Challenges of Retrofitting Existing WTPs with UV Disinfection for Microbial and DBP Rules Compliance

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1 David S. Briley, P.E. Hazen and Sawyer, 4011 WestChase Boulevard, Suite 500, Raleigh, NC ABSTRACT Some water systems are turning to UV disinfection for compliance with the new USEPA rules; Long-Term 2 Enhanced Surface Water Treatment Rule (LT2ESWTR) and the Stage 2 Disinfectant/Disinfection ByProducts Rule (DBPR). Water systems with Cryptosporidium hits in their water supplies may be required to meet higher levels of inactivation. This presentation will briefly present the Microbial Toolbox options available to water systems for achieving higher levels of Cryptosporidium inactivation. The advantages, disadvantages, and risks of each option will be discussed. In addition, water systems classified as Bin 1 (no additional Crypto. inactivation) according to the LT2ESWTR are also considering UV disinfection as a technology for DBP reduction. In this approach, UV disinfection would provide partial or full Crypto. and Giardia inactivation credit and allow a reduction in free chlorine contact within the plant; thereby reducing DBP formation at the point of entry into the distribution system. There can be significant challenges with retrofitting existing water plants with UV disinfection. Available head within the existing plant hydraulic profile is a key consideration since UV systems can have 2 to 4 feet of headloss. Site constraints can also be a significant challenge as plant sites may have been originally laid out for expansion, but not for additional processes. Key challenges and considerations for retrofitting existing WTPs that will be discussed include: Hydraulic considerations Site constraints and facility footprint Water quality and UV transmittance Electrical requirements Maintaining disinfection under adverse operating conditions A brief update will also be provided on current trends in regulatory approval of UV disinfection systems for Crypto. and Giardia inactivation credit throughout the U.S. INTRODUCTION The US EPA promulgated two new rules simultaneously in January 2006; the Stage 2 Disinfectants and Disinfection Byproducts (DBP) Rule and the Long Term 2 Enhanced Surface Water Treatment Rule (LT2ESWTR). These two rules are part of the Microbial-DBP Cluster, which is a set of interrelated regulations that address risks from microbial pathogens and DBPs. These rules have conflicting goals and compliance can be challenging for some systems. The LT2ESWTR and other rules require water systems to provide protection from microbial pathogens through use of a disinfectant residual, while simultaneously minimizing health risks from disinfection byproducts, which are formed by interaction of chlorine and natural organic matter. Overview of LT2ESWTR The United States Environmental Protection Agency (USEPA) finalized the LT2ESWTR in January 2006 concurrently with the Stage 2 Disinfectants and Disinfection Byproducts Rule (Stage 2 D/DBPR) and is part of a series of rules called the Microbial-Disinfectants/Disinfection Byproducts Cluster (M-DBP

2 Cluster). These rules were intended to improve protection from microbial pathogens while minimizing public health risks with disinfection byproducts (DBPs). Cryptosporidium required for filtered water systems is based on the source water monitoring results summarized in Table 1. The LT2ESWTR specified that filtered systems should use at least one of the management and options listed in the Microbial Toolbox to meet the additional Cryptosporidium requirements listed for each bin. Systems classified in Bins 3 and 4 (highest Cryptosporidium levels) with conventional filtration must achieve at least 1.0-log of the total additional required by using one or a combination of the following: bag filters, bank filtration, cartridge filters, chlorine dioxide, membranes, ozone or UV, as specified in the Microbial Toolbox. Table 1: Level of Cryptosporidium Treatment Required for Filtered Systems Bin Classification Source Water Cryptosporidium Concentration (oocysts/l) Conventional Filtration (including Softening) Direct Filtration Slow Sand or Diatomaceous Earth Filtration Alternative Filtration Technologies 1 < No additional No additional No additional No additional 2 > and < log 1.5-log 1.0-log (1) 3 > 1.0 and < log 2.5-log 2.0-log (2) 4 > log 3.0-log 2.5-log (3) Notes: (1) As determined by State such that total Cryptosporidium inactivation/removal is at least 4.0-log. (2) As determined by State such that total Cryptosporidium inactivation/removal is at least 5.0-log. (3) As determined by State such that total Cryptosporidium inactivation/removal is at least 5.5-log. The Stage 2 M DBP Advisory Committee recommended the Microbial Toolbox to provide Public Water Systems with broad flexibility in selecting cost-effective LT2ESWTR compliance strategies. Most options in the Microbial Toolbox carry prescribed credits toward Cryptosporidium requirements. Systems receive these credits by demonstrating compliance with required design and operational criteria. In addition, North Carolina may award credits other than the prescribed credit through a demonstration of performance, which involves site-specific testing by the System with state-approved protocol. Watershed Control Program The watershed control program (WCP) option provides only 0.5-log Cryptosporidium credit. The WCP plan requires the identification of an area of influence (an area outside which there is little chance for Cryptosporidium or fecal contamination to affect the drinking water intake), identification of potential and actual sources of Cryptosporidium contamination and their relative impact on the source water quality, an analysis of control measures that could mitigate the Cryptosporidium sources, and a plan that establishes goals and defines/prioritizes specific actions to reduce source water Cryptosporidium levels. Once the WCP credit is obtained, the State will also require submittal of an annual WCP status report and to conduct a watershed sanitary survey every three years. No water system has petitioned the NCDENR PWS Section to achieve Cryptosporidium credit by implementing a WCP. The process for review and approval of a WCP has not been developed by NCDENR. For many water systems, the area of influence

