Treatment for high-concentration liquid crystal wastewater with a novel Fenton–SBR–microwave pyrolysis integrated process

With the development of organic synthesis, environmental pollution caused by organic pollutants has attracted researchers' attention (Hu et al. 2018; Kim et al. 2018). The contamination caused by liquid crystal wastewater is typical organic pollution. Alkyl glucoside (Zafiu et al. 2016; Amato & Beolchini 2018) or alkyl sulfonate (Srinivasan et al. 2019) and surface active agents (Li et al. 2016) are the dominant components in liquid crystal wastewater. Many investigations have proved that surface active agents are potential carcinogenic substances to humans (Kasuya & Hatanaka 2016; Xu & Gye 2018). Unfortunately, alkyl sulfonate, perfluorosulfonic acid and phthalate esters diffuse in water worldwide (Hansen et al. 2002; Pistocchi & Loos 2009; Zhao & Zhang 2009), and they are even detected in the blood system of human beings (Zhang et al. 2010; Poothong et al. 2017). Normally, liquid crystal industrial wastewater has four characters: complex components, high chemical oxygen demand (COD) concentration, severe bio-toxicity and low biodegradability.

Now, Fenton and electron-Fenton are the common methods for this special wastewater. Colades et al. (2015) treated this wastewater with electron-Fenton and found that COD removal was only 79%, suggesting that advanced oxidation processes could not completely decompose organic pollutants or transformation products. Furthermore, the disposal of iron mud produced in the electron-Fenton reaction was not discussed in their research. Su et al. (2013) carried out a fluidized-bed Fenton process to remove monoethanolamine and phosphate in liquid crystal wastewater and found that degradation kinetics for monoethanolamine and phosphate conformed to the first-order reaction. Fenton, or electron-Fenton, is an important treatment for high-concentration liquid crystal wastewater, and organics are degraded by ·OH which has very powerful oxidation and decomposes almost all organic pollutants (Zhao et al. 2016). Although Fenton is efficient for this special wastewater, the produced iron mud is a type of hazardous waste and should not be ignored. The higher the concentration of COD, the greater the volume of iron mud that is in the Fenton reaction. Iron mud should be handled in a safe, economical and effective manner. Microwave pyrolysis has many advantages, such as shorter pyrolysis time and lower running cost, compared with conventional pyrolysis (Deng et al. 2016; Zhang et al. 2018). Furthermore, microwaves have powerful sterilization abilities by destroying the cell structure of harmful microorganisms (Zhang et al. 2016; Auksornsri et al. 2018). So, it seems that microwave pyrolysis is a friendly disposal technology to realize reduction, resources and harmlessness.

Biological treatment is economical for wastewater which must possess low bio-toxicity and high biodegradability (B/C value) (Kumar et al. 2010). Badawy et al. (2009) studied the effect of the B/C value on the efficiency of the activated sludge treatment process and found that only 61% COD was removed when the B/C value was lower than 0.25, but 92% COD was removed when it was higher than 0.5. The SBR process is not directly suitable for highly toxic liquid crystal wastewater, as the inhibitory and toxic effects for microbes increase drastically with the concentration of organic wastewater (Tennekes & Sánchez-Bayo 2013; Wang et al. 2019). Considering COD removal, economical cost and resource recovery, we performed a novel Fenton–SBR–microwave pyrolysis integrated process in which frontal Fenton–SBR was for wastewater and the following microwave pyrolysis was for iron mud produced in the Fenton reaction.

H₂O₂, FeSO₄·7H₂O, H₂SO₄ and NaOH are all analytical reagents purchased from Tianjin Chemical Reagent Company, China. High purity water (18.2 MΩ cm^–1) used in all experiments was prepared with a water purification instrument (Unique-R20, Research Scientific Instruments Company, China). Sludge was withdrawn from the Haibo River sewage treatment plant (Qindao, China), and its concentration and volume index were 8,500 mg L^–1 and 90%, respectively. The qualities of raw liquid crystal wastewater from the industrial plant were as follows: COD = 150,000 mg L^–1, pH = 4.8, BOD = 2,200 mg L^–1, T_N = 36 mg L^–1, T_P = 5 mg L^–1.

