EPA-R2-72-104
November 1972 Environmental Protection Technology Series
Photochemical Methods For Purifying Water
Office of Research and Monitoring
U.S. Environmental Protection Agency
Washington, D.C. 20460
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RESEARCH REPORTING SERIES
Research reports of the Office of Research and
Monitoring, Environmental Protection Agency, have
been grouped into five series. These five broad
categories were"established to facilitate further
development and application of environmental
technology. Elimination of traditional grouping
was consciously planned to foster technology
transfer and a maximum interface in related
fields. The five series are:
1. Environmental Health Effects Research
2. Environmental Protection Technology
3. Ecological Research
H. Environmental Monitoring
5. Socioeconomic Environmental Studies
This report has been assigned to the ENVIRONMENTAL
PROTECTION TECHNOLOGY series. This series
describes research performed to develop and
demonstrate instrumentation/ equipment and
methodology to repair or prevent environmental
degradation from point and non-point sources of
pollution. This work provides the new or improved
technology required for the control and treatment
of pollution sources to meet environmental quality
standards..
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EPA-R2-72-10ij-
Noveraber 1972
PHOTOCHEMICAL METHODS FOR PURIFYING WATER
By
C. Y. Cha and J. M. Smith
Project 17020 EVQ
Project Officer
Robert H. Wise
Environmental Protection Agency
National Environmental Research Center
Cincinnati, Ohio 1*5268
Prepared for
OFFICE OF RESEARCH AND MONITORING
U.S. ENVIRONMENTAL PROTECTION AGENCY
WASHINGTON, D,C. 20i+6o
For sale by the Superintendent of Documents, U.S. Government Printing Office
Washington, D.C. 20402 - Price 70 cents
-------
EPA Review Notice
This report has been reviewed by the Environmental
Protection Agency and approved for publication.
Approval does not signify that the contents necessarily
reflect the views and policies of the Environmental
Protection Agency, nor does mention o'f trade names or
commercial products constitute endorsement or recom-
mendation for use.
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ABSTRACT
(including Conclusions and Recommendations)
The kinetics of photochemical pollutant removal and chlorine dis-
appearance have been developed in a form suitable for use for various
lamps, reactor-lamp geometries, and wastewaters. Numerical kinetic equa-
tions also are presented for secondary effluents from the Sacramento,
California area. The rate of pollutant removal is directly proportion-
al to the UV energy absorbed by pollutant. The rate of disappearance
of free available chlorine is directly proportional to the UV energy ab-
sorbed, not only by free available chlorine but by pollutant. The rate
of pollutant removal for a conventional, low-pressure mercury lamp is
about 6-fold larger than the rate for a high-pressure mercury lamp, for
the same light absorption.
A trough reactor with parabolic reflector was found to be most suit-
able for tertiary treatment. Design equations for electric power and
chlorine requirements have been developed. Also an equation for calcu-
lating the fraction of UV energy which reaches the surface of water has
been obtained. Low- and high-pressure lamps were compared, and the
G64T6 low-pressure mercury lamp (General Electric Co.) was chosen as the
most effective UV-energy source now available. Optimum values for chlor-
ine concentration and reactor-reflector geometry have been determined
for tertiary treatment.
Costs have been calculated approximately for a plant (capacity of
one-million gallons per day of secondary effluent) to reduce TOG from
10 to 4 mg/liter. Total plant investment was estimated to be $563,200.
Treatment cost was estimated at $323,600 per year, or 89.5<:/(1,000 gal.),
of which the major costs were; labor, 17.8c/(1,000 gal.); electricity,
13.9
-------
important to determine specific kinetic equations for removal of var-
ious types of organics (i.e. acids, phenols, ketones, etc.)- With
this information it should be possible to design the optimum photo-
chemical treatment for a given wastewater.
A second area where more research is needed, prior to careful
economic evaluation of photochemical processing, is small pilot-plant
studies of reactor-lamp-reflector geometry. This work is necessary
in order to substantiate efficiency calculations, such as those con-
sidered in Section IV of this report.
This report was submitted in fulfillment of Project Number 17020 EVQ,
under the partial sponsorship of the Environmental Protection Agency.
iv
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CONTENTS
Pace
I. ABSTRACT iii
II. INTRODUCTION 1
III. KINETICS OF ORGANIC POLLUTANT REMOVAL 3
Unsensitized Rate 3
Sensitized Rate 3
IV. PROCESS DESIGN 5
Reactor Type and Design Conditions 5
Relationship Between Absorption of Secondary
Effluent and Chlorine 6
Design Equations for Electrical Energy Required 7
Chlorine Consumption 10
Selection of Lamp 12
Design Calculations for 1,000 gal/hr Throughput 13
Design Calculations for 10 gal/day Plant 18
V. EVALUATION OF ECONOMICS FOR TERTIARY TREATMENT 21
Fixed-Capital Investment 21
Treatment Cost Per Year 22
VI. ACKNOWLEDGEMENT 23
VII. NOMENCLATURE 25
VIII. REFERENCES 29
-------
Figures
Figure Page
1 Trough reactor with parabolic reflector Sa.
2 Effect of ratio of height to width on
efficiency r^ 15a
3 Arrangement of reactor and reflectors I8a
4 Process flowsheet for photochemical treatment
of secondary effluent 19a
Table
Electricity and chlorine costs 1°
vi
-------
II. INTRODUCTION
The primary objective of this work was to develop and evaluate
the economics of a continuous photo-oxidation process for reducing
the organic pollutants in secondary effluents from municipal waste-
water treatment plants. To determine the technical feasibility of
employing the photo-oxidation process for tertiary treatment, it is
first necessary to develop the reaction kinetics of pollutant removal.
