Validation Protocol for Performance Testing and Scaling Ballast Water Treatment Systems Employing UV Disinfection and Filtration

Transcription

1 Validation Protocol for Performance Testing and Scaling Ballast Water Treatment Systems Employing UV Disinfection and Filtration Jim Cosman Regulatory Affairs Manager Trojan Technologies 3020 Gore Road London, Ontario Canada Jim Fraser Product Architect, Ballast Water Trojan Technologies Glen Latimer Engineering Team Leader, Ballast Water Trojan Technologies Jennifer Gerardi-Fraser Product Platform Manager, Ballast Water Trojan Technologies Abstract Several ballast water treatment system vendors are employing UV disinfection and filtration in systems designed to meet the IMO D2 Ballast Water Performance Standard. A key issue that has been identified is the application of performance test results for one ballast water treatment system to an up or down-scaled version of the exact same system so that type approval can be given to a range of capacities. A proposed validation protocol for performance testing and scaling ballast water treatment systems employing UV disinfection and filtration is discussed. In addition, the concepts of UV dose delivery and factors that impact the performance of UV systems, UV transmittance, and bioassay validation protocol for performance testing UV systems are discussed. In addition, the factors that impact the scaling of filtration systems are discussed. Introduction Over the past three decades, ultraviolet (UV) systems have gained increasing popularity as a means of disinfecting wastewaters, drinking water, and industrial process waters. Further, several ballast water treatment system vendors are also employing UV disinfection in systems designed to meet the IMO D2 Ballast Water Performance Standard. UV disinfection is considered a viable technology for ballast water treatment because it is simple to operate, no additional chemicals are required (no chemical storage or handling), there are no residuals, and the efficacy of UV is not impacted by salinity or ph. In addition, the performance of some UV lamps is not impacted by water temperature variations. 1

2 Several design standards and UV system validation protocols have evolved to support the effective implementation of UV technologies. Examples include the National Water Research Institute (USA) Ultraviolet Disinfection Guidelines for Drinking Water and Water Reuse, the ÖNorm 2001 (Austria) and DVGW Standard W294 (Germany) developed for drinking water applications, and the USEPA UV Design Guidance Manual (UVDGM) which was developed to support the implementation of UV disinfection for drinking water applications in the United States. Not all of these protocols are relevant to the ballast water treatment market, however key principles and concepts derived from these protocols may be applied when considering what factors need to be accounted for when findings for one BWMS are applied to an up (or down-) scaled version of this system, or a type approval is issued for a range of capacities. Background Dose Delivery Historically, the concept of average dose has been used to estimate the dose delivered by a UV reactor. This technique is based on the assumption that the delivered dose is equal to the average intensity within the reactor multiplied by the average retention time of the fluid within the reactor. An average UV dose calculation is only relevant for an ideal UV reactor. An ideal UV reactor is defined as a reactor in which every targeted organism receives the exact dose it requires, no more, and no less, and where no UV-C photons are wasted or absorbed by the walls of the reactor. In a real flow-through reactor, no two microbe, or particle, trajectories are the same and thus each microbe flowing through the reactor will receive a unique dose. (See Figure 1). Thus, in a real UV reactor, the interplay of flow fields and light fields determine true dose delivery. Low Dose Microbe Path Medium Dose Microbe Path High Dose Microbe Path UV Lamp UV Lamp Fluid Flow Figure 1. Computational Fluid Dynamics Model Depicting Particles Flowing Through a UV Reactor 2

