Optimization of a Textile Effluent Treatment System and Evaluation of the Feasibility to Be Reused as Influents in Textile Dyeing Processes

Abstract

Textile effluents derived from azo-reactive dyeing processes represent a severe problem for aquatic ecosystems and human health. The large amounts of water used in this process and the poor quality of the discharges urge the need to develop treatment systems that involve reusing treated water. In this research, we present the optimization of a feasible, simple, and efficient treatment system that improves the quality of the effluents from the cotton fabric dyeing process. Through the characterization of the influents and effluents, we have identified seven parameters that have allowed the optimization of the treatment. Analytical techniques, such as nephelometry, EDTA, gravimetry, and BOD5, among others, and specialized equipment, such as the spectrophotometer, have been used for these purposes. The results showed that using combustion gases in the neutralization stage and new flocculant-coagulant reagents improved parameters, such as pH, total solids, hardness, and conductivity. The quality of the effluents thus obtained allowed their reuse only in the stages before the dyeing bath without affecting the final quality of the cotton fabrics in dark colors. This effort implies savings in water and supplies, and opens the door to future research on the treatment of textile effluents that help improve the environmental conditions of our region.

1. Introduction

The scarcity of safe water has been increasing yearly and is affecting more regions worldwide. According to data from the Water Organization, nearly 800 million people do not have access to safe water, while 1.7 billion people lack access to adequate sanitation [1]. It is estimated that by 2050, water scarcity could affect 75% of the world’s population [2]. Several factors have increased water stress, among which agricultural activities stand out. Studies regarding this liquid vial have mentioned that about 70% of the water extracted worldwide is used in these activities [2,3]. However, other productive activities also have a high demand for this resource. An example is the manufacturing industry, whose global water demand is 5% and is expected to increase to 20% by 2050 [4].

This high demand for water resources from these industries has generated a large volume of wastewater [5,6]. In particular, the textile industry is considered one of the main sewage sources [7,8,9], producing around 20% of all polluted industrial effluents worldwide [10,11]. The large volume of these effluents is because most stages involved in dyeing and finishing fabric treatments use water as a primary input. In addition, the quality demands of processes require that operating baths are used only once, so the water is changed several times, especially in the washing and rinsing stages [12]. Consequently, it is estimated that an average-sized textile mill can use between 70 and 200 m${}^3$ for each ton of fabric dyed [7,13,14]. Unfortunately, these textile effluents often contain a wide variety of chemicals (dye residues, solvents, metals, salts, detergents, etc.) that are highly harmful to aquatic life and human health when discharged without proper treatment [14,15].

The water quantities used in dyeing processes depend on various factors, such as the amount of material processed, the type of equipment, and the finish of the fabrics, among others [16,17]. However, the fiber kind and the physicochemical properties of the dye will determine the process stages required [18,19,20,21]. Different types of fibers are used as raw materials in the textile industry, either synthetic (nylon, polyester, etc.) or natural (wool, cotton) [18,22]. Nevertheless, cellulosic fibers, such as cotton, are the type of fibers that require the most significant amount of water in the dyeing process. In a study by Shaikh, the author compared the water volume needed for wet finishing operations of different fibers [16]. His results showed that, on average, for a ton of fabric in the dyeing process, cotton required a maximum of 300 m${}^3$ of water. In contrast, most synthetic fibers (rayon, nylon, acrylic, and polyester) needed a maximum of 34 m${}^3$, and only acetate fiber required 50 m${}^3$.

It is estimated that, on average, 80% of the water used in textile processes is discharged as effluents into the sewage network [19,23]. These large volumes of wasted water need to be addressed with the development of new treatment techniques that contemplate its reuse without affecting the quality of the final product. Therefore, many research groups have been tasked with developing treatment methodologies that are increasingly safer and more environmentally friendly. These methodologies range from biological treatments (microorganisms, bacteria) [14], conventional (adsorption) [24,25], and, recently, nanotechnology and molecular modeling that allows focusing the problem according to the type of dye used [26].

During discharges, color is the most distinctive characteristic of textile effluents. Nevertheless, although it depends on the dye used, the color of water bodies has adverse effects on aquatic ecosystems [27]. Even in small amounts, dyes can causes severe affectations, such as: preventing the entry of sunlight, inhibiting the photosynthesis of aquatic organisms, affecting the solubility of gases, causing a low level of oxygen in the water, and increasing the metabolic stress of fauna [27,28,29]. Due to this, one of the main targets in designing and manufacturing textile dyes is that they are ecological and non-toxic [30,31]. Unfortunately, for the dye to be fixed on the fibers, a series of chemical reactions must occur that can give rise to contaminating products harmful to aquatic life and human health [28,32,33].

