Tube-in-Tube Membrane Microreactor for Photochemical UVC/H2O2 Processes: A Proof of Concept

Vítor J.P. Vilara,*, Pello Alfonso-Muniozgurenb, Joana P. Monteiroa, Judy Leeb, Sandra M. Mirandaa,*, Rui A.R. Boaventuraa

aLaboratory of Separation and Reaction Engineering - Laboratory of Catalysis and Materials (LSRE-LCM), Department of Chemical Engineering, Faculty of Engineering, University of Porto, Rua do Dr. Roberto Frias, 4200-465, Porto, Portugal

bChemical and Process Engineering, University of Surrey, Guildford, GU27XH

*Corresponding authors:

Tel.: +351 22 041 4937; E-mail address: sandra.miranda@fe.up.pt (Sandra M. Miranda)

Tel.: +351 918257824; E-mail address: vilar@fe.up.pt (Vítor J.P. Vilar)

Graphical abstract

Highlights

- Tube-in-tube membrane microreactor for photochemical UVC/H2O2 oxidation systems;

- Radial addition of H2O2 through the membrane porous into the annular reaction zone;

- “Virtually” unlimited number of H2O2 dosing points across the membrane length;

- Helical motion of water around the membrane shell-side enhances H2O2 radial dispersion;

- Homogenous axial and radial distribution of H2O2 molecules in the annular reaction zone;

- More homogeneous oxidation rates through the annular reaction zone.

Abstract

With the purpose of increasing the efficiency of UVC/H2O2 coupled systems for tertiary treatment of urban wastewaters, an innovative setup is proposed. The principle novelty of the new tube-in-tube membrane microreactor for photochemical UVC/H2O2 processes relies on the radial addition of H2O2 through the porous membrane into the annular reaction zone. This procedure maximises the use of H2O2 by maintaining a homogeneous distribution and constant concentration of the injected chemical across the whole length of the reactor. The proposed novel reactor consists of a ceramic ultrafiltration membrane inner tubing and a concentric quartz outer tubing that compose the annulus of the reactor (path length of 3.85 mm). The ultrafiltration membrane is used as a dosing system to deliver small amounts of H2O2 into the annulus of the reactor. The number of H2O2 dosing points available across the membrane length is “virtually” unlimited. In the annulus, where a 2 mg/L of oxytetracycline (OTC) solution flows, UVC light is provided via four mercury lamps located externally to the outer tube. The efficiency of the photochemical UVC/H2O2 process was evaluated as a function of the OTC flowrate, H2O2 dosage (H2O2 stock solution concentration vs H2O2 dosing rate), H2O2 dosage method and water matrix. An OTC removal of 36% with a residual H2O2 of 14 mg/L was obtained with a synthetic wastewater, while 7% OTC oxidation and 12 mg/L of residual H2O2 was measured when treating an urban wastewater fortified with the same OTC concentration. Besides providing a good performance using low UVC fluence (34 mJ/cm2) and reactor residence time (4.6 s), the reactor has the advantage of an easy upscaling into a real plant by integrating multiple parallel membranes into a single shell.

Keywords: Tube-in-tube Membrane Microreactor; UVC-H2O2; H2O2 dosing method; Process Intensification.

1. Introduction

The occurrence of organic microcontaminants in aquatic ecosystems has become an emerging concern due to the inability of current treatment methods to remove such compounds [1]. These contaminants of emerging concern (CECs), even in concentrations of nanograms or micrograms, constitute a potential threat to both the ecosystem and human health [2]. Consequently, more efficient wastewater treatment methods are required to reduce the discharge of such compounds into the environment, mainly from urban wastewater treatment plants (WWTPs) [3].

