Environ. Sci. Technol. 1999, 33, 3210-3216
Comparison of the Effectiveness of Photon-Based Oxidation Processes in a Pilot Falling Film Photoreactor GIANLUCA LI PUMA AND PO LOCK YUE* Department of Chemical Engineering, The Hong Kong University of Science & Technology, Clear Water Bay, Kowloon, Hong Kong
Progress on the exploitation of photon-based oxidation processes for the treatment and purification of water and wastewater could be achieved by combining the most effective oxidation processes with optimal reactor designs. In this work, the effectiveness of six different photonbased processes on the oxidation of a model organic pollutantssalicylic acidswas studied in a pilot plant falling film photoreactor designed for optimal light absorption. The photon-based processes studied were UVAphotocatalysis, UVA-photocatalysis-peroxidation, UVCphotolysis, UVC-photolysis-peroxidation, UVC-photocatalysis-photolysis, and UVC-photocatalysis-photolysisperoxidation. The effects of major process variables on each of these processes were examined. The highest oxidation rates were obtained with concomitant photocatalysis, photolysis, and UV-peroxidation. The oxidation rates of salicylic acid obtained by this combined process were 1 order of magnitude higher than with UVA-photocatalysis alone, 2-fold higher than UVA-photocatalysis-peroxidation, and 3-fold higher than UVC-photolysis-peroxidation. In addition, a high degree of mineralization of salicylic acid was obtained. An economic analysis shows the cost of treatment by this combined process to be much lower than that of other processes, making this process a suitable photon-based oxidation technology for further development and commercialization.
Introduction Photon-based oxidation processes have been found to be effective for the purification of potable water supplies and the treatment of wastewater (1). During the past two decades, these processes have received considerable attention, but their commercial potential has yet to be fully realized. Selected examples that are illustrative of the various categories of photon-based oxidation processes are shown in Table 1. A notable advantage of photon-based oxidation processes is that they could achieve a higher degree of pollutant mineralization as compared with other oxidation processes such as peroxidation, chemical oxidation, wet oxidation, and others. These processes may be classified into three categories: photolytic, photocatalytic, and combined photocatalyticphotolytic (12). With the exception of homogeneous photolysis, photon-based oxidation processes exploit the high oxidation potential of hydroxyl and peroxyl radicals (13). In * Corresponding author phone: (852)258-7122; fax: (852)2335 9030; e-mail:
[email protected]. 3210
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photolytic oxidation, hydroxyl and peroxyl radicals are generated by high-energy ultraviolet photons acting on strong oxidizing species such as hydrogen peroxide or ozone. The photolytic oxidation of organic substrates in water often results in a rapid disappearance of the primary organic substrate followed by a slow conversion of intermediate products to carbon dioxide. In some cases, intermediates are formed that are resistant to further degradation by photolytic processes. A high degree of mineralization of the organic substrate is often difficult to achieve. Photolytic oxidation by UV-peroxide and UV-ozone are processes that have already been commercialized (14), but the operating costs associated with the production and/or consumption of the oxidant can be high (15, 16). In photocatalytic processes, hydroxyl and peroxyl radicals are formed by the interaction of low-energy ultraviolet photons with a suitable semiconductor in the presence of an electron acceptor, (17). The most notable photocatalytic process is TiO2 heterogeneous photocatalysis, which has been shown to be able to mineralize a wide range of organics (18). Although the degree of mineralization obtained with photocatalysis is usually much higher than that with photolytic processes, the application of heterogeneous photocatalysis for large-scale wastewater treatment has not advanced rapidly. This is primarily due to the low rates of oxidation and the lack of pilot plant studies to validate models for the scale up, design, and optimization of photocatalytic reactors. The cost of the