There is a vast range of dyes on the market, including azo-reactive dyes, which are widely used in the textile industry and industries such as paper, drug, food, etc. [34]. About 60% of textile plants use these types of dyes in the cellulosic fiber sector due to their final dyeing quality, color varieties, and ease of application [14,31,35]. Nevertheless, the most significant disadvantage of these dyes is their poor level of fixation in the fabrics (close to 50%), which means a large part of them are discarded in the effluents [14,21]. To solve the fixation problem, chemicals, such as inorganic salts, are added to reduce the dye-fiber repulsion and, thus, improve the percentage of exhaustion in the process [30,36]. However, this generates more significant ecological problems (soil alkalinization, high water conductivity, and so forth) and increases effluent treatment costs [37].

Due to their reactive nature, high hydrolyzability, and low biodegradability, effluents containing azo-reactive dyes are very difficult to treat [32,38]. Therefore, different types and treatment methods have been proposed to reduce the presence of these dyes in effluents [7]. These treatments can be physical (filtration techniques, adsorption, etc.), chemical (oxidation, ozonation, chemical or UV degradation, cavitation, etc.), biological (degradation of matter by microorganisms or algae, microbial fuel cell, etc.), or combination of these processes [7,39,40,41,42]. In particular, coagulation-flocculation methods are widely used to reduce the coloration of effluents since they optimize processing times, effluent quality, and treatment costs [35,43].

One of the significant tasks in developing textile effluent treatment systems is being able to reuse the treated water within their processes. This would allow optimizing resources and significantly reduce the amount of water used in its processes [44,45]. Thus, this study aims to evaluate the quality of the treated water obtained by optimizing a textile effluent treatment system. Through a neutralization process based on combustion gases and new flocculant-coagulant reagents, it has been possible to reuse the effluents in various stages of the dyeing process without affecting the quality of cotton fabrics dyed in dark shades.

2. Materials and Methods

2.1. Study Area Description

The present research work was carried out in the facilities of the textile company identified by the code FRSA-01. The company is located in the District of Yanahuara in Arequipa, in southern Peru. The treatment plant for effluents is within the facilities derived from the dyeing processes. The company works with cotton fiber fabrics and uses reactive dyes for its dyeing processes. Once the water process is transformed into an effluent, it has a collection and temporary storage circuit prior to its acidification to neutralize its alkalinity. Subsequently, flocculant-coagulant products are added to achieve clarification in the sludge settler. Finally, the treated effluent is emitted into the municipal sewerage network once it is verified that it meets the maximum allowable values (MAV) for non-domestic discharges, according to Peruvian Supreme Decree 010-2019-VIVIENDA [46].

2.2. Water Sample Collection

All samples were collected in the company’s dyeing process. The sampling consisted of three stages to characterize both the influents and the effluents of the process. In the first stage, influent and effluent samples were collected without treatment. Only the effluents treated with the previous methodology were sampled in the second stage. Finally, for the third stage, only samples were obtained from the effluents subjected to the proposed new treatment. These effluents were reused in a specially designed dyeing process line to determine the feasibility of their reuse.

The samples were collected in previously sterilized polyethylene or glass containers of different volumes depending on the analysis performed. The containers were transported inside thermal boxes and stored at 4 ${}^{°}$C for subsequent laboratory analysis. All tests were performed at the Testing and Quality Control Laboratory of the Universidad Católica de Santa María in Arequipa, Peru.

2.3. Physicochemical Parameters and Methods of Analysis

The standard quality norms, equipment, methodology, volume used in the tests, and the storage conditions used to characterize the samples are included in

Table 1. Applied methodology in the characterization of influents and effluents in the fabric dyeing process.