The use of advanced oxidation processes (AOPs) in general and particularly the combination of UVC/H2O2 have proved to be a suitable approach for the removal of CECs [4-12]. When combining H2O2 with UVC light, two OH radicals are formed through the photolytic cleavage of one H2O2 molecule that lead to a chain reaction [13, 14], improving the performance of the individual processes [7, 9, 11]. Generally, the photoreactor comprises a cylindrical shell of stainless steel housing concentric quartz sleeves filled with UVC lamps and the wastewater to be treated flows between the concentric tubes (annular reactor). Oxidant is supplied upstream of the photoreactor inlet, with the help of static mixers, being able to improve the in line mixing of the chemical with the wastewater to be treated. The oxidant concentration rapidly decreases along the reactor length, due to its homolytic cleavage by UVC light and its decomposition with organic and inorganic species present in the wastewater. Consequently, an oxidant concentration profile along the reactor length is observed, generating a non-homogenous reaction rate inside the photoreactor, limiting the length of reactors to be used in full-scale applications. In order to minimize radial and axial concentration gradients inside the annular reaction zone (ARZ) and thus maximize the efficiency of the photochemical process, higher doses of oxidant in the feed stream are required. However, an excessive concentration of H2O2 can act as a radical scavenger mainly in the reactor entrance and higher amounts of residual oxidant are wasted and must be eliminated before the discharge of the treated water into the environment. Consequently, more efficient photoreactors configurations are required to achieve fast reaction rates in the entire annular reactor zone with low residual concentration of chemicals at the reactor outlet.

Therefore, this work presents a new reactor setup, an UVC coupled tube-in-tube membrane microreactor for continuous addition of small H2O2 doses along the reactor length. This system intent to maximise the use of H2O2 by maintaining a homogeneous distribution and constant concentration of the injected chemical across the whole length of the reactor. The efficiency of the photochemical UVC/H2O2 reactor will be tested for the oxidation of oxytetracycline (OTC), a commonly antibiotic used as a model CEC, as a function of the OTC flowrate, H2O2 dosage (H2O2 stock solution concentration vs H2O2 dosing rate), H2O2 dosage method (radial permeation through the porous inner membrane or supplied upstream of the photoreactor inlet) and water matrix. The idea is to maximise the H2O2 usage and therefore, reduce the residual H2O2 concentration in the treated water. This would minimise (or even avoid) the use of subsequent treatment methods needed to remove residual H2O2, as well as the storage capacity and transportation costs of the chemical, maximising the efficient use of H2O2 in real WWTPs.

This type of reactors have been mainly evaluated for chemical synthesis using gas/liquid reactions [15-22], but none of the authors used the proposed reactors with wastewater treatment purposes. For example, Cui, et al. [23] proposed a continuous tube-in-tube membrane reactor for the catalytic N-oxidation of alkylpyridines with hydrogen peroxide, inspired by the idea of a tubular reactor with radial distribution of H2O2 given by Pineda-Solano, et al. [24]. No UV lamps were used in this case. A single tube-in-tube model, as well as a multiunit reactor bundle were proposed for real scale applications using a ceramic membrane for the segregated feeding of H2O2. Similarly, a liquid-liquid tube-in-tube semipermeable membrane microreactor was employed by Buba, et al. [25] for the synthesis of N-Methylated amino acids. For this experimental study, a Teflon AF 2400 membrane was used as an improved mixing system, injecting the solvent by a syringe pump in the annulus of the reactor.

2. Material and methods

2.1. Reagents

Oxytetracycline hydrochloride (OTC, MW = 496.89, 96% purity, CAS# 2058-46-0) was supplied by ThermoFisher and used as a model compound. Hydrogen peroxide (HYPE-30P-1K0 from Labbox, purity 30% (v/v)) was used as oxidant. Na2SO3 (Merck, p.a.) was added to samples for H2O2 elimination. Ammonium monovanadate (Merck, p.a.) was used to determine H2O2 concentration. For OTC analysis by HPLC-DAD, acetonitrile, methanol and oxalic acid dehydrated were supplied by Merck. Pure water (Panice® reverse osmosis system) and ultrapure water (Millipore® Direct-Q system) were used to prepare the synthetic OTC solutions and the mobile phases for HPLC system, respectively. Methylene blue was provided by Merck and used to prepare tracer solutions.

A real wastewater sample was collected after the secondary settling tank of an urban WWTP from Northern Portugal, and kept refrigerated. Table 1 presents its main physicochemical characteristics. The synthetic wastewater (SW) and the urban wastewater (UW) fortified with 2 mg OTC per liter were used as inlet streams of the tube-in-tube reactor.

