A review of color and COD removal from textile effluent by coagulation and advanced oxidation processes

The textile sector has a long history, and it makes significant contributions to the national economy. Disposing of colored wastewater poses serious environmental risks. Over the past few decades, the elimination of color from textile industries has proven to be quite challenging because it contains a high percentage of organic and inorganic ingredients, which makes it difficult to handle. The textile industry poses a severe environmental hazard due to its heavy use of water. Therefore, wastewater treatment is an environmental challenge. Several wastewater treatment processes are available. However, Research is still ongoing to find low-cost and high-efficiency treatment methods. Keeping in mind the permissible limitations of the wastewater systems, treatment techniques for water consumption are studied again. Recently, coagulation, flocculation, and advanced oxidation processes (AOPs) have been studied for water treatment in textile factories around the globe. There are several treatment options for the waters where AOPs were used for only tissue pretreatment or in combination with biological, physical, and chemical treatments. The objective of this research is to investigate the main variables influencing process effectiveness, namely COD, color BOD, pH, and temperature. According to research, the combination of treatment technologies helps improve water quality and create an environmentally friendly climate, regardless of the physical and chemical characteristics of the effluent.


Introduction
Because it employs a sizable fraction of the workforce, one of the most profitable industries is the textile business, particularly in emerging nations.However, as the textile industry expands, environmental problems are becoming more serious.Due to the water and chemicals used in each process, the production of textiles generates a large volume of wastewater.In 2017, estimations showed that toxic dyeing effluent produced more than 700,000 tons annually, and approximately 200,000 tons of wastewater are released without adequate treatment [1].According to the World Bank, textile coloring and finishing account for 17% and 20% of all industrial effluent [2].Textile mills fall into two categories depending on the amount of garbage and effluent they produce: wet preparation and dry preparation.The effluent produced by the textile industry includes washing effluent, industrial effluent, non-contact cooling effluent, and vapor water.In dry cleaning stations, most solid organic waste is generated, while in Wet irrigation plants, The majority of liquid trash is produced by textiles, where unprocessed textile wastewater can be identified by its COD, BOD, color, TSS, TDS, heavy element content, and so on [3], as shown in Table 1 [4].Most of the dyes used in dyeing and printing processes are the primary source of coloration in wastewater [5].In addition to the aesthetic issues with colored effluent, dyes also greatly absorb sunlight, which hinders the ability of aquatic plants to photosynthesize and poses a significant threat to the entire environment [6].It is extremely concerning that textile wastewater seriously pollutes the water supply.Because of minimal oxygen consumption and light penetration, the highly colored effluent has a significant negative influence on aquatic ecosystems and aquatic plants' ability to photosynthesize [7].Before releasing wastewater into the environment, it needs to be properly treated [8].Electrocoagulation [9], ozone [10], adsorption [11], flotation [12], reverse osmosis, and chemical oxidation [13] are a few techniques for the elimination of pollutants from wastewater.Physical-chemical and biological treatments include coagulation/flocculation followed by settling [14].However, the research indicates that conventional wastewater treatment has several drawbacks such as sludge production [15], [16] and ineffective in eliminating salts and resistant substances.This method of treatment produces treated water that is contaminated with COD and salt [17].In addition to these negative aspects, the requirements for acceptable wastewater are becoming more stringent [18].The requirements for reusing treated wastewater are no longer met by conventional treatment [19].The shortcomings of traditional processing can be solved by using advanced processing techniques, such as advanced oxidation processes (AOPs), bioremediation, adsorbent, coagulation/flocculation, and hybrid processes.Reviews of the literature have assessed methods for treating textile liquid water, concentrating on sophisticated and chemical techniques [20].However, a thorough evaluation of the methods utilized to eliminate accidental pollutants found in textile effluents still has to be done.Therefore, this study provides a comprehensive literature review of wastewater treatment systems utilized for effluent management, as well as highlighting the unique application needs, problems, and technological advancements of each technique.The study focuses on the basic factors that affect process efficiency, which include COD, color, BOD, pH, and temperature.According to research, the combination of treatment technologies helps to improve water quality and create an environmentally friendly climate regardless of the physical and chemical properties of wastewater.The industrial waste treatment methods reviewed, critical factors, removal efficiencies, and specific study data from the comprehensive review are shown in Table 3.The methods were divided into three categories: process, coagulation/flocculation, advanced oxidation, and hybrid (see Figure 1) [22].