oxidant in photocatalytic processes is generally very low (16); however, additional costs may be incurred if the catalyst has to be recovered in a downstream process. Combined photocatalytic-photolytic oxidation processes have received much less attention. Li Puma and Yue (11) have observed that the efficiency of a falling film photocatalytic system can be enhanced by combining it with other photolytic processes. In their study, performed in a pilot plant-scale continuous flow falling film (FF) photoreactor, both the UVC-photocatalysis-photolysis and the UVCphotocatalysis-photolysis-peroxidation processes showed considerably higher oxidation rates than UVA-photocatalysis alone. A further enhancement on the observed oxidation rates with photocatalytic and combined photocatalytic-photolytic processes can be obtained with an optimum reactor design. The fundamentals for photoreactor modeling and design have been comprehensively reviewed by a number of workers in the field (19-21). The major limitations in current designs of photocatalytic reactors have recently been discussed by Li Puma and Yue (22) and Ollis and Turchi (23). The authors showed that inefficiencies in photocatalytic reactors may be caused by (a) light scattering, (b) noncorrespondence between the radiation field and fluid residence time, (c) filming on the transparent wall of photoreactors, and (d) mass transfer limitation of reactants. Li Puma and Yue (22) analyzed a number of common reactor configurations with respect to these factors and showed that the laminar falling film slurry (LFFS) photocatalytic reactor should provide the most efficient reactor configuration for wastewater treatment by heterogeneous photocatalysis. They concluded that for LFFS photocatalytic reactors irradiated by artificial sources of UV radiation, the inner wall configuration (LFFSIW), in which the liquid film descends along the internal wall of a column with the lamp sited in the middle of the column, provides the optimal design. The most notable advantages of this configuration are as follows: (a) The cylindrical symmetry of the system enables a reasonably uniform illumination of all parts of the photo10.1021/es9811795 CCC: $18.00
1999 American Chemical Society Published on Web 08/07/1999
TABLE 1. Selected Applications of Photon-Based Oxidation Processes for Water Treatment photon-based oxidation process
process category
contaminant oxidized
UVC-photolysis UVC-peroxidation UVC-ozonation UVC-ozonation-peroxidation UVA-photo-Fenton UVA-photocatalysis UVA-photocatalysis-peroxidation UVA-photocatalysis-ozonation UVC-photocatalysis-photolysis UVC-photocatalysis-photolysis-peroxidation
photolytic photolytic photolytic photolytic photoassisted photocatalytic photocatalytic photolytic-photocatalytic photolytic-photocatalytic photolytic-photocatalytic
pentachlorophenol (2) 1,4-dioxane (3) atrazine (4) aromatics (5) perchloroalkanes (6) chlorinated hydrocarbons (7) parathion (8) dichlorophenoxy acetic acid (9) chlorophenols (10) salicylic acid (11)
reactor to be obtained without losses in efficiency due to unequal illumination of some parts of the reactor. (b) Improved photon utilization, as backscattered photons have a low probability of escaping from the reactor and can be recaptured by the liquid film. (c) There is no need for a light reflector, the use of which inevitably introduces a loss in photon utilization and adds to the overall cost of the unit. A mathematical model for a LFFSIW falling film photocatalytic reactor that may be used for preliminary reactor design by means of dimensional analysis has recently been reported (24, 25). In this study, we compare the effectiveness of photolytic, photocatalytic, and combined photocatalytic-photolytic processes in a pilot plant-scale falling film photoreactor specifically designed for optimal light absorption. A simplified economic analysis of each of these processes is also presented. The photon-based oxidation processes examined are UVAphotocatalysis, UVA-photocatalysis-peroxidation, UVCphotolysis, UVC-photolysis-peroxidation, UVC-photocatalysis-photolysis, and UVC-photocatalysis-photolysisperoxidation. It is hoped that, with the combination of the most effective oxidation process and optimal reactor design, the development of new photon-based oxidation processes may make further progress for use in the industrial environment.