Insert Table 1

2.2. Lab-scale prototype

The proposed novel tube-in-tube reactor consists of an inner ceramic ultrafiltration membrane (γ-Al2O3 membrane from Inopor GmbH) and a quartz outer tubing (Figs. 1 and 2). The main characteristics of the tube-in-tube reactor are summarized in Table 2. The tubes ends are tightly sealed by two movable polypropylene flanges (Fig. 1a). The membrane outlet is connected to a back-pressure regulator (BPR). The H2O2 stock solution enters from the inlet of the membrane, fed by a syringe pump (Nexus 6000 from Chemyx Inc.), permeates through the porous ceramic membrane, and contacts directly with the concurrently fed OTC stream in the annulus (Fig. 3). The bottom and top flanges have inlet and outlet pipes located perpendicularly to the wastewater flow direction and tangentially to the quartz tube, in horizontal plane and at the top in opposite sides (Fig. 2b). This configuration induces a helical motion of wastewater around the membrane shell-side, as can be seen at Fig. 2c using a tracer solution. The fluid movement drags the small H2O2 drops from the membrane shell-side to the bulk of the ARZ. The wastewater is pumped from a jacketed vessel (connected to a F12-MA chiller from Julabo) to the ARZ using a gear pump (model BVP-Z from Ismatec). Radiation is provided by four UVC lamps (Puritec HNS 6 W G5 from Osram) located externally to the quartz tube and with a nominal power of 6 W (λmax = 254 nm; useful power = 1.4 W). Photon flow (1.7 W) inside the ARZ was measured by H2O2 (35 mM) actinometry [26] (overall quantum yield, ΦT = 1.11, molar absorptivity, P = 18.6 M-1 cm-1, path length, b = 3.85 mm). The setup is surrounded by an aluminium shell to avoid direct eye contact with UVC lamps.

Insert Figure 1

Insert Figure 2

Insert Figure 3

Insert Table 2

2.3. Experimental procedure

The ceramic membrane was internally filled with the H2O2 stock solution, with the BPR fully open. After that, with the BPR fully closed, the syringe pump is used to gradually administer the H2O2 solution, until the first drops of H2O2 appears in the shell side of the membrane. This indicates that the membrane pores are completely filled with the H2O2 solution. Then, 1 L of pure water was pumped through the ARZ to clean the membrane surface. Subsequently, 5 L of an OTC solution ([OTC]inlet = 2 mg/L; T = 25ºC) was pumped through the ARZ. At the same time, the syringe pump administer the H2O2 solution at low dosing rates and UVC lamps were turned on. Samples were collected at different periods until reaching steady state conditions and further analysed in terms of OTC and H2O2 concentration. Immediately after samples collection for OTC analysis, Na2SO3 in a Na2SO3:H2O2 molar ratio of 1:1 was added to quench H2O2.

The concentration of hydrogen peroxide in the annular reaction zone at steady state conditions in the absence of reaction (without OTC) was first evaluated for different water inlet flow rate, H2O2 stock solution concentration and H2O2 dosing rate . After that, OTC oxidation tests were performed at different reaction conditions using the synthetic OTC solution and H2O2 permeation method: i) direct photolysis (no oxidant permeation), ii) only with H2O2 (absence of radiation) and iii) UVC/H2O2. The following operating conditions were tested: OTC solution inlet flow rate (Qinlet,OTC = 10, 20, 40, 60 or 80 L/h), H2O2 stock solution concentration ([H2O2]stock solution = 7.5, 15 or 30 g/L) and H2O2 dosing rate (0.35, 0.50, 0.70, 1.4, 2.1, 2.8 or 3.5 mL/min). Assays with the real wastewater sample were performed at the following conditions: Qinlet,OTC = 40 L/h; [H2O2]stock solution = 7.5 g/L and H2O2 dosing rate = 1.4 or 2.1 mL/min.

The system performance was also evaluated using the conventional oxidant dosing method- supplied upstream of the photoreactor inlet (direct injection). Table 3 presents the experimental conditions used in all tests.

Insert Table 3

2.4. Analytical procedure

OTC concentration was determined by HPLC using a Hitachi ELITE LaChrom (Merck-Hitachi, Tokyo, Japan), equipped with a L-2130 pump, a L-2200 autosampler, a L-2300 column oven, a Purospher® RP-18e 125–4 (5 μm) (Merck) column and a L-2455 DAD. A detailed description of the analytical method can be found elsewhere [27]. H2O2 concentration was determined by the metavanadate method (λ = 450 nm) [28]. Dissolved organic carbon (DOC), chemical oxygen demand (COD), total suspended solids (TSS), volatile suspended solids (VSS), total phosphorous, pH, temperature and turbidity, as well as inorganic anions and cations concentrations were assessed according to the procedures already described by Moreira, et al. [29].

