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Evaluation of the efficiency of ultraviolet disinfection systems
Wat. Res. Vol. 23, No. 3, pp. 317-325, 1989 Printed in Great Britain.All rights reserved
0043-1354/89 $3.00+0.00 Copyright ~ 1989PergamonPrels pie
EVALUATION OF THE EFFICIENCY OF ULTRAVIOLET DISINFECTION SYSTEMS RoasgT G. QUALLS*, MARK H. DORF~L,~Nand J. DONALD JOHNSON~ Department of Environmental Sciences and Engineering, The University of North Carolina at Chapel Hill, Chapel Hill, NC 27514, U.S.A. (First received March 1988; accepted in revised form October 1988) Abstract--One problem in using ultraviolet light (u.v.) for full scale wastewater disinfection is the difficulty in measuring the dose of u.v. and in monitoring the immediate results of disinfection. A bioassay method for measuring the average u.v. intensity and the residencetime distribution (RTD) was tested on two small and two large multiple-lamp u.v. systems. Spore suspensions of u.v. resistant Bacillus subtilis were injected into the u.v. units as a spike and collected at a known time from injection in the effluent so that the exposure time was known. The survival could be related to a standard curve of survival vs u.v. dose. In one unit the bioassay measurements of intensity corresponded well with the predictions of a method of calculating intensity across a wide range of fluid u.v. absorbance values. The problems with interpreting the results of a continuous flow of the bioassay spores were demonstrated. The measurement of average intensity, RTD, volume of the chamber and the lamp u.v. output enabled us to assign measures of capacity and efficiency to the systems. The analysis illustrated a means of quantitatively isolating the factors involved in the overall efficiencyof the disinfection system and a means of comparing different systems. Key words--ultraviolet light (u.v.), disinfection, modeling, bioassay, flow dispersion, coliform, bacteria, intensity
INTRODUCTION Disinfection of water using ultraviolet (u.v.) light has been used for specialized small-scale applications for many years. Recently, many large scale u.v. wastewater disinfection systems have been built or planned in response to concern over meeting standards for discharge of chlorinated organics (White et al., 1986). While disinfection of wastewater had been regarded as expensive and unreliable, several pilot and fullscale test projects demonstrated its effectiveness (Whitby et al., 1984; Severin, 1980; Scheible and Bassel, 1981; Scheible, 1987; Quails et al., 1983; Johnson and Quails, 1985). Full scale u.v. disinfection of wastewater has been found to be economically competitive with chlorination (Scheible and Bassel, 1981). One problem that has been cited as a serious disadvantage in the use of ultraviolet disinfection is the difficulty of measuring the u.v. dose. Unlike chlorination and ozonation, there is no measurable chemical residual This makes the immediate control of the process difficult. In a practical u.v. disinfection system, the intensity patterns are complex and the average intensity cannot be directly measured with a detector. Many u.v. disinfection studies have been hampered by inadequate means of calculating intensity. Recently, however, a means of calculating *Corresponding author. Current address: Institute of Ecology, University of Georgia, Athens, GA 30602, U.S.A.
intensity in absorbing solutions has been adapted to u.v. disinfection systems and an indirect method of measuring average intensity with a bioassay has been developed (Quails and Johnson, 1983). This same difficulty in measuring u.v. dose has made it hard to evaluate new designs for u.v. systems and there has been no systematic means of comparing u.v. disinfection systems (Tobin et al., 1983). Engineers choosing u.v. disinfection systems must usually rely on estimates of average dose based on unsubstantiated estimates of intensity times theoretical residence time (volume/flow rate) (Nehm, 1983). The effects of residence time distribution have usually been ignored. Most current u.v. disinfection systems employ tubular germicidal lamps enclosed in a second quartz tube submerged in a chamber through which the fluid flows. Flow may either be parallel or perpendicular to the lamp axes. In one novel system, the fluid flows through Teflon tubes interspersed among the lamps (see Scheible, 1986; White et al., 1986). Our approach to evaluating the efficiency and capacity of u.v. disinfection systems is first to isolate the factors of intensity, residence time distribution and size of the chamber so that advantages or problems with each can be diagnosed and addressed separately. Then we outline ways of integrating these separate components to compare different systems using a simple model. More specifically, the objectives of this study were: (1) to develop and demonstrate methods of measuring intensity and residence time distribution (RTD) in multiple lamp u.v. units; (2) to substantiate a method of calculating intensity 317
ROBERT G.
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Fig. 1. Cross sections of Units l and 2 (not to same scale). Diagonal lines represent fluid filled areas.
patterns in one multiple lamp unit; and (3) to illustrate measurements o f capacity and efficiency used to c o m p a r e u.v. disinfection systems. METHODS The Aquafine CSL-6 unit (Unit l) contained 6 germicidal lamps (445cm total arc length) enclosed in submerged quartz tubes in a cylindrical chamber (Fig. 1). Helical baffles were included to mix flow across intensity gradients. The 3-1amp Enerco unit (Unit 2) was an experimental prototype containing approximately the same total arc length as Unit 1. However, the lamps were suspended in an air space in an aluminum cylinder and the liquid flowed through partially u.v. transparent Teflon pipes. The two larger scale units tested in this study were part of a pilot project located at the Port Richmond Water Pollution Control Plant in New York City. The project was operated by HydroQual Inc. and funded by the Environmental Protection Agency and the City of New York. These units are described by Scheible (1986, 1987). Unit 3 was a rectangular array of 100 submerged lamps, spaced 7.3 cm between lamp axes. Flow was perpendicular to the lamp axes. Unit 4 was a large scale modification of Unit 2, with an array of 32 Teflon tubes situated among an array of 72 lamps. Twenty-four Teflon tubes were 5.99 era in dia and a group of 8 tubes were 8.89 cm in dia. These units were designed to receive flows of 0.95--4.5 M1/d. All measurements of intensity at 254 nm were made with an International Light IL-500 radiometer with an SEE-240 detector calibrated periodically by the manufacturer. Output of the lamp at the 254-nm wavelength was measured as described in Barrows (1951) and Quails and Johnson (1983). Radiometer measurements perpendicular to the lamp center in air could then be used as an index to the output. Output of the lamps was measured with the outer quartz tube covering the lamp for units using an outer tube. A temperature probe was attached to the lamp surface and ouptut was measured as a function of temperature as the lamp was warmed or cooled. Thus output of the lamps inside the unit at a given lamp temperature could be closely estimated. The survival of spores of Bacillus subtilis (ATCC 6633) was measured as a function of u.v. dose (intensity x exposure time) to prepare a standard batch inactivation curve. Survival is defined here as N,/No, where No is the initial concentration of viable bacteria and N, is the concentration of survivors. The u.v. dose in an unknown
intensity field could then be measured by determining the survival and referring to the standard curve (Quails and Johnson, 1983). One lot of spores of Bacillus was grown in Schaeffer's medium (Munikata and Rupert, 1972). Another lot of spores (from the same inoculum) was grown on the surface of BBL A-K Agar in glass baking dishes. The spores grown on the surface of A-K agar were much more u.v. resistant than those grown in Schaeffer's medium, so much higher dose levels could be assayed. Both lots were purified as detailed in Quails and Johnson (1983). The suspension was washed by centrifugation, resuspended in distilled water, heated to 80°C for l0 rain to kill vegetative cells, sonicated to lyse the vegetative cells and washed repeatedly by centrifugation in distilled water. Viable spores were enumerated by pour-plating in triplicate on Nutrient agar (Difco Labs). To prepare the standard curve of log survival vs u.v. dose, the spore suspensions were irradiated in a collimated beam of germicidal u.v. light in a stirred Petri dish (Quails and Johnson, 1983). To obtain maximum intensity in a collimated beam, the apparatus described in Quails and Johnson 0983) was modified by using a cluster of four germicidal lamps, cooled by a blower, with a parabolic reflector. The standard curves of log survival vs dose (Fig. 3) were very reproducible over several months. The intensity in the unknown systems was found by (i) determining the survival (Ns/No), (details of determining the NOand N, are described later); (ii) reading the dose corresponding to the observed survival from the standard curve (Fig. 3); and (iii) using the measured exposure time to calculate the average intensity dose assayed average intenstiy = exposure time"
(1)
Injection bioassay Because of the complex relationship between the average survival and the residence time distribution (RTD) in a flowthrough system (Quails and Johnson, 1983), the bioassay of intensity cannot simply be determined using a continuous flow of spores along with a measurement of the average RT. To eliminate the ambiguities in survival caused by the flow dispersion, the bioassay was done in a manner analogous to a tracer injection study. This method has been demonstrated in a laboratory scale flowthrough system (QuaUs and Johnson, 1983). Spores were suspended in 1 M MgSO4 at a concentration of 106-107 spores/ml and injected with a 50 ml syringe near the entrance to the irradiation chamber of the units (Fig. 2). A sample was withdrawn at
Efficiency of ultraviolet disinfection systems
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in the collection syringe to obtain N,. However, it was possible to perform the bioassay in undisinfected effluent by using a sufficiently high density of injected spores, since the spores were considerably more u.v. resistant than the other bacteria. For Units 1 and 2, these bioassays were performed as a function of u.v. absorbance, applied line voltage, flow rate and at various points on the RTD. To vary the absorbance of the water beyond the range found in the wastewater, dechlorinated tap water with or without added dissolved humic acid (Aldrich Chemical Co.) was used. The absorbance at 254 nm was measured with a spectrophotometer using a special cell to correct for light scattering (11). An estimate of the average intensity in Unit 1 was also made with the point-source summation calculations (Jacob and Dranoff, 1970; Quails and Johnson, 1983). This calculation required measurement of the lamp output and the absorbance of the fluid.
