The Importance of Water Absorbance/Transmittance on the Efficiency of Ultraviolet Disinfection Reactors

Transcription

1 The Importance of Water Absorbance/Transmittance on the Efficiency of Ultraviolet Disinfection Reactors James R. Bolton, Mihaela I. Stefan Bolton Photosciences Inc. 92 Main St., Ayr, ON, Canada NB 1E Robert S. Cushing and Erin Mackey Carollo Engineers West Explorer Drive, Suite #2, Boise, ID, Introduction It is generally known that the efficiency of ultraviolet disinfection reactors decreases as the absorbance of the water increases or the percent transmittance decreases; however, little is known about the quantitative impact of this parameter, unless extensive computational fluid dynamics (CFD) calculations are undertaken or biodosimetry tests made. In this paper calculations, using a new Multiple Point Source Summation (MPSS) Model, are presented in which the quantitative effect of water transmittance is assessed for several aspects of the performance of a UV reactor. For UV reactors containing low pressure or low pressure-high output UV lamps, the important parameter is the absorbance or percent transmittance at 254 nm. However, for broadband UV lamps, such as medium pressure or pulsed UV lamps, it is important to measure the full scan of absorbance or transmittance in the germicidal region from 2 to 3 nm. Thus, where necessary, measured the molar absorption coefficient spectra have been collected for a variety of compounds that may be present in, or added to, drinking water. This data will be presented in tabular and graphical formats. The Multiple Point Source Summation (MPSS) Model has proven useful in mapping the fluence rate a distributions and in calculating the volume averaged fluence rate in UV reactors. Recently, Bolton 1 presented a new MPSS model that takes account of the reflection and refraction that occurs at the interface between the quartz sleeve and the water in the UV reactor. This model has been used to calculate several characteristics of the fluence rate and fluence, as a function of the percent trasmittance of the water, in an annular UV reactor with a single UV lamp in the center of a quartz sleeve: 1. Volume averaged fluence rate multiplication of this parameter by the hydraulic residence time (s) gives the maximum fluence a that the UV reactor is capable of delivering for a given flow rate. It is a maximum value because the calculation assumes perfect radial mixing. a In this paper, we are using the terms and definitions recommended in Ref. 2. Most authors have used the terms irradiance or intensity for the quantity here designated as fluence rate and UV dose for the quantity here designated as fluence. See the Section on Terms and Definitions below.

2 2. Irradiance at a fixed position in the UV reactor this position, for example, could be the position of a UV sensor. This will provide information on how the sensor will respond to changes in the percent transmittance of the water. 3. Penetration depth these calculations provide plots of the penetration depth (cm) at which the fluence rate drops by 95% and 99% of its value at the quartz sleeve. This information can be quite useful in the design of UV reactors. Terms and Definitions (see Ref. 2) Absorbance (A) the absorbance of a solution at a given wavelength is given by A = log(e o /E l ), where E o and E l are the irradiances incident on the cell and transmitted through a path length l, respectively. In practice, because of reflections at the quartz/air interfaces, E o is the spectrophotometer reading with pure solvent in the cell and E l is the reading with the solution of interest in the cell. Absorption Coefficient (a) the absorbance divided by the path length (l). Units m 1 or cm 1. Transmittance from the Beer Lambert Law, the percent transmittance in a 1 mm path length (%T 1 ) is related to absorption coefficient (a) by: %T 1 = 1 1 -al, where l = 1 cm and a is in cm 1. Molar absorption coefficient the absorbance (A) of a solution at a given wavelength is given by the product εcl, where ε is the molar absorption coefficient (M 1 cm 1 ), c is the concentration (M) of the absorber and l is the path length (cm). Irradiance (symbol E; units W m 2 or mw cm 2 ) is defined as the total radiant power incident from all upward directions on an infinitesimal element of surface of area da containing the point under consideration divided by da. Fluence rate (symbol E ; units W m 2 or mw cm 2 ) is defined as the total radiant power incident from all directions onto an infinitesimally small sphere of cross-sectional area da, divided by da. Fluence (also called UV dose) (symbol H, units J m 2 or mj cm 2 ) is the total amount of radiant energy from all directions incident on an infinitesimally small sphere of crosssectional area da, divided by da. Note that fluence rate is the appropriate term (rather than irradiance) for UV disinfection applications because a microorganism can receive UV from any direction. Also, fluence is more appropriate term than UV dose, since only a small fraction of the UV incident on a microorganism is absorbed.

3 Measurement of Absorbance Spectra for a Variety of Possible Absorbers in Drinking Waters Solutions of various absorbers were made up within the concentration range ~1 4 1 M, so that reasonable absorbance readings could be taken, and then the absorption spectra (in 1 cm quartz cells) were recorded against DI water as a blank on a Hewlett Packard Model 845 Diode Array Spectrophotometer. Where required, the ph was adjusted with sodium hydroxide solution. Table 1 gives a summary of the molar absorption coefficients at 254 nm for the compounds studied, and Figures 1a and 1b show the absorption spectra for those compounds that absorb significantly in the 2 3 nm range. Table 1. Summary of Molar Absorption Coefficients at 254 nm Compound Molar Absorption Coefficient (M -1 cm -1 ) ph Ammonia (NH 3 ) 11.5 Ammonium (NH + 4 ) 7. Calcium ion (Ca 2+ ) 6.5 Ferric [Fe(OH) 2+ ] 4, Ferrous (Fe 2+ ) Hydrogen peroxide (H 2 O 2 ) Hydroxide ion (OH - ) 13.3 Hypochlorite (ClO - ) Magnesium ion (Mg 2+ ) 6. Manganous ion (Mn 2+ ) 3.6 Ozone (O 3 ) (aqueous) 3,25 7 Permanganate ion (MnO - 4 ) Phosphate ion species (H 2 PO - 4, HPO - 4 ) 5-9 Sulfate (SO 2-4 ) 7. Sulfite (SO 2-3 ) Zinc ion (Zn 2+ )

