Efficient textile salt recovery: membrane technology revolutionizes how to reuse NaCl and Na₂SO₄

One of the major environmental issues in the textile industry is the intensive use of salts, particularly NaCl (common salt or sodium chloride) and Na₂SO₄ (Glauber’s salt or sodium sulfate), which are essential in dyeing processes.

Beyond the high procurement costs, their production and transportation generate significant CO₂ emissions, contributing to air pollution. Additionally, when saline residues are discharged into rivers at high concentrations, they disrupt aquatic ecosystems, causing severe environmental damage.

Finding innovative solutions to reduce salt consumption and enhance the industry’s sustainability is now one of the primary challenges for industrial wastewater treatment plants.
Research is increasingly focused on developing more eco-friendly alternatives for the textile sector. This is why this article explores the textile industry’s production processes, where salts come into play, their environmental impact, and, most importantly, the groundbreaking innovation that is making salt recovery from textile wastewater more efficient than ever: the Multi Membranes Salt Recovery System.

CONTEXT

The key stages of textile processing and the use of salts

The textile sector operates through a series of key processes that transform raw fibers into fabrics used for clothing, home furnishings, and accessories. From the initial preparation phase to dyeing, each step is crucial to ensuring the quality, durability, and resistance of the materials.

The main production processes are sizing, desizing, bleaching, mercerizing, dyeing, and printing. Steps 2 (desizing), 3 (bleaching), and 4 (mercerizing) are all cleaning and preparation stages. Once the fabric is purified and prepared, the next phase (dyeing) is where salts play a key role in ensuring vibrant, long-lasting colors. This phase is, in fact, the central focus of this discussion.

1. Sizing

This initial step involves applying potato starch, resins, and polyvinyl alcohol to strengthen the fabric. The treatment enhances the yarn’s resistance during processing by using oils, waxes, and hydrocarbons to reduce friction between fibers. This process is especially important for delicate or high-tension fabrics, as it prevents breakage during weaving.

2. Desizing

In this phase, the potato starch and resins applied during sizing are removed through a washing process known as scouring or degreasing. This treatment eliminates impurities and prepares the fabric for the subsequent stages. Enzymatic desizing, which uses specific enzymes to break down starch, is an increasingly popular method due to its lower environmental impact compared to traditional chemical processes.

3. Bleaching

The fabric is treated to remove residual oils and hydrocarbons, achieving a uniform white color. This step is crucial to prepare the fabric for dyeing, ensuring better absorption and color consistency. Hydrogen peroxide (H₂O₂) is commonly used for cotton bleaching due to its efficiency and reduced environmental footprint compared to chlorine-based alternatives.

4. Mercerizing

Applied exclusively to cotton and plain-weave fabrics, mercerization involves a rapid treatment with caustic soda (sodium hydroxide). This process enhances the fabric’s luster, improves dye uptake, and increases tensile strength. It also makes the fibers smoother and more resistant to shrinkage, which is especially beneficial for high-end textile products.

5. Dyeing

Fabric dyeing involves immersing the textile in a bath containing dye, hot water, and salt, which is an essential element that helps fix the color to the fibers. Specific salts, such as NaCl (sodium chloride or common salt) and Na₂SO₄ (sodium sulfate or Glauber’s salt), are added to facilitate dye absorption. Darker shades require more intensive processing, often involving 5 to 10 washing cycles to remove excess dye. Notably, black dyeing demands exceptionally high salt concentrations (twice the salinity of seawater) to ensure deep, consistent coloration. This high salt usage significantly contributes to the environmental challenges associated with textile wastewater.

6. Printing

Certain fabrics undergo advanced printing techniques to create intricate patterns and designs. These methods include digital printing, which enables precise, detailed imagery while using less water than traditional processes, and roller printing, where engraved rollers apply dye while releasing rinse water. Both techniques transform raw textiles into finished products suitable for the fashion and home furnishing industries.

ANALYSIS

Drawbacks of evaporators in salt recovery

Historically, water desalination has been carried out using reverse osmosis. This filtration method, derived from seawater desalination processes, separates water into two streams:

• permeate stream: water that is free of salts;
• reject stream: saline waste containing chlorides and phosphates.

Advancements in reverse osmosis have led to the development of multi-stage processes, which can now achieve water recovery rates between 90% and 95%.

Although this high recovery rate is beneficial, it also brings significant drawbacks. The reject stream is a highly concentrated saline solution that requires further treatment using evaporators to separate the salts. Evaporators pose several challenges: they are expensive, energy-intensive, and responsible for substantial CO₂ emissions.

Are there currently technologies that are more efficient than evaporators and capable of overcoming their drawbacks?

FOCUS

From evaporators to membranes: the revolution in salt recovery

A cutting-edge technology has recently been developed that promotes the circularity of production processes, reduces CO₂ emissions, and allows textile manufacturing companies to recover and reuse the expensive salts used in dyeing.

This Multi Membranes Salt Recovery System, based on a multi-stage reverse osmosis system, is capable of ensuring a water recovery rate between 95% and 98%.

This technology effectively overcomes the drawbacks of evaporators. Instead of relying on energy-intensive evaporators, it uses specialized membranes to process the reject stream produced during the first salt recovery phase (ie. reverse osmosis). In this second phase, the membranes extract and separate the salts from the reject stream, specifically NaCl (common salt) and Na₂SO₄ (Glauber’s salt), enabling their reintroduction into the production cycle.

This is a revolution because this innovative approach minimizes the use of traditional evaporators or MVR. As a result, resource management is not only optimized, but there are also significant economic, environmental, and social benefits for the textile industry. The first company in the world to implement this salt recovery technology is WaterNext, an Italian firm with several active projects and facilities in regions with a high density of textile manufacturers, such as India and Mexico. The company had already distinguished itself for its ability to reduce the size of evaporators, cutting initial investment and subsequent operational costs for textile manufacturers by 50%.

CONCLUSIONS

Benefits of the Multi Membranes Salt Recovery System

As seen, the Multi Membranes Salt Recovery System is a cutting-edge solution that serves as an effective, cost-efficient and sustainable alternative, enhancing industrial water treatment and recovery processes.
The recovery of dyeing salts is now both economically viable and feasible. In fact, the purchase of Glauber’s salt is drastically reduced, leading to significant cost savings in textile production.