3 Several authors have pointed out that hydraulic profiles and intensity gradients within UV reactors give rise to a distribution of delivered doses as opposed to a fixed value (Qualls et al., 1989; Scheible, 1985; Chiu et al., 1997). A dose histogram of a real UV reactor achieved by Computational Fluid Dynamics (CFD) modeling is depicted in Figure 2. The key to predicting real UV reactor performance is in the ability to accurately quantify the dose distribution for the reactor at each UV transmittance (UVT), flow rate and lamp power condition. Roughly, dose equals the intensity of the UV-C imparted on the target multiplied by the time the target is exposed to this intensity. In a normal reactor, the goal is to give all targets an equal dose. However, in real life, each target receives varying intensities and retention times during its path through the UV system. The accumulation of dose of the target at the end of the path is what matters, and it is what determines the efficacy of the reactor as a whole. Each reactor is unique in its hydraulic flow patterns and its intensity distribution. UV Transmittance (UVT) UV dose depends on the UV intensity, the flow rate, and the UV transmittance (UVT). Thus, UV transmittance is also a key parameter to consider when designing UV systems. UVT is the percentage of light passing through a water sample over a specified distance relative to distilled water. The UVT is usually reported for a wavelength of 254 nm and a path length of 1-cm. UVT is often represented as a percentage and is related to the UV absorbance (A254) by the following equation (for a 1-cm path length): % UVT = 100 x 10 -A. If the average UVT of ballast water reported in the literature is approximately 90%, this would mean that 90% of the light penetrates one centimeter of water, and 81% of the light would pass the second centimeter (Figure 3). If the UVT value is 50%, this would mean that 50% of the light penetrates one centimeter of water and only 25% of the light would pass the second centimeter (Figure 4). The lower the UVT of the water, the more energy that must be applied to achieve the desired inactivation of organisms. It should be noted that a typical secondary effluent from a municipal wastewater plant can range between approximately 50% to 65%, thus there is a great deal of experience designing UV equipment to treat water of a similar nature to ballast water. Ballast 3

4 water treatment systems must of necessity address water from different sources, and the UVT of these source waters can be expected to vary dramatically from low values for sediment rich harbor waters (river run-off) to higher values for cleaner waters such as protected Mediterranean harbors that do not have a river inflow. 100% 100% 90% 60% 90% 81% 1 cm 2 cm Figure 3. Water that absorbs less light, has a higher UVT. In this example, the water has 90%/cm UVT. The amount of incident light remaining at 2 cm is 81%. 100% 100% 50% 50% 25% 1 cm 2 cm Figure 4. Water that absorbs more light, has a lower UVT. In this example, the water has 50%/cm UVT. The amount of incident light remaining at 2 cm is only 25%. 4

5 Impact of UV Transmittance on System Design The UV transmittance of the water being treated must be taken into account when assessing the appropriate application of UV reactors for ballast water treatment. Two examples illustrate the importance of this statement. Example 1 is a BWTS that was designed for 50% UVT water. A system designed for this UVT will typically have lamps that are closely spaced together compared to a system designed for a higher UVT. The close spacing ensures that no water being treated gets in a low intensity or low dose region. This is required as only 50% of the light emitted by the lamp will penetrate the first 1 cm of the fluid being treated (Figure 3). This system can be utilized for fluids with a much higher UVT (eg. 99%) because target organisms are receiving much more than their required minimum target dose at higher UVT levels. The system illustrated in Example 1 will be effective at treatment across the entire water quality range (50-99% UVT). Example 2 is a BWTS system that was designed for 90% UVT water. As the design UV transmittance of the water being treated increases, lamps can typically be spaced further apart. In this scenario, 90% of the light emitted by the lamp will penetrate through the first cm of water being treated (Figure 3). This system will work for water with UVT greater than 90%, however, it will not work for water with UVT less than 90%. For example, at 50% UVT this system will have dark zones or zones of inadequate UV intensity. If this is the case, the dose delivered in these dark zones will be greatly reduced. As a result, many organisms will not receive their required minimum target dose. Therefore, the system illustrated in Example 2 will not be effective at treatment across the entire water quality range (50-99% UVT), rather its effectiveness will be limited to the range greater than 90% UVT. In summary, a UV system should be designed to be effective at the lowest UVT value of the waters to be treated. It should also be noted that filtration and separation technologies will likely impact UV system performance. Filtration may sometimes improve the UVT of the fluid, and the required dose levels will change depending on the filtration pore size utilized in the system. UV Transmittance versus Turbidity The unit of measurement of turbidity is NTU, or Nephelometric Turbidity Unit. The device that measures this value is a nephelometer. The nephelometer is oriented 90 degrees to the light source and measures the light scattered from the suspended solids in the water rather than measuring the percentage of light absorbed from the particulate matter in the water, or inversely, the percentage of light transmitted through the water. The particle colour, shape, and size can affect the NTU value. It is important to note that turbidity measurements are made using a light source with natural light wavelengths (ie. visible light), as apposed to light at the wavelength of 254nm that is used for UVT measurements. As discussed previously, UVT is the percentage of light passing through a water sample over a specified distance (typically 1 cm) relative to distilled water. Turbidity can influence UVT, but they are not linearly connected, or actually connected in any concrete way. For example, a fluid with a turbidity of 20 NTU could have a UVT of 60% or higher, or of 5% or lower. Conversely, a fluid with a turbidity of 1 NTU could have a UVT of 90% or greater, or 5% or lower. Fluid that is visibly clear to the human eye may have contaminants in solution that do not block the long wavelengths of light (ie. visible light) that a nephelometer uses to make a measurements, but these contaminants may block a 5

