H2O2 reactors

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Design aspects of UV/H2O2 reactors B.A. Wols, D.J.H. Harmsen, T. van Remmen, E.F. Beerendonk, C.H.M. Hofman-Caris

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Received date: 10 March 2015 Revised date: 3 June 2015 Accepted date: 24 June 2015 Cite this article as: B.A. Wols, D.J.H. Harmsen, T. van Remmen, E.F. Beerendonk, C.H.M. Hofman-Caris, Design aspects of UV/H2O2 reactors, Chemical Engineering Science, http://dx.doi.org/10.1016/j.ces.2015.06.061 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Design aspects of UV/H2 O2 reactors B.A. Wolsa,b,d , D.J.H. Harmsena , T. van Remmenc , E.F. Beerendonka , C.H.M. Hofman-Carisa a

KWR Watercycle Research Institute, Groningenhaven 7, 3430 BB Nieuwegein, The Netherlands b Delft University of Technology, The Netherlands c Van Remmen UV Techniek, The Netherlands d Corresponding author, tel: +31 (0)30 60 69 604, fax: +31 (0)30 60 61 165, e-mail: [email protected].

Abstract The design of UV/H2 O2 reactors is studied. First an analytical approach is followed, that provides characteristics (maximum fluence and maximum possible degradation) for an ideal system. An efficiency parameter is introduced that relates the actual performance of a reactor with the best possible performance for a reactor with a certain water flow, lamp power and UV transmittance. From the analytical model, several design parameters were investigated. The desired treatment level influences the choice of a design parameter, as the fluence distribution becomes less important at lower desired treatment levels. As the desired degradation levels in UV/H2 O2 applications are lower than the desired inactivation levels for UV disinfection, a UV/H2 O2 reactor may be designed with a larger water layer depth than UV disinfection reactors. Also, manipulating the velocity profile towards a profile that mimics the fluence rate profile is beneficial, as well as increasing mixing. Increasing the number of lamps (while the total energy consumption remains the same) is beneficial, as it results in a more uniform fluence rate profile. Enlarging the quartz sleeve has limited effect. Changing the distribution of hydrogen peroxide in the reactor also has limited effects. UsPreprint submitted to Chemical Engineering Science

July 3, 2015

ing the design parameters, new UV/H2 O2 reactor types were developed with CFD simulations and tested experimentally. An increase in log degradation up to 30% was demonstrated by the improved reactor design. Keywords: UV, advanced oxidation process, hydrogen peroxide, pharmaceuticals, water treatment, modelling

1

2

1. Introduction The UV/H2 O2 process is an important barrier against organic micropol-

3

lutants (OMPs). The use of UV/H2 O2 processes for treatment of (waste)waters

4

is stimulated by the growing occurrence of organic micropollutants in water

5

sources. One of the drawbacks of the UV/H2 O2 process is the energy con-

6

sumption. For the degradation of OMPs the UV dose should roughly be a

7

factor of 10 higher than for UV disinfection. Nevertheless, most effort on

8

improving the energy efficiency of UV reactors has been done for the UV

9

disinfection process. For the optimization of UV reactors, computational

10

fluid dynamics (CFD) models are often used to account for the complex

11

hydraulics (Janex et al., 1998; Chiu et al., 1999; Wright and Hargreaves,

12

2001; Pan and Orava, 2007; Wols et al., 2011; Xu et al., 2013; Zhang et al.,

13

2014). The hydraulics of the UV disinfection reactors are optimized by

14

reducing short-circuit flows, associated with low fluences (Schoenen, 1996;

15

Chiu et al., 1999). Assessment of commercial designs or representatives of

16

commercial designs reveals big differences in reactor performance (Wright

17

and Hargreaves, 2001; Pan and Orava, 2007; Wols et al., 2011), related to the

18

hydraulics inside the reactor. Different measures to reduce short-circuits and

19

stagnant zones have been investigated: placing baffles (Janex et al., 1998;

20

Chiu et al., 1999), changing inflow and outflow piping (Wright and Harg-

21

reaves, 2001; Xu et al., 2013), and changing lamp positions (Xu et al., 2013). 2

22

Design aspects, such as length of reactor, inner and outer reactor radius, and

23

multiple lamp configurations were investigated for annular systems by Xu

24

et al. (2013).

25

Less effort has been put in optimizing UV/H2 O2 reactors, although the

26

energy consumption and therefore the potential in energy gain is much

27

higher. Chong et al. (2010) recognize the need for more effective designs

28

of photo-catalytic reactor systems. Taghipour and Sozzi (2005) considered

29

two reactor designs for UV photolysis and found that short-circuits have a

30

large impact on reactor efficiency. In a parametric study they show that

31

the better design becomes more important at low flow rates and high UV

32

output. The effect of reactor radius in annular reactors has been a topic

33

of study (Shen and Wang, 2002; Johnson and Mehrvar, 2008; Mohajerani

34

et al., 2012), indicating that there is an optimal water layer distance for

35

each specific application of a UV reactor. However, no general conclusion

36

on water layer distances were obtained.

