Site water reuse: RO + EDI polish + MD to near-ZLD

The full hybrid a large chemical site builds when it is reusing its own effluent rather than buying river water: reverse osmosis does the bulk desalting, electrodeionisation polishes the permeate to boiler-feed quality, and the RO concentrate is warmed by waste heat and pushed to salt by membrane distillation and a crystalliser. It is the companion to industrial-water-reuse-ro-md-zld, which stops at reuse-grade water; the delta here is the polishing leg, and it is a genuinely different technology rather than a tighter membrane.

Why EDI and not more RO. RO is pressure-driven and pushes water, so its energy scales with the volume produced. Electrodialysis drags ions out under a field and the water never crosses, so its energy scales with the salt removed -- which is why it is hopeless on seawater and excellent on an RO permeate that is already 99% desalted. This model shows that rather than asserting it: at the same stack and current the moles removed are fixed by Faraday's law regardless of how salty the feed is.

The reason the polisher is resin-filled is the interesting part. Plain electrodialysis has a limiting current density proportional to the diluate concentration, so as the water approaches pure the boundary layer runs out of ions to deliver and the salt flux plateaus -- ED cannot reach ultrapure at any voltage, and the voltage runs away as the water stops conducting. Packing the diluate compartment with ion-exchange resin fixes both: the resin carries the current, and the water splitting that occurs above the classical limiting current -- a fault in a solution-filled stack, causing a pH excursion and scale -- is exactly what regenerates the resin in place. That is why an EDI polisher needs no chemical regeneration, and why the same operating point means opposite things in the two stacks.

Honesty note: the stack's mass-transfer coefficient, membrane area resistance and current efficiency are measured properties of a specific stack at a specific velocity, supplied here as inputs -- no ED-stack databank ships, for the same reason no membrane or dust databank does. The same applies to the water-splitting overpotential, and it matters more than it sounds: the ohmic terms alone give a fraction of a volt per cell pair while a real EDI stack runs at 1-2 V, so with that input left at its zero default the model reports an energy floor rather than a prediction. It is set explicitly here (1.2 V/cell pair) so the number below is worth quoting. One lumped 1:1 salt, so no per-ion selectivity and no monovalent-selective membrane. Pretreatment (filtration and pH adjustment) sits upstream of this boundary because the thermo carries no particulate phase, and the concentrate side has no scaling model -- screen it with the silica and cooling-water tools, which is what actually decides how far the brine loop can be pushed. The fouling that sets the CIP interval on the RO stage is likewise a separate calculation (the fouling and CIP tool), not part of this steady-state balance.