The Use of Coagulation–Flocculation for Industrial Colored Wastewater Treatment—(I) The Application of Hybrid Materials

Featured Application

Application of some hybrid materials as coagulation–flocculation agents in single or combined treatment steps for colored wastewater (industrial dye-containing effluents).

Abstract

Polluting species released in industrial-colored effluents contaminate water, degrading its quality and persisting in the aquatic environment; therefore, it must be treated for safe discharge or onsite reuse/recycling to ensure a fresh water supply. This review has the principal goal of facilitating understanding of some important issues concerning wastewater (WW) treatment systems, mainly based on a coagulation–flocculation step, as follows: (i) the significance of and facilities offered by specialized treatment processes, including the coagulation–flocculation step as a single or associated step (i.e., coagulation–flocculation followed by sedimentation/filtration or air flotation); (ii) the characteristics of industrial-colored WW, especially WW from the textile industry, which can be reduced via the coagulation–flocculation step; (iii) primary and secondary groups of hybrid materials and their characteristics when used as coagulants–flocculants; (iv) the influence of different process operating variables and treatment regimens on the efficiency of the studied treatment step; and (v) the benefits of using hybrid materials in colored WW treatment processes and its future development perspectives. The consulted scientific reports underline the benefits of applying hybrid materials as coagulants–flocculants in colored textile WW treatment, mainly fresh, natural hybrid materials that can achieve high removal rates, e.g., dye and color removal of >80%, heavy metals, COD and BOD of >50%, or turbidity removal of >90%. All of the reported data underline the feasibility of using these materials for the removal of colored polluting species (especially dyes) from industrial effluents and the possibility of selecting the adequate one for a specific WW treatment system.

1. Introduction

The wastewater (WW) produced by different economic sectors is characterized by large variations in terms of composition and concentration, as well as various water consumption levels and, thus, the different flowrates of the WWs produced. The European annual reports of water consumption in different economic sectors (2015–2023 period) indicate that the highest water consumption (around 40% of the total water consumption) is required in agriculture and at a decreasing rate in the energy sector (27.80%); mining, manufacturing, and construction industries (18%); domestic consumption (11.70%); and other services (2.50%) [1,2]. In the world, especially developing states, numerous effluents from different economic sectors, especially colored effluents from various manufacturing industries, are directly discharged into aquatic receptors, leading to serious environmental and health concerns such as reducing the dissolved oxygen (DO) concentration in water bodies due to sunlight penetration being blocked [3] and resistance to photochemical reactions [4,5] or carcinogenic and mutagenic potential effects for all kinds of water consumers (aquatic flora, fauna, and even humans) [4,5,6].

WW can affect aquatic life due to the presence of colored compounds such as various dyes and other colorants in numerous water bodies. The most important content of residual dyes in natural water resources is caused by discharge from major manufacturing industries (Figure 1), such as the textile industry, which is considered to constitute 54% of existing dye-containing effluents in the world [5,6]. The annual growth rate of textile dyes proposed by Global Market Research Future is 8.13% (over 2016–2022) [5]. The economic activities in Europe require ca. 243,000 cubic hectoliters of water per year, according to the Water Exploitation Index (WEI) of the European Environmental Agency (EEA) [1,7]. A high percentage of this water (>55–60%) is returned to the aquatic environment, i.e., more than 140,000 cubic hectoliters per year, but loaded with solid impurities or dissolved pollutants, including dangerous residual chemicals such as persistent colored compounds, as synthetic dyes, emerging compounds, or heavy metals [1,8].

There are several restrictions on the discharge limits of dangerous compounds generated during production processes (especially consolidated organic pollutants) and the residual levels that are allowed in water for human consumption (European legislation for water—2000/60/CE—with its subsequent updates) [8]. Emerging pollutants (i.e., pesticides, industrial chemicals, surfactants, pharmaceuticals, and personal care products) are not officially limited in terms of direct discharges in the water (i.e., groundwater, surface water, municipal wastewater, drinking water, and food sources) but are known to be dangerous [6]. Usually, emerging pollutants of synthetic or natural origin (occurring compounds or microbes) are not monitored in the aquatic environment but can negatively influence ecological or public health [8,9] due to their environmental occurrence or specific acute and chronic (long-term) effects (i.e., endocrine disruption, immunotoxicity, neurological disorders, cancers, etc.).

Highly colored WWs contain different colored compounds (such as biobased or synthetic dyes, in liquid or solid/powder form), and, as illustrated in Figure 1, the majority are the residuals of industrial effluents from textile and dyestuff industries, distilleries, and tanneries, among other industries [10,11]. Synthetic dyes (i.e., soluble as anionic (acid, reactive, direct, mordant, stuff, and indigoid) and cationic (basic) dyes, or insoluble as insoluble azo, disperse, Sulphur, vat, solvent dyes) are considered recalcitrant compounds due to their high resistance to degradation and have numerous applications in paper printing, color photography, use in inks, cosmetics, in food, textiles, and leather industries, but awareness of their toxic, carcinogenic, and other health-related effects was not general until recently. A few examples of synthetic dyes that have been reported are as follows: (i) acidic dyes, applied in cosmetics, food, leather, nylon, silk and wool dyeing, paper printing, and inks (e.g., Acid Blue 45); (ii) basic dyes, used in inks, modified nylon and polyester dyeing, paper printing, and medicine (e.g., Basic Orange 5); (iii) direct dyes, applied in leather, cotton, nylon and silk dyeing, and paper printing (e.g., Congo Red); (iv) mordant dyes, used for the preparation of anodized aluminum, natural fibers, and leather and wool dyeing (e.g., Mordant Black 11); (v) reactive dyes, applied in cellulosic, cotton, nylon, and silk and wool dyeing (e.g., Reactive Blue 19); (vi) azo dyes, used in acetate, cellulose, cotton, and rayon and polyester dyeing (e.g., Methylene blue); (vii) disperse dyes, applied to acetate, acrylic fibers, cellulose, nylon, polyamide, polyester, cotton, and plastic dyeing (e.g., Disperse Blue 5); (viii) sulphur dyes, applied in cotton, leather, polyamide fibers, rayon, silk dyeing, and paper printing (e.g., Leucon Sulphur Blue 11); (ix) solvent dyes, used for fats, gasoline, inks, lacquers, lubricant, oils, plastics, stains, and wax manufacturing (e.g., Solvent Red 146); and (x) vat dyes, applied in the dyeing of cotton, cellulosic fibers, polyester–cotton, rayon, and wool (e.g., Vat Orange 15) [3,10,12,13,14,15,16,17,18,19,20,21,22,23,24,25].

Due to the leading positions that textile dyeing, printing, and finishing industries have in relation to water consumption and their implicit production of colored industrial WW, the findings of this review are focused on the characteristics of the WWs produced by the textile industry and a possible coagulation–flocculation treatment step using new and innovative hybrid materials with comparative perspectives referring to conventional coagulants–flocculants used on a large scale, and the treatment techniques that can be used to prevent conventional persistent organic pollutants (POPs) from entering WW, especially synthetic dyes.

In addition, it must be underlined that the specific water consumption rates of different industrial operations and processes of textile finishing (i.e., washing (3%), alkaline treatment (18%), whitening (37%), optical whitening (46%), soft, medium and/or dark dyeing and rinsing (50–73% from total water consumption of the textile company) varies depending on the textile operation or process type, such as presented in

Table 1. Total water consumption in various textile operations or processes.

Textile Process	Water Consumption (×103 m3/kg of Textile Product)	Textile Operation or Process	Water Consumption (×103 m3/kg of Textile Product)
Wool finishing	110.90–657.20	Raw wool washing	4.20–77.60
Fabric finishing	10.80–276.90	Fiber finishing	3.30–557.10
Carpet finishing	8.30–162.60	Yarn finishing	33.40–930.70
Cloth finishing	5.80–392.80

[10,11].

For example, 70 L of freshly softened water is used per kg of cotton material during dark color reactive dyeing [10,12,13,14], but with further optimization, the consumed fresh water can be reduced to 25–40 L per kg of cotton [12]. Moreover, the US EPA indicated a minimum of 40 L of clean water when coloring only 1 kg of cloth, a water volume that can increase according to the textile material and the requirements of the dyeing process [15].

A lot of recent studies have underlined the importance of removing color and dyes from WW before discharge to the aquatic environment, and several research studies have reported a few treatment methods (physical, chemical, and biological) that are able to reduce and even eliminate dyes as well as heavy metals from colored WWs within a short period of time (minutes to days) without causing secondary contamination or leading to additional metabolites [10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27]. These scientific reports on textile WW discharge in the aquatic environment highlight numerous direct effects (e.g., suppression in the streams of re-oxygenation capacity; groundwater pollution due to the leaching of contaminants through the soil; poor sunlight penetration in receptor damaging the ecosystem flora, fauna; aesthetic problems due to a change of color) and indirect effects (e.g., suppression of the human immune system; the acceleration of genotoxicity and micro ecotoxicity due to allergens present in colorings; eutrophication; the killing of aquatic life, such as fishes, plants, mammals, etc.) together with their interconnections [28].

Due to the complexity of textile effluents, no single technique (unitary operation or process) has been capable of achieving satisfactory treatment results to fulfill the regulatory requirements relating to compliance with limitations. Thus, a combination of operations and processes is used in practice in each colored WW treatment step to achieve efficient treatment and obtain very good WW quality (especially concerning the cost-efficiency criteria) [10,12].

2. Principal Textile Colored WW Characteristics and Composition

Due to the diversity of industrial processes (e.g., textile finishing processes such as alkaline or acid pre-treatments, dyeing, printing, washing or multiple rinsing, etc.), industrial effluents have variable compositions and are loaded with complex mixtures of inorganic and organic chemicals that must be treated before being discharged or must undergo onsite reuse/recycle. Colorants are usually classified into natural and synthetic dyes. Synthetic dyes can be easily produced that cover the entire color palette and are characterized by their fastness, which makes them more widely used than natural dyes [4].

Considering the generated volume and composition of industrial effluents, one can consider colored WW from textile industries to be the most polluting of all industrial sectors. Thus, a lot of contaminants can be present in textile effluents, such as dyes, surfactants, metal ions, salts, and other hazardous organics. Commonly, dyes are used in combination with other chemicals, such as acids, alkalis, salts, fixing agents, carriers, dispersing agents and surfactants, in dyeing or printing operations and finishing processes, and are partly or almost completely discharged in the produced WW. Dye fixation rates onto textile fibers vary considerably, being dependent on the class of dye used (e.g., the reactive dyes used for cotton dyeing have low fixation rates, and 20–50% residuals remain in the dyestuff, or 30–40% in the case of residual sulphur dyes) [10,12]. The most used colorants from all dye classes are azo dyes, most of which are non-toxic, but their metabolites (after oxidation without azo reduction) may be toxic due to the formation of highly reactive electrophilic diazonium salts. Commonly, substituted benzene and naphthalene rings are constituents of azo dyes and have been identified as potential carcinogens.

A complete investigation of all dyestuffs available on the market is still impossible to perform. Therefore, it is essential to understand the dye structures, their degradation process and the formation of toxic by-products because the coloring of WW is one of the major concerns facing industries involved in dyeing processes.

Dye-containing effluents are high in color and pH and have increased levels of total suspending solids (TSSs), turbidity (T), organics expressed in terms of chemical oxygen demand (COD-Cr), biochemical oxygen demand (BOD₅) [4,10], metal ions [16,17], temperature [18], and dissolved salts (TDSs) [10,12,15]. The permanent monitoring of these characteristics in WWs (non-treated and treated) is required before discharging the corresponding effluent into an aquatic receptor to compare the real values to standardized acceptable limits and to appreciate the efficiency of each WW treatment step to improve and maximize its performance in mitigating pollution. In addition, it is necessary to evaluate WW toxicity and ecotoxicity by testing the whole effluent stream of aquatic organisms (a cost-effective method), such as fishes, daphnia, bacteria (e.g., duckweed—Lemna sp.), other non- and vertebrated microorganisms, or plants (algae); however, the identification of all toxic compounds used within the textile manufacturing industries is quite impossible due to the huge variety of chemicals used and the lack of data about their toxicity.