Coagulation/Flocculation (CF)
Because they require little incubation time and are easy to use, CF techniques are among the most effective methods for treating chemical waste.They are widely used to treat textile waste.Aluminum sulfate, also known as alum [Al2(SO4)3.18H2O], and ferric salts such as [FeCl3.7H2O]or [Fe2(SO4)3.7H2O],are the most often utilized coagulants in wastewater treatment processes [23].The coagulant/flocculation/ozone combination outperformed the coagulant/flocculation/absorption combination in terms of color and COD elimination.The biodegradability of the wastewater is enhanced by both techniques (CF/ozone and CF /desorption).However, CF/ozone demonstrated greater BOD / COD rates, enabling subsequent environmental remediation [24].In addition, the Fenton method has been used to enhance the biodegradability of effluents for cleaning technologies [25].The combination of these methods resulted in an excellent COD removal rate (93.7%) and also color (89.5%) [25].Some AOPs are suitable for use as a final therapy after treatment procedures to eliminate any residual persistent contaminants, or within biological treatments to enhance effluent biodegradation.Since larger oxidation doses were usually required when used.Lastly, the fiscal potential of some AOPs can be enhanced together with biological mechanisms and ozone processes, where the color change is 96% [26], as shown in Table 5.

Advanced Oxidation Processes (AOPs)
AOPs use photochemical, chemical, and electrochemical processes that utilize hydroxyl radicals (OH) as the primary redox agent and produce them during the process.AOPs are highly adaptive and promote environmental harmony by eliminating persistent pollutants [26].AOPs can be divided into six classes based on the various methods used to produce oxidation agents: Ultraviolet, Peroxide of Hydrogen, Processes using UV/H2O2, Photo-Fenton, Fenton, and ozonation, as shown in Figure 2 [27].

Figure 2. Categorization of advanced oxidation processes (AOPs) [27]
Zinc oxide-assisted photocatalysis is one of the AOPs methods listed in Table 3 [28], demonstrating the greatest removal effectiveness (97.7%) for COD.Photo-Fenton technology [29] showed high COD elimination (97%) by performing electrochemical oxidation with acid blue 22 on a boron-doped diamond anode (BDD).Because OH was generated on the diamond surface, the bulk electrolysis results showed that the electrochemical method was suitable for completely removing COD and efficiently decolorizing the wastewater.Rising temperatures accelerate the chemical breakdown of electrolyte peroxide sulfate, leading to worldwide declines [30].However, according to the electrolytic decomposition procedure suggested by [31], the least amount of COD was removed from the tissue wastewater (55%), which is achieved at pH 7.7, a voltage of 4 V, and NaCl content of 1 g/L.In this paper, it was found that electrocoagulation using aluminum electrodes is a viable tool for treating mixed industrial wastewater.However, this method was implemented as a pretreatment for biological therapy.
Compared to the bipolar connection, the unipolar connection allows for more affordable treatment [31].After a 30-minute trial, ozone also improved the biodegradability of post-tanning residues by more than 0. .This set of technologies is hopeful for the treatment of effluents due to their high ability to degrade bio-refractory compounds and the absence of sludge generation, except for Fenton-based methods.The works assessed for the design and management of effluent treatment using AOPs.However, are very complicated (related to high expenses).Additionally, intermediary molecules that are more toxic than the initial ones could be released.Therefore, additional research that considers expenses and residual harm is still required to advance scientific understanding in this area.

