Advanced treatment of triazole fungicides discharged water in pilot scale by integrated system: Enhanced electrochemical oxidation, upflow biological aerated filter and electrodialysis

Accepted Manuscript Advanced treatment of Triazole fungicides discharged water in pilot scale by integrated system: enhanced electrochemical oxidation, upflow biological aerated filter and electrodialysis Tao Cui, Yonghao Zhang, Weiqing Han, Jiansheng Li, Xiuyun Sun, Jinyou Shen, Lianjun Wang PII: DOI: Reference:

S1385-8947(17)30040-2 http://dx.doi.org/10.1016/j.cej.2017.01.039 CEJ 16341

To appear in:

Chemical Engineering Journal

Received Date: Revised Date: Accepted Date:

7 November 2016 31 December 2016 11 January 2017

Please cite this article as: T. Cui, Y. Zhang, W. Han, J. Li, X. Sun, J. Shen, L. Wang, Advanced treatment of Triazole fungicides discharged water in pilot scale by integrated system: enhanced electrochemical oxidation, upflow biological aerated filter and electrodialysis, Chemical Engineering Journal (2017), doi: http://dx.doi.org/10.1016/ j.cej.2017.01.039

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Advanced treatment of Triazole fungicides discharged water in pilot scale by integrated system: enhanced electrochemical oxidation, upflow biological aerated filter and electrodialysis Tao Cui, Yonghao Zhang, Weiqing Han*, Jiansheng Li, Xiuyun Sun, Jinyou Shen, Lianjun Wang**

Key Laboratory of Jiangsu Province for Chemical Pollution Control and Resources Reuse, School of Environment and Biological Engineering, Nanjing University of Science and Technology, Nanjing 210094, China

Corresponding author: *Weiqing **Lianjun

Han, Tel/Fax: +86-25-84315795, E-mail address: [email protected]. Wang,

[email protected];

Tel/Fax:

+86-25-84315941,

E-mail

address:

ABSTRACT In this work, a novel integrated system in pilot plant scale of electrochemical oxidation, upflow biological aerated filter and electrodialysis was investigated for high level standard reuse of triazole fungicides discharged water. In order to enhance biodegradable property of discharged water to be easier to recycled, a novel enhanced electrochemical oxidation reactor was designed and applied as pretreatment process. Effects of operating parameters on the performance of each process were studied with the discussion of economic evaluation. The results indicated that the optimal condition was current density of 5 mA/cm2, flow velocity of 3 m3/h, pH value of 5.0 and without supporting electrolyte. The target species including N-heterocyclic contaminants (tricyclazole, 1H-1,2,4-triazole and propiconazole) were removed over 90% from the discharged water by this process. Within upflow biological aerated filter, the remained COD would decrease further to the level less than 60 mg/L from 250 mg/L. Then the salt was removed efficiently during electrodialysis. Final effluent revealed a very low level of COD of 58.32 mg/L, TOC of 20.56 mg/L, EC50,48h of 73.1±2.1%, consistent with an excellent removal of the target species of 94.19% tricyclazole, 90.11% 1H-1,2,4-triazole and 100% propiconazole, >99% salt and a low operating cost of $0.85.

The excellent performance as well as the low energy

consumption confirmed that this integrated system is highly applicable for the advanced treatment of triazole fungicides discharged water.

Keywords TFs discharged water; Pilot scale test; Recycling; Enhanced electrochemical oxidation; UBAF; Electrodialysis

1. Introduction Triazole fungicides (TFs) are a diverse group of commercial fungicides commonly used in the planting of vegetables and fruits, cereal crop protection programs, wood preservation and lawn care [1,2]. Although the traditional technology, to a certain extent, can treat discharged water generated by the production of triazole fungicides, it still has shortcomings of containing a number of different kinds of contaminants such as nitrogen heterocyclic rings (NHCs), organic acids and salts. Tricyclazole (TC), 1H-1,2,4-triazole (Tz) and propiconazole (PPC), known as three typical nitrogen heterocyclic rings compounds have been identified in this kind of discharged water. The exposure of these pollutants to surface and groundwater may damage the whole environment, especially the aquatic ecosystem [3,4]. Besides, they are toxic to plants, microorganisms, animals and humans [5,6]. A case that water discharged by a pesticide production company contains these organic pollutants does exist in Changzhou, China. Moreover, the concentration of TC, Tz, PPC and COD of this discharged water reaches as high as 175 mg/L, 67 mg/L, 40 mg/L, 240 mg/L, respectively. If we can take advanced treatment and achieve resource recycling of discharged water, we will not only reduce the cost of processing water, but also avoid the damage to lives and the environment [7]. Conventional treatments of wastewater include biological degradation [8], advanced oxidations [9], distillation [10] and incineration [11]. However, nitrogen heterocyclic rings in discharged water are very difficult to biologically decompose due to the toxicity of triazole-ones. If we can firstly break the ring of the nitrogen heterocyclic compounds and turn into low molecular weight organic matter with low toxicity, biological degradation will be the optimal choice. Based on practical experience, distillation and incineration would not be appropriate for local WWTP

