Close the membrane balance first
Use mass per unit time where density changes with concentration. For feed F, permeate P and concentrate C, the overall balance is F = P + C. For a retained component, F·xF = P·xP + C·xC, with x its measured mass fraction in each stream. Given F and the three compositions, C can be calculated as F·(xF − xP)/(xC − xP).
This equation describes only the chosen component. UF may retain particles while dissolved salts or organic substances pass; total solids must not be treated as fully retained in that case. Choose a suitable analytical tracer, check closure with simultaneous samples and treat any discrepancy as uncertainty or accumulation within the system.
- Measure flow and density for every stream over the same period.
- Analyze suspended solids, dissolved solids and organics separately.
- Include purges, backwashes and recirculation in the balance boundary.
Convert concentrate flow into evaporation duty
If C feeds the evaporator, establish another balance: C = V + R, where V is vapour or condensate and R is final residue. For a nonvolatile solute, C·xC = V·xV + R·xR. Only when xV is negligible can R be approximated as C·xC/xR. Evaporation mass is V = C − R; compare it with the no-pre-concentration case at the same final xR.
Reduced evaporator feed does not automatically mean an equal reduction in energy use. Inlet temperature, boiling-point elevation, heat recovery, cleaning, operating hours and evaporator type all matter. An EPA verification report for an MVR evaporator illustrates why streams and balance closure need checking in real operation.
- State the final concentration target and condensate-quality requirement.
- Calculate vapour load per operating campaign and at peak hourly feed.
- Obtain manufacturer limits for viscosity, scaling and allowable solids.
Account for the permeate destination
Water separated by UF does not disappear from the project. It may need further treatment, conditional reuse or disposal. If it carries salts and soluble COD, its quality may prevent the intended destination. A system-level benefit exists only when all three outputs — membrane permeate, evaporator condensate and final concentrate — can be handled acceptably.
Trial the system on variable feed and assess fouling, flux recovery and permeate quality. Pre-concentration that lowers thermal load but causes frequent cleaning or unpumpable concentrate may increase total cost.