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UNIT I
Fig. 2. Apparatus for injecting and collecting spore suspension and simultaneously measuring residence time distribution. the exit of the chamber as the slug of spores passed the collection syringe. The very short exposure times typical of u.v. disinfection, ranging from 1 to 15s, present special problems in measuring the RTD and exposure times by tracers. To accurately measure the short exposure times, a switch on the injection syringe initiated the timed sweep on a Houston Omniscrible electric pen recorder. A switch on the collection syringe operated a pen lift mechanism on the recorder. A I.,eeds and Northrup conductivity probe, designed to be used in a pipe flowstream, was positioned behind the collection syringe and indicated the RTD of the injected salt (suggested by O. K. Scheible) on the recorder. The holes at the end of the syringes were enlarged and plugged with paraffin film so that injection and sample collection could each be accomplished in a fraction of a second. Injection time and initial dilution were measured by injecting salt at the position of the collection syringe and recording the conductivity. The injection time was <0.15 s and the sample collection was estimated to require 0.25 s. The exposure time was corrected for the injection and collection time, and the time to traverse the short distance between the syringes and the chamber. In order to calculate the survival (N,/No) of the spores it was necessary to measure the dilution of the spores between injection and collection. The concentration of viable plus inactivated spores in the collection syringe (No) was measured by the dilution of the injected tracer: N,
N~ C|Nj
No
N|N, COOL
319
(2)
where Nlwj and N~ are the concentrations of viable spores in the injection and collection syringes, respectively and C[w~ and CcoL are the concentrations of tracer in the injection and collection syringes, respectively. The relative concentration of salt tracer in the collection syringe was measured by comparing the conductivity to dilutions of the injected suspension. Having measured the N s and N0, the average intensity in the irradiation chamber was calculated as outlined by equation (1) using the survival of the spores, the standard curve and the measured exposure time. Injection bioassays were performed in Units 1 and 2 in most cases while partially disinfected secondary effluent flowed through the unit. The effluent was initially pumped through the disinfection unit and into a tank to reduce the background count due to bacteria in the effluent. This background count was subtracted from the bacterial density
In Unit 4 the design was such that injections and collections could be made at the entrance and exit of individual Teflon tubes. Flow distribution studies and spore bioassay studies were done, by methods similar to those for Unit 2, on two tubes representing the inner and outer positions in the array. Only those for the 5.99 cm tubes are reported here. Bioassays were done in flowing tapwater (absorbance=0.035) and in flowing wastewater (absorbance = 0.16). In Unit 3 the front and rear planes of the lamp battery were open to flow, so it was necessary to consider the flow regime at various representative points on the cross-section perpendicular to flow. For this reason the unit was considered to consist of a 5-by-5 array of parallel chambers. Flow tests and bioassay injections were performed at points representing the central area, and the sides. A thorough characterization of the flow regime was performed by HydroQual Inc. (Scheible, 1986). The submerged entrance and large dilution of injected fluid required larger adaptation of a syringe. The end of a 60 ml plastic syringe was cut off and the syringe was mounted in the end of an aluminum tube. A section of a rubber stopper was placed on a screw driven into the plunger, so that a 75 ml aliquot contained between the plunger and stopper could be released or collected in a fraction of a second with minimum velocity. A switch activated a chart recorder. For flow distribution studies, a mixture of dye and salt allowed us to both observe and record the passage of the injected aliquot and to assure that segregation of the aliquot was not a problem. For spore injections, however, 0.1 M LiCI was mixed with the spore suspensions to measure dilution [equation (2)] because of the unusually high conductivity of the wastewater. Lithium concentrations were measured by atomic absorption spectroscopy.
Continuous flow bioassay In some cases accessibility may make it difficult to use the injection method for the bioassay. While using a continuous flow of spores is in some ways simpler, the interpretation of the data is more difficult. Bacillus spores were mixed in a tank of previously irradiated secondary effluent to give a concentration of about 5 x 104 spores/ml. The mixture was pumped through Unit 1. The average survival was measured from samples of the inflow and outflow. The RTD was measured by salt injection. To assay the intensity from the observed average survival, we used a simulation of average survival in a flowthrough system based on the No, the RTD, the standard dose-survival curve (Fig. 3), and an assumed value for average intensity. The simulation, detailed in Quails and Johnson (1985), calculates the number of survivors in small increments of the RTD and sums them over the RTD to calculate the average survival. The simulation was run using
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Fig. 3. Standard dose-survival curve for B. subtilis spores harvested from Schaeffer's medium (solid circles) or A-K agar (open circles). a range of intensity values, and then the simulated average survival was plotted against the assumed intensity. The intensity corresponding to the measured average survival was then interpreted as the assayed average intensity. Disinfection of coliforms Units 1 and 2 were compared in their ability to kill total coliforms in the secondary effluent. The effluent was pumped through the u.v. units from the same batch of effluent and samples of inflow and outflow were collected at various flow rates. Samples were iced, kept in darkness and total coliforms were enumerated by the membrane filter procedure (APHA, 1981; Quails et al., 1984). RESULTS
AND
DISCUSSION
Bioassays The normalized residence time distribution for the two smaller units (Units 1 and 2) are shown in Fig. 4(a). The R T D for Unit 2 approached plug flow because the irradiation chamber was essentially a
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long Teflon pipe. Unit 1 had considerably more dispersion because of the baffles in the unit used to insure mixing across intensity gradients. The dispersion in both units was, however, considerably better than the dispersions of most of the small u.v. disinfection units tested in a National Sanitation Foundation study (Bellen et al., 1981). Measurements of the average residence time based on the moment of the R T D matched the volume/flow rate (V/Q) measurements quite well for both units; usually within 5%. The normalized R T D remained the same over the range of recommended flow rates in both units (1.9-6.1 l/s). At a given flow rate, the average residence time in Unit 2 was much longer since the volumes of Units 1 and 2 were 11.2 and 30.11, respectively. For Unit 3, curves were normalized for separate sections of the unit with different flow velocities, so the average residence time at an overall flow rate of 15.8 l/s is indicated for the center and side sections in Fig. 4(b). The flow distribution in the central areas of Unit 3 was fairly similar to that of Unit 1. However, flow velocity was considerably lower and the dispersion was greater near the sides of the unit. Thus, although the R T D in the central portion of Unit 3 was only moderately dispersed, the flow traversed the unit in less time than if the flow velocity was evenly distributed throughout the cross section. It appeared that the distribution of flow velocities entering the unit in the plane perpendicular to flow was a greater problem than dispersion caused by the lamps themselves. The R T D in the Teflon tubes of Unit 4 was very similar to that of Unit 2 and is not shown. Scheible (1986) reported similar Morrill Dispersion Indices for Unit 3 and for another Teflon unit such as Unit 4. The injection bioassay measurements in Unit 1 matched the point-source summation (PSS) calculations well across a wide range of absorbance (Fig. 5). In Fig. 5, each point represents one injection
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Fig. 5. Average intensity in the Units 1 and 2 as a function of u.v. absorbance line for Unit 1 represents the theoretical point-source summation calculations, while the line for Unit 2 was drawn through the points by hand.