because their huge investment and new inevitable hazardous wastes. To overcome the drawbacks mentioned above, attempt to find a new and efficient treatment method has been made in recent years. For the past few years, electrochemical oxidation treatment, which was mainly for the removal of toxic and persistent organic pollutants, has attracted wide attention, such as pesticide wastewater [12-14], textile dyes effluents [15,16], landfill leachate wastewater [17], explosive wastewater [18] and pharmaceutical wastewater [19,20]. The outstanding characteristic of this kind of treatment lies in the ability to reach the total mineralization without any new toxic wastes produced. In 2014, our research group have conducted the electrochemical degradation of TC in aqueous solution on a Ti/SnO2-Sb/PbO2 anode [21]. Though results showed that Ti-substrate PbO2 anodes coated with a SnO2 + Sb 2O3 interlayer could increase electro-catalytic performance of electrodes and service life, anode modified by electrodeposition seemed not suitable for industrial production considering that this process needed reduplicative electrodeposition and different calcination programs. Moreover, traditional plate electrode is limited by mass transfer. Based on these considerations, in early 2015, our group invented a small-scale electrochemical oxidation reactor using tubular macro-porous titanium membranes electrode which had overcame the diffusion control bottlenecks and obtained an excellent performance at relatively low current density [22]. To continue previous work, this paper established a pilot-scale tubular reactor for the industrialized application. Biological treatment is relatively an economical method because of its cost-effective removal of organic compounds [23]. Upflow biological aerated filter (UBAF) is the most representative technology which is an alternative to the traditional activated sludge process commonly used in biological wastewater treatment. It has

been developed extensively due to several advantages, such as less occupied area [24,25] and excellent performance at much higher hydraulic loading than that of conventional biological process and high removal efficiencies for SS, COD, biodegradable organic substances and ammonia nitrogen [26]. At present, the treatment aimed at wastewater with high salinity is fairly single which has been one of intractable issues for so long. Making cyclic utilization after advanced treatment will not only alleviate serious pressure for water use but also greatly reduce environmental pollution. Li et al. [27] used artificial fast ooze biological filter for deeply treating organic pesticide wastewater. Wang et al. [28] presented a combined process with membrane bioreactor, advanced oxidation and ultrafiltration to treat heterocyclic pesticides wastewater. However, these studies only directed at organic pesticide in discharged water but lack of researches on the removal of salinity. Applied to advanced treatment of discharged water in the pesticide production, electrodialysis can achieve desalination and separation of small molecule organic salt so as to reduce the effluent concentration of COD. Therefore, in this paper, we put forward a combined process including enhanced electrochemical oxidation, upflow biological aerated filter (UBAF) and electrodialysis methods. The optimal parameters of this process, which used to purify and reuse the discharged water preferably, was also discussed.

2. Materials and Methods 2.1. Characterization of TFs discharged water Wastewater in this project was acquired from final sedimentation tank in Jiangsu Fengdeng environmental technology service co., LTD located in Changzhou China which was a real-raw TFs discharged water. The physical and chemical properties of

the TFs discharged water were shown in Table 1.

2.2 Pilot plant The combined process for the TFs discharged water consists of electrochemical oxidation, upflow biological aerated filter (UBAF) and electrodialysis. All the process equipment was built in target corporation. A flow diagram of this combined treatment process was presented in Figure 1. The whole treatment comprises three stages: electrochemical oxidation, UBAF and electrodialysis.

2.2.1 Enhanced electrochemical oxidation Before the pilot plant test, to determine the optimal current density and electrochemical oxidation elapsed time, we carried out pilot test. The apparatus of electrochemical oxidation consists of 40 pairs of electrodes, a DC power supply (SOYI-15300, 0-15V, 0-300A) and two vertical centrifugal pumps (ISG30-125, 5.0 m3/h, 0.75 Kw). It was important to make sure that each anode and cathode cannot short out and every joint is sealed in case of leaking water. The anode was made of macro-porous titanium membranes electrode (ɸ60 mm×1.15 m, external surface area 130.0 cm2, aperture 50 µm) coated with RuO2, while tubular and fenestrated stainless steel (ɸ90 mm×1.2 m) was used as cathodes; the distance between each pair of electrodes was 1 cm. Six cubic meters TFs discharged water was pumped into the reaction tank (2.5 m×2.5 m×1.5 m). Before starting electrochemical oxidation, we took water sample to confirm the initial concentration of TC, Tz, PPC. At the bottom of the reaction tank, a set of perforated pipe sparger which are connected with an air compressor were fitted out to play a role of stirring. As was shown in Figure 1, the