Once rate equations are obtained, reactor design calculations are re-
quired for process evaluation. Since the cost of the photo-oxidation
process can depend heavily upon the efficiency of energy utilization,
it is necessary to analyze the reflector-lamp-reactor geometry before
economic evaluation.
This report consists of three major parts: the first describes
the reaction kinetics of organic pollutant removal [the details of
this part are given in references (2), (3), (4)]; the second part
discusses process design for tertiary treatment; the third part is
an economic evaluation and utilizes the results published in ref-
erence (5) .
-I-
-------
III. KINETICS OF ORGANIC POLLUTANT REMOVAL
Rate of Pollutant Removal Due to A I and C_; Unsensitized Rate
_ p tot _ p_ _
The rate of pollutant disappearance was found to be first order
in the amount of radiation absorbed, directly proportional to oxygen
concentration at low C , and independent of C at high oxygen concen
trations. Reference (2) describes details of procedure to determine
the rate, using a high-pressure mercury lamp. The final form of the
rate equation obtained for a high-pressure lamp is
1.47 x 10~3 c
When the shorter wave length radiation of a low-pressure mercury
lamp is used, the quantum yield increases 14 times. The experimental data
are given in Reference (4). The final rate equation obtained when
using a low-pressure lamp is
[ \ 2.07 x 10~2 C
n -- a — — A r _ (2)
I P)LP 6.5 x icf8 + CQ p tot
It will be seen later that the rate of pollutant disappearance increases
significantly when chlorine is applied.
Rate of Pollutant Removal Due to A I . and C_., ; Sensitized Rate
__ _ p tot _ oi _
When chlorine is added to secondary effluent, the rate of pollutant
disappearance Increases significantly for both high- and low-pressure
lamps. The rate, ft , consists of two contributions: one for the non-
sensitized reaction with oxygen, given by Eqs. (1) and (2), and a second
for reaction between pollutant and available chlorine. The experimental
data and method of correlation for the rate, using the high-pressure
mercury lamp, are given in detail in (2) and (3). The final expression
for the sensitized rate for the high-pressure lamp is
1.47 x 10"3 CQ 2.1 x 10"2 Ccl
+
6.5 x 10"8 + CQ 1.1 x 10"9 +
A I
The sensitized rate also can be improved by using the low-pressure
mercury lamp. The experimental data for a low-pressure lamp are given
-3-
-------
in (4). The rate equation for the low-pressure lamp is
PLp
2.07 x 10~2 Cn 1.01 x 10'1 C
,
6.5 x 10~8 + C 1.1 x 10~9 + C
Comparing Eq. (3) with (4), we see that the quantum yield is approx-
imately six times greater for the low-pressure mercury lamp.
The rate of free available chlorine disappearance was found to be
directly proportional to the UV energy absorbed by both free available
chlorine and pollutant. The method of obtaining the rate, fi , for
low- and high-pressure lamps is described in (4). The final results
are as follows:
for a high-pressure lamp
°-062 *
1.1 x 10 + C_. P
d J.
for a low-pressure lamp
0.057 Cr1
(6)
= 0.52 A_,I +
Comparison of Eq. (5) with (6) shows that the low-pressure lamp gives
about 1.4-times higher rate, for the same intensity of radiation.
In order to avoid the complication of the effects of specific com-
ponents of secondary effluent on kinetics, the rate equations were form-
ulated in terms of A . However, it would be desirable in future
work to study a variety of secondary effluents in order to accumulate
information on the relation between A and measurable properties of
the effluents. Nevertheless, A can be calculated from transmittance
measurements and thus not require such a relation. For design calcu-
lations for a specific secondary effluent, A should be expressed in
terms of concentration of pollutants. A depends on the composition of
secondary effluent and on the amount of chlorine added initially. This
dependence for a Sacramento effluent will be discussed in the follow-
ing section.
Note that the rate equations given in this part are not restricted
to any particular reactor-reflector geometry.
-4-
-------
IV. PROCESS DESIGN
Reactor^ Type and Design Conditions
Since methods for obtaining the rate of removal of organic carbon
[Eqs. (3) and (4)] and for the efficiency of UV-energy transfer to the
surface of water (5) have been obtained, it is now possible to design
a photoreactor for a tertiary treatment process. First, it is nec-
essary to decide upon a type of reactor which is feasible for photo-
chemical treatment on a large scale. The secondary effluent is a
colloidal solution which may deposit on walls. These deposits will
decrease the transmittance of light so that the fraction of light energy
which reaches the surface of the reacting solution will be reduced.
Therefore, a system having no surfaces between the lamp and reactor is
desirable. Such a system is also attractive from the economic point
of view since it would be expensive to construct transparent walls
for a large-scale system.