6 relatively short wavelength of light such a 254 nm. High NTU values can be an indicator that the water being evaluated may have a low UVT, but any further assumptions are very risky and possibly incorrect. Accurate UVT measurements are critical to properly assess the impact of water quality on UV system performance. In addition to hydraulics and UV transmittance, other factors will impact dose delivery within a UV reactor. Figure 5 below outlines the various factors that impact the performance of the UV system. Figure 5 Factors Affecting UV Disinfection Performance Bioassay Protocols The non-ideal behavior of real UV reactors and the complexity of their designs prohibits reliance solely on theoretical calculations to reliably predict the UV dose delivered by the reactor, and requires that the dose delivered by the reactor be validated using an empirical testing protocol. The bioassay protocol is the standard approach provided by all current regulatory guidance, and is currently the globally accepted approach for validation of the dose delivery performance of a UV disinfection reactor. The bioassay protocol is divided into three parts: firstly the development of a UV dose-response curve with an ideal laboratory reactor for a culture of challenge organism (bacterium, bacterial spore or virus); secondly the passing of the challenge organism from the same culture through the reactor being validated while it operates under specified conditions of flow rate, lamp power level and water quality; and thirdly, a comparison of the inactivation of the challenge organism following passage through the reactor with the laboratory dose-response curve to determine which dose delivered by the ideal reactor gives the same challenge organism inactivation. For those specified conditions of operation, the reactor is thereby validated to deliver the Bioassay Equivalent Dose read from the doseresponse curve. For more extensive validation under different operating conditions (flow rate and/or power level and/or water quality (UVT), the protocol is repeated from step two for each operating condition. 6

7 Comparing Average (Ideal) Doses to Bioassay Doses Bioassay determined dose versus average (ideal) dose for a UV reactor system under varying flow rates, UVTs and lamp power settings is plotted in Figure 6 (Petri and Olson, 2001). Poor correlation exists between the ideal model and actual data, and using ideal dose calculations to size UV reactor systems is, therefore, inappropriate, for several reasons: (a) the spatial distribution of UV intensity is very difficult to model, especially since the absolute UV lamp output is difficult to quantify (b) hydraulic effects generally account for 20% to 50% of reactor inefficiency, meaning the ideal model could lead to undersizing by a factor of 2 or more. A simple example illustrates the fallacy of using ideal dose for sizing. Consider a reactor delivering a dose of 100 mj/cm 2 for 99% of the flow and 0 mj/cm 2 for 1% of the flow. Dose Value Discussion Average (Ideal) 99 mj/cm 2 Average UV intensity within reactor multiplied by average residence time Bioassay 40 mj/cm 2 99% of the reactor achieves 5 log inactivation of the target, while 1% of the reactor achieves 0 log inactivation. Only 2 log inactivation can be achieved overall (100,000/100 ml organisms to /100 ml) Table 1. Comparison of Average (Ideal) Versus Bioassay Dose Calculation Ideal dose calculations would average out the dose to give dose delivered by the reactor as 99 mj/cm 2. Clearly, only 2 log inactivation can be achieved by such a reactor. However, the ideal dose of 99 mj/cm 2 leads one to believe that if the reactor were challenged with MS2 Phage, where a dose of approximately 20 mj/cm 2 is required for one log inactivation, nearly 5 log inactivation would be achieved. The preferred method to size UV reactor systems is through bioassays, or bioassay validated computational tools. Figure 6. Comparison between bioassay dose versus average (ideal) dose for a UV reactor system 7