37

This work focuses on the improvement of UV/H2 O2 reactors, although the

38

methodology could be used for UV disinfection systems as well. First an

39

analytical approach is applied, that provides characteristics (maximum flu-

40

ence and maximum possible degradation) for an ideal system. An efficiency

41

parameter is introduced that relates the actual performance of a reactor

42

with the best possible performance for a reactor with a certain water flow,

43

lamp power and UV transmittance. Using the analytical approach, differ-

44

ent design parameters are thoroughly studied. Using these design aspects,

45

new types of UV/H2 O2 reactors were designed with CFD modelling. Sub-

46

sequently, the new reactors were built and experiments were performed in

47

a pilot-scale setup to verify the improved degradation of different organic

48

micropollutants. 3

49

2. Materials and methods

50

2.1. Experimental conditions

51

Pilot-scale UV/H2 O2 reactor tests were performed. Two reactors (VR130

52

and VR200), supplied by Van Remmen UV Techniek, were both equipped

53

with 1 monochromatic (low-pressure) lamp (Hereaus NNI 125-84-XL) with

54

an electrical power output of 120 W (UV-C output of 38 W). A calibrated

55

plug-in UV sensor unit (sensor certified by ONORM M5873-1), placed at

56

the outer wall of the reactor, recorded the UV irradiance to ensure that the

57

lamp output remained stable. An electromagnetic flow meter (Endress +

58

Hauser, PROMAG 50 50H22-10x7/0) was used to measure the water flow.

59

Pharmaceuticals were spiked from a stock solution (with a flow of 10 L/h)

60

upstream of the reactor. The targeted concentration of pharmaceuticals in

61

the main stream ranged from 1-5 µm. Hydrogen peroxide (from a stock so-

62

lution of 5 g/L and spiked with 2 L/h) was added to the main stream. The

63

side streams of pharmaceuticals and hydrogen peroxide were mixed with the

64

main stream by means of a static mixer. Influent and effluent samples were

65

taken in triplicates with one hydraulic retention time (HRT) in between the

66

samples. Spiking of the pharmaceuticals started after the lamp output and

67

flow had become stable. The samples were analysed in the KWR laboratory

68

by means of (U)HPLC/MS/MS to determine pharmaceuticals concentra-

69

tions. Also, separate samples were taken to determine H2 O2 (Prominent

70

Dulcotest DT3B photometer), UV-T (spectrophotometer Dr Grbel 50 mm

71

cuvette) and water matrix components ((bi)carbonate, DOC, pH and NO− 3 ).

72

In a buffer tank (10 m3 ), tap water from the city of Wijhe was collected.

73

The UV-T was increased from 86% to 89% by pre-treating the water with

74

UV/H2 O2 at a fluence of 1600 mJ/cm2 . For the experiments with lower UV-

4

75

T, 0.8 mg/L pHBA (4-hydroxybenzoic acid 99%, from Sigma Aldrich, CAS-

76

nr 99-96-7) was added to the water resulting in a UV-T of 80%, . The follow-

77

ing pharmaceuticals were used: Atenolol (29122-68-7), Bezafibrate (41859-

78

67-0), Carbamazepine (298-46-4), Clenbuterol (37148-27-9), Clofibric acid

79

(882-09-7), Cortisol (50-23-7), Cortisone (53-06-5), Cyclophosphamide (50-

80

18-0), Diatrizoic acid (737-31-5), Diclofenac (15307-86-5), Erythromycin

81

A (114-07-8), Fluoxetine (54910-89-3), Furosemide (54-31-9), Gemfibrozil

82

(25812-30-0), Ifosfamide (3778-73-2), Lincomycin (154-21-2), Metformin (657-

83

24-9), Metoprolol (51384-51-1), Metronidazole (443-48-1), Naproxen (22204-

84

53-1), Paracetamol (103-90-2), Paroxetine (61869-08-7), Pentoxifylline (6493-

85

05-6), Phenazone (60-80-0), Pindolol (13523-86-9), Propranolol (525-66-6),

86

Sotalol (3930-20-9), Sulfachloropyridazine (102-65-8), Sulfadiazine (68-35-

87

9), Sulfamethoxazole (723-46-6), Sulfaquinoxalin (59-40-5), Tramadol (27203-

88

92-5), Trimethoprim (738-70-5), Venlafaxine (93413-69-5). More details on

89

these pharmaceuticals, analysis method and limits of detection can be found

90

in Wols et al. (2013). Table 1: Overview of pilot-scale UV/H2 O2 reactor tests.

Exp.

Reactor

Flow

T10,w a

Power

H2 O2

pHBA

m3 /h

%

W

mg/L

mg/L

1

VR130

1.00

88.6 (±0.1)

143

10.6 (±0.5)

0

2

VR130

1.00

88.2 (±0.5)

106

10.2 (±0.4)

0

3

VR130

1.02

80.8 (±0.6)

139

11.8 (±0.9)

0.8

4

VR200

1.02

88.5 (±0.9)

139

10.7 (±0.2)

0

5

VR200

1.01

88.9 (±0.4)

106

11.4 (±0.6)

0

6

VR200

1.02

77.1 (±2.0)

139

10.8 (±0.2)

0.8

a

UV transmittance over 10 mm at 254 nm

5

91

2.2. Analytical modelling

92

The reactor design of UV systems is investigated analytically to find an opti-

93

mal performance for any flow-through system. The reactor is schematized by a 2D

94

system (x-z), where the water is flowing in x direction, and the fluence rate and

95

velocity may be a function of z. The concentration of micropollutants in the sim-

96

plified reactor is calculated by a mass transport equation. A similar approach was

97

followed by Shen and Wang (2002); Johnson and Mehrvar (2008). The diffusion is

98

neglected - which represents a worst-case scenario - in the transport equation and a

99

first-order reaction (representing compound degradation or microbial inactivation,

100

both referred to as degradation) is added as a sink term: ∂C ∂C + u(z) ∂t ∂x

= −k(z)I(z)C

(1) (2)

101

Since we are interested in the steady state solution, the differential equation reduces

102

to:

103

dC k(z)I(z) =− C (3) dx u(z) where I(z) represents the fluence rate [W/m2 ], u(z) the velocity in x-direction

104

[m/s] and k the first-order reaction rate constant [m2 /J]. To facilitate an analytical

105

solution of equation 3, it is assumed that the fluence rate is independent of C. As

106

a result, the water absorption cannot be reduced by the degradation of C. This

107

assumption holds as long as C is small compared to other compounds in the water

108

matrix. The solution of equation 3 reads: 

k(z)I(z)x C(x, z) = C0 exp − u(z)

 (4)

109

The main interest is the mean degradation, so that the (mass) flow of concentration

110

is averaged over the depth (for this 2D system): Z 1 h uCdz, (5) C= q 0 Rh where q is the specific flow rate (= 0 u(z)dz [m2 /s]). Combining equation 4 and

111 112

equation 5 results in the depth-averaged concentration.

