United States
Environmental Protection
Agency
Municipal Environmental
Research Laboratory
Cincinnati OH 45268
Research and Development
EPA-600/S2-84-136 Sept. 1984
Project Summary
Pilot-Scale Evaluation of
Photolytic Ozonation for
Trihalomethane Precursor
Removal
William H. Glaze, Gary R. Peyton, Brigitte Sohm, and Dean A. Meldrum
Ozone used in combination with
ultraviolet (UV) radiation was studied
on the pilot scale for removing
trihalomethane (THM) precursors from
potable water. The effects of varying
parameters such as ozone dose rate and
UV intensity were first studied using a
synthetic feedwater reconstituted from
a humic concentrate. This concentrate
was obtained by sodium hydroxide
elution of granular activated carbon
(GAC) columns used for the adsorption
of humic material from water in a
previous project. The pilot plant was
then operated for 8 months at the
Sabine River Water Treatment Plant in
Longview, Texas.
A laboratory investigation was also
conducted on the mechanism of the
ozone/UV process. Kinetic analysis
was performed on data collected during
ozone photolysis experiments in the
presence and absence of known
hydroxyl radical scavengers. This anal-
ysis indicates that ozone photolysis in
aqueous solution leads directly to the
formation of hydrogen peroxide, which
then produces hydroxyl radical by
secondary reaction of peroxy anion
(HO2~) and subsequent species with
ozone.
The mechanistic results predict a
maximum yield of % hydroxyl radical
for each ozone molecule used in the
proposed mechanism. The mechanistic
results also predict a plateau in treat-
ment efficiency as a function of the
fraction of ozone photolyzed and thus
as a function of UV power input. A
simple model was developed for ozone
mass transfer with chemical and
photochemical reaction. Correlation of
this model with mechanistic and pilot
plant data is ongoing.
Operating data from the pilot plant
and analysis of the residual trihalometh-
ane formation potential (THMFP) after
treatment provided the basis for
projecting costs for removing THMFP
by photolytic ozonation. At a 1-mgd
plant capacity and an initial THMFP of
300 fjg/L, projected treatment costs
ware $0.55, $0.70, and $0.91 per
thousand gallons for 60%, 70%, and
80% THMFP removal, respectively,
assuming an electrical cost of $0.10 per
kWh. At this capacity, projected
treatment costs were still decreasing
rapidly with increased plant capacity.
This Project Summary was developed
by EPA's Municipal Environmental
Research Laboratory. Cincinnati, OH,
to announce key findings of the research
project that is fully documented in a
separate report of the same title (see
Project Report ordering information at
back).
Introduction
The chlorination of natural water for
potable use is now known to produce
trihalomenthanes (THM's), which have
been shown to be carcinogenic in
laboratory animals. A preliminary
laboratory study reported by this research
group in 1980 (EPA-600/2-80-110)
showed that the potential of natural
waters for producing THM's upon
chlorination could essentially be elimi-
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•*''"f
•A
nated by simultaneous treatment with
ozone and ultraviolet (UV) radiation.
However, the ozone doses required in the
laboratory experiments were rather high.
The present report describes an attempt
to understand and refine the ozone/UV
system; also described is a pilot-scale
evaluation using photolytic ozonation to
destroy THM precursors in water.
The project was conducted in four
parts: (1) a laboratory and theoretical
investigation of the mechanism of photo-
lytic ozonation, (2) construction and initial
testing of the pilot plant using a reconsti-
tuted lake water concentrate for f eedwa-
ter consistency, (3) continuous operation
of the pilot plant using feedwater from the
Sabine River Water Treatment Plant
located in Longview, Texas, and (4) a
laboratory study of byproducts formed
upon photolytic ozonation of natural
waters. In addition to these four phases, a
simple model for ozone mass transfer
with photochemical reaction has been
developed to use with the mechanistic
results for interpreting the experimental
results. Data obtained during pilot-scale
operation were used in treatment cost
projections for a full-scale system.