An attractive type is the trough reactor (Figure 1) which is
irradiated from a lamp above the water and surrounded by a parabolic
reflector at the top. The system may operate as a stirred-tank re-
actor or as an integral-flow reactor. The integral arrangement would
be more suitable for tertiary treatment. This follows from the follow-
ing reasoning. Since the electric-energy requirement is likely to be
a major cost in photoreactors, it is more economical to arrange the
reactor such that a constant, large fraction of light energy is ab-
sorbed along the flow path in the reactor. This may be achieved by
continuously increasing the depth as pollutant concentration decreases
due to oxidation. This can be accomplished in the integral-flow type,
but not in a stirred-tank reactor. At low concentrations of free
available chlorine the rate increases directly with Cr-. Therefore,
multiple injections of small quantities of chlorine along the flow
path, always operating at low C 1, are desirable. The contribution of
C> -L
oxygen to the rate is insignificant for a high-pressure lamp but not
negligible for low-pressure lamp operation. Hence, multiple injections
of oxygen are also desirable. However, it would be more economical
to use air instead of oxygen, considering that the effect of concentra-
tion of oxygen on the rate is much smaller than that of chlorine. There-
fore, the following design calculations will be based upon an integral-
flow, trough reactor with multiple injections of chlorine and air.
The basis for design calculations are:
1. The concentrations of chlorine and oxygen are maintained reasonably
constant throughout the reactor by supplying chlorine and air along
the reactor length. The numerical value for oxygen concentration is
—6 3
0.28 x 10 g moles/cm , corresponding to saturation of water with
air at room temperature.
2. A constant fraction of UV energy is absorbed along the reactor flow
length (length of trough) by changing the depth of the reactor. The
-5-
-------
-5a-
Lamp
Water in
Parabolic
RtflKtor
Water Out
Trouch Reactor
Figure L Trough Reactor with Parabolic Reflector
-------
numerical values for chlorine concentration and fraction of UV en-
ergy will be determined by minimizing the total cost of electricity
and chlorine.
3. The radiation from the reflector and lamp is assumed to be perpendic-
ular to the water surface.
Relationship Between Absorption of Secondary Effluent and Chlorine
Concentration
In design calculations it is necessary to express the rate equa-
tion in terms of concentrations. To do this, A must be given in terms
of absorption, (A°), of secondary effluent before adding chlorine, and
in terms of the concentration of free available chlorine.
The absorption of secondary effluent decreases when chlorine is
applied, until a large amount of chlorine is added. This is due to
formation of low-absorbing substances by reaction between chlorine and
pollutants. Such a decolorizing effect more than balances the absorp-
tion due to chlorine, until C . becomes large. The empirical relation-
0*4-
ship between the absorption and amount of chlorine added, (Cp1 ) , was
obtained (3) as L t
Ap -
where the constant K depends on the components of secondary effluent
(NH , --- etc). The relationship between (C ,| and C .., as given
J I w-U f Li-L
in Reference (4), was found to be linear:
(8)
where a and b are constants depending on the composition of
secondary effluent. From Eqs. (7) and (8), A for any secondary effluent
may be written as P
Ap - a- + Ccl <»>
where a' = 1 + K a and bf - K b.
The detailed chemical and optical properties of secondary effluent when
chlorine is applied are given in (3),
-6-
-------
For a trough reactor, with perpendicularly incident light directed
inward, the energy absorption per unit length of optical path (of
secondary effluent) before adding chlorine is obtained from Lambert's
law as
I
-------
fl =
p
0
}0 \6.5 x 10
-8
'Cl
1.1 x 10
A I(z)/w
P
(14)
where $ and $ are constants depending on UV spectral distribution,
PQ Pri
but not on the characteristics of the secondary effluent. Here I(z)
is the light intensity at the surface of the trough of water in Einsteins/
(cm)(sec). Note that A is the average fraction of UV energy absorbed
per unit length of optical path. According to condition 2, Ay is con-
•\ P
stant with respect to z. Therefore,
1 -
tot
is constant.
Then y,y = constant = y,y , and
- xp)y
(15)
where u, is the attentuation coefficient of the feed stream, and y
A 0
is the depth at z = 0. From Eq. (15) we obtain the depth at any con-
version as
y =
1 - x
(16)
Substituting Eqs. (9) to (14) in Eq. (11) yields
"""PC 9 C ~\
Pr» ^ p Cl
-8 -9
6.5 x 10 + C 1.1 x 10 + CP1
— U L» J_»
where
I(z) T|_ dz
a' + b' Ccl
(17)
tot
(18)
Note that HO represents the fraction of light energy absorbed by
secondary effluent. Since x =» 0 at z = 0, x = x at z = L, where
P P Pf
L is the length of reactor, integration of Eq. (17) yields
,6'5
i C $ C , ~
PO ° u PCI cl
fi -9
x 10 + C_ 1.1 x 10 + C_n
O L±«J
n3 Xt
a' -L h1 T
a + b Ccl
(19)
where
-8-
-------
It = I I(z)dz
(20)
I is the total intensity (Einsteins/sec) at the surface of the water
in the trough. We define efficiencies 1) and n9 as follows:
_ total UV output _ _o f lamp (Einsteins/sec) _ ot
1 total power input (watts) ~ P
total UV energy at the surface of the water _ t
2 total UV output of lamp ~ I
(21)
(22)
Then
(23)
Substituting this expression for I in Eq. (19) gives
ru r|_ r\- P
1 2 J t
x C
Pf Po
P0 ° , PC1 C1
8 9
6.5 x 10 + C_ 1.1 x 10 + Cnl
bk O (j J^-
1
a1 + b' Ccl
, cm /sec (24)
or in terms of gal/hr
q' = 0.95 q
(25)
Then the electric power required per gal of secondary effluent will be
1.05 x
Pf P0
(a' + b' Ccl)
0 C
po °
6.5 x 10~8 + CQ
^
1.1 x 10~9 + Cnl
Li-'
watts/gal
(26)
Equation (26) shows that the electric power requirement decreases with
increasing values of the efficiencies T) , r\ , and ru- Tne value of
r). can be calculated from known reactor dimensions, using the equations
in (5). It can also be seen in Eq. (26) that there is an optimum chlor-
ine concentration which gives the minimum required electric energy.
