The use of dyes dates back to the advent of the neolithic age for artistic works and colouring fabric. Over time, the demand for dyes has increased due to their ability to improve aesthetics or commercial value of products. Dyes are now used in textile, paint, cosmetics, paper, food industries and printing technologies. They adhere to compatible surfaces by forming various bonds or complexes with salts or metals, by physical adsorption or by mechanical retention. Most of the naturally occurring dyes originate from plant sources like leaves, berries, barks, fungi, flowers etc. whereas the synthetic dyes which has the most application is derived from petrochemicals. Ever since the advent of synthetic dye technology, synthetic dyes have overtaken the use of natural dyes due to availability of broad range of colour, stronger bonding and colour stability ability at a lesser cost.
The dyestuff sector is one of the core chemical industries in India. It is also the second highest export segment in chemical industry. The Indian dyestuff industry is made up of about 1,000 small scale units and 50 large organized units, who produce around 1,30,000 tonnes of dyestuff. Maharashtra and Gujarat account for 90% of dyestuff production in India due to the availability of raw materials and dominance of textile industry in these regions. The major users of dyes in India are textiles, paper, plastics, printing ink and foodstuffs. At present, India contributes about 6% of the share in the global market with a CAGR of more than 15% in the last decade. The dye markets are mostly dominated by reactive and disperse dyes.
Dyes and dye intermediates consist of basic dyes; azo acid and direct dyes; disperse dyes; fast color bases; reactive dyes; sulfur dyes; vat dyes; organic pigments; naphthols; and optical brighteners. Market demand for dye and dye intermediates is expected to grow at a Compounded Annual Growth Rate (CAGR) of 4.7%, from 652,000 tonnes in 2004-05 to 900,000 tonnes in 2010-11 and estimated to reach astonishing volume of 5722000 increasing at CAGR of 9.11% by 2020- 2024 . The organized sector dominates, with 65% share of the total market, while the unorganized sector controls the remaining 35% of the market. However, owing to stringent environmental regulations and awareness among customers, the cost of operations for small, unorganized players is likely to increase, thereby shrinking their share in the industry. The demand for dyes and dye intermediates is expected to grow at around 6% during 2019-20, backed by strong demand from the textiles, leather, and inks industries, which are expected to register a growth rate of 6%, 4%, and 11%, respectively. Exports of dyes are also expected to increase by 6.4% due to the shift of production bases from developed countries to India on account of stringent pollution control measures being adopted in those countries.
It is evident that the dye industry commands a large portion in chemical industries and has the one of the highest water usage volumes in the industry. Without adequate water supplies it is impossible to run a dye and intermediate industry. Now with such tons of dyes being produced every year, the water used pre and post production needs to be treated. As the govt. norms get stricter and stricter with every passing day, waste water treatment has become a priority for the dye industry today. Let us first explore some major contributors to the pollution load in dyes.
Based on their chemical properties, colour properties, and applications dyes can be characterized into acidic dyes, basic dyes, azoic dyes, nitro dyes, direct dyes, reactive dyes and Sulphur dyes. The fuctional groups in dye that impart colour are known as auxochromes and the groups that intensify this imparted colour are known as chromophores. A point to be noted is that more often than not, the chemical structure of these dyes decides their tenacity and their biodegradability as well. More tenacious dyes have low biodegradability rates and are therefore harder to treat when released in waste waters.
The dye imparts colour to anything it mixes with, including the surface or ground water that comes in contact with the wastewater effluents. This disturbs the potability and aesthetic value of water and blocks the penetration of light through the water affecting photosynthesis in water bodies. Apart from this visible damage the dye effluents also has several compounds that are toxic, mutagenic, carcinogenic and enzyme inactivators. Oral ingestion or inhalation can lead to acute toxicity triggering irritation of skin and eyes. Workers having excessive exposure to the dyes can experience dermatitis, allergic conjunctivitis, occupational asthma or other allergic reactions. The genotoxic effect of dyes can lead to chromosomal disturbance. Dyes like azure can affect human behaviour due to its impact on the central nervous system. Since mutagenesis is trigger for cancer, the azo and nitro dyes can cause cancer over period of time. A plethora of dyes have similarly been found to have severe mutagenic and carcinogenic effects in human and animal models.