Raw liquid crystal wastewater (5 L) was transferred into a conical Fenton pool and the pH value was first adjusted to 3, and then Fenton reagents were added to start the reaction. After 30 min, the Fenton reaction finished. To accelerate the flocculation, the pH value was adjusted to 8 by adding 5 mol L^–1 NaOH aqueous solution. Clear water from the Fenton pool and the other wastewater from washing cans containing methanol (the quantity of cleaning water almost equalled that of the Fenton outlet in this chemical plant) were mixed and discharged to the SBR pool in which active sludge had been activated for about 24 h. Iron mud from the Fenton pool was withdrawn and concentrated mechanically, and then pyrolyzed by microwaves in a microwave stove (M1-L213B, Midea, China). In the SBR stage, hydrolytic time was controlled at 24 h, initial mixed liquor suspended solids (MLSS) were 4,500 mg L^–1 and bubbling air was set at 60 mL min^–1. To maintain the stability of the SBR process, certain nutrition and phosphorus elements were added. The procedure of the Fenton–SBR–microwave pyrolysis integrated process for wastewater is shown in Figure 1.

Chemical oxygen demand (COD) was analyzed with a spectrophotometer (Hach-DR 2800, USA) equipped with a digestion instrument (MH-2800D, Hach). Samples were first digested at 150 °C for 2 h in the digestion instrument and then cooled to room temperature. Their concentrations were recorded by the spectrophotometer at 620 nm wavelength. Before analysis, all samples were diluted to satisfy the detection range. Total organic carbon (TOC) was analyzed by a TOC analyzer (TOC-4200, Shimadzu, Japan) connected to an automatic sampler system, and N₂ was the carrier gas. All samples were also diluted to satisfy the calibration method. The crystal phase of Fe₂O₃ was analyzed via its X-ray diffraction (XRD) pattern, which was monitored using a D/max RB system (Cu Kα₁ irradiation, λ = 1.5406 Å, voltage = 40 kV, current = 30 mA, scanning rate = 0.01°/s, scanning range = 10°–80°).

The Fenton dosage and molar ratio of Fe²⁺ to H₂O₂ (mFe²⁺:mH₂O₂) were investigated and their effects on COD removal are displayed in Figure 2(a) and 2(b). From Figure 2(a), it is observed that COD removal increases with the enhancement of Fe²⁺ concentration (namely Fenton dosage), and almost 95% COD is removed when the Fe²⁺ concentration is 0.3 mol L^–1. The reason is because the more Fenton is added, the more ·OH radicals are produced. The reaction equations are as follows: Fe²⁺ + H₂O₂ + H⁺ → Fe³⁺ + ·OH, ·OH + pollutants → intermediates. From Figure 2(b) it can be seen that COD removal is first enhanced and then decreases with the enhancement of mFe²⁺:mH₂O₂, and the optimal value of mFe²⁺:mH₂O₂ is 1:3. When mFe²⁺:mH₂O₂ is lower than 1:3, more HO· radicals are produced and remain, resulting in higher COD removal. However, when mFe²⁺:mH₂O₂ is higher than 1:3, more excessive Fe²⁺ ions react with HO· radicals, forming a disputed state between Fe²⁺ and organic pollutants with HO· (Vorontsov 2019), so COD removal declines.

Liquid crystal wastewater is a type of special industrial wastewater, which has high toxicity and complex components. Other methods for this special wastewater are listed in Table 1. It is observed that Fenton is the most popular method and it is significant in handling high-concentration liquid crystal wastewater, however, a biological method, such as MBR, is only for low-concentration liquid crystal wastewater.