3. Results and discussion

3.1. H2O2 permeation

Fig. 4 shows the H2O2 concentration profile on the water flow side (annular zone) at the outlet of the tube-in-tube reactor. H2O2 concentration increases with time until steady state conditions after t/ ≥ 90. The H2O2 steady state concentrations are very similar to the theoretical values considering the annular flow rate, H2O2 stock solution concentration and H2O2 dosing rate. A similar trend was observed when varying the annular flow rate, H2O2 stock solution concentration and H2O2 dosing rate (data not showed).

Insert Figure 4

3.2. Effect of the OTC annular flow rate

The OTC concentration profiles at the outlet of the tube-in-tube reactor for different H2O2 permeation doses, displayed in Fig. 4, shows that steady state conditions are obtained after stabilization of H2O2 in the reactor outlet (t/ ≥ 90), in agreement with the results reported in section 3.1. The shaded areas in Fig. 4 corresponds to the amount of H2O2 consumed during each reaction for the different H2O2 permeation doses.

Therefore, the OTC conversion for all assays will be calculated by taking into account the OTC concentration in the inlet and outlet streams of the tube-in-tube reactor after t/ ≥ 100, corresponding to an OTC working volume of 5 L.

According to Fig. 5: i) UVC light was not able to break down efficiently the OTC molecules considering the low residence time inside the ARZ; ii) H2O2 molecules showed an oxidation power over OTC molecules, resulting in a maximum OTC conversion of 12%; iii) the combination of UVC light and H2O2 boosted the OTC oxidation efficiency, achieving values in average of 30%, mainly associated to the generation of hydroxyl radicals.

Fig. 5 also shows the effect of the annular flow rate (Qinlet,OTC) in the OTC conversion using the same amount of H2O2 injected per unit of time (10.5 mg H2O2 per min). The H2O2/UVC process led to higher OTC conversions with the increment of Qinlet,OTC from 10 to 20 L/h (Fig. 5), indicating a change in the hydrodynamic conditions inside the ARZ. A Re number of 1028 (Q = 20 L/h, = 9.1 s) allowed to a 1.5-fold increase on OTC conversion comparing with a Re of 514 (Q = 10 L/h, = 18.3 s). This can be explained by the poor mixing of H2O2 in the ARZ (the helical motion of water around the membrane shell-side only starts to occur for annular flow rates ≥ 20 L/h), decreasing the photolysis of H2O2 by the UVC light and further interaction of the reactive species and UVC photons with the OTC molecules, even in the presence of a higher H2O2 residual concentration and higher residence time.

Insert Figure 5

A decrease on OTC conversion can be observed for flow rates higher than 40 L/h (Re >2056; < 4.6 s). Under these mixing conditions, the limiting factors are the residence time and H2O2 concentration in the ARZ. Fig. 6 shows also the effect of the degree of mixing and residence time on OTC oxidation (assay 14: Qinlet,OTC = 40 L/h, [H2O2]stock solution = 7.5 g/L, H2O2 dosing rate = 1.4 mL/min; assay 9: Qinlet,OTC = 80 L/h, [H2O2]stock solution = 15 g/L, H2O2 dosing rate = 1.4 mL/min), considering the same concentration of H2O2 on the annular reaction zone at the steady state conditions (15.8 mg/L). A 2.5-fold increase on OTC oxidation efficiency is observed when the flow rate decreased from 80 L/h to 40 L/h, showing that under these mixing conditions, the residence time is the limiting factor.

Insert Figure 6

Therefore, considering the highest OTC conversion (27±3%) at lower residual H2O2 concentration, a feed flow rate of 40 L/h was selected for the next set of experiments. However, for Qinlet,OTC = 60 L/h, the addition of 24 mg H2O2 was requested per mg OTC oxidised, against 29 mg H2O2 added per mg OTC oxidised for Qinlet,OTC = 40 L/h.

3.3. Effect of H2O2 stock solution concentration

The effect of H2O2 stock solution concentration in OTC conversion was evaluated by maintaining the amount of H2O2 injected per unit of time (10.5 mg H2O2 per min) for a Qinlet,OTC = 40 L/h. According to Table 1 (Assays 1, 6 and 14), the OTC conversion remains on average near 30% for all conditions. This indicates that the helical motion of wastewater around the membrane shell-side drags rapidly the small H2O2 drops from the membrane shell-side to the bulk, resulting in a homogenous distribution of H2O2 molecules in the ARZ. This minimizes local points near the membrane surface with much higher oxidant concentration than in ARZ bulk.