113

6

114

Similarly, the fluence H 0 can be calculated by solving the following differential

115

equation (replacing the concentration by the fluence in equation 1 and using the

116

fluence rate as a source term): ∂H 0 ∂H 0 + u(z) = I(z) ∂t ∂x

117

For the steady state solution, the equation reduces to: dH 0 I(z) = dx u(z)

118

Z

H = 0

120

(7)

resulting in: 0

119

(6)

L

I(z)L I(z) dx = u(z) u(z)

The depth-averaged fluence is given by: Z 1 h u(z)H 0 dz H0 = q 0 Using equation 8, the mean fluence can be reduced to: Z L h H0 = I(z)dz q 0

(8)

(9)

(10)

121

In other words, the mean fluence is independent of velocity distribution! The

122

velocity distribution in such a system only determines the fluence distribution, but

123

does not affect the mean fluence.

124

The micropollutant concentration can be written as a function of fluence (combining

125

equation 8 with 4): C = C0 exp (−k(z)H 0 (z))

(11)

126

An ideal system will have a zero fluence distribution1 (Dirac delta function), from

127

which the mean degradation (or inactivation) can be directly calculated from the 1

Considering equation 11: According to Jensen’s inequality, the average value of an

exponential function is always equal to or larger than the exponential of the average value of that function. In other words, using an average fluence in the calculation of compound concentration results in the smallest concentration. So, for the most ideal situation, all the fluences are the same (and thus equal to the average fluence).

7

128

mean fluence (assuming a constant k over the depth): C 0 max = C0 exp −kH 0




(12)

129

So, the expected maximum degradation can be easily determined, once the fluence

130

rate distribution is known. This will be further elaborated for an annular UV

131

reactor.

L

P r rq Q

R

Figure 1: Annular UV reactor.

132

Example annular reactor. An annular reactor is considered as an axisymmetric

133

system and the water is flowing parallel to the UV lamp (Figure 1). The fluence rate

134

profile is determined from an infinite line source model and using absorption from

135

a one-dimensional Beer-Lambert law (see Shen and Wang (2002), using I0 =

136

as fluence rate at the outer quartz tube): I(r) =

P exp (−α(r − rq )) 2πL r

P 2πLrq

(13)

137

where P is the total power of the UV lamp [W], L the length of the lamp [m], rq

138

the radius of the quartz sleeve, and α the (napierian) absorption coefficient of the

139

water, given by: α=−

ln(T10,w ) 0.01[m]

8

(14)

140

where T10w is the 10 mm transmittance of water. Since we are dealing with an

141

axisymmetric system, the expression for the mean fluence becomes: Z Z 1 2π R+rq u(r)H 0 (r)rdrdθ H0 = Q 0 rq Z Z L 2π R+rq I(r)rdrdθ = Q 0 rq

(15) (16)

142

where R is the (radial) water layer distance (not to be confused with the total

143

radius of the system R + rq ). The flow rate is given by: Z

2π

Z

R+rq

Q=

u(r)rdrdθ 0

(17)

rq

144

The expression for the mean fluence becomes after substitution of equation 13 into

145

16: H0 =

P (1 − exp (−αR)) . Qα

(18)

146

The mean fluence can thus be easily determined as a function of lamp power, flow

147

rate, UVT and water layer distance. For the calculation of degradation, we need

148

to define a velocity distribution, which will be discussed later on. First, from the

149

expression of the mean fluence, the maximum degradation (assuming a fluence

150

distribution of zero) in an annular system can be derived: ln

C 0 max P (1 − exp (−αR)) = −k C0 Qα

(19)

151

The mean fluence as well as the maximum degradation increase with the distance R

152

from the quartz sleeve to the outer wall. So, the maximum achievable mean fluence

153

is

154

expressions can be very useful to relate them to real reactors, and use them for an

155

efficiency parameter to examine how close a design is to its theoretical optimum.

156

For disinfection, Xu et al. (2013) introduces an efficiency parameter that relates the

157

actual log inactivation to the log inactivation with a zero fluence distribution or the

158

log inactivation at a constant fluence (Xu et al., 2015). However, the theoretical

159

optimum may be higher, as the mean fluence may differ per reactor type. From a

P Qα ,

P and the maximum achievable (natural) log degradation is k Qα . These simple

9

160

practical point of view, we are interested in a reactor with the highest log degrada-

161

tion (with the same energy input, water flow and water quality). The efficiency of

162

164

a UV reactor is therefore determined as follows: C ln C0 η= (20) P k Qα C where ln is the actual log degradation. So the efficiency parameter relates the C0 actual degradation to the maximum achievable degradation that uses all radiation

165

power and has a zero fluence distribution. Note that it will be difficult to reach

166

an efficiency of 1 in practice, as there will be losses at the quartz tube, such as

167

absorption and reflection. The velocity distribution determines whether the optimal

168

degradation is reached, or that a smaller degradation will be obtained due to a

169

suboptimal fluence distribution. The following velocity profiles were considered

170

(Figure 2) :

163

171

1. Velocity profile similar shaped as the fluence rate profile: u=

Qα exp (−α(r − rq )) 2πr 1 − exp (−αR)

(21)

172

The fluence rate profile is then equal to the mean fluence given in equation 18.