Water qualities, including COD, TOC, biochemical oxygen demand (BOD) and T_N and T_P, were analyzed before starting the SBR process, and the values were as follows: COD = 2,300 mg L^–1, BOD = 932 mg L^–1, TOC = 3,386 mg L^–1, T_N = 28 mg L^–1, T_P = 3 mg L^–1. It is observed that the ratio of BOD to COD climbed from 0.015 at the beginning to 0.41 after the Fenton process, revealing that treated water from the Fenton process is biodegradable. To analyze the running stability of the SBR process, COD removal, mixed liquid suspended solids (SSLV) and sludge volume index (SVI) were measured, respectively, and the results are presented in Figure 3. It is observed that 90% COD is removed, suggesting that the SBR process is stable and credible. Besides water qualities, the activated sludge is also analyzed. It can be seen that the concentration of SSLV decreases firstly before ten running cycles and then increases slowly to the original state after 19 running cycles. Furthermore, the variation trend of SVI is similar to that of SSLV, as shown in Figure 3(b). Here, the hydraulic residence time (HRT) of every cycle is 24 h. Before the first ten cycles, the microbes in the activated sludge received a sudden impact from the liquid crystal wastewater, forming a serious inhibition and resulting in depopulation, so the values of SSLV and SVI both decline. After ten cycles, organisms grew slowly and reached a stable period. Coincidentally it was found that the sludge became compact at the beginning, forming numerous micelles; however, it became looser after several running times and changed to denser again in the later stage.

The dominant microbes existing in the activated sludge were identified with a high-power inverted microscope (XDY-100, New Optics Company, China); see Figure 4. The representative microbes were metazoans such as flagellates, Arcella and spiral worm in active sludge, as shown in Figures 4(a1)–4(a2) and S1 (flagellates' video presented in the Supplementary Material), and the probable reason is that the activated sludge was withdrawn from the aeration tank, which was mainly occupied by metazoans. Furthermore, there are micelles in the activated sludge, showing that the biological activity of sludge is perfect. However, at the beginning of the SBR process, sludge-loading increased so suddenly that some metazoans could not endure the attack of pollutant, resulting in the decline of activity (there is an Arcella corpse identified within a purple square in Figure 4(b)). Compared with the standard microorganism, the microbes of Figure 4(c1)–4(c5) are assigned to rotifer, Epistylis galea, Opercularia coarctata, Vorticella and mormon worm (a kind of amoeba) in turn. Here, Epistylis galea, Opercularia coarctata, Vorticella and mormon worm can secrete many extracellular polymeric substances, which is good for the formation of micelles and the flocculation of activated sludge, so the above-mentioned microbes are regarded as the characteristic indicator organisms to evaluate the successful operation of the SBR process in many wastewater treatment plants. Li et al. (2013) studied the effects of Vorticella and rotifer on aerobic sludge granules in an SBR reactor and found that Vorticella anchored into the granules by stalks, while rotifer attached or adhered to the surface of the granules. The attachment and accumulation both improved the features of the sludge, thus the values of SSLV and SVI both picked up again, showing that the flower shape of rotifer and branch shape of Opercularia coarctata and Epistylis galea have important roles in the recovery of sludge, proved by Li's research. Furthermore, there was an abundance of rotifers and mormon worm after the 20th cycle; S2 and S3 in the Supplementary Material are videos of mormon worm and rotifers recorded by microscope in this stage. In fact they only appear in good water quality, indicating that the water quality is much improved.

Microwaves are a kind of electromagnetic wave and can excite polar materials to form a high-temperature field where the crystal phase transformation can be immediately completed. Zhao et al. (2015) successfully synthesized β-Ga₂O₃ photocatalyst using the microwave-assisted hydrothermal method. Fortunately, iron is a good substance for absorbing microwaves. Iron mud containing dominant Fe(OH)₃ was treated by microwave pyrolysis and the effects of microwave time and power on pyrolysis were studied. The results are presented in Figure 5(a). An obvious conclusion that iron mud weight decreases with the enhancement of microwave irradiation time and power can be achieved from this 3-D figure. Because a high-temperature field is formed during the microwave pyrolysis process, Fe(OH)₃ as a kind of polar material absorbs microwaves quickly and forms Fe₂O₃ by losing crystal water. The corresponding reaction equation is: Fe(OH)₃ → Fe₂O₃ + H₂O.

