Evaluation of advanced oxidation processes for the treatment of nanofiltration membrane concentrate considering toxicity and oxidation by-products

Increasing amount of water consumption by industries leads to a decrease in water resources. Recycle and reuse of industrial wastewater is an important issue due to the decrease in water resources and increasing water costs. With its higher water consumption, the textile industry is the most notable industry in this regard. Different manufacturing processes and various textile chemicals (such as dyes, salts, acids, bases, softeners, surfactants, organic materials, etc.) are used in the textile industry (Sundrarajan et al. 2007). Among these, dyeing is the most important process in terms of both water consumption and environmental pollution. The effluent produced from the textile industry usually contains high chemical oxygen demand (COD), color, inorganic salts, complex organics, temperature, pH and salinity (Ghaly et al. 2013; Raman & Kanmani 2016). Discharge of colored textile wastewater into the water bodies without any treatment can cause direct (ground water pollution, blockage of sunlight penetration through, depletion of dissolved oxygen of receiving bodies, damage to fauna and flora) and indirect (acceleration of genotoxicity and microtoxicity, causes of various human illnesses, eutrophication) consequences. Textile industries typically produce about 250–350 m³ of wastewater per ton of finished product resulting in an average pollution of 100 kg COD per ton of fabric. Textile manufacturing is the second industry in the sector with higher water consumption, equal to 15% of all industrial water consumption in Turkey.

Many studies that are related to the recovery and treatment of textile industry wastewater using biological, chemical and advanced treatment technologies have been published. Physico-chemical methods have some disadvantages such as being less efficient, expensive, having limited application and sludge production (Robinson et al. 2001). Conversely, biological processes are well known and cost effective but most of the dyes used in the textile industry are not biodegradable (Van der Bruggen et al. 2003). Unfortunately, conventional treatment technologies cannot provide sufficient effluent standards and needed quality for water reuse. Although conventional treatment technologies provide effluents within satisfactory discharge limits, these processes produce large amounts of sludge, which makes these processes not economically feasible. Non-conventional methods, such as electrodialysis (Chandramowleeswaran & Palanivelu 2006; Balcik-Canbolat et al. 2020), Fenton's reagent oxidation (Duarte et al. 2013), adsorption (Ahmad & Hameed 2009), ozonation (Bes-Pia et al. 2004), etc. have been utilized for the treatment of dyes and reuse from the effluent. The most promising technology with a high water quality for reuse of industrial wastewater is membrane technology (Cheng et al. 2012; Lin et al. 2015a, 2015b). Consequently, it offers protection of natural sources and sustainable water management. However, membrane processes have some limitations such as treatment of the concentrate stream and fouling, which are major problems of concern for membrane systems (Van der Bruggen et al. 2017). Proper disposal of membrane concentrates is needed to increase the applicability of membrane processes.

The treatment of membrane concentrates produced from different sources such as sea water, leachate wastewater, brackish water, etc. has been investigated in the literature. Unfortunately, there are only a few studies related to textile membrane concentrate. The existing applications for the treatment of textile wastewater membrane concentrate usually suggest reducing the production of the concentrate stream or returning the concentrate stream to the feed. Different treatment approaches can be applied for the sustainable treatment of membrane concentrate.

Many existing researches center on physicochemical process for the treatment of concentrate streams. Forward osmosis (FO)-coagulation/flocculation (CF) combined system has been investigated by Han et al. (2016) to provide an alternative solution for the effective treatment and reuse of textile wastewater (Han et al. 2016). The FO process was performed to reuse the textile effluent; CF was carried out to the FO highly concentrate line for dye removal with high efficiency and low chemical concentrations. It was stated that CF can effectively eliminate more than 95% of the dyes with a low dosage of coagulants and flocculants at 500–1,000 ppm. Ozonation/aerobic treatment of textile membrane concentrate was investigated in another study by Yaman et al. (2015). It was stated that biodegradability of the membrane concentrate was nearly three times raised by the ozonation in this study. Lopez et al. (1999) also focused on the ozonation of real nanofiltration (NF) membrane concentrate and obtained an increase in biological oxygen demand (BOD₅) from 0 to 75 mg/L. Also, they observed 50% of COD and 30% of total organic carbon (TOC) removal efficiencies.