This optimum problem will be considered in a later section. The electric
power requirement also depends on the spectral distribution of the lamp,
since $ and $ are dependent on the lamp. The higher the conversion,
Po PC1
-9-
-------
the higher the feed concentration of pollutants, and the higher the
initial chlorine demand, the higher will be the required electric
power. Note that Eq. (26) can be applied to all lamps, all secondary
effluents*, and at all levels of conversion.
Chlorine Consumption
The concentration of chlorine is maintained constant throughout the
reactor. The mass balance of chlorine for a volume element dV is
dv = d(Fci}
(27)
where d(F - ) is amount of chlorine consumed in dV due to the UV
energy absorbed. The rate of free available chlorine disappearance,
given in Eqs. (5) and (6), may be written as
Cl
*ci Aci -
$ c
Cl Cl
p
-9
1.1 x 10 + C
P
Kz)/w
(28)
where
Cl
7x -yxci5
e
'tot
(29)
It has been shown in Reference (3) that the light energy absorbed by
chlorine is very small and independent of pollutant. Then expanding
the exponential term in Eq. (29) in a series, and neglecting terms
higher than first-order, we obtain
X,
Cl
tot
XC1 = aCl CC1
(30)
where a
Cl
2 F,
, ,
tot
is the average absorptivity of chlorine. Sub-
stituting Eqs. (9), (12), (16), (28), and (30) in Eq. (27) gives
$
+
L-X
C1
~9
(a' + b' Ccl)(l.l x 10~ + CC1J
Ccll(z)dz
(31)
*0f the types for which rate measurements were made
-10-
-------
Solving Eq. (17) for I(z)dz and substituting this in Eq. (31) gives
q C C^1 dx
po Cl p
6.5
$ C
p 0
*o
x 10~8 + <
+ $pci CGI
:_ 1.1 x io~9 + c_.
U d -I—
Cl
(a1 + b1
1.1 x 10
~9
(32)
Integrating Eq. (32) from x = 0 to x = x gives total chlorine consumed
due to the light-energy absorbed: "f
Cl
n cn
pci cl
6.5 x 10 8 + Cn 1.1 x 10 9 + C
- U
(a' + b1 Ccl) In
1 " XpfJ 1.1 x 10~9 +
g moles/sec
(33)
Or in terms of Ibs/hr
= 564 F , Ibs/hr
(34)
Since the initial amount of chlorine required to obtain C is given
i ^-*i
by
(F -) = (a + b C )q , g moles/sec
Cl
Cl
the total amount of chlorine required will be
-11-
-------
(F'cl) =
+ 594 (a + b
Ibs/hr
(35)
Note that Eq. (35) is not restricted to any particular lamp or second-
ary effluent.
Selection of Lamp
In order to determine what radiant energy source is best suited
for secondary-effluent treatment, it is necessary to determine which
lamp produces the greatest amount of energy, of the most useful wave
length, for the least cost. From Eqs. (26) and (33) we see that lamps
which have higher values of , n , and r\ give lower operating costs.
The efficiency r) depends only upon the lamp. On the other hand, the
efficiency r|- depends on both the spectral energy distribution of the
lamp and the attenuation coefficients of secondary effluent. Note
that the efficiency n_ can be controlled by the depth of the reactor.
High- and low-pressure mercury arcs are normally considered to be the
most efficient and most useful sources of ultraviolet energy which are
commercially available.
High-pressure lamps emit much more intense radiation than low-
pressure lamps of about the same physical size. Although the quantum
yield is greater for a low-pressure lamp, the number of lamps required
would be much greater for the high-pressure variety. However, the
useful life of the high-pressure mercury arc is relatively short—
usually less than 1,000 hrs. Low-pressure lamps have long lifetimes —
usually 7,500 hrs, or longer. High-pressure lamps are the more ex-
pensive but do not have higher efficiencies, H-,- Further, high-pressure
lamps require air cooling to obtain the best performance.
Since electricity is a major operating expense, we should compare
the electric power requirement for both lamps. Neglecting the contri-
bution of oxygen to the rate, the ratio of the electric power require-
ments for both lamps [as obtained from Eq. (26)] and for the same ab-
sorption will be
CVLp
(VHP
Lp
- pci „ _
(36)
The high- and low-pressure lamps which have the highest efficiencies
r\- among the lamps available are, respectively,
(a) 2100-watt, high-pressure, mercury lamp (78A Hanovia):
TI, - 1.054 x 10~6 Einsteins/(watt) (sec)
-12-
-------
and
(b) 65-watt, low-pressure mercury lamp (G64T6 General Electric)
H-, = 5.86 x 10 Einsteins/(watt) (sec)
The values for $~ are given in Eqs. (3) and (4). Substituting numeri-
Cl
cal values for ru and $ in Eq. (36), we see that the electric power
1 PC1
requirement is three times higher for the high-pressure lamp. Further,
the ratio of chlorine consumption for both lamps [obtained from Eq. (33)]
and for the same absorption is approximately given by
IFCI)HP . (*Pq)i.p '*
(F \ (0
FCI]LP f
UV)
Lp
Cl|Hp_ (37)
The values for 4> and $ .. are given in Eqs. (3) to (6). Substitution
PC1 C1
of numerical values for $ and $ - in Eq. (37) shows that chlorine
PC1 C1
consumption is 3.4 times larger for the high-pressure lamp. Therefore,
the best source appears to be a 65-watt, low-pressure, mercury lamp
(G64T6 General Electric). This lamp will be chosen for economic calcu-
lations .