8 Filtration A robust filtration system design is critical in ballast water treatment. The filtration system loading rates which are designed for use in the ballast water treatment market are among the highest in the filtration industry. This is done mainly to keep the size of the filtration equipment within reason so it can be incorporated into an already limited available space aboard an existing ship or a new build design. Loading rate is key in filtration, especially in the simple screen filtration technologies which are used in ballast water treatment and which have no depth component to them. The higher the loading rate as measured in flow rate per area of filter media (e.g. GPM/FT 2 ) the more differential pressure is produced. Higher differential pressure leads to degradation of filter performance (more available force to drive retained particles through the thin filter media, particle shearing ). A filtration system not only has to catch the particles being removed, it also has to retain them throughout the filtration cycle as well. It is very important to note that the filtration system must be matched with UV equipment design to achieve an efficient overall ballast water treatment system. It is important to design systems in such a way as to optimize the strengths of both the separation and the disinfection technology. You do not want to size the filtration system to remove particles of the size that the UV system can easily inactivate and you do not want to oversize the UV system to inactivate particles that can be easily removed by the separation technology. It is equally important to align the two technologies so that there is no gap in performance between them as such a gap would lead to a non-compliant system. The balance between filtration system performance (particle size removal) and UV disinfection system performance (UV dose delivery to individual targets and smaller particles) is critical to developing a robust, cost effective, and efficient ballast water treatment system. Discussion For drinking water UV reactors, there are no convenient indicator organisms, like total coliforms, that are abundantly available for routine monitoring of UV disinfection system performance for wastewater applications; therefore, it is important that the full scale reactor be bioassay validated to ensure delivery of the target UV dose whether the water being treated has the indicator or not. As a result, the practice of scaling UV reactors up and down is not permitted for drinking water applications in most jurisdictions around the world. To address higher flow drinking water applications, duplicate validated reactors can be used in parallel to increase total system capacity. In addition, duplicate validated reactors are sometimes employed in series to deliver higher doses. Scaling up is permitted under specific conditions for water reuse applications according to the National Water Research Institute (NWRI) (USA) Ultraviolet Disinfection Guidelines for Drinking Water and Water Reuse. Such applications generally use modular systems where the units are scaled up by repeat units of identical geometry. For reclaimed-water application only (as they are of lower risk to the public), if the velocity field for both the test and full-scale reactors can be measured and the uniformity of the velocity field can be verified by empirical measurements then larger reactors can also be used in full-scale applications. The full-scale and test reactors must have identical lamp spacing. In addition, the full-scale reactor must be operated at the 8

9 same velocity range and flow per lamp used for performance validation. The scale-up factor for a given reactor is limited to 10 times the number of lamps used in the test reactor. Filtration technologies are scalable by either of two ways; the first is by adding more of the same model tested in a modular approach which could involve a header type arrangement or some other method. This approach while technically accurate can become difficult to install and manage depending on the number of overall units. The amount of plumbing, valves, connections, and control mechanisms increases making the maintenance of such a system cumbersome. The second way to scale a filtration technology is to extrapolate the important design factors into a larger size unit. Unlike scaling up a UV system, a filtration system s size can be typically increased within reason, without sacrificing the integrity of the design or system performance. One of the key design parameters to maintain throughout the scaling up of various models is filter media surface area. Maintaining the same filter loading rate (flow rate per unit of filter media area) from a smaller test unit tested should yield similar filtration performance in a larger model. Recommendation It is paramount that any validation protocol for a filtration-uv ballast water treatment system accounts for: 1. Overall system performance changes due to filter performance changes resulting from changes in: a. Hydraulic pressure and b. Loading rate per unit filter surface area. 2. Overall system performance changes due to UV system performance changes resulting from changes in: a. Flow rate b. UV transmittance of the water being treated c. Changes in lamp power setting In addition, the interrelationship between the type of filtration system and UV dose must also be accounted for. A ballast water treatment system (BWTS) employing a smaller sized screen (e.g. 30 microns) in the filtration system is likely to require a lower delivered UV dose for the UV system than a BWTS employing a filter with a 50 micron screen. Therefore, the determination of dose values over which a system is validated should be related to the filtration system utilized in the overall system. Scaling Filtration Systems For filtration system scaling, if done within a reasonable factor and as long as best engineering practices are used in the extrapolation of the key design parameters, a small scale (250 M³/hr) filtration system should perform similarly to a larger scale filtration system (1250 M³/hr). For ballast water filtration systems it is recommended that testing to validate encompass a worst case scenario with respect to water quality. Typically the types of filtration systems used 9

