NANOFILTRATION IN WATER SUPPLY SYSTEMS 1 NANOFILTRATION IN WATER SUPPLY SYSTEMS

Generally, there are four types of membrane processes, 2 Generally, there are four types of membrane processes, Microfiltration. Ultrafiltration. Nanofiltration. Reverse Osmosis. MEMBRANE PROCESSES

Operating Pressure,psi Types of Materials Removed Filter type Symbol Pore Size,m Operating Pressure,psi Types of Materials Removed Microfilter MF 1.0-0.01 <30 Clay, bacteria, large viruses, suspended solids Ultrafilter UF 0.01-0.001 20-100 Viruses, proteins, starches, colloids, silica, organics, dye, fat Nanofilter NF 0.001-0.0001 50-300 Sugar, pesticides, herbicides, divalent anions Reverse Osmosis RO < 0.0001 225-1,000 Monovalent salts

Applications of Micro- and Ultrafiltration: Conventional water treatment (replace all processes except disinfection). Pretreat water for R.O and nanofiltration. Iron/Manganese removal (after oxidation). Removal of DBP precursors. Applications of R.O. and nanofiltration: R.O. application mostly desalination. Nanofiltration first developed to remove hardness. Nanofiltration can be used to remove DBP precursors.

HISTORY OF NANOFILTRATION 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.

NEED FOR NANOFILTRATION Increasing demand of good quality water due to increasing population. Reducing the wastage and reuse of water. Better reliability and durability of filter membranes. To reduce the overall cost of operation.

Materials used in NF membranes Different polymers used are polyethersulfone, polysulfone, polyphenylsulfone, polytetrafluoroethylene, polyvinylidene fluoride, polyacrylonitriles, nylon, polypropylene, cellulose acetate (CA), regenerated cellulose, and composites. Ceramic and cintered metals. 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 Low cost of operation. Low energy cost. Lower discharge and less waste water than typical Reverse Osmosis system. Reduction of Heavy Metals (removes 95%). Reduction of water hardness. Reduction / Removal of viruses, bacteria, VOC’s, and Pesticides.

Reduction of Nitrates and Sulphides. Reduction of the salt content (brackish water). Chemical - Free filtration (No use of salt). pH of the water can be altered for better health. Ideal for municipal water supply, well water, river and rain water. 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: 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.

Various Applications: Industrial applications: Food and dairy sector. edible oil processing sector. Petroleum industry. Drug industry. Paper pulp industry Water treatment. Desalination of water. Water softening. LINK\app.docx

Drawbacks of the process of Nanofiltration: Membrane fouling. Insufficient separation. Treatment of concentrates. Membrane lifetime and chemical resistance. 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. Pretreatment of feed water greatly influences the performance of NF installations. The application of a pre-treatment has several benefits: 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.

CASE STUDY Nanofiltration Membranes for Removal of Colour and Pathogens in Small Public Drinking Water Sources:

Small public water supplies that use surface water as source for drinking water are frequently faced with elevated levels of colour and NOM which are precursors for chlorinated DBP formation. NF systems can prevent DBP formation by removing colour and NOM before chlorination. Research studies were conducted in Fall Lake water in Minnesota and dechlorinated potable water spiked with NOM in Ohio by using nanofiltration(Fyne process).

Fyne process Developed in Scotland in 1994 to treat highly coloured surface waters. Requires no pretreatment of feed water other than a coarse screen to prevent the entry of large solids. Chosen because of its minimal pretreatment and chemical cleaning requirements and its unique mechanical cleaning feature. The process uses a tubular semi permeable membrane to allow clean filtered water to pass through while retaining microbial contaminants and most of the colour producing organic materials dissolved in water.

Surface water treatability studies were conducted in Fall lake water, Minnesota, for removal of microorganisms and organic matter in 2008 and 2009. Additional studies were conducted with the same Fyne process pilot unit using dechlorinated potable water spiked with NOM in Ohio. Pilot unit was fitted with two 3 ft vertical membrane modules made of acrylonitrile butadiene styrene plastic. Each module contained 72 individual 0.91m tubes. PCI type ES404 polyethersulfone membrane with a 4,000 Dalton MWCO, PCI type CA2PF cellulose acetate membrane with a 2,000 Dalton MWCO and in 2009, the ES404 membrane was tested again in Minnesota, along with a PCI type AFC30 polyamide film membrane with a MWCO of 350 Daltons, potentially offering better DBP precursor removal.

NF membrane flux results from Minnesota (2008)

Apparent colour removal 99% 95% Removal of particles>2µm 2.5log Results ES404 CA2PF Flux rates 624L/m2/day 608L/m2/day Apparent colour removal 99% 95% Removal of particles>2µm 2.5log 2.4log Iron and Manganese Complete removal Calcium 25% Magnesium 13% Flux declined by 18% & 7% respectively for the two membranes.

NF membrane flux results from Minnesota(2009)

Colour removal efficiency 90.7% 93.3% Removal of particles>2µm Results ES404 AFC30 Flux rates 624L/m2/day 400L/m2/day Colour removal efficiency 90.7% 93.3% Removal of particles>2µm 2.2log 2.3log Iron and Manganese Complete removal Calcium 42% 63% Magnesium 29% 100% Both flux rates are greater than design flux rates.

DBP studies in Minnesota. In 2008, the ES404 and CA2PF membranes removed 92 and 73% of TOC respectively. ES404 and CA2PF reductions of ultraviolet (UV) 254 were 96 and 69%, respectively. In 2009, the ES404 membrane removed only 76% of TOC, whereas the AFC30 removed greater than 91%. The results of the tests indicate that ES404 and AFC30 membranes would be capable of reducing DBP formation to acceptable levels.

Particle count results from Minnesota (2008)

Above table represents the 2 & 3µm particle removal results, same size range of Cryptosporidium. Influent water-average particle count of 3440/ml. Effluent water, for ES404-average particle count-14/ml. Effluent water, for CA2PF- ES404-average particle count-17/ml. Thus,99.6 and 99.5% removal, respectively for both the membranes.

Colour removal results from Minnesota.

TOC and colour removal results from Ohio (2009)

CONCLUSIONS: In Minnesota, the ES404 membrane showed noticeably better performance in colour removal than the CA2PF membrane. The ES404 and CA2PF membranes both showed 99.5% removal efficiency for particles in the 2 and 3 μm size ranges (the size range for Cryptosporidium). The colour removals achieved by the ES404 and AFC30 membranes in treating Fall Lake water were also satisfactory. In Ohio, the Fyne process membranes demonstrated good colour and TOC removal.

The results of this study indicate that the Fyne process nanofiltration system could be used at very small (serving 25–500 people) public water systems to reduce colour and microbial pathogens in drinking water. The TOC removal exhibited by the nanofiltration membranes would also reduce the formation of DBPs after chlorination. Thus, It is evident that nanofiltration will play a vital role in providing a quality, usable form of water in the future.

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