Procedures
Laboratory Study
Ozone photolysis was studied in the lab-
oratory by monitoring the accumulation
of ozone, hydrogen peroxide, and total
oxidants in a laboratory scale, stirred-
tank reactor, with and without irradiation
by UV light and in the presence and
absence of known hydroxyl radical
scavengers. The experiment was set up
so that the ozone concentration in the gas
stream into and out of the reactor could
be measured. The analyses indicatedthat
in the absence of organic compounds, the
total oxidant concentration was equal to
the sum of the ozone and hydrogen
peroxide concentrations. Ozone was
measured using indigo disulfonate
bleaching; hydrogen peroxide was
measured by the colorimetric detection of
the peroxide-titanium complex, and total
oxidants by molybdate-catalyzed iodimetry.
Pilot-Scale Studies
Pilot studies were conducted in a pilot
plant housed in a 42-ft-long drop-frame
trailer. The plant had been previously
used in a pilot study of ozonation com-
bined with GAC for THM precursor
removal. This facility was moved to the
SumX Corporation in Austin, Texas,
where it was modified by the addition of
UV lamps to the existing ozone contactor
and the addition of tubular photochemical
reactors (bubble columns with UV lamps
installed in quartz lamp wells) for
comparison studies.
Operating parameters were varied in
the initial phase, which used synthetic
lake water reconstituted from a
concentrate. The concentrate was
obtained by sodium hydroxide elution of
several GAC columns that had been used
for several weeks to treat water with a
high natural organic content. The
consistency of feedwater provided by this
concentrate during a several-month
study to determine optimum treatment
parameters outweighed arguments
about the concentrate's representative
nature. A stirred-tank reactor equipped
with a turbine contactor was compared
with the tubular photochemical reactors
(TPR's) while the ozone dose rates and UV
intensities were varied in both.
Following the study to determine opti-
mum treatment parameters, the pilot
plant was moved to the Sabine River
Water Treatment Plant in Longview,
Texas, and operated for 8 months. Feed-
water was of two types: alum-settled
water before and after lime clarification.
The stirred-tank reactor and the TPR's
were compared while ozone dosages and
UV intensities were varied. Treatment
effectiveness of various configurations
was evaluated by withdrawing water
samples after treatment, chlorinating
them for 7 days in the laboratory at an
applied chlorine dose of 30 mg/L, and
measuring them for THM formation by
liquid-liquid extraction and gas chroma-
tography/electron capture detection
(GC/ECD) analysis.
Byproduct Studies
Treated and untreated water samples
from the pilot plant were passed through
columns of the macroreticular resins
XAD-4 and XAD-8 to trap organic bypro-
ducts of photolytic ozonation. In the first
of two experiments, the water was
acidified to pH 1.8, a water-to-resin
•volume ratio of 20 was used, and the
resin was eluted with methanol. To
derivatize carboxylic acids, the sample
was evaporated to dryness by gentle
warming in a stream of inert gas after
adding tetrabutylammonium hydroxide
(TBAH) to pH 9. The residue was taken
back up in a small quantity of acetonitrile
containing a slight excess (based on
TBAH) of ethyl bromide and allowed to
stand at room temperature for 1 hour or
longer. This procedure produces the ethyl
esters of carboxylic acids.
In the second experiment, the water
was passed through the resins after it
was adjusted to pH 10, the resin was
eluted with ethyl ether, the water was
adjusted to pH 2 and put back through the
column, and the column was again eluted
with ether. Extracts from both
experiments were analyzed by capillary
gas chromatography/flame ionization
detection (GC/FID) and by gas
chromatography/mass spedrometry
(GC/MS) at overall concentration factors
of 1300 and 1200, respectively, for the
first and second experiments.
Results and Discussion
Laboratory Study of
Active Species
The main thrust of this portion of the
project was to identify the principal active
specie(s) in photolytic ozonation and, if
possible, to determine the reaction
mechanism. The primary barrier to
elucidating the mechanism was
identifying the products of the first step,
ozone photolysis. The literature indicated
that this reaction would produce either
two hydroxyl radicals or hydrogen
peroxide, followed in either case by a
complex network of secondary reactions.
