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Recovery is permeate divided by feed flow. Salt rejection is the fraction of dissolved solids kept out of the permeate, typically 97-99.5% for brackish water membranes. High recovery concentrates the reject stream — at 75% recovery the concentrate is four times feed salinity, which is where scaling risk arises.
Recovery and rejection
Recovery % = permeate / feed x 100; rejection % = (feed TDS - permeate TDS) / feed TDS x 100
Concentration factor
CF = 1 / (1 - recovery)
Scaling of sparingly soluble salts — calcium carbonate, calcium sulphate, barium sulphate and silica — in the concentrate. Antiscalant dosing and acid addition raise the achievable recovery.
Because the average feed concentration along the array rises, and salt passage is proportional to concentration. The last element sees the most concentrated water and produces the poorest permeate.