3.4. Effect of H2O2 dosage

As concluded by Crittenden, et al. [30], “there exists an optimum hydrogen peroxide dosage for each set of reactor conditions with regard to the organic pollutant removal efficiency of the H2O2/UV process“. Thus, the concentration of H2O2 in the reactor annulus was varied while maintaining an OTC annular flow rate of 40 L/h (Fig. 6a). This was carried out by modifying the H2O2 stock solution concentration and H2O2 dosing rate. Overall, OTC conversion increases with the increment on H2O2 concentration in the ARZ until 15.8 mg/L. A further increment on H2O2 doses until 31.5 mg/L led to a similar OTC conversion (Fig. 6a.1). In fact, H2O2 in excess can act as hydroxyl radicals scavenger [10, 31].

In order to increase the treatment capacity, the OTC inlet flow rate was increased to 80 L/h. Under these conditions, to achieve an OTC conversion near 30%, an H2O2 concentration in the ARZ of 39.4 mg/L is required, resulting in a much higher residual H2O2 concentration (Fig. 6b.2).

Considering the above and taking into account both OTC conversion and residual H2O2 as reactor performance parameters, the best approach was obtained with a OTC inlet flow rate of 40 L/h and H2O2 concentration on the ARZ of 15.8 mg/L.

3.5. Effect of H2O2 dosage method

The H2O2 radial permeation through the porous inner membrane was compared with the conventional supply of the chemical upstream of the photoreactor inlet (direct injection). The direct injection was carried out using a H2O2 dosing rate of 1.4 mL/min and H2O2 stock solution of 7.5 g/L. Similar OTC conversion and H2O2 residual concentrations were obtained using both oxidant dosage methods. This can be related to the constant axial gradient profile of the oxidant in the ARZ, as a consequence of the low H2O2 consumption in short reaction times (4.6 s). However, in full-scale applications, the oxidant is rapidly consumed along the reactor length, due to its homolytic cleavage by UVC light and its decomposition with organic and inorganic species present in the wastewater. Consequently, an axial oxidant profile will be generated when using tubes length in the order of 1 m, resulting in different OTC conversion rates across the reactor length, limiting the length of reactors to be used in full-scale applications. In order to minimize radial and axial concentration gradients of the oxidant inside the ARZ and thus maximize the efficiency of the photochemical process, higher doses of oxidant in the feed stream are required. However, an excessive concentration of H2O2 can act as a radical scavenger mainly in the reactor entrance and higher amounts of residual oxidant are wasted and must be eliminated before the discharge of the treated water into the environment. Therefore, the uniform addition of H2O2 to the reaction zone through “virtually” unlimited dosing points along the membrane shell side can enhance the transformation rate of target contaminants and can also reduce the residual H2O2 concentration at the reactor outlet.

3.6. Effect of water matrix

In order to test this best approach for real applications, OTC removal by the photochemical UVC/H2O2 system was also evaluated for an urban wastewater after secondary treatment fortified with 2 mg/L of OTC (Fig. 7). Paralleling the reaction rates in both matrices, a substantial decrease in OTC conversion was observed in the presence of the real wastewater, adding the same H2O2 amount to the ARZ (15.8 mg/L). This is mainly related to the presence of light absorbing and reactive oxygen scavenger species (NOM, nitrite, etc.) [32]. Although the OTC conversion using the real wastewater almost doubled with the increment on [H2O2]ARZ from 15.8 to 23.6 mg/L, the residual H2O2 concentration also increased in similar extend. Therefore, a much higher amount of oxidant is required to overcome those effects to obtain OTC conversions similar to the synthetic wastewater. However, it is important to highlight that the aforementioned results were obtained using a low UVC fluence (45 mJ/cm2), being significantly lower than those used in real treatment plants (600-1000 mJ/cm2) [33].