vertical centrifugal pump between the reaction tank on the left and the airtight stainless steel water tank on the right worked 2 minutes per 15 minutes to pump water from the anode chamber to the right and then back to the left tank. According to the jar-test investigated by our group before [22,29], it indicated that the optimal conditions for the electrochemical reactor was flow rate of 0.31 L min-1, pH value of 5.0, current density of 5 mA cm-2 and without supporting electrolyte. Therefore, the current density, initial pH, flow velocity and duration of the reaction in pilot-plant scale were set and discussed at 1.5~5.5 mA/cm2, 3~9, 3 m3/h and 24 h, respectively. At 3 hours’ intervals, 10 milliliter samples were taken to measure the characteristic organic pollutants concentration in the reactor. The treated wastewater after electrochemical oxidation was continuously transported to UBAF by high pressure power pump.

2.2.2 Upflow biological aerated filter (UBAF) Upflow biological aerated filter (UBAF) was placed after the process of electrochemical oxidation. Four cubic meters’ bio-ceramic (diameter 5~8 mm approximately) need to go through bio-film colonization for 30 days which the activated sludge used to make the film was obtained from the PACT reaction tank in the same wastewater treatment plant (Jiangsu Fengdeng co., LTD). Activated sludge with mixed liquor volatile suspended solids (MLVSS)/mixed liquor suspended solids (MLSS) = 0.75. The moisture content was determined to be 99.56% and the volatile suspended solid (VSS) concentration was set to 5.8 g/L. Organic matters in discharged water served as carbon source, and were cultivated in biochemical reactor with aeration furnish in which the gas-liquid ratio was (1~10):1. The hydraulic retention time (HRT) increased gradually from 10 hours to 24 hours. In the condition of

continuous aeration, through microscopic examination, when the bio-film appeared tawny and a large number of zoogloea, rotifer and ciliates which belong to protozoon can be seen, it can be judged that biological aerated filter had bio-film colonization succeeded and could put into operation. We regarded it as a cycle that water spilled over from the overflow on the top of the filter into the balance tank on the right and then returned to filter on the left. The sampling was set at 10 mL per 12 hours.

2.2.3 Electrodialysis According to high salinity of wastewater, electrodialysis set was determined to be placed after biological aerated filter. The wastewater to be treated was collected from the effluent tank of biological aerated filter in this pilot plant test. The structural component of this set was presented below. The membrane stack used in this work consisted of 40 cell pairs, i.e., 40 AEMs and 41 CEMs are assembled in total. Considering the availability of commercial membranes and investment cost, the domestic heterogeneous membrane combination (products of Hangzhou Ion-tech electrodialysis treatment material Co. Ltd., China) was mainly used. The stack was equipped with polypropylene spacer with a dimension of 42 cm (length) × 22 cm (width) × 0.75 cm (thickness), in which the polypropylene turbulence accelerating mesh net was embedded. During the ED experiments, a desired fixed electrical voltage or current was supplied by a variable power source (0–40 V, 0–10 A) by means of a couple of electrodes made of titanium coated with ruthenium with aid of 1 % sodium chloride aqueous solution (conductivity 10000 µs/cm approximately) used as electrode rinsing solution. Three plastic magnetic pumps (MP15R, Shanghai Magnetic Pump Co. Ltd., China.) were employed to drive flows into stack. The flow rates were monitored by specified flow meters (ZXLDG-35,

Nanjing Huapu Water Instrument Factory, China).

2.3. Analytical part The measured value of COD, tricyclazole, 1H-1,2,4-triazole and propiconazole all took standard methods for examination of wastewater. COD adopted titration method after digestion of the samples by potassium dichromate. Analysis of 5 day biological oxygen demand (BOD5) was carried out by a WTW BOD OxiTop-IS® 6 System (WTW Company, Germany). TOC was determined by a Vario TOC Cube analyzer using the non-purgeable organic carbon (NPOC) method. The concentration of TC, Tz, PPC was analyzed by Waters Alliance-HPLC-e2695. The diode array detector was performed at 35°C with a rate of 1 mL min−1 at isocratic mode. Injection volume was 10 µL and samples were detected as collected without diluted. The concentration of the diluent and the concentrate was determined according to the conductivity of solution. Hence, the relation between concentration and conductivity was examined at 20 ºC (Lei-ci conductivity meter: DDSJ-308A) and regressed in different concentration ranges. The results of the acute toxicity tests are expressed as EC50, 48 h (%, V/V), the concentration responsible for the death in 50% of the tested population after exposure. Further, economic of electrochemical treatment was mainly determined by electric energy consumption and operating cost. The electric energy consumption (EEC) and operating cost (OC) for per ton wastewater were calculated by Eq. 1 [30] and Eq. 3 [31]. EEC =