Efficiency of ultraviolet disinfection systems
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Fig. 6. Assay of dose as a function of time from injection in Unit 1. trial. The absorbances ranged from that of tapwater to that typical of raw wastewater. The accuracy of the PSS calculations depended on accurate measurements of lamp output which depended on accurate measurements of relative lamp temperature. In sets of 3 or 4 replicate bioassays the standard deviation averaged 5% of the mean. Intensities reported here for Unit 1 are somewhat higher than reported for the same unit in an earlier study (Quails and Johnson, 1985) because newer lamps were used in this study. The PSS calculations were not made for Unit 2 because of its dependence on reflection from the aluminum walls. The PSS calculations cannot consider such effects due to the complex geometry of the reflection. However, in chambers such as Unit 1 in which the water is contained by the walls, the walls rapidly lose all reflectivity. The role of reflection from the walls in Unit 2 was demonstrated by covering the walls with non-reflective paper, or with cleaned, moderately reflective aluminium foil during some bioassays. Few materials reflect 254 nm light well except polished aluminum which has at best about 70% reflectivity (Koller, 1958). The assayed intensity with the reflection blocked was 1.5 mW/cm 2 compared to 3.0 mW/cm 2 with the regular reflection. The aluminum foil only enhanced the intensity about 8% over that of the regular reflector. It can be seen in Fig. 1 that less than half of the arc around the lamps is intercepted directly by the Teflon pipes and most of the light must be reflected at least once before striking a Teflon pipe. The PSS calculation gave a good estimate of the average intensity in the Aquafine unit, suggesting this method would serve well in multiple lamp units where reflection is unimportant. It is valuable for interpolation across the range of absorbance and can provide a way of evaluating changes in design easily. Mixing across intensity gradients t
A series of bioassays performed at various times on the RTD provides a useful way of illustrating the importance of the initial fractions of the RTD, as well as serving as a diagnostic tool to test mixing across intensity gradients (Fig. 6). The open circles in WR. 23/~.-E
Fig. 6 represent the concentration of viable plus inactivated spores (No) which reflects the RTD in Unit 1. Closed circles are the concentration of survivors (N,) in separate samples taken at various times from injection and triangles represent the assayed dose for each sample calculated from the corresponding log N J N o standard curve. The line through the dose values is the regression forced through the origin. The number of survivors as a function of time from injection, compared with the number of injected spores, shows that the spores traversing the unit in the first half of the RTD dominate the average survival because of the logarithmic relationship of survival to dose. The bioassayed dose for each fraction of the RTD was fairly linearly related to the time from injection. Thus, the average intensity to which the various flow fractions were exposed was fairly constant. If, for example, the first flow fractions to emerge were able to traverse the chamber in an area of low intensity, then the apparent intensity for the first-flow fractions would be lower than those for the later points and the data would systematically deviate from a linear dose vs exposure time relationship. The theoretical relationship of survival to average intensity in the case of a distribution of residence times and a distribution of doses within each fraction of the residence time distribution is described in detail in Quails and Johnson (1985). If the experiment indicates insufficient mixing across intensity gradients, then the bioassay results for those flow fractions cannot be interpreted as the average intensity (Quails and Johnson, 1985). Insufficient mixing across intensity gradients can result in a distribution of doses within each fraction of the residence time distribution. In fractions of the residence time distribution containing subfractions segregated across intensity gradients, survivors subject to lower doses will contribute disproportionally to the number of survivors. These survivors will skew the results of the bioassay. The bioassay results for those fractions should only be interpreted qualitatively. Such problems can seriously affect the average survival. Cortelyou et al. (1954) noted an increase in disinfection efficiency as baffles
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ing for the distribution of residence times can lead to gross errors. For example, if we multiply the intensity from the injection assay (13.9mW/cm 2) times the average residence time (4.37 s) we get an "average" dose of 60.7 mWs/cm 2. The corresponding log survival on the standard curve (Fig. 3) would be -3.10, far from the observed log survival of -1.67. Use of the volume/flow as the average residence time would give similar results since it was close to the average from the RTD. The steep slope and logarithmic nature of the dose-survival curve (Fig. 3) causes the first flow fractions to contribute most to the average survival as was pointed out in Fig. 6.
Applicability of the bioassay procedure
, 17
were added to one u.v. unit to increase mixing across intensity gradients. The bioassay analysis shown in Fig. 6 provides a useful means of diagnosing problems in mixing across intensity gradients. For example, the bioassay experiments could be used to find the minimum number of baffles needed in Unit 1 while avoiding the shadowing caused by unnecessary baffles. It should also be noted that higher u.v. absorbance results in steeper intensity gradients. Consequently, diagnosis of problems with mixing across gradients should be done at the high range of absorbances encountered. In Units 2 and 4, the partially transparent Teflon tubes allowed observations of injections of suspended particles and dye. These indicated rapid turbulent mixing throughout the pipe diameter and, therefore, across intensity gradients. In Unit 3 there was no consistent trend in the bioassays of apparent average intensity as a function of time from injection: assayed intensities were 5.3, 6.5, 5.3, 5.2 and 6.5 mW/cm 2 for residence times of 12.0, 12.6, 12.9, 13.2 and 18.2s, respectively, in wastewater.
We estimate the costs of materials for the bioassay procedure to be about $250 plus the costs of a chart recorder with an event marker mechanism and a conductivity probe. Testing of Units ! and 2 would require about 80 man-hours, and an equal time would be required for Units 3 and 4. The injection method requires a far smaller volume of spores than a continuous flow method does. Its applicability is limited to units where access to the entrance and exit to the irradiation chamber is possible, including submerged arrays where all parts of entrance and exit planes can be reached with a pipe. In very large array units with many more rows of lamps in the direction of flow than were encountered in these units, and which also have very dispersed, turbulent flow, dilution of the injected slug may be greater than about 106-fold, which would preclude detection o f survivors. Such dilution may be measured by dye injection before attempting a bioassay. Application of the PSS calculation of intensity is much easier, and Scheible et al. (1986) present calculations for the most common lamp configurations. However, the role of bioassay measurements of intensity should be to verify the PSS calculation for various general reactor configurations, not necessarily for routine measurements. In some unusual reactor configurations such as Unit 2, the bioassay may be the only way to reliably measure intensity.
Continuous flow bioassay
Measures of capacity and efficiency
The continuous flow bioassay gave an assayed intensity close to that given by an injection bioassay performed immediately before (Fig. 7). The observed average log survival, using the continuous flow of spores, was - 1.67. On the line of simulated survival vs given intensity values, the observed survival corresponded with an intensity of 13.6 mW/cm ~. In injection bioassays performed immediately before (Fig. 6), the average intensity was estimated as 13.9 ( + 0.9) mW/cm 2 (indicated by arrow in Fig. 7). While the results of these two methods of using the bioassay were close, the injection method is more accurate because it is more direct and the interpretation is simpler. Interpretation of the Bacillus spore survival in the continuous flow experiment without account-
Perhaps the ultimate measure of disinfection capacity of a particular unit is the maximum flow rate at which a given target level of survival can be achieved. We chose a total coliform log survival of - 3 . 5 as a desirable target to meet disinfection standards, allowing for some photoreactivation. A direct comparison of total coliform survival vs flow rate between Units 1 and 2, using the same batch of secondary effluent, is shown in Fig. 8. At a given flow rate, the survival in Unit 1 was lower. By interpolation, the flow rate corresponding to - 3 . 5 log survival units was 6.6 and 4.01/s for Unit 1 and Unit 2, respectively (Table 1). This sort of comparison is difficult to generalize since even the relative flow rates necessary to obtain this survival may vary with the
Fig. 7. Bioassay of intensity using average survival of a continuous flow of spores.