Design Calculations for Treatment of 1^000 gal/hr ^econdary Effluent
This section shows the form of design calculations and does not
provide a practical design for a small (1,000 gal/hr) unit. The numerical
calculations will be based on the following design conditions:
(a) 1,000 gal/hr of secondary effluent from the Howe Avenue treatment
plant in Sacramento, California (qf = 1,000 gal/hr).
(b) Organic pollutant is to be reduced from 10 mg/liter (TOG) to 4[C -
10 mg/liter, or — ^ 10 g moles/cm , x = 0.6].
(c) The numerical values for a, b, and K for the secondary effluent are:
a - 1.38 x 10~7, b - 1.52, K - 2.5 x 106
These values were obtained for the secondary effluent from the Howe
Avenue plant in Sacramento [see References (3) and (4)].
(d) The reflectance (p) of reflector is 0.7, as found by measurement
for aluminum [see Reference (5)].
—6 3
(e) The concentration of oxygen is 0.28 x 10 g moles/cm , correspond-
ing to saturation of water with air at room temperature.
-13-
-------
(f) The costs of electricity and chlorine chosen are 1C/KWH and 3.75<:/lb,
respectively. The numerical values for the parameters in the rate
equations for a low-pressure lamp [see Eqs. (4) and (6)] are;
ot
3.81 x 10
-5
2.07 x 10
-2
*
C1
Cl
0.057
2.6 x 10'
Einsteins/sec
g moles/Einstein
g moles/Einstein
g moles/Einstein
g moles/Einstein
cm /g mole
The efficiency n2> given in (5), may be written as
1/2
where
a
16y
2
1/2
1 -
a
1+
1-61
1/2
al-
l/2
(38)
L
I
Y • ~
a
L
I
h
w
2h
length of reflector (» reactor), ft
length of lamps, ft
height of reflector above water surface, ft
width of'reflector (- reactor), ft
-14-
-------
Figure 2 shows the efficiency r\ as functions of the ratios, y
and a, when p = 0.7 and 6 = 1.3. We see from this figure that the
efficiency, r) , increases as both the ratio of height to width and the
ratio of height to lamp length decreases. This indicates that wide,
low, and long reflectors give higher efficiencies. One restriction
to height is the necessary distance between the lamp and the surface
of water to prevent excessive deposits on the lamp. We choose six
inches as the minimum safe distance. Then from the equation for a
parabola, the height in terms of y is
h
ft
(39)
1 -
16y
Since the power input per unit length of low-pressure lamp is
small (0.411 watts/cm), the length of lamp required for 1,000 gal/hr
wastewater treatment is expected to be very large. Therefore,
a = -T— -3^0
and we choose 6 = = 1. As noted from Figure 2, the
efficiency f"u increases when the ratio y decreases. However,
for y < 0.3 the change in
r\ with respect to
y is very small.
Hence y = 0.3 is chosen as the optimum value. Note that the volume
of the reactor increases as y decreases. Substituting these values
for a, y, and 6, and p = 0.7, in Eq. (38) gives
= 0.833
The low-pressure mercury lamp emits very little radiation other
than that at 2537 A. Hence, this lamp may be considered as a mono-
chromatic ultraviolet source. Then the efficiency, r|_ (Eq. (18)), be
comes
,
- 1 - e
y9<-o7 = 0.264 for the secondary effluent from the Howe Avenue
£* J J /
where
plant (corresponding to TPC of 10 mg/ liter.
With the chosen design conditions, substituting values for TU ,
ru, Ho and the parameters of the rate equations in Eqs. (26) and (35)
gives the electric power and the amount of chlorine required. The
results for a treatment rate of 1,000 gal/hr, as functions of the chlor-
ine concentration and the initial depth of reactor, are:
107. 55 "(1.345 + 0.038 Crl x 108)(0.11 + Cr1 x 108)
(1 - e
yo) (0.185 + 11.78 C x 10°)
UJ-
KWH
(40)
15-
-------
-15a-
0.840
0.830
CM
O
UJ
O
0.790
0.1
0.2
Figure 2. Effect of Ratio of Height to Width on Efficiency
-------
0.061 y C_. x 108 (1.345 + 0.038 C... x 108)(0.11 + C_. x 108)
/pi \ _ o l-l Cl Cl
01 t " (1 - e°'264 yo)(0.185 + 11.78 GCI x 108)
1.69 C x 108
+ — ;r + 0.009 C x 10 + 0.082, Ibs/hr (41)
0.185 + 11.78 c_. x 10° Li
C J_
Since the costs of electricity and chlorine are Ic/KWH and 3.75c/lb,
respectively, the total cost of electricity and chlorine is obtained
from Eqs. (40) and (41) as
(1.345 + 0.038 Cpl x 108)(0.11 + CP1 x 108)(107.55 + 0.0229 y C_, x 108)
Q _ LiJ. \jj- O Li-L
0 (1 - e°'264 yo)(0.185 + 11.78 Cpn x 108)
OJ.