Experimental Equipment and Procedures Experiments were conducted in a pilot plant falling film photoreactor (Figure 1). The reactor was specifically designed for optimal light absorption according to the methodology described in refs 22 and 26. The design yielded a reactor column, 1600 mm in length and 108 mm in diameter, that was vertically mounted in a support structure. The lamp was placed exactly in the middle of the reactor column. The lamps used in these experiments were supplied by Philips Lighting (The Netherlands) and were of two types: UVA blacklight model TLD36W/08 and UVC germicidal model TUV36W Longlife (Table 2). The spectral power distribution of the blacklight lamp has been reported previously (25). The UVA blacklight was shielded by a Pyrex UV-transparent sheath with a radiation cutoff at the wavelength of 310 nm. Both lamps have identical dimensions and input power. Their capital and operating costs are low and require neither a lamp cooling system nor an expensive power supply system. The UVC germicidal lamp selected is specially designed by Philips for a very high UV efficiency and an extremely long lifetime. The overall efficiency of a photocatalytic system using low-wattage UV lamps is expected to be higher than one using high-wattage UV lamps because quantum yields are reduced at high irradiation intensities (27). UV energy fluxes were measured using a Cole & Parmer VLX-3W radiometer that could be fitted with a 254 nm or a 365 nm UV sensor. The intensity of the incident radiation at the lamp wall was varied by connecting the lamp to ballast systems with different power ratings. Appropriate protection
FIGURE 1. Overview of pilot plant falling film photoreactor rig shown in continuous mode with a partial recycle. of skin and eyes from UV radiation was required. In the unlikely event of explosion of the lamps, the stainless steel reactor column would provide a safe shield. The results presented in this study pertain to operating the pilot plant in a continuous mode with a partial recycle, i.e., a fraction of the product stream is recirculated back to the liquid distributor system (Figure 1). Solutions of substrate, catalyst, and oxidizing agent in deionized water were prepared in a 480-L feed tank. The solutions from the feed tank were pumped to the liquid distribution system at the top of the reactor and collected in a receiver tank at the bottom outlet of the reactor column. The solutions could be recycled from the receiver tank back to the liquid distributor system and/or discharged from the system through an overflow. The falling film reactor was operated at a Reynolds number of 1400-1450 to provide matching between fluid residence time and radiation field while maintaining a high processing rate and a minimum optimum catalyst loading. The estimated film thickness was in the range of 0.474-0.479 mm (25). The feed and the receiver tanks were fitted with mixer units, oxygen spargers, and temperature-controlling systems. The operating temperature in each of the experiments was kept in the range of 22-24 °C. All wetted parts of the pilot plant were made of 316 stainless steel, Teflon, Viton, or borosilicate glass to eliminate contamination. VOL. 33, NO. 18, 1999 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 2. Lamp Specifications and Costs As Supplied by Philips Lighting (The Netherlands) lamp
length × diameter (mm) nominal power (W) emission spectrum UV power (W) UV efficiency (%) lifetime (h) average depreciation (%) unit cost (US$)c a
Per 1000 h.
b
Philips TLD36W/08 blacklight
Philips TUV36W Longlife germicidal
1200 × 28 36 polychromatic at 300-420 nm; peak at 365 nm 7.5 21 2000 5a 15
1200 × 28 36 monochromatic at 253.7 nm 14 39 8000 15b 30
After 5000 h. c Source: Philips Hong Kong Limited (July 1996).
TABLE 3. Range of Experimental Variables for Each Photooxidation Processa
photooxidation process UVC-photolysis UVC-photolysis-peroxidation UVA-photocatalysis UVC-photocatalysis-photolysis UVA-photocatalysis-peroxidation UVC-photocatalysis-photolysis-peroxidation aFeed
UV wavelength (nm) 253.7 253.7 310-380 peak ) 365 253.7 310-380 peak ) 365 253.7
Results and Discussion The complete mineralization of salicylic acid by oxygen or hydrogen peroxide oxidants can be represented by the following stoichiometric relations:
C7H6O3 + 7O2 w 7CO2 + 3H2O C7H6O3 + 14H2O2 w 7CO2 + 17H2O The results of the steady-state oxidation experiments conducted in the pilot plant are presented in terms of conversions of salicylic acid, χSA; of salicylic acid to carbon dioxide, χSAwCO2; 9
TiO2 loading (g L-1)
H2O2 loading (ppm)
salicylic acid (feed) (ppm)
9.6 6.8-17.2 5.8-12.6b
0 0 0.05-5.00
0 50-5680 0
60 60 10-100
6.8-17.2 7.7b
2.75 2.75
0 1000
10-100 60
6.8-17.2
0.05 - 4.00
50-5680
10-100
flow rate ) 24 L h-1; recycle ratio ) 0.9459; oxygen flow rate ) 1 L min-1.
