Environ. Sci. Technol. 1989, 23,550-555
(18) Behymer, T. D.; Hites, R. A. Environ. Sei. Technol. 1985, 19, 1004-1006. (19) Albano, C.; Blomquist, G.; Coomans, D.; Dunn, W. J.; Ed-
(20) (21) (22) (23)
(24) Smith, I. M. PAH from coal utilisation-emissions and effects; IEA Coal Research: London, 1984. (25) Oberg, T.; Bergstrom, J. G. T. Chemosphere 1987, 16, 1221-1230. (26) Schwartz, T. R.; Stalling, D. L.; Rise, C. L. Environ. Sei. Technol. 1987, 21, 72-16. (27) Bossert, I. D.; Bartha, R. Bull. Environ. Contam. Toxicol. 1986,37,490-495. (28) Rheadman, T. W.; Mantoura, R. F. C.; Rhead, M. M. Sci. Total Environ. 1987, 66, 73-94.
lund, U.; Eliasson, B.; Hellberg, S.; Johansson, E.; Norden, B.; Sjoestroem, M.; Soederstroem,B.; Wold, H.; Wold, S. Proceedings of the Symposium on Applied Statistics; NEUCC, RECKU, and RECAU, DTH: Lyngby, Denmark, 1981; pp 183-218. STATGRAPHICS; STSC Inc.: Rockville, MD, 1986. Nicholls, P. H. Pestic. Sei. 1988, 22, 123-137. Vogt, N. B. Principal component variable discriminant plots-a novel approach to interpretation of multiclass data. J. Chemometrics, in press. Masclet, P.; Mouvier, G.; Nikolaou, K. Atmos. Environ.
Received for review December 21, 1987. Revised manuscript received July 19, 1988. Accepted December 16, 1988. We are grateful to the Welsh Office for financial support during this
1986,20, 439-446.
work.
A Means of Determining Depth and Volume Requirements for Indirect Photolysis Treatment Systems Richard J. Watts”
Department of Civil Engineering, University of Nevada, Reno, Nevada 89557 V. Dean Adams
Center for the Management, Utilization and Protection of Water Resources, Tennessee Technological university, Cookeville, Tennessee 38505 E. Joe Middlebrooks
Provost and Vice President for Academic Affairs, University of Tulsa, Tulsa, Oklahoma 74104 Indirect photolysis in reactors under solar radiation has been proposed for the pretreatment of organic industrial wastes and for wastewater effluent disinfection. An equation with terms for light intensity and sensitizer absorbance was used to calculate indirect photolysis rate constants as a function of wastewater depth as a basis for pilot system design. The equation was initialized by using batch, bench-scale tests in which the surface photolysis rate constant was determined. Calculation of an empirical efficiency factor and estimation of light intensity allowed the determination of a rate constant profile for the water column. The accuracy of the calculations was verified by using laboratory reactors. The first step in the design of a photolysis reactor was the calculation of mean photolysis rate constants for successive depths. For each depth, the mean rate constant was used in the plug flow design equation to calculate reactor volume and area. Reactor area declined exponentially as a function of depth. The beginning of the asymptotic region of the exponential curve was the optimum reactor depth and area.
Introduction Photolysis of organic pollutants in water and wastewater can occur by direct or indirect routes. Direct photolysis rates under solar radiation may be slow, with parent compound half-lives in the range of days to months (1, 2). Indirect photolysis, which is mediated by sensitizers, can proceed a t rates with parent compound half-lives in minutes ( 3 ) . As a result of the rapid reaction rates that can be achieved, indirect photolysis has been proposed for the pretreatment of toxic and refractory organic wastes and for wastewater effluent disinfection. The primary advantage of using indirect photolysis for waste treatment is that energy requirements are likely to be minimal if the process is conducted in reactors under solar radiation. This is not true for ultraviolet irradiation and ozonolysis. 550
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Indirect photolysis is promoted by photosensitizers, many of which absorb light in the visible region. The sensitizer transfers energy to organic substrates which results in their degradation. Indirect photolysis mechanisms include singlet oxygen generation (3,4),free radicals (5),organic hydroperoxides (6),and hydrogen peroxide and hydroxyl radical (7,8). The process has been effective in degrading pesticides (9),polycyclic aromatic hydrocarbons (IO),phenols (11, 12),and chloroanilines (13, 24). Krinsky (15) reviewed the effects of indirect photolysis on biological systems and concluded that the process is effective in degrading biological molecules such as vitamins, nucleic acids, and amino acids. The ability to degrade biological molecules makes indirect photolysis an attractive unit process for wastewater effluent disinfection. Sargent and Sanks (11, 12) performed a bench-scale treatability study for the indirect photolysis of phenol and o-cresol. They determined optima for pH, sensitizer concentration, and waste concentration. Gerba et al. (16,17) similarly investigated optimum indirect photolysis conditions for secondary wastewater effluent disinfection and virus removal. Although indirect photolysis has been effective in decomposing organic compounds and disinfecting effluents in the laboratory, no design criteria have been developed for pilot- or full-scale photolysis systems. Indirect photolysis rate as a function of depth per unit surface area has been described for completely mixed systems under sunlight (18, 19). The average wavelength-dependent indirect photolysis rate constant is
lzA = 2.310aveAC[Sens]
(1)
where IOaV is the average scalar irradiance in the water column, eA is the sensitizer molar absorptivity, [Sens] is the molar sensitizer concentration, and C is an efficiency factor which is related to the indirect photolysis quantum yield. The use of eq 1 is limited by known values for C,
00i3-936X/89/0923-0550$0i.50/0
0 1989 American Chemical Society
which are sensitizer- and substrate-specific. Equation 1 also applies only to completely mixed systems; no design criteria have been developed for plug flow systems. Therefore, the purpose of this research was to develop a procedure by which a form of eq 1can be used to determine the optimum depth and volume of plug flow indirect photolysis treatment systems based on bench-scale treatability studies.