Hybrid process
Effluents containing unconventional materials have the potential to be treated with hybrid processes.The combination of two or more processes enhances and increases the disposal efficiency of these substances due to their synergistic effects, such as coagulation, Fenton oxidation (Fe 2+ /H2O2), and ozone.For the wastewater being evaluated, the Fenton process appeared to be the most effective treatment technique, mainly because it was more financially feasible than the other available treatments [37].Compared with coagulationflocculation/desorption, the coagulation-flocculation/ozone combination succeeded in removing more COD and pigment.Both processes (CF/ozone and CF/desorption) enhanced the biodegradability of the effluent, but CF/ozone showed higher BOD / COD ratios, making subsequent bioremediation easier [24].Additionally, the Fenton technique has been applied to improve the biological remediation of wastewater's biodegradability [25].The combination of these methods resulted in the effective removal of COD (93.7%) and color (89.5%) [25].AOPs can either be applied during bioremediation to enhance the biodegradability of the effluent or used as a downstream treatment to eliminate any strong contaminants remaining after bioremediation.The economic potential of some AOPs may be considered when combined with a biological process because they usually require high doses of oxidants when used as a definitive treatment.The combined treatment process biological with ozone achieved a decolorization rate of 96% [26], as shown in Table 3.The outcomes show that, even at very high dye concentrations (500 mg/L), Ozone can partially oxidize Remazol Black B dye and entirely decolorize effluent at pH levels ranging from 3 to 11 [44]

Conclusion
It has been proven that the industrial sector is one of the necessities for living and providing manpower, so it must be able to meet the requirements of production and national economic development.However, expansion in this sector has been shown to lead to the creation of large volumes of wastewater rich in organic matter, colors, COD, and suspended particles.Because this can hurt living organisms and the environment, and to ensure the safety of the environment and its ecosystems, basic treatment methods must be implemented.According to the literature, since there is no single approach the advanced coagulation-flocculation-sedimentation and oxidation procedures can be used separately or in combination to obtain optimal wastewater treatment efficiency for the textile sector.As a result, it was noted: coagulants has the same effect as using mineral coagulants, but at a lower cost and with less environmental impact.
2. 2. Many new pre-hydrogenated coagulants, such as (PACl), poly aluminum ferric chloride, and ferrous sulfate, are more effective and have been proposed for the decolorization of textile effluent.
3. In chemical coagulation, iron and alum salts are common inorganic coagulants.These salts are involved in neutralizing the charge.Polymers can help, for example, with coagulation.It is also used to improve mass density.Most researchers believe salts are superior.To varying degrees, iron salt coagulants can remove contaminants.According to the literature study, AOPs are effective in removing organic matter from textile sector wastewater, and reducing toxicity to acceptable and environmentally friendly levels.
5. The formation of HO radicals improves the ozonation process under many conditions (acid solution, addition of H2O2, radiation, and catalyst).According to the literature review, using the ozonation technique to remove organic carbon and colors, turbidity, and TSS from industrial wastewater has highly efficient.
6.The two most commonly used ways for removing COD and BOD from textile factory water, respectively, are AOPs and biological treatment techniques.7. It is possible to improve biodegradability by using AOPs and adsorption before traditional biological therapy.
In conclusion, the CF method and AOPs show huge potential for industrial wastewater treatment, with a future study focusing on wastewater reuse for agricultural and drinking water applications.

Figure 1 .
Figure 1.The benefits and drawbacks of existing dye removal techniques from industrial wastes.

4 .
The work hour is affected by variables such as H2O2, Fe 2+ , and pH.It has been determined that the use of ultraviolet light and Ultrasound improves the effectiveness of the Fenton process.

. Textile effluent treatment methods are being used.
4 [32].This is a useful application of electrochemical oxidation and ozone pretreatment methods.Compared to other AOPs, both methods use little energy.The average amount of energy used by electrochemical decomposition was 70 kWh m 3 COD [31].Moreover, ozone uses less energy than other AOPs (O3/UV, UV, UV/ H2O2, and H2O2/O3)[33].Photolytic electrolysis achieved 100% decolorization of the textile effluent[34].High color removal of effluent water was also achieved using the catalytic treatment technique (88-90%)[35].The percentage of color removal when using ozone was (90%)[32].When compared to other AOPs, catalytic, photo analytical, photocatalytic, electro-oxidative, and photocatalytic therapies have the benefit of elimination because they can be used to simultaneously decrease chromium and oxidize organic matter[36].When compared to the stimulated treatment, the stimulatory treatment method reduces chromium (VI) and color more effectively, similar results were discovered previously[34]

Table 3 .
Application of industrial liquid waste treatment methods.