6.348 C x 108 ft
+ — £ + 0.0338 C - x 10 + 0.307, Cents (42)
0.185 + 11.78 C_- x 10
CJ.L
The total costs calculated from Eq. (42) at various C _ and y are
summarized in Table 1
Table 1. Electricity and Chlorine Costs
Depth yo, cm
Cost, cents/1,000 gal
C... • 0.5 x 10~8,g tnoles/cm3 18.46 17.23 16.60 16.27 16.09 16.00 16.02
= 1.0 x 10~8 17.49 16.37 15.80 15.51 15.38- 15/30 15.34
= 1.5 x 10~8 17.38 16.31 15.78 15.51 15.40 15.39 15.44
= 2.0 x 10~8 17.50 16.45 15.94 15.71 15.63 15.63 15.72
The optimum chlorine concentration and optimum depth which give the mini-
mum cost of electricity and chlorine are found from Eq. (42) to be
—8 3
CGI - 1.1 x 10 g moles/cm (0.78 mg/liter)
yQ = 17 cm (0.56 ft)(n3 = 0.9887)
Cost - 15.21$ (Electricity 13.88<: and chlorine 1.33$)
With these values, the electric power required for 1,000 gal/hr waste-
water treatment is E « 13.88 KWH/1,000 gal. If four parallel lamps are
placed at the focus of the parabolic reflector, the power input per foot
-16-
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(using 65-watt low-pressure lamps) is 53.8 watts/ft. Then, the total
length of lamps needed is
x 103 f e
J ->. O
The amount of chlorine required can be found from Eq. (41):
(F' ) = 0.355 (Ibs/hr)
C1 t
The optimum values for geometry and chlorine concentration for 1,000
gal/hr of secondary effluent from the Howe Avenue plant are summarized
as follows:
(1) Reflector (parabola)
width - 4.92 ft
height - 1.64 ft
distance between lamp and surface of water = 0.5 ft
length = 257 ft
n2 = 0.833
(2) Lamp: 65 watt, low-pressure mercury lamp (G64T6 General Electric)
total length = 257 ft
total number = 214
(3) Reactor (Trough)
depth: inlet = 0.56 ft
outlet = 1.4 ft
length = 257 ft (length parallel to lamps)
width - 4.92 ft
volume - 1240 ft3
concentration of chlorine = 0.78 mg/liter
(4) Operating cost (electricity and chlorine)
electricity - 13.88 KWH or 13.88
-------
increased from 5.86 x 10~ Einsteins/(watt)(sec) to 1.172 x 10~ Einsteins/
(watt)(sec), the electric power requirement for 1,000 gal/hr treatment
would be reduced to 6.94 KWH.
The Hanovia Lamp Company attempted to build a more intense low-
pressure lamp with higher n in response to our request, but the first
attempt did not succeed.
Design Calculations for One Million gal/day Treatment Plant
The plant is designed to reduce TOG from 10 to 4 mg/liter, using
G64T6, low-pressure, mercury-arc lamps. This lamp is in commercial
production and has the highest efficiency, n among low-pressure lamps
available. The UV output is 18.5 watts for an electrical power input
of 65 watts. A plant would handle one million gal/day. All other de-
sign conditions are the same as for the 1,000 gal/hr calculations given
in the previous section. The concentration of chlorine is maintained
constant at C - = 0.78 mg/liter throughout reactor.
(1) Determination of electric energy and chlorine requirements
The efficiency, r)9, is the only parameter in the design equation
which may be affected by scale-up. However, for low-pressure lamps the
total length of lamps in one reflector is long enough for the ratio
*7Vi
a = -p— to be negligible. Note that r)_ does not change for a < 0.05,
even if the total length of lamps is changed, provided that other parameters
in Eq. (38) are constant. In order that ru be high, a reflector is
chosen which has the same height and width as for the 1,000 gal/hr de-
sign calculations. Hence, a value of ru = 0.833 will be used. With
these conditions the electric energy and chlorine requirements are
directly proportional to the flow rate. For a feed rate of one-million
gal/day, we obtain, from Eqs. (26) and (35X,
Electricity = 578 KWH
Chlorine =1.48 Ibs/hr
(2) Reactor-reflector geometry and plant description
The reactor shown in Figure 3 represents an arrangement such that
r\ = 0.833, which is the highest efficiency obtainable for the given
design conditions. Then the total number of lamps is 8,890. The ratio
of width to length of the reactor is unimportant as long as a < 0.05
and the total number of lamps = 8,890. As a practical possibility we
choose
width of reactor = 172 ft
length of reactor = 309 ft
-18-
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-18a-
Parabolic Reflector
Parabolic Reflector
Lamp
Water In
Trough Reactor
Fifure 3. Arrangement of Reactor and Reflectors.
-------
As shown in Figure 3, four parallel lamps are placed near the focus of
the reflector. The reactor cover consists of 35 parabolic reflectors.
Each reflector has 254 lamps and then about 64 lamps are connected in
series at the line of focus. The inlet and outlet depths of reactor
are chosen to be the same as for 1,000 gal/hr treatment, since the con-
centration of free available chlorine is maintained at C =0.78 mg/liter.
Cf-L
The optimum values for reactor-reflector geometry (to reduce the
TOG from 10 to 4 mg/liter) for secondary effluent from Howe Avenue plant
are summarized as follows:
(1) Reflector (parabolic)
width = 4.92 ft
height = 1.64 ft
distance between lamp and surface of water -• 0.5 ft
length = 309 ft
number of reflectors = 35
n2 = 0.833
(2) Lamp: 65 watt, low-pressure mercury lamp (G64T6 General Electric)
total number of lamps required = 8,890
number of lamps in each reflector = 254
(3) Reactor (trough)
depth: inlet = 0.56 ft
outlet = 1.4 ft
length = 309 ft (length parallel to lamps)
width - 172 ft
volume = 52,400 ft3
concentration of chlorine = 0.78 mg/liter
A process flowsheet is given in Figure 4. Chlorine would be supplied
as vapor from a 1,400 gal liquid chlorine tank. Automatic, flow-control
equipment would regulate the chlorine flow to each injection point in
order to maintain the constant chlorine concentration (C - » 0.78 mg/liter)
There are 35 and 64 injection points across and along the trough reactor,
respectively.