Salicylic acid (Riedel-de Hae¨n, g99.8%) was selected as a model organic substrate for all experiments because it is a substrate suitable for the safe operation of the pilot plant and because of the ease of its analysis. Salicylic acid has a relatively complex molecular structure that it similar to many toxic aromatic compounds, and experiments have shown that the rates of photocatalytic oxidation of salicylic acid in TiO2 aqueous suspensions are comparable with those of chlorophenols and other aromatic substrates (28). The photocatalyst was untreated titanium dioxide Degussa P25. Hydrogen peroxide (30% solution) was supplied by Merck. The samples collected during the steady-state experiments were promptly analyzed. The degradation of salicylic acid and the formation of intermediate products were followed by HPLC analysis on a Hewlett-Packard 1090 series II liquid chromatograph. The degree of mineralization of salicylic acid to carbon dioxide was followed by total organic carbon (TOC) analysis on a Shimadzu TOC-5000 total organic carbon analyzer. Both analytical instruments were equipped with autosampling units. Details of the analytical procedures including calibration, accuracy, and repeatability have been described elsewhere (11, 26). The concentration of hydrogen peroxide was measured by iodometry (29).
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intensity at lamp wall (mW cm-2)
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b
Spectral averaged value, see ref 25.
and of salicylic acid to intermediates, χSAwI:
χSA )
Cfeed - Cout Cfeed
χSAwCO2 )
TOCfeed - TOCout TOCfeed χSAwI ) χSA - χSAwCO2
The removal rates of salicylic acid and of total organic carbon are obtained from the products of the feed flow rate and the removal of salicylic acid (Cfeed - Cout) and TOC (TOCfeed TOCout), respectively, under steady-state conditions. The efficiency of mineralization, which is the fraction of degraded salicylic acid converted to carbon dioxide, was calculated from the ratio (χSAwCO2/χSA). The apparent quantum yield, Φ, was calculated from the ratio of the removal rate of salicylic acid or total organic carbon divided by the theoretical maximum rate of photon absorption (assuming that all photons emitted by the lamp are absorbed by the solution and that photon losses from the column ends are negligible). Table 3 shows the range in which each of the significant parameters was varied for each of the photooxidation processes. A summary of the most significant results obtained is reported in the following subsections. The conversion of H2O2 with the various processes employing H2O2 was found to be 95% or higher. UVC-Photolysis. Homogeneous photolysis of salicylic acid resulted in a rapid degradation of the primary organic substrate without substantial conversion to carbon dioxide. Typical results are shown in Table 4. The efficiency of mineralization achieved by this process was only 14.2%. UVC-Photolysis-Peroxidation. Figure 2 shows the results of the experiments conducted with increasing concentrations of hydrogen peroxide in the feed stream of the UVC-photolysis-peroxidation process. The results showed a significant enhancement of conversions in comparison with UVC-photolysis. The observed conversion of salicylic acid reached a plateau at concentrations of hydrogen peroxide
TABLE 4. Conversions and Mineralization Efficiency of Salicylic Acid under Optimal Experimental Conditions of Each Photooxidation Processa
photooxidation process UVC-photolysis UVC-photolysis-peroxidation UVA-photocatalysis UVC-photocatalysis-photolysis UVA-photocatalysis-peroxidation UVC-photocatalysis-photolysis-peroxidation
UV wavelength (nm) 253.7 253.7 310-380 peak ) 365 253.7 310-380 peak ) 365 253.7
intensity at lamp wall (mW cm-2)
TiO2 loading (g L-1)
H2O2 loading (ppm)
χSA (%)
χSAwCO2 (%)
χSAwI (%)
χSAwCO2/χSA (%)
9.6 9.6 7.7b
0 0 2.75
0 1000 0
28 57 2.6
4 10 1.7
24 47 0.9
14.2 17.5 65.4
9.6 7.7b
2.75 2.75
0 1000c
6.9 24
4.1 15
2.8 9
59.4 62.5
9.6
2.75
1000
48
28
20
58.3
Feed flow rate ) 24 L h-1; salicylic acid (feed) ) 60 ppm; recycle ratio ) 0.9459; oxygen flow rate ) 1 L min-1; power supply to lamp ) 36 W. b Spectral averaged value, see ref 25. c The concentration of hydrogen peroxide was selected without experimental verification that this was the optimal value. a