173

In this particular case, the mean degradation will be equal to the maximum

174

degradation given in equation 19. This velocity profile is in fact the most

175

ideal, but will not be easy to obtain in practice.

176

2. Constant velocity profile: u=

Q πR (2rq + R)

(22)

177

An analytical expression for the mean degradation cannot be obtained with

178

this function of u. Numerical integration is required to determine the mean

179

degradation.

180

3. Fully developed laminar flow (Bird et al., 2002):  0  r 2 1 − κ2 R 1− − ln 2Q R0 ln(1/κ) r u=
2 2 πR02 1−κ 1 − κ4 − ln(1/κ)

10

(23)

rq R0 .

181

where R0 = R + rq and κ =

182

solution for the mean degradation can be obtained.

For this expression of u, again no analytical

183

Obtaining a velocity profile that matches the mean fluence is difficult to obtain in

184

practise, but efforts could be made by using baffles and perforated plates.

Figure 2: Different velocity profiles

185

Reaction rate of microorganisms. For microbial inactivation, the inactivation rate

186

constant is a scalar that depends on the type of microorganism (assuming a linear

187

relation between UV dose and log inactivation). An overview of these inactivation

188

constants is given by Hijnen et al. (2006).

189

Reaction rate of organic micropollutants. During the UV/H2 O2 process, the degra-

190

dation of OMPs depends on a complex kinetic scheme (Crittenden et al., 1999; Wols

191

et al., 2014). An approximation can be made by assuming quasi steady-state con-

192

centration of hydroxyl radicals and a negligible reduction of peroxide, resulting in

193

a reaction rate of: k(z) =

ln(10) Uλ

 ΦM εM + 2ΦH εH

kM CH (z) kS + kH CH (z)

 ,

(24)

194

which may account for a spatial varying hydrogen peroxide concentration (CH (z)),

195

so that the coefficient k depends on z. The water quality is represented in the

196

parameter kS , showing the competition of background compounds for OH radicals

197

(such as DOC, (bi)carbonate and all the organic micropollutants). The compound

198

specific parameters are ΦM , εM and kM . An overview of these parameters for

199

different chemical species can be found in Wols and Hofman-Caris (2012b).

11

200

Desired treatment level. The efficiency of a UV reactor may depend on the actual

201

treatment level. Short-circuits associated with low fluences may have a larger im-

202

pact when a higher treatment level is desired. For example, if a UV/H2 O2 reactor

203

has a short-circuit of 1% that receives zero fluence (so 1% of the water flows at po-

204

sitions in the reactor where the fluence rate is zero), the degradation of this reactor

205

will be limited to 99%. If higher degradation levels are desired, the efficiency of

206

this reactor will drop. We therefore introduce the concept of desired treatment level

207

(more specifically, desired degradation level for UV oxidation and desired inactiva-

208

tion level for UV disinfection), which is characterized by the maximum achievable

209

P log degradation or inactivation (k Qα ) obtained from the analytical model. A design

210

parameter for a UV reactor may therefore, next to operating conditions and water

211

matrix, also depend on the desired treatment level.

212

2.3. CFD modelling

213

Different reactor designs were assessed by CFD modelling. The commercial

214

package COMSOL v4.3 was used to model the hydraulics. The hydraulic model

215

solves the Reynolds averaged Navier-Stokes equations and the standard k-ε turbu-

216

lence model was used as a closure model. As the k-ε model uses a simplification

217

to account for the turbulence, validation of the CFD model is important, which

218

was done in previous work for the hydraulics (Wols et al., 2010) and for compound

219

degradation (Wols et al., 2015). The computational domain was meshed by an

220

unstructured tetrahedral mesh. An inflow velocity was prescribed at the upstream

221

boundary and a zero pressure at the downstream boundary. The walls were mod-

222

elled by built-in wall functions. The fluence rate was modelled by the MSSS model

223

(Liu et al., 2004). Particle tracking was used to obtain the fluence distribution.

224

In the particle tracking routine, particles are displaced by advection and diffusion

225

according to the velocity and turbulent viscosity calculated by the hydraulic model

226

(Wols and Hofman-Caris, 2012a). A virtual amount of 5000 particles was released

227

in the piping upstream of the reactor. A kinetic model was applied to each of

228

the particle paths to calculate the degradation of a target compound along the

12

229

particle trajectory. By averaging over all particles, the degradation of the UV re-

230

actor is obtained. Using the particle tracking method for calculating degradation

231

implies a simplification, but this error is small for UV/H2 O2 systems (Wols and

232

Hofman-Caris, 2012a). A validation of the CFD model was performed previously

233

(Wols et al., 2015), showing that the use of particle tracking method and the stan-

234

dard k-ε turbulence model proved to accurately predict the degradation of several

235

pharmaceuticals in UV reactors.

236

3. Results and discussions

237

3.1. Analytical model

238

The log degradation or inactivation and efficiency are calculated by the ana-

239

lytical model for an annular reactor. For the three velocity profiles, the effect of

240

UV transmittance, water layer distance and desired treatment level are investigated

241

(Figures 3 to 5). The water layer distance, defined as the distance from reactor wall

242

to quartz sleeve, is multiplied with the absorption α to obtain a dimensionless wa-

243

ter layer distance (Rα). For oxidation, desired levels of degradation are typically

244

between 1.5 (corresponding to 78% of degradation) and 3 (95% of degradation)

245

(Kruithof and Martijn, 2013), whereas for disinfection desired log inactivation will

246

usually be higher than 5 (more than 99% inactivation). For a constant velocity pro-

247

file (Figure 3), the log degradation or inactivation increases with increasing desired

248

treatment level, however not in a one to one relation. This is visible in the effi-

249

ciency that decreases if a higher treatment is desired. At higher desired treatment

250

levels, poor reactor performance becomes more apparent, because the micropollu-

251

tants that received lower fluences contribute to a larger extent. Or, to put in other

252

words, short-circuits associated with low treatment will limit the total treatment.

