Physical coagulation, simply known as coagulation, has been widely used in textile wastewater treatment for the removal of color. Often, coagulation can remove color and meet standards set by law or agreement. The technology is simple and there are a variety of products on the market to accomplish this goal. Coagulation methods are useful in reducing the amount of color in textile waste, but they only remove a portion of the color and reduce COD.
There are at least three major problems with coagulation. The first is that the process can be expensive. The user pays for chemicals that are only intended to be disposed of. The solids generated are added to the solid waste load of the textile mill or to the municipal waste treatment facility. Second, dyes with high water solubility resist coagulation and more coagulants may be required to achieve reasonable removal of the dye from the effluent. Naturally, this increases the cost and the solid waste load and also sometimes has a detrimental effect on the oxidation of ammonia to nitrite and nitrate (nitrification) in the biological treatment plant.
Third, the generation of solid waste requires that a suitable means of removing and disposing of coagulated dye waste must be added to the other solid wastes generated by the textile dyeing and finishing operations. Increasingly, pressure is being exerted by environmental regulators to reduce the solid waste load, and any process that adds to the solid waste load creates additional headaches for the textile mill environmental manager. A review article by Hao et al. 5 in 1999 discussed a wide range of dye removal methods, including coagulation. As noted, the coagulation process is often used in conjunction with other processes because of its inability to completely remove some classes of dyes. For example, coagulation may be combined with oxidation using a Fenton reagent or with pretreatment with ozone.
Over the past decade, reported research on coagulation for dye removal has continued uninterrupted.
Gao et al. 6 evaluated a polymer aluminum silicate composite as a coagulant for water treatment. Depending on the method of composite production, better coagulation results were obtained but less stability was observed over longer storage times. Sanghi and Bhattacharya 7 used a combination of powdered activated carbon and bentonite clay, together with polyaluminum chloride (PALC) as a coagulant for the decolorization of dye solutions of direct, mild, and basic dyes. PALC not only enhanced the color removal but also formed a rapidly settling sludge that could be used for color removal.
A comparison of the removal of disperse and reactive dyes by chemical coagulation with Fenton oxidation was made by Kim and Park . 8 Their work showed that disperse dyes are more easily decolorized than reactive dyes due to their lower solubility and lower ratio of dissolved oxygen demand (SCOD) to total chemical oxygen demand (TCOD). Reactive dyes have high solubility and are more resistant to removal from solution. Conversely, they are therefore more available to be degraded by oxidative methods.
investigated the removal of reactive dyes from synthetic wastewater by a combination of Al(III) coagulation and carbon adsorption.
The dyes were CI Reactive Red 45 and CI Reactive Green 8. The combination of treatments resulted in almost total removal of both dyes from the solutions. A secondary advantage was that minimal sludge was formed, which reduced both initial and solid waste disposal costs. Gao and Yue 10 evaluated the coagulation efficiency of a series of polyaluminum chloride silicates (PACS) with different OH/Al and OH/Al/Si ratios. These were prepared by copolymerization and composite methods. The results were compared with polyaluminum chloride (PAC) . PACS prepared by copolymerization gave the best results in coagulation and at lower residual aluminum in the wastewater. Another report by the same group focused on the PACS particle distribution and zeta potential and their effect on coagulation performance. Two other papers by Gao’s group address aluminum species formed by different methods. 11, 12
Lee and Choi13 evaluated a combination of adsorption and coagulation in the removal of two reactive dyes, Orange 16 and Black 5. The adsorbent was coconut-based powdered activated carbon, and aluminum chloride was used as the coagulant. Coagulation followed by adsorption was more effective than the reverse. They concluded that the combined process could reduce the amount of coagulation and adsorption, resulting in less sludge production. Joo et al14 decolorized a high-concentration reactive dye effluent using a combination of a polymeric flocculant and an inorganic coagulant. The four model reactive dyes used were Black 5, Blue 2, Red 2, and Yellow 2. The polymer was synthesized from cyanoguanidine and formaldehyde and applied with alum or iron salt. The inorganic salt alone was not effective, but with the addition of polymer, the combined treatments yielded dye removal efficiencies of up to 60%.
Lee et al. 15 evaluated a submerged hollow fiber microfiltration membrane combined with a combination of coagulation and adsorption by activated carbon in the removal of reactive dyes, Orange 16 and Black 5. Under optimal conditions, they were able to achieve 99% removal for Black 5 and 80% for Black 5. Orange 16. The combined process was found to be significantly superior to the individual processes in the removal of both dyes. They concluded that this process reduced the use of coagulants and adsorbents. Petzoid and Schwarz 16 investigated the removal of dyes by flocculation for two types of sludge and for pure dyes using polyelectrolytes and polyelectrolyte-surfactant complexes. The dyes studied were Acid Yellow 3 and Acid Blue 74.
They concluded that the flocs forming the new particles could be “engineered” to completely remove the dye over a wide concentration range. Saraso et al. 17 followed an ozonation treatment of industrial wastewater with a Ca(OH)2 treatment to remove almost all of the compounds remaining after the application of ozone. However, aniline and chloroanilines were formed after the coagulation step, probably caused by the strong basic environment.
In short, the coagulation method by mineral salts alone will probably continue to be used. However, given the limitations of sludge disposal and the quantities of these materials required to remove highly soluble dyes, it seems inevitable that unique polyelectrolytes or different combinations of coagulation with oxidation, physicochemical techniques will be used. At least in those countries or US states with strict environmental standards, such treatments should be used.