Parameter	Methodology and Equipment				Sampling Requirements
	Norm	${}^{\mathit{a}}$ Method	Equipment	Unit	Container	Sample vol.	Preservation/ Concentration	${}^{\mathit{b}}$ Max. SL
physicochemical characteristics
Temperature	-	Direct measuring	Multimeter	${}^{°}$C	P or G	25 mL	Inmediatly	15 min
pH	-	Direct measuring	Multimeter	-	P or G	100 mL	Inmediatly	15 min
Conductivity	-	Direct measuring	Multimeter	$\mathsf{\mu }$s/cm	P or G	500 mL	Cool at 4 ${}^{°}$C	28 days
Total solids	AOAC Official Method 920.193	Gravimetric, Dried at 110 ${}^{°}$C	Analytical balance	mg/L	P or G	100 mL	Cool at 4 ${}^{°}$C	2–7 days
Dissolved Solids	AOAC Official Method 920.19	Gravimetric, Dried at 180 ${}^{°}$C	Analytical balance	mg/L	P or G	100 mL	Cool at 4 ${}^{°}$C	2–7 days
Suspended solids	NMX-F-527-1992	Gravimetric, Dried at 110 ${}^{°}$C	Analytical balance	mg/L	P or G	100 mL	Cool at 4 ${}^{°}$C	2–7 days
Turbidity	APHA-AWWA-WEF part 2000 method 2130-b	Nephelometric	Turbidimeter	NTU	P or G	100 mL	Cool at 4 ${}^{°}$C	24 h
COD	NMX-AA-030-SCFI-2001	Open Reflux, Colorimetric	Laboratory, Spectrophotometer	mg/L	P or G	100 mL	Cool at 4 ${}^{°}$C, H${}_2$SO${}_4$, pH < 2	28 days
Ammonia nitrogen	NMX-AA-026-SCFI-2001	Distillation, Titration	Analytical or Granataria balance, Potentiometer	mg/L	P	2000 mL	Cool at 4 ${}^{°}$C, H${}_2$SO${}_4$, 1.5 < pH < 2	7 days
Sulfates	NTP 214.023.2000	Turbidimeter	Turbidimeter	mg/L	P or G	100 mL	Cool at 4 ${}^{°}$C	28 days
Sulfides	EPA 376.1	Volumetric	Laboratory	mg/L	P or G	100 mL	${}^c$ Cool at 4 ${}^{°}$C	7 days
Oil/Grease	Method SM 5520 E	Gravimetric-Extraction	Laboratory, Soxhlet extractor system	mg/L	G	500 mL	Cool at 4 ${}^{°}$C, H${}_2$SO${}_4$, pH < 2	28 days
Total hardness	NTP 214.018.1999	EDTA complexometric method	Laboratory	mg/L	P or G	500 mL	Cool at 4 ${}^{°}$C, HNO${}_3$, pH < 2	6 months
ASMB	NMX-AA-039-SCFI-2001	Colorimetric	Laboratory, Spectrophotometer	mg/L	P	600 mL	Cool at 4 ${}^{°}$C, H${}_2$SO${}_4$, pH < 2	7 days
Metal traces
Metals	EPA 200.7	ICP	ICP-AES	mg/L	G	200 mL	Cool at 4 ${}^{°}$C, HNO${}_3$ (for P, H${}_2$SO${}_4$), pH < 2	28 days
Microbial activity
BOD	NMX-AA-028-SCFI-2001	DBO${}_5$ at 20 ${}^{°}$C)	Incubator	mg/L	P or G	1000 mL	Cool at 4 °C	48 h
Total Coliforms	APHA, AWWA, WPCF, 9221 Method B	Technique, Fermentation	Laboratory	NMP/100 mL (35 ${}^{°}$C)	P or G	100–500 mL	${}^a$ Cool at 4 °C	24 h

^a Sodium Thiosulfate is added if the water contains residual chlorine. ^b Maximum Shelf Life. ^c Add four drops of zinc acetate 2 N/100 mL; add NaOH until pH > 9. P = Polyethylene, G = Glass. COD = Chemical Oxygen Demand. ASMB = Active Substances to Methylene Blue. BOD = Biological Oxygen Demand.

.

2.4. Fabrics Dyed Assessment

The dyeing quality was obtained using a DATA Color double beam spectrophotometer using the CIELab color space system. This system is three-dimensional, whose coordinates are luminosity (L, L = 0 indicates greater darkness, =100 greater luminosity); the green/red coordinate a, − = green, + = red), and the blue/yellow coordinate (b, − = blue, + = yellow). The total color, $\Delta E$ difference was calculated according to the following equation:

$$\Delta E={(\Delta L^2+\Delta a^2+\Delta b^2)}^{0.5}$$

(1)

where $\Delta L$, $\Delta a$, and $\Delta b$ are the differences between the sample and reference color values. The reference values were those obtained on fabrics treated with clean influent.

Four colors were used for the dyeing test, three from the light gamut (antique, pink, aqua) and one from the dark gamut (atlantic). The Pantone codes for these colors are 220011, 240220, 260345, and 276531, respectively.

