**2. Environmental impacts of untreated textile wastewater effluents**

The textile and clothing industry is considered a vital industry, with a global market of over U\$450 billion, in terms of nominal sales, contributing with 7% of the total world exports and employing around 35 million workers around the world. Additionally, it is one of the most polluting industrial sectors [8, 9]. The major activities from textile industry that may cause severe environmental impacts are associated with—(1) energy consumption throughout the production of man-made fibers, in yarn manufacturing, in finishing processes, and in washing and drying clothes; (2) solid waste generated from textile and clothing manufacturing and, mostly, from the disposal of products at the end of their cycle life; (3) direct carbon dioxide (CO2) emissions, particularly related to transportation processes within globally-dispersed supply chains; and (4) water large volumes consumption and chemical products requirement associated with fiber growth, wet pretreatment, dyeing and finishing activities, and laundry [9, 10].

The United Nations Framework Convention on climate change indicated that textile and fashion industries produced about 20% of global wastewater in 2018 [11]. Textile mills are also responsible for being a 20% contributor to the world's industrial water pollution, using thousands of toxic chemicals during production, some of them cataloged as carcinogenic compounds [12]. A broad type of chemical product is used in the textile industry including inorganic compounds, polymers, and organic products with very complex compositions. The effluents from textile processes are characterized by alkaline reaction, significant salinity, intensive color, and toxicity since they contain dyes, heavy metals (like chromium, cobalt, and copper), pentachlorophenol, chlorine bleaching, halogen carriers, carcinogenic amines, free formaldehyde, biocides, salts, surfactants, disinfectants, solvents, and softeners [13, 14]. **Table 1** shows the average values of main wastewater parameters according to different types of textile processing, variability in pollutant concentrations, and operating conditions present in the discharged effluent.

From **Table 1**, process type (raw wool scouring) may reach high values of chemical oxygen demand (COD) and biochemical oxygen demand (BOD), which are difficult to degrade by using conventional biological treatments. By contrast, type 3 (woven fabric finishing) with the highest BOD/COD ratio suggest that biological-based treatment can be a suitable option. On the other hand, process type 7 (stock and yarn dyeing and finishing) is of major concern since selected dyes are mainly water-soluble and nonbiodegradable with an important content of recalcitrant compounds. In this context, according to the processing type carried out will be the complexity of pollutant loads generated, and thus, the type of wastewater treatment required by textile effluent discharged.

More than 100,000 commercial dyes are currently available worldwide, with over 1 million tons of dye-stuff produced annually [16]. However, 10% of produced dyes are released to the environment in the form of toxic effluents [13, 17]. Many dyes are difficult to decolorize due to their complex structure and synthetic origin. Brightly colored, water-soluble reactive azo and acid dyes are the main concerns. Indeed, they normally pass through conventional biological treatments without suffering metabolic degradation [18], textile dyes may cause allergic reactions, carcinogenicity, mutagenicity, and cytotoxicity effects on various plants, rats, fishes, mollusks, microbes, and mammalian cells [19]. Even, low concentrations of dyes in effluents are highly visible and undesirable.

The environmental effects caused by textile pollutants include the detriment of the visual aspect of superficial water bodies, which in turn interferes with aquatic biological processes, prevents penetration of light and, causes eutrophication in


*Categories description—(1) raw wool scouring; (2) yarn and fabric manufacturing; (3) wool finishing; (4) woven fabric finishing; (5) knitted fabric finishing; (6) carpet finishing; (7) stock and yarn dyeing and finishing. TSS, total suspended solids. Adapted from [15].*

### **Table 1.**

*Effluent characteristics from different production processes.*

water bodies [20]. It is known that effluent with a high concentration of COD (from 800 to 30,000 mg/L) indicates the presence of recalcitrant toxic compounds that can lead to depletion of dissolved oxygen in the receiving water bodies [21]. The strong development of synthetic dyes has caused several detrimental effects on the environment and human health as an improper discharge of strongly colored effluents and the associated metabolites in aqueous ecosystems to reduce sunlight penetration, causing inhibitory effects on photosynthesis, and the presence of aromatic amines generated when dyes are broken anaerobically, which are toxic, carcinogenic and mutagenic [22, 23].

