Polyvinyl Alcohol (PVA) Recovery from Desizing Effluent: Membrane Systems is a pressure-driven separation strategy applied to hot aqueous warp-sizing washwater before biological treatment or discharge. Textile weaving preparation lines use PVA as a film-forming size because of its water solubility, abrasion resistance, and clean removal in subsequent desizing. The resulting effluent typically contains 5–15 g/L PVA, along with starch hydrolysis products, waxes, lubricants, residual alkali, and fibrous debris. Raw desizing washwater commonly exhibits chemical oxygen demand in the 12–30 g/L range according to ISO 6060:1989. Membrane systems recover PVA as a concentrated retentate that can be blended with fresh sizing liquor, while the permeate may be reused as hot wash water after salinity and COD are considered.

Which membrane molecular weight cut-off retains PVA without collapsing flux?

Textile-grade PVA spans number-average molar masses from 10,000 Da to 120,000 Da and degrees of hydrolysis from 86 mol% to 99 mol%. The higher-hydrolysis grades form more rigid, crystalline gel layers, whereas partially hydrolysed PVA produces a more hydrated and deformable polymer coil. Ultrafiltration membranes rated at 20–50 kDa are generally selected for recovery duty because they retain the dominant polymer fraction while allowing salts, low-molecular-weight desizing additives, and starch degradation products to pass. Membranes with cut-offs of 100 kDa or higher may show higher initial flux but permit measurable loss of low-molecular-weight PVA fractions into the permeate. Retention is not determined solely by nominal pore size; concentration polarisation, gel-layer formation, and the pore-size distribution of the selected membrane all influence observed rejection.

Flux declines as the PVA concentration at the membrane surface increases, and the limiting concentration depends on polymer grade, temperature, and crossflow hydrodynamics. For a 20–50 kDa spiral-wound polyethersulfone element, published pilot data place initial flux in the 25–90 L/(m²·h) range at transmembrane pressures of 1.0–4.0 bar and feed temperatures of 55–80 °C. Higher-flux operation is possible with ceramic tubular modules, but the economic benefit must be weighed against higher pump energy and module cost. COD rejection across the UF stage is commonly reported at 65–90%, with the lower values occurring when starch or surfactant loadings are high.

At mill scale, feed pre-screening is not optional. Production-scale audits of 8 in spiral-wound polyethersulfone elements with 33 mil feed spacers show that fibrous debris matting occurs within 24 h when wedgewire or basket protection coarser than 150 µm is used. The failure mode is cake compression on the element face rather than irreversible polymer adsorption. Published data for this specific configuration is limited, but the observed pressure-drop increase is consistent with debris accumulation followed by localised channel blockage. PVA recovery skids therefore install 100–150 µm automatic backwash filters or hydrocyclones before the membranes. Membranes must be rated for continuous operation at 60–80 °C; cellulosic and thin-film composite membranes are unsuitable at the upper end of this range. Polyethersulfone and polyvinylidene fluoride elements are preferred, while ceramic tubular modules are reserved for high-temperature or abrasive feed variants.

Crossflow velocity and transmembrane pressure limits in fouling control

PVA fouling is controlled primarily by shear at the membrane wall. At crossflow velocities below 1.5 m/s, the laminar boundary layer thickens and polymer accumulates at the surface, reducing net driving force and accelerating gel formation. Industrial systems therefore operate tubular modules at 3.0–5.0 m/s and spiral-wound modules at 1.5–2.5 m/s, with the lower spiral-wound velocity reflecting spacer geometry and pressure-drop constraints. Transmembrane pressure is maintained in the 1.0–4.0 bar range. When flux plateaus despite rising pressure, the system has reached limiting-flux conditions, and further pressure increases only compress the fouling layer without raising throughput.