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u.v. absorbance and the sensitivity of coliforms in the particular effluent under consideration. Furthermore, this measure provides no information on the contributions of lamp output, light use efficiency and flow dispersion to the overall efficiency. We have used several measures of both efficiency and capacity to evaluate the u.v. units. Measures of efficiency provide a way to compare not only units of the same size, but units of different size. However, given two units which are equally efficient, one may be much larger and, hence, able to treat much larger volumes. Consequently, we also need measures of capacity which incorporate the size of the units and are proportional to the flow rates which can be handled. Several detailed measures of capacity and efficiency are shown in Table 1. The values for average intensity shown in Table l are all based on bioassays. While Units 1 and 2 had similar total lamp lengths and nominal power input (240W), Unit 2 was less efficient in converting power to 254 nm u.v. output. In fact, we measured the output of the lamp with the surrounding outer quartz tube in place in Unit 1,
323
which should reduce the u.v. output compared to the naked lamp. The product of average intensity times the volume of the irradiation chamber is a factor which is directly proportional to the capacity to treat specified flow rates of water with a given dose under ideal flow conditions. This factor serves to isolate the effectiveness of the intensity regime from the effects of non-ideal flow. The utility of using measures based on the intensity volume product is not that they alone can be used as a rating system, but in isolating how effectively intensity is used in the design. While average intensity in Unit 2 was much lower, its volume was larger. Still, the intensity-volume product was lower. To compare the efficiency of units with different total lamp lengths we used the intensity x volume/ nominal input watts (Table 1). In this respect Unit 2 was only 53% as efficient as Unit 1. A measure of the "light use efficiency" is the intensity x volume/u.v. output. By comparing these two efficiencies, it can be seen that about 1/3 of the difference in efficiency was due to less efficient u.v. output of Unit 2 and the remainder was due to less efficient use of the u.v. output in the irradiation chamber. The lamp ballasts in Unit 2 were found to be relatively inefficient. The less efficient use of the u.v. light output can be explained by the differing geometry of the two units. In Unit 1, the solution surrounds the quartz tubes covering the lamps. In Unit 2, however, the liquidcontaining Teflon intercepted less than half of the arc around the u.v. lamps directly, so that much of the light must be reflected at least once before striking the Teflon. In addition Teflon transmits less u.v. than does quartz. The better residence time distribution of Unit 2 did not compensate for the lower intensity-volume product in the coliform disinfection comparison (Table 1, Fig. 8). As pointed out in Quails and Johnson (1983)
Table I. Comparison of disinfection units Characteristic (I)
Average intensity in wastewater
Unit I
Unit 2
Unit 3
Unit 4
12.1"
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5.8*
2.8t
(mW/crn2) (2)
Average intensity in tap water:[:
25.6
5.7
(3) (4)
Fluid volume (I) Intensity x volume (roW/ore2 x 1) Ultraviolet output/power input
11.2 136
30.2 72
378 2191
90/240
71/240
1160/4000
0.57
0.30
0.55
0.13t
1.5
1.0
1.9
0.52t
2.27
1.22
1.85, 2.08**
1.30
6.6
4.0
89.7¶1
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(5) (6) (7) (8) (9)
(mW/cm2)
(w)
I x V/input watts (mW/cm 2 x l/w) I x V/u.v. output (mW/cm x l/w) Morrill dispersion Index tgo/ho Simulated max. flow (I/s) for
-3.5 log S and abs ffi0.2
10.9
4.0 314~ 754t 1462/5760
*At an absorbance (base 10) of 0.2. t a t an absorbance (base 10) of 0.16. ~:At an absorbance (base 10) of 0.035. §Assuming all tubes in unit are small size tubes. ¶lSimulation yields an extrapolated value which is probably an overestimation since such high flow rates could not be used in that unit. **Center and side, respectively.
324
ROBERTG. QUALLSe t al.
the log survival vs dose curves of coliforms in wastewater effluents have a "steep" portion and a relatively level portion at higher doses. The RTD of Unit 1 was good enough so that an insignificant fraction of the flow received doses corresponding to the "steep" portion of the coliform dose-survival curve where the dispersion would seriously affect the survival. Using the information in Table 1 on the relative contribution of various factors to the efficiency of the prototype Unit 2, the design of the unit was modified by the manufacturer. Unit 3 had not only the highest intensity-volume product, but it had the highest light-use efficiency (Table 1). The proximity of the chamber walls acting to absorb some of the available u.v. radiation in Unit 1 apparently reduced the efficiency somewhat compared to Unit 3. However, the average RT of the flow in the central portions of Unit 3 was 0.82 times the theoretical RT for the entire unit. This uneven flow distribution might have significant effects on the average survival, if substantial amounts of flow receive doses corresponding to the steeply declining portions of the dose-survival curve (Quails and Johnson, 1983). For the purpose of making the comparisons shown in Table 1, we made the assumption that all the tubes in Unit 4 were 5.99 cm dia tubes although 8 were 8.89 cm in dia. The bioassay of Unit 4 was not performed at the same fluid absorbance as for Unit 3, so the capacity and efficiency terms were not directly comparable. However, while Unit 4 had the largest volume and lamp wattage, the efficiency was lowest of all the units and would have appeared even lower at an absorbance of 0.20. This was due partly to the low output which we measured from the bulbs, many of which were old, but mainly to low light-use efficiency. Scheible et al. (1986) presented extensive coliform disinfection data comparing Units 3 and 4. They also concluded from disinfection data that Unit 4 was not as energy efficient as Unit 3. Scheible (1986) noted that the u.v. transmission of the Teflon tubes decreased significantly over several months, even after cleaning. Characteristics 4 through 8 in Table 1 give information on the efficiency and capacity of specific components of the overall system. There is no simple single parameter based on physical measurements alone which integrates these components into a balanced measure of the ultimate effectiveness of the system. However, the maximum flow rate at which a - 3 . 5 log survival of coliforms can be achieved is a measure which integrates the effects of the intensity, volume and flow characteristics as well as providing an easily interpreted and realistic comparison of units operated under different conditions. However, it would be impossible to directly measure this parameter, as was done in Fig. 8, for many different units at different locations because of varying dose-survival curves and absorbances. A simulation using measured intensity and RTD data, along with a
standard coliform dose-survival curve, has the advantage of wide applicability. Simulations were run using (i) the average intensity data (Table 1), and RTD data gathered in this study (Fig. 4); (ii) a simple model detailed in Quails and Johnson (1985); and (iii) a standard coliform dose-survival curve based on the average of 36 samples from 6 types of wastewater effluents (Fig. 2 in Quails et al., 1985). The simulated maximum flow rates for each unit are listed in Table 1. Because of the good flow characteristics of Units 1, 2 and 4, the estimated maximum flows were close to that predicted by simply dividing intensity × volume by the dose corresponding to - 3 . 5 log survival (20mWs/cm) on the standard curve. The value for Unit 3 represents a considerable extrapolation because flow rates that high could not be pumped into the installation and we could not determine whether the RTD remained the same high flow rates. Nevertheless, the uneven flow distribution was not bad enough to seriously effect the average survival under the model conditions. The simulation of the maximum flow rate necessary to achieve - 3 . 5 log survival provides a single easily interpreted overall parameter for comparison. In cases where more specific data on u.v. absorbance and the dose survival curve can be anticipated, these may be substituted for standardized data in the simulations. The relative capacities of various units may change as the u.v. absorbance changes. CONCLUSIONS
The analysis presented here illustrates a means of quantitatively isolating the factors involved in the overall efficiency of a u.v. disinfection system. This approach should be useful not only in developing designs but provides a uniform means of comparing u.v units. The information also provides the necessary input to model the performance of a unit at a particular installation where dose-survival data are available. Acknowledgements--We gratefully acknowledge the cooperation of O. K. Seheible and W. Dunn of HydroQual Inc., and A. Forndran and G. Cox of the City of New York, DEP, without whose help the demonstration of these procedures on the large scale systems would have been impossible. We also appreciate the contributions of students in the Department: C. M. Dumais, J. C. Chang, S. F. Ossoff, D. Lobe; J. Cruver and T. Creeden of Enerco Corp.; and the personnel of the Chapel Hill wastewater treatment plant for allowing construction of a pilot unit, as well as the comments of anonymous reviewers. This research was funded by Grant No. CEE-8205274 from the National Science Foundation, Dr Edward Bryan, project officer. REFERENCES
APHA (1981) Standard Methods for the Examination of Water and Wastewater, 15th edition. American Public Health Association, Washington, D.C.
Barrows W. E. (1951) Light, Photometry and Illuminating Engineering, 3rd edition. McGraw-Hill, New York. Bellen G. E., Gottler R. A. and Dorman-Herera R. (1981)
Efficiency of ultraviolet disinfection systems Water Disinfection Technology. National Sanitation Foundation, Ann Arbor, Mich. Cortelyou J. R., McWhinnie M. A., Riddiford M. S. and Semrad J. E. (1954) Effects of ultraviolet irradiation on large populations of certain water-borne bacteria in motion--I. The development of adequate agitation to provide an effective exposure period. Appl. Microbiol. 2, 262-269. Jacob S. M. and Dranoff J. S. (1970) Light intensity profiles in a perfectly mixed photoreactor. A.LCh.E. JI. 16, 359-363. Johnson J. D. and Qualls R. G. (1985) Ultraviolet disinfection of secondary effluent: measurement of dose and effects of filtration. EPA-600/2-84-160, U.S. EPA, Cincinnati, Ohio. Koller L. R. (1958) Ultraviolet Radiation. Wiley, New York. Munikata N. and Rupert C. S. (1972) Genetically controlled removal of "spore photoproduct" from deoxyribonucleic acid of ultraviolet-irradiated Bacillus subtilis spores. J. Bact. III, 192-198. Nehm P. H. (1983) Operating experience disinfecting secondary effluent with pilot scale ultraviolet units, In Municipal Wastewater Disinfection (Edited by Venosa A. D.). U.S. Environmental Protection Agency, Washington, D.C., EPA-600-9-83-009. Quails R. G. and Johnson J. D. (1983) Bioassay and dose measurement in UV disinfection. Appl. envir. Microbiol. 45, 872-877. Quails R. G. and Johnson J. D. (1985) Modeling and efficiency of ultraviolet disinfection systems. War. Res. 19, 1039-1046. Qualls R. G., Flynn M. P. and Johnson J. D. (1983) The role of suspended particles in ultraviolet disinfection. J. Wat. Pollut. Control Fed. 56, 1280. QuaUs R. G., Chang J. C. H., Ossoff S. F. and Johnson J. D. (1984) Comparison of methods of enumerating coliforms after UV disinfection. Appl. envir. Microbiol. 48, 699-701.