1000UIt (C0 - Ct )V

(1)

The operating costs of combined process consisted of: OC =OCEC + OCUBAF +OCED

(2)

Among them, the operating cost (OC) for per ton wastewater was calculated by: OC =

EEC (C0 - Ct ) × CE =UIt × CE 1000

(3)

Where U is the voltage (V), I is the current (A), t is the electrolysis time, C0 and Ct are the initial COD concentration (mg L-1) and COD concentration at t time (mg L-1), V is the volume of the electrolyte (L), CE represents the cost of one kilowatt-hour of electricity in the locality ($ 0.14).

3. Results and discussion 3.1. Enhanced electrochemical oxidation Discharged water from the final sedimentation tank first went through the electrochemical oxidation. To determine the operating parameters and optimize conditions of electrochemical oxidation, jar-test had been performed in our lab before in order to determine the operating parameters and optimize the conditions of electrochemical oxidation in the combined process. The aqueous degradation of triazole-ones has been reported that the traditional physical and chemical methods cannot be degraded effectively [32]. In consequence, the electrochemical oxidation process was placed as a pre-treatment procedure in the overall combined process. Benefited from its high degradation efficiency and economical cost against organic contaminants. The effects of current density, mechanism of electrochemical oxidation, initial pH on electrochemical oxidation of triazole-ones treatment were determined and then the overall integrated system of continuous treatment was performed. Figure 2 showed the results of effects of operating parameters on electrochemical oxidation.

3.1.1. Effects of current density In the electrochemical system, current density has a major role during the anodic

oxidation process [33]. The performance of the triazole-ones degradation was evaluated at various current densities ranging from 1.5 mA/cm2 to 5.5 mA/cm2. From Fig. 2(a) ~ (e), the removal of TC, Tz, PPC, COD and TOC were 51.49%, 40.58%, 56.50%, 34.56% and 30.57% at 1.5 mA/cm2 current density after 24 hours. The maximum removal of TC, Tz, PPC, COD and TOC were 85.35%, 82.54%, 95.31%, 59.79% and 48.22% with the current density of 5.5 mA/cm2 after 24 hours. It could be indicated that increasing current density has a positive influence on the decomposition of triazole-ones in the range 1.5~5.5 mA/cm2, similar results have been reported by our

group

while

studying

electrochemical

oxidation

of

5-Fluoro-2-Methoxypyrimidine by using a tubular porous electrode electrocatalytic reactor [29]. The removal of contaminants increased slowly when the current density increased from 4.5 mA/cm2 to 5.5 mA/cm2. Moreover, a remarkable phenomenon occurred in Figure 2(d), it appeared that during the first 15 hours of reaction, COD removal rate was negative which can be interpreted as a rise of COD concentration. This was mainly because triazole-ones were not easily oxidized by potassium dichromate because its oxidation property is not strong enough. However, the N-heterocycle were divided into fragments under the attack of hydroxyl radicals during electrochemical oxidation which can be detected by COD national standard detection method.

This is the reason why causing COD concentration to rise first

and then decrease. The phenomenon could be ascribed to the fact that a higher current density led to a higher production of hydroxyl radicals that originated from Reaction (1) which was presented below [34]. The remaining concentration of COD was composed of yet unoxidized organic compounds as well as the intermediate products after electrochemical oxidation. Electrochemical oxidation not only removed more than

half of the COD concentration, it also cut down its toxicity and non-biodegradable property to facilitate subsequent treating (upflow biological aerated filter).

3.1.2. Effect of mechanism of electrochemical oxidation The mechanism of electrochemical degradation of organic matter at anodes has been put forward by Comninellis [35]. Water is electrolyzed by anodic catalysis to produce adsorbed hydroxyl radicals [36]. H2O + MOx→ MOx [•OH] + H+ + e−

(1)

The adsorbed hydroxyl radicals may form chemisorbed active oxygen. MOx [•OH] → MOx+1 + H+ + e−

(2)

Meanwhile, the hydroxyl radicals will react with each other to form molecular oxygen to complete the electrolysis of the water molecules. MOx [•OH] → M + O2 + H+ + e−

(3)

The mechanism of electrochemical oxidation of TC has been carried out in our previous works [37]. The radicals [•OH] have a very short lifetime due to their high oxidation potential and the capacity of oxidizing organic compounds (i.e. direct oxidation). They can also turn into other oxidants (such as O2, O3 and H2O2) and then diffused to the water away from anode thus continuing the oxidation process (indirect oxidation). The direct oxidation can effectively degrade pollutants because of its strong ability to convert all the organic matters into water and carbon dioxide which the indirect oxidation did not have. Using a Ti/RuO2 tubular macro-porous membrane anode, the pollutants are mainly removed by direct electrolysis mediated by electro-generated hydroxyl radicals; indirect electron-transfer oxidation can be negligible.

