Volume balance comes before energy savings
For a stream with non-volatile solids or solutes, estimate retained mass and the feed, permeate and concentrate flows at the target concentration. If the membrane removes water as permeate and that permeate has a suitable use or outlet, less volume reaches the evaporator. If permeate needs costly treatment, include that cost within the same system boundary.
Do not simply multiply avoided volume by the latent heat of water and call the result savings. Actual consumption per unit evaporated depends on thermal configuration, heat recovery, inlet temperature, fouling and final concentration. The US Department of Energy explains how multiple-effect and vapour-recompression systems reuse heat differently.
| Variable | Why it changes economics |
|---|---|
| Water evaporated | The volume to which specific thermal cost applies. |
| Permeate quality | It may need further treatment or enable reuse. |
| Concentration limit | Viscosity, scaling and product quality may limit membranes. |
| Evaporator type | Steam, electricity and heat recovery are not interchangeable. |
| Final residue | Remaining mass needs an outlet even when volume falls. |
A transparent scenario calculation
Assume, solely as a balance illustration, a 100 m³/day feed carrying 2 tonnes/day of conservative solids and a concentrate at 4% by mass. If both densities are approximately 1 tonne/m³, concentrate would be near 50 m³/day and permeate near 50 m³/day. This idealised calculation predicts neither flux, rejection, permeate quality nor 50% energy savings; it identifies the volume that belongs in the thermal quotation.
Price UF/RO and evaporation energy separately, along with chemicals, cleaning, replacement, labour, installed capital and the evaporator's minimum stable load. Existing thermal equipment can operate less efficiently away from its design point; request its part-load curve before assuming proportional savings.
- Use the site's actual steam or electricity tariff, including relevant charges.
- Check whether recoverable waste heat exists and already has another use.
- Compare scenarios with the same final residue and recovered-water destination.
Investment criterion
The pilot should establish achievable concentration, permeate quality, cleaning and availability across feed variability. Ask the evaporator supplier to quote the preconcentrated stream rather than a generic mixture. Calculate a range of annual total cost and payback sensitive to energy, downtime and membrane life; if ranges overlap, measure the uncertainty that drives the decision.
Common questions
Does halving evaporation volume halve the thermal bill?
Not necessarily. Specific consumption and part-load behaviour may change, while preconcentration energy, capital and maintenance must be included.