The purpose of this paper was to investigate the feasibility of the treatment of membrane concentrate produced from the textile industry, using the Fenton, Advanced Fenton (AF), ozone and hydrodynamic cavitation (HC) and combination of these processes. The study has been structured to investigate the optimum oxidant and catalyst concentrations, optimum operational conditions and comparison of these processes.

Textile industry where wastewater was provided has produced 1,400 m³ daily wastewater. A biological wastewater treatment plant sited in the textile industry has been operated as a sequencing batch reactor (SBR). A biological treatment plant is composed of an equalization tank and SBR tank. Textile wastewater is firstly collected into the equalization tank. A cycle of SBR was designed to treat 350 m³ of wastewater and four cycles were carried out in a day. The operation of SBR consists of 3.5 hours of fill step, 0.5 hours of react step, 0.75 hours of settle step and finally, 1.25 hours of decant step. A cycle of SBR process takes about 6 hours. SBR discharge wastewater has been used in this study.

Biologically treated textile wastewater was fed to a pilot scale cross flow membrane unit. The system was formed of a pretreatment unit (cartridge filter), membranes, feed, permeate and concentrate tanks, and the programmable logic control (PLC) system. NF and reverse osmosis (RO) membranes with an effective filtration area of 2.6 m² were linked individually. Cross flow was supplied with a high pressure pump. NF/RO hybrid system was performed to evaluate a feasible process to treat textile wastewater. Before the membrane processes, biologically treated textile wastewater was firstly filtered through a 10 μm cartridge filter to eliminate SS.

Then, the wastewater firstly was filtered by NF membrane. Then, NF permeate was considered as feed water for the RO membrane. While RO permeate can be utilized as process water, NF concentrate was planned to treat with HC, Fenton, AF and ozone. Characterization of NF membrane concentrate is given in Table 1. The detailed information about the characteristics of the membranes and the results of membrane systems could be found elsewhere (Balcik-Canbolat et al. 2017). Integrated membrane process was carried out in batch operation modes. The concentrate was recirculated to the feed tank during the operation. Pilot cross flow membrane unit was performed until the 85% (for NF) and 80% (for RO) of water recovery was achieved.

A pilot scale hydrodynamic cavitation reactor was used in this study. The reactor consists of a cylindrical tank with a capacity of 10 L, a centrifugal pump, a venture cavitation device (5 mm throat diameter), a by-pass line, an iron column with zero-valent iron (ZVI) pieces (for AF experiments), pressure gauges and valves. The venturi cavitation device is detachable so that the HC reactor can be used both stand-alone oxidation processes and HC with oxidation processes. Mixing and pressure were supplied with the pump. The temperature was kept constant using an external cooling bath during the experiments. HC experiments were conducted stand-alone and with Fenton, AF and ozone. The performance of the HC reactor was determined according to the cavitation number, which differs from inlet pressure. Operation of variable CN from 0.1 to 0.8 (0.1, 0.2, 0.4, 0.6 and 0.8) was investigated. The effect of advanced oxidation processes such as Fenton, AF and ozone in combination with HC were also examined to enhance the efficiency of HC.