3.1.4. Effect of solution pH Some reports showed that the electrochemical oxidation process was more favorable in acidic media [38]. While some other reports showed that the degradation efficiency was enhanced in alkaline media [39]. Thus, different pH value, varying from 3.0 to 9.0 which adjusted by adding sulfuric acid or sodium hydroxide, was selected to study the effect of pH on the removal of triazole-ones on the condition of current density 5.5 mA/cm2, As shown in Figure 2(f), it appeared that the lowest original pH value (pH 3) led to the highest oxidation efficiency of all. The removal efficiency of TC, Tz, PPC decreased from 85.35%, 82.54% and 95.31%, to 60.24%, 50.14% and 62.01%, respectively, when the pH increased from 3 to 9 after oxidation time of 24 hours. This could be due to that the increase of the solution pH decreased the oxygen evolution potential and consequently increased the flow rate of oxygen at the electrode surface, slowing the diffusion rate of the substances from the bulk solution toward the electrode [40]. Furthermore, the decrease of the solution pH favors oxidation reaction which can be testified by the decrease of COD removal efficiency between pH 3 and 9. However, strong acid solution would shorten the service life of electrode [41]. Thus, during the treatment, the initial pH was kept at 5.0. The operating cost of discharged water treatment should be reduced as much as possible with the guarantee of the high removal efficiency of contaminants. In order to avoid additional costs generated by adjusting pH, the pH 5.0 was chosen as the optimum condition.

Therefore,

the optimal operating parameters for

the

electrochemical oxidation were of current density of 5.5 mA/cm2, flow velocity of 3 m3/h and pH value of 5.0.

3.1.5. Biodegradability evaluation In addition to the above for COD monitoring, BOD5 values were measured before and after electrochemical oxidation in order to assess the biodegradability and the BOD5/COD ratio was used as a biodegradability indicator. The BOD5/COD ratio of the discharged water is 0.028, the triazole-ones were thus non-biodegradable. After electrochemical oxidation at pH value of 5.0 and current density of 5.5 mA/cm2 for 24 hours, the BOD5/COD ratio of the effluent was 0.46, showing that the biodegradability was significantly improved as the fact that some biodegradable by-products were generated during the EC process. As shown in Fig. 2(e), with increasing current density in EC process, the BOD5/COD ratio increased steadily from 0.25 to 0.46 after 24 hours’ oxidation. Similar conclusion could be drawn from Figure 2(f) which showed that BOD5/COD ratio increased as the initial pH value decreased. Increments in current density and acid–fortified had positive effects on biodegradability. Indeed, an effluent can be considered as easily biodegradable if its BOD5/COD ratio is above 0.4 [42]. Therefore, oxidation of triazole-ones by EC process provided a valuable pretreatment technique for subsequent upflow biological aerated filter.

3.2. Upflow Biological aerated filter (UBAF) UBAF system was successfully operated for about 90 days. Performance of UBAF in terms of biofilm colonization and contaminant removal had been systematically studied before. The inflow and effluent COD concentration along with COD removal rate during the first 15 days were shown in Figure 3(a). Particles which gave rise to turbidity were greatly rejected by bio-ceramic, so their fluctuation in the influent had little effect on the performance of the UBAF system. Part of low molecular weight organic matters could be biodegraded by microorganisms attached on the bio-ceramic. Therefore, in this section, different factors affected the performance of organic contaminant would be presented and discussed below.

3.2.1. Effect of HRT on COD removal Hydraulic retention time (HRT) played a crucial role in biological wastewater treatment. The effect of HRT (10.0-24.0 h) on COD removal efficiency was shown in Figure 3(b). At the very start, COD removal efficiency increased rapidly with the increase of HRT from 10.0 to 18.0 h. We guessed that under the short HRT, heterotrophic microorganisms suspended and on the surface of biological film were washed out by the strong shear force formed by the fast flow rate of air and water which led to the ineffective degradation of organic matters. However, as HRT increased, water velocity and shear force reduced which increased heterotrophic microorganisms in the outer layer of the biofilm and suspension. As a result, organic matters in the wastewater could be degraded effectively with COD removal rate increased gradually [43]. When HRT became longer, like 18.0 h to 24.0 h, COD removal rate changed slowly because fewer biodegradable organic compounds remained in the wastewater which slowed down the reproduction of heterotrophic

microorganisms in outer layer of the biofilm among the surface of bio-ceramic. Generally speaking, COD removal efficiency was stable as HRT increased from 10.0 h to 18.0 h. Based on these results, HRT should not be less than 18.0 h so that COD concentration could be reduced to 70 mg/L or less.