Currently, colored WWs contain numerous polar and non-polar compounds, predominantly polar compounds, which are non-biodegradable and often incompletely eliminated. Many of these compounds are present in WW due to the large number of chemicals used, such as detergents and caustics, sizing agents, oils, latex and glues, dyes, fixing agents and many inorganics (e.g., acids, alkali-as, heavy metals, etc.), a wide variety of special chemicals used as softeners, stain-releasing agents, wetting agents, etc. Considering some wet textile processes and operations, the main characteristics of WW produced from the textile industry, i.e., a cotton fabric manufacturing plant, are summarized in

Table 2. Main characteristics of WWs produced from a textile manufacturing plant (wet cotton fabric processing).

Process	Singering/De-Sizing	Scouring	Bleaching	Mercerizing	Dyeing/Printing/ Finishing
Indicators *
pH	-	10–13	8.5–9.6	5.5–9.5	5–10
Color (ADMI), [color units]	-	650–700	153–190	-	1450–4750
Total suspended solids (TSSs), [mg/L]	16,000–32,000	7600–17,400	2300–14,400	600–1900	500–14,100
Total dissolved salts (TDSs), [mg/L]	-	-	4800–19,500	4300–4600	50–500
COD-Cr, [mgO2/L]	4600–5900	8000–8600	6700–13,500	1600–1700	1100–4600
BOD5, [mgO2/L]	1700–5200	100–2900	100–1700	50–150	10–1800
Water usage, [L/kg cotton]	3–9	26–43	3–124	232–308	8–300

* Principal quality indicators for textile WW produced from wet cotton processing (adapted from a report by Zaharia and Suteu (2012) [10]).

[10].

The composition of textile WW varies from country to country (factory to factory or mill to mill) and depends on the equipment used for each operation/process, the type of processing, the types of textiles produced (i.e., fabric, fiber, carpet, yarn, etc.), the type of fabric, fabric mass/weight/length, season, and fashion trends [10,19]. Some textile effluent characteristics reported in different countries for different types of textiles are summarized in

Table 3. Textile WWs characteristics reported from different sources and certain countries.

Continent	Asia							Europe					Arabia	America
Country [Reference]	India [20,21,22,23,24]	Pakistan [25]	Malaysia [26,27]	Bangladesh [29]	Thailand [30]	China [31,32]	Turkey [41,42]	Spain [33,34,35]	Italy [36]	Austria [4]	Romania [10]	Greece [4]	Iraq [37,38,39]	Mexico [40]
Textile WW Source	Raw WWs of a Textile Factory (Tirupur/ Sumukh)	Raw WWs from 7 Mills/ Treatment Steps	Effluent Garment Factory/Pen Fabric Mill	Raw WWs from a Mill	Real Effluents from Dyebath	Raw Dye WW/Jinyang Industry/ Biostep	Real Effluent Factory/ Bursa/ Eskisehir	Raw WW of Rinsing Baths/ColorTex Industry	Real Effluents from Dyebath	Mixed WWs from Poly-Ester Fibers Processing	Real WWs from Cotton Fabrics Factory	Real WWs of Epilektos SA Factory	WWs of Al-Hilla Factory/ 14 Mills	Rinsing Step of a Denim Factory
Quality Indicator
pH	4.8–10	7.5–11	5.5–10	3.9–14	5.59	7.4–8.3	7.8–9.1	6.9–7.8	8.5–9.5	6.36–6.67	6.6–8.3	8–8.2	5–8.5	6.84–7
Turbidity, [NTU]	240–290	-	63–74	-	417–423	40–137	-	8–15	-	-	205–280	-	-	100–105
Color [ADMI/ Pt-Co] *	420–2500	-	680–750	-	420–3000	310–325	1400–3000	390–540	60–660	-	150–390	-	85	330–600
CODCr [mg O2/L]	381–4400	125–705	231–990	41–2430	750–2600	61–1350	200–1953	200–806	972–1075	1380–6033	665–950	150	80–90	344–500
BOD5 [mg O2/L]	130–1750	115–653	-	10–876	25–530	6–10	-	-	8.8–100	177–720	250–320	80	50–60	85–120
TSS [mg/L]	8300–101,580	200–1619	39–11,689	25–3950	90–140	-	115–245	46–112	-	75–220	350–500	-	312–400	180–3000
TDS [mg/L]	2070–4800	2469–7295	14,000–107,500	90–5980	4800–5700	-	-	1456–1568	-	-	2000–2500	-	400–1350	2000–2100
Chlorides [mg/L]	980–4320	950–2750	1500	600	-	-	-	270–365	950–2750	-	80–500	-	542–550	300–330
Dissolved phosphates [mg/L]	72–87	-	5–10	-	-	-	-	-	3.1–3.3	-	5–15	-	0.64	50–287
Sulphates [mg/L]	2050–2250	600–1000	250	100	-	-	-	124–176	300–450	-	100–800	-	410–580	200–230
Total N [mg/L]	-	-	-	-	-	-	-	-	-	7.53–5.2	20	-	-	-
NO3-N [mg/L]	3.6–627	-	-	-	-	-	-	-	3.7–3.9	0.26–1.1	3.2–3.5	-	-	1.9
NH4-N [mg/L]	1.7	-	2.1–3.8	-	-	-	-	-	36–44	0.76–3.7	2.5	-	-	-
Carbonate (CO32−) [mg/L]	110–120	-	-	-	-	-	-	96–98	-	-	-	-	-	0.35
HCO3− [mg/L]	555–1464	-	-	-	-	-	-	800–1000	-	-	-	-	-	101.5

* ADMI—American Dye Manufacturing Institute unit; TSS—total suspended solids; TDS—total dissolved salts (fixed residues); NO₃-N—nitrate nitrogen; NH₄-N—ammonia nitrogen.

[29,30,31,32,33,34,35,36,37,38,39,40,41,42].

Numerous researchers have examined textile WW treatment technologies and the efficiency of pollutant removal using simulated synthetic WWs in the form of aqueous solutions (individual or mixed dye solutions) or prepared synthetic textile WW (i.e., a diluted prepared dyeing bath at ratios of 1:100, 1:1000, or even 1:10,000 dyebath/tap water or deionized/distilled water) considering the validated dyebath formulations of a few textile factories for certain colors used in textile fabrics/fibers of different origins (natural: wool and cotton; synthetic: polyester, polyacrylic, polyamide, etc.) [40,41,42,43,44,45,46,47,48,49,50,51,52,53,54,55,56,57,58,59,60,61,62,63,64,65,66,67,68,69].

Different effluent mixtures can be found in industrial sewer systems from certain textile productive that contain water used for rinsing/washing or even rainfall and stormwater, which is why the composition of non-treated textile WWs can be constituted of numerous inorganic and organic species, including different types of residual dyes (for different colors of textile products) at different concentrations (0.05–3000 mg/L) used for dyeing/finishing steps but also auxiliary agents for each step of the wet textile finishing processing (e.g., starch, waxes, and enzymes used in de-sizing; NaOH, surfactants, soaps, fats, pectin, oils, and waxes used in scouring; H₂O₂, sodium silicate, organic stabilizers, and alkalis used in bleaching; metals, salts, surfactants, and pH adjustment reagents used in dyeing; solvents, formaldehyde, metals, and urea used in printing; and softeners, solvents, resins, and waxes used in other finishing processes [4,10,12,15].

In this context, a lot of scientific reports have discussed the experimental results obtained in relation to textile WWs, especially simulated dye-containing effluents, the application of different single or combined treatments for the removal of persistent colored contaminants, the presence of emerging pollutants, and the fulfilment of standardized limitations of WW quality indicators for further discharge into aquatic receptors or intended for reuse/recycle. Certain prepared synthetic WWs that reflect some dyebath formulations are briefly presented in

Table 4. Constituents of prepared synthetic dye-containing WW and its applied treatment method from several reported works.

Chemical Constituents of Textile Colored WW	Concentration, [mg/L]	Dye Type	Reported Treatment Method	Testing Country	Reference
Starch, Acetic acid, Sucrose, NaOH, H2SO4, Na2CO3, NaCl, Sodium lauryl sulphate, Dyes mixture: Reactive Black 5+ Congo Red + Disperse Blue 3	1000, 200, 600, 500, 300, 500, 3000, and 100 200	Reactive Acid Disperse	Coagulation–flocculation	India	[28,43]
Starch, Ammonium sulphate, Disodium phosphate, Reactive Violet 4	2.78, 5.56, 5.56 80	Reactive	Electrocoagulation	Marocco	[44]
NaCl, NaOH, Na2CO3, Reactive dye	40,000, 1500, 2000, 600	Reactive	Adsorption by shale column	Thailand	[30]
Meat extract, Urea, K2HPO4, NaCl, CaCl2.H2O, MgSO4.7H2O, Acid Orange 7	110, 30, 28, 7, 4, 2, 20	Acid	Adsorption by PAC and activated sludge	Spain	[45]
NaCl, Dyes mixture: Everzol Black + Everzol Blue + Everzol Red	500, 60 (mixture)	Reactive	Ultrafiltration/nanofiltration	Spain	[34]
Na2CO3, NaOH, NaCl, Reactive Black 5	40, 20, 600, 130	Reactive	Nanofiltration	Austria	[46]
Polyvinyl alcohol, Remazol Turquoise Blue G-133, Irgapadol MP, Reactive Black 5, Disperse Yellow 211, Vat Yellow 46	100, 50, 2000, 30, 30, 30	Reactive, Vat Disperse	Advanced oxidation–Fenton process	Turkey	[47]
Acetic acid, NaCl, Na2CO3, NaOH, Polyether based co-polymer micro-dispersion, Acryl co-polymer-phosphorus mixture, Acryl phenol polyglycol ether, Procion Blue HERS, Procion Crimson HEXL, Procion Yellow HE4R, Procion Navy HEXL, Procion Yellow HEXL	790, 41,000, 13,000, 510, 1200, 850, 500, 6.83, 40.6, 15, 86.3, 33.3	Reactive	Advanced oxidation (AO)	Australia	[48]
Polyvinyl alcohol, Reactive Blue R94H	125, 20	Reactive	AO–Fenton process	Taiwan	[49]
Starch, NaCl, Remazol Red	465, 10,000, 10	Reactive	Anaerobic biofilm reactor	India	[21]
Starch, Lab Lemco, Ammonium sulphate, MgSO4.7H2O, CaCl2, FeSO4.7H2O, NiSO4.7H2O, MgCl2. H2O, ZnSO4.7H2O, Boric acid, CoCl2.7H2O, CuSO4.5H2O, Maxilon Red	1280, 400, 353, 108, 40, 0.75, 0.50, 0.50, 0.50, 0.10, 0.05, 0.005, 25–50	Reactive	Suspended biofilter using activated sludge	UK	[50]
D-glucose, NaCl, FeCl3.H2O, ZnSO4.7H2O, MgSO4.7H2O, Boric acid, CuSO4.5H2O, MgCl2.2H2O, Ammonium molybdate, MnCl2. 2H2O, Al2(SO4)3. 6H2O, CaCl2.2H2O, CoCl2.6H2O, Thiamine-HCl hydrogen, Reactive Yellow 22	100, 50, 7100, 1, 5000, 1, 1, 1, 1, 80, 80, 550, 10,000, 2000–250, 500, 1000	Reactive	Biological treatment by algae	India	[51]
Cotton Blanc KRS, Biavin BPA, Meropan DA, Na3PO4, NaOH, Na2SO4, Acetic acid, Ammonium sulphate, Disperse Blue 1, Disperse Orange 3	330, 330, 170, 330, 1000, 900, 170, 600, 12, 20	Disperse	Anoxic-aerobic photo-bioreactor	Spain	[52]
Slipper, Mollan, Na2CO3, NaOH, NaCl, Acetic acid, Na2S2O3, Procion Marine HEXL	1000, 125, 10,000, 1320, 63,000, 500, 2000, 20	Reactive	Catalytic wet hydrogen peroxide oxidation (FeY11.5 or Y5)	Germany	[53]

[21,28,30,34,43,44,45,46,47,48,49,50,51,52,53].