253

In the case of a fully developed laminar flow (Figure 4), similar trends as for the

254

constant velocity profile can be found. The laminar flow has a slightly better per-

255

formance than the constant velocity profile, because the laminar flow profile reaches

256

its highest velocity closer towards the UV lamp (see Figure 2), where the fluence

13

257

rate is higher. The best performance is obtained when the velocity profile is similar

258

as the fluence rate profile (Figure 5). The efficiency reaches 1 for the lower UV

259

transmittances. In fact, for the higher UV transmittances, the fluence distribution

260

will still be optimal but the mean fluence is lower since UV radiation gets lost to

261

the outer wall (as can be seen from equation 18).

14

Figure 3: Log removal and efficiency as a function of dimensionless water layer distance and desired treatment level for a constant velocity profile

Figure 4: Log removal and efficiency as a function of dimensionless water layer distance and desired treatment level for a laminar velocity profile

Figure 5: Log removal and efficiency as a function of dimensionless water layer distance and desired treatment level for a fluence shaped velocity profile

262

Optimal water layer distance. An optimal water layer distance can be determined,

263

for which the efficiency is maximal. These optimal values differ per velocity profile

264

and desired treatment levels. For each desired treatment level, the optimal water

265

layer distance and associated efficiency was determined by the analytical model

266

(Figure 6). For the constant and laminar flow profile, the optimal distance is mostly

267

between an Rα of 1 and 2, which is decreasing when a higher treatment level is

268

desired. However, the efficiency is rather low (≈0.5) and also decreases with desired

269

treatment level. At very low treatment values, the efficiency approaches unity, as

270

the fluence distribution becomes irrelevant at such low levels and the performance

271

is mainly determined by the mean fluence. For the fluence shaped profile, the

272

optimal water depth is infinity (the calculations were only done up to a Rα of 5,

273

however the additional treatment above Rα = 4 is limited). The efficiency for the

274

fluence shaped profile can be up to two times higher than for the other profiles. A

275

remark is that no mixing is considered here. If mixing would be maximal (e.g., a

276

CSTR), resulting in a narrow fluence distribution, the results will be similar as for

277

the fluence rate profile, regardless of velocity profile.

278

As a design criterion for UV/H2 O2 reactors, a larger water layer distance than

279

for disinfection reactors can be chosen, as the desired treatment levels are lower

280

(although one should notice that the UVT may decrease by the addition of H2 O2 ,

281

about 1%/cm for 10 mg/L H2 O2 ). An optimal Rα of 4 is a good choice, which is in

282

fact higher than most UV reactors in practice use. For a UV transmittance of 80%

283

and 90%, this will result in a water layer of 18 cm and 38 cm, respectively. Also,

284

the efficiency can be increased by manipulating the velocity profiles closer towards

285

the fluence rate profiles or increase mixing (which also holds for UV disinfection

286

reactors).

16

1

4

0.8

3

0.6 η

Rα

5

0.4

2

1

0 0

0.2

fluence profile constant laminar 2

4 6 kP/(Qα)

8

0 0

10

fluence profile constant laminar 2

4 6 kP/(Qα)

8

10

Figure 6: Optimal (dimensionless) water layer distance (left) and efficiency (right) as a function of desired treatment level for the three velocity profiles.

17

287

Multiple lamps (uniform velocity profile). It was shown that a UV system with a

288

velocity distribution that mimics the fluence rate distribution performed well. Since

289

changing the velocity distribution is difficult and energy demanding, an alternative

290

would be to change the fluence rate distribution by applying multiple lamps with the

291

same total amount of energy as the single lamp in the annular system. UV systems

292

with 1 to 63 lamps were investigated (Figure 7). The fluence rate is calculated for

293

each lamp from equation 13 (relative to the middle of the lamp), translated towards

294

the lamp position in the reactor and summed up for all lamps. A lamp to lamp

295

distance of 10 cm was chosen, whereas the lamp to wall distance was 5 cm (except

296

for the single lamp system, where the lamp to wall system is 10 cm). The lamp

297

radius was set to 2 cm. The efficiency while varying the UV-T is shown in Figure

298

7. Note that a uniform velocity profile is chosen in the calculations. The efficiency

299

is indeed improved by applying more lamps, because the fluence rate distribution

300

becomes more uniform.

301

Effect of quartz sleeve size. Another way to realize a more uniform fluence rate

302

profile is applying larger quartz sleeves (Figure 8). The single lamp efficiency is

303

improved due to the more uniform fluence rate profile, however no improvements

304

were found for the multiple lamp systems compared to the smaller quartz sleeves.

305

In addition, quartz losses are not taken into account, which may even reduce the

306

efficiency of systems with larger quartz sleeves.

307

18

Maximum degradation of 1.5 log (78%)

Maximum degradation of 5.0 log (99%)

1

1 1 lamps 7 lamps 19 lamps 38 lamps 63 lamps

0.8

1 lamps 7 lamps 19 lamps 38 lamps 63 lamps

0.8

η

0.6

η

0.6

0.4

0.4

0.2

0.2

0 0

1

2

3

4

0 0

5

1

2

Rα

3

4

5

Rα

Figure 7: Effect of multiple lamps, lamp radius is 2 cm. Upper panel: fluence rate distribution for different configurations of multiple lamp systems (color shows the relative difference in fluence rate distribution, red=high, blue = low). Lower panel: efficiency of multiple lamp configurations for typical conditions for oxidation (left) and disinfection (right).