FIGURE 2. Effect of hydrogen peroxide on the photooxidation of salicylic acid with the UVC-photolysis-peroxidation process. Feed flow rate ) 24 L h-1; salicylic acid (feed) ) 60 ppm; recycle ratio ) 0.9459; oxygen flow rate ) 1 L min-1; power supply to lamp ) 36 W; intensity at lamp wall ) 9.6 mW cm-2. higher than 1000 ppm. This was a result of hydroxyl radicals being consumed when peroxide was present in large excess (30). Under these conditions, the absorption of UV radiation does not have a strong effect on the process. The efficiency of mineralization was found to be of the same order of magnitude as with UVC-photolysis. UVA-Photocatalysis. The optimum concentration of the photocatalyst was varied. The optimal concentration of the photocatalyst in a falling film photoreactor is a function of the absorption and scattering coefficients of the photocatalyst, averaged over the range of wavelengths of irradiation. It is also dependent on the hydrodynamics of the liquid film and the radiation field. The optimal concentration of TiO2 in the UVA-photocatalysis process was found to be 2.75 g L-1 for a film Reynolds number in the range of 1400-1450 (25). The experiments performed using this concentration of titanium dioxide showed that the dependence of the oxidation rate of salicylic acid on the intensity of the incident radiation was half-order and that it was negative in power with respect to the concentration of salicylic acid (25). Consequently, experiments performed at higher concentrations of salicylic acid showed slower oxidation rates. The conversions observed with the UVA-photocatalysis process were the lowest of the six photon-based oxidation processes examined in this study. However, the efficiency of mineralization was high, reaching up to 50-65%. UVC-Photocatalysis-Photolysis. Conversions observed with the UVC-photocatalysis-photolysis process were ap-
proximately 2-4 times higher than the conversions observed with the UVA-photocatalysis process, but the efficiency of mineralization was of the same order of magnitude. The dependence of the reaction rates on the intensity of the incident radiation and the substrate concentration were similar to that observed with UVA-photocatalysis (11). UVA-Photocatalysis-Peroxidation. The results in Table 4 show that with the UVA-photocatalysis-peroxidation process, in the presence of 1000 ppm hydrogen peroxide, reactor conversions were enhanced by approximately 1 order of magnitude in comparison with conversions observed with the UVA-photocatalysis process. The efficiency of mineralization was of the same order of magnitude as that with UVA-photocatalysis. UVC-Photocatalysis-Photolysis-Peroxidation. The optimal concentration of titanium dioxide was found to be 2.38 g L-1 for a film with a Reynolds number in the range of 14001450. However, at a catalyst concentration 2.75 g L-1, conversions were substantially the same (11). The highest conversions were observed with this photooxidation process. The conversion of salicylic acid to carbon dioxide was 1 order of magnitude higher than the corresponding conversion observed with UVC-photocatalysis-photolysis and 2-fold higher than the conversion observed with UVC-photolysisperoxidation. However, the conversion of salicylic acid was reduced by 16% in comparison with the homogeneous UVCphotolysis-peroxidation process. Figure 3 shows the results of the experiments conducted with increasing concentrations of hydrogen peroxide in the feed stream of the UVCphotocatalysis-photolysis-peroxidation process. Despite a decrease in the extent of adsorption of salicylic acid in aqueous suspensions of TiO2 in the presence of hydrogen peroxide (11), there was a significant enhancement of conversions in comparison with the UVC-photocatalysisphotolysis process. The observed conversion of salicylic acid to carbon dioxide reached a maximum at a concentration of 1000 ppm hydrogen peroxide in the feed stream, with the conversion of salicylic acid attaining a constant plateau value as previously observed with the UVC-photolysis-peroxidation process. A linear relationship between average exposure time and removal of organic carbon (TOCfeed - TOCout) was observed in the range of 0-200 s, but the removal of salicylic acid (Cfeed - Cout) was not found to increase in a linear fashion. Process Comparison. An initial comparison of these photon-based oxidation processes was carried out on the data obtained under optimal experimental conditions. In this comparison, the loadings of hydrogen peroxide and/or the photocatalyst were optimal in relation to the hydrodynamics of the liquid film and the radiation profile. The power supplied to the UV lamp used in each of the processes was 36 W, as rated by the lamp manufacturer for optimal UV VOL. 33, NO. 18, 1999 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 5. Estimation of Apparent Quantum Yields of Processes from Results in Table 4
photooxidation process
photon emission rate of lamp (einstein s-1) × 105
removal rate of salicylic acid (mol s-1) × 107
removal rate of TOC (mol of C s-1) × 107
ΦSA (mol einstein-1) × 102
ΦSAwCO2 (mol of C einstein-1) × 102
UVC-photolysis UVC-photolysis-peroxidation UVA-photocatalysis UVC-photocatalysis-photolysis UVA-photocatalysis-peroxidation UVC-photocatalysis-photolysis-peroxidation
2.16 2.16 2.38a 2.16 2.38a 2.16
8.11 16.51 0.75 2.00 6.95 13.9
8.10 20.26 3.44 8.31 30.39 56.73
3.8 7.6 0.3 0.9 2.9 6.4
3.8 9.4 1.5 3.9 12.8 26.3
a
Spectral averaged value, see ref 25.
