|
| Physical method |
| Adsorption | Refers to a process where a substance or material is concentrated at a solid surface from its liquid of the gaseous surrounding. |
| Irradiation | Involves the use of radiations usually obtained from monochromatic UV lamps working under 253.7 nm. It is a simple and effective technique for removing a wide variety of organic contaminants and disinfecting harmful microorganisms. |
| Filtration processes | Microfiltration: it is mostly employed for the treatment of dye baths containing pigment dyes. |
| Ultrafiltration: this technique can remove polluting substances such as dyes only 31–76% but can be used to eliminate macromolecules and particles. |
| Nanofiltration: it is employed for the treatment of colored effluents from the textile industry mostly in a combination of adsorption and nanofiltration as NF modules are very sensitive to fouling by colloidal materials and macromolecules. |
| Reverse osmosis | This technique is used to eliminate hydrolyzed reactive dyes, most types of ionic compounds, and chemical auxiliaries in a single step. |
|
| Chemical method |
| Electrochemical | This process is very simple and is based on applying an electric current to wastewater by using sacrificial iron electrodes to produce ferrous hydroxide. |
| Oxidative processes | Oxidation by ozonation is a technology initially used in the 1970s, and it is carried out by ozone generated from oxygen. |
| Chemical oxidation is the conversion of pollutants by chemical oxidizing agents (such as chlorine, ozone, Fenton reagents, UV/peroxide, and UV/ozone). |
| Oxidation with sodium hypochlorite: in this treatment, azo bond cleavage is initiated and accelerated by the attack of the dye molecule by Cl+ at the amino group. |
| The oxidation processes with hydrogen peroxide (H2O2) can be used as wastewater treatment in two systems: (1) homogenous systems based on using visible or ultraviolet light, soluble catalysts such as Fenton reagents which are strong oxidants compared to H2O2 and other chemical activators such as ozone and peroxidase. (2) Heterogenous systems based on using semiconductors, zeolites, and clays with or without ultraviolet light. |
| Photochemical oxidation: the UV treatment of dye-containing wastewater in the presence of H2O2 can break down the dye molecules into smaller organic molecules or even ultimate products such as CO2 and H2O and other inorganic oxides. |
| Coagulation | Coagulation of dyes and other auxiliaries in textile effluents has been successfully done by aluminum, iron slats, organic polymer, flocculants, etc. |
| Electrocoagulation | Electrocoagulation is an advanced electrochemical treatment for dye and color removal. It involves processes such as electrolytic reactions at electrodes, coagulation in the aqueous effluent and adsorption of soluble pollutants on coagulants, and, finally, their removal by sedimentation |
|
| Biological method |
| Bacterial | Aerobic biological treatment: use bacteria and oxygen (from injected air) to remove dissolved organic load (COD/BOD) from dye-containing wastewater. The process is controlled by oxygen sensors in the activated sludge (aeration tank), and residual bacteria (waste activated sludge) can be separated in various ways. |
| Anaerobic biological treatment: anaerobic biodegradation of water-soluble dyes including azo dyes is mainly reported to take place by a redox reaction with hydrogen leading to the formation of methane, carbon dioxide, hydrogen sulfide, and other gaseous compounds and releasing electrons. |
| Fungal | Fungal organisms can decolorize a wide range of dyes. |
| Microalgal | Algae are capable of decolorizing colored wastewater through mechanisms of enzymatic pathways as well as adsorption on algal biomass. |
| Enzymatic | The enzyme that is produced from microbes and plants is used to degrade or decolorize dyes. |
|
