A membrane or, more properly, a semipermeable membrane, is a thin layer of material capable of separating substances when a driving force is applied across the membrane. Membrane filtration enables to separate particles with a diameter smaller than the pore diameter in the membrane from the liquid feed, by applying a driving force (pressure) over the membrane.
The membrane technology consists principally of Microfiltration (MF), Ultrafiltration (UF), Nano filtration (NF) and Reverse Osmosis (RO).MF is essentially employed as a clarifying operation to remove macro-materials and suspended solids, milk fat globules, bacteria and colloidal particles. The pore sizes range from0.1 to 10micron and the operating pressures are in the range of 1 to 25 psi g. UF membrane allows the particles from 10,000 to 75,000 Daltons. The operating pressure ranges between 10 to200 psi g. RO membranes are characterized by a molecular weight cut off of nearly 100 Daltons. Only water passes through while everything else including ions, organic molecules remain in the concentrate. The pressures involved are 5-10 times higher than those used in UF. NF allows divalent ions to pass through while retaining the organic molecules. It separates particles with molecular weights in the range of 300-1000Daltons.The operating pressures required are nearly 300-psi g. Advantages of Membrane Processes
1.Operation at ambient temperature, thus, thermal and oxidative degradation problems, common to evaporation processes can be avoided or minimized in RO/UF process. Thus, the nutritional and functional properties of milk constituents like vitamins; proteins, etc are least affected.
2.No need of any complicated heat transfer or heat generating equipment, for the membrane operation, which required only electrical energy to drive the pump motor, can be situated far from the prime power generating plant. Also no additional steam capacity needs to be installed to handle the RO/UF unit. Since no condensers are needed, problems like thermal pollution and overloading of sewage treatment systems are avoided.
3. Minimal changes in microenvironment (pH, ionic strength). 4. Improved product yield.
5. Improved product consistency.
6. Greater efficiency due to reduced processing time.
8. Membrane technology, especially UF and RO can be used to alleviate the pollution and disposal problem and recover the nutritious whey solids. RO is particularly helpful in reducing the pollution problem as well as increasing the byproduct recovery. Concentration of whey by RO up to 30% TS reduced the BOD of whey to 500mgO2 /l under certain operational conditions. By the combined process of UF and RO, BOD of cheese whey (sweet whey) can be reduced by 99% and of cottage cheese whey (acid whey) by 97%. Limitations: The RO process is quite limited in its upper solids limits As an example, current Technology permits milk to be concentrated by multi-effect evaporation to about 50 per cent total solids, while the upper limit using RO is about 25 per cent total solids for skim milk and 28 per cent for whole milk. T he osmotic pressure of the feed and the concentration polarization limits the concentration level in the RO/UF process. Other problems that plagued early membrane applications were the fouling of membranes, poor clean ability of some early modules, and restricted operating conditions, although some of these problems have been overcome through the development of superior membrane materials and improved module design.
A} REVERSE OSMOSIS (RO) Reverse osmosis is a high-pressure, energy-efficient technique for dewatering process streams, concentrating low-molecular-weight substances in solution, or purifying waste water. It has the ability to concentrate all dissolved and suspended solids.RO is widely used in the desalination of seawater. Principles
In principle RO works by passing a dilute product stream...