TABLE 6. Process Conditions Used for Economic Comparison of Processes process conditions reactor column plant operating time volumetric flow rate, QO recycle ratio, η feed concn, CO salicylic acid plant conversion χsalicylic acidwCO2 TOC removal rate of plant electricity cost hydrogen peroxide (100% purity) a
FIGURE 3. Effect of hydrogen peroxide on the photooxidation of salicylic acid with the UVC-photocatalysis-photolysis-peroxidation process. Feed flow rate ) 24 L h-1; salicylic acid (feed) ) 60 ppm; TiO2 loading ) 2.75 g L-1; recycle ratio ) 0.9459; oxygen flow rate ) 1 L min-1; power supply to lamp ) 36 W; intensity at lamp wall ) 9.6 mW cm-2. output and lamp life. Although the average intensity of the incident radiation at the lamp wall was 20% less with the UVA blacklight lamp than with the UVC germicidal lamp, the average photon emission rate was 10% higher with the former lamp (2.38 × 10-5 einstein s-1) than the latter (2.16 × 10-5 einstein s-1). The feed flow rate (24 L h-1), the recycle ratio (0.9459), and the concentration of salicylic acid in the feed (60 ppm) were constant. The basis for comparison of the various photooxidation processes are relative to the above experimental conditions. The results of these experiments are shown in Table 4. The highest conversion of salicylic acid, χSA, was 58%, which was obtained with the UVCphotolysis-peroxidation process. The lowest value was 2.6%, which was obtained with the UVA-photocatalysis process. Conversely, the highest conversion to carbon dioxide, χSAwCO2, was obtained with the UVC-photocatalysis-photolysisperoxidation process (28%), and the lowest value was with the UVA-photocatalysis process (1.7%). The mineralization efficiency of the photocatalytic and photolytic-photocatalytic processes was in the range of 58-65%, far above that of photolytic processes (which was in the range of 1418%). The higher mineralization efficiency of photocatalytic and photocatalytic-photolytic processes could be attributed to the powerful action of TiO2 in catalyzing the oxidation reaction. On the other hand, in photolytic processes, it is possible that free radical reactions could yield condensation products that are more resistant than the parent organic compound to further degradation. For example, photolysis of pentachlorophenol produces toxic compounds of high molecular weight such as octachlorodibenzo-p-dioxins (31); photolysis of 2,4-dichlorophenol yields tetrachlorodihy3214
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1600 mm × 108 mm 7000 h yr-1 24 L h-1 0.9459 60 ppm (36.5 ppm C) 80% 700.8 mg of C h-1 0.1 US$/kWh 1.1 US$/kga
Source: Chemical Week, January 31, 1996.