Commonly, the testing of different dye(s) solutions of various concentrations (trace and low/medium/high concentrations in water or an aqueous solution) and synthetic dye-containing WWs is required when establishing the basis for an action mechanism to be applied to the WW treatment system and the variation in the field of each testing/operating process parameter. However, testing is especially relevant to finding the best treatment dye capture solution in terms of the cost-efficiency criteria. Further WW treatment processes will be modeled and validated in relation to large industrial-scale applications for real industrial WW (e.g., real-time industrial-scale practice modeling), and various technical optimization methodologies will be found to obtain the highest treatment degrees related to color and dye(s) content associated with treatment supervision (e.g., process monitoring and operator support—knowledge-based systems—with integrative design and control using computers for signal treatment and monitoring).

3. Textile WW Treatment Processes for Dye(s) and Color Removal—Chemical Coagulation–Flocculation Technologies and Their Performance

3.1. Textile WW Treatment Processes for Dye(s) and Color Removal

Equalization and sedimentation were considered viable as preliminary treatment operations in the primary mechanical treatment step of textile WWs until 1990; they were used for dye and color removal since no limits were imposed on their content in the treated effluent [3,54]. The primary treatment step was usually followed by the secondary step based on specific biological–mechanical processes. After the approval of restrictive/limitative quality standards for treated effluent discharged to different aquatic receptors, technological treatment processes were introduced that were available and efficient techniques used for color and dye removal. The majority were based on discoloration and degradation processes [10,55,56,57,58,59,60], which are active in the primary (physical–chemical–mechanical treatments based on precipitation, coagulation, flocculation, sedimentation, filtration, or air flotation, etc.), secondary (biological–mechanical treatments based on biodegradation, adsorption, redox and/or ionic exchange, sedimentation, and filtration), and tertiary (advanced physical–chemical treatments based on advanced oxidation, precipitation, membrane processes, filtration on multiple adsorptive materials’ layers, etc.) treatment steps but also in sludge treatment (achieved via supervised tipping, chemical–mechanical conditioning, recycling, especially for sludge dehydration, or even incineration).

Discoloration processes are especially used in removing colors from textile WWs, but the treated effluent can remain loaded with a significant content of organics expressed through high values of COD-Cr, BOD₅, and/or TOC, commonly exceeding the standard limits. For both color and dye removal from textile effluents, degradation processes are prominently used, which are processes involving the destruction of complex dye and auxiliary organic structures based on decomposition or chemically breaking down dye molecules into small molecular structures, i.e., degradation products such as carbon dioxide, water, simple minerals, and organic by-products (e.g., organic acids and alcohols, small organics with low molecular weights).

All states of the European Community enforce strict legislative norms and measures concerning the presence of coloring and dyes in WW produced within the community and natural water resources; however, there is no official document that lists the different limits of numerous classes of dyes present in treated effluent, only a limitative sum of dye (<1 mg/L) and color (<10–50 HU) content that can be present in the treated effluent, which is dependent on a country’s strategic policy and the norms imposed by environmental authorities/regulators on the compliance plan of certain textile companies. Moreover, in UK, a law that declares ‘no synthetic chemicals should be discharged into the marine environment’ [54], including synthetic dyes, is still active.

In recent years, the E.U., Canada, USA and Australia have permitted environmental legislative authorities to specify the threshold concentration levels of different polluting species in treated WW, including dyes. Morocco and Turkey use the EU model, while Thailand adopted the USA system. In Pakistan, India, and Malaysia, effluent discharge limits are regulated by specific directives of their Central Pollution Control Board, but the limits for azo dyes are not specified, and other dye classes are considered separate groups, unrelated to the other physical-chemical characteristics of treated WW (e.g., the total dissolved solids (TDSs) content or color index) [58,59,60,61,62,63,64,65,66,67].

The scientific literature reports various treatment schemes for textile WWs while referring to their initial compositions, but an important indicator when forming a decision is the COD_Cr/BOD₅ ratio, which estimates the biodegradability of WW [58,59,60,61,62,63,64,65,66,67,68,69,70,71,72]. If this ratio is <2.5, the colored effluent contains at least 40% biodegradable organic compounds that are susceptible to biological treatments (which are efficient, commonly used, and low-cost) [73,74,75,76,77,78,79,80,81,82,83,84,85,86,87,88,89,90,91]. If the ratio is between 2.5 and 3.5, a comparative study of biological and physical–chemical treatment process efficiency is required (various treatment processes can be applied); if the ratio is >3.5, physical–chemical processes are mostly indicated (at least as a pre-treatment before the application of a biological treatment step) due to the high fraction of non-biodegradable compounds [58,59,60,92,93,94,95]. Commonly, technological treatment processes of colored effluents can use different methods that are grouped as follows: (i) conventional methods: coagulation/flocculation, precipitation, biodegradation, sand filtration (lent or rapid), and adsorption using activated carbon (AC); (ii) recovery methods: solvent extraction, evaporation, oxidation, electrochemical treatment, membrane separation, treatment in the membrane bioreactors, ion exchange, and incineration; and (iii) emerging removal methods: advanced oxidation, adsorption onto non-conventional solids (e.g., ‘low cost’ adsorbents), biosorption, living biomass growth, nanofiltration, etc. [12,54,55,56,57,58,59,60,61,62,63,64,65,66,67,68,69,70,85,86,87,88,89,90,91].

A better and deeper understanding is needed that considers all biological, physical, ecological, social, and economic interactions or other connections surrounding a textile WW treatment system. The techniques used to describe and solve the concerns and treatment problems are those used by chemical, biochemical, hydrological, environmental and/or technological engineers who select the adequate technological process in terms of cost-efficiency, develop models based on mass balances or conversion efficiencies relating to different technological treatment steps, minimize residual concentrations of different polluting ionic and molecular species in the treated WWs or water resources considered for supply or discharge, and avoid the development of health effects or the dispersion of toxic compounds.

Some treatment processes used for colored WW that are commonly applied to textile effluents are summarized in Figure 2 in association with their principal advantages and disadvantages. Each treatment process has specific constraints considering the treatment cost, feasibility, efficiency, practicability, reliability, environmental impact, sludge production, difficulties in operation and processing, pre-treatment requirements, and the possibility of producing potentially toxic by-products [12,41,42,43].

In general, physical–chemical and biological treatments can remove most pollutants from industrial effluents, but it is important to concentrate research on cheaper and more effective combined/mixed treatments or new alternatives that can be possibly applied in each country (impoverished, developing, or even highly industrialized nations) of our changing world. Therefore, biological treatments are based on biodegradable conversion processes of contaminants/pollutants from colored WW into more simple and harmless products with the help of different groups of microorganisms (bacteria, fungi, yeasts, and algae) (via adsorption and biodegradation with the help of active bacteriological biomass). These methods produce less sludge and also require fewer chemicals, are economically feasible in poor and developing countries (no large investment and operational costs), have energy-saving features, and permit the complete mineralization of the dyes [3,10,12,57,69,70,71].

The chemical processes used for dye removal from dye-containing effluents are often more expensive than physical and biological treatments (except electrochemical techniques) and require chemicals, specific equipment, and electricity and can produce toxic by-products (secondary metabolites) that entail additional disposal and treatment problems [3,10,12,72,73].

The representative chemical treatments are as follows:

(i) advanced oxidation processes (AOPs) operating in the presence of UV light and/or oxidizing agents (hydroxyl radicals, persulphate radicals, ozone, hydrogen peroxide, and other oxidizing agents in association or not with various catalysts) under specific conditions of temperature and pressure as stand-alone or hybrid technologies can be used [74,75,76,77,78,79];

(ii) coagulation processes operating under vigorous mixing conditions for the charge neutralization of fine particles in WW followed by flocculation under gentle mixing for fine solid agglomeration, and further floc separation via sedimentation can be applied. They are natural and synthetic coagulants/flocculants of inorganic and organic natures; recently, however, increasing interest has been on the development of hybrid materials (of inorganic–inorganic, inorganic–organic, organic–natural, inorganic–natural, or inorganic–organic–natural origin) [60,80,81,82,93,94];

(iii) electrochemical treatments can be used that operate in electrochemical cells/reactors with two metal electrodes connected at a direct current source in which the coagulant is in situ generated at the anode and hydrogen gas evolves at the cathode [83,84,85]. In addition, electro-Fenton and anode oxidation are considered electrochemical treatments that enable the removal of dyes in two stages based on a combination of oxidation and coagulation processes named usually electrochemical AOPs [86,87,88];

(iv) ion exchange treatments developed based on the strong interactions between the functional groups of ion exchange resins and charged dyes are highly efficient, low-cost methods but can only be used for low and medium concentrations of contaminants [10,12].

The physical processes used for the removal of dye from colored effluents involve mass transfer processes and are low-cost treatments with high efficiency (85–99%), simple designs, easy operating units, fewer chemical requirements, and no inhibitory effect due to the presence of toxic species. Such physical processes are adsorption, membrane filtration as microfiltration (MF), ultrafiltration (UF), nanofiltration (NF), reverse osmosis (RO), which are all known to be membrane separation processes that remove contaminants from WW; the difference between these separation processes especially relating to pores size, which can be 0.1–10 μm (MF), 0.001–0.1 μm (UF), and 0.5–2.0 nm (NF) [89,90,91].

In the case of certain textile effluents, the scientific literature reports that WW treatments can be commonly based on the following three steps: [10,61,62,63]: (i) a coarse prefiltration of mixed textile effluents to remove any fibrous matter; (ii) the chemical treatment of the effluent with a 10% lime slurry into a first tank to yield a pH of 11.30, after which an iron (II) sulphate solution can be added into a second tank, forming a precipitate of iron (II) hydroxide, and finally, the addition of polyelectrolyte to aggregate the floc particles into a third tank; and (iii) S/L separation via a faster and more effective particles settlement in the settlement tank; the sludge is automatically pumped to a sludge tank using a vacuum extraction device, and the supernatant passes through two further tanks where the pH is adjusted to values between 7.5 and 8.5 by injecting carbon dioxide under pressure or, in some instances, concentrated hydrochloric acid. The treated effluent is further pumped to the biological plant for treatment [61,62,63].

The abovementioned technological treatment process is successful in decolorizing strongly colored dyehouse effluent. Frequently, the agglomeration of fine solids to settle or improve filtration (e.g., single or multi-bed filtration) requires only the action of an electric or magnetic field due to the indirect formation of specialized coagulation–flocculation agents (via dissolution on different iron- or aluminum-based electrodes or the specific action of metal species present in WWs). Commonly, there are necessary specific chemical materials that need to be introduced to the colored WW to achieve the coagulation and flocculation processes for high and even the complete removal of small suspended and colloidal particles of various compositions. Chemical precipitation and coagulation–flocculation processes remove more than 50% of the BOD₅ of the raw effluent but the decolorized effluent has COD-Cr and BOD₅ loads that are too high for discharge into the watercourse (river, lake, or lagoon); therefore, additional treatment steps are required to fulfil the compliance plan requirements for discharge limits of all of the imposed WW quality indicators. Usually, typical mechanical–chemical–biological technological treatment processes for textile effluents consist of the following steps: WW collection and storage (preliminary step), equalization (WW mixing and cooling step), pH adjustment, coagulation–flocculation + aggregate/floc separation, biological treatment (bio-oxidation) + sludge separation (sludge thickening), filtration, and the disinfection and discharge of the treated WW. The chemical treatment step based on coagulation–flocculation with aggregates/flocs separation reduces the turbidity, suspended solids, oil, organic matter, color, and COD/BOD ratio, as illustrated in Figure 3.

