An apparatus for treating a fluid to be injected into a subterranean hydrocarbon-bearing formation includes a filtration unit having one or more filtration membranes including either or both ultra-filtration membranes and microfiltration membranes, and an ionic species removal plant, wherein fluid to be injected is first treated by the filtration unit and then treated by the ionic species removal plant. In a preferred embodiment, the ionic species removal plant is a sulfate removal plant which incorporates nano-filtration membranes. A method of treating fluid to be injected into a subterranean formation is also disclosed. A cleaning system and method for use in an apparatus for treating injection fluid is also disclosed.