droxybiphenyls (32). It is possible that photolytic processes would be better exploited if performed in photoreactors with higher optical thickness than those used in a falling film photoreactor. The apparent quantum yield, as defined previously, is lower than the actual quantum yield because the actual rate of photon absorption is less than the rate of photon emission of the lamp. Table 5 shows the apparent quantum yields of salicylic acid degradation and mineralization obtained from the results in Table 4. The results of this comparison clearly suggest that TiO2 photocatalyst is necessary in order to achieve substantial conversion to carbon dioxide. Overall, the UVC-photocatalysis-photolysis-peroxidation process appears to be the most effective process in the present comparison as it shows the highest mineralization χSAwCO2, the second highest conversion χSA, and a high mineralization efficiency and apparent quantum yield. Conversely, UVA-photocatalysis was the least effective process in the oxidation of salicylic acid. Process Economics Analysis. The adoption of the optimal oxidation technology in the industrial environment would depend on favorable process economics. In the case of the six photon-based processes studied, the process economics are primarily dependent on the costs of electricity, lamp replacement, and added oxidant/catalyst. Table 6 shows the process conditions upon which the economic comparison of the processes was made. Table 2 shows the specifications and costs of the lamps. In the estimation of the operating cost, a negligible cost of catalyst recovery/regeneration and a linear dependence of the removal rate on the average exposure time was assumed. This linear dependence was experimentally verified in the case of the UVC-photocatalysis-photolysis-peroxidation process (11, 26). The capital cost of the plant was considered to be directly proportional to the number of reactor units. The cost difference between Pyrex and quartz lamp sleeves was not included in the economic analysis as the use of a lamp sleeve is primarily for support to the lamp in the present experiments but is not a necessary requirement in the design of a falling
TABLE 7. Process Conditions and Cost Estimates That Minimize the Total Operating Cost of Each Photooxidation Process photooxidation process UVC-photolysis UVC-photolysis-peroxidation UVA-photocatalysis UVC-photocatalysis-photolysis UVA-photocatalysis-peroxidation UVC-photocatalysis-photolysis-peroxidation a
annual costs in US$ no.of H2O2 loading H2O2 flow (ppm) rate (g h-1) lamps electricity H2O2 reactors totala 20 9 47 19.5 6 4
0 740 0 0 660 340
0 17.8 0 0 15.8 8.1
525 237 2470 512 315 106
504 227 1186 491 151 101
0 137 0 0 122 63
1029 (168) 6.1 601 (98) 3.6 3656 (596) 21.8 1003 (163) 6.0 588 (96) 3.5 270 (44) 1.6
Numbers in parentheses are US$/kg of TOC mineralized. Numbers in italics are US$/1000 L of treated effluent.
TABLE 8. Sensitivity Analysis on Relative Operating and Capital Costs of Processes
photooxidation process
increase reduce lamp process reduce conditions electricity reduce H2O2 increase H2O2 costs by 50% lifetime of 7.5 US$a or blacklight to from Tables cost by 50% cost by 50% cost by 50% 8000 h 15 US$b 2 and 6 0.05 US$/kWh 0.55 US$/kg 1.65 US$/kg
UVC-photolysis UVC-photolysis-peroxidation UVA-photocatalysis UVC-photocatalysis-photolysis UVA-photocatalysis-peroxidation UVC-photocatalysis-photolysis-peroxidationc
Relative Operating Costs 3.81 3.53 2.23 2.21 13.54 13.93 3.71 3.44 2.07 2.33 1 (270) 1 (220)
4.47 2.31 15.9 4.36 2.26 1 (230)
3.40 2.22 12.10 3.32 2.15 1 (302)
3.52 2.21 11.11 3.43 1.98 1 (218)
UVC-photolysis UVC-photolysis-peroxidation UVA-photocatalysis UVC-photocatalysis-photolysis UVA-photocatalysis-peroxidation UVC-photocatalysis-photolysis-peroxidationd
Relative Capital Costs 5 5 2.3 2.3 11.8 11.8 4.9 4.9 1.5 1.5 1 (4) 1 (4)
6.67 3 15.7 6.5 1.7 1 (3)
5 2.3 11.8 4.9 1.5 1 (4)
5 2.3 11.8 4.9 1.5 1 (4)
a Blacklight lamp. of reactors.
b
Germicidal lamp. c Numbers in parentheses are annual operating cost in US$.