Insert Figure 7

4. Conclusions

This study proves the viability of using tube-in-tube membrane microreactors to promote photochemical UVC/H2O2 oxidation processes for wastewater treatment. The oxidant stock solution enters from the inlet of the membrane, permeates through the porous ceramic membrane, and contacts with the concurrently fed wastewater in the annulus. The helical motion of wastewater around the membrane shell-side drags rapidly the small H2O2 drops from the membrane shell-side to the bulk. The homogenous axial and radial distribution of H2O2 molecules in the annular reaction zone can enhance the transformation rate of target contaminants and can also reduce the residual H2O2 concentration at the reactor outlet. At the same time, the proposed set-up could be easily scaled-up by integrating multiple parallel membranes into a single shell. In order to provide a better light distribution in the ARZ, multiple concentric quartz sleeves filled with UVC lamps can be incorporated parallel to the several membranes.

The tube-in-tube membrane microreactor showed an OTC conversion near 36%, for a synthetic wastewater containing an OTC concentration of 2 mg/L, resulting in a residual H2O2 of 14 mg/L. Although lower OTC conversions were observed for an urban wastewater after secondary treatment, fortified with the same amount of OTC, a low UVC fluence (45 mJ/cm2) was applied, when compared to those used in real treatment plants (600-1000 mJ/cm2). The authors would like to highlight that this type of reactor configuration can be used to promote different chemical, catalytic, electrocatalytic and photo-driven processes, through the dosing of any kind of oxidant (either gas or liquid) or catalyst solutions along the reactor length.

Acknowledgements

This work was financially supported by Associate Laboratory LSRE-LCM - UID/EQU/50020/2019 - funded by national funds through FCT/MCTES (PIDDAC). V.J.P. Vilar acknowledges the FCT Investigator 2013 Programme (IF/00273/2013).

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Figure Captions

Figure 1. Photographs of the tube-in-tube reactor a) and respective components (b): 1 - polypropylene flanges; 2 – quartz outer tube; 3 - γ-Al2O3 ultrafiltration membrane inner tube; 4 - UVC lamp.

Figure 2. a) Reactor scheme; b) Reactor flanges design; c) Images of the helical motion of the tracer solution (methylene blue dye solution) in the annular reaction zone.

Figure 3. Sketch a) and photograph b) of the proposed setup.

Figure 4. a) H2O2 concentration profiles (open symbols) on the water flow (Qinlet,water = 40 L/h) side at the reactor outlet in the absence of UVC light; b) OTC (full symbols) and H2O2 (semi-filled symbols) concentration profiles on the water flow (Qinlet,OTC = 40 L/h) side at the reactor outlet in the presence of UVC light. [H2O2]stock solution = 7.5 g/L; H2O2 dosing rate (mL/min) = 0.7 (, , ); 1.4 (, , ); 2.1 (, , ).

Figure 5. a) Effect of the annular flow rate in the OTC conversion; b) Residual H2O2 concentration vs Qinlet,OTC: [H2O2]stock solution = 15 g/L; H2O2 dosing rate = 0.7 mL/min. - UVC/H2O2; - H2O2 (absence of radiation); - UVC photolysis (no oxidant permeation).

Figure 6. Effect of the H2O2 dosage in the ARZ on OTC conversion: a.1 and a.2: Qinlet,OTC = 40 L/h, [H2O2]stock solution = 7.5 g/L (), 15 g/L (), 30 g/L (); b.1 and b.2: Qinlet,OTC = 80 L/h, [H2O2]stock solution = 15 g/L ().

Figure 7. OTC conversion a) and residual H2O2 concentration b) for synthetic () and real matrices () fortified with 2 mg OTC per liter: Qinlet,OTC = 40 L/h; [H2O2]stock solution = 7.5 g/L; H2O2 dosing rate = 1.4 or 2.1 mL/min.

Figures

Figure 1

Figure 2

Figure 3

b)

a)

Figure 4

Figure 5

Figure 6

Qinlet,OTC = 40 L/h

Qinlet,OTC = 80 L/h

Figure 7

Tables

Table 1. Characteristics of the tube-in-tube reactor.

Components

Characteristics

Quartz tube

Ø External

3.2 cm

Ø Internal

2.8 cm

Total length

20 cm

Illuminated length

17.4 cm

Ultrafiltration membrane

Ø External

2.03 cm

γ–Al2O3

Ø Internal

1.55 cm

(Cut-off: 20 kDa)

Total length

20 cm

Illuminated length

17.4 cm

Pore size

10 nm

Porosity

30 - 55 %

UVC lamp

Lamp power

6 W

HNS 6W G5

Radiation flux (254 nm)

1.4 W

Table 2. Main physicochemical characteristics of the real wastewater sample collected after the secondary settling tank of an urban WWTP.

