Srabani Mallick

REUSE OF WASTEWATER in textile dyeing process

The amount of water retention and use in the textile industry is huge. It is a water-intensive industry.  Water is used in various steps in this industry from the cleaning of raw materials to the textile dyeing process.  As a result, a large amount of wastewater generated poses various challenges.  Many of the world’s producing countries are facing various problems due to the impact of water pollution, freshwater scarcity and strict environmental regulations.  Hence more attention is being paid to large-scale wastewater treatment, recycling and sustainable water management.

What do you mean by textile wastewater?

The textile industry, one of the water-consuming industrial sectors, releases large amounts of wastewater. Textile wastewater (TWW) is characterized by dark-brown strong color and a high amount of chemicals whose composition mainly depends on the dyeing process, type of fabric and auxiliaries used during dyeing. Currently, more than 100,000 dyes are used in various industrial processes. Dyes are classified according to functionality. Such effluents have high pH values, manganese, sodium, chloride, nitrate, lead, copper, chromium, iron, and high BOD and COD values ​​resulting in severe degradation of aquatic ecosystems.

Does textile wastewater need to be treated?

soil/groundwater pollution by textile

According to UNICEF and the World Health Organization, one in three people worldwide does not have access to safe drinking water, and the United Nations reports that nearly one and a half million people die each year from diseases caused by contaminated drinking water. Textile dyeing is the second largest polluter of water worldwide. The fashion industry produces 20 per cent of global wastewater which affects the ecosystems, animals, human food chains and soil/groundwater pollution. Water pollution is a major problem in our textile industry. Currently, many industries in Bangladesh are working to reduce water pollution. Water consumption is a major part of the textile industry; it is used for scrolling, bleaching and dyeing processes. The pollution aspect mainly comes from wastewater. If this contaminated water is not treated before it is released into water bodies, this wastewater can reduce the oxygen concentration, which can be harmful to both aquatic life and aquatic ecosystems in general. Textile dyeing is the second largest polluter of water worldwide. We can give the example of cotton as this aspect. It takes about 20,000 litres of water to produce a cotton t-shirt and jeans. Conventional cotton production also involves the high use of fertilizers and pesticides, which can pollute surrounding water bodies. Obviously, the textile industry’s impact on the Earth’s water can be dire, but the good news is that there are ways to reduce its toll on the environment. In fact, some big businesses are working to reduce their harmful effects on the environment.

Textile wastewater treatment

wastewater treatment Plant

Primary and Secondary treatment: The first step involved in wastewater treatment is the primary treatment.  Screening and sedimentation of solid wastes are performed to remove major contaminants present in the wastewater.  Total suspended solids (TSS) can be reduced by primary treatment, but this alone cannot meet the requirements of wastewater standards for other parameters.  Hence the secondary treatment is required.  When the waste flows from primary treatment to secondary treatment it is further purified by oxidation. The biological treatment method for wastewater treatment in the textile industry is the activated sludge system. This is a process with a high concentration of microorganisms like bacteria, protozoa, fungi etc. present as loose aggregates of fine particles which were determined to be an effective process for the removal of organic chemicals, ammonia, nitrogen etc. from wastewater. In this system, sewage from primary treatment is mixed with air and bacteria-loaded sludge for several hours. The bacteria then slowly break down the organic matter into harmful substances. About 85% of the organic matter is removed in the second stage through the use of biological processes.

Tertiary treatment plant

Tertiary treatment: Tertiary treatment may be applied after secondary treatment to remove non-biodegradable residual dyes and chemical compounds. Generally, activated carbon and ozonation are applied in tertiary treatment for adsorption. Activated carbon is used to purify liquids and gases in a variety of applications including taste and odour control, organic chemical removal, industrial pollution control etc. On the other hand, Ozonation is a highly effective advanced oxidation process (AOP) that uses ozone to instantly neutralize microorganisms such as viruses and bacteria. Ozone is a better disinfectant than chlorine that not only disinfects water; It also oxidizes metals like iron and manganese.

Advanced techniques for treatment

Treated wastewater from primary treatment processes, especially in the dyeing industry, may not achieve the same or very similar quality to fresh water. Therefore, in addition to primary, secondary and tertiary treatment, advanced treatment techniques should be used to recycle wastewater in the textile industry. Advanced treatment techniques consisting of membrane systems have been used in most industries for the past few decades. On the basis of pore size, pressure-driven membranes can be four types –

Microfiltration (100 nm – 10 µm)

Microfiltration is a membrane filtration method used as a pre-filter for ultrafiltration. Here a pressure difference occurs between the liquid containing all the harmful substances, particles and bacteria and the filtered liquid. The contaminated liquid is forced through the membrane with the required pressure. As a result, all dissolved substances, particles and bacteria can be removed from the fluid through the membrane pores.

Ultrafiltration (2 – 100 nm)

Ultrafiltration removes unresolved viruses from a liquid using a pressure difference. With the help of pores in this filter and necessary pressure, the contaminated liquid is forced into the membrane which reduces the bacteria and viruses.

Nanofiltration (1 – 2 nm)

Very energy-intensive Nanofiltration is a filtration method that can remove not only undissolved substances but also dissolved substances such as chalk and PFAS. In this method, micropollutants and polyvalent ions are removed from the liquid using a membrane with the required pressure.

Reverse Osmosis (0.1 – 1 nm)

Reverse osmosis is similar to distillation through which about 90% of COD can be removed. The pore size of the reverse osmosis membrane is very small, so separation based on diffusion occurs. At the required pressure, both micro-impurities, single ions (dissolved salts) and polyvalent ions (dissolved substances) are removed from the liquid. Only gases and low-molecular substances can (partially) pass through the reverse osmosis membrane filter.

What are the benefits of wastewater reuse in the textile industry?

 Textile industries have a significant impact on the human life cycle.  The use of large quantities of water and chemicals has made the global textile industry one of the most polluting and waste-producing industries in the world.  Water shortages or strict environmental laws are increasingly making wastewater recycling more imperative due to the large water demand in this industry.  Below are some important reasons for this:

wastewater treatment plants

1) Various chemicals are used in the textile industry along with water in various processes.  So these harmful chemicals are mixed in the wastewater and affect the surrounding areas which is a great threat to the environment. So reusing the wastewater through proper wastewater treatment plants will reduce environmental problems.

 2) Effectiveness of wastewater reuse also affects improved performance. Eco-friendly processes of wastewater treatment help in increasing company performance.

 3) Reuse of wastewater can be economically beneficial to the industry.  Not only the textile industry, but many other industries are also reaping economic benefits by treating wastewater and converting it into a significant resource.

4) Produced water after wastewater treatment is always important for the textile industry and other industries where a large quantity of water is required.  Treated water can be reused for manufacturing processes that meet water demand and save money.

 5) Textile industry is dependent on freshwater for various processes of functioning.  Wastewater reuse can significantly reduce dependence on freshwater.  Water treatment becomes very important for textile industries, especially in drought-affected regions, where there is a massive demand for fresh water.

 6) Reusing wastewater is obvious to improve the quality of the end product.  Because it is absolutely free of dangerous reagents.  Wastewater reuse is a mechanized process that requires minimum manpower to implement so no additional manpower is required.

Challenges of Wastewater Reusing…….

The challenges of textile wastewater recycling are mainly related to technology, skilled manpower and economic conditions. Membrane systems are prone to fouling and require skilled manpower for regular maintenance. Although these membrane designs and technological advances are economically beneficial the capital cost is high for medium and small-scale industries. Also, these advanced systems require large amounts of energy resulting in high electricity consumption and operational costs.