10 in ballast water filtration will work as well and in most cases better, in relativity clean water than they do in the most challenging waters. There are several water quality parameters that need to be tested such as turbidity, total suspended solids, particle counts and biological loading while monitoring and recording the differential pressure through out the filtration cycle. Each of these parameters needs to be tested at the pre and post filtration sample locations to determine overall filtration performance throughout the filtration cycle. A filtration cycle begins with a clean filtration system and continues until the differential pressure measured across the filtration system reaches the terminal head loss point as determined by the system manufacturer. When scaling a filtration system, the test water should have turbidity influent levels of a NTU range that is representative of harbor waters. In addition, the test waters should have total suspended solids levels that are appropriate for a challenge. Influent particle counts should have an appropriate particle size distribution so as to challenge the filtration system and the effluent particle counts should be monitored. Biological loading should consist of organisms that are representative of the IMO guidelines with regards to numbers, size, and type. This filtration testing is to be done in conjunction with the UV system testing protocol to insure total system compliance. Scaling UV Systems In ballast water treatment, there are not necessarily going to be convenient indicator organisms universally and always present in the untreated ballast water, and therefore monitoring of the treated effluent is not currently a practical solution. For UV systems, the following approach is recommended. First, at least one system configuration (e.g. 250 m 3 /h) should be bioassay tested at a specific set of operating variables (design flow, minimum UVT, power) in accordance with IMO testing protocols and requirements at an approved test facility. To increase the flow capacity of the system, two approaches, multiplication and scaling may be utilized. First, unlimited multiples, N, of the tested system (e.g. 250 m 3 /h) could be used in parallel to increase the total flow capacity of the system. In this example, a 1000 m 3 /h system may employ N or four of the 250 m 3 /h previous validated systems to attain this flow requirement. Again, there should be no limitation on the parallel multiples allowed given that the unit has been extensively performance tested. If a parallel arrangement is not appropriate, another option would be to use a scaled up higher flow system that may be more cost effective in treating the higher flows. The scaling should be limited to five times the flow of the validated unit as long as the following conditions are met: 1. The scaled up UV system has the same lamp spacing and hydraulic configuration of the bioassay tested validated unit. 2. The scaled up UV system employs the exact same lamp and power level as the bioassay tested validated unit. 3. The scaled up UV system has a similar flow/velocity per lamp. The intent of the above conditions is to ensure that the scaled up UV system has a similar UV dose distribution as the base validated unit. By ensuring that the scaled up system has the same lamp and lamp spacing, an attempt is being made to ensure that the light intensity is similar between units. In addition, the requirement to have a similar flow/velocity per lamp attempts 10

11 to manage hydraulic flow patterns in the UV system. It should be noted that this suggested approach falls between the accepted drinking water validation approach of extensively bioassaying each model of UV system and that of the grey water approach for scaling that is used by NWRI in North America. To manage the above factors, Computational Fluid Dynamics and Light Intensity modeling may be used. Over a decade of experience exists in using these models to manage the above factors. Specifically, ballast water treatment system manufacturers should be required to submit this modeling to demonstrate the proper scaling of different configurations. These models may also be used to scale system pipe diameters or other components that affect the inflow or outflow of the BWTS so as to correlate the scaled up design to the performance tested validated configuration. References Chiu, K., Lyn, D.A. and Blatchley, E.R. (1997). Hydrodynamic behaviour in open-channel UV systems: Effects on microbial inactivation. CSCE/ASCE Env. Eng. Conf., DVGW UV Disinfection Devices for Drinking Water Supply Requirements and Testing. DVGW W294. German Gas and Water Management Union DVGW), Bonn, Germany. Lawryshyn, Y.A. and Cairns, B. (2003). UV disinfection of water: the need for UV reactor validation. Water Science and Technology: Water Supply Vol 3 No 4 pp Petri, B.M. and Olson, D.A. (2001). Bioassay-validated numerical models for UV reactor design and scale-up. Proc. the First Int. Congress on Ultraviolet Tech., Washington DC., USA, June. Qualls, R.G., Dorfman, M.H. and Johnson, J.D. (1989). Evaluation of the efficiency of ultraviolet disinfection systems. Wat. Res., 23(3), Scheible, O.K. (1985). Development of a rationally based design protocol for the ultraviolet light disinfection process. 58th Annual WPCF National Conf., Kansas City, Missouri, USA. USEPA Ultraviolet Disinfection Guidance Manual. United States Environmental Protection Agency, Office of Water, Washington, D.C. Wright, H.B. and Lawryshyn, Y.A. (2000). An assessment of the bioassay concept for UV reactor validation. Proc. Wat. Env. Specialty Conf. on Disinfection, New Orleans, Louisiana, USA, March. 11