Membrane Filtration and Reverse Osmosis

Membrane filtration separates dissolved and suspended pollutants via semi-permeable membranes enabling water recovery for reuse achieving 90-99% removal of color, organics, salts, and producing high-purity permeate suitable for process water substitution reducing freshwater consumption 50-80%. Membrane types by pore size and separation mechanism include microfiltration MF (pore size 0.1-10 μm removing suspended solids, bacteria, turbidity, operating pressure 1-3 bar, used for pretreatment or biomass separation in MBR), ultrafiltration UF (pore size 0.01-0.1 μm or MW cutoff 10-100 kDa removing colloids, macromolecules, viruses, dyes with high MW, operating pressure 2-5 bar, used for pretreatment before RO or direct treatment for color removal), nanofiltration NF (pore size 0.001-0.01 μm or MW cutoff 200-1000 Da removing divalent salts, small organics, dyes, operating pressure 5-15 bar, partial desalination retaining 70-90% of NaCl while rejecting 90-98% of divalent salts, calcium, magnesium, sulfate, color, used for water softening, dye recovery, partial reuse water production), and reverse osmosis RO (effective pore size 0.0001 μm rejecting >95% of dissolved salts, >98% of organics, >99% of dyes, bacteria, viruses producing high-purity water <500 mg/L TDS from feed 3,000-10,000 mg/L TDS, operating pressure 15-50 bar depending on feed salinity, used for complete desalination, high-quality reuse water production). Membrane materials include polymeric (polyethersulfone PES for MF/UF, cellulose acetate CA for RO, polyamide PA for thin-film composite TFC-RO membranes, polysulfone PSF, polyvinylidene fluoride PVDF), ceramic (aluminum oxide, titanium dioxide, zirconium oxide for MF/UF, high chemical resistance, high-temperature tolerance, expensive $500-2,000/m² vs. polymeric $20-200/m², used in harsh conditions, high-fouling applications), and composite (TFC with PA active layer on PSF support, most common RO, high flux 20-40 L/m²/hr, high rejection 98-99.5%, moderate cost $30-100/m²). Membrane configurations include spiral-wound (flat sheet membranes spiraled around permeate tube, compact, high packing density 300-900 m²/m³, used in UF, NF, RO, standard modules 2.5-8 inch diameter, 40 inch length, 2-40 m² area), hollow fiber (self-supporting fiber bundles, very high packing density 600-9,000 m²/m³, outside-in or inside-out flow, used in MF, UF, some RO, excellent for MBR applications), tubular (membranes inside tubes 5-25 mm diameter, low packing density 20-100 m²/m³ but high fouling resistance, easy cleaning, used in high-solids streams, food, pharmaceutical), and plate-and-frame (flat membranes stacked with spacers, easy maintenance, high operating cost, used in small-scale, lab, specialty applications). RO process comprises pretreatment (cartridge filtration 5-10 μm removing particles preventing membrane abrasion, antiscalant dosing 2-5 mg/L preventing calcium carbonate, calcium sulfate, silica precipitation, pH adjustment to 6-7 optimizing performance, preventing scaling, sodium bisulfite 2-5 mg/L scavenging residual chlorine which degrades polyamide membranes), high-pressure pumps (15-50 bar depending on feed salinity, osmotic pressure of 3,000 mg/L TDS = 2 bar, 10,000 mg/L TDS = 8 bar, operating pressure must exceed osmotic pressure to drive permeate flow), membrane modules (arrays of spiral-wound or hollow-fiber modules in pressure vessels, typically 4-8 modules per vessel, multiple vessels in parallel and series stages), permeate collection (high-purity water <500 mg/L TDS, <5 NTU turbidity, >95% color removal, >98% COD removal), concentrate recirculation (rejecting 50-80% of feed as concentrate containing 2-5× feed concentration, partially recycling to increase recovery to 70-85%, or disposing with concentrate containing all rejected salts, organics, dyes), and energy recovery (pressure exchangers, turbochargers recovering 30-50% of energy from high-pressure concentrate stream reducing net energy consumption to 1-3 kWh/m³ permeate vs. 3-6 kWh/m³ without recovery). Permeate quality depends on membrane type and feed composition: UF permeate (removing 85-95% color, 40-60% COD, minimal TDS removal <10%, suitable for low-salinity reuse—process water in pretreatment, dyeing requiring makeup), NF permeate (removing 90-98% color, 70-85% COD, 70-90% hardness, 20-40% monovalent salts like NaCl, suitable for moderate reuse—dyeing, washing, cooling towers), RO permeate (removing >98% color, >95% COD, >97% TDS, ultra-pure suitable for critical reuse—boiler feed, high-pressure dyeing, final rinse, or potable quality after post-treatment disinfection). Membrane fouling limits performance: particulate fouling (suspended solids, colloids depositing on surface, prevented via pretreatment cartridge filtration, coagulation), organic fouling (oils, dyes, humic substances adsorbing, blocking pores, mitigated via pretreatment coagulation, activated carbon, periodic cleaning), biological fouling (biofilm growth, prevented via chlorination of feed or periodic biocide shock treatment, UV disinfection), scaling (inorganic precipitation—CaCO3, CaSO4, BaSO4, SiO2—prevented via antiscalant, pH adjustment, operating below saturation limits), and irreversible fouling (chemical degradation, compaction reducing flux over years, requiring membrane replacement every 3-7 years). Cleaning protocols include routine chemical cleaning (every 1-4 weeks depending on fouling rate, cleaning with acid pH 2-3 for scale, alkali pH 11-12 for organics, surfactants for oils, enzymes for biofilm, restoring 80-95% of flux), and physical cleaning (periodic backwash in MF/UF reversing flow, air scour, increasing crossflow velocity). Performance monitoring includes flux (permeate flow per unit area L/m²/hr typically 15-40 for RO, 50-150 for UF, declining with fouling indicating cleaning needed), pressure drop (increasing across modules signaling fouling, cleaning if pressure increases 20-30% above baseline), salt passage (monitoring permeate conductivity, TDS, rejection >95% normally, declining rejection indicating membrane damage requiring element replacement), and recovery rate (permeate flow / feed flow %, targeting 70-85% for RO, limited by concentrate saturation, scaling potential). Costs include capital ($1,000,000-10,000,000 for 1,000-5,000 m³/day RO system including pretreatment, pumps, membranes $30-100/m², vessels, energy recovery, controls), operating ($0.50-1.50/m³ permeate dominated by energy 1-3 kWh/m³, membrane replacement every 3-7 years $0.10-0.30/m³ amortized, chemicals for pretreatment and cleaning $0.05-0.15/m³, labor, maintenance), concentrate disposal (20-30% of feed volume at 3-5× concentration requiring disposal via sewer with surcharge $0.20-0.60/m³, evaporation ponds in arid regions, deep well injection, or zero liquid discharge ZLD). Applications include water reuse (recovering 70-85% of effluent after biological and pretreatment, substituting for freshwater in non-potable applications—process water, cooling, toilet flushing, gardening, reducing water purchase costs $0.50-2/m³, justifying RO investment via payback 2-5 years in water-scarce regions), salt recovery (NF or RO concentrating salt from dyeing effluent, evaporative crystallization producing sodium sulfate for reuse or sale $50-150/tonne offsetting treatment costs), dye recovery (UF or NF retaining dye while permeating water, enabling dye reuse in subsequent batches reducing dye purchase 10-30%, limited to single-color campaigns, batch dyeing), and zero liquid discharge ZLD (RO followed by evaporation or brine crystallizer completely eliminating discharge, recovering salt and water, high cost $5-15/m³ total water treated but mandatory in water-scarce or discharge-prohibited areas like India GPCB ZLD mandate for clusters, China environmental zones). Challenges include membrane fouling (requiring robust pretreatment, frequent cleaning increasing costs, downtime), concentrate disposal (20-30% volume at high salinity, color, COD creating secondary pollution problem, requiring expensive disposal or further treatment ZLD), high energy (1-3 kWh/m³ for RO contributing 30-50% of operating cost, requiring energy recovery systems minimizing), and capital intensity (RO systems $1-5 million for 1,000-5,000 m³/day capacity vs. biological treatment $500,000-2,000,000, justified by water reuse value in scarcity scenarios).