3.2.2. Effect of A/L on COD removal A/L means gas water ratio that is, aeration than liquid. It is well known that the microbe bioactivity within the reactor was markedly influenced by DO concentration so that changes of DO concentration in the UBAF would indirectly influence the removal of COD [44]. Figure 3(c) showed the UBAF performance for COD removal with hydraulic loading of 0.25 m3/ (m2·h) at different DO concentrations produced by different A/L. The UBAF was operated continuously at each ratio for at least one month. DO concentrations in the system not only depended on the A/L ratio but also on the organic loading rate and the oxygen transfer efficiency (OTE) [45]. At the A/L of 1:1, the COD removal efficiency was not satisfactory (32.00%) probably due to that the OTE was not high enough. Then the A/L ratio was gradually increased, firstly to 2:1, then to 4:1 and finally up to 8:1. By this strategy, COD removal rate increased rapidly. This was partly due to the increasing DO concentration in the wastewater (especially at the entrance of the UBAF reactor contributed by air compressor) made A/L enhanced thus promoting degradation of organic substrate by aerobic heterotrophic bacteria. When A/L increased from 8:1 to 15:1, COD removal rate decreased (23.00 %). It is likely that the increasing water flow rate with higher A/L generated strong shear force from the water flow [46]. Heterotrophic bacteria in the outer layer

of the biofilm among the surface of bio-ceramic was easily detached and washed out from the UBAF reactor and COD removal rate decreased due to the reduced practicable biofilm. Therefore, to obtain high COD removal rate, 8:1 was a better choice for A/L than other ratios As was shown in Figure 3(c), different COD removal efficiencies were observed in the UBAF when DO concentrations increased. The average COD removal efficiencies were 39%, 48%, 51% and 53%, respectively, when DO concentrations increased from 2.4 to 6.1 mg/L. These results confirmed that for the steady organic loading and hydraulic loading, microbe bioactivity could be enhanced which represented the increase of DO improved the removal of COD. On the other hand, with the increase of DO, diffusion function was accordingly strengthened which also contributed for the decrease of COD in the effluent [47]. However, it should be noted that with DO concentration rising from 4.1 to 5.0 mg/L, average COD removal efficiencies only increased 3% but the aeration intensity enhanced 1.5 times. Therefore, in order to achieve good COD removal efficiency and power consumption simultaneously, the UBAF should be operated with DO concentration of about 4 mg/L, under which average COD in the effluent was 60 mg/L.

3.3. Electrodialysis (ED) 3.3.1. Effect of cell voltage Cell voltage is one of the most important operational parameters in electrodialysis process which is the impetus of the ED process considering that it determines the migration velocity of Na+, K+, SO42-, Cl- and other ions across the membranes. In this test, the effluent of upflow biological aerated filter (COD 85 mg/L, salinity 4500 mg/L) was treated at different cell voltages as the following: 10 V, 20 V, 30 V,

40 V, respectively. Both the diluted water and concentrated water flux were 600 L/h and the electrode rinse flux was 200 L/h. As was shown in Figure 3(d), the conductivity of diluted water decreased from 204.7 µs/cm at 10 V to 78.5 µs/cm at 20 V rapidly but slightly from 78.5 µs/cm at 20 V to 41.1 µs/cm at 40 V. It seems that higher voltage was better for water quality but the ED increased linearly from 0.54 kWh/m3 to 4.58 kWh/m3 with the voltage increased from 10 V to 40 V. Since the conductivity of 132.0 µs/cm at 20 V was good enough to achieve the water reuse requirement, 20 V would be the optimal voltage for ED process if taking water quality and energy consumption into consideration. As for the power consumption, the ED was only 0.89 kWh/m3 under the condition of 20 V.

3.3.2. Effect of water flux In this test, the water flux of diluted water and concentrated water were kept at the same at six levels, 100 L/h, 200 L/h, 400 L/h, 600 L/h, 800 L/h and 1000 L/h. The cell voltage was fixed on 20 V. As was shown in Figure 3(e), the curve of diluted water showed that the optimal range of water flux would be 400–600 L/h. As for electrochemical oxidation, it declined gradually from 3.50 kWh/m3 to 0.85 kWh/m3 with the increasing water flux. Based on these results, if the ED process was operated at the water flux range of 400–600 L/h, the conductivity of diluted water would be under 50 µs/cm and the electrochemical oxidation would be less than 1.87 kWh/m3 of diluted water which indicated that the diluted water of ED process could be reused as cooling water and flushing water.