3.2. Coagulation–Flocculation Technologies and Its Performances in Colored Textile WWs Treatment

The scientific literature clearly indicates that colored colloids from textile effluents cannot be separated via simple gravitational means, and some chemicals (e.g., lime, ferrous and ferric salts, aluminum salts, various polymers containing these metallic ions, such as poly aluminum chloride (PAC), poly aluminum sulphate (PAS), or cationic, anionic, and nonionic organic polymers/co-polymers (polyelectrolytes) based on poly acrylamide, polyacrylic acid, polyimides, other macromolecular structures, such as processed hybrid materials of inorganic, organic, and mixed/hybrid origins) [60,61,62,63,64,65,66,67,68,69,70,71,92,93,94,95,96] are added to cause the fine solids to separate via settlement or/and filtration. These chemicals destabilize colloidal suspended small particles (e.g., dyes, clay, iron, heavy metals, organic solids, and oil in WW) and emulsions entrapping solids (coagulation) and/or lead to the agglomeration of these particles to flocs large enough to settle (flocculation) or highly improve further filtration/biofiltration [12,60,64,65,66,73,74,75,76,77,78,79,80,81,82,83,84,85,86,87,88,89,90,91,92,96,97,98,99,100,101,102,103,104]. Anionic, nonionic, and cationic polymers can be used for the flocculation process. If an over dosage with coagulants and/or flocculants was applied, it is possible to intervene in a particle charge reversal due to the adsorption of excess ions, and thus stable colloid particles are presented [60,92,96,105,106,107,108,109,110,111,112,113,114,115,116,117,118,119,120,121,122,123].

The scientific literature has reported four major coagulation–flocculation mechanisms to explain the agglomeration of colloidal particles at higher particle size dimensions and, consequently, the ease of separation via gravity sedimentation or filtration [60,92,93,94,95,96,97,98,99,100,101,102,103,104,105,106,107,108,109,110,111,112,113,114,115,116,124,125,126,127,128,129,130,131,132,133,134,135,136,137,138,139,140,141,142,143,144,145,146,147,148,149,150]:

(i) double layer compression: The colloids present in WW are commonly charged and often described in terms of an electrical double layer. The charge serves to attract opposite counterions from the surrounding aqueous medium and, thus, it forms a layer adjacent to the colloid surface, which leads to ion exchange properties (Figure 4a). The stability of colloids and suspended solids in water is maintained because the small, charged particles repel each other. If they come sufficiently close, the repulsive forces of the surface charge are balanced by the attractive forces of counterions, and aggregation can occur with the formation of larger and settable flocs. One destabilization mode occurs when there is a high concentration of electrolyte in the water (acting as a coagulant), representing a source of counterions that accumulate around the solid surface and reduce the thickness of its double layer. The space charge density of ions in water rapidly decreases with distance from the particle surface, and the potential exponentially declines as a function of distance (DLVO model: Derjaguin–Lindau–Verney–Overbeek). For the concentration of monovalent electrolyte (NaCl) in water of 0.1 M and 10⁻³ M, the double layer thickness is of 11 Å and 101 Å, and, in the case of certain salts, small concentrations may be enough for particle aggregation. The double layer thickness decreases markedly with increasing counterion valence (Hardy rule). Therefore, the interaction energy (E_T) is defined as follows:

E_T = E_R + E_A ≤ 0

(1)

where E_A is the van der Waals attraction energy, and E_R is the coulombic repulsive energy of the double layer.

The critical concentration of the coagulant (commonly a metal-containing salt or a certain metal-containing hybrid material) used for the aggregation of colloidal particles depends on the co-ions rather than the counter ions. The Hofmeister series rule must be considered that relates to coagulation effectiveness as SO₄²⁻ > Cl⁻ > NO₃⁻ > I⁻.

(ii) charge neutralization and adsorption: The retention of species on colloid surfaces can be produced via charge neutralization and specific binding. Adsorption phenomena (physical process) is based on electrostatics (coulombic electrostatic forces that are weak in comparison to covalent, coordinative, or hydrogen bonds) when the charge density on both the colloid and water/WW species determines the extent of adsorption. The colloid–coagulant, coagulant–WW, and colloid–WW interactions are important when compared with coulombic energy (i.e., colloids–surfactant-like molecules such as dodecyl ammonium chloride). When enough counterions are adsorbed, charge reversal takes place, and re-stabilization occurs. If long-chain counterions (polymers, such as polyelectrolytes, and certain polymeric hybrid materials) are attached to the colloid surface, the effective charge outside of the shear layer is reduced in contrast to double-layer repression (double-layer compression), which alters the charge distributions within the diffuse layer (Figure 4b).

(iii) entrapment in a precipitate (co-precipitation) and adsorption: The addition of electrolytes as coagulants (e.g., Fe³⁺ and Al³⁺ salts) in colloid-containing WW leads to the formation of polynuclear hydrolysis products, such as M(OH)_n^z+, which are adsorbed at solid–WW interfaces (e.g., hydrous metal oxide interfaces). It is possible that the formation of a surface complex is due to cation and anion adsorption at hydrous colloid interfaces and the establishment of ligand exchange equilibrium, e.g., the coordination/complexation of cations and anions onto amphoteric hydrous metal oxides.

(iv) interparticle bridging: Polyelectrolytes (e.g., polymer/co-polymer/or macromolecular compounds with multiple ionizable functional groups on its chain, which are soluble in water and have macromolecular structures with flexible chains and charges) and certain polymeric hybrid materials can modify the surface of mineral solids, leading to floc formation. The use of excess polymer, prolonged agitation, or a lack of intramolecular adsorption onto solids can re-stabilize colloidal systems (Figure 4c).

In practice, all four mechanisms can act on a system, but the first two mechanisms are implicated in solid particle neutralization and agglomeration, and the last two are responsible for the growth in aggregate size and/or aggregation as a flocculation process (Figure 4). In the case of using hybrid materials as coagulants/flocculants, all mechanisms act together and depend on the type, composition (metal-containing species and charged functional groups of organic polymeric chains and metal salt content), WW characteristics, and imposed operating conditions. As a result, the coagulation–flocculation process applied in colored WW treatment can act according to the following [71,105,117,118,119,120,121,122,123]:

–

Attraction forces: these decrease the surface charge and enhance the aggregation of solids in distinct, separable aggregates or flocs (coagulation process).

–

The simple electrostatic adsorption of counterions: effectively neutralizes the solid surface charge and decreases the surface potential (dependent on ionic species or large, complex molecules, and ordinary adsorption) (coagulation–flocculation process).

–

Precipitation: hydrated metal hydroxides (precipitates) are formed that can adsorb on the solid surface with other existing colloids and neutralize the surface charge (pH-sensitive, with the characteristic value of the isoelectric point of metal hydroxide) (coagulation–flocculation process).

–

Enmeshment in an agglomerate precipitate and adsorption: is when organic polymers are used (cationic, anionic, and nonionic ones). The existing ions in the WW interact with the polymeric chains, forming solid aggregates (flocculation process).

Flocculation usually refers to the post-destabilization process in which large flocs are produced as a result of the collision of small aggregates due to rapid stirring (peri-kinetic flocculation via Brownian motion) or slow stirring (orthokinetic flocculation via velocity gradients). Both rapid and slow stirring are required for good and complete flocculation with conventional coagulants/flocculants [95,96,97]. When hybrid materials are applied as coagulant–flocculants, a single stirring regime can be used at an adequate constant velocity, yielding increased treatment efficiency.

Commonly, the separation of dense produced aggregates or flocs from WW is achieved by (i) sedimentation, which is the settling of the flocs without stirring for quiescence settling or (ii) filtration/biofiltration, which is the separation of flocs using a free vacuum or under pressure by passing them through a granular solid layer of certain porosity and density (e.g., graded sand, garnet, coal, and resins) supported by gravel layers and/or porous underdrains (depth filters) or precoat filters [123,124,125,126,127,128,129,130,131,132,133,134,135,136,137,138,139,140,141,142,143,144,145,146,147,148,149]. Certain characteristics of coagulation–flocculation processes are summarized in

Table 5. Destabilization characteristics in terms of coagulation–flocculation process.

Operating Characteristics	Coagulation by Double Layer Compression	Coagulation by Charge Neutralization and Adsorption	Co-Precipitation with Coagulation and Adsorption	Flocculation by Interparticle Bridging
Destabilization chemicals (coagulant/flocculant)	Non-hydrolyzing counter ions	Hydrolyzing salts (Fe3+, Al3+ salts); superficial active counter ions, soluble polynuclear compounds	Metal ions and anions, Hybrid materials	Polymers/co-polymer, Polyelectrolytes, Hybrid materials
Electrostatic effects	Predominant	Important	Important	Sub-ordinated
Chemical and adsorption effects	Usually do not take place	Important	Important	Predominant
Zeta potential for aggregation	Almost zero	Almost zero	Often, different to zero	Frequent, different of zero
Physical properties of formed aggregates	Dense and resistant coagulation	Dense, filtrable, easily dehydrated coagulation	Easily filtrable and settable, tridimensional	Easily breakable, voluminous, tridimensional, easy filtrable flocs
Addition of agent	Without effect	Re-stabilization because of charge exchange	Re-stabilization because of charge and ionic exchange	Re-stabilization because of complete surface covering
Surface covering degree for destabilization	Not observable	0 < ϕ < 1	0 < ϕ < 1	ϕ = 0.5
Critical content of destabilization/ re-stabilization agent	Independent of disperse phase concentration.	Stoichiometric towards superficial concentration of disperse phase	Dependent on disperse phase concentration and charge	Stoichiometric

[150].

The efficiency of WW coagulation–flocculation processes in relation to solid separation via sedimentation is important. The highest removal of suspended solids and turbidity after this WW treatment step must be higher than 60–90%, ideally 100% (in a mechanical–chemical WW treatment system). In practice, the effect of coagulation–flocculation on sedimentation performance is beneficial, improving the removal of suspended solids and turbidity but not completely (48–92%) [12,60,71,96,105,123,124,125,126,127,128,129,130,131,132,133,134,135,136,137,138,139,140,141,142,143,144,145,146,147,148,149,150,151,152,153,154,155,156,157,158,159].

The main disadvantage of this WW treatment step is that the process control is a little difficult, possibly due to precipitation rate and floc size growth involving contaminants or residuals such as non-ionic detergents remaining in the effluent, sludge production, which must be settled, dewatered, and pressed into a cake for subsequent landfill tipping, the necessity for further detoxifying and valorization (useful compounds recovery), or incineration [98,99,100,101,102,103,104,105,106,107,108,109,110,111,112,113,114,115,116,117,118,119,120,121,122,123].

Very effective chemical coagulation–flocculation (C-F) methods and the precipitation of phosphorus and existing carbonates in different colored WWs (e.g., textile WWs) have been reported, which reduce the load of the biological treatment, working with relatively high concentrations of inorganic coagulants based on lime, iron, and aluminum salts (e.g., more than 200–300 mg/L); however, very good results were also reported when using a combination of an inorganic coagulant and a polymeric flocculant (coagulation aid), such as for a reference model of textile WWs reported in the scientific literature, such as a poly (aluminum chloride) (PAC) along with an organic polymer [117] or ferrous/ferric chloride and a commercial organic coagulant aid (e.g., sodium alginates) at a pH of 6.7–8.3 (color removal > 80%) [118], alum at pH = 8.2 (54–81% color removal) with the addition of bentonite (3 g/L) for Remazol Violet dye-containing effluent [119], or ferric chloride and two commercial polyelectrolytes, cationic Prodefloc CRC 301 and anionic Ponilit GT-2 polyelectrolyte at a pH of 7.38–7.83 (turbidity removal of >86.12%, color removal of 48.22%, and COD removal of >36.84%) [63].