3. Results and Discussion

3.1. Determination of the Process Parameters to Be Optimized

FRSA-01 company facilities include cotton fabric weaving, dyeing, and cutting plants. The dyeing plant has 13 baths: 7 unit-process baths (scouring, neutralizing, biopolishing, dyeing, soaping, and softening) and 6 rinsing baths (Figure 1a). In addition, as mentioned above, the company has a small effluent treatment plant in order to comply with environmental regulations and discharge effluents into the municipal sewer system (Peruvian Supreme Decree 010-2019-VIVIENDA [46]). This treatment consists of only two stages: a filtration stage for retaining cotton fiber and solid materials, and another for neutralization and flocculation of the effluent (Figure 1b).

As a first step to optimizing the treatment of effluents in the dyeing process, different parameters in which the previous treatment was inefficient according to environmental standards were determined. For this, both the influent and effluents without treatment and treatment were characterized. These parameters were divided into the physicochemical, presence of metals, and microbial activity, and they are shown in Table 1. The results obtained are represented in Figure 2. For the physicochemical parameters, an essential difference can be observed in conductivity, suspended solids, turbidity, and hardness (Figure 2a). In addition, for both conductivity and suspended solids, the treatment improves the quality of the effluent by 83.8% and 23.1%, respectively. However, for the turbidity and hardness parameters, the treatment considerably increases the discharge values compared to the effluent without treatment (137.0% and 562.2%).

The effluent characterization regards the metal trace values (Figure 2b) showed high values for selenium, potassium, manganese, and aluminum. These last two have much higher values than effluent without treatment, 4553.6% and 2686.0%, respectively. Finally, for microbial activity (Figure 2c), the increased presence of coliforms in treated effluent indicates high contamination of the treatment equipment or tanks. These results showed that the most critical characteristics present in the effluents, both the dyeing process and the one obtained after the first treatment, are the presence of solids, the alkalinity of the effluents (mainly in the untreated effluent), and the turbidity or opacity.

These analyses confirmed that the previous treatment was no longer adequate for several of those parameters and could compromise environmental discharge requirements. Therefore, in order to determine the target properties to be used in the optimization of the treatment system, two fundamental facts were taken into account. First, the limitations of the study would be governed by the needs and possibilities of the company in terms of the process, facilities, and budget. Secondly, and according to one of the main objectives of this study, these properties should have been used in other research regarding the reuse of treated textile wastewater. Consequently, seven properties were chosen based on these two points: suspended and total solids, temperature, pH, absorbance, total hardness, and conductivity.

Results obtained in monitoring these properties can be seen in

Table 2. Parameters used to optimize the effluent treatment from the dyeing process.

Parameter	Unity	Influent	Effluents
		Soft Water	Untreated	Previous	Optimized
T. Suspend Solids	mg/L	0.0	312.0	240.0	-
Temperature	${}^{°}$C	34.6	39.9	26.9	26.2 ± 1.4
pH	-	6.5	10.5	6.5	7.3 ± 0.44
Total Solids	mg/L	440.0	2080.0	5644.0	4900.0 ± 1560.0
Absorbance	-	0.014	3.02	1.5	0.6 ± 0.4
Total Hardness	mg/L	<0.01	26.7	176.8	12.9 ± 3.9
Conductivity	mS/cm	0.6	54.0	9.2	5.3 ± 2.3

. In the following sections, it will be detailed each of these properties.

3.2. Optimization of the Effluent Treatment System

The dyeing process in this plant consumes an average of 10 L of water per kilogram of dyed fabric in each process bath. This consumption means 130 L in a total of clean water considering the respective rinses, so the optimization and reuse of effluents were deemed critical in the operation of the plant. Therefore, the proposed system considered a series of improvements in facilities, civil works, and the conditioning of the infrastructure with the existing equipment. As seen in Figure 3a, in the previous treatment plant, the coagulant-flocculant agents were added to the effluents in a small well and taken to a channel for mixing. Once mixed, they were taken to the sedimentation wells, where the solids were trapped, and the effluent was discharged to the municipal sewerage. In the new treatment system, the treatment circuit was redesigned considering the new effluent neutralization process. These improvements allowed a higher level of mixed with the coagulating-flocculant agents, taking advantage of the gas pressure in this new well (Figure 3b). In addition, new pre-sedimentation and sedimentation wells were added, reaching laminar velocities for better sedimentation of the flocs. Figure 3c shows the flow diagram of this new treatment system.