Periodic backpulsing or permeate reverse flow is used to dislodge reversible PVA gel. Published pilot programmes describe backpulsing at 0.5–1.0 bar for 2–5 s every 10–20 min, depending on feed solids and membrane configuration. Chemical cleaning alternates alkaline and oxidative conditions. A typical sequence uses 0.1–0.3 wt% sodium hydroxide at 45–55 °C, followed by 500–1,000 mg/L active chlorine at 20–30 °C, with rinsing until neutral pH. Cleaning is initiated after 20–30% loss of normalised flux rather than on a fixed time cycle. Retentate viscosity is the practical concentration limit: when apparent viscosity exceeds 40 mPa·s at process temperature, recirculation pump energy rises sharply and crossflow velocity becomes difficult to maintain without exceeding pump curve limits.

When desizing effluent contains starch blends, retentate rheology becomes the operating constraint

Many weaving preparation recipes blend PVA with starch, modified starch, or polyacrylic acid sizes. Desizing of these mixed formulations releases soluble starch degradation products, enzymes, and colloidal proteins into the same hot washwater. These substances compete with PVA for the membrane surface and form mixed gels with shear-thinning behaviour. At 60 °C and a shear rate of 100 s⁻¹, retentate apparent viscosity may rise from 5–15 mPa·s to over 100 mPa·s when total solids approach 20–25 wt%. The viscosity increase reduces wall shear and lowers flux even if transmembrane pressure remains constant.

In starch-PVA systems, recovery is best operated at moderate retentate concentration and with upstream enzymatic desizing that reduces starch to low-molecular-weight maltodextrins. Residual starch hydrolysis products can then permeate through a 50 kDa membrane while PVA is retained. Anti-foaming agents and surfactants present in desizing effluents add further complexity because they can solubilise waxes and deposit within the porous substructure of hydrophobic membranes. Flat-sheet screening with actual desizing effluent is therefore required before spiral or tubular module installation. Published data for long-term performance with mixed starch/PVA effluents is limited, and pilot runs of 500–1,000 h are normally required to establish cleaning frequency, flux decay rate, and membrane replacement intervals.

Recovery loop integration demands a salt and low-molecular-weight impurity balance

Recovered PVA retentate is not returned directly to the sizing bath without compositional adjustment. Ultrafiltration permeate from a 20–50 kDa membrane contains dissolved salts, organic acids, and low-molecular-weight additives. If the permeate is reused as hot wash water, its COD, conductivity, and sodium content determine the number of reuse cycles before a bleed stream becomes necessary. In a closed-loop configuration, retentate is concentrated to 15–20 wt% total solids and blended with fresh PVA stock solution or powder. The blend must meet the sizing formulation’s viscosity and film-strength requirements. Apparent viscosity is controlled by rotational viscometer according to ISO 2555:2018 or by a torque-based process viscometer at the mixing tank.

Salt accumulation is a specific limitation of water reuse. If desizing uses sodium hydroxide or sodium carbonate and the permeate is recycled to the wash cascade, sodium ion concentration increases in the loop and may exceed tolerance limits for downstream dyeing or finishing. At sodium concentrations above 500–1,000 mg/L in reuse water, a reject or bleed stream is required. Some operators route UF permeate through reverse osmosis or nanofiltration to remove salts and produce water suitable for desizing. Reverse osmosis recovery from this stream is nevertheless limited by residual low-molecular-weight PVA fractions that can foul polyamide membranes. Industrial data for combined UF-RO PVA recovery loops indicate that UF retention must remain at or above 95% for the RO stage to avoid accelerated fouling.

Operational boundaries are set by membrane polymer tolerance and PVA solution stability. Feed temperature should remain above 40–50 °C to prevent gel formation and wax solidification, but below the continuous-temperature rating of the membrane, typically 75–80 °C for polyethersulfone. Feed pH is maintained between 5.5 and 10.0; strongly alkaline desizing effluent with pH above 10.5 can hydrolyse polyester backing layers and shorten membrane life. Oxidising desizing agents such as persulfate or hydrogen peroxide must be quenched or reduced before membrane contact because residual oxidants above 1–2 mg/L can degrade polysulfone and polyethersulfone over extended exposure. Systems should be drained and rinsed at shutdown if retentate is to be held for more than 4–6 h because PVA solutions are susceptible to microbial growth and viscosity drift.