system.