Interpreting UV Reactor Validation Results: Impact of Key Design Parameters on Performance of Closed Vessel Reactors

Transcription

1 Interpreting UV Reactor Validation Results: Impact of Key Design Parameters on Performance of Closed Vessel Reactors Robert Kelly, Bruno Ferran, Shanshan Jin Infilco Degremont, Inc. Degremont North American Research & Development Center 510 East Jackson Street Richmond, VA Abstract The Long Term 2 Enhanced Surface Water Treatment Rule (LT2ESWTR) dictates that for a DWTP to receive credit for UV inactivation of chlorine resistant pathogens such as Cryptosporidium and Giardia, validation testing must be performed to demonstrate that the UV reactor equipment delivers the required dose. This paper summarizes an extensive bioassay validation program undertaken at the DVGW testing center in Germany with the objective to evaluate the performance of two closed-vessel UV reactors for the disinfection of drinking water. A cross-flow in-line reactor and an axial-flow bottom-through-top reactor operating with 4-kWatt medium pressure lamps were tested using the UV intensity set-point method and Bacillus subtilis spores as a pathogen indicator. The paper reviews the impact of key design parameters on the disinfection performance as measured during bioassay validation testing. Key design parameters evaluated include sensor to sleeve distance, approach hydraulics, reactor inlet and outlet configuration and doped versus non-doped sleeves. It was determined that optimization of sensor to sleeve distance is vital to yielding maximum reactor performance. Increasing the initial sensor to sleeve distance produced beneficial results. Under conditions where the inlet pipe diameter was smaller than the diameter of the cross-flow reactor, the resulting approach hydraulics yielded a high inlet velocity, described as a core jet, and subsequent degradation of performance. The implementation of a stilling plate restored performance to some extent particularly with low flow conditions. No significant impact on reactor performance was observed by using doped versus non-doped sleeves. Careful consideration and optimization of the design parameters outlined herein is a vital component of any reactor validation program, ensuring that UV reactors can deliver the proper UV dose over a wide range of water quality and flow conditions experienced at water treatment facilities. Introduction Since the discovery in the late 1990's that chlorine resistant pathogens such as Cryptosporidium and Giardia could be effectively inactivated by ultraviolet radiation, the disinfection of drinking water using UV technology has been the focus of increasing attention from municipalities and water treatment professionals in the United States. The US EPA is currently working on a new set of regulations to further control microbial pathogens and disinfection byproducts, which include the Long Term 2 Enhanced Surface Water Treatment Rule (LT2ESWTR) and Stage 2 Disinfectants and Disinfection Byproducts Rule (S2DBPR). UV disinfection will be proposed as one of the disinfection alternatives covered 1

2 by these new treatment rules. With the objective to provide guidance for the utilities, state authorities and equipment manufacturers, the US EPA will include in this new set of regulations a guidance manual on UV disinfection of drinking water (US EPA, 2003). In this guidance manual it is recommended that a UV reactor delivered dose (RED) be validated for a utility to receive credit for inactivation of a target microorganism such as Cryptosporidium, Giardia or Adenovirus. From the validation work, a set of operating conditions that can be easily monitored in the field must be determined to ensure that sufficient disinfection is provided. UV reactors can be validated through on-site or off-site testing. On-site validation is more accurate in representing the hydraulic conditions and water quality of a specific UV installation. However, the range of operating conditions for the UV reactor is often limited and concerns with releasing surrogate microorganisms in the plant discharge represent significant drawbacks to on-site validation. On the contrary, off-site validation provides results that can be applied to a wide variety of treatment scenarios making this a cost alternative. For this reason, most equipment manufacturers choose off-site validation as the first option for UV reactor validation. It is crucial to ensure that the range of conditions tested during off-site validation covers the design criteria and hydraulic configurations of the future full-scale UV installations. In order to achieve this goal, the validation protocol of the off-site validation testing needs to be carefully developed. Between September 2002 and February 2003 an extensive bioassay validation program was undertaken by IDI at the DVGW testing center near Koln, Germany in order to evaluate the performance of two closed-vessel UV reactors for the disinfection of drinking water using medium pressure (MP) lamps as a light source. The impact of a series of key design parameters on the disinfection performance as measured during bioassay validation testing was evaluated throughout the testing process. Because the UV intensity setpoint approach was chosen as the means to monitor dose delivery in IDI s drinking water UV reactors the first parameter evaluated was the UV sensor to sleeve distance. Additional key parameters included reactor approach hydraulics, inlet and outlet piping configuration, dose additivity with downstream reactors and doped versus non-doped quartz sleeves. The use of doped versus non-doped quartz material for lamp sleeves has recently raised some controversy. German UV reactors using MP lamps are required to incorporate doped quartz sleeves, which cut off wavelengths below 240 nm (DVGW, 2003) mainly to prevent nitrate (NO 3 - ) conversion to nitrite (NO 2 - ). However, the use of such sleeves has been reported to reduce germicidal UV output of MP lamps thereby increasing the cost of UV reactor systems. US EPA regulations do not require the use of doped sleeves to prevent disinfection by product (DBP) formation. In fact, a study on MP UV disinfection using non-doped quartz sleeves and surface water indicated that with non-doped sleeves, the nitrate (NO 3 - ) to nitrite (NO 2 - ) conversion was below the more stringent European regulations (Sharpless and Linden, 2001) for a typical range of applied UV doses (40 mj/cm 2 and higher). The objective of this paper is to review the impact of the aforementioned key design parameters in order to demonstrate proper design consideration and data interpretation to be useful for a wide range of field applications. 2