Design Procedure The depth-specific form of eq 1 is k,,, = 2.3I,,,~,C[Sens]
In C/Co = -kmeaV/Q (2)
where Iz,,is the scalar irradiance a t depth z. The functional rate constant may be obtained by integrating eq 2 over the wavelengths the sensitizer absorbs light:
k , = 2.3ihPI,,,C[Sensla 1 dX
(3)
The use of eq 3 is based on the batch, bench-scale determination of k for the surface reaction in an optically thin solution. Bench-scale tests are performed under the same conditions and minimum light intensity for which the reactor will be designed. The rate constant is deterA n e d by measuring chemical oxygen demand removal rate or, for more homogeneous wastes, by the disappearance of a toxic or refractory compound. The rate of disinfection may be determined by coliform bacteria measurements as a function of time. Light quantity and quality are measured a t the time of the bench-scale study. The sensitizer extinction coefficient a t wavelength A, E,, is determined from E,, data, which are available in the literature, and the absorption spectrum of the sensitizer, which may be measured experimentally. Equation 3 is then numerically integrated for the surface conditions, solving for C. The efficiency factor, C, is then used to calculate k at depth z. Turro (20) reported that quantum yield does not vary with light intensity; therefore, C calculated for the surface reaction may be used to determine k a t lower light intensities. The first step in determining k a t depth z is the calculation of Is, using Beer's law:
I , , = Io,,lO-*"
where It,, is the mean rate constant, ki is the rate constant for the ith control volume, W is the control volume width, and z is the depth selected for the reactor. Determination of optimum depth for the photolysis reactor involves finding k,, for successive depths. For each k,,, a t depth z , the volume of the reactor may be obtained from the plug flow design equation
(4)
where CY, is the decadic absorption coefficient for the wastewater treatment system, I,,, is the light intensity a t depth z, and Io,,is the light intensity at z = 0. If the waste contains light-scattering compounds, a diffuse attenuation coefficient, K,, should be determined for the waste and used in eq 4 in place of CY, (21,22). The final calculation uses C for the surface reaction and IZ,,to numerically integrate eq 3, solving for k,. The sensitizer concentration in a photochemical treatment system would likely be kept constant for optimum system performance. Therefore, the [Sene] term of eq 3 remains constant through the two integration steps. The functional form of eq 3 then becomes
where
k,' = k2/2.303[Sens] With data generated by eq 5, indirect photolysis rate profiles may be used to find reactor volume a t any depth z by first calculating the mean rate constant and summing the rate constants from horizontal control volumes weighted by width:
(7)
where Co is the influent waste concentration, C is the desired effluent waste concentration, Q is wastewater flow rate, and V is reactor volume. The resulting reactor area is A = V/z where z is the selected depth for which k,,, lated.
