GATE 2025 ES – Question 58
A 2 MLD treatment plant uses RO. TDS rejection is 95%,water recovery70%,and feed TDS8000 ppm. Find reject-stream TDS in g/L, to one decimal.
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Correct answer: 25.6 to 25.8
Explanation
For dilute-water units, feed concentration is 8 g/L. Permeate concentration is $(1-.95)8=.4$ g/L. On a unit feed-volume basis, permeate and reject flows are .7 and .3. Salt balance gives $8=.7(.4)+.3C_r$,so $C_r=(8-.28)/.3=25.7333$ g/L. Answer **25.7 g/L**. Solute rejection and water recovery are different percentages.