The textile industries of the world release wastes during different stages of production. Water pollution has always been a rising concern, and many new technologies have been introduced to combat the problem of effluent wastes of textile dyeing. The industry is assumed to consume more than 80,000 tons of reactive dyes every year. Stringent laws and increasing ecological awareness is making textile dyeing units, switch to more sustainable options of dyeing and treating waste water to upcycle.
The dyeing of fabrics or fibre uses water to transmit dyes in the form of vapour to heat the treatment baths. Printing being a subset of dyeing also contributes to releasing toxic wastes. Hence textile pre-treatment, printing, dyeing, and finishing processes are responsible for using and discharging all kinds of chemical waste. These processes generate effluents that comprise of high concentration of salts, and demonstrate high biological oxygen demand (BOD) or chemical oxygen demand (COD) values. Water treatment plants and processes must be incorporated by textile industries.
Most of the textile dyeing waste gets discharged into water bodies like lakes and rivers. The waste water disposed reduces the depths of the penetration of sunlight and results in decreasing the process of photosynthesis, and consequently dissolves oxygen. Several techniques are developed to treat water waste and effectively reduce chemicals before releasing them into rivers. The following are a few processes widely used in the textile industry.
Physiochemical effluent treatment, where the first step is to mix and equalize the effluent waste that is released from different stages at different intervals and time. Another primary treatment used is equalization and homogenization, where in waste substances that have similar characteristics in terms of pollution, temperature, and pH are separated. Floatation is a technique that divides fibre from the waste water. This process releases micro-bubbles to form substances in three phases of water, gas, and solids. To remove organic substances coagulation flocculation treatments are used conventionally. This process adequately eliminates insoluble dyes but does not remove soluble dyes.
With the amount and extent to which textile industries churn out effluents in water, more advanced and efficient methods to treat water are required. The following are some developed and proficient procedures adopted to meet desired waste water treatment standards.
Adsorption is the most commonly used method in which soluble organic pollutants and color can be removed. Toxic chemicals like pesticides, phenols, reactive dyes and azo dyes, and cyanides can be done away with using this process. An adsorbent used more often than not is Active carbon. Apart from this kaolin and silicon polymers are also used. By using adsorption, 92.17% of the chroma and 91.15% of the COD can be decreased and be brought to minimum levels and the water can be re-used for washing purposes. The high cost of activated carbon makes the adsorption technique an expensive process.
Membrane filtration is a relatively new approach to treat waste water. It provides the potential applications of incorporating treatment of waste water with in-plant water circuits. Depending on the end outcome, the different courses of membrane process like reverse osmosis, nanofiltration, ultrafiltration, and microfiltration can be chosen. The ability to separate harmful substances from waste water is higher using these processes. Membrane filtration techniques consume less energy, can be operated with ease, and cause no pollution.
Reverse osmosis membranes have a higher retention rate of 90% and induce a high quality of permeate in case of ionic compounds. By this simple process the waste can be decolorized and the chemical auxiliaries can be removed. Higher the amount of dissolved salt greater is the energy required for separating. Mineral salts, hydrolyzed reactive dyes, and chemical auxiliaries can be eliminated with reverse osmosis.
Nanofiltration is used for treating water that is highly concentrated and has complex solutions in the textile industry. Nanofiltration membranes absorb organic compounds with low molecular weight, large monovalent ions, divalent ions, hydrolyzed reactive dyes, and dyeing auxiliaries. This treatment to a large extent can be the possible solution to the problem of accumulation of dissolved solids in streams of water. It is also in favor of meeting environment regulations.
Ultrafiltration helps in getting rid of macro molecules but does not do complete justice while treating dyes. Hence the water after this process can be used for rinsing and washing, where water salinity is not an issue. It can be used as a pretreatment process for reverse osmosis or in combination with biological processes. Microfiltration also works in similar manner as ultrafiltration, and can be only used for pre-treatment or in combination processes.
Ozonation uses one of the strongest oxidizers, Ozone, to purify palatable water. Ozonation breaks detergents, phenols, and complex organic molecules into simpler compounds. Besides this, it is also used to deodorize and decolorize the waste water from industries.
Multiple effective evaporation is a method in which the waste water is evaporated and the concentrated liquid is then sent to the next level of evaporation, so on and so forth, until the liquid is circulated again and again, in order to meet a required concentration of salt or desired density. The liquid is evaporated at a low temperature because of the vacuum created by condensers.
Crystallization is another new technique to dispose chemicals and purify the water. The liquid is first supersaturated in order to crystallize, wherein the liquid contains more dissolved solids than it normally accommodates at that temperature. The process can extract chemicals like sodium sulphate, sodium chloride, calcium chloride, and calcium sulphate from effluents.
Using such unique techniques and technologies the textile industry can improve poor recycling practices, substantially reduce the water pollution, and meet ecological standards set by governments. With increasing demand for textiles manufactured in an environment friendly way, the textile industries must adopt these treatments to reduce effluents and to cut back on wastages.