On the other hand, textile wastewaters also contain many persistent organic pollutants, such as phenols, aromatic amines, dioxazine, anthraquinone, consider toxic chemicals that can be transported by wind and water. Moreover, these pollutants may persist for long periods in the environment and can be accumulated, passing from one species to the next through the food chain [15]. **Figure 1** shows the sources, transport pathways, and fates of persistent organic pollutants of textile wastewater. The impacts on human health and the environment are also shown.

From **Figure 1**, a portion volume of textile wastewater is usually mixed with municipal wastewater, to be treated in wastewater treatment plants through conventional biological processes, while the remaining volume is directly discharged into rivers. Also, another portion volume of untreated hospital and agricultural wastewater is discharged to the rivers contributing to severe contamination to receiving water bodies. The discharge of a wide pollutants variability from all these sources generates wastewater very complex which is transported and bioaccumulated by rivers, which severely impacts to marine biota presents in the superficial waters, to the exposed population and reached environment. It is estimated that 10% of textile chemicals are potentially toxic to human health (i.e., carcinogenic compounds), and about 5% of these substances are highly toxic to the environment [19, 25]. Therefore, the minimization of the discharges of untreated textile wastewater into the environment together with improving the processes for textile wastewater treatment through more efficient and sustainable technologies arises as an obligatory task.

*A Critical Review on Algal-Bacterial Granular Sludge Process as Potential Economical… DOI: http://dx.doi.org/10.5772/intechopen.99973*

**Figure 1.**

*Source, transport pathways, and the fate of persistent organic pollutants in the environment. Adapted from [24].*

### **3. Operating cost of main AOPs used for textile wastewater treatment**

AOPs are processes based on the production and utilization of hydroxyl (OH·) radicals [18]. These processes can be broadly classified into four groups—photocatalytic process (H2O2/UV, O3/H2O2/UV, UV/TiO2, H2O2/TiO2/UV, O3/TiO2/UV); the Fenton reaction-based processes (Fe2 + /H2O2, Fe2 + /H2O2/UV, Electro-Fenton), ozone-based processes (O3/UV, O3/H2O2, O3/Fe++, O3/metal oxide catalyst, O3/ activated carbon, O3/ultrasound); and other processes which may include activated persulfate, ionizing radiation or electron beam technology [2], the AOPs can be applied either individually or in combination among them. Thus, the operating costs of AOPs applied to wastewater treatment may widely vary according to the type and the amount of energy/reagent used.

Although the mechanisms of AOPs rely on the formation of OH· radicals, the formation pathways might be different under different operating conditions [26], showing a strong impact on the estimated cost for the selected treatment process. Moreover, AOPs for complete pollutant mineralization are generally expensive because the intermediates formed during treatment tend to be even more resistant to complete chemical degradation; the intermediates treatment also represents a substantial part of energy and chemicals, which increase with the treatment duration [27].

In this sense, the cost-effectiveness of each technology is one of the main concerns of decision-makers. It must be considered the different costs involved and the efficiencies achieved in the proposed treatment system. The total costs estimation


### **Table 2.**

*Operating cost in terms of energy and chemical consumption for textile wastewater treatments.*

comprises installation, operation, maintenance and additional requirements of the AOP used and that could arise during the process. **Table 2** presents the operating cost related to energy and chemical consumption for AOPs commonly used for the treatment of textile wastewaters.

### **3.1 Ozone-based processes**

Ozone (O3) is a strong oxidant, characterized by its extremely high oxidative potential (E0 2.07 V), which can decompose many hardly degradable pollutants [32, 33]. During ozonation, the organic compounds can be oxidized in two ways. In the first oxidation way, the generated ozone, which is a highly selective oxidant, can react directly with dissolved organics at variable rates. In the second way, ozone is involved in a chain reaction mechanism to form hydroxyl radicals, which are responsible for pollutant decomposition/oxidation [3]. The two pathways can lead to several final products, with different transformation kinetics and represent different treatment costs.

