Dye Removal Techniques

Environmental contamination resulted from the emission of effluents of dyeing industries is a global problem, therefore different methods of effluents treatment have been used in an attempt to minimize the problems resulted from this contamination. Coagulation, foam flotation, precipitation, ozonation, ion exchange, filtration, solvent extraction, electrolysis, chemical oxidation, membrane separation technology, liquid-liquid extraction and adsorption on activated carbon have been used for the removal of dye contaminants from wastewater [32, 52]. The technologies can be divided into three broad categories: physical, chemical and biological [53]. The advantages and disadvantages for dyes and colour removal of industrial wastewater applied over the time into different industrial units are summarized in Table 3.

Agricultural Solid Wastes in Aqueous Phase Dye Adsorption: A Review

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Table 3. Advantages and disadvantages of dyes removal techniques updated and

modified from [2]

Separation Techniques	Advantages	Limitations
Physical-Chemical processes
Precipitation, Coagulation- flocculation	Short detention time and low capital costs. Relatively good removal efficiencies.	Sometimes expensive, Agglomerates separation and treatment. Specific to operating condition.
Electrokinetic coagulation	Economically feasible	High sludge production.
Solid-Liquid physical adsorption such as:
Activated carbon	Economically attractive. Good removal efficiency of wide variety of dyes.	Very expensive; cost intensive regeneration process
Peat	Effective adsorbent due to cellular structure. No activation required.	Surface area is lower than activated carbon.
Industrial solid wastes such as ash, red mud etc	Economically attractive. Good removal efficiency.	Slow kinetics. Specific surface area for adsorption are lower than activated carbon.
Raw wood chips/ Wood sawdust Other agricultural solid wastes such as pine cone, pine leaves, eucalyptus bark etc.	Effective adsorbent due to cellular structure. Economically attractive. Good adsorption capacity for acid dyes. Sustainable cost-effective good adsorbents. Moderate to high removal efficiency	Slow kinetics and activation may be required Sometime slow kinetics and activation may be required
Silica gels Ion Exchange	Effective for basic dyes Effective with ionic compounds removal and easy regeneration	Side reactions prevent commercial application Specific and effective for dyes removal and softening of water
Biological separation processes
Aerobic process	Partial or complete decolonization for all classes of dyes.	Not cost-effective
Anaerobic process	Resistant to wide variety of complex coloured compounds. Bio gas produced is used for stream generation.	Longer acclimatization Phase, odour
Microbial processes	Good removal efficiency for low volumes and concentrations. Very effective for specific colour removal.	Culture maintenance is cost intensive. Cannot cope up with large volumes of wastewater.

Table 3. (Continued)

Separation Techniques	Advantages	Limitations
Advanced separation processes
Electrochemical oxidation	No additional chemicals required and the end products are non-dangerous/hazardous.	High cost
Other advanced oxidation process	Complete mineralization ensured. Growing number of commercial applications. Effective pretreatment methodology in integrated systems and enhances biodegradability.	Cost intensive process
Fenton process	High efficiency for colour removal	Generation of sludge and expensive
Membrane filtration	Removes all dye types; recovery and reuse of chemicals and water.	High running cost. Concentrated sludge production. Dissolved solids are not separated in this process.
Photo catalysis	Process carried out at ambient conditions. Inputs are no toxic and inexpensive. Complete mineralization with shorter detention times. No sludge production	Effective for small amount of coloured compounds. Expensive process.
Photochemical process Irradiation	Effective oxidation at lab scale	Formation of by-products Requires a lot of dissolved oxygen (O₂).
Sonication	Simplicity in use. Very effective in integrated systems.	Relatively new method and awaiting full scale application.
Redox mediators	Easily available and enhances the process by increasing. electron transfer efficiency	Concentration of redox mediator may give antagonistic effect. Also depends on biological activity of the system.
Engineered wetland systems	Cost effective technology and can be operated with huge volumes of wastewater	High initial installation cost. Requires expertise and managing during monsoon becomes difficult.