In the case of industrial beverage WW treatment applied for the removal of trace metals, such as total Fe (Fe²⁺ + Fe³⁺), total Cr (Cr³⁺ + CrO₄²⁻) and Zn (Zn²⁺) ions, the effectiveness of polymer/co-polymer or hybrid material addition to the coagulation–flocculation process was verified, especially when both FeCl₃ (300 mg/L) and an organic polymer (a non-ionic polyacrylamide, 65 mg/L) were added individually, using a FeCl₃–polymer/co-polymer combination (hybrid material); in the case of the individual use of a ferric-based coagulant, high removals of metal species were reported, such as total Cr(III, VI) ion removal (91%), Zn(II) (72%), and total Fe(II, III) (54%), and the addition of polymer/co-polymer increased the efficiencies of the processes to about 95%, 87%, and 88%, respectively. Another case is that of synthetic WWs (models of industrial WW from cosmetic manufacturing) containing Cu²⁺, Ni²⁺, Zn²⁺, and Pb²⁺ ions together with vegetal oil (cedar oil), which were treated with inorganic precipitation agents (sodium carbonate and lime) at pH = 8.5–9.3 and anionic Ponilit GT-2 polyelectrolyte (0.25–0.75 mg/L) individually or in association with anionic Ponilit GT-4 polyelectrolyte as flocculants (co-polymers based on maleic acid, acrylic aldehyde and/or acid) [120] by using chemical precipitation and coagulation–flocculation followed by air flotation and rapid filtration with relatively to very good removals of 54.60–96.20% for Cu(II), 51.52–96.10% for Ni(II), 68.68–96.80% for Zn(II), 68.90–96.08% for Pb(II), and 82.30–98.30% for oil [121].

Numerous scientific reports have noted that the combination of ferric chloride and polymer/polymeric hybrid materials at different ratios achieved high removal efficiency in relation to the removal of metal species and color from WW [123,124,125,126,127,128,129,130,131,132,133,134,135,136,137,138,139,140,141,142,143,144,145,146,147,148,149,150,151,152,153,154,155,156,157,158,159].

In the presence of an electric field, the efficiency of the coagulation–flocculation processes improves due to the good separation of the S/L phases using electrochemical processes, especially electrocoagulation–electro flotation, in which multiple processes are involved, such as electrolytic reactions at the electrodes, the formation of coagulants when treating the effluent, the adsorption of soluble or colloidal pollutants on solid coagulants, and removal after settlement or dissolved air flotation [63,64,65,122]. This electrochemical treatment is efficient even at high pH for color and COD-Cr removals, and it is strongly influenced by the current density and duration of the reaction. The EC treatment was applied with high efficiency for textile WWs. Thus, the EC efficiency in WW containing Orange II and Acid Red 14 dye was found to be higher than 98% for color removal [123,124], and in industrial effluent containing Yellow 86, high turbidity, COD-Cr, and extractible substances were 87.20%, 49.89%, 94.67%, and 74.20% after 30 min of operation at a current intensity of 1 A with monopolar electrodes [122], where iron was the sacrificial anode (producing an iron-based coagulant). Discoloration performance in EC treatment was reported to be in the range of 90–95%, and for COD-Cr, it was reported to be in the range of 30–46% under optimal conditions [96].

The same efficient effect has been noted for magnetic fields in relation to the separation of different solid agglomerates (agglomerated metal co–precipitates) from WWs, especially new metal-based formed aggregates that are easily separable from treated WW with the help of efficient magnets (Figure 4b). In both electric and magnetic fields, the WW treatment process can be performed using advanced electrochemical or magnetic separation processes with very good results, usually considered recovery treatment methods.

4. Hybrid Materials and Their Performance in Colored WW Treatments Based on Coagulation–Flocculation Processes

4.1. Hybrid Materials Used as Coagulation–Flocculation Agents in Colored WW Treatment

Commonly, the coagulants applied in WW treatments are classified into two groups: synthetic and natural coagulants. To enhance the formation of flocs, the introduction of coagulant aids is sometimes required to increase the density of flocs for better separation via gravity or filtration (rapid/lent), such as different types of clays or mineral materials (e.g., kaolin, bentonite, montmorillonite) that improve the mechanical separation of suspended solids from WW after an adequate stirring regime as well as WW discoloration treatment. This is one of the main reasons for the application of hybrid materials as coagulant–flocculants, which have higher densities due to the multiple materials present in their composition (inorganic, organic, natural biopolymers, etc.) that permit the improved removal of suspended solids and dissolved colored species from WW. Good S/WW separation improvements in terms of sedimentation (>20–50%) and discoloration (>10–35%) were achieved, especially in the case of treated effluents from beverage industries, local small wine manufacturing, textile or cosmetic industries, and municipal WWs associated with improved sedimentation and multiple filtration layers (rapid multiple-bed filtration) or biofiltration.

Hybrid materials used as coagulant/flocculant in WW treatment systems are materials obtained by adding or introducing certain efficient components/functional chemical groups into the original material to enhance the agglomeration capacity, thus achieving superior aggregation and (S/WW) separation performance than that of individual/initial conventional coagulant materials [71,103,105,130]. These materials can be used as convenient alternative materials in WW treatment systems with combined actions (e.g., as coagulant/flocculants, adsorbents, co-precipitation, or bridging/cross-linking agents, etc.), reducing the operation time in a single operation when discharging large volumes of industrial WW.

A lot of terms have been used in the scientific literature to define these hybrid materials, such as composite coagulants [144], composite flocculants [145], composite polymers [146], hybrid coagulants [147], hybrid flocculants [148], hybrid polymers [103,149,150,151,152,153,154,155,156,157,158,159,160], among others. Hybrid materials are commonly grouped into three primary groups: (i) structurally hybridized materials (composites), (ii) chemically bound hybridized materials, and (iii) functionally hybridized materials.

The first group, known as composites, consists of a combination of materials at the macroscopic level, such as physical mixtures prepared by the blending of inorganic and organic materials at room or high temperature without containing new chemical species (e.g., polyferric chloride–poly dimethyl diallyl ammonium chloride (PFC–PDMDAAC) hybrid, or polyaluminium chloride–epichlorohydrin–dimethylamine (PAC–EPI–DMA) hybrid). The composite properties follow the rule of mixture, wherein the properties of all of the components’ materials are combined. The composite efficiency in the coagulation–flocculation step can be enhanced due to the synergetic effect of all of the material components.

The second group of hybrid materials consists of special combinations and mixtures of molecules and atoms at the molecular level with excellent properties and efficiencies due to chemical bonds at the interface between the component materials produced via chemical modification/transformation due to new chemical groups introduced to the molecular chains of the materials. Inorganic polymeric coagulants have greater performance in the coagulation–flocculation step than basic inorganic coagulants (e.g., poly aluminum chloride (PAC) vs. aluminum chloride, or poly ferric sulphate (PFS) vs. ferric sulphate) but they have weaker performance than organic polymer flocculants (e.g., polyacrylamide—PAM) due to lower-molecular-weight of inorganic polymeric coagulants. To increase the molecular weight of the hybrid material, a new chemical group can be introduced to the molecular chain to form chemically bound hybridized materials (e.g., in the case of PAC, poly silicic acid was introduced to form the hybrid material of PASiC). Chemical modifications to produce chemically hybridized materials are performed via hydroxylation–pre-polymerization, co-polymerization, and chemical grafting/cross-linking [161,162,163,164,165].

The third group consists of materials that have a harmonizing function due to the utilization of interface functions to obtain new functions or super functions and, thus, coagulation–flocculation can be performed in a single step and not in two steps for each type of WW treatment process. Usually, these hybrid materials are developed from inorganic and organic materials, natural polymers, or biopolymers.

The performance of these primary groups of hybrid materials depends on the WW characteristics and time variations, resistance to WW flowrate fluctuation or shock, operating regime, and working conditions. Some informative data on the performance of the primary groups of hybrid materials applied to some colored industrial WWs as coagulant–flocculants are summarized in

Table 6. Comparative performance of different WW treatments of primary groups of hybrid materials used as coagulant–flocculants.

Treated WW	Pollutant	Efficiency as Coagulant-Flocculant, [%]
		Conventional Coagulants-Flocculants	I—Structurally Hybridized Materials (Composites)	II—Chemically Bound Hybridized Materials	III—Functionally Hybridized Materials
Effluent from antibiotics’ production company [155,160,161,163]	Antibiotic (amoxicillin, diclofenac, tetracyclin)	- Sodium alginate: 20–39% diclofenac	- chitosan-based magnetic hybrid materials: 80–88% antibiotics	- 3D alginate-based MOP, biopolymer-based hybrid material: 80% diclofenac in 60–90 min	- Chitosan-Fe(II)/Ni(II) nano hybrid material: 93% antibiotic, 85% diclofenac
Textile effluent [72,108,116,125,126,127,130,142,156,157,158,159,162] from: - fabric laundry - fabric dyeing mill	Organics as dyes (COD), fine solids (T), suspended solids (SS)	- alum: 20% T, 42% BOD, 43.2–65% COD, 74% color, - FC: 71% dye, 98% SS - FS: 90% dye - PAC: 80% direct dyes	- PAC-extract Hibiscus Rosa-Sinensis leaf: 77.8% COD, 99.4% SS, 78.4% color	- PAM-Alum hybrid material: 68.2% COD, 61.4% SS	- Opuntia ficus-indica-Al(III) hybrid material: 64.77–87.19% COD, 91.26–93.62% T
Hospital WW [160]	Organics expressed by COD, fine particles (T)	- PAC: 74–82% COD, 60–78% T	- clay–silica–biopolymer hybrid material: 50–68% COD, 75–84% T	- biopolymer-based hybrid materials: 80% T, 60–80% COD	- PAC-M. oleifera seed hybrid material: 50% COD, 73.80% T
Effluent from palm oil mill (POME) [72,159]	Organics (COD, BOD, color), suspended solids (SS)	- alum: 15% T, 46% BOD, 41.8–55% COD, 45% color	- Alum-Cassia obtusifolia seed gum hybrid material: 48.22% COD, 81.58% SS	- Alum–silica monolites-titanium dioxide hybrid material: 70–90% in 60 min	- Alum-PFC-PAM hybrid material: 70–90% COD, 70–95% SS
Grey water [103,151,152,153,154]	Organics (COD, BOD, color), antibiotics	- alum: 35% T, 48% BOD, 43.2–63% COD, 71% color,	- laccase immobilized on Fe3O4/SiO2–DTPA hybrid nanocomposite: 99% diclofenac	- soybean peroxidase (SBP)-oxide nano particles-poly(styrene-co-maleic anhydride (SMA): 90% 2,4 dichlorophenol	- Alum-Cassava peels starch: 56.89% COD, 77.48% T, 77.34% SS

. More complex polymeric hybrid materials continue to be synthetized and tested, e.g., tests toward Cd(II) ions, acetaminophen, and diclofenac, and the experimental results are favorable for their use due to their high removal efficiency, i.e., 78% for Cd(II), and 85–99% for acetaminophen and diclofenac. The reusability of the tested complex hybrid materials and enhanced removal action against emerging contaminants and metals are considered in the case of core–shell magnetic sub-microparticles—such as complex magnetite.