3.4. Integrated system for TFs discharged water treatment 3.4.1. Toxicity assays of integrated system

Figure 4 shows the toxicity of discharged water, the effluent of electrochemical oxidation, UBAF and electrodialysis. The higher EC50,48h value means a lower toxicity to Zebrafish. It can be observed that the EC50,48h value of discharged water was only 18.6±2.5%, indicating that the organic is highly toxic. Moreover, the EC50,48h value of the effluent of the three process was 68.2±3.2%, 72.6±4.2% and 73.1±2.1%, respectively. Discharged water exhibited the highest acute toxicity among these samples. What is more important is that the EC50,48h value of electrochemical oxidation was much higher than that of discharged water, suggesting a significant reduction in the biological toxicity via electrochemical oxidation. The low toxicity of electrochemical oxidation and UBAF indicated that the actual wastewater might be acceptable for recycling after electrodialysis in the system.

3.4.2. Continuous operation performance of integrated system The effects of factors mentioned above on the overall continuous integrated system were shown in Figure 3(f) and Table 2. COD, TC, Tz, and PPC of wastewater with 240, 100, 70 and 40 mg/L, was treated in this process. As pretreatment of the overall system, the contribution of electrochemical oxidation in terms of COD, TC, Tz and PPC removal was from 53.06%, 82.79%, 80.47% and 94.78%, to 58.41%, 85.87%, 84.16% and 96.54% under the same condition of current density of 5.5 mA/cm2, mass transfer rate of 3 m3/h and pH value of 5.0. The contribution of UBAF process to COD was 34.97%. In the overall combined process, COD, TC, Tz, PPC and conductivity removal efficiency achieved 76.34%, 94.19%, 90.11%,100% and 100% respectively. We found the triazole-ones in the continuous integrated system were almost removed and the effluent included only COD below 60 mg/L and conductivity below 50 μs/cm.

3.4.3. Economic evaluation of combined process Through the comparison of operating cost between our work and other published pilot plant works which was presented in Table 3. We could be proud to say that the OC of our pilot plant ($0.85 per ton) is lower than industrial water price in the locality ($0.92 per ton). This indicated that this combined process was more cost-saving in recycling treatment with residual toxicity and non-biodegradable discharged water. Therefore, our pilot plant showed big advantages in several aspects, such as larger electrochemical surface area, lower current density, higher treatment size and considerably less operation cost.

4. Conclusions In this work, an integrated system of electrochemical oxidation, UBAF and electrodialysis in pilot scale for over three months has been successfully applied for advanced treatment of triazole fungicides discharged water. As a result, a series of optimized parameters can be obtained and many findings can be verified: The triazole-ones were efficiently degraded by EC while the toxicity was significantly reduced in this section. The triazole-ones were first fragmented by EC, then most low-molecular compounds were removed by UBAF and the left were finally rejected by ED process for over 90%. Salinity in the discharged water were acted as electrolytes in EC and were mostly removed by ED. The final effluent demonstrated a low content of COD under 60 mg/L and conductivity under 50 μs/cm, which could meet the standard of high level recycle for industrial water. Especially, both the processing capacity and operating cost of this intergrated system in pilot scale was more extraordinary than the other conventional advanced treatment. The above results

indicated this combined process acquired obvious adaptability and pertinence in advanced treatment for chemical industry discharged water, especially for those with unbiodegradable organic contaminants and high salinity, which exhibited great potential application value for more actual employment.

Acknowledgement We thank the Jiangsu Fengdeng environmental technology service co., LTD for its technical support and special venue sponsor for pilot plant test. Financial support was obtained from National Science Technology Support Plan of China (2014BAC08B03) and Natural Science Foundation of China (51578287).

Appendix A. Supplementary data The electrochemical oxidation kinetics of triazole-ones degradation at different current density including TC, Tz and PPC. The effect of current density on triazole-ones degradation in electrochemical oxidation process. HPLC maps for the degradation of triazole-ones (including TC, Tz and PPC) in electrochemical oxidation process under the condition of 5.5 mA/cm2 of current density, 3 m3/h of flow velocity, pH 5.

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Graphical abstract

Figure 1. Flow diagram of combined process for TFs discharged water advanced treatment. (1) Electrochemical oxidation reaction tank, (2) electrode pair. (3) DC supply, (4) air compressor, (5) vertical centrifugal pump, (6) airtight stainless steel water tank, (7) upflow biological aerated filter, (8) balance tank. D: Dilute water compartment. E: Electrolyte Water Compartment. C: Concentrate water compartment.

