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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
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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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PII: DOI: Reference:
S0009-2509(15)00469-8 http://dx.doi.org/10.1016/j.ces.2015.06.061 CES12464
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Chemical Engineering Science
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
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