325
Quails R. G., Ossoff S. F., Chang J. C. H., Dorfman M. H., Dumais C. M., Lobe D. C. and Johnson J. D. (1985) Factors controlling sensitivity to UV disinfection in secondary effluents. J. Wat. Pollut. Control Fed. 57, 1006-1046. Scheible O. K. (1986) Ultraviolet radiation. In Design Manual: Municipal Wastewater Disinfection, pp. 157-247. EPA No. EPA/625/1-86/021, U.S. Environmental Protection Agency, Cincinnati, Ohio. Scheible O. K. (1987) Development of a rationally based design protocol for the ultraviolet light disinfection process. J. Wat, Pollut. Control Fed. 59, 25-31. Scheible O. K. and Bassel C. D. (1981) Ultraviolet disinfection of a secondary wastewater treatment planet effluent. EPA-600/S2-81-52, U.S. Environmental Protection Agency, Washington, D.C. Scheible O. K., Casey M. C. and Forndran A. (1986) Ultraviolet disinfection of wastewaters from secondary effluent and combined sewer overflows. EPA No. 600/$2-86/005, U.S. Environmental Protection Agency, Cincinnati, Ohio. Severin B. F. (1980) Disinfection of municipal wastewater effluents with ultraviolet light. J. Wat. Pollut. Control Fed. 52, 2007-2018. Tobin R. S., Smith D. K., Horton A. and Armstrong V. C. (1983) Methods for testing the efficiency of ultraviolet light disinfection devices for drinking water. J. Am. Wat. Wks Ass. 75, 481. Whitby G. E., Palmateer G., Cook W. G., Maarschalkerwerd J., Huber D. and Flood K. (1984) Ultraviolet disinfection of secondary effluent. J. Wat. Pollut. Control Fed. 56, 844-850. White S. C., Jernigan E. B. and Venosa A. D, (1986) A study of operational ultraviolet disinfection equipment at secondary treatment plants. J. Wat. Pollut. Control Fed. 58, 181-192.
0043-1354/89 $3.00+0.00 Copyright ~ 1989PergamonPrels pie
EVALUATION OF THE EFFICIENCY OF ULTRAVIOLET DISINFECTION SYSTEMS RoasgT G. QUALLS*, MARK H. DORF~L,~Nand J. DONALD JOHNSON~ Department of Environmental Sciences and Engineering, The University of North Carolina at Chapel Hill, Chapel Hill, NC 27514, U.S.A. (First received March 1988; accepted in revised form October 1988) Abstract--One problem in using ultraviolet light (u.v.) for full scale wastewater disinfection is the difficulty in measuring the dose of u.v. and in monitoring the immediate results of disinfection. A bioassay method for measuring the average u.v. intensity and the residencetime distribution (RTD) was tested on two small and two large multiple-lamp u.v. systems. Spore suspensions of u.v. resistant Bacillus subtilis were injected into the u.v. units as a spike and collected at a known time from injection in the effluent so that the exposure time was known. The survival could be related to a standard curve of survival vs u.v. dose. In one unit the bioassay measurements of intensity corresponded well with the predictions of a method of calculating intensity across a wide range of fluid u.v. absorbance values. The problems with interpreting the results of a continuous flow of the bioassay spores were demonstrated. The measurement of average intensity, RTD, volume of the chamber and the lamp u.v. output enabled us to assign measures of capacity and efficiency to the systems. The analysis illustrated a means of quantitatively isolating the factors involved in the overall efficiencyof the disinfection system and a means of comparing different systems. Key words--ultraviolet light (u.v.), disinfection, modeling, bioassay, flow dispersion, coliform, bacteria, intensity
INTRODUCTION Disinfection of water using ultraviolet (u.v.) light has been used for specialized small-scale applications for many years. Recently, many large scale u.v. wastewater disinfection systems have been built or planned in response to concern over meeting standards for discharge of chlorinated organics (White et al., 1986). While disinfection of wastewater had been regarded as expensive and unreliable, several pilot and fullscale test projects demonstrated its effectiveness (Whitby et al., 1984; Severin, 1980; Scheible and Bassel, 1981; Scheible, 1987; Quails et al., 1983; Johnson and Quails, 1985). Full scale u.v. disinfection of wastewater has been found to be economically competitive with chlorination (Scheible and Bassel, 1981). One problem that has been cited as a serious disadvantage in the use of ultraviolet disinfection is the difficulty of measuring the u.v. dose. Unlike chlorination and ozonation, there is no measurable chemical residual This makes the immediate control of the process difficult. In a practical u.v. disinfection system, the intensity patterns are complex and the average intensity cannot be directly measured with a detector. Many u.v. disinfection studies have been hampered by inadequate means of calculating intensity. Recently, however, a means of calculating *Corresponding author. Current address: Institute of Ecology, University of Georgia, Athens, GA 30602, U.S.A.
intensity in absorbing solutions has been adapted to u.v. disinfection systems and an indirect method of measuring average intensity with a bioassay has been developed (Quails and Johnson, 1983). This same difficulty in measuring u.v. dose has made it hard to evaluate new designs for u.v. systems and there has been no systematic means of comparing u.v. disinfection systems (Tobin et al., 1983). Engineers choosing u.v. disinfection systems must usually rely on estimates of average dose based on unsubstantiated estimates of intensity times theoretical residence time (volume/flow rate) (Nehm, 1983). The effects of residence time distribution have usually been ignored. Most current u.v. disinfection systems employ tubular germicidal lamps enclosed in a second quartz tube submerged in a chamber through which the fluid flows. Flow may either be parallel or perpendicular to the lamp axes. In one novel system, the fluid flows through Teflon tubes interspersed among the lamps (see Scheible, 1986; White et al., 1986). Our approach to evaluating the efficiency and capacity of u.v. disinfection systems is first to isolate the factors of intensity, residence time distribution and size of the chamber so that advantages or problems with each can be diagnosed and addressed separately. Then we outline ways of integrating these separate components to compare different systems using a simple model. More specifically, the objectives of this study were: (1) to develop and demonstrate methods of measuring intensity and residence time distribution (RTD) in multiple lamp u.v. units; (2) to substantiate a method of calculating intensity 317
ROBERT G.