3 Materials and Methods UV Reactors The two reactors subjected to validation testing were closed - vessel reactors for drinking water applications. Reactor A is 20" in diameter and contains six medium pressure lamps operating with 4-kWatt electrical per lamp. Because water flows across the reactor and lamp arc, reactor A is referred to as a cross flow, in - line reactor (Figure 1). Reactor B is 8" in diameter and is a single lamp reactor. The type of UV lamp is the same as that used in reactor A. Flow for reactor B runs through 6 flanges from the bottom to the top of the reactor and parallel to the lamp arc. Reactor B is therefore referred to as an axial flow, bottom - through - top reactor (Figure 2). Inlet Outlet Figure 1 Reactor A: Cross-Flow, In-line Reactor Figure 2. Reactor B: Axial-Flow, Bottom - Through - Top Reactor 3

4 Testing Site and Source Water The validation testing facility was the Meindorf DWTP located at Sankt Augustin, Germany. Figure 3 illustrates the flow schematic of the testing setup according to DVGW W 294 standards. The surrogate microorganism and a UV absorbant are injected in-line upstream of the UV disinfection unit(s). Static mixers are installed upstream and downstream of the inlet and outlet sampling ports, respectively, to provide proper mixing of the microorganisms and UV absorbant with the water matrix. Direct groundwater was used as the water matrix for the validation testing (Well No 4). The table below summarizes the characteristics of this source water. Property Value HPC at 20 o C and 36 o C Below detection limit E. coli Below detection limit Fecal coliforms Below detection limit UV Transmittance (T 254 ) 97.9% Conductivity 56 ± 2 ms m -1 Temperature 11 ± 0.5 o C During testing lignin sulfonate (Zewakol MG55S Zell Wildshausen, Ltd., Düsseldorf, Germany) was used to adjust UV Transmittance of the water. The spectral transmittance of lignin sulfonate is shown in Figure 4. Figure 3. Schematic of Testing Setup according to DVGW Standard W 294 (Adapted from Hoyer 2001) 4

5 Figure 4. Spectral transmittance of lignin sulfonate used as UV absorbant for the validation testing. Value indicated in each curve is the absorption coefficient of the water in m -1 as measured at 254 nm. Biodosimetry Biodosimetry testing was conducted according to DVGW Standard W 294 part 2 (DVGW 2003). Bacillus subtilis spores (ATCC 6633) were used as the surrogate microorganism. For each batch of spores, a UV inactivation curve was produced using a bench scale collimated beam irradiation apparatus following the methods described in W 294 part 2, chapter 7. During reactor challenge testing with B. subtilis spores, five samples were taken in duplicate upstream and downstream of the UV reactor. For each inlet sample, triplicate plate counts were performed to obtain the average spore count for each sample. The Log counts from the five samples were then averaged to represent initial spores count (Log N 0 ). For each downstream sample, 1 to 3 ml of sample was applied per plate in triplicate to reach the appropriate dilution resulting in a colony count between 20 and 200 per plate. Once the appropriate volume per plate was known, all 15 counts from the five triplicates were averaged to obtain the survival spore count N and Log N. Note that the test run results were deemed valid when the standard deviation on the final spore count for both influent and effluent samples was close to 0.05 with a maximum allowable value of 0.2. A Reduction Equivalent Dose (RED) was then assigned to the UV reactor by comparing the reactor log inactivation to the collimated beam dose response curve for the same batch of spores. UV Intensity Set-point Method Each lamp of reactor A is equipped with a duty SDW1 sensor, as illustrated in Figure 5 below. Each SDW1 sensor fits in a port equipped with three external grooves that allow for the position of the port with respect to the sleeve to be changed. Note that reactor design prevents adjustments of sensor ports 3 and 6. Reactor B, a single lamp reactor, is therefore equipped with only one SDW1 duty sensor and the identical port design. 5

