Nanofiltration Between RO and UF: Where Charged Membranes Pay Off on Industrial Wastewater

Nanofiltration is a membrane separation technology driven by a pressure differential, sitting between reverse osmosis and ultrafiltration and retaining nanometer-sized particles from water. It is used to purify drinking water and industrial water, to treat wastewater, and to concentrate valuable components in process fluids. Most nanofiltration membranes derive from reverse osmosis membranes - CA and CTA membranes, aromatic polyamide composite membranes, sulfonated polyethersulfone membranes - but operating pressure is lower, which is why nanofiltration is also called low-pressure reverse osmosis or loose RO.

Why Charge Changes the Picture

Nanofiltration membranes are charged membranes capable of electrostatic adsorption, and for the same water quality and conditions they need less pressure than a reverse osmosis membrane. Pore size and surface characteristics set their performance, and they exhibit different Donnan potentials toward ions of different charge and valence. The separation mechanism combines sieving with solution-diffusion while also involving charge repulsion. The practical result: strong removal of divalent and multivalent ions and of substances with a molecular weight above 200, and partial removal of monovalent ions and of substances below that molecular weight.

Against ultrafiltration and microfiltration its separation performance is clearly superior; against reverse osmosis it offers partial monovalent-ion removal, low process osmotic pressure, low operating pressure and energy savings. It is for this reason that membranes with a cut-off of only a few hundred still remove inorganic salts at very low pressure.

Operating pressure is generally around 1.0 MPa, and because monovalent ions and small molecules pass relatively freely, process osmotic pressure stays small - under identical conditions nanofiltration saves about 15 percent energy compared with reverse osmosis.

Eight Industrial Duties

Petroleum industry wastewater

This stream carries inorganic salts and organics from crude oil extraction and refining and is extremely complex. A nanofiltration membrane can split crude oil wastewater into an oil-rich aqueous phase and an oil-free brine phase; the oil-rich phase returns to the oil-washing step with fresh feed water, recovering crude oil and saving water at once. Placing nanofiltration ahead of reverse osmosis also solves the severe membrane fouling that RO alone suffered on this duty. On phenolic wastewater, phenol removal exceeds 95 percent, and high-valence heavy metal ions such as cadmium, nickel, mercury and titanium are removed efficiently at relatively low pressure and far lower cost than reverse osmosis.

Pesticide wastewater

Ordinary treatment cannot remove low-molecular-weight organic pesticides. Rejection studies on non-phenolic pesticides found that apart from dichlorides, rejection exceeded 96.7 percent, and the adsorption capacity of each pesticide on the membrane was influenced by its hydrophobicity.

Chemical fiber and dyeing

Most dyes have molecular weights from several hundred to several thousand, so the membrane passes inorganic salts and small molecules while retaining the dye. After a crude dye slurry passes through a nanofiltration system, the dye concentrates and the inorganic salt concentration falls: desalination exceeds 98 percent and dye loss stays below 0.1 percent, and the system can run at high temperature. Nanofiltration also treats and recycles oily wastewater from fiber processing.

Domestic sewage

Biodegradation followed by chemical oxidation consumes a lot of oxidant and leaves considerable residue. Inserting nanofiltration between the two lets biodegradable small molecules below a molecular weight of 100 pass, while non-biodegradable large organics above 100 are retained for chemical oxidation and then biodegraded. Biodegradation is used fully, oxidant or activated carbon is saved, and final residue drops.

Thermal power plant secondary wastewater

This water comes from ash flushing, dust removal and cooling systems, carrying heavy suspended solids and ash, high salt content and some organics. Microfiltration first removes all suspended particles along with 99 percent of BOD, 98 percent of COD, 73 percent of total nitrogen and 17 percent of total phosphorus, and reduces total bacterial count to 3 to 4 per litre; acid is then added to lower pH and remove CO2; finally nanofiltration desalination brings the water to boiler feedwater quality. The Eraring power station of Australia's Pacific Power has used nanofiltration on this duty, processing 1,000 to 15,000 m3 per day, easing load on the municipal supply and saving the plant US$800,000 a year. With planned capacity expansion, the volume treated was estimated to reach 5,000 m3/d by 2010.

Pickling waste liquid

In steel pickling, steel is immersed in a sulfuric acid bath at a mass fraction of about 20 percent. As pickling proceeds, acid concentration falls while ferrous sulfate rises; once acid drops to 6-8 percent and ferrous sulfate exceeds 200 to 250 g/L, the pickling rate declines and the bath must be replaced. Nanofiltration exploits the difference in rejection between sulfuric acid and ferrous sulfate: ferrous sulfate is retained in the concentrate and sent to a cooling crystallization tank to crystallize FeSO4-7H2O, while the permeate passes through another module retaining sulfuric acid and is concentrated back to 20 percent for reuse. Both acid and ferrous sulfate are recovered.

Papermaking

Replacing conventional chemical treatment with nanofiltration removes dark-colored lignin more effectively. The chlorinated lignin produced during wood pulp bleaching is negatively charged and is retained by the negatively charged membrane without fouling it. Since there is no strict requirement to remove sodium across the process, reverse osmosis is unnecessary, and ultrafiltration combined with nanofiltration works well on kraft paper manufacturing wastewater.

Household chemical wastewater

Applied research shows nanofiltration membranes resist acid and alkali, offer excellent rejection and remove heavy metals well, with no fouling problem. Because operating costs are lower than reverse osmosis and rejection of small organic molecules is good, nanofiltration may eventually cover more than 90 percent of household chemical wastewater treatment.

Drinking Water: The Qualifications

Sibille and colleagues studied the groundwater of Auvers-sur-Oise in France, comparing nanofiltration with biological drinking water treatment using ozone and biological activated carbon filtration. Nanofiltration significantly improved drinking water quality, reducing bacterial counts and organic matter concentrations, which makes subsequent disinfection more effective and reduces chloroform formation. The study also noted that small amounts of readily assimilated biodegradable organic matter (BOM) and assimilable organic carbon (AOC) can pass through the membrane.

Work on tap water drawn from the heavily polluted Huai River found that a recirculating nanofiltration process achieved higher water yield at the same low pressure than a single-pass arrangement, while lowering energy consumption and reducing concentrate discharge. Even at a high recovery rate of 80 percent, total organic carbon (TOC) in the membrane effluent was still 50 percent lower than in tap water, and removal of mutagenic substances was marked enough to turn Ames-test-positive water negative.

In China, large-scale engineering application is not yet mature and remains at trial stage, mainly because performance indicators of domestically produced membranes are not yet good enough. There are working examples: the Nanhuangcheng nanofiltration demonstration project on Changdao in Shandong, the country's first industrial large-scale membrane softening system, designed by the Hangzhou Water Treatment Center of the State Oceanic Administration, began producing fresh water in April 1997 and ran normally and continuously for 27 months with desalinated water meeting the national drinking water hygienic standard.

What Still Has to Be Solved

Membrane preparation, characterization and separation mechanisms still carry open questions, inexpensive high-performance membranes still need developing, and users need accurate performance parameters. Beyond that, three fundamental problems of membrane separation remain the main reason nanofiltration-based water treatment is hard to apply widely, and specialists worldwide are working on them.

The direction of travel is nonetheless clear. Nanomaterials differ from ordinary materials in mechanical, magnetic, optical, electrical and thermal behaviour, showing new characteristics in radiation, absorption, catalysis and adsorption. Nanotechnology concerns the special properties of matter within the 0.1 to 100 nm range, and research into how those properties act on pollutants in water suggests nanoscale water treatment technologies - including nano-TiO2 photocatalytic oxidation alongside nanofiltration - are not far off.