The increasing popularity in recent years of ozone applications is mainly explained by two factors—(1) costs associated with ozone production have considerably decreased in the last decade, and (2) ozone presents some environmental advantages over chlorine. The benefits of ozonation in wastewater treatment plants include sludge reduction and removal of recalcitrant organic contaminants from hazardous wastewater. However, the ozone is an unstable gas that must be generated *in situ* and the associated generation process is still considered an expensive technology [2].

### *A Critical Review on Algal-Bacterial Granular Sludge Process as Potential Economical… DOI: http://dx.doi.org/10.5772/intechopen.99973*

In this sense, ozone treatment costs for textile wastewater involve installation and maintenance costs in the site. Ozonation technology cost is defined by the cost of an ozone generator and its cooling system. The process is also affected by the cost of a pretreatment unit for drying the oxygen (or air) that fed the ozonator; and a post-treatment system for treating the residual ozone in the off-gas, that is, a catalytic ozone destruction unit [3]. Depending on the water quality requirements and treatment objectives, the estimation of operating cost is impacted by different design variables, including flow rate, site constraints, type of manufacturer, among others, which determine the applied ozone doses [31]. Operation and maintenance costs are based on the energy consumption and replacement part costs. In the case of ozone-based treatment, its high costs are mainly related to energy consumption, and the cost of equipment for oxygen or air generation [31, 34].

The removal efficiencies using an ozone-based process in the treatment of textile wastewater may attain values of 97% for color removal and 60% for phenol removal [35, 36]. For water contaminated with phenol, treatment costs are \$0.03 and \$0.51/L using O3 and O3/UV, respectively. This same tendency is observed in the case of water contaminated with reactive azo dye, with treatment costs of \$0.04 and \$0.24/L for O3 and O3/UV processes, respectively [3]. From **Table 2**; the ozonation process is the most expensive AOP with operating costs up to 10 and five times higher than Fenton and UV-irradiation processes, respectively. Even, it has higher costing than UV/H2O2 and UV/persulfate combined processes up to 265.7 and 14.8%, respectively. Although the ozonation process is an effective method to degrade several toxic recalcitrant pollutants, it is still viewed as an expensive technology in application aimed to complete substance mineralization [37], compared with other AOPs.

### **3.2 Fenton and Photo-Fenton processes**

The Fenton process is based on the enhanced oxidative potential of hydrogen peroxide (H2O2) when iron (Fe) is used as a catalyst under acidic conditions. The Fenton reaction mechanism is well-known where—the Fe+3 ion, dissolved in water, form different complexes. For instance, at pH close to 3, the pentaqua-iron (III) hydroxide ([Fe(H2O)5(OH)]2+) becomes the predominant stable species [38]. On the other hand, the combination of H2O2 and UV radiation with a Fe2+ or Fe3+ ion, Fe(OH)2+, Fe-radical, etc., produces more hydroxyl radicals and in turn, increases both the degradation rate of persistent organic pollutants, as the applicability of the process. The latter process is known as the Photo-Fenton process [39].

The Photo-Fenton application in textile wastewater is able to remove a wider range of pollutants than the Fenton-based process since is considered the most effective treatment for the decolorization of wastewater. It also provides high energy efficiencies compared to other AOPs [25]. Several studies have shown that the treatment of textile wastewater using Fenton and Photo-Fenton processes resulted in 74 and 87% of COD removal, respectively, while color removal through Fenton's oxidation process for direct blue 71, and acid orange 24, reach efficiencies of 94 and 92.7%, respectively [38, 40, 41].

Additional advantages of Fenton's technologies include a simple application procedure, low investment cost, lack of residues, ability to treat complex compounds, and low environmental impacts [5]. However, the main drawbacks of the Fenton and Photo-Fenton process are the sludge production and the discarded unused ferrous ions, especially in the case of the homogenous processes. Additionally, textile wastewater is usually generated at alkaline conditions (see **Table 1**), while the Fenton processes usually require a pH of around 3. Furthermore, the addition

of strong acids may even prove counterproductive for ensuring optimal treatment conditions since precipitation phenomena can appear at low pHs [30].

Commonly the iron species involved in the oxidation is normally found at concentrations between 10 and 150 mg/L in textile wastewater, which may cause a hindrance in the effluent and thus prevent water reuse in the textile industry. High iron concentrations can cause fabric stains and wear during bleaching and dyeing [42]. In this sense, the Fenton processes are better recommended for the pretreatment of textile wastewater.

