Abstract: The present invention provides subsea seawater filtration and treatment system, comprising a filtration assembly (1,3) for filtering out particles and detritus from the seawater; a first pump (5) comprising an inlet in fluid communication with the filtration assembly (1,3) and an outlet; a sulphate removal unit (6) comprising an inlet in fluid communication with the outlet of the first pump (5), a first fraction outlet for sulphate depleted seawater and a second fraction outlet for sulphate enriched seawater; at least one second pump (7) comprising an inlet in fluid communication with the first fraction outlet, and an outlet for the sulphate depleted seawater; wherein the second fraction outlet is in fluid communication with the filtration assembly (1,3), such that the sulphate enriched seawater may be used for backwashing at least a part of the filtration assembly during use.