A sequence of Fenton's experiments was performed at different Fe²⁺ and H₂O₂ concentrations in the wide range of 1–35 mM and 5–175 mM, respectively. Fenton's oxidation of 500 mL membrane concentrate samples was conducted in a glass beaker at room temperature as follows; firstly, the pH of the membrane concentrate was adjusted to 2.95 ± 0.5 (e.g. the optimal pH of Fenton's reaction (Pignatello 1992; Kiwi et al. 1993; Kang et al. 1999)) with concentrated H₂SO₄ solution (6N). Subsequently, Fe²⁺ catalyst was added to the membrane concentrate sample from a daily prepared FeSO₄.7H₂O solution under a continuous stirring speed of 200 rpm. The reaction was started by adding the desired concentration of H₂O₂ (35%w/w) to the solution. After 40 min, the reaction was stopped by adding the NaOH solution (6 N) to the samples to increase the pH to 7–8. The samples were filtered through 45 μm membranes to remove the formed Fe(OH)₃ flocs. Interferences of unreacted H₂O₂ remaining in the samples on the COD results were eliminated with catalase made from Micrococcus lysodeikticus.

AF experiments were performed in an HC reactor with a volume of 10 L with zero-valent iron (ZVI) pieces and H₂O₂. HC reactor was modified by adding an iron column. Prior to the experiments, the pH of the membrane concentrate was adjusted to 2.95 ± 0.5, the same as the Fenton process. The temperature was kept constant using an external cooling bath during the experiments. Stirring was supplied with the pump. Samples were taken at certain time intervals and analysed for TOC, COD, color and H₂O₂. Before the analysis, the reaction was stopped after 30 min by adding the concentrated NaOH solution (6 N) to the samples to increase the pH to 7–8. The samples were filtered through 45 μm cut-off Millipore membranes to remove the formed Fe(OH)₃ flocs.

Ozonation experiments were performed in a 1,000 mL glass reactor in which the 750 mL membrane concentrate sample was bubbled by ozone through a gas diffuser which was placed at the bottom of the glass reactor. Ozone gas was supplied by an ozone generator (Sabo, capacity: 7,000 mgO₃/sa) which was fed high purity oxygen. The ozonation experiments were determined at pH 3.8 (original pH) and 10. The inlet and off-gas ozone concentrations were determined via iodometry to measure ozone absorption efficiencies after each experiment. Ozonation experiments were carried out for 60 min at two different ozone doses.

A commercial assay kit marketed as BioTox^™ was used to measure acute toxicity changes during the application of Fenton, AF and ozone processes. A lyophilised preparation of the marine photobacterium Vibrio fischeri (V. fischeri) was used as the reagent. The assay is based on the reduction in light emission of V. fischeri resulting from its exposure to the toxicant. Before the assay, the pH of the samples was adjusted to 7.0 ± 0.2 with H₂SO₄ or NaOH solutions. NaCl was added to have a final chloride concentration of 2%, w/v in the samples.

Raw and treated wastewater samples were extracted with hexane. The extract volumes were reduced to approximately 5–6 ml final volume with a rotary evaporator. After concentration, samples were cleaned up on a silica gel column containing 3 g of deactivated silicic acid. The column was pre-washed with 50 ml hexane. Then, the PCB fraction was eluted with a mixture of n-hexane: dichloromethane. Then, the purified samples were concentrated to 0.2 ml under a nitrogen stream. Internal standards were added before GC-MS analysis.

A total of 19 individual PCB compounds (PCB 1, PCB 5, PCB 18, PCB 31, PCB 52, PCB 44, PCB 66, PCB 101, PCB 87, PCB 110, PCB 151, PCB 153, PCB 138, PCB 141, PCB 187, PCB 183, PCB 180, PCB 170, PCB 206) were determined on Agilent gas chromatograph with mass spectrometer (AGILENT 5975 GC-MS) using a DB-608 capillary column (30 m × 250 µm × 0.25 µm). High purity He was used as the carrier gas, at a flow rate of 2 ml/min. The temperature of the injection port was determined at 250 °C. The GC oven program was set as follows: starts at 110 °C and holds for 0.5 min; then raised to 280 °C at 15 °C/min and holds for 8 min.

