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Dyeing for denim

Denim trousers

With fashion month drawing to its close, we look at the dyeing and treatment of denim, one of the world’s largest textile activities. It is a process which presents challenges with respect to the effluent.

24 February 2020

Despite extensive research on textile wastewaters, very few studies have been carried out on the effluent from the denim production process. Denim plants use a stonewashing method, which is not found in other textile plant processes. Stonewashing distresses the denim creating a worn look. To achieve this pumice stones and denim are spun together, with the spinning time adjusted to create the desired finish. Pumice stone, a volcanic rock with a highly vesicular and perforated structure, gets broken down during the stonewashing process so the wastewater contains a high concentration of stone/inorganic particulates in a wide size distribution. With the particulate matter having a significant abrasion impact on the mechanical equipment, it is critical that appropriate treatment technology is selected.

Further complication arises during denim processing in that a variety of sequences are needed depending on the garment, hence the wastewater characteristics will exhibit variations with time, as the process sequence is adjusted to different types of production.

 A research team, supported by the Scientific Research Fund of Istanbul Technical University, has carried out a study to define a sustainable pre-treatment scheme for achieving full removal of colour and pumice stone and providing an optimum adjustment of wastewater characteristics for biological treatment.

Conducting their investigations at a denim plant located in Ҫorlu, Turkey, researchers completed a comprehensive evaluation of the plant effluent in terms of major quality indicators. These indicators included pH, pumice stone content, chemical oxygen demand, colour, along with total nitrogen and phosphorous. Thirty daily composite samples were collected during the period February to April 2018 and analysed for the selected quality indicators.

The research team found that pumice stone content was a significant factor in the effluent. ‘This significant pollutant, if not effectively eliminated, would totally block the activated sludge process as it would accumulate in the aerated reactor,’ the research team stated. The results did indicate that an integrated physical-chemical treatment, including an initial plain settling phase, could potentially remove the entire range of pumice stone, down to 50-60mgL-1 levels. The researchers also found that chemical treatment using poly-aluminium chloride with strong aluminium oxide content and high basicity achieved almost total colour removal, which is a major problem for all types of textile effluents.

The team concluded that an integrated physical-chemical pre-treatment scheme was an effective and reliable approach, resulting in an effluent free of colour and total suspended solids, as well as a chemical oxygen demand level that could be eliminated through biological treatment.

Journal of Chemical Technology and Biotechnology: DOI:10.1002jctb.6216

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