(8)
was calcu-
Experimental Section Materials. Bromacil (5-bromo-3-sec-butyl-6-methyluracil) and terbacil(5-chloro-3-tert-butyl-6-methyluracil) were obtained from E. I. du Pont de Nemours, Inc., and recrystallized from 2-propanol. Methylene blue and riboflavin (Sigma Chemical Co.) were used without further purification. Ethyl acetate was reagent grade. NaH2P04 and Na2C03were purchased from Fisher Scientific. All solutions were prepared with water obtained from a Milli-Q deionizing system. General Reaction Conditions. Indirect photolysis rate was measured in batch reactors under constant conditions. Two discrete substrate-sensitizer systems were studied: bromacil-methylene blue and terbacil-riboflavin. The solutions were buffered with 500 mg/L NaH2P04and 500 mg/L Na2C03 and were aerated with compressed air passed through an aqueous filter system to remove contamination. Analysis. Disappearance of substrate was monitored by gas chromatography after a 1:l ethyl acetate extraction. A Hewlett-Packard 5880A gas chromatograph with flame ionization detector and 61 cm X 2 mm (i.d.) glass column packed with 1% SP-2250 on sO/l00 Supelcoport was used. Chromatographic conditions were as follows: injector port temperature, 250 "C; initial oven temperature, 170 OC; final oven temperature, 230 "C; program rate, 20 OC/min; detector temperature, 300 OC;nitrogen gas carrier flow rate, 40 mL/min. Indirect Photolysis Rate as a Function of Depth. The accuracy of eq 5 was evaluated under laboratory lights where eight reaction chambers were used to simulate the depths of a photolysis reactor. The chambers consisted of a 150 mm X 15 mm Pyrex glass Petri dish with a 15cm-high cylinder of tripled 6-mil black plastic taped to the lower section of the dish. The plastic supported the Petri dish and allowed it to sit in a level position. Under the Petri dish-plastic sleeve assembly sat the reaction vessel, a 15 cm diameter X 5.5 cm deep amber glass dish with black electrical tape covering the outside. The plastic sleeve attached to the Petri dish contained two ports: an air diffuser port and a sampling port (Figure 1). Depth was simulated by filling the Petri dishes 1.0 cm deep with a concentrated solution of the sensitizer. The sensitizer concentrations in the eight Petri dishes were chosen to simulate succeedingly deeper cross sections of a reactor. For example, a depth of 60 cm in a reactor containing 1 mg/L sensitizer was simulated by using a 60 mg/L senEnviron. Sci. Technol., Vol. 23, No. 5, 1989
551
Petri dish light filter
I Sampling port
\
i I
Lr W
n n
Reaction vessel Reaction vessel
0
/w
r
(I)
A
0.1 o 0.50 1.0
2.0
m g / ~ ethylene mg/L Methylene m g / ~Methylene m g / ~M e t h y l e n e
Blue Blue Blue Blue
\Glass air diffuser
Flgure 1. Reaction chamber used to measure indirect photolysis rate at simulated depths.
Table I. Efficiency Factors for Four Methylene Blue and Riboflavin Concentrations
[methylene blue], mg/L
C
0.1
0.00139
1.0
0.00610
0.5
0.00179 0.00311 0.00517
2.5
5.0
0.0142 0.0194
10.0
0.0301
1.0 2.0
[riboflavin],
mg/L
C
sitizer solution 1.0 cm deep in the Petri dish. Bromacil and terbacil concentrations were 30 mg/L. Laboratory verification experiments using eight depths were performed with 0.1,0.5,1.0, and 2.0 mg/L methylene blue and 1.0, 2.5, 5.0, and 10 mg/L riboflavin. Bromacil-methylene blue solutions were adjusted to pH 10.5 with 3 N NaOH terbacil-riboflavin solutions were adjusted to pH 3 with 20% HC1. These pH regimes were used to obtain high photolysis rates under the relatively low intensity of the laboratory lamps (23).Bromacil and terbacil were not added to the Petri dish filters since there is no competitive light absorption between the substrates and the sensitizers. Light was provided by a bank of 10 Duro-Test Vitalite lamps, which are 95% corrected to the quality of sunlight. The total radiation falling on the reaction chambers was 107 W/m2 measured by a Lambda LI-185 radiometer. Aeration was provided by glass air diffusers (Figure 1) a t a flow rate of 0.1 L/min, which maintained dissolved oxygen a t saturation. Experiments were begun by adding the sensitizer to the reaction vessel containing the buffered substrate solution. Reactions proceeded for 2 h with substrate concentrations measured a t 30-min intervals.
Results and Discussion Indirect photolysis rate followed first-order kinetics. Figure 2 shows first-order rate constants for bromacil and terbacil photolysis as a function of simulated depth for the four concentrations of methylene blue and riboflavin. For a reactor with high sensitizer concentration (e.g., 2.0 mg/L methylene blue), a rapid reaction rate was maintained a t the surface with a sharp rate decline with increasing depth. With a low sensitizer concentration (e.g., 0.1 mg/L methylene blue), the surface photolysis rate was low but the rate did not significantly decrease with depth. The efficiency factors, C, which were calculated by using eq 5 for the four concentrations of methylene blue and riboflavin,are shown in Table I. Although these empirical factors were smaller 552
k(min-1)
Envlron. Sci. Technol., Vol. 23, No. 5, 1989
1.0
mg/L
Riboflavin
2.5
mg/L
Riboflavin
A
5.0
mg/L
Riboflavin
o
10.0
mg/L
Riboflavin
m
Figure 2. Indirect photolysis rate constants at simulated depths sensitized by (upper) methylene blue and (lower) riboflavin.
when lower sensitizer concentrations were used, a low sensitizer concentration, on the order of 0.1 mg/L for methylene blue, may be optimum for photolysis system design. Acher and Saltzman (3)and Sargent and Sanks ( I I , I 2 ) found 2-10 mg/L methylene blue to be optimum for the photolysis of bromacil(3) and o-creso1(11,12). The difference in the results of the previous studies and this research may be explained by the differences in light path length. The previous studies used small reaction chambers with short path lengths (