TOX was analyzed using the micro coulometric titration method. Firstly, 100 mL of raw and treated wastewater samples were adjusted to pH < 2.0 with concentrated HNO₃. After that, 5 mL of stock nitrate solution and 40 mg of activated carbon (Jena, Germany) were added. Then, the mixtures were shaken at 180 r/min for 1 h, and then filtered through a quartz column to retain the activated carbon. The quartz columns were transferred to the TOX analyzer to determine the TOX concentration.

pH, salinity, total dissolved solids (TDS) and conductivity were analyzed using a multi parameter. COD, BOD, chloride, color and sulfate were determined according to the Standard Methods (APHA 2012).

CN is the most important parameter that has an impact on cavitational conditions and efficiency of an HC reactor (Rajoriya et al. 2018). For this purpose, the effect of CN on the treatment of membrane concentrate was investigated by varying CN from 0.8 to 0.1. The obtained COD and color removal results are shown in Figure 1. It was observed that COD and color removal increased with a decrease in CN from 0.8 to 0.1. CN decreases with increasing inlet pressure. An increase in COD and color removal with a rise in inlet pressure or reduction in CN may be ascribed to the fact that more cavities are produced at higher inlet pressure, causing the production of more HO^• radicals. An increase in CN beyond an optimum value causes a cavity cloud formation, which decreases the cavitational intensity due to the coalescence of cavities (Rajoriya et al. 2016; Rajoriya et al. 2017). At CN below 0.1, the cavity cloud begins to form, which causes a reduction in cavitational intensity. Although maximum removal efficiency of COD and color was obtained as 18.8% and 60.7% respectively at CN of 0.1 in 30 minutes, the percentage of COD and color removal efficiencies were only 1.9% and 53.5% respectively at CN of 0.8 in 30 minutes. Results showed that there was no significant reduction in COD, and color at CN of higher than 0.1. However, CN in the HC reactor was kept at 0.1 for all the remaining experiments. Similar results were informed for the treatment of wastewater in the literature. Gore et al. (2014) investigated the degradation of reactive orange 4 dye using hydrodynamic cavitation and stated that the maximum TOC removal efficiency of 22.22% was observed at 0.15 of CN and the decolorization rate decreased with further increase in the inlet pressure (Gore et al. 2014).

In the present study, the efficiency of the Fenton process for the removal of TOC, COD and color from textile NF membrane concentrate was optimized by studying the effect of initial catalyst dosage and H₂O₂ concentration. Fenton experiments were conducted at the optimal Fe²⁺:H₂O₂ (1:5) ratio with different Fe²⁺ (1, 5, 10, 17.5, 20 and 35 mM) and H₂O₂ (5, 25, 50, 87.5, 100 and 175 mM) concentrations. The pH of membrane concentrate was adjusted to 2.95 ± 0.5, which is known as the optimum pH of Fenton's reaction (Pignatello 1992; Kiwi et al. 1993; Kang et al. 1999). The total removal efficiency of COD and TOC increased with an increase in Fe²⁺:H₂O₂ concentrations (mM:mM) from 1:5 to 35:175. It can be seen from Figure 2 that maximum COD and TOC removal efficiency was found to be 87.1% and 80.8% respectively at molar concentrations of 35 mM Fe²⁺ and 175 mM H₂O₂ and pH 3 for 30 minutes. Whereas, only 11.2% of TOC and 4.1% of COD removal efficiencies were obtained at molar concentrations of 1 mM Fe²⁺ and 5 mM H₂O₂ and pH 3 for 30 minutes. Since the rate of removal is proportional to the COD and TOC values, the COD and TOC removal rate increased with an increase in concentrations of Fe²⁺ and H₂O₂. Similarly, 99% of maximum color removal efficiency was observed at molar concentrations of 35 mM Fe²⁺ and 175 mM H₂O₂ and pH 3 after 30 minutes. In all experiments performed in this study, H₂O₂ was completely utilized after 30 min oxidation, indicating that the chosen Fe²⁺:H₂O₂ ratio was appropriate.