One can imagine the level of damage such compounds could induce once they enter our ecosystem. Hence it becomes necessary to get rid of such compounds before they are discharged into water bodies. The textile industry should comply to various standards set for the discharge of effluent such as COD <250 ppm, BOD<30 ppm, colour <150 PCU, phenolic compounds<1 ppm NH3-N < 50 ppm and TDS <2100 ppm and in order to meet these standards industry has to deploy various treatment methods of outgoing effluent.
Synthetic dyes used in industries are recalcitrant in nature. The reason for such recalcitrant nature of compounds is due to the chemical structures of these compounds. In the process of dye production, raw materials like are benzene, naphthalene, anthraquinone are most commonly used in addition to other compounds which chelates with minerals or salts to generate waste water containing salts, acids, alkali, halogen, hydrocarbons, nitro, amines, dyes and other substances. Most of the dye contains aromatic rings with one or more -N=N- groups which makes it resistant to degradation via ozone, light, and biological activity.
Out of all the dyes produced and utilized globally, the dye with azo compounds accounts for more than 60% of annual dye production due to its wide variety of application. These dye compounds are used in textile, cosmetics, food, and printing industry, and amongst them textile industry is the the largest consumer. The major issue with the dye is not the quantum of its production but rather the process of its application. During the dyeing process, not all the dye bind to the fabric and the unbounded dye is lost in waste water. Almost 2% of basic dye to 50% of the reactive dye is lost in waste water leading to high level of surface and ground water contamination. The presence of very small amount of azo dyes in water (<1ppm) are highly visible affecting transparency and water- gas solubility of water.
A lot of methods have been devised to treat dye containing waste water which includes physicochemical process and biological processes followed in effluent treatment plants (ETPs) as well as other sophisticated process including RO, distillation , filtration. A highly optimized of specific Primary, Secondary, Tertiary treatment strategies can ensure maximum removal of water from wastewaters.
A variety of physicochemical processes have been deployed to treat the colour of dyes from effluent water. Flocculation and coagulation techniques have been extensively used. Coagulants and flocculants like alum, ferrous sulphate, polyaluminium chloride, polyamines etc. at specific dosages help in dye reduction during primary effluent treatment. These polyelectrolytes neutralize the negatively charged dye material to form particle-polymer-particle complexes which precipitates in the form of chemical sludge containing good amount of chemical residue which again requires safe disposal. Adsorption is another effective method to remove dye from industrial waste water. Commercial activated carbon is well known absorbent. Various factors such pH, contact time between adsorbate (dye) and absorbent (carbon) affect the removal of dye from effluent. Advanced Oxidation Process (AOP) is another method used for removal of dye from waste water. In this process strong oxidizing agents in combination with an irradiation source like ultraviolet rays or ozone (O3) generate hydroxyl radical to destroy hazardous and refractory chemicals in waste water. But this process is cost and energy intensive.
Several microorganisms such as fungi, bacteria, algae are known to decolorize the azo dyes under certain environmental conditions. The microorganisms by several aerobic and anaerobic pathways can decolorize azo dyes and can degrade the aromatic amines. Microorganisms secrete several enzymes like laccases, azoreductase and different peroxidases. These ezymes either transforms the dye structure or mineralizes the dyes. Microbes such as Polyporus sanguineus, Irpex flavus, Coriolus versicolor, Phanarochaete chrysosporium known for their capability to adsorb and degrade dyes, but on the retention time required for the treatment is very high. Anaerobic decolorization involves an oxidation-reduction reaction with hydrogen which allows azo and other water-soluble dyes to be decolorized. The anaerobic degradation process includes a decolorization stage as well. Studies have revealed that certain steps in anaerobic pathway viz. acidogenesis and methanogenesis are processes that contribute in dye degradation. Gamma proteobacteria and sulfate reducing bacteria during acidogenesis and Methanosaeta species, Methanomethylovorans hollandica during methanogenesis are known to be dominant species for dye removal. The challenge remains to equip treatment plants with organisms that can survive, thrive and treat effluents at the rate required.
