Generally, there are four types of membrane processes. They are microfiltration, ultrafiltration, nanofiltration and reverse osmosis. Microfilter(MF) - Operating Pressure(psi) is less than 30. Types of materials removed are Clay, bacteria, large viruses, suspended solids. Ultrafilter(UF) - Operating Pressure(psi) is between 20 and 100. Types of materials removed are Viruses, proteins, starches, colloids, silica, organics, dye and fat. Nanofilter(NF) - Operating Pressure(psi) is between 50 and 300. Types of materials removed are Sugar, pesticides, herbicides, divalent anions. Reverse Osmosis(RO) - Operating Pressure(psi) is between 225 and 1000. Types of materials removed are Monovalent salts.
Applications of Micro and Ultrafiltration are Conventional water treatment (replace all processes except disinfection), Pretreat water for R.O and nanofiltration, Iron/Manganese removal (after oxidation) and Removal of DBP precursors. Applications of R.O. and nanofiltration are R.O. application are mostly used for desalination. Nanofiltration first developed to remove hardness. Nanofiltration can be used to remove DBP precursors.
During 1970s RO membranes with greater operating pressures was developed. This resulted in considerable increase in energy cost. Thus, low pressure RO membranes were developed and came to be known as NF membranes. By the second half of 1980s,NF became established. Starting in early 1990s, it had became common and various applications were found out.
We need nanofiltration due to increase in demand of good quality water due to increasing population, Reducing the wastage and reuse of water, Better reliability and durability of filter membranes and to reduce the overall cost of operation. Different polymers used for nanofiltration are are polyethersulfone, polysulfone, polyphenylsulfone, polytetrafluoroethylene, polyvinylidene fluoride, polyacrylonitriles, nylon, polypropylene, cellulose acetate (CA), regenerated cellulose, and composites, Ceramic and cintered metals and carbon nanotubes. Two types of membranes. Spiral membranes - Cheapest, more sensitive to pollution. Tubular membranes - Most used, not easily polluted. But surface area of spiral membranes is greater than that of tubular membranes and hence greater capacity.
Benefits of Nanofiltration
1) Low cost of operation.
2) Low energy cost.
3) Lower discharge and less waste water than typical Reverse Osmosis system.
4) Reduction of Heavy Metals (removes 95%).
5) Reduction of water hardness.
6) Reduction / Removal of viruses, bacteria, VOC’s, and Pesticides.
7) Reduction of Nitrates and Sulphides.
8) Reduction of the salt content (brackish water).
9) Chemical - Free filtration (No use of salt).
10) pH of the water can be altered for better health.
11) Ideal for municipal water supply, well water, river and rain water.
12) Removes Iron, Lime and other problem causing chemicals often neglected by water softeners.
Distinct properties of Nanofiltration
The pore size of a NF membrane corresponds to a molecular weight cut-off of 300-500g/mol. Hence, separation of these components from higher molecular weight components can be achieved. Nanofiltration membranes have a slightly charged surface. The dimensions of pores are close to dimensions of ions and hence charge interaction takes place. It can be used to separate ions with different valences. Two basic types of exclusion mechanisms are
Steric exclusion mechanism - Similar to sieving. Geometric exclusion of solute particles larger than pore size. Charge based exclusion mechanisms:
Donnan exclusion - Due to charged nature of NF membrane. Dielectric exclusion: Due to difference in dielectric constant.
Drawbacks of the process of Nanofiltration
1) Membrane fouling.
2) Insufficient separation.
3) Treatment of concentrates.
4) Membrane lifetime and chemical resistance.
5) Insufficient rejection for individual components.
There are various ways to reduce the fouling such as Periodic pulsing of feed. Periodic pulsing filtrate (backwashing). Increasing shear by rotating membrane. Vibrating membrane. Pretreatment of feed water greatly influences the performance of NF installations. The application of a pre-treatment has several benefits such as Membranes have a longer life-span when pre-treatment is performed. The production time of the installation is extended. The management tasks become simpler. The employment costs are lower.