6 Figure 5. Reactor A Cross Sectional View Sensor and Sleeve Position With the UV intensity set point method, the reactor dose delivery is monitored based on flow and UV intensity without consideration of UV transmittance. For this monitoring approach to be efficient the reactor sensors should be located at a set distance from the lamp sleeve that yields an intensity reading proportional to the UV delivered dose, given any combination of lamp power setting and water quality condition. The position of sensor ports 1, 2, 4 and 5 of reactor A were adjusted during challenge testing to optimize the sensor readings in relation to the reactor disinfection performance expressed as RED. Note that for reactor B the sensor to sleeve distance initially determined based on CFD modeling was found to be optimal and did not require any adjustment during challenge testing. Each reactor challenge test consisted of first determining an Application Layout Setting (ALOS) for a given sensor to sleeve distance. Under this ALOS the lamp output level was dimmed to 70%, which is the expected end-of-life lamp output level, and the UV transmittance was set to the lowest design value in the range of 80% to 95%. The readings obtained from the DVGW reference sensor (MUV 505) in ports 1 and 5 of reactor A and in the only port of reactor B were considered as the sensor setpoint readings under the ALOS. Note that initial work performed on reactor A showed that sensor readings from ports 1 and 5 were equivalent to sensor readings from ports 2 and 4. Each reactor was then set at the maximum lamp output available (4 kwatt electrical) with the water transmittance lowered using lignin sulfonate until the ALOS sensor setpoint reading was reached. This setting resulted in the Test H condition. Conversely, the test L condition was obtained using the untreated source water with the highest transmittance available (97.9%) while the lamp power was dimmed to reach the ALOS sensor setpoint reading. The sensor position for reactor A or B was deemed optimal when the difference in delivered dose (RED) between the H and L test conditions was less than +/- 2 mj/cm 2, which corresponds to the uncertainty of the biodosimetry testing. 6

7 Hydraulic Configuration Reactor A was tested on two different test rigs to simulate different approach hydraulic conditions. The medium or M-rig, as illustrated in Figure 6a, was equipped with 300 DN inlet and outlet piping, which is smaller than the reactor diameter of 20. Two 90 o bend elbows (A & B) were placed 500 mm apart immediately before the reactor inlet. The large rig or L-rig, as illustrated in Figure 6b, was equipped with 600 DN inlet and outlet piping and the two elbows (A & B) were placed 1,300 mm apart, immediately before the reactor inlet. The purpose of the double elbow configuration was to downgrade the approach hydraulics, which in theory should decrease UV disinfection performance and therefore provide a conservative delivered dose for use in the field. Because reactor B is relatively small in size there is greater potential for irregular inlet and outlet piping configurations. Using CFD modeling, a set of internal baffles was designed with the objective to reduce the impact of reactor inlet and outlet piping configurations on disinfection performance. With the objective to confirm the effectiveness of the baffles reactor B was bioassayed with various combinations of inlet and outlet piping configurations, as illustrated in Figure 7. A B A B Figure 6. a) M-rig Piping Configuration b) L-rig Piping Configuration AXIAL Configuration Figure 7. Inlet and Outlet Piping Configurations Tested for Reactor B 7