Bacteria are not directly able to use dye compounds as their food source hence addition of carbon sources like glucose, starch, acetate or other sugar sources is essential for microbial activity. Out of the different biological treatment reactors that have been assessed for efficacy of dye degradation and treatment, the anaerobic filter and the UASB reactor have shown promising results with good colour removal efficiency. However, one must note that post anaerobic treatment, azo dyes are converted to colourless but potentially hazardous aromatic amines. The second phase for treatment aromatic amines requires to be carried out under aerobic conditions.
A sequential UASB tank followed by aerobic sludge blanket has proven to result in a significant dye reduction. In some cases of effluent treatment for dye industry, anaerobic treatment followed by aerobic treatment seems more efficient approach for dye degradation. Around 70 % of dye removal can be achieved by UASB or baffled reactor while rest of COD can be further oxidised under aerobic conditions. Aerobic treatment includes bacterial and fungal degradation of dye. Many bacterial strains like bacillus species of B.megaterium, B.licheniformis, B.subtilis, and some gram negative species like Pseudomonas luteola, Aeromonas hydrophilia were isolated with good azo degrading and decolorization activity. Fungal species like white rot fungus are also known for their decolorizing acitivity. They produce enzyme that are capable of dye degradation.
When compared to physicochemical methods, biological degradation method is more environment friendly and cost effective. With decades of first hand industry experience and industry expertise, Organica Biotech has helped boost biological treatment for many dyestuff industries. One such pertinent case study is as follows:
A dye industry located in Vapi Gujarat, had a simple functioning ETP setup of Activated Sludge Process (ASP). The treatment of wastewater was performed by operating only primary and tertiary methods whereas biological process was completely skipped by the Environment Health and Safety department. The biggest challenge to treat the effluent at hand was the decolorization of azo dyes and COD control. After initial effluent assessment, studies were conducted on the effluent and an assessment on the toxicity of the effluent and its treatability was made. After a thorough analysis at lab scale, a robust aerobic secondary treatment using our flagship product Cleanmaxx was strategized for effluent treatment considering the reactor parameters.
Cleanmaxx® is one of a technologically backed effective biological aerobic wastewater treatment solution. It is a specialised heterogenous concentrated consortium of uniquely functional bacteria with a high proliferative capacity and tenacity to withstand hostile effluent waters. The aerobic wastewater treatment solution – Cleanmaxx® is capable of rapid biomass development and can withstand fluctuations in wastewater quality. Cleanmaxx® accelerates COD/BOD reduction through bacteria mediated organic load degradation. This rapidly reduces the time required for effective aerobic wastewater treatment, as well as reduces energy spent in aeration & agitation thereby cutting CAPEX/OPEX costs. The uniqueness of Cleanmaxx® lies in the flexibility of the bacterial consortium to sense, adapt & effective aerobic wastewater treatment of any industry origin creating a unique microbial fingerprint acclimatised to the wastewater. Cleanmaxx® proves that aerobic wastewater treatment gives better results and can be used in a wide spectrum of industries for all suspended and attached growth processes including Activated Sludge Process (ASP), Sequence Bed Reactor (SBR), Lagoon process, Moving Bed Bioreactor (MBBR), Membrane Bioreactor (MBR), Rotating Biological Contractor (RBC) and Fluidised Air Bed Reactor.
Cleanmaxx was added in the aeration tank which had a retention time (RT) of 2 days. A dosing structure for 60 days was created with daily dosing and each and every parameter was closely monitored. Positive results were observed within first week of addition only and the decolorization improved up to 45% in first week which improved significantly 94% after two months of treatment. Similarly the COD was reduced by 83% from an initial COD of 7500 ppm. The aerobic treatment of azo dye provides us a low cost process and non-toxic by-products due to mineralization of dyes
Organica Biotech has extensive experience in treating effluents from a varied spectrum of dye industries. The usage and production of new dyes will continue to pose challenge to treat the dye effluents. With continuous research, bioremediation products and processes can be updated to face such challenges efficiently.
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