Maximum degradation of 1.5 log (78%)

Maximum degradation of 5.0 log (99%)

1

1 1 lamps 7 lamps 19 lamps 38 lamps 63 lamps

0.8

1 lamps 7 lamps 19 lamps 38 lamps 63 lamps

0.8

η

0.6

η

0.6

0.4

0.4

0.2

0.2

0 0

1

2

3

4

0 0

5

Rα

1

2

3

4

5

Rα

Figure 8: Effect of multiple lamps, lamp radius is 10 cm. Upper panel: fluence rate distribution for different configurations of multiple lamp systems (color shows the relative difference in fluence rate distribution, red=high, blue = low). Lower panel: efficiency of multiple lamp configurations for typical conditions for oxidation (left) and disinfection (right).

19

308

Hydrogen peroxide injection. The hydrogen peroxide concentration is mostly uni-

309

formly distributed over the reactor , as the hydrogen peroxide is added upstream

310

of the reactor and mixed by means of a static mixer. However, as the fluence rate

311

is strongly non-uniform, manipulating the hydrogen peroxide profile may be bene-

312

ficial, for example to generate higher hydrogen peroxide concentrations close to the

313

lamps (with high fluence rates). Three different hydrogen peroxide profiles were

314

tested:

315

316

317

318

• A constant hydrogen peroxide profile, which is most commonly used. Results will be similar as for a general k independent of z. • A profile with the same shape as the fluence rate distribution, so that areas with high fluence rates also have high concentrations of hydrogen peroxide.

319

• A profile with the inverse shape of the fluence rate distribution, so that areas

320

with high fluence rates have low concentrations of hydrogen peroxide. In this

321

way the OH radical concentration is more uniform.

322

In practise the hydrogen peroxide profile could be manipulated by placing injection

323

points in the reactor at several positions. It will be difficult to exactly match the

324

proposed profiles, but these extremes were chosen to investigate what the effect

325

would be. These hydrogen peroxide profiles were investigated in combination with

326

the laminar flow profile. The only degradation pathway was the reaction with

327

OH radicals, no direct photolysis was considered. The efficiencies as a function of

328

desired degradation levels show that the constant hydrogen peroxide profile mostly

329

results in the highest efficiencies (Figure 9). Only for low desired degradation levels

330

(less than 40% treatment), the fluence shaped peroxide profile results in higher

331

efficiencies.

20

1

0.8

η

0.6

0.4

0.2

0 0

constant fluence shaped fluence inverse shaped 2

4

6

8

10

kP/(Qα)

Figure 9: Effect of different hydrogen peroxide profiles on the degradation efficiency as a function of desired degradation level. A uniform H2 O2 profile, a H2 O2 profile similar as the fluence rate and a H2 O2 profile equal to the inverse of the fluence rate were applied.

332

3.2. Performance of real reactors

333

Fluence rate distribution. In the analytical approach, an idealized distribution of

334

fluence rate is assumed, without refraction, reflection, quartz loss etc. In real reac-

335

tors, these effects may result in substantial losses. By using 3D fluence rate models,

336

as incorporated in the CFD modelling framework, such as multiple segment source

337

summation (MSSS) or multiple points source summation (MPSS), see Liu et al.

338

(2004), an estimate of these losses was made. This was done for a UV lamp of 0.5

339

m placed in the middle of a reactor of 1 m height, while using an Rα of 10 to ensure

340

that all the radiation remains within the domain (no losses at boundaries). Table

341

2 shows that without these losses, indeed an efficiency of 1 is found. The effect of

342

using segments (incorporated in the MSSS obeying Lambert’s cosine law) instead

343

of points sources (MPSS) is largest, followed by reflection and quartz absorption.

344

Consequently, the most optimal mean dose and therefore the maximum efficiency

345

for the MSSS model is limited to 0.73. Note that this value is slightly dependent

21

346

on the choice of the quartz sleeve diameter and quartz sleeve thickness, this value

347

of 0.73 is calculated for a quartz sleeve thickness of 1.5mm and sleeve diameters

348

smaller than 5cm.

349

Table 2: Fluence rate modelling

Fluence rate model

Options

ηmax

MSSS

Standard1

0.73

MSSS

No quartz absorption (T10,q = 1)

0.74

MSSS

No reflection

0.78

MSSS

No refraction

0.78

MPSS

No segments (MPSS)

0.91

MPSS

All of above

1.00

1

nsources =200; refractive indices: nwater =1.33, nquartz =1.54; transmittance T10,q =0.96, αR=10; size lamp/quartz: rlamp =0.01m, rquartz =0.0015m; lamp dimensions: hlamp =0.5m, hreactor =1m.

350

Reactor design with CFD modelling. Improvement of UV/H2 O2 reactor designs was

351

assessed by CFD. First, a standard disinfection reactor manufactured by Van Rem-

352

men UV Techniek was used. This reactor had been optimized for UV disinfection

353

purposes. This type of reactor design was improved by increasing the water layer,

354

as demonstrated by the analytical model, resulting in the VR200 reactor. This re-

355

actor is also equipped with a flow plate to obtain a velocity profile that mimics the

356

fluence rate profile. Furthermore, another type of reactor was designed using the

357

larger water layer as well as oblique baffles to increase mixing. The effect of shad-

358

owing of baffles was not considered in the fluence rate model. This may give a small

359

overestimation of the fluence rate, however the baffles are placed at the outer walls

360

where the fluence rate is largely reduced (also because of the large water layer).