Parameters (units)

Color

Pale yellow

Odor

n.d.a

Turbidity (NTU)

7.1

pH

8.1

Temperature (°C)

24

Dissolved Oxygen (mg/L)

4.4

Total dissolved carbon (mg/L)

91.7

Inorganic carbon (mg/L)

68.3

Dissolved organic carbon (mg/L)

23.4

Chemical oxygen demand (mg/L)

117

Absorbance at 254 nm (AU)

0.272

Total suspended solids (mg/L)

2.7

Volatile suspended solids (mg/L)

0.68

Ammonium – N-NH4+ (mg/L)

1.1

Nitrite – N-NO2- (mg/L)

0.2

Nitrate – N-NO3- (mg/L)

< 0.1

Chloride – Cl- (mg/L)

167

Sulfate – SO42- (mg/L)

78

Phosphate – PO43- (mg/L)

12.6

Total Phosphorous – P (mg/L)

4.1

a) n.d. – not detected

29

Table 3. Experimental conditions used in all tests.

[H2O2]stock solution (g/L)

H2O2 dosing rate (mL/min)

[H2O2]ARZ (mg/L)

QInlet,OTC (L/h)

τ (s)

Re

mgH2O2/mgOTC

Efficiency (%)

[H2O2]Residual (mg/L)

Experiment number

Radial Permeation

Synthetic OTC solution

30

0.35

15.8

40

4.6

2056

26

31±2

10±2

1

0.50

22.5

40

28±1

16±2

2

0.70

31.5

48

33±2

21.8±0.3

3

15

0.70

63.0

10

18.3

514

171

18±3

41±1

4

31.5

20

9.1

1028

59

27±4

25±2

5

15.8

40

4.6

2056

29

27±3

10±3

6

10.5

60

3.0

3084

24

22±3

6.8±0.5

7

7.9

80

2.3

4112

52

8±1

2.4±0.7

8

15

1.4

15.8

80

2.3

4112

74

11±3

10±0.5

9

2.1

23.6

49

24±4

18.9±0.5

10

2.8

31.5

58

27±1

27±2

11

3.5

39.4

59

33±1

36.7±0.5

12

7.5

0.70

7.9

40

4.6

2056

25

16±3

5.6±0.7

13

1.4

15.8

22

36±2

14.3±0.7

14

2.1

23.6

36

32±2

22.8±0.3

15

UW fortified with OTC

7.5

1.4

15.8

40

4.6

2056

121

6.5±0.5

12±1

16

2.1

23.6

97

12±1

21±2

17

Direct Injection

Synthetic OTC solution

7.5

1.4

15.8

40

4.6

2056

30

39±2

16±3

18

0

5

10

15

20

25

30

35

40

45

[

H

2

O

2

]

Outlet

(mg/L)

7.9

23.6

15.8

020406080100

0.0

0.2

0.4

0.6

0.8

1.0

t/t

[OTC]

Outlet

/[OTC]

Inlet

1020406080

0

10

20

30

80

100

(a)

1-[OTC]

Outlet

/[OTC]

Inlet

(%)

Q

Inlet,OTC

(L/h)

1020406080

0

10

20

30

40

(b)

[

H

2

O

2

]

Residual

(mg/L)

Q

Inlet,OTC

(L/h)

0

10

20

30

40

90

100

(a.1)

1-[OTC]

Outlet

/[OTC]

Inlet

(%)

0

10

20

30

90

100

1-[OTC]

Outlet

/[OTC]

Inlet

(%)

(b.1)

7.915.822.523.631.5

0

5

10

15

20

25

30

[

H

2

O

2

]

Residual

(mg/L)

[H

2

O

2

]

ARZ

(mg/L)

(a.2)

7.915.823.631.5039.4

0

5

10

15

20

25

30

35

40

(b.2)

[

H

2

O

2

]

Residual

(mg/L)

[H

2

O

2

]

ARZ

(mg/L)

15.823.6

0

10

20

30

40

50

90

100

1-[OTC]

Outlet

/[OTC]

Inlet

(%)

[H

2

O

2

]

ARZ

(mg/L)

(a)

15.823.6

0

5

10

15

20

25

(b)

[

H

2

O

2

]

Residual

(mg/L)

[H

2

O

2

]

ARZ

(mg/L)