3 may extend beyond the extraterritorial jurisdiction of the City or County, and therefore, would require coordination with other municipal governments in order to implement the needed control measures. Alternate Source/Intake The use of an alternative water source may not be a feasible option for many water systems as sufficient supply may not be available. It would require considerable effort and would take many years for a water system to find an alternative source and to obtain regulatory approval to use this alternative raw water source. Also, changing the timing of withdrawals to obtain lower source water Cryptosporidium concentration for a lower bin placement is not practical. This would require a firm understanding of the timing of higher Cryptosporidium risk in order to manage timing of source water withdrawals, and could present operational issues for a water system as it manages its available raw water storage and yield. Pre-Sedimentation with Coagulation The pre-sedimentation basin with coagulation option would require construction of a redundant process upstream of the existing water plant, and would only provide a 0.5-log credit. During months when source water turbidity is low, it may be difficult to achieve the 0.5-log (68 percent) turbidity removal criteria. This option may be suitable for high turbidity sources. In addition, there would be operational challenges as the water system would be operating a dual coagulation process, which could complicate the coagulation chemistry for the existing coagulation-sedimentation processes currently practiced. Once the raw water has undergone the initial coagulation/settling, there may be no further turbidity removal in the second stage of, or the turbidity levels may increase in the second stage of since the first stage process would remove much of the organics, silts and sediment. The second stage of might also require the use of a coagulant aid polymer if particle agglomeration and settling difficulties arise. Bench and pilot scale testing would be necessary to identify these potential challenges and document the appropriate chemical scheme needed to ensure good settled water quality from the second stage of. The filterability of the floc created in the second stage settling process would also need evaluation in order to assess expected filter run times. Further, the pre-sedimentation option with chemical would require additional attention from the plant staff due to operational complexity. For water systems without an existing presedimentation basin, the capital costs for this option could be significant. Lime Softening The two-stage lime softening option is not applicable to most water systems in North Carolina since source waters are typically low in hardness, and do not require softening. Bank Filtration The bank filtration option provides either 0.5-log (for 25 ft setback) or 1.0-log (for 50 ft setback) when using the bank of the lake as a natural filter. While this may be technically feasible depending upon the aquifer characteristics adjacent to the water supply source, capital costs could be significant to install bank filtration well systems. If Rainey-type wells systems are used, capital costs could be on the order of $750,000 per mgd of capacity, not including piping costs to connect the wells to existing raw water pumping facilities. In addition there would be the associated electrical cost to operate pumps for this system. Combined and Individual Filter Performance The combined filter performance option provides 0.5-log credit, if the CFE turbidity measurements are less than or equal to 0.15 NTU for 95 percent of the samples during that month. The CFE turbidity samples are required to be collected at 4-hour intervals or less. Most water plants consistently meet turbidity requirements required by regulations (i.e. < 0.3 NTU), and many water plants could meet the 0.15 NTU goal, especially Partnership for Safe Water members.

