The proper treatment of heavily contaminated textile effluent is a complicated process. Although the technologies and methods have been developed in the last few decades, there are still many challenges to recycle the treated wastewater. As water scarcity and shortage become more pressing problems, wastewater recycling and sustainable water management is gaining more attention.
The annual water consumption of the textile industries (including cotton farming) is nearly 93 billion m3 which is almost 4% of the global freshwater withdrawal. A large portion of this freshwater turns into industrial effluents which contain a wide range of dyestuffs and chemicals. As a result, many of the production countries around the world are facing major challenges with water pollution and freshwater scarcity.
Local legislations obligate textile industries to treat their wastewater before discharging it into the environment. But in most countries, the local discharge standards are focused more on the conventional parameters, for example, Biochemical Oxygen Demand (BOD), Chemical Oxygen Demand (COD), and Total Suspended Solids (TSS) [Figure 1]. Unfortunately, treatment of the textile effluents that considers only these conventional parameters does not always ensure the water quality is good enough to be reused or recycled. The textile effluent with a high pollution load must go through advanced treatment before it could be recycled again into the process.
Fig 1: National discharge standards of 100 countries (WHO, 2017b)
Textile Wastewater Treatment
The present-day methods for textile wastewater treatment have been developed gradually over the years. The well-known techniques, for example sedimentation, equalisation, and flocculation, have been applied for more than 100 years in Europe. Although these techniques are now used around the world, the installation and operational costs can vary in different regions.
The pollution load of the textile effluent depends upon the type of wet processing units and the chemicals used in the process. Typically discharge from the dyeing industries need more extensive treatment compared to the washing or printing industries. The most commonly used sequence of treatment techniques could be seen in Figure 2.
Fig 2: Textile effluent treatment processes (Sustainable Fibres and Techniques, 2017)
The first step involved in a traditional wastewater treatment plant is the primary treatment. By screening and sedimentation of solid waste, larger contaminants within the wastewater are removed. The primary treatment can reduce the Total Suspended Solids (TSS), but it alone cannot meet the requirements of the wastewater standards for the other parameters. From the primary treatment, the effluent flows to the secondary treatment where further purification is conducted through oxidation. The most commonly used biological treatment method in the textile industry is the activated sludge system. In this system, sewage from the primary treatment is mixed with air and sludge loaded with bacteria. The mixture is kept for several hours and the bacteria slowly break down the organic matter into harmless substances. By using a biological process, about 85% of the organic matter is removed in the secondary stage.
Tertiary treatment may be applied after the secondary treatment to remove the remaining organic matter. Non-biodegradable residual dyestuffs and chemical compounds are adsorbed in this step. Typically, activated carbon and ozonation are applied in tertiary treatment for adsorption. Many industries reuse a small portion of the treated water from the tertiary stage in gardening, toilet flushing, agricultural fields, or for some washing purposes.
Advanced Techniques to Recycle Wastewater
The treated water from the effluent treatment plant could be recycled and used again in the textile process only if it has the same or very similar quality as the freshwater. But in most cases, especially in the dyeing industries, the treated wastewater from the basic treatment methods cannot achieve that quality. Therefore, a textile industry which plans for a higher percentage of wastewater recycling in the process needs to install advanced treatment techniques in addition to the primary, secondary, and tertiary treatment.
The advanced treatment techniques mostly consist of membrane systems, which have been in use in industries for water reuse applications for the last few decades. The pressure-driven membrane filtration process can remove both dissolved and suspended solids. Based on the pore size, the pressure-driven membranes could be classified into four types – Microfiltration (100 nm – 10 µm), Ultrafiltration (2 – 100 nm), Nanofiltration (1 – 2 nm), and Reverse Osmosis (0.1 – 1 nm). Since the pore size is relatively larger, Microfiltration (MF) and Ultrafiltration (UF) are for low-pressure applications. On the other hand, for Nanofiltration (NF) and Reverse Osmosis (RO) much higher pressure is needed to force the solute to pass through the membrane. Therefore, they are also very energy-intensive methods. With the Reverse Osmosis process, nearly 90% of the COD could be removed. The filtration ability of these membranes could be seen in Figure 3. These filtration techniques could be also used to recover salt from the dyeing effluent and reuse them again in the process.
Fig 3: Membrane Technology Comparison (Koch Membrane Systems)
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Challenges of Wastewater Recycling
The major challenges of textile wastewater recycling are mostly related to the technologies and their economic feasibility. Although the technological advances in membrane design and application have reduced the cost significantly, the capital cost is still considerably high for small and medium scale industries. If freshwater in that production region is very expensive, only then the advanced treatments are worth the investment. Moreover, the high-pressure-driven membrane systems require a large amount of energy. For countries that have high electricity costs, the operational cost of effluent treatment plants would also be higher. As a result, per m3 of treated recycled water would be more expensive than per m3 of freshwater, which is simply not financially viable. Membrane systems are also prone to fouling and need regular maintenance. It is also challenging in many regions to have the skilled manpower to operate these advanced technologies.
Is Wastewater Recycling Going to be the Obvious Choice?
Freshwater is a finite resource and it represents only 2.5% of the total available water on Earth. Many regions of the world including a few textile production countries, for example, India, China, and Pakistan are facing major challenges with water scarcity. Since the textile industry is a water-intensive industry, these water-stressed production countries need to find appropriate solutions to ensure their long-term water sustainability. Therefore, minimising the freshwater requirement and recycling the treated wastewater would be the appropriate choice.
In recent years, few international brands and retailers are advocating for water recycling and sustainable water management in the supply chains. For example, Swedish fashion giant H&M has set up a goal of 15% recycled water to be used in production in its Water Roadmap 2018-2022. German sportswear manufacturer PUMA obligates its core suppliers to perform wastewater testing according to a stringent wastewater guideline and also promotes water reusing and recycling in its environmental policy. Moreover, strict environmental regulations require textile industries in a number of regions to plan for Zero Liquid Discharge (ZLD). From the linear approach of wastewater treatment and discharge, the industry is slowly shifting toward a circular approach of water reuse and recycling. As a result, the demand for innovative and cost-effective advanced water treatment solutions is increasing. With the growth of the global population, the availability of freshwater will be more challenging. The higher cost of freshwater will result in a higher production cost. This might force the textile industry and the policymakers to reassess the current approach to water management and develop a circular water economy.