Moreover, external addition of H₂O₂ also enhanced the pollutant removal rates and efficiencies (Girit et al. 2015; Dogan et al. 2016; Arslan-Alaton & Olmez-Hanci 2017), since ZVI could serve as a slow-releasing source of dissolved Fe (Fe²⁺), which reacts with H₂O₂ to produce additional HO^•.

In order to evaluate the effect of AF on the treatment of membrane concentrate, a series of assays were conducted with ZVI pieces and H₂O₂. The HC reactor was modified with an iron column containing 1 kg of ZVI pieces. All experiments were carried out in the HC reactor without using a cavitation device to investigate the efficiency of the AF process only. The HC reactor was used for comparison of the results obtained for AF under the same conditions with and without HC.

As shown in Figure 3, the COD and TOC removal efficiency at pH 3 and molar concentrations of 175 mM H₂O₂ after 30 min was 82.2% and 71.2%, which was much higher than at molar concentrations of 25 mM H₂O₂, 57.2% and 50.1%, respectively. The enhancement in the COD, TOC and color removal efficiencies with increasing molar concentrations of H₂O₂ could be attributed to the production of additional HO^•. It was clearly observed that COD and TOC removal efficiencies increased sharply at the beginning of the experiment as indicated by the slopes of all curves, which started to climb straight in the first 5 min. These results overlapped with the H₂O₂ consumption rate, which indicated that the removal efficiencies were enhanced by the appropriate addition of H₂O₂. As can be seen in Figure 3, after the 30 min of reaction time at molar concentrations of 175 mM H₂O₂, the decolorization efficiency was almost 96.1%. It was stated that the appropriate addition of H₂O₂ plays a key role in decolorization.

Figure 4 illustrates the COD and TOC removal efficiencies at different pH and ozone concentrations. As expected, there is a tendency toward an increase in COD removal efficiency with increasing ozonation time. This can be explained as the ozone gas moving through the column and reacting with pollutants. It can be seen from Figure 4 that the COD removal efficiency enhanced by increasing the supplied ozone concentration. For example, when the ozonation time was 60 min, the COD removal efficiency was about 23.5% at a dose of 2,760 mg/L, increased to 31.3% at a dose of 3,168 mg/L. Unfortunately, it was observed that it was difficult to increase the COD removal efficiency only by increasing the ozone concentration.

The decolorization efficiencies of membrane concentrate after 60 min are also shown in Figure 4. As shown in Figure 4, it can be said that there was no significant effect of pH values on COD and dye removals at concentrations of 2,760 mg/L ozone after 60 min. Different results were reported regarding the effect of pH on color removal by ozonation by other researchers. Hsu et al. (2001) stated that the decolorization rate by ozonation was more rapid in acidic conditions (Hsu et al. 2001). On the contrary, Wu et al. (2005) reported that the decolorization rate in an ozone system was higher at pH 10 (Wu et al. 2005). Ozone reaction is direct and occurs through molecular ozone at acidic pH, while the reaction is indirect at basic pH, which leads to the formation of HO• and HO₂•. The various results on the effect of pH on color removal may be attributed to different parameters, which are important for process performances, such as the molecular structure of the dye molecule and its configuration in the liquid medium, oxidation mechanism, temperature, dye concentration and others.

It was reported in the literature that stand-alone HC has a limited effect on COD and color removal. Maximum COD removal efficiencies reported were in the ranges of 10–27% depending on the characteristics of the wastewater and HC reactor for dye or textile wastewater treatment (Gogate & Bhosale 2013; Parsa & Zonouzian 2013; Gore et al. 2014; Gagol et al. 2018). For this reason, a combination of different AOPs with HC has been used to improve the degradation of the pollutants. In order to examine the combined effect of HC and Fenton, AF and ozonation on the COD, TOC and color removals of membrane concentrate, experiments were carried out at optimum CN of 0.1. The obtained results are shown in Figure 5.