Figure 2. (a) Effects of current density on TC removal efficiency. (b) Effects of current density on Tz removal efficiency. (c) Effects of current density on PPC removal efficiency. (d) Effects of current density on COD removal efficiency. (e) Effects of current density on TOC removal rate and the BOD5/COD ratio. (f) Effects of initial pH value on triazole-ones, COD removal efficiency and the BOD5/COD ratio.

Figure 3. (a) The COD removal of UBAF during the first 15 days ( , effluent of electrochemical oxidation treatment and

, COD effluent of UBAF treatment). (b)

Effects of HRT (10.0-24.0 h) on COD removal efficiency. (c) UBAF performance for COD removal with hydraulic loading of 0.25 m3/ (m2·h) at different DO concentrations produced by different A/L. (d) Effects of voltage on electro-dialysis removal efficiency. (e) Effects of different water flux on conductivity removal rate. (f) The effects of optimal operating parameter on the overall continuous integrated system.

Figure 4. Toxicity of each process unit(Ⅰ Ⅰis discharged water, Ⅱ, Ⅲ and Ⅳ is the effluent of electrochemical oxidation, UBAF and electro-dialysis, respectively.)

Table 1 Characteristic of TFs discharged water Parameter

Range

Average

Temperature (℃)

18-25

23

pH

7-7.5

7.2

SS (mg/L)

800-1000

900

Conductivity (µs/cm)

8000-9000

8500

COD (mg/L)

200-300

250

BOD5 (mg/L)

5.6-8.4

7.0

BOD5/COD

0.025-0.032

0.028

Tricyclazole (mg/L)

150-200

180

1H-1,2,4-Triazole (mg/L)

50-75

70

Propiconazole (mg/L)

25-55

40

Table 2 The effects on COD, tricyclazole(TC), 1H-1,2,4-triazole(Tz) and propiconazole(PPC) and salinity at the overall continuous combined process Process unit

COD mg/L

Raw Electrochemical

Tricyclazole(TC)

Propiconazole(PPC)

Salinity

mg/L

Removal(%)

mg/L

Removal(%)

mg/L

Removal(%)

µs/cm

Removal(%)

168.20

0(0)

72.89

0(0)

45.28

0(0)

7850

0(0)

115.70 53.06(53.06)

28.95

82.79(82.79)

14.24

80.47(80.47)

2.36

94.78(94.78)

7430

5.35(5.35)

65.24

73.53(43.61)

10.49

93.76(63.77)

8.47

92.50(40.52)

0

100(100)

7260

7.52(2.29)

58.32

76.34(10.61)

9.78

94.19(6.77)

7.21

90.11(14.88)

0

100(100)

41.1

99.48(99.43)

246.48

Removal(%)

1H-1,2,4-triazole(Tz)

a

b

0 (0 )

oxidation UBAF Electro-dialysis Wastewater reuse criterion

≤60.00

≤10.00

a

Cumulative removal efficiency of overall process.

b

Removal efficiency of unit process.

≤10.00

≤10.00

≤200.0

Table 3 The contrast of operating cost between our laboratory test, pilot plant test and other pilot scale test using similar process in publication

Research Group

Electrochemical surface area

Pair of electrode

cm2/pair A.J.C. da Silva et al

M.T. Fukunaga et al

Katsoni, A et al.

63.5 (Ti/IrO2–Ta2O5 electrode) 1800.0 (TiO2-RuO2 tubular electrode) 70.0 (BDD plate electrode)

Current density

Actuat daily capacity

Operation cost

References

mA/cm2

m3/d

$/m3·d(a)

1

20.0

0.007

1.93

[48]

1

100.0

0.12

51.81

[49]

1

257.0

0.60

1.40

[50]

1.05 G. Ren et al

120.0 (Ti-PbO2 mesh electrode)

10

10.0

0.016

(including electric charge and chemical cost

[51]

of Fe2+ and H2O2) Our laboratory test

Our pilot plant test a

69.0 (tubular porous Ti-RuO2 anode) 1884.0 (tubular porous Ti-RuO2 electrode)

1

3.0

0.02

0.78

40

5.5

6.00

0.85

[29] This study

Operating cost of each project was calculated by same COD removal efficiency (obtained from Figure 2). The charge of power according

to the national standard is settled as $0.14 per wat. The industrial water price in the locality is $0.92 per ton.

(1) Efficient removal of N-heterocyclic contaminants and salts by a novel integrated process. (2) A novel pilot-scale tubular electrochemical reactor which overcame diffusion control bottleneck. (3) Each process was optimized and integrated with relatively low energy consumption. (4) High level quality of final effluent for industrial water reuse.