About 97% of the world’s water resources are salty, which makes the water undrinkable, and less than 1% water is potable. Unfortunately, the volume of the potable water used in industry and farming is still increasing, and numerous regions on our planet suffer from a water deficit [1
]. Moreover, many industrial branches produce salty wastewater that contains a huge concentration of low-molecular-weight salt (LMWS), like NaCl and Na2
. Therefore, a large amount of LMWS is emitted into the environment, which disturbs living conditions in the biosphere [2
As far as salty wastewater is concerned, the textile industry is one of the greatest polluters. LMWSs are commonly used in dyeing processes as auxiliaries—even 1.5 kg of LMWS per 1 kg of textiles can be used in these operations [4
]. It should be kept in mind that during industrial dyeing by reactive dyes, the application of LMWS is inherent. LMWSs are remediating agents, which increase a dye distribution from the dyebath into the textile material by changing the equilibrium conditions of the dyeing liquor [8
]. Therefore, LMWS cannot be eliminated from a typical exhaustion-fixation dyeing process. Consequently, thousands of tons of LMWS are used daily by textile manufacturers [2
]. The regions of huge areas, like the Panjab region in India, have been polluted by the emission of textile wastewater released into the environment, without any previous purification, which increases general salinity of surface water [3
It is difficult to find an efficient and inexpensive treatment method that could be used to clean wastewater polluted by LMWS. Especially, biological treatment, which is the most commonly used method, could be severely affected by a high concentration of LMWS. The use of membrane filtration methods could be an option to deal with salinity; however, among them, reverse osmosis (RO) is the only method that can eliminate LMWS from wastewater. On the other hand, RO is not suitable for COD removal from wastewater because of fouling. The use of microfiltration (MF) or ultrafiltration (UF) as a pretreatment step before nanofiltration (NF) and RO is possible, but this solution cannot eliminate the fouling problem entirely. Another problem is the huge volume of highly contaminated concentrate and backwashes. At present, the operating cost of membrane filtration is still high when industrial wastewater treatment is concerned [9
An application of ozone as a non-discharge treatment method can be an alternative for the purification of textile wastewater contaminated by LMWS. In contrast to membrane filtration, ozonation cannot remove LMWS but can serve as a technique for brine recovery from wastewater. Ozone oxidation can give good results when many poorly degradable contaminants, including textile dyes, are taken into consideration [9
]. It can be concluded that the effectiveness of dye decomposition by ozone treatment has been proven in many papers, and the field seems to be well covered by the literature (Table 1
). However, it should be noted that only a few authors have dealt with by-products detection [11
], scaling-up [16
], or purified wastewater recycling [17
]. A publication that includes all these topics has not been found.
The main objective of the study presented in Part 1 was to investigate the recycling of purified wastewater. For this purpose, a highly contaminated industrial textile wastewater (the bath after the reactive dyeing of cotton, with a residual LMWS concentration equal to 30 g/L) was treated by ozone to remove color and then recycled as a source of ‘ready to use’ brine for the next dyeing. Therefore, the key idea was not to treat the LMWS as a pollutant, but as a resource that can be recycled. Consequently, the challenge was to keep LMWS concentration at a constant level, while the dye residuals were removed. To this end, the same wastewater was recycled four times (five cycles of fabric dyeing and four cycles of ozonation, carried out in a laboratory scale). The additional goal was to analyze the by-products that could have accumulated due to recycling, and their potential influence on the possibility of brine recycling.
The substances used for textile dyeing and present in the wastewater were dyes and auxiliaries. The dyes were: Synozol Yellow KHL (C. I. Reactive Yellow145), Synozol Red K3BS150% (C. I. Reactive Red 195), purchased from KISCO (Eksoy Chemical Industries, Adana, Turkey), and C. I. Reactive Black 5 (purified dye, Boruta-Zachem (Bydgoszcz, Poland)). The auxiliaries were: an industrial dyeing assistant—Perigen LDR (SAA—a naphthalene sulfonic acid and carboxylates mixture, Textilchemie Dr. Petry Co. (Reutlingen, Germany)), as well as NaCl, NaOH, and Na2CO3 (technical products from Tomchem (Łódź, Poland)).