COD, TOC and color removal efficiencies for Fenton and a combination of Fenton and HC are shown for two different concentrations of catalyst and oxidant (low: Fe²⁺:H₂O₂ 17.5 mM:87.5 mM and high: Fe²⁺:H₂O₂ 35 mM:175 mM)). From Figure 5, COD and TOC removal efficiencies at molar concentrations of 175 mM H₂O₂ and 35 mM Fe²⁺ and pH 3 after 30 min were 87.1% and 80.8%, which were higher than at molar concentrations of 87.5 mM H₂O₂ and 17.5 mM Fe²⁺, 71.6% and 68.1%, respectively. However, the combined approach of HC and Fenton process has shown 87.4% of COD and 80.6% of TOC removal efficiencies at molar concentrations of 175 mM H₂O₂ and 35 mM Fe²⁺ and pH 3 after 30 min, which was 83.5% and 76.5% at molar concentrations of 87.5 mM H₂O₂ and 17.5 mM Fe²⁺, 71.6% and 68.1%, respectively. It was clearly observed that the synergetic effect of the combined HC and Fenton process was much higher at low oxidant concentration. As mentioned before, the addition of a higher concentration of H₂O₂ up to an optimum value causes a decrease in removal efficiencies. The higher amount of H₂O₂ addition can result in the scavenging of the generated HO^•, resulting in lower removal efficiencies (Kumara et al. 2018). Rajoriya et al. (2018) have also studied the performances of stand-alone Fenton and Fenton enhanced with HC process for the treatment of textile wastewater (Rajoriya et al. 2018). According to the obtained results, 9.5% and 38.1% of COD removal efficiencies were achieved with Fenton and Fenton enhanced with HC process, respectively. The concentration of H₂O₂ used in this study was very low (5 mL/L). It can be stated that the effect of HC process on the Fenton process was easily observed at lower H₂O₂ concentrations. Similarly, Wang et al. (2014) have focused on the treatment of dye solution with low concentrations of H₂O₂ (100 mg/L) with stand-alone HC, stand-alone Fenton and Fenton enhanced with HC. 14, 62 and 91% of color removal efficiencies were found for stand-alone HC, stand-alone Fenton and Fenton enhanced with HC, respectively (Wang et al. 2014). It should also be noted that the optimal concentration of H₂O₂ is always dependent on the operational conditions (pH, operation pressure and catalyst), equipment configuration and the type of wastewater (Chakinala et al. 2008).

Similarly, Mishra & Gogate (2010) studied the effect of HC and coupled HC and Fenton processes on Rhodamine B removal. Although they did not specify the individual Fenton results, they achieved 25 and 99% of color removal efficiencies in HC and coupled HC and Fenton processes (Mishra & Gogate 2010). In their study, the synergistic effect of the HC process on the Fenton oxidation process cannot be fully observed. When the results of coupled HC and Fenton processes are considered without the results of individual Fenton oxidation, coupled HC and Fenton processes appear very attractive. However, when the performances of individual Fenton and coupled HC and Fenton processes are compared in the literature, it is stated that the great effects of the HC process cannot be observed. Consequently, the use of high oxidant concentrations masks the synergistic effect of HC on Fenton processes due to the scavenging effect. It is also clear that Fenton and HC combination is more feasible and efficient at lower concentrations of oxidant.

The combined approach of HC and AF process has shown 78.6% of COD and 94.7% of color removal efficiencies at molar concentrations of 175 mM H₂O₂ and 35 mM Fe²⁺ and pH 3 after 30 min, which was 82.2 and 96.1% for AF process alone at the same molar concentrations of H₂O₂ and Fe²⁺. Additionally, no significant effect was observed for AF enhanced with HC.