3.2.1. Suspended Solids

One of the problems detected was a large amount of solids in the effluents. The total suspended solids (TSS) readings reached values of 240 mg/L, while for the total dissolved solids (TDS), the recorded value was 5404 mg/L. These solids, composed mainly of yarns, fibers, and particles of chemical additives (molecules and ions) used in the dyeing process, are common in textile effluents [47,48]. Moreover, the TDS is related to the salinity and conductivity of the wastewater, while TSS is related to turbidity [49,50]. In the case of high TDS concentration, a future bioremediation treatment using bacteria is planned for this purpose [51,52]. It has demonstrated that using bacterial strains offers the advantage of removing heavy metals [53,54], another of the problems that this dyeing process has and has been studied and implemented in other works by our research group [55,56]. Due to this, in this research work, only the improvement in the treatment of TSS is presented.

Being larger, TSS is usually removed by physical means or sedimentation. In the previous treatment, a “Y” type filter was used, which offers several advantages: high operational pressure (up to 200 psi), temperature range from −29 to 65 ${}^{°}$C (effluent outlet is close to 30 ${}^{°}$C), and the ability to clean it without the need to interrupt the process. The diameter size of the filter used in this treatment is 4.8 mm. In order to improve the filtration process, a second “Y” type filter was implemented in series (Figure 4a), but now with an orifice diameter of 1.6 mm. The results in solids retention are shown in Figure 4b.

The results show a critical operational improvement of close to 51.8 ± 6.8% on average. Another important fact is that 34.0 ± 3.1% of the total solids are retained in the second filter, which shows the importance of reducing the diameter of the filter. Although it is a minor change, the values obtained increased the quality of the effluent and avoided the wear of inputs in the later stages of the treatment.

3.2.2. Temperature and pH

Using chemical agents in the dyeing and finishing process resulted in a high fluctuation in the pH values of the wastewater [57]. The dyes used in our dyeing process require fixing agents that work in alkaline conditions. Consequently, high pH values (≈11.0) were registered, so it was necessary to neutralize them with acidifying agents. It is common to use mineral or organic acids origin for this process. However, its use implies costs of production, equipment, and management, among other disadvantages, and may also be associated with other methods that damage the environment [58].

In the previous treatment, acetic acid (CH${}_3$COOH) was used as a buffering agent for the effluents. However, several alternatives for this process are currently more eco-friendly and efficient. One of these alternatives is using combustion gases to neutralize alkaline wastewater. This process has been tested in various industries, including textiles, with excellent results [58,59,60,61,62]. One of its main advantages is the reduction in the emission of polluting greenhouse gases, such as CO${}_2$, which can be reduced by up to 15,000 tons per year [59]. It should be remembered that in the dyeing process, the influent is heated until it reaches the appropriate operating temperature according to the process bath or rinses. Therefore, implementing this technology eliminates two problems in the plant: it reduces contamination by combustion gases and helps neutralize the effluent without additional energy consumption. The corresponding civil works were carried out to implement this technology in the dyeing process, and the Centrox Aerator equipment (FUCHS Enprotec GmbH, Stocktal 2, Mayen, Germany) was purchased (Figure 5b. This equipment is responsible for subtracting the flue gases produced by the industrial boiler, which uses LP gas as fuel, and diffuses them in the neutralization tank.

Nine sampling points were established over all the layout stages to assess the efficacy of the treatment regards the pH parameter (Figure 5a). Moreover, in this monitoring, temperature variations were also measured to ensure the effluent did not heat up due to flue gas energies. Results can be seen in Figure 5c,d. The results show that the effluent temperature does not increase despite the injection of the flue gases. The variation range was 6.8 ± 1.4 ${}^{°}$C throughout the treatment, ensuring the systematization of the process (Figure 5c). The average temperature reached by the new treatment was 25.5 ± 1.5 ${}^{°}$C, with the average temperature of the influent being 34.6 ${}^{°}$C.

Regarding the pH, the efficiency of the process is evident. In point 1 (P1), there were decreases of up to four units. The pH measured between P1 and P2 was 7.1 ± 0.5 on average. It was also observed that this value is maintained throughout the treatment. Noteworthy, the specified pH value by the supplier was 8.5 at this stage, which shows the high efficiency of this neutralization process. If the measurement is made at point P8, the average value is 7.3 ± 0.5. These data were essential when considering the reuse in the dyeing process since the initial pH of the influent using soft water is 6.5.