22

361

This resulted in the annular mixer 1 lamp reactor, which was further up-scaled to

362

the annular mixer 4 lamps reactor (Figure 10). The dimensions (diameter, length)

363

were as follows: VR130 (130mm, 1050mm), VR200 (200mm, 1050mm), annular

364

mixer 1 lamp (300mm, 2300mm), annular mixer 4 lamps (450mm, 2050mm).

365

The degradation efficiencies as a function of UV transmittance are shown for the

366

four reactors in Figure 11. The degradation efficiency depends on the desired treat-

367

ment level, as the fluence distribution becomes more critical at higher degradation

368

levels. In the annular mixer reactors, the difference between a low and high degra-

369

dation is smaller, because the mixing elements narrow the fluence distribution. The

370

degradation efficiency reduces at higher UVTs, because UV radiation is lost at the

371

outer walls (assuming there is no reflection at the outer walls). An efficiency of 1

372

could not be obtained, even at low UVTs, due to losses by quartz absorption and

373

reflections. The improvement of the new reactor designs, visualized as the increase

374

in log degradation of the new reactor compared to the VR130 reactor, is shown

375

for different UVTs in Figure 12. The desired treatment level is 1.5 log (78%) - a

376

typical value for oxidation purposes. A substantial increase in degradation can be

377

obtained by the new reactor designs.

378

Validation of design. The CFD model predicted an increase in degradation effi-

379

ciency of 20%-30% for the VR200 reactor compared to the VR130 reactor. This

380

reactor has been built and tested experimentally. Figure 13 shows the measured

381

increase in degradation for different pharmaceuticals and UVTs. For most of the

382

pharmaceuticals, an improvement of about 30% was found at a high UVT of 88%,

383

and an improvement of about 10% at a UVT of 78%. These measured improvements

384

are in good agreement with the improvements found by the CFD modelling.

23

Figure 10: Overview of reactor designs optimized by CFD. From left to right: VR130, VR200, annular mixer 1 lamp and annular mixer 4 lamps.

24

Figure 11: Reactor performance simulated by CFD, efficiency as a function of UV transmittance. The desired treatment level is shown by the color, corresponding to a natural log degradation from 1 (blue) to 5 (red). The black line shows the 1.5 log degradation level (78% degradation). The maximum efficiency of 0.73 using a realistic fluence rate model (with losses) is shown by the dotted lines.

annular mixer 4 lamps

annular mixer 1 lamp

VR200 UVT 80% UVT 85 % UVT 90 % UVT 95 % 0

20

40

60 η increase (%)

80

100

120

Figure 12: Improvement in degradation compared to the standard VR130 reactor. The desired level of degradation is 1.5 log (78% degradation).

25

at en ol be ol za ca fib rb am rate az ep in cl e en bu te cl ro of l ib ric ac id co rti so cy co l cl rti op so ho ne sp ha di m at riz ide oi c ac id di cl of er en yt hr ac om yc in a flu ox et in fu e ro se m id ge e m fib ro zi ifo l sf am id lin co e m yc in m et fo rm m in et op m ro et lo ro l ni da zo le na pr ox pa en ra ce ta m pa ol ro pe xeti ne nt ox ify ph lline en az on e pi nd ol pr ol op ra no su lo lfa l ch so lo ta ro lo py l rid az su i lfa ne su di lfa az m in et e ho xa su zo lfa le qu in ox al tra in m ad tri m ol et ho pr ve im nl af ax in e

% improved 100

90 VR200, UVT=88% VR200, UVT=78%

80

70

60

50

40

30

20

10

0

Figure 13: Experimental increase in degradation by new reactor design (VR200) compared

to the standard reactor (VR130) for a UVT of 88% (dark blue) and a UVT of 78% (light

blue)

26

385

4. Conclusions

386

The design of UV/H2 O2 reactors has been investigated systematically. An ana-

387

lytical model is developed, from which an efficiency parameter is derived that relates

388

the actual performance of a UV/H2 O2 reactor to the optimal performance, based

389

upon operating conditions as flow, UV power and UV transmittance. From the an-

390

alytical model, several design parameters were investigated. The desired treatment

391

level influences the choice of a design parameter, as the fluence distribution becomes

392

less important at lower desired treatment levels. As the desired degradation lev-

393

els in UV/H2 O2 applications are lower than the desired inactivation levels for UV

394

disinfection, a UV/H2 O2 reactor may be designed with a larger water layer depth

395

than in UV disinfection reactors. Also, manipulating the velocity profile towards a

396

profile that mimics the fluence rate profile is beneficial, as well as increasing mixing.

397

Increasing the number of lamps (while the total energy consumption remains the

398

same) is beneficial, as it results in a more uniform fluence rate distribution. En-

399

larging the quartz sleeve has limited effect. Changing the distribution of hydrogen

400

peroxide in the reactor also has limited effects.

401

New UV/H2 O2 reactor types were developed with CFD simulations and tested ex-

402

perimentally. An increase in log degradation up to 30% was demonstrated by the

403

improved reactor design.

404

5. Acknowledgments

405

This work was performed in the TTIW-cooperation framework of Wetsus, cen-

406

tre of excellence for sustainable water technology (www.wetsus.nl) and this work

407

is supported by the joined Dutch Water Supply Companies. Wetsus is funded by

408

the Dutch Ministry of Economic Affairs. The authors would like to thank the par-

409

ticipants of the research theme ’clean water technology’ for the fruitful discussions

27

410

and their financial support.