Ozone photolysis studies were
designed to differentiate between the two
possibilities for the primary step in
photolytic ozonation. The studies led to
conflicting results when two different
known hydroxyl radical scavengers were
used. When 0.015M acetic acid/acetate
was present, hydrogen peroxide
accumulated in solution upon continued
ozonation and irradiation at pH values in
the range of 3 to 8. However, peroxide
levels were too low to be quantified in the
presence of 0.015M sodium bicarbonate
at pH 7. Experiments run in distilled water
with no scavenger added gave results
intermediate between the two scavenger
experiments, indicating the presence of
some additional complicating effect in at
least one set of experiments. Kinetic
analysis of the complex reaction system,
using rate constants from the literature
and assuming first one initiation step and
then the other, indicated that the initial
step of the reaction mechanism is the
photolysis of aqueous ozone to produce
hydrogen peroxide. The ensuing second-
ary reaction system is primarily that of the
ozone/hydrogen peroxide system. This
system includes the dissociation of
hydrogen peroxide to yield the anion,
which in turn reacts with ozone to yield
hydroxyl radical and superoxide (O2~).
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Superoxide reacts very quickly with
additional ozone to yield ultimately
another hydroxyl radical. Thus in the net
reaction, three ozone molecules and one
photon yield two hydroxyl radicals. The
implication of this net reaction
stoichiometry is that, in a system where
the hydroxyl radical is effectively
scavenged by the substance to be
destroyed (organic substrates), the
optimum hydroxyl radical yield is
achieved when one-third of the ozone
transferred into solution is photolyzed.
On the other hand, if ozone photolysis
resulted directly in the production of two
hydroxyl radicals, the maximum yield
would be realized when all the trans-
ferred ozone was photolyzed.
Pilot Studies
The studies to determine optimum
operating parameters indicated that the
stirred-tank reactor performed better
than the tubular photochemical reactors,
even on the basis of ozone consumed.
Thus the greater effectiveness in the
former case was not simply the result of
better mass transfer. These studies also
indicated that an optimum level of UV
power existed, above which performance
increased very little. These data support
the proposed mechanism.
The studies of optimum operating
parameters also indicated that in the
stirred tank with 0.1 to 0.3 watt/L of
applied UV radiation, an ozone dose of 30
mg/L was required to reduce the total
trihalomethane formation potential
(TTHMFP) of the reconstituted lake water
concentrate to 1/e (37%) of its original
value. (Original THMFP values ranged
from 106 to 192 /jg/L in this phase of the
study). In the tubular photochemical
reactors, 40 mg/L ozone was required to
achieve the same extent of removal,
using applied UV radiation of 0.5 to 0.7
watt/L of reactor volume.lt was found in
most cases that upon ozone/UV
treatment, the THMFP first increased
from its initial value, then dropped expo-
nentially with continued treatment. This
is attributed to the formation of precursor
sites from nonprecursor organic material.
During pilot operation in Longview, the
stirred tank also proved superior to the
tubular photochemical reactor. In an
attempt to test whether gas-phase ozone
photolysis was detrimental to process
efficiency, delayed irradiation (Dl)
configurations were also tested. In these
configurations, ozone contacting took
place in the stirred tank reactor, followed
by ozone photolysis in a separate vessel.
The Dl configurations were less effective
than simultaneous ozone contacting
and photolysis in the stirred tank, but
more effective than the tubular reactors
alone, based on the amount of ozone
removed from the gas stream. The ozone
doses required for TTHMFP removal to
1/e of the original value are listed in
Table 1, along with those required for the
reconstituted lake water used in the
parameter studies. Correspondence of
results obtained on the two different
feedwaters is good when expressed in
terms of the ozone dose required to reach
a particular extent of removal. Note that
the desired TTHMFP removal could be
reached using ozone alone, but that the
required ozone dose is prohibitive.
The pilot-scale operation at Longview
also snowed that the lowest dose rate
that could achieve the desired removal
proved to be the most efficient. In
addition, the efficiency with which ozone
was used in the ozone/UV reactions was
relatively insensitive to pH and other
matrix components in clarified, filtered
Sabine River water.