3.3. Effluent Decolorization Process (Total Solids and Turbidity)

The main characteristic of textile effluents is their coloration. Thus, various methods are used to decolorize effluents: filtration, absorption, oxidizing agents, and advanced oxidation process (AOP), among others [63,64]. However, the coagulation-flocculation process is still widely used due to its efficiency, low cost, and benefit, mainly by small and medium-sized industries [65]. Due to the infrastructure conditions of the FRSA-01 plant, this coagulation-flocculation process was used in the previous wastewater treatment. As improvements to the treatment, it was decided to build new compartments that would allow the separation of the flocs by flotation or sedimentation more efficiently (Figure 6b, P3). The new pre-sedimentation well, with a volume of 20.4 m${}^3$, allowed reaching laminar flow conditions, leading to more significant action of the coagulating and flocculant reagents. It was also decided to change these chemical reagents for higher-performance others.

Two new flocculating agents, Ferrocryl^®7276 and Chemlok^®2040, belonging to the polyacrylamide family, were used to optimize the effluent decolorization process. Ferrocryl agent is used as a cationic flocculant, while Chemlok is an anionic flocculant. Both are used in different applications in treating industrial effluents from dyeing industries, especially in waters with high concentrations of ionic particles and pH between neutral and alkaline [66,67,68,69]. Finally, aluminum sulfate, Al${}_2$(SO${}_4$)${}_3$, was used as a coagulation reagent [70,71]. Three tanks with a semi-automated dosing system were used to add these reagents to the treatment. Each tank has an interconnection to a radar, activated when the effluent discharges into the pre-sedimentation well, sending a signal to a pump to start dosing the chemicals. The dosing points were chosen to seize some process characteristics. Thus, for the Ferrocryl agent, the dosage was carried out at the effluent inlet to take advantage of the turbulence generated by the discharge and allow the flocculant agent to mix (point P0). For the Al${}_2$(SO${}_4$)${}_3$ and Chemlok reagents, both dosing points were set in the neutralization tank (point P1). The dosage was carried out at the CO${}_2$ diffusion point for the first reagent, while for the second reagent, the dosage was close to the aerator to seize the generated turbulence (Figure 6b, P0, and P1). The doses and working parameters of each reagent can be seen in

Table 3. Summary of working parameters of the coagulation-flocculation reagents.

Reagent	Quantity (kg)	Volume (L)	Dosage (L/h)	Stirring Speed (rpm)	Contact Time (min)
FERROCRYL®7276	9.0	60.0	1.9	100	5
Al${}_2$(SO${}_4$)${}_3$	6.8	225.0	28.1	100	10
CHEMLOK 2040	0.19	375.0	46.9	30	20

.

The amount of total solids (TS) and turbidity were determined to assess the capacity of the new treatment to decolorize the dyeing effluents. These two properties are directly related to coloration, so absorbance was calculated as the variable to determine the effectiveness of the treatment. Furthermore, some reports mention that values above 10 mg/L in TS cause stains and unevenness in the fabrics [72]. On the other hand, it has been determined that an influent has staining quality if it does not exceed 10 mg Pt-Co/L, equivalent to an absorbance reading of 0.015 [73]. Therefore, the same sampling points were used to calculate these properties (Figure 6b).

It can be observed in the TS graph (Figure 6a) a marked increase in point 2 due to the use of the new coagulant-flocculant products. At this point, the most significant amount of sludge and solids skimming is generated, which the sedimentation traps retain. At point P1, it can be seen that the TS concentration is relatively low. This decrease may be due to the turbulence caused by the confluence of the flue gases (CO${}_2$ intended for the neutralization process) and the entry of the alkaline effluent current. In addition, the centrifugal force caused by the neutralization venturimeter leads to many solid particles being thrown toward the walls of the well, where they remain attached. Finally, it is up to point P3 where the coagulant-flocculant effect of the new chemical products intended for it can be measured. From this sampling point, the TS concentration maintained an average of 4900 ± 1560 mg/L. Finally, the action of the new coagulant-flocculant agents can be appreciated at the lamellar sedimentation well (Figure 6b, points 4–8).

Figure 6c shows the difference between the values obtained in the new treatment with those obtained in the influent (soft water, 440 mg/L) and those recommended by Costa et al. (<10 mg/L) [72]. Taking the TS value of soft water as a reference, the treated effluents exceed the amount of TS on average by more than 1000%. However, this research aims to evaluate the possibility of reusing this treated water in the dyeing process so that this process is susceptible to future improvements.

