Module 6. Membrane processing

Lesson 27


Reverse Osmosis (RO) is the tightest possible membrane process in liquid/liquid separation. Water is in principle the only material passing through the membrane; essentially all dissolved and suspended material is rejected. The more open types of RO membranes are sometimes confused with nanofiltration (NF). Reverse osmosis is a process which separates small molecules and ions (molecular weight less than 1000; molecular size less than 0.001µm) from the solvents.

It is a pressure driven membrane system essentially used for dewatering of fluid foods.


Fig. 27.1 Principle of RO (a)


Fig. 27.1 Principle of RO (b)

Table 27.1 Characteristics



Fig. 27.2 Osmotic pressure of foods at room temperature

Materials for membrane manufacturing

  • Cellulose acetate
  • Polymers (polysulphones, polyamides, PVC, polystyrene, polycarbonates, polyethers).
  • Composite or ceramic membranes (porous carbon, zirconium oxide, alumina).


The pressure difference across the membrane (the trans-membrane pressure) is found using:

where P (Pa) is trans-membrane pressure, Pf (Pa) is pressure of the feed (inlet), Pr (Pa) is pressure of the retentate (outlet) (high molecular weight fraction) and Pp (Pa) is pressure of the permeate (low molecular weight fraction).

Water flux is measured as:

where J (kg /h) is flux, K (kg m-2 h -1 Pa-1) is mass transfer coefficient, A (m2) is area of the membrane, ΔP (Pa) is applied pressure and ΔΠ (Pa) is change in osmotic pressure.

Osmotic pressure Π = MRT

where T (ºK) is absolute temperature, R is universal gas constant, M is molar concentration and ΔΠ (Pa) is osmotic pressure.


1. The removal of water is accomplished without a change in phase or state of the solvent.
2. The process can be operated at ambient or up to 50°C temperature. Thus thermal degradation of nutrients is minimum.
3. There is negligible loss of volatiles and eating quality.
4. Complicated heat transfer or heat generating equipments are not required.
5. Lower labour and operating costs.


1. Variation in the product flow rate when changes occur in the concentration of feed liquor
2. Higher capital costs than evaporation
3. A maximum concentration to 30% total solids
4. Fouling of the membranes (deposition of polymers), which reduces the operating time between membrane cleaning.


1. Concentration of milk, whey & UF permeate.
2. Preparation of indigenous dairy products like khoa, chakka etc.
3. Concentration and purification of fruit juices, enzymes, fermentation liquors and vegetable oils.
4. Concentration of wheat starch, citric acid, egg white, coffee, syrups, natural extracts and flavours.
5. Clarification of wine and beer.
6. Demineralization and purification of water from boreholes or rivers or by desalination of sea water.

Last modified: Saturday, 3 November 2012, 7:27 AM