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patterns in one multiple lamp unit; and (3) to illustrate measurements o f capacity and efficiency used to c o m p a r e u.v. disinfection systems. METHODS The Aquafine CSL-6 unit (Unit l) contained 6 germicidal lamps (445cm total arc length) enclosed in submerged quartz tubes in a cylindrical chamber (Fig. 1). Helical baffles were included to mix flow across intensity gradients. The 3-1amp Enerco unit (Unit 2) was an experimental prototype containing approximately the same total arc length as Unit 1. However, the lamps were suspended in an air space in an aluminum cylinder and the liquid flowed through partially u.v. transparent Teflon pipes. The two larger scale units tested in this study were part of a pilot project located at the Port Richmond Water Pollution Control Plant in New York City. The project was operated by HydroQual Inc. and funded by the Environmental Protection Agency and the City of New York. These units are described by Scheible (1986, 1987). Unit 3 was a rectangular array of 100 submerged lamps, spaced 7.3 cm between lamp axes. Flow was perpendicular to the lamp axes. Unit 4 was a large scale modification of Unit 2, with an array of 32 Teflon tubes situated among an array of 72 lamps. Twenty-four Teflon tubes were 5.99 era in dia and a group of 8 tubes were 8.89 cm in dia. These units were designed to receive flows of 0.95--4.5 M1/d. All measurements of intensity at 254 nm were made with an International Light IL-500 radiometer with an SEE-240 detector calibrated periodically by the manufacturer. Output of the lamp at the 254-nm wavelength was measured as described in Barrows (1951) and Quails and Johnson (1983). Radiometer measurements perpendicular to the lamp center in air could then be used as an index to the output. Output of the lamps was measured with the outer quartz tube covering the lamp for units using an outer tube. A temperature probe was attached to the lamp surface and ouptut was measured as a function of temperature as the lamp was warmed or cooled. Thus output of the lamps inside the unit at a given lamp temperature could be closely estimated. The survival of spores of Bacillus subtilis (ATCC 6633) was measured as a function of u.v. dose (intensity x exposure time) to prepare a standard batch inactivation curve. Survival is defined here as N,/No, where No is the initial concentration of viable bacteria and N, is the concentration of survivors. The u.v. dose in an unknown
intensity field could then be measured by determining the survival and referring to the standard curve (Quails and Johnson, 1983). One lot of spores of Bacillus was grown in Schaeffer's medium (Munikata and Rupert, 1972). Another lot of spores (from the same inoculum) was grown on the surface of BBL A-K Agar in glass baking dishes. The spores grown on the surface of A-K agar were much more u.v. resistant than those grown in Schaeffer's medium, so much higher dose levels could be assayed. Both lots were purified as detailed in Quails and Johnson (1983). The suspension was washed by centrifugation, resuspended in distilled water, heated to 80°C for l0 rain to kill vegetative cells, sonicated to lyse the vegetative cells and washed repeatedly by centrifugation in distilled water. Viable spores were enumerated by pour-plating in triplicate on Nutrient agar (Difco Labs). To prepare the standard curve of log survival vs u.v. dose, the spore suspensions were irradiated in a collimated beam of germicidal u.v. light in a stirred Petri dish (Quails and Johnson, 1983). To obtain maximum intensity in a collimated beam, the apparatus described in Quails and Johnson 0983) was modified by using a cluster of four germicidal lamps, cooled by a blower, with a parabolic reflector. The standard curves of log survival vs dose (Fig. 3) were very reproducible over several months. The intensity in the unknown systems was found by (i) determining the survival (Ns/No), (details of determining the NOand N, are described later); (ii) reading the dose corresponding to the observed survival from the standard curve (Fig. 3); and (iii) using the measured exposure time to calculate the average intensity dose assayed average intenstiy = exposure time"
(1)
Injection bioassay Because of the complex relationship between the average survival and the residence time distribution (RTD) in a flowthrough system (Quails and Johnson, 1983), the bioassay of intensity cannot simply be determined using a continuous flow of spores along with a measurement of the average RT. To eliminate the ambiguities in survival caused by the flow dispersion, the bioassay was done in a manner analogous to a tracer injection study. This method has been demonstrated in a laboratory scale flowthrough system (QuaUs and Johnson, 1983). Spores were suspended in 1 M MgSO4 at a concentration of 106-107 spores/ml and injected with a 50 ml syringe near the entrance to the irradiation chamber of the units (Fig. 2). A sample was withdrawn at
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in the collection syringe to obtain N,. However, it was possible to perform the bioassay in undisinfected effluent by using a sufficiently high density of injected spores, since the spores were considerably more u.v. resistant than the other bacteria. For Units 1 and 2, these bioassays were performed as a function of u.v. absorbance, applied line voltage, flow rate and at various points on the RTD. To vary the absorbance of the water beyond the range found in the wastewater, dechlorinated tap water with or without added dissolved humic acid (Aldrich Chemical Co.) was used. The absorbance at 254 nm was measured with a spectrophotometer using a special cell to correct for light scattering (11). An estimate of the average intensity in Unit 1 was also made with the point-source summation calculations (Jacob and Dranoff, 1970; Quails and Johnson, 1983). This calculation required measurement of the lamp output and the absorbance of the fluid.
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Fig. 2. Apparatus for injecting and collecting spore suspension and simultaneously measuring residence time distribution. the exit of the chamber as the slug of spores passed the collection syringe. The very short exposure times typical of u.v. disinfection, ranging from 1 to 15s, present special problems in measuring the RTD and exposure times by tracers. To accurately measure the short exposure times, a switch on the injection syringe initiated the timed sweep on a Houston Omniscrible electric pen recorder. A switch on the collection syringe operated a pen lift mechanism on the recorder. A I.,eeds and Northrup conductivity probe, designed to be used in a pipe flowstream, was positioned behind the collection syringe and indicated the RTD of the injected salt (suggested by O. K. Scheible) on the recorder. The holes at the end of the syringes were enlarged and plugged with paraffin film so that injection and sample collection could each be accomplished in a fraction of a second. Injection time and initial dilution were measured by injecting salt at the position of the collection syringe and recording the conductivity. The injection time was <0.15 s and the sample collection was estimated to require 0.25 s. The exposure time was corrected for the injection and collection time, and the time to traverse the short distance between the syringes and the chamber. In order to calculate the survival (N,/No) of the spores it was necessary to measure the dilution of the spores between injection and collection. The concentration of viable plus inactivated spores in the collection syringe (No) was measured by the dilution of the injected tracer: N,
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where Nlwj and N~ are the concentrations of viable spores in the injection and collection syringes, respectively and C[w~ and CcoL are the concentrations of tracer in the injection and collection syringes, respectively. The relative concentration of salt tracer in the collection syringe was measured by comparing the conductivity to dilutions of the injected suspension. Having measured the N s and N0, the average intensity in the irradiation chamber was calculated as outlined by equation (1) using the survival of the spores, the standard curve and the measured exposure time. Injection bioassays were performed in Units 1 and 2 in most cases while partially disinfected secondary effluent flowed through the unit. The effluent was initially pumped through the disinfection unit and into a tank to reduce the background count due to bacteria in the effluent. This background count was subtracted from the bacterial density
In Unit 4 the design was such that injections and collections could be made at the entrance and exit of individual Teflon tubes. Flow distribution studies and spore bioassay studies were done, by methods similar to those for Unit 2, on two tubes representing the inner and outer positions in the array. Only those for the 5.99 cm tubes are reported here. Bioassays were done in flowing tapwater (absorbance=0.035) and in flowing wastewater (absorbance = 0.16). In Unit 3 the front and rear planes of the lamp battery were open to flow, so it was necessary to consider the flow regime at various representative points on the cross-section perpendicular to flow. For this reason the unit was considered to consist of a 5-by-5 array of parallel chambers. Flow tests and bioassay injections were performed at points representing the central area, and the sides. A thorough characterization of the flow regime was performed by HydroQual Inc. (Scheible, 1986). The submerged entrance and large dilution of injected fluid required larger adaptation of a syringe. The end of a 60 ml plastic syringe was cut off and the syringe was mounted in the end of an aluminum tube. A section of a rubber stopper was placed on a screw driven into the plunger, so that a 75 ml aliquot contained between the plunger and stopper could be released or collected in a fraction of a second with minimum velocity. A switch activated a chart recorder. For flow distribution studies, a mixture of dye and salt allowed us to both observe and record the passage of the injected aliquot and to assure that segregation of the aliquot was not a problem. For spore injections, however, 0.1 M LiCI was mixed with the spore suspensions to measure dilution [equation (2)] because of the unusually high conductivity of the wastewater. Lithium concentrations were measured by atomic absorption spectroscopy.