The scientific literature also reports on a second classification of hybrid materials used in coagulation–flocculation applications based on material type (i.e., inorganic, organic, natural polymer, and biopolymer) and their possible combinations such as follows:

–

Inorganic–inorganic hybrid materials (e.g., PASiC, PFC–Na–bentonite, PFC–magnetic nanoparticles, clinoptilolite–Al₂(SO₄)₃, oxo titanium sulphate–Al₂(SO₄)₃, FeCl₃–PAC, iron–aluminum polymer hybrid),

–

Inorganic–organic hybrid materials (e.g., Al(OH)₃–PAM, Al(OH)₃–P(AM–co–AA), CaCl₂–PAM, MgCl₂–PAM, PFC–PAM, PAC–PAM, Al₂(SO₄)₃–PDMDAAC, FeCl₃–PDMDAAC, PFC–PDMDAAC, PAC–starch–graft–PAM, and PAC–EPI–DMA, etc.),

–

Inorganic–natural polymer hybrid materials (e.g., Al₂(SO₄)₃–chitosan, PAC–chitosan, poly (aluminum ferric silicate)–chitosan, chitosan–PAC–Na₂SiO₃, rectorite–amylose, red mud–hydrochloric pickle liquor of bauxite, etc.),

–

Inorganic–biopolymer hybrid materials (e.g., pullulan–PAC, microbial flocculant GA1–PAC, MBF (Aspergillus niger)—zeolite);

–

Organic–organic hybrid materials (e.g., poly (acrylamide–co–acrylic acid),

–

Organic–natural polymer hybrid materials (e.g., sodium alginate grafted PAM, chitosan–g–N,N–dimethylacrylamide, PAM–g–carboxymethyl starch, CMC–starch, starch–g–PAM, chitosan–g-N–vinyl formamide, starch–g-PAM–co-sodium xanthan);

–

Natural polymer–natural polymer hybrid materials (e.g., cationic starch–chitosan cross-linking co-polymer).

Preparation of Hybrid Materials: There are various ways to prepare hybrid materials, but the main preparation methods are as follows:

–

Physical blending (at ambient temperature) (e.g., for structurally hybridized materials such as PDMDAAC, PAM, PFC–PDMDAAC, PAC–PDMDAAC, PFC–PDMDAAC, FeCl₃–PDMDAAC), PAC–chitosan, PAC–EPI–DMA, MgCl₂–PAM, PFC–PDMDAAC, etc.);

–

Elevated temperature blending (e.g., for structurally hybridized materials such as PFS–PDMDAAC, PFS–PAM, CaCl₂–PAM, FeCl₃–PAM, etc.);

–

Hydroxylation–pre-polymerization (e.g., for chemically bound-hybridized materials such as PASiC, PAFC, PAFSiC, PMAS, PFSiS, PASiC, poly aluminum silicate, PAC, etc.);

–

Co-polymerization (e.g., for chemically bound-hybridized materials as Al(OH)₃–PAM, Al(OH)₃–P(AM–co–AA), PGS–PAM (polygorskite–polyacrylamide), etc.);

–

Chemical grafting/cross-linking (e.g., for chemically bound hybridized materials such as PAM–g–CMS (polyacrylamide grafted carboxymethyl starch), chitosan–g–N,N–dimethylacrylamide, CMC–starch, SAG–g–PAM (sodium alginate grafted polyacrylamide), starch–g–PAM–co sodium xanthate).

Hybrid material characterization: The application of hybrid materials in the coagulation–flocculation processes is controlled by a few key properties such as the chemical, physical, thermal, morphological, and structural properties of the component materials, which is correlated with the aggregation effect of different solids in the WW. Some basic characteristics of the hybrid materials used as coagulant–flocculants are summarized in

Table 7. Main important properties of hybrid materials in coagulation–flocculation processes.

Property Type	Specific Property	Analysis Technique	Registered Data	Observations
Chemical	Chemical structure	FT-IR	- infrared spectrum of absorption, emission, photoconductivity, or Raman scattering, e.g., new peak in PFS–PAM spectra (blending at 50 °C) [152]	- bands associated with –OH vibrations of water or bridging OHs and with –O bond vibrations; - peaks’ intensity variation with molar ratio of constituents
	Chemical species distribution	- monitoring of hydrolysis–polymerization process for finding changes of Fe(III) and Al(III) active species by - Ferron complexation timed spectro-photometry. - NMP spectroscopy	- difference of complex reaction rate between Ferron and chemical species (Al and Fe) (after reference time = 3 h) E.g., PFC–PDMDAAC, 7% organic excess decreases Fea and Fec but increases Feb [152]	- content/proportion of all kinds of Fe and Al species (Al a,b,c and Fe a,b,c)
Physical	Molecular weight (MW)/intrinsic viscosity (η)	- Mark–Houwink eq.; - static light scattering analysis. - ultrafiltration followed by Al–Ferron timed complex colorimetry	- use of eq. η = f (MW) - selection of size for aggregating actions. - bridging ability efficiency, e.g., CMS-PAM, higher PAM% increases η [153]	- medium and high MWs requested for bridging with multiple interactions with/inter particles
	Conductivity	- ionic content in aqueous solution	- increase/decrease of conductivity with variation of distinct species molar ratios in aqueous solutions - variation of Fe/Si ratio dependent on degree of hydrolysis	- increase in OH/Fe resulted in slight decrease in conductivity related to the degree of polymerization, e.g., PFSiS [151]
	Zeta potential	- electric surface potential measurement	- critical values for destabilization of dispersions, or neutralization, or various composition of materials [152]	- zeta potential value at different critical pH values for stabilization/destabilization of colloids in aqueous systems
Thermal	- Differential scanning calorimetric analysis (DSC). - Thermal gravimetric analysis (TGA)	- DSC: finding of temperature or heat flow during phase transformations and transitions in solids - TGA: thermal decomposition of materials with temperature elevation	- critical temperature for each decomposition step; - weight loss (%) to determine the thermal stability of material. - DT – mass change region, e.g., stability of MgCl2–PAM decreases with increasing MgCl2 content [154]	- thermal decomposition dependent on temperature, heating rate. - hybrid materials with a higher positive value of activation energy (Flyn and Wall’s model) have better thermal stability
Morphological	- visual analysis, or - (SEM) scanning electron microscopy; - (TEM) transmission electron microscopy	- SEM: morphology and microstructure of solid hybrid sample. - TEM: molecular structure of solid hybrid in liquid form	- microstructure varies with the composition, functional groups, and reaction time [134,135]	- short chain-like and less branchy inorganic–inorganic hybrid material is a less favorable structure; a multi-branched structure with larger size / fractal dimension is desired
Structural	- XRD—energy-dispersive X-ray diffraction. - EDX—X-ray spectroscopy	- identification of the presence of organic material (usually amorphous for inorganics)	- identification of crystalline and amorphous phases, e.g., crystal-line phase prominent when increases (Al + Fe)/Si ratio in XRD [155] - atomic distribution of hybrid materials	- limited studies on the atomic distribution of hybrid materials. - introduction of hygroscopic component increases oxygen content

.

Usually, hybrid materials are designed to improve the coagulation and flocculation steps in a water/WW treatment system. Therefore, the scientific literature reports a high number of applications for three principal groups of hybrid materials used in various treatments of lake/river water: kaolin suspension, landfill leachate, dye-containing WW, different solutions, effluents with single or multiple heavy metals and other organics contents, and the conditioning of concentrated sludge.

4.2. Influencing Factors of Hybrid Materials’ Performance in Coagulation–Flocculation Processes

The treatment of textile WW quality in the coagulation–flocculation step is directly influenced by the characteristics of the WW (i.e., pH, turbidity, color, COD, content of phosphates, carbonates, total nitrogen, heavy metal concentration, light absorbance, and transmittance), coagulant/flocculant characteristics (type, critical concentration/dosage, and mixture ratio or percentage of non-polymeric/polymeric components), and operating regime (static / dynamic regime, flowrate, temperature, stirring rate, and other facilities). Certain influencing factors in the textile WW coagulation–flocculation and agglomeration separation steps via sedimentation and/or filtration are discussed below.

4.2.1. The Effect of pH on Textile WW Treatment via Coagulation–Flocculation

One of the key operating parameters in WW treatment based on the coagulation–flocculation processes is pH, which can control process efficiency. Thus, in the case of electrolyte coagulants (i.e., monomeric aluminum and iron salts), various hydrolysis reactions rapidly take place with the assistance of the HO^– group, which is pH-sensitive. Commonly, when using non-polymerized or very low polymerized coagulants (metal salts), the coagulation efficiency is mainly based on the formation of Al (OH)₃ precipitates in the pH range of 6.5–7.0 rather than a charge neutralization mechanism. The increasing pH leads to a drastic decrease in coagulation efficiency due to the predominant formation of ionic Al (OH)₄⁻ species. High coagulation efficiency will only be achieved in an adequate pH range or at the optimal pH, which efficiently favors suspended particles and colloid destabilization and further aggregation into easily separable flocs. Improvements to the coagulation–flocculation process can be performed by using polymeric coagulants/flocculants under wider pH conditions, i.e., many inorganic polymeric coagulants, such as PAC, PFC, and PFS produced by controlling the HO/Al or HO/Fe ratio or in association with additives to prepare much more resistant and dense hybrid materials, such as PASiC, PFSiS, PAC-PDMDAAC, PFC-PDMDAAC, PAC-EP-DMA, etc. A wider corresponding coagulation pH is present in PASiC (6.0–8.5), which is related to unmodified PAC (6.0–8.0), or PFSiS (5.0–9.0), which is related to PFS (5.0–8.0). All of the mentioned hybrid coagulants have significantly higher turbidity removals than monomeric iron-based coagulants, i.e., Fe₂(SO₄)₃. When pH is out of the corresponding coagulation pH range, color removal rapidly decreases.

In the case of inorganic–organic hybrid materials, the influence of pH is insignificant, and the coagulation pH can be extended to a wide range of 3.0–10.5; e.g., PFC-PDMDAAC. Thus, the use of certain hybrid coagulants can improve the coagulation–flocculation efficiency and eliminate the pH effect.

4.2.2. Effect of Hybrid Material Dose on Textile WW Treatment Using Coagulation–Flocculation

The characteristics and polluting load of textile WWs influence the critical concentration of hybrid material applied in the coagulation–flocculation step for high color and solid removal efficiencies. The coagulant/flocculant concentration is dependent on the suspended solids content or turbidity, color, and organics in the textile WWs. Usually, the increase in coagulant/flocculant concentration increases the treatment efficiency until a critical value where the reverse effect is noted in terms of solid re-stabilization (reversal of particle surface charge). Over dosage can cause treatment efficiency to decrease, i.e., PASiC requires a lower dosage than PAC for turbidity removal, and higher efficiencies were obtained. The color removal efficiency is improved when the PFC–PDMDAAC hybrid material is used, and the optimal concentration is relatively reduced compared to that of the individual use of PFC or PDMDAAC [156]. Removal improvements were also proven in relation to PFC-magnetic nanoparticles, PFS–PDMDAAC, and PAC–PDMDAAC [152,153,154,155,156]. The inorganic–organic hybrid materials are found to be more effective in textile WW treatment.

4.2.3. Effect of Stirring Speed and Time on Textile WW Treatment Using Coagulation–Flocculation

Good contact between textile WW and coagulant/flocculant must be present for the initiation/inducing of the coagulation–flocculation process and the formation of concentrated flocs. Commonly, the coagulation–flocculation process develops in two stages of the mixing regime, i.e., (i) a rapid stirring in the range of 75–7000 rpm for 0.5–3.0 min to yield good dispersion of the coagulant–flocculant for the destabilization of colloids and suspended matter existing in the textile WW, and (ii) a slow stirring at 30–150 rpm for 5–30 min to grow the formed flocs and limit the breakdown of aggregates. If functionally hybridized materials are applied as coagulant–flocculants, the two stirring stages can be combined into one stage due to the fact that they are simultaneously involved in the coagulation and flocculation processes. The treatment efficiency decreases when the stirring speed is rapid and the stirring time is long. Moreover, inadequate contact between colloids, suspended solids, and the coagulant/flocculant occurs when the stirring speed and time are slow and short, and the formation of concentrated flocs fails. Therefore, the optimum stirring speed and time must be found for each specific textile WW treatment system when hybrid coagulants are applied.