8 Delivered Dose Additivity with Downstream Reactors Optimum UV disinfection efficiency should be accomplished through the use of a reactor having plug flow with a small amount of axial mixing. When two reactors are connected in series the axial mixing created by the upstream unit may improve hydraulics in the downstream unit therefore increasing its disinfection performance. This assumption was put to the test during the biodosimetry testing of two A type arranged in series on the L rig. Doped versus Non-doped Quartz Sleeves The disinfection performance of reactor B with both doped and non-doped sleeves was compared at flows of up to 0.72 MGD. The ALOS transmittances tested for both types of sleeves were 90% and 95%. The non-doped sleeve was made of standard GE 214 quartz manufactured by Enterprise Q Ltd. in Manchester, UK. The doped sleeve was made of PN235quartz and manufactured by Quarzschmelze llmenau GmbH. in Langewiesen, Germany. Results and Discussion Impact of Sensor Position For reactor A, a 40 +/- 2 mm gap between sensor port and sleeve was first selected for all sensors of the reactor. This initial sensor to sleeve distance was based on fluence rate modeling and particle dose tracking. Clearly, these calculations were more qualitative due to the complexity of the modeling and this sensor position was only used as a starting point. As illustrated in Figure 8, the delivered dose obtained from the test L conditions was much higher than that obtained from test H. Although, it is logical to expect a lower delivered dose with test H since it involves operating the reactor under a low transmittance, the difference with test L should not be so great. Consequently, sensor ports 1, 2, 4 and 5 were moved away from the lamp sleeves to 75 +/- 2 mm and finally to 110 +/- 2 mm distance with the objective to reduce the difference in delivered dose between tests L and H. As shown on Figure 8 the delivered dose values from test H and test L became closer to each other as the sensor to sleeve distance increased. At a sensor to sleeve distance of 110 +/- 2mm, the delivered dose from test L was only 5 mj/cm 2 higher than that of test H. 8

9 35 30 RED (mj/cm 2 ) Sensor to Sleeve Distance (mm) MGD - 90% UVT 6.0 MGD - 95% UVT Layout Setting Figure 8. Test L RED Minus Test H RED for Various Sensor to Sleeve Distances (M-rig) When reviewing the overall results from the bioassay testing of both reactors A and B the test H delivered doses were consistently lower than the results obtained for test L. This was probably because the Lignin Sulfonate absorbs UV light significantly below 240 nm (Figure 4). The test H settings (high absorbance, high lamp output) correspond to a worst case condition resulting in conservative delivered doses, which are deemed final for reactor performance evaluation. It is indicated from the above analysis that optimization of sensor to sleeve distance is vital to yielding maximum reactor performance. The fluence rate modeling and particle tracking calculations underestimated the optimal sensor to sleeve distance. Increasing initial sensor to sleeve distance produced beneficial results. A sensor to sleeve distance that is not optimized would lead to a low delivered dose under the test H conditions, which then results in a low reactor final delivered dose and a significant over-sizing of full-scale UV systems. Impact of Approach Hydraulics with Reactor A Figure 9 shows that for all flows and ALOS water transmittances tested reactor A yielded delivered doses that were consistently lower when mounted on the M - rig as compared to the L - rig. 9

10 Delivered Dose (mj/cm 2 ) % 90% 95% Mrig, 95% UVT Mrig, 90% UVT 15 Mrig, 85% UVT 10 Lrig, 95% UVT Lrig, 90% UVT 5 Lrig, 85% UVT Flow Rate, MGD Figure 9. Test H Delivered Doses from Reactor A (M-rig versus L-rig Configuration). From the data presented in Figure 9 the reactor A when mounted on the L-rig produced a delivered a dose of 40 mj/cm 2 under flow conditions of 9.0, 5.5 and 3.5 MGD for ALOS transmittances of 95%, 90% and 85% respectively. When reactor A is mounted on the M- rig a much lower flow of 6.5 MGD at an ALOS transmittance of 95% resulted in a delivered dose of 40 mj/cm 2. It is thought that under conditions where the inlet pipe diameter is smaller than the diameter of the cross-flow in-line reactor such as M - rig configuration, the resulting approach hydraulics yielded a highly unbalanced inlet velocity profile, described as a core jet and subsequent degradation of performance. With the objective to homogenize this inlet velocity profile a stilling plate was inserted approximately 8 upstream of reactor A. Figure 10 below shows the stilling plate, which was designed to block 32 percent of the reactor cross sectional area. Figure 11 depicts the impact of the stilling plate on the test H delivered dose. 10