Raw knitted fabric, made of 100% cotton, was obtained from Sontex DK ApS (Ikast, Denmark); this fabric was previously bleached by a hydrogen peroxide method.
2.2. Experimental Procedure
The experiment was carried out as a sequence of textile dyeing and ozonation steps, according to the scheme presented in Figure 1
Textile dyeing: Cotton dyeing was carried out in a LABOMAT BFA-12 system (laboratory dyeing machine made by Mathis AG, Oberhasli, Switzerland), according to a standard exhaustion-fixation procedure at a temperature of 60 °C (Figure 2
). The weight of each sample was 10 g and the liquor ratio was set to 1:12. The alkalis, which were NaOH and Na2
aqueous solutions, were partially dosed to avoid excessive dye hydrolyzation. The electrolyte (NaCl) was dosed only into fresh water dyes, not recycling ones.
Ozonation: A semibatch glass reactor (heterogeneous gas-liquid system) with a capacity of 1 L was used during the experiment. A gas mixture containing ozone was delivered into the reaction solution through a porous plate. Mixing was performed with a magnetic stirrer (type ES 21, Wigo, Pruszków, Poland). Ozone was produced by an Ozonek Ozone Generator (Ozonek, Lublin, Poland). The oxygen used for ozone production was supplied from a compressed gas cylinder (O2 purity 99.5%). The ozone concentration was measured at the inlet and outlet of the reactor using a BMT 963 Vent ozone analyzer. The reactor was thermostated. The temperature and pH inside of the reactor were monitored using an Elmetron C411 device (Elmetron, Zabrze, Poland). The progress of the reaction was stopped by the addition of 0.01 M Na2SO3 to the samples.
2.3. Analytical Methods
The color of the samples collected at specified time intervals was measured by a spectrophotometer (Helios Thermo Fisher Scientific, Waltham, MA, US). A calibration plot based on Lambert-Beer’s law was used to determine the concentration of the dyes in the wastewater samples.
Chemical oxygen demand (COD) was obtained using the standard method with a HACH-LANGE apparatus (DR 3800) through dichromate (VI)—LCK 514 and 314 tests.
By-product analysis was carried out by thin layer chromatography (TLC) using TLC Silica gel 60 F254 plates and ethyl acetate: n-butanol: water (1:1:8) and ethyl acetate: n-propanol: water (6:1:3) as eluents.
The colors of the textile samples were measured using a DataColor 400 reflection spectrophotometer (Datacolor AG, Dietlikon, Switzerland) in accordance with ISO 105-J03 [66
]. Each measured DECMC
(DE of Color Measurement Committee) value is an average of at least three measurements (measured up to a maximum error value—standard deviation from the average value set at 0.1).
Based on the presented results, it can be concluded that ozonation is an efficient method for the decolorization of textile wastewater, characterized by high salinity and alkalinity. More than 90% of color removal was achieved within 30 min of the treatment. The investigation showed that the brine produced by wastewater decolorization could be successfully used in at least one subsequent dyeing of cotton fabrics. Even though the experiment indicated an efficient decolorization of textile wastewater by ozonation, the COD removal was rather poor. No more than 20% of the COD reduction could have been achieved. Correspondingly, the detected by-products, which stayed after the process, contributed low COD removal.
The multi-recycling-loop experiment indicated an accumulation of the oxidation by-products that were analyzed by TLC and chemical analysis (coupling reactions with di-azo 4-nitroaniline), as naphthol and phenol derivatives. The occurrence of these specific by-products could suggest that an indirect oxidation mechanism via free radicals can occur during ozonation of the textile wastewater. The presence of the by-products resulted in a lower color removal rate by ozone treatment in subsequent recycling loops. At the same time, the increase in the COD values and the decrease in pH values of the wastewater could be observed. The formation of a stable buffer of pH 9.5–10.0, caused by the by-product’s accumulation, could also be observed. The color shades of upcycled fabrics were slightly influenced by the occurrence of by-products. Because of the detection of by-products, the findings of this study, Part 1, encourage discussion of the industrial applicability of textile wastewater treatment by ozone. It is possible that a more complex, multi-step treatment, could result in a more efficient removal of by-products. Consequently, industrial wastewater ozonation in the upscaled system was investigated in Part 2 of the study, to determine the feasibility of industrial implementation.