4.2.4. Effect of Temperature on Textile WW Treatment Using Coagulation–Flocculation

Commonly, temperature is not considered a key operating parameter in the coagulation–flocculation process and has been discussed in only a few scientific reports. The reported findings conclude that higher temperatures produce better coagulation efficiencies in the case of inorganic–inorganic hybrid materials (5–40 °C), e.g., PASiC in turbidity removal [156] or red mud–hydrochloric pickle liquor of bauxite in phosphate removal from colored WWs [157]. Contrarily, in the case of organic–organic hybrid materials, the flocculation efficiency decreases as the temperature increases from 5 to 40 °C, e.g., when cationic starch–chitosan cross-linking hybrid is used to treat WW sewage. In addition, the flocculation time at a lower temperature is shorter than that at a higher temperature; e.g., the cationic starch–chitosan hybrid acts better at low temperatures. The temperature influence on WW treatment efficiency varies with applied coagulant/flocculant and WW characteristics and must be particularly evaluated for each colored textile WW.

4.2.5. Process Kinetics of Coagulation–Flocculation

Treatment efficiency correlated with process kinetics and dynamics has been studied by numerous researchers, for example, Moussas and Zouboulis [151]. All of these researchers used general kinetic models and the photometric dispersion analyzer (PDA), which permits the relative comparison of formed floc growth rate, floc size, and variance of floc size as well as the extent of aggregation for certain hybrid materials according to their master curve plotting a typical ratio (mean relative concentration and size of disperse phase, C₀/C_i or d_i/d₀) vs. time (s). In fact, the master curve describes the coagulation–flocculation kinetic stages (relative ratio of solid content = ratio_i = f(t), where i = 1…n, number of aggregated particles or flocs) [151,156]. The proposed master curve mainly consists of three distinct phases: the lag phase (I), floc growth phase (II), and steady-state phase (III) (Figure 5). The rate of each floc growth phase is determined from the slope of the growth region phase, and thus, the variance of floc size (Δ) can be evaluated. A large variation of values in the steady-state region is related to a wide-ranging floc size distribution, but a low value of variance implies a narrow floc size distribution, indicating a more homogeneous, dense, and less porous floc structure [105,135,136,137,138,139,140,141,142,143,144,145,146,147,148,149].

The kinetics of the coagulation–flocculation treatment is significantly affected by (i) the composition of the hybrid materials, e.g., an increased organic/inorganic ratio increases the average ratio of the steady time interval, which is significantly prominent with a relatively high concentration of solids and (ii) the solid concentration in the WW, i.e., for higher solid concentrations, the growth rate of flocs is higher than that present in WW with a lower solid concentration. The logical explanation is that the increased frequency of particle collisions relates to increasing particle concentration, which, in turn, increases the growth rate and formation of larger flocs, which is attributed to different mechanisms, such as (i) interparticle bridge formation, which occurs when adjacent particles are adsorbed onto the available sites of extended polymeric chains, e.g., as with case of PAM in textile WW with a high concentration of solids (as in Figure 4c), and (ii) charge neutralization, e.g., as in the case of PFS with lower variance than that of the PFS-PAM hybrid, indicating floc formation with a narrow size distribution.

In the case of inorganic–inorganic hybrid materials, the addition of silica affects floc formation and the extent of the aggregation, e.g., increasing the silica concentration within the hybrid coagulant leads to the rapid formation of flocs (higher ratios)—the growth region is shorter than that of the non-hybridized component, ferric sulphate. This fact is attributed to a bridging mechanism with polymeric materials with high MW since polymeric loops tend to extend further to reach adjacent particles during bridge formation.

The scientific literature reports on the very good performance of hybrid coagulants/flocculants in dynamic applications (e.g., desalination membranes, fixed-bed columns) due to their excellent strength, regeneration potential, high adsorption and aggregation capacity, low-cost/eco-friendliness, and high efficiency in retaining both anionic and cationic pollutants, a fact which is a superior attribute when compared to other materials.

Some researchers proposed some models to describe the treatment of dye-containing WW when using hybrid materials based on first- and second-kinetic-order or interparticle interaction or different adsorption-based kinetic models used in static and dynamic regimes because adsorption is still considered one of the possible action mechanisms of coagulation–flocculation. In our opinion, these kinetic models (adsorption kinetics) can be better used in the case of WW treatment based on specific adsorption/sorption/biosorption treatment steps and not for the coagulation–flocculation step. Commonly, kinetic models, which describe the coagulation–flocculation processes very well, correspond to the model equations summarized in

Table 8. Orthokinetic flocculation versus perikinetic coagulation [95,96,150].

Characteristics	Orthokinetic Flocculation	Perikinetic Coagulation
Interparticle processes involved [96]	Hydrodynamic fluid motion or agitation in laminar (a) or turbulent (b) regimes $-\frac{dn}{dt}=\frac{1}{\alpha }·k·n^2·G^0{d}^3$ (a), or $-\frac{dn}{dt}=k·n^2·G^0{d}^3$ (b)	Brownian interparticle contact $\frac{dn}{dt}=\propto ·\frac{4·k·T}{3· \eta ·d}·n^2$
Maxwell–Boltzman distribution associated with sedimentation [95,96]	${\left(\frac{dN}{dt}\right)}_{ok}=-\left(\frac{2\alpha G{d}^3}{3}\right)\cdot N^2$, ${\left(\frac{dN}{dt}\right)}_{ok}=-\left(\frac{4\alpha G\Phi }{\pi }\right)\cdot N$, ln N/N0 = −4afGt/π, $G=\sqrt{\frac{P}{V\cdot \eta }}=k\cdot \sqrt{\frac{P}{V}}$ tF = 2/k1N0, or tA = −ln(1 − α)k12N0, ϕ = πd3N/6	${\left(\frac{dN}{dt}\right)}_{pk}=-\left(\frac{4\alpha kT}{3\mu }\right)\cdot N^2$ or N = N0/[1 + (4αkTN0/3μ)t], or t1/2 = 3μ/(4αkTN0) = 1.6·1011/(αN0)
where d—particle diameter (m); D—Brownian diffusion coefficient (D = kT/(3πμd)); G0—initial velocity gradient; G—velocity gradient; k—Boltzmann constant (J/K); k1, k12—appropiate rate constants; n (N)—number of particles (flocs)/volume unity (m−3); P—real dissipated power (m2 kg/s3, or W); T—absolute temperature (K); tF—flocculation time; tA—adsorption time; V—volume occupied of water (m3); α—fraction of efficient collisions to agglomeration; μ—absolute viscosity; ϕ—volume fraction of colloidal particles; η—dynamic viscosity (Kg/m·s).
Maxwell–Boltzman distribution associated with filtration [95,150]	For packed-bed filtration: $-\left(\frac{dN}{dl}\right)=\frac{3}{2}\left(\frac{1-f}{d}\right)\alpha \eta N$ where f—porosity; (1 − f)—the volume of filter media per volume unit of filter bed; d—bed depth, η—a single collector efficiency, reflecting the rate at which particle contacts occur between suspended particles and filter bed; N—number of agglomerated particles/flocs; α—fraction of efficient collisions to agglomeration.
Overall rate for diminishing of particles [95,150]:	$-\frac{dN}{dt}=4\alpha N\left[\frac{kT}{3\mu }N+\frac{G\Phi }{\pi }\right]$, usually G = 10/s, ϕ = 10−4, α = 10−1, and t = 103 s.

.

4.3. Performance of Different Hybrid Materials Used in Colored Textile WW Treatment

Various types of hybrid coagulants/flocculants have been applied to treat synthetic and real dye-containing WWs via a coagulation–flocculation step. Polymeric functionalized hybrids produced via cross-linking, grafting, and impregnation techniques were prepared to obtain derivative adsorbents and hybrid coagulants/flocculants for colored textile WW treatment. The scientific reports indicated 370% improved uptake capacities and fivefold faster kinetics when compared with conventional clay material [158].

In

Table 9. Performance of certain hybrid coagulants/flocculants in textile WW treatment using coagulation–flocculation steps.

