Dyeing textiles without effluent

At present about 24,000 cubic meters of wastewater are treated with membrane elements and ion exchangers from LANXESS in the Tiruppur region in Tamil Nadu.

Clean water is essential for human life. Contaminated water is the primary cause of many infectious diseases, especially in developing countries. In the developing world, and elsewhere, untreated wastewater is still dumped into rivers. This makes it difficult, or even impossible, to obtain unpolluted drinking water. On March 22, 2017, the United Nations World Water Day reminded us of this problem. The aim is to identify and utilise wastewater as a resource.

For example, in the private sector wastewater is produced in sanitary facilities and wherever water is required for production processes. Traditionally, industries such as leather-making and textile production use large quantities of water and generate a lot of effluent. Nowadays, however, liquid waste can be minimised, or even completely eliminated, thanks to highly efficient treatment methods using reverse osmosis and ion exchange. Lewabrane and reverse osmosis elements and Lewatit ion exchange resins from LANXESS play an important role in this.

Modern treatment in India

In Tiruppur, in the province of Tamil Nadu in southern India, at the heart of the Indian cotton treatment industry, there are high-performance treatment plants that serve as a shining example of how modern wastewater management works. In the 1990s hundreds of textile factories, especially the many cotton dyeing factories there, still dumped large amounts of untreated effluent into the Noyyal River, a vital source of drinking water for the entire region.

The effluent situation improved when the Tamil Nadu Pollution Control Board developed special methods of treating the effluent from cotton dyeing factories as part of a country-wide programme. Dedicated treatment plants for large textile companies, as well as for small and medium-sized dyeing factories, were and are still being built. Such plants can be ideally combined with existing production facilities. In the case of newly constructed factories, specially optimised production processes offer further opportunities for recycling processed water and avoiding the generation of wastewater.

At present about 24,000 cubic meters (cu m3) of wastewater are treated with membrane elements and ion exchangers from LANXESS in the Tiruppur region. For example, in one textile factory in Tiruppur a total of 154 Lewabrane ROS400 HR membrane elements and 7,500 liters of Lewatit CNP 80 WS cation exchange resin have been used since August 2016 to process about 85 cu m3 of wastewater per hour.

Effluent treatment for dyeing factories

The multi-stage “end-of-pipe” method – i.e., retrofitted environmental protection measures that do not change the production process but do reduce environmental pollution – starts with biological treatment, separation of sludge and coarse filtration. This removes most of the organic content and dispersed particles, e.g., fibres. After this, the filtrate is bleached and then softened with the aid of ion exchangers. In the next step, a low-salt, colorless permeate, containing less than one percent of the originally dissolved salts and no organic contamination, is produced by reverse osmosis. This can often go right back into the industrial process.

“The use of ion exchange is a highly efficient method for pre-treating water before reverse osmosis,” explains Alexander Scheffler, Membrane Business Director from the LANXESS Liquid Purification Technologies Division. The salt-enriched low-chloride retentate can either be re-used directly in the dyeing process or further concentrated. Finally the salts, primarily sodium sulfate and sodium chloride, are separated into distinct solids. The sulfate can be re-used in the dyeing process and the chloride can be disposed of. With this process, wastewater is no longer produced. This not only protects the environment, but also has potential to save the textile industry money, mainly thanks to the reclamation of salts. Experts are convinced that all of this could be implemented with almost no effect on production costs if primarily regenerative energy sources were used.

“The example of the plant in India is an impressive demonstration of how very salty water that is also severely contaminated with organic material can be very efficiently purified with a combination of ion exchangers and reverse osmosis,” Scheffler explains.

New RO membrane elements impress

New reverse osmosis (RO) membrane elements from LANXESS with ASD feed spacers have undergone a trial in Germany’s largest industrial water treatment plant. More than 50,000 cu m3 of water are needed every day for pulp manufacturing at Zellstoff Stendal GmbH, Arneburg, and this is treated using reverse osmosis and ion exchange resins. The new grades impressed with their performance and consistently high level of rejection.Optimised for applications in brackish water, these Lewabrane-branded elements are characterised by very low energy consumption (LE = low energy) and high fouling resistance (FR = fouling-resistant).

“The outstanding results of our ASD products in benchmark tests with other manufacturers at one of the largest pulp manufacturers in Europe once again underline the extraordinary quality of our products,” says Scheffler.

Consistently good performance

One year ago, Zellstoff Stendal launched a project to test LANXESS’s newly developed Lewabrane ASD range of membrane elements. This involved installing conventional reverse osmosis (RO) elements with a standard feed spacer and FR elements with LANXESS’s new ASD spacer in parallel in a single pressure vessel. Feed spacers are essential components for spirally wound membrane elements. They are made of polymer material and are optimised to ensure constantly good performance for a wide range of raw water compositions and various process parameters. The LANXESS spacers have a special “netting” that reduces biological growth and particle fouling by minimising dead zones. The netting has an alternating strand design (ASD) of thin and thick filaments, which is what gives the spacer its name.

Frank Gorges, plant technician for water treatment at Zellstoff Stendal GmbH in Arneburg, was excited about the result.

Fall in energy consumption

After one year, the elements were removed and taken to the applications laboratory LANXESS operates in Bitterfeld for an “autopsy”. Compared to a standard element, there was hardly any increased pressure drop along the element when using the ASD grades. This indicates less blockage from fouling or biological growth.

Computational fluid dynamic (CFD) simulations had previously indicated that RO elements with ASD feed spacers would indeed be capable of achieving higher performance levels than standard grades. Furthermore, the tendency toward fouling in the element is reduced. “As we calculated, using the new elements reduces energy requirements while at the same time increasing service lives,” says Dr Jens Lipnizki, head of Technical Marketing Membranes in the Liquid Purification Technologies (LPT) business unit at LANXESS.