Cost Analysis
Data for removing 60%, 70%, and 80%
of the TTHMFP in the optimum configura-
tion was used to project an approximate
treatment cost, using cost data supplied
by the U.S. Environmental Protection
Agency (EPA) for small systems (0.1 and
0.5 mgd). These data were extrapolated to
other system sizes using the equation T —
LQ n, where T is the cost, Q is the plant or
unit capacity, and L and n are parameters
evaluated by fitting the equation to the
EPA-supplied data for capital and
operating costs for ozone generation and
feed, UV light, stirring, etc., at 0.1- and
0.5-mgd capacities. Capital costs were
subjected to a multiplicative factor of 1.4
to cover engineering, contractors, over-
head and profit, site preparation, interest
during construction, and legal and admin-
istrative fees. An interest rate of 8% was
assumed over 20 years.
Cost projections for various levels of
removal strongly depended on plant
capacity in the 0.1 - to 0.5-mgd range, and
a method was sought to estimate
treatment costs over a larger range of
plant size. To assess the validity of
extrapolation to larger systems, suppliers
of ozonation equipment were contacted
and asked about ozone generation and
feed equipment costs for systems in the
size range of 20 to 1000 Ib/day. The
hardware costs thus obtained were fit to
the same exponential equation given
above and then extrapolated upwards (in
plant capacity) for comparison with the
installed costs for actual large systems
installed in recent years. A ratio of
hardware costs to installed costs for large
systems was obtained from this compari-
son. When this ratio was applied to the
manufacturer-supplied hardware costs
applicable to 0.1 - and 0.5-mgd plants, the
resulting value predicted the EPA-
supplied cost figures (including the factor
of 1.4) within 10% to 15%, thus indicating
the validity of the above extrapolation.
Projected treatment costs are given in
Table 2 for 0.1- to 5.0-mgd capacities.
Treatment costs are still strongly capacity-
dependent in the range of 5.0-mgd
capacity.
Byproduct Study
The methanol extract derived from the
first experiment was analyzed by GC/FID
andGC/MS. Several peaks were reduced
to a few percent of their original size by
ozone/UV treatment (17.7 mg/L of ozone
used), and only two new peaks were
found. The latter corresponded with 9 and
Table 1. Ozone Dose Required to Reduce TTHMFP of Sabine River Water and Reconstituted
Lake Water to 1/e of Its Original Value*
Ozone Dose
Reactor
Stirred Tank
Stirred Tank
Tubular
Stirred Tank
Mode
Oi/UV-CPj
O3/UV- Dl\
O3/UV-CP]
Ozone only
Sabine River Water,
Longview, Texas
27 mg/L
34 mg/L
41 mg/L
48 mg/L
Reconstituted
Lake Water
30 mg/L
§
40 mg/L
§
* 1/e = 37%; initial THMFP values ranged from 106to 192 g/L in reconstituted lake water and 120
to 437 g/L in Sabine River water.
f CP = simultaneous ozone contacting and photolysis.
t Dl = ozone contacting followed by photolysis in a separate vessel.
§ Not run.
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Table 2. Projected Treatment Costs for THM Precursor Destruction Using Photolytic Ozonation*
Dollars/1000 gallons
Plant Capacity
0. 1 MOD
0.5 MOD
1.0 MOD
5.0 MOD
60% Removal
$2.16
0.83
0.55
0.21
70% Removal
$2.43
1.02
0.70
0.29
80% Removal
$2.75
1.27
0.91
0.42
* An electrical cost of $0.10 per k Wh was used, but costs are only 5% to 10% lower at an electrical
cost of $0,05 per kWh.
2 //g/L, assuming compound response
factors equal to that of the internal stand-
ard d,0-anthracene. In the second experi-
ment, GC/FID analysis of the ether
extract revealed several compounds in
the treated water that were not present in
the untreated sample. The concentration
of these compounds ranged from a few
/ug/L to several tens of /ug/L, assuming
response factors equal to that of the
internal standard. No GC/MS identifica-
tion of these compounds has yet to be
made.
Also in that sample, a small group of
peaks present in the untreated water
disappeared upon treatment, reconfirm-
ing the ability of the ozone/UV process to
remove trace quantities of potentially
harmful organic pollutants during
treatment for THM precursors.
Comparison with the internal standard
indicated that the compounds were origi-
nally present in the water at less than 1
ppb. The mass spectra indicated that the
compounds were probably a homologous
series of alkyl benzenes. Further analysis
of the GC/MS data reported here is
ongoing.