Type of Hybrid Materials	Hybrid Materials	Dosage	Wastewater Type	Wastewater Characteristics	Experiments Conditions	Removal Efficiency (%)	Reference
Inorganic–inorganic hybrid polymer	PFSA (Polyferric-silicate-acetate)	16 mg/L	Simulated dye wastewater (Congo Red)	0.1 g/L, pH 7.50 ± 0.10; temperature, 20 ± 2 °C	Rapid mixing 300 rpm for 2 min, slow mixing 60 rpm for 10 min, settled for 20 min	Dye: 93.3%	[125]
Inorganic–inorganic hybrid polymer	PAC (Polyaluminum chloride)	30 mg/L	Real textile wastewater	Color: 91.7 ± 11.4 Pt/Co, BOD5: 278.54 ± 65.23 mg O2/L, COD: 1346.17 ± 123.36 mg O2/L, TSS: 178.28 ± 23.82 mg/L	Rapid mixing at 150 rpm for 4 min, slow mixing at 40 rpm for 20 min, settle for 1 h	Dye: 44.5% COD: 40% BOD5: 34% TSS: 23.7%	[126]
Inorganic–inorganic hybrid polymer	PFTS (Poly-ferric-titanium-silicate-sulfate)	0.4 mmol/L	Synthetic dye wastewater (Disperse Blue 56)	0.1 g/L pH 8–9	Rapid stirring at 200 rpm for 1.5 min, slow stirring at 40 rmp for 15 min for floc formation; rapid stirring at 200 rmp for 5 min for breakage; slow stirring at 40 rmp for 15 min for recovery	Dye: 95.5% Residual turbidity: 7.0 FTU	[127]
Inorganic–inorganic hybrid polymer	PFTS (Poly-ferric-titanium-silicate-sulfate)	0.4 mmol/L	Synthetic dye wastewater (Reactive Yellow)	0.1 g/L pH 8.0–9.0	Rapid stirring at 200 rmp for 1.5 min, slow stirring at 40 rmp for 15 min for floc formation; rapid stirring at 200 rmp for 5 min for breakage; slow stirring at 40 rmp for 15 min for recovery	Dye: 49.5% Residual turbidity: 6.4 FTI	[127]
Inorganic–inorganic hybrid polymer	MgSiPC (Magnesium silicate)	62.0–78.0 mg/L	Reactive dye simulated wastewater (Reactive Yellow2)	100 mg/L pH 12.08/12.0	Rapid stirring at 300 rpm for 2 min, slow stirring at 60 rpm for 10 min, settle for 20 min.	Dye (RY2): 90–93%	[128]
Inorganic–inorganic hybrid polymer	PAC (Polyaluminium chloride)	1000 mg/L	Aqueous mixed solutions	7 mg Pb(II) /L, 5 mg Zn(II)/L, pH 8.7–9.2	Mixed at 60 to 65 rpm for 3 min; settled for 30 min	Pb (II): 92% Zn (II): 98%	[129]
Inorganic–organic hybrid material	PAC-EPI-DMA (Polyaluminum chloride-epichloro-hydrin dimethylamine)	10.8 mg/L	Synthetic dying solution (Reactive Brilliant Red K-2BP)	100 mg/L pH 8.45	Rapid mixing at 120 rpm for 3 min, slow mixing at 40 rpm for 12 min, settling time 20 min	Color: 90%	[130]
Inorganic–organic hybrid polymer	PACl–PAMIPCl (Polyaluminum chloride–poly(3-acryl-amido-isopropanol chloride)	50 mg/L	Synthetic dying solution (Reactive Cibacron Blue F3GA)	pH 6.5–6.9, COD 70–80 mg O2/L, color 1050–1100 Pt/Co	Rapid mixing for 3 min at 120 rpm, slow agitation for 12 min at 40 rpm, settling time 30 min pH 7.5	COD: 92% Color: 95%	[131]
Inorganic–organic hybrid polymer	PACl–PAMIPCl (Polyaluminum chloride–poly(3-acrylamido-isopropanol chloride)	20 mg/L	Synthetic dying solution (Disperse Terasil Yellow W-4G)	pH 7.0–7.4, COD 140–150 mg O2/L, color 4550–4700 Pt/Co	Rapid mixing for 3 min at 120 rpm, slow agitation for 12 min at 40 rpm, settling time 30 min, pH 3	COD: 93% Color: 96%	[131]
Inorganic–organic hybrid polymer	MCPAM (Magnesium chloride-polyacrylamide)	1200 mg/L	Simulated reactive dye WW (Cibacron Red FN-R)	200–500 mg/L, pH 12; temperature, 20 °C	Agitation speed of 100 rpm for 5 min, settling time 30 min	Dye: 99%	[132]
Inorganic–organic hybrid polymer	MgCl2-PEO (Magnesium chloride-polyethylene oxide)	1020 mg/L	Synthetic aqueous dye solution (Cibacron Blue F3GA)	173 mg/L, pH 11.13	Mixing speed 150 rpm for 6 min, settling time 30 min	COD: 92.09% Color: 99.76%	[133]
Inorganic–organic hybrid polymer	FeCl3–PAM (ferric chloride–polyacrylamide)	500 mg/L	Synthetic dye wastewater (Terasil Red R)	200 mg/L, pH 5.58–5.95, COD 278–412 mg O2/L, color 3860–4320 Pt/Co	Rapid mixing 200 rpm for 3 min, slow mixing 100 rpm for 6 min, settling for 30 min, pH 5	COD: 89% Color: 99%	[134]
Inorganic–organic hybrid material	MgCl2-PAM (magnesium chloride-polyacrylamide)	1000 mg/L	Real textile WW	T = 44.2–46.5 °C, pH 11; turbidity, 24.9–26.2 NTU; conductivity, 1919–1967 µS/cm; TDS, 962–987 mg/L; color, 810–850 Pt/Co; COD, 762–784 mg O2/L	Mixing speed 100 rpm for 5 min	COD: 26.4% Color: 82.8%	[135]
Inorganic–organic hybrid polymer	Al(OH)3-PAM (aluminium hydroxide-polyacrylamide)	700 mg/L	Dye wastewater (Reactive Blue 19)	1000 mg dye/L pH 5–6	Rapid mixing of 500 rpm for 1 min, slow mixing 200 rpm for 10 min, settling time 60 min	COD: 82% Dye: 90%	[136]
Inorganic–natural hybrid material	ALAV (Aluminium sulphate-Aloe vera)	3000 mg/L	Dye wastewater (Methylene Blue)	10 mg MB /L, pH 6	Mixing speed 100 rpm, settling time 30 min	Dye: 50–55%	[137]
Inorganic–natural hybrid material	MGAV (Magnesium sulphate -Aloe vera)	3000 mg/L	Dye wastewater (Methylene Blue)	10 mg MB /L, pH 12.5	Mixing speed 100 rpm, settling time 30 min	Dye: 60–70%	[137]
Inorganic–natural polymer hybrid materials	PSAF–CTS (Polysilicate Aluminum Ferric-Chitosan)	18.0 mg/L	Wastewater containing heavy metals ((CrO4)2−, Ni2+)	pH 9	Rapid stirring at 150 rpm for 5 min, slow stirring at 80 rpm, settle for 30 min.	Cr6+: 100% Ni2+: 82.2% Turbidity: 99.5%	[138]
Inorganic–natural polymer hybrid materials	CMNP (Chitosan-Coated Magnetite Nanoparticles)	1.5 g/50 mL sample	Aqueous solution containing Pb2+ and Cu2+ ions	0.1 mmol/L, pH 4	Stirring at 100 rpm for 60 min	Lead (Pb2+): 98% Coopper (Cu2+): 98%	[139]
Inorganic–natural polymer hybrid materials	CMCTS-g-P(AM-CA) (Carboxymethyl chitosan—acrylamide—ammonium dithiocarbamate)	50 mg/L	Simulated heavy metal-containing wastewater	25 mg Pb2+/L 25 mg Cd2+/L pH 5–6	Rapid mixing 300–400 rpm for 3–5 min, slow mixing 50–70 rpm for 10–15 min, settling time 15 min	Pb2+: 95.24% Cd2+: 95.72%	[140]
Inorganic–natural polymer composite material	PAFC-Starch-g-p(AM-DMDAAC) (polyaluminium ferric chloride-starch graft co-polymer with acrylamide and dimethyl diallyl ammonium chloride)	0.2 mg/mL dye	Synthetic textile wastewater (Brilliant Blue KN-R)	100 mg/L, pH 3.54, conductivity, 23.7 mS/cm; temperature, 80 °C	Mixing at 120 rpm for 1 min, 80 rpm for 5 min, 30 rpm for 15 min, and settling for 30 min	Dye: 81.22%	[141]
Natural–inorganic composite material	Extract of Moringa oleifera Lam seeds (5 g) in 100 mL of 1M NaCl (1600 mg/L) and KCl solution—1000 mg/L Al2(SO4)3	pH 5–6 and 820 mg/L AS; pH 2 and 2064 mg/L MO-KCl/2774 mg/L MO-NaCl	Real wastewater from industrial laundry containing reactive dyes, RP-HE8B and OP-HER	pH 10.9, color 4500 mg Pt-Co/L, COD, 5820 mg O2/L; turbidity, 66.8 NTU	Mixing for few min and settling for 30 min	Color: 82.2% COD: 83.04% RP-HE7B dye: 78.4% OP-HER dye: 89.7%	[142]
Natural–organic polymer composite material	Lignin-METAC (lignin-[2-(methacryloyloxy) ethyl] trimethyl ammonium chloride)	120 mg/L	Simulated dye solutions (Reactive Black 5)	100 mg/L pH 2–8	Temperature 30 °C, mixing of 150 rpm for 10 min, centrifuged at 1500 rpm for 10 min	RB5: 98% COD: 95%	[143]
Natural–organic polymer composite material	Lignin-METAC (lignin-[2-(methacryloyloxy) ethyl] trimethyl ammonium chloride)	105 mg/L	Simulated dye solutions (Reactive Orange 16)	100 mg/L pH 2–8	Temperature 30 °C, mixing of 150 rpm for 10 min, centrifuged at 1500 rpm for 10 min	RO16: 94% COD: 95%	[143]

, treated textile WWs are characterized with reference to suspended solids content, turbidity, COD-Cr, BOD₅, dyes, and different heavy metal contents. Effective dye, turbidity, color, metals, UV₂₅₄ (WW quality indicator for organics with aromatic rings or unsaturated carbon bonds (double or triple) in their molecular structure that greatly absorbed light), and COD removals via a coagulation–flocculation process were achieved for raw WW samples when hybrid materials were applied due to the higher content of medium–high polymerized species, i.e., Al species in PAC, which are more effective in COD removal at normal or alkaline pH (25 mg/L PAC at pH 8) since the utilized Al_b species show high neutralization abilities (the removal order is as follows: turbidity > color > metal ions > UV₂₅₄ > COD).

The working conditions (pH, stirring speed and time, temperature, and hybrid material dose) and coagulation–flocculation efficiency in the removal of polluting species are summarized in Table 9 for the fourth secondary group of hybrid materials.

The results underline the benefits of the application of hybrid materials as hybrid coagulants/flocculants in colored textile WW treatment due to their high removal rates (dye and color removal > 80%, and for heavy metals, COD, BOD, and turbidity > 40%, even 90% for turbidity removal, etc.).

5. Future Perspectives on the Use of Hybrid Materials in Coagulation–Flocculation Treatment of Colored Textile WWs

WW treatment improvements are always necessary to enable treated WW recycling for freshwater savings and the security/safety of direct discharge into natural receiving basins or receptors, such as natural watercourses, lakes, or groundwater. In this context, the coagulation–flocculation processes of colored textile WWs must be improved through the use of cost-efficient alternatives, some of them including the application of new hybrid materials as coagulants/flocculants. The preparation of low-cost, high-performance hybrid materials with a wide range of functionalization options can improve interactions between colored pollutants and hybrid materials and the formation of larger flocs easily separable via sedimentation/filtration.

A few synthetic polymers (e.g., styrene–acrylonitrile co-polymer) indicated greater potential in colored WW treatment, but their complexity and toxicity, as well as the high cost, have made them less desirable.

Cellulose-based materials that are cheap, reusable, and have longer life cycles are important components for the preparation of new hybrid materials and their application in the treatment of colored WW [164]. Composite materials based on natural polymers (chitosan, alginates, pullulan, etc.) will have higher production costs; however, a more convenient one will be obtained after optimizing the existing production technology. Chemically functionalized nanocomposite materials can improve dye and color removal via coagulation–flocculation steps by adding carboxylic groups. Moreover, the formed flocs can be rapidly separated if magnetic nanocomposite materials are used (magnetite-based hybrid materials) [165].

Some industries produce colored WWs that have high temperatures, consuming greater amounts of time for cooling and normalizing, thus increasing the overall WW treatment time. The application of hybrid materials should be considered in future work, as well as the possibility of regenerating and recycling the residual sludge. Hybrid coagulants should be explored to minimize the residual sludge volume or for their use in concentrated sludge conditioning.

As a result, the application of polymeric composite materials in textile WW treatment is beneficial because of their nature, structure, and versatility. Researchers have proven the advantages of the application of hybrid materials in the removal of various polluting species from industrial effluents (e.g., turbidity, suspended solids, oil, organic matter, heavy metals, color, and COD) and for recycling purposes or the conditioning of concentrated sludge treatment. Under optimal operating conditions, hybrid materials are efficient as coagulants–flocculants compared to conventional inorganic coagulants (electrolytes, such as aluminum and iron salts). These hybrid materials are environmentally friendly because of their cost-efficiency, renewability, adaptability, low toxicity, and residual sludge formation, as well as biodegradability, and are more stable and resistant than standard inorganic coagulants for storage requirements. Additional optimization studies will finalize and select the optimal operating conditions required for the removal of highly polluting species from colored textile WWs.

6. Conclusions

The permanent demand of modern society for water consumption across different industrial and domestic activities involves an increasing requirement for effective facilities that can ensure the treatment of the produced WW for onsite reuse, recycling, and safe/non-polluting discharge of the final effluents to natural aquatic environments. In this review, a few fundamental aspects of WW treatment using different physical, chemical, and biological processes were discussed, with the central goal being focused on the coagulation–flocculation step. Therefore, the role of the coagulation–flocculation step when applied to the treatment of colored textile WW and the advantages and disadvantages of using different chemicals as coagulation–flocculation agents in some industrial WW treatment systems as well as hybrid materials were presented in association with their increased efficiency in comparison to conventional ones. To better understand these hybrid materials in terms of WW treatment, essential aspects were discussed concerning their classification (three primary and four secondary groups) based on their structure and composition, material origin/type, adequate operating conditions, possible mechanisms, and kinetics for the removal of highly polluting species as well as the benefits of using hybrid materials in colored WW treatment processes and future perspectives. All our findings underline the benefits of using natural and synthetic hybrid materials, especially the fact that synthetic hybrid materials possess overall acid-resistant properties and are capable of working in very low pH conditions. Natural or semi-synthetic hybrid materials provide certain advantages based on their biodegradability and ready abundance.

In addition, all of the reported findings are focused on improvements to the quality of WW treatment and improved removal values in relation to polluting species when hybrid materials are applied, as well as the possibility of utilizing only one continuous stirring speed and contact time and the insignificance of pH variation related to monomeric and low-polymerized conventional coagulants–flocculants applied to the treatment of colored textile WW. This work will be continued with new data and information on the performance of other hybrid materials in WW treatment processes and new issues on the control, modeling, and process optimization of coagulation–flocculation using hybrid materials for very good removal performance.