Textile salt recovery: electrodialysis vs nanofiltration

A dyehouse should compare electrodialysis and nanofiltration against the same segregated liquor and the same reuse specifications. Electrodialysis moves ions electrically; nanofiltration uses pressure and membrane selectivity. Neither recovery percentage establishes value until the recovered salt, water or dye can return to production without causing quality defects or increasing treatment elsewhere.

Wastewater segregation and separation mechanisms

The first design choice is where to collect the feed. A concentrated stream from a known dye recipe retains information about its salt, colourants and auxiliaries. Once mixed with general plant wastewater, it may become harder to separate into reproducible products. Segregation can therefore determine whether recovery is economical before a membrane is selected. It also carries costs: additional piping, tanks, scheduling and controls belong in the investment comparison.

Pressure-driven systems already provide a relevant operating baseline. Tintoria Emiliana describes an operating recovery train containing microfiltration, ceramic ultrafiltration and a 50 cubic metre per day nanofiltration section. The operator reports recovery of up to 80% of sodium chloride. These figures apply to that installation and its process; they are not a general membrane guarantee. The useful lesson is that pretreatment, selective recovery and residual treatment function as a connected train.

Electrodialysis offers a different separation mechanism. An electric field drives dissolved ions across membranes, while larger dye molecules may be retained by a suitably chosen barrier. The 2025 study on nanoporous electrodialytic dye and salt fractionation investigates this approach with a size-selective membrane. Its research setting must be distinguished from a long-running dyehouse plant. Replacing one membrane component in an experimental stack does not establish industrial cleaning intervals, module life or performance on every commercial dye formulation.

Recovered-stream quality and usable salt yield

Salt recovery is valuable only within a defined quality envelope. Conductivity indicates ionic content but cannot establish that a recovered stream has the correct balance of sodium chloride, sulphate, hardness and organic impurities for reuse. The same applies to dye recovery: retained coloured material may include hydrolysed dye or altered mixtures. Procurement should therefore require practical dye-bath tests, shade consistency and the relevant product-quality measurements, together with chemical analysis of the recovered streams.

Energy figures need matching denominators. A result in kilowatt-hours per cubic metre of feed cannot be compared directly with one per kilogram of transferred salt. Require both the complete feed mass balance and the accepted recovered output. Include pumping, electrical power, pretreatment, cleaning and any later concentration or oxidation. A membrane that uses little energy itself may still produce a residue that is expensive to manage. Conversely, an apparently demanding separation can be worthwhile if it removes a larger burden downstream.

As an illustrative recovery calculation, 100 cubic metres per day containing 20 kilograms of recoverable salt per cubic metre carries two tonnes of salt daily. If 80% becomes usable recovered salt, the avoided purchase is 1.6 tonnes per day. The commercial credit is lower if contamination or recipe mismatch prevents all of that output being reused. This simple distinction between separated mass and usable mass is essential when comparing quotations or calculating a payback period.

Membrane fouling and treatment-train integration

Fouling and cleaning should be observed through representative production changes. Surfactants, suspended solids and finishing agents can influence a membrane differently from a prepared dye–salt solution. Track flux or electrical resistance, cleaning frequency, lost dye, pressure drop and membrane replacement, alongside output quality. A pilot should cover the relevant recipe range and repeated cleaning, with retained records of feed composition. A short sequence of laboratory cycles cannot establish multi-year consumption or maintenance costs.

The decision concerns the whole treatment architecture. DuPont’s account of the Foshan Jialida project describes commercial recovery using multiple membrane stages, illustrating the scale of the incumbent engineering approach. A new electrodialysis module earns its place if it preserves useful materials or avoids equipment and operating costs elsewhere. Where it merely adds a specialist stage with no measurable gain in reuse or total cost, retaining a qualified pressure-driven route may be the stronger investment.