To confirm this conclusion, Fe₂O₃ was purified several times with alcohol–water aqueous solution (alcohol:water = 1:1) and characterized by XRD analysis. The XRD pattern is displayed in Figure 5(b). It is clear that the crystal phase of Fe₂O₃ belongs to the ɑ-phase, for there appear apparently characteristic diffraction peaks for the (012), (104), (110), (113), (024), (116), (018), (214) and (119) crystal planes of the rhombohedral phase of ɑ-Fe₂O₃ crystalline structure (JCPDS No. 33-0664). No additional peaks of any other impurities were identified in this figure, suggesting that ɑ-Fe₂O₃ is the only byproduct of microwave pyrolysis. Marinho et al. (2014) coincidentally prepared ɑ-Fe₂O₃ nanorods by the microwave heating method; ɑ-Fe₂O₃ was extracted from iron mud via microwave pyrolysis, realizing the effective recovery of waste resources. It is valuable for microwave pyrolysis not only to avoid environmental pollutants but also to obtain an efficient and commercial recovery for the ɑ-Fe₂O₃ byproduct.

It is apparent that ɑ-Fe₂O₃ is the sole product from clear iron mud pyrolyzed by microwaves, according to Figure 5(b). In fact, there are other ingredients in raw iron mud generated from the Fenton reaction, such as NaOH, FeSO₄ and Na₂SO₄. To make certain of the probable components in raw iron mud, XRD of raw iron mud was carried out and is displayed in Figure 5(c). Compared with standard diffraction peaks, Na₂SO₄, Fe(OH)₃ and NaOH were identified. Na₂SO₄ and Fe(OH)₃ both come from the Fenton reaction, and NaOH is from pH adjustment.

TOC removals in the Fenton–SBR process were analyzed and the results are displayed in Figure 6(a) and 6(b). It can be seen that TOC decreases quickly in the Fenton process, illustrating that Fenton is an efficient method for liquid crystal wastewater. At optimal conditions, 76.8% TOC is removed by the Fenton process within 0.33 h and 87.2% TOC is removed by the SBR process within 24 h; see Figure 6(b). TOC declines from 50,000 to 398 mg L^–1. Furthermore, TOC removal kinetics in the Fenton and SBR processes were investigated and the results are displayed in Figure 6(c). From this figure, it is observed that TOC removals in Fenton peroxidation and SBR processes both fit with pseudo-first-order kinetics, and the reaction rate constants were 4.608 and 0.098 h^–1, respectively. The corresponding half-life time is 0.15 and 7 h for Fenton and SBR processes, respectively, on which basis TOC removal can be easily forecast in actual treatment for wastewater.

Except for the costs of instrument and labor, the fees for chemical reagents and electricity charge for the Fenton–SBR–microwave pyrolysis integrated process were calculated. In the Fenton process, the cost mainly focuses on chemical reagents such as H₂O₂, FeSO₄·7H₂O, H₂SO₄ and NaOH, and in SBR and microwave pyrolysis the main cost is electricity. The estimated cost of this Fenton–SBR–microwave pyrolysis integrated process is approximately US$320 per ton of wastewater.

High-concentration liquid crystal wastewater was efficiently and safely decomposed by the Fenton–SBR–microwave pyrolysis integrated process, where not only wastewater was degraded by the Fenton–SBR process but also iron mud was handled by microwave pyrolysis, realizing liquid–solid synchronous treatment.

This work was financially supported by Shandong Provincial Natural Science Foundation (Nos. ZR2016EEM15, ZR2019MEE097) and The National Water Pollution Control and Treatment Science and Technology Major Project (No. 2017ZX07101-002-05). Furthermore, we especially thank Prof. Rogers-Carpenter for the revision in English grammar and sentence structure.

S1, S2 and S3 are uploaded on the following youtube address: https://www.youtube.com/channel/UC9elA_h-AlsrjBVMK6lEYxQ.