Consistency Testing of an Integrated SWRO-Brine Valorization Architecture: Alkalinity as the Binding Constraint

This version reverses the conclusions of the previous one. Peer review identified two errors in the earlier analysis; correcting them removes both the claimed energy advantage over conventional SWRO and the favourable levelized cost. The brine concentrator as originally specified is thermodynamically inadmissible. The reversible minimum work of separation for the modelled brine is 0.94 kWh per cubic metre of brine processed; the assumed range of 0.5-1.0 kWh/m3 spans 0.53 to 1.06 times that minimum. A bounded estimate from the minimum and a 25-55% second-law efficiency is 1.7-3.8 kWh/m3, raising modelled total specific energy consumption to a median 13.1 kWh/m3. Magnesium recovery is limited by alkalinity, not by magnesium. Two equivalents of base are required per mole of Mg(OH)2 however the base is supplied. Costed explicitly, soda ash and lime come to $1.70 per cubic metre of permeate against a realisable mineral credit of $1.25, and carry 10.0 kg CO2/m3 of embodied carbon against 5.2 kg from grid electricity. Assessments restricted to electricity account for about a third of the total. The calcium balance does not close. Lime dosing returns 5.3 times more calcium than the preceding carbonate step removes, and total dissolved solids rise across precipitation from 121.7 to 124.8 g/L, so selective precipitation slightly increases the crystallizer duty rather than reducing it. Monte Carlo propagation over thirteen parameters (300,000 samples, seed 20260919) gives a net levelized cost of $2.20/m3 (90% interval $1.38-$3.01) against $0.76/m3 for conventional SWRO. No sampled configuration undercut the comparator and none was carbon negative. Every specific-energy figure is a bounded model estimate rather than a measured or predicted plant value. No experimental data were used.