Continuous flow bioassay In some cases accessibility may make it difficult to use the injection method for the bioassay. While using a continuous flow of spores is in some ways simpler, the interpretation of the data is more difficult. Bacillus spores were mixed in a tank of previously irradiated secondary effluent to give a concentration of about 5 x 104 spores/ml. The mixture was pumped through Unit 1. The average survival was measured from samples of the inflow and outflow. The RTD was measured by salt injection. To assay the intensity from the observed average survival, we used a simulation of average survival in a flowthrough system based on the No, the RTD, the standard dose-survival curve (Fig. 3), and an assumed value for average intensity. The simulation, detailed in Quails and Johnson (1985), calculates the number of survivors in small increments of the RTD and sums them over the RTD to calculate the average survival. The simulation was run using
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Bioassays The normalized residence time distribution for the two smaller units (Units 1 and 2) are shown in Fig. 4(a). The R T D for Unit 2 approached plug flow because the irradiation chamber was essentially a
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Fig. 6. Assay of dose as a function of time from injection in Unit 1. trial. The absorbances ranged from that of tapwater to that typical of raw wastewater. The accuracy of the PSS calculations depended on accurate measurements of lamp output which depended on accurate measurements of relative lamp temperature. In sets of 3 or 4 replicate bioassays the standard deviation averaged 5% of the mean. Intensities reported here for Unit 1 are somewhat higher than reported for the same unit in an earlier study (Quails and Johnson, 1985) because newer lamps were used in this study. The PSS calculations were not made for Unit 2 because of its dependence on reflection from the aluminum walls. The PSS calculations cannot consider such effects due to the complex geometry of the reflection. However, in chambers such as Unit 1 in which the water is contained by the walls, the walls rapidly lose all reflectivity. The role of reflection from the walls in Unit 2 was demonstrated by covering the walls with non-reflective paper, or with cleaned, moderately reflective aluminium foil during some bioassays. Few materials reflect 254 nm light well except polished aluminum which has at best about 70% reflectivity (Koller, 1958). The assayed intensity with the reflection blocked was 1.5 mW/cm 2 compared to 3.0 mW/cm 2 with the regular reflection. The aluminum foil only enhanced the intensity about 8% over that of the regular reflector. It can be seen in Fig. 1 that less than half of the arc around the lamps is intercepted directly by the Teflon pipes and most of the light must be reflected at least once before striking a Teflon pipe. The PSS calculation gave a good estimate of the average intensity in the Aquafine unit, suggesting this method would serve well in multiple lamp units where reflection is unimportant. It is valuable for interpolation across the range of absorbance and can provide a way of evaluating changes in design easily. Mixing across intensity gradients t
A series of bioassays performed at various times on the RTD provides a useful way of illustrating the importance of the initial fractions of the RTD, as well as serving as a diagnostic tool to test mixing across intensity gradients (Fig. 6). The open circles in WR. 23/~.-E
Fig. 6 represent the concentration of viable plus inactivated spores (No) which reflects the RTD in Unit 1. Closed circles are the concentration of survivors (N,) in separate samples taken at various times from injection and triangles represent the assayed dose for each sample calculated from the corresponding log N J N o standard curve. The line through the dose values is the regression forced through the origin. The number of survivors as a function of time from injection, compared with the number of injected spores, shows that the spores traversing the unit in the first half of the RTD dominate the average survival because of the logarithmic relationship of survival to dose. The bioassayed dose for each fraction of the RTD was fairly linearly related to the time from injection. Thus, the average intensity to which the various flow fractions were exposed was fairly constant. If, for example, the first flow fractions to emerge were able to traverse the chamber in an area of low intensity, then the apparent intensity for the first-flow fractions would be lower than those for the later points and the data would systematically deviate from a linear dose vs exposure time relationship. The theoretical relationship of survival to average intensity in the case of a distribution of residence times and a distribution of doses within each fraction of the residence time distribution is described in detail in Quails and Johnson (1985). If the experiment indicates insufficient mixing across intensity gradients, then the bioassay results for those flow fractions cannot be interpreted as the average intensity (Quails and Johnson, 1985). Insufficient mixing across intensity gradients can result in a distribution of doses within each fraction of the residence time distribution. In fractions of the residence time distribution containing subfractions segregated across intensity gradients, survivors subject to lower doses will contribute disproportionally to the number of survivors. These survivors will skew the results of the bioassay. The bioassay results for those fractions should only be interpreted qualitatively. Such problems can seriously affect the average survival. Cortelyou et al. (1954) noted an increase in disinfection efficiency as baffles
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13 15 I NTENS ITY
ing for the distribution of residence times can lead to gross errors. For example, if we multiply the intensity from the injection assay (13.9mW/cm 2) times the average residence time (4.37 s) we get an "average" dose of 60.7 mWs/cm 2. The corresponding log survival on the standard curve (Fig. 3) would be -3.10, far from the observed log survival of -1.67. Use of the volume/flow as the average residence time would give similar results since it was close to the average from the RTD. The steep slope and logarithmic nature of the dose-survival curve (Fig. 3) causes the first flow fractions to contribute most to the average survival as was pointed out in Fig. 6.
Applicability of the bioassay procedure
, 17
were added to one u.v. unit to increase mixing across intensity gradients. The bioassay analysis shown in Fig. 6 provides a useful means of diagnosing problems in mixing across intensity gradients. For example, the bioassay experiments could be used to find the minimum number of baffles needed in Unit 1 while avoiding the shadowing caused by unnecessary baffles. It should also be noted that higher u.v. absorbance results in steeper intensity gradients. Consequently, diagnosis of problems with mixing across gradients should be done at the high range of absorbances encountered. In Units 2 and 4, the partially transparent Teflon tubes allowed observations of injections of suspended particles and dye. These indicated rapid turbulent mixing throughout the pipe diameter and, therefore, across intensity gradients. In Unit 3 there was no consistent trend in the bioassays of apparent average intensity as a function of time from injection: assayed intensities were 5.3, 6.5, 5.3, 5.2 and 6.5 mW/cm 2 for residence times of 12.0, 12.6, 12.9, 13.2 and 18.2s, respectively, in wastewater.
We estimate the costs of materials for the bioassay procedure to be about $250 plus the costs of a chart recorder with an event marker mechanism and a conductivity probe. Testing of Units ! and 2 would require about 80 man-hours, and an equal time would be required for Units 3 and 4. The injection method requires a far smaller volume of spores than a continuous flow method does. Its applicability is limited to units where access to the entrance and exit to the irradiation chamber is possible, including submerged arrays where all parts of entrance and exit planes can be reached with a pipe. In very large array units with many more rows of lamps in the direction of flow than were encountered in these units, and which also have very dispersed, turbulent flow, dilution of the injected slug may be greater than about 106-fold, which would preclude detection o f survivors. Such dilution may be measured by dye injection before attempting a bioassay. Application of the PSS calculation of intensity is much easier, and Scheible et al. (1986) present calculations for the most common lamp configurations. However, the role of bioassay measurements of intensity should be to verify the PSS calculation for various general reactor configurations, not necessarily for routine measurements. In some unusual reactor configurations such as Unit 2, the bioassay may be the only way to reliably measure intensity.
Continuous flow bioassay
Measures of capacity and efficiency
The continuous flow bioassay gave an assayed intensity close to that given by an injection bioassay performed immediately before (Fig. 7). The observed average log survival, using the continuous flow of spores, was - 1.67. On the line of simulated survival vs given intensity values, the observed survival corresponded with an intensity of 13.6 mW/cm ~. In injection bioassays performed immediately before (Fig. 6), the average intensity was estimated as 13.9 ( + 0.9) mW/cm 2 (indicated by arrow in Fig. 7). While the results of these two methods of using the bioassay were close, the injection method is more accurate because it is more direct and the interpretation is simpler. Interpretation of the Bacillus spore survival in the continuous flow experiment without account-
Perhaps the ultimate measure of disinfection capacity of a particular unit is the maximum flow rate at which a given target level of survival can be achieved. We chose a total coliform log survival of - 3 . 5 as a desirable target to meet disinfection standards, allowing for some photoreactivation. A direct comparison of total coliform survival vs flow rate between Units 1 and 2, using the same batch of secondary effluent, is shown in Fig. 8. At a given flow rate, the survival in Unit 1 was lower. By interpolation, the flow rate corresponding to - 3 . 5 log survival units was 6.6 and 4.01/s for Unit 1 and Unit 2, respectively (Table 1). This sort of comparison is difficult to generalize since even the relative flow rates necessary to obtain this survival may vary with the
Fig. 7. Bioassay of intensity using average survival of a continuous flow of spores.
Efficiency of ultraviolet disinfection systems -2
,
i
,
I
i
I 4
'
I
._1
-3t~.
14 L9 O ...I
-5~
i
i
FLOW
i
~
i
8
(I/s)
Fig. 8. Log survival of total coliforms in a Chapel Hill secondary effluent as a function of flow rate.