Regarding the absorbance (Figure 7a), it is observed that at the beginning (point P0), the average was 3.02 ± 1.12, the point of greatest coloration. At points P1 and P2, the action of the coagulant-flocculant agents can be observed when the minimum values are obtained. On average, at the end of the treatment (point P8), the absorbance reached a value of 0.59 ± 0.36, again, very far from the optimal value (0.014) for the dyeing process (Figure 7b). However, similar absorbance values have been observed in treatments with other coagulants or flocculants [65,74] and even using different methodologies for treating textile effluents [75,76].

3.4. Total Hardness and Conductivity

The ion concentrations of magnesium (Mg${}^{+2}$) and calcium (Ca${}^{+2}$) cause problems of scale formation in the pathways through which they pass (equipment, pipes, etc.) so that, as far as possible, they must be eliminated in their entirety [72]. Although the ideal is to reach a total hardness equivalent to 0.0 mg/L (soft water) in the effluent, values even lower than 20.0 mg/L are acceptable in practice. Figure 7c shows the evolution of total hardness through the different sampling points. The average value reached by the treatment was 12.93 ± 3.96 mg/L. Considering that flocculating and coagulating agents were also used in the previous treatment, the results suggest that the decrease in total hardness could be due to the use of combustion gases in the neutralization process. Furthermore, Choi et al. demonstrated that their use could reduce calcium hardness by up to 90% in alkaline water treatment [59]. On the other hand, Zheng et al. decreased up to 64.1% the concentration of Ca${}^{2+}$, Al${}^{3+}$, and Cl${}^{-}$ ions in wastewater from dyeing processes [62]. However, further studies are necessary to confirm our hypothesis.

On the other hand, conductivity indicates the concentration of dissolved salts within the effluent. Values above 2.00 mS/cm cause dyeing abnormalities [72]. However, the process with reactive dyes uses large amounts of NaCl as an electrolyte in neutralizing charges to achieve good finishing in cotton fabrics [72]. The new treatment reached an average value of 5.32 ± 2.26 mS/cm (Figure 7d). For reference, deionized water has a conductivity of 0.0055 mS/cm, drinking water between 5.00 and 50.00 mS/cm, and seawater between 5000.00 and 6000.00 mS/cm [77]. This conductivity value means the process conditions would be similar to dyeing with drinking water. In addition, since NaCl is added to the baths in some stages of the dyeing process, it was considered that these conductivity values could be favorable in the reuse of treated effluents.

3.5. Fabric Dyeing Using the Treated Effluent as Influent

The dyeing process at the FRSA-01 plant consists of 13 baths: 7 unit processes and 6 rinses (Figure 1a). Each bathroom has its independent water intake and outlet. In addition to the optimization of effluent treatment, this research was aimed at the reuse of this treated water in order to make the most of water resources. The dyeing quality was assessed according to the color properties obtained from the CIELab space parameters [78]. This methodology is widely used in the textile industry to determine the tolerances required by customers. In addition, it can determine the quality of the treatment used in effluents that will be reused in the process lines [79,80,81]. Therefore, various tests were carried out to evaluate which baths the treated water could be used, affecting the dyeing finish as little as possible. In the present work, four of these tests are presented, which were the ones that obtained the best results in fabric finishing. In all of them, the treated water was used in the first baths of the dyeing process. Our results showed that the dyeing lost quality considerably by involving more stages.

Four dyes of shades from light to dark were used to carry out the tests and understand the effect of the reuse of effluents on the quality of the dyeing. It should be mentioned that cotton fabrics undergo standard treatment with NaOH and other surfactants for the scouring stage. However, when a medium or light shade color is dyed, the substrate is subjected to an additional wash with H${}_2$O${}_2$, called half-bleaching. This process was also not altered in performing the dyeing tests.

The first two series tests (A0 and A1) used treated water as an influent, as obtained from the treatment plant. Therefore, the difference between A0 and A1 consisted of the number of baths used in these tests (

Table 4. Dyeing process stages in which the reuse of treated effluents was considered.

Process Conditions	Process Bath
	1	2	3	4	5	6	7	8	9	10	11	12	13
	Scouring	Rinse	Neutralizing	Biopolishing	Rinse	Dyeing	Rinse	Rinse	Soaping	Soaping	Rinse	Rinse	Softening
A0
A1
A2
A3

). On the other hand, for the A2 series tests, the effluent was passed through the treatment twice to obtain lower turbidity of the treated water. Thus, these tests used the treated water in the first six process baths. Finally, in the A3 tests, twice-treated effluent was used again and mixed with soft water in a 50:50 ratio in the first five process baths.