Moreover, the conventional Fenton process is mainly influenced by the cost of the required chemical reagents as hydrogen peroxide, ferrous iron, and those aimed at pH adjustment. Many studies have addressed the significant impact of hydrogen peroxide on process costs for Fenton-based technologies [3]. For this process, the costs of Fenton reagents like H2O2 and FeSO4; and acidification/neutralization chemicals (H2SO4 and NaOH) are commonly used for operating cost calculation, while energy consumption is considered negligible. However, in the Photo-Fenton processes, energy plays an important role, which has a considerable impact on the treatment costs of the process [27].

Photo-Fenton processes are considered more energy-efficient than other AOPs [25, 39]. However, Fenton-based processes are considerably less expensive than Photo-Fenton technology in terms of total costs. From **Table 2**, the Fenton process is the most economical AOP among all reported processes with an annual operating cost of about 92% lower compared to the ozonation process. However, the operating cost of the Photo-Fenton process rises over the UV-based and UV/H2O2 combined processes in 47.2 and 12.12%, respectively. The above is explained by the electrical energy requirement for treatment application. In this sense, the Photo-Fenton is ranked as the second most expensive AOP when it is applied as a single treatment process.

### **3.3 UV-based processes**

Ultraviolet (UV) light (200–400 nm) is usually utilized for degrading organic compounds by direct photolysis [33]. The compounds absorb UV light and undergo degradation from their excited state. The types of lamps commonly applied in photolysis reactions are low-pressure UV and medium-pressure UV lamps [3], which define the cost of process installation and operation.

Advantages of UV-irradiation processes in the treatment of textile wastewater treatment include no sludge production, no use of hazardous chemicals, and no generation of unpleasant odors [29]. However, UV-based systems are widely known as energy-intensive processes with important maintenance costs. Electricity consumption of UV irradiation is the major contributor to the total operating cost. Moreover, it has been reported that routine replacement of key parts in a UV system may equate to about 45% of the annual electrical power consumption costs [26].

This technology has been applied as a single treatment for textile wastewater [33, 43], although it is commonly combined with other techniques such as UV/ H2O2, UV/O3, or UV/H2O2/O3 and thus the operating costs may vary depending on contact time, light intensity, dose, among other operating conditions [44]. It has been reported that treatment cost wastewater contaminated with phenol and reactive azo dyes using H2O2/UV might reach values of \$1.64 and \$0.32/L, respectively [3].

The combination of these advanced oxidation processes tends to substantially improve the removal efficiencies of textile pollutants since possess the ability to provide high color removals in the range of 80–100% after 45–120 minutes of reaction, and different behaviors in terms of COD removal [42]. However, Paździor *A Critical Review on Algal-Bacterial Granular Sludge Process as Potential Economical… DOI: http://dx.doi.org/10.5772/intechopen.99973*

*et al.* [30] reported that investment costs, particularly in UV lamp equipment, may significantly increase the total costs of wastewater treatment (approximately 47% compared to ozonation). The authors demonstrated that operational costs of AOPs based on UV radiation may increase 44% in comparison to other oxidation technologies, including the Photo-Fenton process, and UV-process/ozonation.

From **Table 2**, the operating cost of the UV-irradiation process is competitive compared to other AOPs. However, the combination of UV-irradiation with other processes (i.e., O3, H2O2, or persulfate) increases up to eight times the final operational cost, which becomes them the process most expensive among all available AOPs. Just the operating cost of the combined UV/O3 process may cost up to \$8.5 m−3, which is approximately equal to the total operating cost of all the individual oxidation processes. Therefore, the application of each treatment process either single or combined will depend on the type of textile wastewater treated, target pollutants, desired level of mineralization of the pollutants, and applicable legislation for the particular case.

The efficiency of these technologies mainly depends on the volume of wastewaters to be treated, the concentration and nature of the specific pollutants, and the co-occurrence of other substances [45]. In this sense, the operating cost in each AOP for wastewater treatment varying according to application strategy and energy/reagent amount used.