Air would be supplied (from an air compressor) uniformly to each in-
jection point. The number and arrangements of air injection points
are the same as those for chlorine.
-19-
-------
Chlorine Pump
1 gpm
Waste Water from
Secondary
Treatment Plant
TOC =10 mg/liter
/
s,
i
)
J Chlorine Storage Tank
1400 -gal Capacity
/~N
r
K
Water Pump
700 gpm
Water Dist
Cl? Injection v.
i
i i
1
]
i
i
ributor
Trough Reactor with
Parabolic Reflectors
i
i
i
t
f
!
Air Injection
„ ,1 ,
I
1 Tertiary Treated Wastewater
i One Million Gallon / Day
! TOC -4 mg/ liter
1 1
P^ «
i
Air Compressor
10 ft3/min
Figure 4. Process Flowsheet for Photochemical Treatment of Secondary Effluent (One Million gal / Day)
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V. EVALUATION OF ECONOMICS FOR TERTIARY TREATMENT
A. Fixed—Cap it al_ Investment
Because prices may change considerably with time and location, a
detailed calculation of costs is not warranted. The results presented
here are estimates, but they do indicate the general level of costs for
photochemical treatment at its present stage of development. A reliable
estimate of costs would require engineering drawings, specifications, and
site surveys would be required for construction of a treatment plant. The
chief reference for costs used here was Peters and Timmerhaus (6).
The plant investment has been estimated for a plant capacity of one-
million gal/day. The design conditions and detailed dimensions of major
equipment are described in the previous part (process design) of the re-
port. Figure 4 shows the major equipment. The cost of installed major
equipment items is $185,400, itemized as follows:
1. Trough reactor; 6" thick concrete
total surface area = 53,040
cost = $88,500 ($15/sq yd)
1 "
2. Reflectors (35) ; -77- thick aluminum
ID
total surface area = 150,000 ft2
cost = $82,500 ($0.55/ft2)
3. Wastewater pump; 700 gpm, $2,000
4. Chlorine pump (stainless steel); 1 gpm, $1,800
5. Air compressor; 10 ft3/min; $5,600
6. Chlorine storage tank; 1,400 gal, $5,000
Other plant costs are:
1. Instrumentation and controls $ 33,300
2. Piping (installed) 38,900
3. Electrical (installed) 27,800
4. Buildings (including service) 44,500
5. Yard improvements 11,000
6. Service facilities (installed) 55,600
7. Land 5,500
8. Engineering and supervision 50,000
9. Construction expense 56,000
10. Contractor's fee 11,100
11. Contingency 44,100
$377,800
This gives a total plant investment of $563,200, excluding lamp costs.
Since a life expenctancy of the G64T6 low-pressure mercury lamp is one
year, it is best to treat lamp costs as a yearly operating cost (in the
next section).
-21-
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B. Annual Operating Expenses
These expenses include direct treatment cost and fixed charges.
Annual costs are based on 360 working days.
The treatment cost was estimated as follows:
Operating labor: 2 men/shift
Raw material
Chlorine: 355 Ibs/day at 3. 75
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VI. ACKNOWLEDGEMENT
Dr. C. P. Jeffreson, Dr. V. Schorr, Dr. B. Boval and Dr. V. Hancil
assisted importantly in carrying out the experimental measurements and
analysis of results.
The support of the project by the Environmental Protection Agency,
and the help provided by Mr. Robert H« Wise, the Grant Project Officer,
is also acknowledged.
-23-
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VII. NOMENCLATURE
A absorption ratio, cm
A = absorption ratio for secondary effluent
A° = absorption ratio for the feed stream of secondary effluent
P before chlorine is added
A = absorption ratio for chlorine
3
a initial chlorine-demand parameter in Eq. (8), g moles/cm
a' = 1 + Ka
b constant in Eq. (8)
b' = Kb
C concentration, g moles/cm
C = concentration of pollutant
C . = concentration of free available chlorine
C = concentration of oxygen
(Cr1) = amount of chlorine added initially
t
E electrical energy requirement, watts/gal
F, fraction of UV energy emitted by lamp of wave length A
F total UV energy emitted by lamp
F_.. amount of chlorine consumed due to UV energy, g moles/sec;
F^, Ibs/hr
(F' ) » total amount of chlorine required, Ibs/hr
C1 t
(Frl ) = amount of chlorine required to obtain given C ,, Ibs/hr
Cl ± Cl
h height of reflector above water, ft
I(z) intensity of radiation, at the surface of water at z, Einstein/.