Figure 8 shows the results obtained on the cotton fabrics and the values obtained from the CIELab parameters. In the case of the A0 tests, it can be seen that the finish of the fabrics is opaque when light dyes are used. Due to this, the $\Delta L$ (luminosity) and $\Delta a$ (green-red) parameters present the highest values of all the tests. On the other hand, in the A1 tests, the finish improves, although opaque tones continue to appear. Not including treated water in the dye bath improved the $\Delta a$ and $\Delta b$ (blue-yellow) parameters, but $\Delta L$ is increased, which enhances the ($\Delta$E) values. In contrast, in both tests, favorable results were obtained in all the parameters for the dark color, obtaining values close to the target color.

On the other hand, the tests carried out with the effluent treated twice showed a significant improvement over the previous tests, especially in A3. Interestingly, this double treatment of the effluent for the pink dye does not improve the total color parameter ($\Delta$E) since its values increase. This is because the parameter $\Delta a$ is increased by 150%, despite the visual improvement of the fabrics. Contrarily, in the case of the atlantic dye, it again presents the best values, both visually and in its CIELab parameters. It is noteworthy that in both situations, using the treatment once or twice, not including treated water in the dye bath, improved the finish of the fabrics and CIELab values.

Some studies consider that the total deviation of $\Delta$E should not exceed a value of 1.02 [80,82]. However, these tolerances depend mainly on customer requirements and may vary. Therefore, tolerance of $\Delta$E ≤ 5.00 was considered for this evaluation. The results showed that in cases A0 and A1, they did not meet the tolerance in light colors, obtaining values between 5.05 and 16.3. Only the atlantic dye was in optimal values. On the other hand, except for the pink color in test A2 ($\Delta$E = 9.95), the rest of the tests for A2 and A3 did meet this specification.

Once the $\Delta$E tolerance has been obtained, it is necessary to set the tolerances for the $\Delta L$, $\Delta a$, and $\Delta b$ parameters [78]. For these parameters, the results showed that the best quality of the dyed was found in the antique and atlantic dyes, both in A3.

Figure 9 shows the results of the values obtained in the CIELab parameters graphically. In the case of the luminosity ($\Delta L$) and the green-red hue ($\Delta a$) parameters, it is evident that for the dark shade, there are more minor deviations concerning the target color (Figure 9a,b). It is also observed that the values improve when the dyeing is done by taking the doubly treated effluent. This behavior is repeated for the blue-yellow hue ($\Delta b$), except for the pink dye values in A2 mentioned above (Figure 9c).

Finally, regarding the total color deviations ($\Delta$E), a large dispersion is observed using light dyes when the dyeing is performed. This parameter fluctuated from 0.94 to 16.30, which would limit the use of treated effluents in this range of colors. However, visually and in the $\Delta$E evaluation (1.02 ± 0.8), the antique tint showed promising results in tests A2 and A3 (Figure 9d). Undoubtedly, the atlantic dye (dark hue) was the one that showed the best results concerning the accurate color in the tests carried out. With a value of $\Delta$E = 1.27 ± 0.27, both with a single treatment or with two, the dyeing process could be carried out without significantly affecting the finished quality of the fabrics.

4. Conclusions

Reusing textile effluents in one or several process stages can lead to substantial savings in water consumption, especially in an industry characterized by high consumption of this vital liquid. In this work, we present the optimization of a treatment system that has allowed us to reuse the treated water during the dyeing process of cotton fabrics. Changes in infrastructure and flocculant-coagulant reagents have allowed us to improve several key physicochemical properties to reuse effluents. In particular, building a neutralization well that uses combustion gases as a source of neutralizing reagents has led to optimizing the process without additional energy costs. Noteworthy, the effect of the neutralizing process could affect other properties besides the pH based on comparing the values of the properties evaluated concerning the previous treatment. However, more advanced studies are necessary to reaffirm this observation.

On the other hand, the evaluation of the use of treated water in the stages before the dyeing bath demonstrated the effectiveness of the treatment by not affecting the dyeing finish on dark-colored fabrics. These results are encouraging because there are still essential parameters to improve in textile effluents. This will undoubtedly impact the better quality of the treated water and, therefore, the prospects for its reuse in the process stages, even after the dyeing bath. Although there are still things to be done to improve the treatment presented, we hope these results will open up future research on this topic.