film reactor. In the present experiments, Pyrex was also used as a mean for adjusting the transmission of UV radiation to the solution. The calculation of the optimal number of reactors that would be needed to meet the process specification in Tables 2 and 6 with the UVC-photocatalysis-photolysis-peroxidation process is shown in Figure 4. Figure 4a shows the TOC removal rates derived from the results in Figure 3 as a function of the inlet mass flow rate of hydrogen peroxide and the number of reactors that would be needed to meet the process specification in Tables 2 and 6. The number of reactors equals the ratio of the TOC removal rate of the plant (Table 6) divided by the value of the removal rate per reactor. Figure 4b shows the annual costs of lamp replacement, electricity, and hydrogen peroxide for a given inlet mass flow rate of hydrogen peroxide. The results of this analysis suggest that the total operating cost is minimized at an inlet mass flow rate of hydrogen peroxide of 8.1 g h-1, corresponding to a concentration of hydrogen peroxide in the feed stream of approximately 340 ppm and four reactor units connected in series. The total cost of electricity, lamp replacement and hydrogen peroxide for the optimal conditions is equal to 270 US$. Table 7 shows the process conditions and cost estimates that minimize the total operating cost of each photooxidation process. The optimal number of reactors was as low as 4 for the UVC-photocatalysis-photolysis-peroxidation process and as high as 47 for the UVA-photocatalysis process. The breakdown of the annual costs of lamp replacement, electricity, and hydrogen peroxide are shown in the three successive columns. The total operating cost is given in the last column in US$/year, US$/kg of TOC mineralized, and US$/1000 L of treated effluent. A sensitivity analysis on relative operating and capital costs of photooxidation
d
3.81 2.23 6.68 3.71 1.30 1 (270) 5 2.3 11.8 4.9 1.5 1 (4)
Numbers in parentheses are the number
processes is given in Table 8. These results clearly show that the UVC-photocatalysis-photolysis-peroxidation is the most economical process. The operating and capital costs of the UVC-photocatalysis-photolysis-peroxidation process are respectively 54% and 33% less than the corresponding costs of the second-best process: UVA-photocatalysisperoxidation. Commercially available blacklight UV lamps have a maximum lifetime of 2000 h. However, if the technology was such that the lifetime of a blacklight UV lamp could be increased to 8000 h as with the Longlife germicidal lamp manufactured by Philips Lighting, then the operating cost of the UVA-photocatalysis-peroxidation process would be considerably reduced, and it would be comparable with that of the UVC-photocatalysis-photolysis-peroxidation process, but the capital cost (six reactors units) would not change. Thus, the competitiveness of the UVA-photocatalysis-peroxidation process may depend on technological advances in lamp manufacturing. The remaining four oxidation processes require a high number of reactor units and therefore would suffer from high operating and capital costs. The UVA-photocatalysis process is the least economical of the processes and shows prohibitive operating and capitals costs. However, these costs can be reduced by 6-8-fold when hydrogen peroxide is used as a supplementary oxidizing agent, viz., the UVA-photocatalysis-peroxidation process. A reduction of 50% in the cost of hydrogen peroxide will give rise to increased competitiveness of the UVC-photocatalysis-photolysis-peroxidation process as both capital and operating costs would be further reduced. In this case, the operation point in Figure 3 shifts from a concentration of hydrogen peroxide of 320 ppm to 820 ppm, which allows for a higher efficiency of oxidation. VOL. 33, NO. 18, 1999 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 4. Number of reactors (panel a) and operating costs (panel b) to meet specifications in Tables 2 and 6 with the UVCphotocatalysis-photolysis-peroxidation process as a function of the inlet mass flow rate of hydrogen peroxide. Removal rates in panel a are calculated from results in Figure 3. The operating cost for the removal of an average carbon adsorber for the removal of benzene or phenol (60 ppm in concentration) by granular activated carbon adsorption is reported to be in range of 0.5-13 US$/1000 L (14). The results in Table 7 show that the UVC-photocatalysis-photolysisperoxidation process would be competitive with carbon adsorption and is worth considering for further development and commercialization. A comparison with the wet oxidation process, which for a composite petroleum waste has a treatment cost in the range of 8-24 US$/1000 L (33), is not appropriate because wet oxidation is more applicable to the treatment of wastewater with a very high COD concentration. Thus from a process economics point of view, an optimized photon-based process is competitive with other treatment technologies.
Literature Cited (1) Yue, P. L. Trans. Inst. Chem. Eng. 1992, 70 (Part B), 145. (2) Ho, T. L.; Bolton, J. R. Water Res. 1998, 32, 489.
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Received for review November 13, 1998. Revised manuscript received May 27, 1999. Accepted June 16, 1999. ES9811795
