RO Recovery, Concentrate Flow, and Concentration Factor
The reverse-osmosis mass balance an operator sets recovery from: recovery R = permeate / feed, concentrate (reject) flow = feed - permeate, and the concentration factor CF = 1 / (1 - R) is how much the rejected salts pile up in the reject. At 75% recovery (10 gpm feed, 7.5 gpm permeate) the reject is 2.5 gpm and CF = 4, so a 500 mg/L feed leaves about a 2,000 mg/L concentrate; 50% recovery is only CF = 2. Pushing recovery higher shrinks the reject but raises CF sharply, and past the scaling limit of the least-soluble salt the membrane fouls -- antiscalant and the LSI govern that ceiling. The membrane manufacturer's projection software and the state primacy agency (for a public system) govern the design.
Formula and source
recovery = permeate_gpm / feed_gpm; concentrate_gpm = feed_gpm - permeate_gpm; concentration_factor = 1 / (1 - recovery); concentrate_tds_mgl = concentration_factor x feed_tds_mgl (near-complete rejection).
AMTA (American Membrane Technology Association) and AWWA membrane-treatment practice, by name; the membrane manufacturer's projection software and the state primacy agency govern the design.
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