u.v. absorbance and the sensitivity of coliforms in the particular effluent under consideration. Furthermore, this measure provides no information on the contributions of lamp output, light use efficiency and flow dispersion to the overall efficiency. We have used several measures of both efficiency and capacity to evaluate the u.v. units. Measures of efficiency provide a way to compare not only units of the same size, but units of different size. However, given two units which are equally efficient, one may be much larger and, hence, able to treat much larger volumes. Consequently, we also need measures of capacity which incorporate the size of the units and are proportional to the flow rates which can be handled. Several detailed measures of capacity and efficiency are shown in Table 1. The values for average intensity shown in Table l are all based on bioassays. While Units 1 and 2 had similar total lamp lengths and nominal power input (240W), Unit 2 was less efficient in converting power to 254 nm u.v. output. In fact, we measured the output of the lamp with the surrounding outer quartz tube in place in Unit 1,
323
which should reduce the u.v. output compared to the naked lamp. The product of average intensity times the volume of the irradiation chamber is a factor which is directly proportional to the capacity to treat specified flow rates of water with a given dose under ideal flow conditions. This factor serves to isolate the effectiveness of the intensity regime from the effects of non-ideal flow. The utility of using measures based on the intensity volume product is not that they alone can be used as a rating system, but in isolating how effectively intensity is used in the design. While average intensity in Unit 2 was much lower, its volume was larger. Still, the intensity-volume product was lower. To compare the efficiency of units with different total lamp lengths we used the intensity x volume/ nominal input watts (Table 1). In this respect Unit 2 was only 53% as efficient as Unit 1. A measure of the "light use efficiency" is the intensity x volume/u.v. output. By comparing these two efficiencies, it can be seen that about 1/3 of the difference in efficiency was due to less efficient u.v. output of Unit 2 and the remainder was due to less efficient use of the u.v. output in the irradiation chamber. The lamp ballasts in Unit 2 were found to be relatively inefficient. The less efficient use of the u.v. light output can be explained by the differing geometry of the two units. In Unit 1, the solution surrounds the quartz tubes covering the lamps. In Unit 2, however, the liquidcontaining Teflon intercepted less than half of the arc around the u.v. lamps directly, so that much of the light must be reflected at least once before striking the Teflon. In addition Teflon transmits less u.v. than does quartz. The better residence time distribution of Unit 2 did not compensate for the lower intensity-volume product in the coliform disinfection comparison (Table 1, Fig. 8). As pointed out in Quails and Johnson (1983)
Table I. Comparison of disinfection units Characteristic (I)
Average intensity in wastewater
Unit I
Unit 2
Unit 3
Unit 4
12.1"
2.4*
5.8*
2.8t
(mW/crn2) (2)
Average intensity in tap water:[:
25.6
5.7
(3) (4)
Fluid volume (I) Intensity x volume (roW/ore2 x 1) Ultraviolet output/power input
11.2 136
30.2 72
378 2191
90/240
71/240
1160/4000
0.57
0.30
0.55
0.13t
1.5
1.0
1.9
0.52t
2.27
1.22
1.85, 2.08**
1.30
6.6
4.0
89.7¶1
20.8t§
(5) (6) (7) (8) (9)
(mW/cm2)
(w)
I x V/input watts (mW/cm 2 x l/w) I x V/u.v. output (mW/cm x l/w) Morrill dispersion Index tgo/ho Simulated max. flow (I/s) for
-3.5 log S and abs ffi0.2
10.9
4.0 314~ 754t 1462/5760
*At an absorbance (base 10) of 0.2. t a t an absorbance (base 10) of 0.16. ~:At an absorbance (base 10) of 0.035. §Assuming all tubes in unit are small size tubes. ¶lSimulation yields an extrapolated value which is probably an overestimation since such high flow rates could not be used in that unit. **Center and side, respectively.
324
ROBERTG. QUALLSe t al.
the log survival vs dose curves of coliforms in wastewater effluents have a "steep" portion and a relatively level portion at higher doses. The RTD of Unit 1 was good enough so that an insignificant fraction of the flow received doses corresponding to the "steep" portion of the coliform dose-survival curve where the dispersion would seriously affect the survival. Using the information in Table 1 on the relative contribution of various factors to the efficiency of the prototype Unit 2, the design of the unit was modified by the manufacturer. Unit 3 had not only the highest intensity-volume product, but it had the highest light-use efficiency (Table 1). The proximity of the chamber walls acting to absorb some of the available u.v. radiation in Unit 1 apparently reduced the efficiency somewhat compared to Unit 3. However, the average RT of the flow in the central portions of Unit 3 was 0.82 times the theoretical RT for the entire unit. This uneven flow distribution might have significant effects on the average survival, if substantial amounts of flow receive doses corresponding to the steeply declining portions of the dose-survival curve (Quails and Johnson, 1983). For the purpose of making the comparisons shown in Table 1, we made the assumption that all the tubes in Unit 4 were 5.99 cm dia tubes although 8 were 8.89 cm in dia. The bioassay of Unit 4 was not performed at the same fluid absorbance as for Unit 3, so the capacity and efficiency terms were not directly comparable. However, while Unit 4 had the largest volume and lamp wattage, the efficiency was lowest of all the units and would have appeared even lower at an absorbance of 0.20. This was due partly to the low output which we measured from the bulbs, many of which were old, but mainly to low light-use efficiency. Scheible et al. (1986) presented extensive coliform disinfection data comparing Units 3 and 4. They also concluded from disinfection data that Unit 4 was not as energy efficient as Unit 3. Scheible (1986) noted that the u.v. transmission of the Teflon tubes decreased significantly over several months, even after cleaning. Characteristics 4 through 8 in Table 1 give information on the efficiency and capacity of specific components of the overall system. There is no simple single parameter based on physical measurements alone which integrates these components into a balanced measure of the ultimate effectiveness of the system. However, the maximum flow rate at which a - 3 . 5 log survival of coliforms can be achieved is a measure which integrates the effects of the intensity, volume and flow characteristics as well as providing an easily interpreted and realistic comparison of units operated under different conditions. However, it would be impossible to directly measure this parameter, as was done in Fig. 8, for many different units at different locations because of varying dose-survival curves and absorbances. A simulation using measured intensity and RTD data, along with a
standard coliform dose-survival curve, has the advantage of wide applicability. Simulations were run using (i) the average intensity data (Table 1), and RTD data gathered in this study (Fig. 4); (ii) a simple model detailed in Quails and Johnson (1985); and (iii) a standard coliform dose-survival curve based on the average of 36 samples from 6 types of wastewater effluents (Fig. 2 in Quails et al., 1985). The simulated maximum flow rates for each unit are listed in Table 1. Because of the good flow characteristics of Units 1, 2 and 4, the estimated maximum flows were close to that predicted by simply dividing intensity × volume by the dose corresponding to - 3 . 5 log survival (20mWs/cm) on the standard curve. The value for Unit 3 represents a considerable extrapolation because flow rates that high could not be pumped into the installation and we could not determine whether the RTD remained the same high flow rates. Nevertheless, the uneven flow distribution was not bad enough to seriously effect the average survival under the model conditions. The simulation of the maximum flow rate necessary to achieve - 3 . 5 log survival provides a single easily interpreted overall parameter for comparison. In cases where more specific data on u.v. absorbance and the dose survival curve can be anticipated, these may be substituted for standardized data in the simulations. The relative capacities of various units may change as the u.v. absorbance changes. CONCLUSIONS
The analysis presented here illustrates a means of quantitatively isolating the factors involved in the overall efficiency of a u.v. disinfection system. This approach should be useful not only in developing designs but provides a uniform means of comparing u.v units. The information also provides the necessary input to model the performance of a unit at a particular installation where dose-survival data are available. Acknowledgements--We gratefully acknowledge the cooperation of O. K. Seheible and W. Dunn of HydroQual Inc., and A. Forndran and G. Cox of the City of New York, DEP, without whose help the demonstration of these procedures on the large scale systems would have been impossible. We also appreciate the contributions of students in the Department: C. M. Dumais, J. C. Chang, S. F. Ossoff, D. Lobe; J. Cruver and T. Creeden of Enerco Corp.; and the personnel of the Chapel Hill wastewater treatment plant for allowing construction of a pilot unit, as well as the comments of anonymous reviewers. This research was funded by Grant No. CEE-8205274 from the National Science Foundation, Dr Edward Bryan, project officer. REFERENCES
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Quails R. G., Ossoff S. F., Chang J. C. H., Dorfman M. H., Dumais C. M., Lobe D. C. and Johnson J. D. (1985) Factors controlling sensitivity to UV disinfection in secondary effluents. J. Wat. Pollut. Control Fed. 57, 1006-1046. Scheible O. K. (1986) Ultraviolet radiation. In Design Manual: Municipal Wastewater Disinfection, pp. 157-247. EPA No. EPA/625/1-86/021, U.S. Environmental Protection Agency, Cincinnati, Ohio. Scheible O. K. (1987) Development of a rationally based design protocol for the ultraviolet light disinfection process. J. Wat, Pollut. Control Fed. 59, 25-31. Scheible O. K. and Bassel C. D. (1981) Ultraviolet disinfection of a secondary wastewater treatment planet effluent. EPA-600/S2-81-52, U.S. Environmental Protection Agency, Washington, D.C. Scheible O. K., Casey M. C. and Forndran A. (1986) Ultraviolet disinfection of wastewaters from secondary effluent and combined sewer overflows. EPA No. 600/$2-86/005, U.S. Environmental Protection Agency, Cincinnati, Ohio. Severin B. F. (1980) Disinfection of municipal wastewater effluents with ultraviolet light. J. Wat. Pollut. Control Fed. 52, 2007-2018. Tobin R. S., Smith D. K., Horton A. and Armstrong V. C. (1983) Methods for testing the efficiency of ultraviolet light disinfection devices for drinking water. J. Am. Wat. Wks Ass. 75, 481. Whitby G. E., Palmateer G., Cook W. G., Maarschalkerwerd J., Huber D. and Flood K. (1984) Ultraviolet disinfection of secondary effluent. J. Wat. Pollut. Control Fed. 56, 844-850. White S. C., Jernigan E. B. and Venosa A. D, (1986) A study of operational ultraviolet disinfection equipment at secondary treatment plants. J. Wat. Pollut. Control Fed. 58, 181-192.