11 Figure 10. Stilling Plate Design Delivered Dose (mj/cm 2 ) % 90% UVT, NSP 90% UVT, SP 95% UVT, NSP 95% UVT SP 85% UVT, NSP 85% UVT, SP 90% Flow rate (MGD) Figure 11. Impact of Stilling Plate on Reactor A Test H Delivered Doses Figure 11 illustrates that for all flow and ALOS transmittance conditions tested that the installation of a stilling plate caused the delivered doses to increase by up to 20%. This clearly improves the disinfection performance of reactor A when mounted on the M-rig. However, use of an inlet stilling plate does result in significant increases of reactor head loss as shown in Figure 12 below. 95% 11

12 Head Loss in Inches of WC Reactor A Flow in MGD Figure 12. Head Loss for Reactor A with and without the Stilling Plate Reactor A + Stilling Plate The results presented in this section demonstrate the importance of approach hydraulics when considering the design of a UV disinfection system using cross-flow in-line UV reactors. Clearly, the piping configuration used in this validation testing where a pipe having a diameter significantly smaller than that of the reactor and with reducers mounted as close as 8 upstream and downstream is an extreme case that should be avoided for fullscale application. The stilling plate improved reactor A delivered dose however at the expense of a dramatic increase in head loss that would be prohibitive for full-scale UV systems that are gravity fed. The draft US EPA Disinfection Guidance Manual (US EPA 2003) specifically recommends that a UV reactor be installed with a minimum 5 pipe diameters of straight piping between the reactor and any upstream hydraulic configuration. Impact of Inlet & Outlet Piping Configuration for Reactor B As shown in Figure 7, reactor B was connected horizontally in AXIAL, CIS and TRANS configurations and bioassayed under ALOS transmittances of 90% and 95%. The table below compares the test H delivered doses obtained between the two aforementioned hydraulic configurations. 12

13 Test Date Flow (MGD) Configuration (*) UVT (%) RED (mj/cm 2 ) 11/18/ AXIAL /8/ CIS /8/ TRANS /12/ AXIAL /8/ CIS /8/ TRANS (*) Refer to Figure 7 The delivered dose values presented in the table above are not statistically different considering the RED uncertainty of +/- 2 mj/cm 2. For flows of up to 0.57 MGD the AXIAL, TRANS or CIS piping configuration has no impact on reactor B disinfection performance. Delivered Dose with Multiple A Type Reactor in Series Figure 13 below depicts the dose flow curves obtained with two reactors A mounted in series on the L-rig Delievred Dose (mj/cm 2 ) % 2, 95%UVT 2, 90% UVT 2, 85% UVT 1+1, 95% UVT 1+1, 90% UVT 1+1, 85% UVT 90% 95% Flow Rate, MGD Figure 13. Test H Delivered Doses from two A type Reactors in Series Typical ALOS transmittances of 85%, 90% and 95% were tested. Figure 13 shows that for each transmittance outlined above and for a delivered dose of 50 mj/cm 2 the flow capacity of two type A reactors in series is significantly greater than the sum of two individual A 13

14 type reactors. A synergistic effect occurred between the two reactors, where the most upstream reactor improved the performance of the downstream reactor by creating additional axial mixing and therefore improving the approach hydraulics. The findings described in this section confirm the conservative nature of the bioassay results from a single reactor A when mounted downstream of a double elbow configuration. Impact of Doped versus Non-doped Sleeves for Reactor B Figure 14 below compares the dose flow curves from reactor B obtained at ALOS transmittances of 80%, 85%, 90% and 95% with non-doped sleeves versus doped sleeves. Delivered Dose (mj/cm 2 ) % Non-doped 95% UVT Non-doped 90% UVT Non-doped 85% UVT Non-doped 80% UVT Doped 95% UVT Doped 90% UVT Doped 85% UVT Doped 80% UVT 30 85% 90% 95% Flow Rate, MGD Figure 14. Test H Delivered Doses from Reactor B with Non-doped and Doped Sleeves As depicted in Figure 14 reactor B was produced a delivered dose of 40 mj/cm 2 under flow conditions of 0.88, 0.55 and 0.35 MGD for ALOS transmittances of 95%, 90% and 85% respectively. Figure 14 also shows that for all ALOS transmittances doped and non-doped sleeves yielded similar delivered doses. Using doped sleeves versus non-doped sleeves did not have a significant impact on reactor B disinfection performance. This finding confirms the generally accepted knowledge that radiation below 240 nm from medium pressure lamps does not significantly contribute to disinfection performance due to the low UV absorbance of DNA bases. However, it should be noted that lignin sulfonate, the UV absorbant used during this testing, is known to absorb more UVC light below 240 nm than most surface waters and other UV absorber chemicals. Since UV emission below 240 nm may have been 14