(cm)(sec)
I total intensity at the surface, Einsteins/sec
I total UV output from lamp, Einsteins/sec
I total intensity of UV energy at the reactor wall, Einsteins/
(cm2)(sec)
L length of reactor (or reflector), ft
£ total length of lamps in one reflector, ft
P total electric power input, watts
-25-
-------
3
q volume flow rate of secondary effluent, cm /sec
q? = gal/hr
x conversion; x refers to organic carbon pollutants
3
v volume of reactor, cm
w width of reflector (or reactor), cm
y depth of reactor, cm
z axial distance, cm
3
TOG concentration of total organic carbon, g moles/cm , or mg/liter
Greek
a ratio of height of reflector to one half of lamp length h/(£/2)
2
OU absorptivity at wave length A, cm /g mole
a = average absorptivity =Y"
— - ou
, tot
Al
6 ratio of length of reflector to that of lamp =
Y ratio of height to width of reflector = h/w
3
fi rate of reaction, g moles/ (cm ) (sec)
fir1 = rate of free available chlorine disappearance
\jj.
£2 = rate of organic pollutant removal
X wave length, cm
$ quantum yield, g moles/Einstein
$p = quantum yield for contribution of oxygen to rate of pollutant
0 disappearance
p
^
= quantum yield for contribution of free available chlorine
to rate of pollutant disappearance
Q1 = quantum yield for contribution of pollutant to rate of free
available chlorine disappearance
$P1 = quantum yield for free available chlorine disappearance due
toAci
VJU attenuation coefficient at wave length A, cm
y, = feed stream of secondary effluent
chlorine
fraction of the electrical energy input which is emitted in the
UV wave-length region of the lamp
-26-
-------
i~|_ fraction of UV energy leaving lamp which strikes the surface
of the secondary effluent
T]~ fraction of the incident UV energy which is absorbed by secondary
effluent
p reflectance
•* parameter in Eq. (38)
Subscripts
Cl free available chlorine
f final, or exit conditions
Hp high-pressure mercury lamp
Lp low-pressure mercury lamp
o feed or entrance conditions
0 oxygen
p pollutant
t total
Superscripts
o feed condition before chlorine is added
-27-
-------
VIII. REFERENCES
1. Melners, A. F., "Light-Catalyzed Chlorine Oxidation for Treatment
of Wastewater," Midwest Research Institute, Kansas City,
Missouri 64110, Report No. 17020 DUE09/70, Water Pollution
Control Research Series, Contract No. 14-12-531.
2. Schorr, V., Boval, Bruno, Hancil, V. and Smith, J. M., "Photo-
oxidation of Organic Pollutants in Municipal Wastewater,"
Ind. and Eng. Chem., Process Design and Development 10, 509
(171).
3. Hancil, V. and Smith, J. M., "Chlorine-Sensitized, Photochemical
Oxidation of Soluble Organics in Municipal Wastewater,"
Ind. and Eng. Chem., Process Design and Development 10,
515 (1971).
4. Cha, C. Y. and Smith, J. M., "Wavelength Effects in Photochemical
Oxidation of Organic Pollutants in Wastewater," Ind. and Eng.
Chem., Process Design and Development 11, 451 (1972).
5. Hancil, V., Schorr, V. and Smith, J. M., "Radiation Efficiency of
Photoreactors," AIChE Journal JJJ, 43 (1972).
6. Peters, M. S. and Timmerhaus, K. D., "Plant Design and Economics for
Chemical Engineers," McGraw-Hill, New York (1968).
II. S. GOVKHMWKNT I'HINTINO OK KICK : l')72 — 514-140/1°°
-29-
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SELECTED WATER
RESOURCES ABSTRACTS
INPUT TRANSACTION FORM
1. Report No.
2.
3. Accession No.
w
4. Title
PHOTOCHEMICAL METHODS FOR PURIFYING WATER
5. Report Date
'.'«.
7. Author(s)
Cha, C. Y. and Smith, J. M.
9. Organization
California University, Davis, Dept. of Chemical
Engineering
8. P-jifoTouRs Organization
Report No.
10. Project No.
17020 EVQ
12. ST 'nsorin: Organ' *ti
11. Contract/Grant No.
•:' Type f Repot i and
Period Covered -
IS. Supplementary Notes
Environmental Protection Agency report
number EPA-R2-72-l()lj-, November 1972.
16. Abstract
The kinetics of photochemical oxidation of organic carbon pollutants, with and
without sensitization by chlorine, have been determined. The results, which are
suitable for use in designing reactors for any type of lamp and lamp-reactor geom-
etry, are based upon data obtained for secondary effluents from Sacramento area
municipal treatment plants.
A trough reactor with parabolic reflector was found to be the most suitable de-
sign for a large-scale, tertiary treatment plant based upon photochemical oxidation.
Cost calculations were made for reducing the TOG from 10 to 4 mg/liter and main-
taining a chlorine concentration of about 0.8 mg/liter. For a capacity of one-
million gallons per day of secondary effluent, treatment costs were 89<: per 1000
gallons with a total plant investment of $563,000.
17a. Descriptors
*0xidation, *Photoactivation, *Sewage treatment, *Waste treatment, *Waste water
treatment, *Tertiary treatment, Water pollution treatment, chemical degradation,
Chlorine, Chlorination, Kinetics, Organic matter, Organic wastes.
17b. Identifiers
Photochemical oxidation, Sewage oxidation, Chlorine-sensitized oxidation,
Photooxidation, Chemical oxidation, Oxidation kinetics.
17c. COWRR Field & Group
05 D
18. Availability
19.
Security Ctass.
(Repot )
*D. Se- 'TityCl fs,
(Page)
21.
No. of
Pages
Send To:
WATER RESOURCES SCIENTIFIC INFORMATION CENTER
U.S. DEPARTMENT OF THE INTERIOR
WASHINGTON. D. C. 2O24O
Abstractor
C. Y. Cha
institution Uaiversity of California
WRSIC IO2(REV.JUNF 137 I)
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