411

412

References

413

Bird, R., Stewart, W., Lightfoot, E., 2002. Transport Phenomena, 2nd Edition. John Wiley &

414

415 416

417 418

419 420

Sons, New York. Chiu, K., Lyn, D. A., Savoye, P., Blatchley, E. R., 1999. Effect of uv system modifications on disinfection performance. Journal of Environmental Engineering 125 (5), 459–469. Chong, M. N., Jin, B., Chow, C. W. K., Saint, C., 2010. Recent developments in photocatalytic water treatment technology: A review. Water Research 44 (10), 2997–3027. Crittenden, J. C., Hu, S., Hand, D. W., Green, S. A., 1999. A kinetic model for h2o2/uv process in a completely mixed batch reactor. Water Research 33 (10), 2315–2328.

421

Hijnen, W. A. M., Beerendonk, E. F., Medema, G. J., 2006. Inactivation credit of uv radiation

422

for viruses, bacteria and protozoan (oo)cysts in water: A review. Water Research 40 (1), 3–22.

423

Janex, M. L., Savoye, P., Do-Quang, Z., Blatchley, E., La, J. M., in´ e, 1998. Impact of water

424

quality and reactor hydrodynamics on wastewater disinfection by uv, use of cfd modeling for

425

performance optimization. Water Science and Technology 38 (6), 71–78.

426

Johnson, M. B., Mehrvar, M., 2008. Aqueous metronidazole degradation by uv/h2o2 process

427

in single-and multi-lamp tubular photoreactors: Kinetics and reactor design. Industrial and

428

Engineering Chemistry Research 47 (17), 6525–6537.

429 430

431 432

Kruithof, J. C., Martijn, B. J., 2013. Uv/h2o2 treatment: An essential process in a multi barrier approach against trace chemical contaminants 13, 130–138. Liu, D., Ducoste, J. J., Jin, S., Linden, K., 2004. Evaluation of alternative fluence rate distribution models. Aqua - Journal of Water Supply: Research and Technology 53 (6), 391–408.

433

Mohajerani, M., Mehrvar, M., Ein-Mozaffari, F., 2012. Photoreactor design and cfd modelling

434

of a uv/h 2o 2 process for distillery wastewater treatment. Canadian Journal of Chemical

435

Engineering 90 (3), 719–729.

436

Pan, H., Orava, M., 2007. Performance evaluation of the uv disinfection reactors by cfd and fluence

437

simulations using a concept of disinfection efficiency. Journal of Water Supply: Research and

438

Technology - AQUA 56 (3), 181–189.

28

439

Schoenen, D., 1996. The influence of inhomogeneous irradiation in uv disinfection - experimental

440

findings and theoretical considerations. Journal of Water Supply: Research and Technology -

441

AQUA 45 (3), 120–129.

442 443

444 445

Shen, Y. S., Wang, D. K., 2002. Development of photoreactor design equation for the treatment of dye wastewater by uv/h2o2 process. Journal of Hazardous Materials 89 (2-3), 267–277. Taghipour, F., Sozzi, A., 2005. Modeling and design of ultraviolet reactors for disinfection byproduct precursor removal. Desalination 176 (1-3 SPEC. ISS.), 71–80.

446

Wols, B. A., Harmsen, D. J. H., Beerendonk, E. F., Hofman-Caris, C. H. M., 2014. Predicting

447

pharmaceutical degradation by uv (lp)/h2o2 processes: A kinetic model. Chemical Engineering

448

Journal 255, 334–343.

449

Wols, B. A., Harmsen, D. J. H., Wanders-Dijk, J., Beerendonk, E. F., Hofman-Caris, C. H. M.,

450

2015. Degradation of pharmaceuticals in uv (lp)/h2o2 reactors simulated by means of kinetic

451

modeling and computational fluid dynamics (cfd). Water Research 75, 11–24.

452

Wols, B. A., Hofman, J. A. M. H., Beerendonk, E. F., Uijttewaal, W. S. J., van Dijk, J. C.,

453

2011. A systematic approach for the design of uv reactors using computational fluid dynamics.

454

AIChE Journal 57 (1), 193–207.

455

Wols, B. A., Hofman-Caris, C. H. M., 2012a. Modelling micropollutant degradation in uv/h 2o 2

456

systems: Lagrangian versus eulerian method. Chemical Engineering Journal 210, 289–297.

457

Wols, B. A., Hofman-Caris, C. H. M., 2012b. Review of photochemical reaction constants of

458

organic micropollutants required for uv advanced oxidation processes in water. Water Research

459

46 (9), 2815–2827.

460 461

Wols, B. A., Hofman-Caris, C. H. M., Harmsen, D. J. H., Beerendonk, E. F., 2013. Degradation of 40 selected pharmaceuticals by uv/h2o2. Water Research 47 (15), 5876–5888.

462

Wols, B. A., Shao, L., Uijttewaal, W. S. J., Hofman, J. A. M. H., Rietveld, L. C., van Dijk,

463

J. C., 2010. Evaluation of experimental techniques to validate numerical computations of the

464

hydraulics inside a uv bench-scale reactor. Chemical Engineering Science 65 (1), 4491–4502.

465 466

Wright, N., Hargreaves, D., 2001. The use of cfd in the evaluation of uv treatment systems. Journal of Hydroinformatics 3 (2), 59–70.

467

Xu, C., Rangaiah, G. P., Zhao, X. S., 2015. A computational study of the effect of lamp arrange-

468

ments on the performance of ultraviolet water disinfection reactors. Chemical Engineering

469

Science 122 (0), 299–306.

29

470

Xu, C., Zhao, X. S., Rangaiah, G. P., 2013. Performance analysis of ultraviolet water disinfection

471

reactors using computational fluid dynamics simulation. Chemical Engineering Journal 221,

472

398–406.

473

Zhang, J., Tejada-Martnez, A. E., Zhang, Q., 2014. Developments in computational fluid

474

dynamics-based modeling for disinfection technologies over the last two decades: A review.

475

Environmental Modelling and Software 58, 71–85.

30