5.3 Applications in water softening and contaminant removal

Nanofiltration membranes are game-changers for water treatment. They soften water by removing calcium and magnesium, reducing scale in pipes. They also partially desalinate brackish water, making it usable for various purposes. This process is more energy-efficient than reverse osmosis.

These membranes are also heroes in contaminant removal. They tackle organic micropollutants like pesticides and pharmaceuticals, as well as inorganic nasties like nitrate. Nanofiltration can even improve water's color and taste by removing natural organic matter.

Water Softening and Partial Desalination

Nanofiltration for Water Softening

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• Nanofiltration membranes effectively remove divalent ions such as calcium and magnesium from water
• Water softening using nanofiltration reduces scale formation in pipes and appliances by removing hardness-causing ions
• Nanofiltration is a more environmentally friendly alternative to traditional water softening methods that use salt and generate brine waste
• Nanofiltration membranes have a high rejection rate for divalent ions while allowing monovalent ions to pass through (sodium, potassium)
• The removal of divalent ions also reduces the total dissolved solids (TDS) content of the water, resulting in partial desalination

Partial Desalination using Nanofiltration

• Nanofiltration membranes can achieve partial desalination by removing a significant portion of the dissolved salts from water
• The level of desalination achieved depends on the membrane properties and operating conditions (pressure, feed water composition)
• Partial desalination using nanofiltration is suitable for brackish water treatment and producing water for industrial processes
• Nanofiltration is more energy-efficient than reverse osmosis for partial desalination as it operates at lower pressures
• The permeate from nanofiltration can be blended with untreated water to achieve the desired water quality for specific applications (irrigation, cooling towers)

Organic Contaminant Removal

Removal of Organic Micropollutants

• Nanofiltration membranes are effective in removing a wide range of organic micropollutants from water
• Organic micropollutants include pesticides, pharmaceuticals, endocrine-disrupting compounds, and personal care products
• The removal of organic micropollutants is achieved through a combination of size exclusion, charge repulsion, and adsorption mechanisms
• Nanofiltration membranes have a molecular weight cut-off (MWCO) range that allows them to retain most organic micropollutants
• The removal efficiency of organic micropollutants depends on factors such as membrane properties, feed water composition, and operating conditions (pH, pressure)

Removal of Specific Organic Contaminants

• Nanofiltration is particularly effective in removing pesticides from water sources (atrazine, diuron)
• Pharmaceutical compounds such as antibiotics, anti-inflammatory drugs, and hormones can be efficiently removed by nanofiltration membranes (sulfamethoxazole, diclofenac, estradiol)
• Nanofiltration can significantly reduce the color of water caused by natural organic matter (humic and fulvic acids) and synthetic dyes
• The removal of color-causing compounds improves the aesthetic quality of water and reduces the formation of disinfection by-products during chlorination
• Nanofiltration membranes can be tailored to target specific organic contaminants by modifying membrane surface properties and pore size distribution

Inorganic Contaminant Removal

Nitrate Removal using Nanofiltration

• Nanofiltration is an effective technology for removing nitrate from groundwater and surface water sources
• Nitrate is a common inorganic contaminant that poses health risks, especially to infants (methemoglobinemia or blue baby syndrome)
• Nanofiltration membranes have a high rejection rate for nitrate ions due to their small pore size and negative surface charge
• The removal efficiency of nitrate depends on factors such as feed water composition, pH, and the presence of competing ions (sulfate, chloride)
• Nanofiltration can achieve nitrate concentrations below the drinking water standard of 10 mg/L as nitrogen set by the World Health Organization (WHO)
• Nanofiltration is a more selective and energy-efficient alternative to reverse osmosis for nitrate removal, as it allows the passage of beneficial minerals (calcium, magnesium)
• The concentrated nitrate stream from nanofiltration can be further treated or used as a nutrient source for agricultural applications (fertigation)