From Figure 5, in the case of the HC and ozonation combination, a maximum 28.4 and 36.2% of TOC and COD removal efficiencies were observed; whereas, 20.4 and 31.1% of TOC and COD removal efficiencies were obtained, respectively, for ozonation alone. The synergetic effect of the HC process on the ozonation process was also reported by other researchers. Rajoriya et al. (2018) have examined the effects of the combined HC with ozone and Fenton processes for the treatment of textile wastewater. The obtained results showed that the ozonation of textile wastewater resulted in only 15% COD removal efficiency while the stand-alone HC process achieved 12% COD removal efficiency. Otherwise, combined ozonation and HC process reached 22% COD removal efficiency. Similarly, with our results, an increase of only 7% in COD removal efficiency was observed with coupled ozonation and HC compared to the single ozonation process (Rajoriya et al. 2018). Raut-Jadhav et al. (2016) studied pesticide wastewater treatment by HC and ozonation. According to the obtained results, 15 and 36.3% of COD removal efficiencies were achieved with only HC and coupled HC and ozonation respectively.

An important risk for the treatment of wastewater with oxidation processes is the possibility of producing oxidation intermediates that are more toxic and difficult to treat than the targeted contaminants (Rivas et al. 2008). The biological response of different organisms affected by chemical substances is very diverse and a battery of bioassays can be used for measuring toxic effects. This effect consists of complex and different chemical compounds, their behavior at different pH values, solubility, antagonism and synergism, biocompatibility, and their measurement is possible with bioassays (Pignatello et al. 2006). Applying bioassays using a series of bioindicators at different trophic levels is a necessary and efficient method for detecting environmental threats in the aquatic environment (Rizzo 2011). It should be noted that different pollutant groups may have no toxic effects on different trophic level organisms.

Raw and treated membrane concentrate was analyzed on the V. fischeri bacteria by measuring the percent relative inhibition (INH, %). From the obtained results shown in Table 2, it can be stated that the toxicity of raw membrane concentrate after Fenton and AF tended to increase, whereas the toxicity of the raw membrane concentrate after ozonation was almost stable. This increase might be attributed to the formation and accumulation of oxidation intermediates that are more toxic than parent compounds towards the V. fischeri photobacteria.

The formation potential of chlorinated organic compounds after oxidation of membrane concentrate membrane with a high chloride content was also investigated. Total organic halogen (TOX) represents the whole of chlorinated, brominated and iodinated organic compounds in a water sample. TOX result includes chlorinated organic compounds (TOCl), brominated organic compounds (TOBr) and iodinated organic compounds (TOI) species. TOX and PCB results are shown in Table 3. As can be seen in the obtained results, there was no increase in ∑PCBs concentration. According to obtained TOX results, TOX concentration showed a drop after the Fenton process. On the contrary, a slight increase in TOX concentration was observed after ozonation. Also, a significant increase in TOX concentration was obtained after AF treatment. Leshem et al. (2006) carried out ozonation studies for the treatment of textile wastewaters. TOX analyses have been carried out for the detection of byproducts that may be formed in wastewater after treatment with ozone. They reported that while the TOX value of raw wastewater was 77 μg/L, it increased to 120 μg/L after ozonation (Leshem et al. 2006).

The present study reports the feasibility of the treatment of membrane concentrate produced from the textile industry, using Fenton, Advanced Fenton (AF), ozone and hydrodynamic cavitation (HC) and combinations of these processes. Maximum removal efficiencies in COD and color were 18.8% and 60.7% respectively using HC alone at a CN of 0.1. All combined treatment strategies; that is, HC + Fenton, HC + AF, and HC + ozone, showed higher COD, TOC, and color removal efficiencies compared to that observed using HC alone. Among all the combined approaches applied in this study, combined HC with Fenton showed the highest percentage removals in COD, TOC, and color. It was also stated that the use of high oxidant concentrations masks the synergistic effect of HC on Fenton processes due to the scavenging effect of excessive (overdose) H₂O₂ concentration.

This study has important potential to assess the real applicability of the advanced oxidation process for the treatment of real membrane concentrate. The results of this study can be useful when designing a pilot reactor to be applied in the treatment of real textile membrane concentrate.

This study was supported by The Scientific and Technological Research Council of Turkey (TUBITAK) Project number: 114Y098.

All relevant data are included in the paper or its Supplementary Information.