How to simulate industrial water reuse: ro + membrane distillation to near-zld
A chemical-site water-reuse train of the kind large industrial water users build when the intake is salinising and freshwater is getting scarce: reverse osmosis makes reuse-grade water, the RO concentrate is warmed by low-grade waste heat and pushed further by membrane distillation, and the MD concentrate is crystallised to salt. Overall water recovery comes out at 94.9%. The point of the flowsheet is why MD is there at all. RO is pressure-driven, so its ceiling is osmotic pressure — and this feed hits that wall hard: at 25 bar the same 75% recovery is **infeasible**, and the model says so with a number rather than a shrug (osmotic pressure 27.2 bar against a 25 bar feed, a structured SPEC_THERMODYNAMICALLY_IMPOSSIBLE error, which is why the feed here runs at 45 bar). Membrane distillation is driven by a vapour-pressure difference instead, so only vapour crosses and there is no osmotic ceiling at all: it takes the RO concentrate from 2.0 mol% to 9.7 mol% salt, recovering 81% of the water RO had to leave behind, and its distillate is solvent-only. The waste-heat coupling is not decoration. At the RO concentrate's own 25 C the MD flux is about 1 kg/m2/h and the module is pointless; warmed to 60 C it runs at 21.5 kg/m2/h, inside the 10-50 kg/m2/h band real DCMD modules achieve. That is the entire commercial case for MD — it is a heat-driven process, so it is only cheap where low-grade heat is already being vented.
- 1Open the ready-made model
Open the "Industrial water reuse: RO + membrane distillation to near-ZLD" model in the MaximaLabs workspace — no install, no license. It loads live on the canvas, ready to edit and run.
- 2Confirm the thermodynamics
This process is modeled with the PENG-ROBINSON property package over water, nacl — already selected, so the phase equilibrium and enthalpy are physically consistent from the first run.
- 3Review the flowsheet
The flowsheet chains RO, Waste HEAT, MD, Cryst. Every block is a real, solvable unit op you can reconfigure on the canvas.
- 4Run the simulation
Click Run. The deterministic solver converges the material and energy balances (recycles included) and fills the live stream table — the AI never invents a number.
- 5Read the results and iterate
Inspect the converged streams, tweak a spec, and re-run — or ask the AI copilot to explain a result or diagnose a failed solve in plain English.
- Thermodynamics
- PENG-ROBINSON
- Components
- water, nacl
- Unit operations
- ROWaste HEATMDCryst
Opens live on the canvas — free, no install.
Explore the model & flowsheetModeling assumptions & limitations
What this model captures, and what it deliberately does not — from the engineers who built it.
- 1The membrane coefficient and the film heat-transfer coefficients are measured module properties supplied as inputs, not correlations — they depend on the membrane's porosity, tortuosity and thickness and on the module hydrodynamics, so the flux is only as good as those numbers. The reported temperature-polarisation coefficient (0.72 here) is the fraction of the measured driving force that actually reaches the membrane, and it is the thing a bulk-temperature model gets wrong. No membrane-wetting model: pore wetting by surfactants is what ends a real MD module's life and is not predicted here. The salt is NaCl only — a real industrial reuse stream carries silica, which is usually what actually caps recovery; screen that separately with the silica saturation tool, since this flowsheet's thermo does not carry it.
Frequently asked questions
- What does the Industrial water reuse: RO + membrane distillation to near-ZLD model simulate?
- A chemical-site water-reuse train of the kind large industrial water users build when the intake is salinising and freshwater is getting scarce: reverse osmosis makes reuse-grade water, the RO concentrate is warmed by low-grade waste heat and pushed further by membrane distillation, and the MD concentrate is crystallised to salt. Overall water recovery comes out at 94.9%. The point of the flowsheet is why MD is there at all. RO is pressure-driven, so its ceiling is osmotic pressure — and this feed hits that wall hard: at 25 bar the same 75% recovery is **infeasible**, and the model says so with a number rather than a shrug (osmotic pressure 27.2 bar against a 25 bar feed, a structured SPEC_THERMODYNAMICALLY_IMPOSSIBLE error, which is why the feed here runs at 45 bar). Membrane distillation is driven by a vapour-pressure difference instead, so only vapour crosses and there is no osmotic ceiling at all: it takes the RO concentrate from 2.0 mol% to 9.7 mol% salt, recovering 81% of the water RO had to leave behind, and its distillate is solvent-only. The waste-heat coupling is not decoration. At the RO concentrate's own 25 C the MD flux is about 1 kg/m2/h and the module is pointless; warmed to 60 C it runs at 21.5 kg/m2/h, inside the 10-50 kg/m2/h band real DCMD modules achieve. That is the entire commercial case for MD — it is a heat-driven process, so it is only cheap where low-grade heat is already being vented.
- Which thermodynamic method does it use?
- The PENG-ROBINSON property package, over water, nacl — already selected. You can switch the method on the canvas before running.
- Which unit operations are in the flowsheet?
- It chains RO, Waste HEAT, MD, Cryst. Every block is a real, solvable unit operation you can reconfigure, add to, or remove.
- Do I need to install software or buy a license?
- No. Industrial water reuse: RO + membrane distillation to near-ZLD runs entirely in your browser on MaximaLabs — free, no install, no license. Open the model to load it live and run the deterministic solver.
More guides like this
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.
RO desalination + ZLD
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