15 inherently cut off by the lignin sulfonate the findings presented in this section may not reflect of the actual impact of doped sleeves on reactor B disinfection performance. The bioassay data generated from reactor B equipped with a non-doped quartz sleeve can be used to size UV installations for operation in European countries that require UV disinfection reactors to be equipped with doped sleeves for the prevention of nitrite (NO 2 - ) formation. Conclusions Design parameters such as approach hydraulics, inlet and outlet piping configuration, sensor to sleeve distance and quartz sleeve material are well known to impact the UV disinfection performance obtained with typical closed vessel reactors such as cross-flow in-line (reactor A) or axial-flow bottom-through-top (reactor B). Well-planned reactor validation testing provided an opportunity to quantify the impact of these key design parameters. This information is very useful for sizing a wide range of UV applications. It was determined that optimization of sensor to sleeve distance is vital to yielding optimum disinfection performance for reactors that use the UV intensity setpoint method to validate, monitor and control the delivered UV dose. A sensor that is not properly located with respect to the sleeve would lead to a significant over-sizing of UV full-scale systems. Although the two reactors studied are operating in significantly different ranges of flow capacity it is noteworthy to point out that each reactor technology reacted very differently to outside hydraulic conditions. The testing performed on the M-rig revealed the importance of the upstream piping design for cross-flow in-line UV reactors. Piping diameters smaller than that of the reactor should be avoided or if not possible, reducers should be located as far as possible from the reactor body. For instance, the draft US EPA Disinfection Guidance Manual (US EPA 2003) specifically recommends that a UV reactor be installed with a minimum 5 pipe diameters of straight piping between the reactor and any upstream hydraulic configuration. Alternatively, a flow straightener like an inlet perforated plate can be used to restore performance for UV reactors that are subject to poor inlet hydraulic conditions. However, such a fix significantly increases system head loss and therefore cannot be applied to all drinking water plants. Unlike the cross-flow in-line reactor (reactor A) a relatively small size axial-flow bottom-through-top reactor (reactor B) was shown to be insensitive to inlet and outlet piping configuration due to the internal baffling design. Finally, no significant impact on reactor performance was noticed when comparing use of doped versus non-doped sleeves. However, this finding may not have held true if a different UV absorbant had been used during validation testing. From the data presented in this paper the cross-flow in-line reactor (reactor A) mounted on the L-rig produced a delivered a dose of 40 mj/cm 2 under flow conditions of 9.0, 5.5 and 3.5 MGD for ALOS transmittances of 95%, 90% and 85% respectively. The disinfection results obtained with two cross-flow in-line reactors arranged in series revealed the degree of conservatism in the delivered doses obtained when such a reactor is bioassayed with an 15

16 upstream double elbow configuration. With the M-rig piping configuration and no inlet stilling plate a much lower flow of 6.5 MGD at an ALOS transmittance of 95% resulted in a delivered dose of 40 mj/cm 2. The axial-flow bottom-through-top reactor (reactor B) was validated to produce 40 mj/cm 2 under flow conditions of 0.88, 0.55 and 0.35 MGD for ALOS transmittances of 95%, 90% and 85% respectively. References US EPA, Ultraviolet disinfection guidance manual (Draft). Washington D.C. German Association on Gas and Water. Technical Standard DVGW 294, UV Systems for Disinfection in Drinking Water Supplies-Requirements and Testing, 2 nd version, Sharpless, C. M. and K. G. Linden, UV photolysis and nitrate: effects of natural organic matter and dissolved inorganic carbon, and implications for UV water disinfection. Environ. Sci. and Tech. 35, 14, Hoyer, Oluf, Testing and monitoring the efficacy of UV-disinfection systems - the German DVGW approach. Proc. 1 st IUVA World Congress, Washington D.C., USA. 16