4 molar absorption coefficient (M -1 cm -1 ) Fe 3+ H 2 O 2 x wavelength / nm OCl x 5 Fe 2+ x 5 molar absorption coefficient (M -1 cm -1 ) MnO 4 SO wavelength / nm Figures 1a (upper) and 1b (lower) showing the molar absorption coefficient spectra of drinking water components that absorb significantly in the 2 3 nm range. O 3

5 Fluence Rate Calculations All of the calculations in this Section were carried out with the software program UVCalc (Bolton Photosciences Inc.), which incorporates the mathematics described in Ref. 1). The parameters for the calculations were chosen to match a 1-lamp annular reactor with the following characteristics given in Table 2. Table 2. Characteristics of the UV reactor used for the calculations Characteristic Value Lamp type Low-pressure mercury Lamp length 12 cm Lamp power 4 W Lamp efficiency (254 nm) 33% Quartz diameter 2. cm Reactor length 13 cm Reactor diameter variable %T 1 of the water variable Volume averaged fluence rate In this case a reactor with the characteristics of Table 2 and a reactor diameter of 1 cm was chosen. The results are given in Figure 2. The fluence is for a flow rate of 5 gpm. Note that this is a maximum fluence, since it assumes perfect plug flow and radial mixing. Real UV reactors will deliver fluences much lower than the indicated values because of imperfect mixing. As can be seen the calculated performance of the reactor depends strongly on the transmittance of the water. ` Irradiance at a fixed position in the UV reactor In this case, the fluence rate was calculated at a position corresponding to the middle of the UV lamp and at the outside wall (5 cm from the lamp). Here the fluence rate should be close to the irradiance, since it is at a moderate distance from the lamp. This position could be that of a UV sensor monitoring the lamp, where the UV sensor would measure the irradiance incident on the sensor at that location and within its acceptance angle. Figure 3 shows the results of this calculation. Note that the calculated sensor reading falls off much more quickly than the fluence does, as seen from the ratio in Figure 3. Thus the sensor reading is not proportional to the fluence, and thus cannot be used (even in a relative sense) to monitor the fluence.

6 Volume Averaged Fluence Rate (mw cm -2 ) Fluence (mj cm -2 ) %T 1 Figure 2. Volume average fluence rate in the UV reactor as a function of the percent transmittance of the water. The fluence on the right axis has been calculated for a flow rate of 5 gpm. The dotted line corresponds to a common minimum fluence standard for disinfection safety. Sensor Irradiance Reading (mw cm -2 ) Ratio (s -1 ) %T 1 Figure 3. Sensor irradiance reading (at the wall 5 cm from the lamp center) (left axis) versus the percent transmittance. The blue curve (right axis) is the ratio of the sensor irradiance reading to the calculated fluence.

7 Penetration depth In this case, the fluence rate distribution was calculated as a function of the percent transmittance. For each %T 1 value, the depth (cm) was calculated at which the fluence rate had dropped by 95% and 99% of its value at the quartz sleeve. Figure 4 gives the results. This graph should be quite useful in the design of annular or near-annular UV reactors. For drinking waters with percent transmittance values >9%, the distance from a lamp to the wall should be >1 cm. For waste water (%T 1 values 5 65%), the corresponding distance should be <4 cm. UV reactors with a diameter larger than these limits will tend to have a low mixing efficiency, since large volumes of the reactor will be essentially dark. UV reactors with diameters much less than these limits will waste UV absorbed at the walls Penetration depth (cm) from quartz sleeve 24 2 drop by 99% drop by 95% %T 1 Figure 4. UV penetration depth (out from from the quartz sleeve) versus percent transmittance. The red curve is for the penetration depth at which the fluence rate has dropped by 99% of its value at the quartz sleeve. The blue curve is the corresponding one for 95%. One should note that the calculations presented in this paper are for an ideally mixed UV reactor. Although real reactors will not be ideally mixed, these calculations nevertheless provide some design guidelines that should allow a better choice of a starting design for further improvements by, for instance, CFD calculations. Conclusions 1. Of the compounds studied, only ferric ion, ferrous ion, hydrogen peroxide, hypochlorite ion, permanganate ion, ozone and sulfite ion absorb significantly in the 2 3 nm region.

8 2. The volume averaged fluence rate and the fluence are strong functions of the percent transmittance of the water. 3. The calculated fluence rate at a sensor position is not proportional to the fluence and drops off more rapidly as the percent transmittance decreases. 4. The penetration depth to 5% and 1% is a strong function of the percent transmittance. References 1. Bolton, J. R. Calculation of ultraviolet fluence rate distributions in an annular reactor: Significance of refraction and reflection, Wat. Res. 2, 34, Bolton, J. R. "Terms and definitions in ultraviolet disinfection"; Water Environment Federation: Proc. Disinfection 2 Conf., New Orleans, LA, 2, Water Environment Federation, Alexandria, VA.