Conclusions
The laboratory study shows that the
primary mechanism of photolytic
ozonation with UV irradiation at 254 nm
is the photolysis of ozone to produce
hydrogen peroxide, which dissociates to
some degree to produce peroxy anion.
The reaction between peroxy anion (and
in a later step, superoxide) and ozone
produces hydroxyl radical, which is the
primary active species of the ozone/UV
process. The above conclusions predict
that (1) the maximum yield of hydroxyl
radical per ozone molecule is 2/3, and (2)
when more than V3 of the available ozone
is photolyzed, little or no increase in
hydroxyl radical production should be
obtained.
In the pilot-scale study, an ozone dose
of 27 mg/L (transferred from the gas
phase to the liquid) was sufficient to
reduce the THMFP of clarified and filtered
Sabine River water to 1/e (37%) of its
original value (300 /ug/L) using a stirred
tank reactor with 0.27 watts of UV/L
applied in the reactor. If UV application
was delayed until after initial ozone
contacting, or if the stirring or UV was
omitted, required dosages (still based on
that amount of ozone transferred out of
the gas) were 34, 41, and 48 mg/L,
respectively. The experimental data
supported the concept of minimal in-
crease in treatment efficiency above a
certain UV intensity. The lowest rate of
ozone addition that effected the desired
THMFP removal was the most efficient in
terms of ozone used.
The projected treatment costs of the
process at a 1-mgd plant for 60%, 70%
and 80% THMFP removals from clarified
Sabine River water were $0.55, $0.70,
and $0.91 per thousand gallons, respec-
tively, at an assumed electrical cost of
$0.10 per kWh. Treatment costs still
strongly depended on plant capacity at
this size. The increase in process
efficiency by the addition of stirring and
UV greatly outweighed the increased cost
over ozonation alone in these cost
projections. The dominant term (65%) in
the projected treatment cost is the
amortized capital cost of the ozone gen-
eration and feed equipment.
Recommendations
Further laboratory-scale investigations
should be made of several points that
were either beyond the scope of this
study or emerged too late in the study to
be systematically investigated. Ozonation
of natural organics in water should be
studied to determine how or whether it is
possible to suppress the formation of new
THM precursor material during the
destruction of that material initially pres-
ent. If that could be accomplished, the
possibility would exist for considerably
lowering the ozone dose requirement for
satisfactory THMFP removal. The model
developed for mass transfer with simul-
taneous chemical and photochemical
reaction should be verified in the labora-
tory under conditions where careful
oxidant measurements can be made. The
use of a simple model substrate rather than
natural organic material would increase
the probability of success, as would the
use of a photolyte that is less susceptible
to side reactions than is ozone. Finally, a
survey of ozone dose requirements for
water samples from different geographi-
cal locations would provide information
concerning the transferability of projec-
ted treatment costs from one water
source to another.
Two further studies are necessary
before photolytic ozonation can be
recommended as a potable water
treatment process. Thorough toxicity and
mutagenicity testing should be done on
concentrates of treated and untreated
water from several different water
sources to help ensure that public health
problems do not arise later. The process
itself should be tested at the field scale to
verify ozone and UVdose requirements in
larger systems and to address the
problems associated with scale-up of the
complicated, photochemically initiated
reaction.
The full report was submitted in
fulfillment of Cooperative Agreement
CR808825 by the University of Texas at
Dallas under the sponsorship of the U.S.
Environmental Protection Agency.
*USGPO: 1984-759-102-10686
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William H. Glaze, GaryR. Peyton, andBrigitte Sohm are with University of Texas at
Dallas, Richardson, TX 75083; and Dean Meldrum is with SumX Corporation,
Austin. TX 78761
J. Keith Carswell is the EPA Project Officer (see below).
The complete report, entitled "Pilot Scale Evaluation ofPhotolytic Ozonation for
Tnhalomethane Precursor Removal." (Order No. PB 84-234 517; Cost: $20.50.
subject to change) will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield. VA 22161
Telephone. 703-487-4650
The EPA Project Officer can be contacted at:
Municipal Environmental Research Laboratory
U S Environmental Protection Agency
Cincinnati, OH 45268
United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
Official Business
Penalty for Private Use $300
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