4 The individual filter performance option can provide another 0.5-log credit (in addition to combined filter performance) if the IFE turbidity is less than or equal to 0.15 NTU in at least 95 percent of the samples each month (samples should be taken at 15 minute intervals or less) and no IFE turbidity readings are greater than 0.3 NTU in two consecutive measurements (taken 15 minutes apart). This goal may be significantly more challenging to achieve, particularly following a filter backwash cycle. For both goals, source water quality variations including fluctuating levels of algae, turbidity, iron, and manganese could make compliance with the IFE and CFE turbidity requirements challenging at time. The above two options may be considered as LT2ESWTR compliance methods but would require demonstration testing to ensure the existing processes can consistently control filtered water turbidity (combined and individual filters) during high manganese or algae events or during periods of other raw water quality excursions due to extreme storm events, such as hurricanes. Demonstration of Performance The demonstration of performance (DOP) option may not provide a higher log credit than the presumptive log credit offered under the LT2ESWTR. Since there is little precedence for a DOP option, it is not likely that NCDENR would grant 1.0-log additional Cryptosporidium credit for a conventional process meeting current turbidity standards. Bag and Cartridge Filters The bag and cartridge filter option is more applicable to small water systems and not to larger municipal water plants. This option would likely require intermediate pumping since bag and cartridge filters can have high headloss across the filters. Therefore, this option may not be feasible for larger municipal water plants. Membrane Filtration Option The membrane filtration option has a high capital cost (at $1.50 per gallon capacity or more) and would generate additional wastewater in the form of reject water, thereby reducing the capacity of existing WTPs. This option may also require intermediate pumping. Second Stage Filtration The second-stage filtration option provides 0.5-log credit and would require construction of a second set of filters after the primary filtration process. This may also require construction of an intermediate pump station at most water plants. The second-stage filtration option would be expected to produce more consistent low turbidity filtered water, and along with the Combined Filter Performance option, could provide a total of 1.0-log Cryptosporidium credit to meet the Bin 2 LT2ESWTR Rule requirements. Slow Sand Filtration The slow sand filtration option can provide 2.5-log credit when used as a secondary filtration step. This option requires a large area for construction of the slow sand filters. Slow sand filtration would also affect the plant hydraulics and would require intermediate pumping. Due to capital and operational costs, this option may not be suitable for many water systems. Chlorine Dioxide The chlorine dioxide option can provide up to 3.0-log credit based on the available CT. Since the residual chlorine dioxide concentration is limited to 0.8 mg/l and the chlorite MCL is 1.0 mg/l (which is a byproduct of chlorine dioxide), this option may not be feasible. Some water systems have experienced taste and odor problems with ClO 2 when used in drinking water for disinfection.

5 Ozone The ozone option can also provide up to 3.0-log credit based on the available ozone contact time (CT). Ozonation could also result in ozonation disinfection byproducts in the distribution system. Some of the ozonation byproducts can result in increased biological activity in the distribution system (unless they are removed at the plant site using biologically active filters). UV Disinfection The UV option can provide up to 4.0-log Cryptosporidium inactivation credit, where the credit is based on the UV dose. UV disinfection does not result in any known byproduct formation. UV is usually incorporated after the filters and before the finished water clearwells. This option could require intermediate pumping depending upon the available head within the existing hydraulic profile. NCDENR has not yet formally granted Cryptosporidium inactivation credit for UV disinfection. However, Hazen and Sawyer and the City of Raleigh have ongoing discussions with NCDENR, and it is expected that inactivation credit for UV disinfection may be granted soon. Background of UV Disinfection Ultraviolet (UV) light has been used for many decades for disinfection of drinking water supplies worldwide, particularly in Europe. UV light for drinking water disinfection has been gaining more interest in North America over the last two decades. UV was long assumed to provide effective inactivation of bacteria and viruses in raw water supplies, but UV light was believed to be ineffective for inactivating protozoa at UV doses that would be cost-effective as a water process. However, in the late 1990s new research demonstrated that UV disinfection was a cost-effective process in the inactivation of Cryptosporidium (Crypto) oocysts. These studies utilized animal infectivity assays to determine the ability of oocysts to replicate and cause disease. In following years, additional studies confirmed these findings and demonstrated the effectiveness of UV disinfection for the inactivation of Giardia cysts, as well. UV light inactivates biological organisms that can be pathogenic to humans. UV light is effective at inactivating many pathogens including Cryptosporidium parvum in water at relatively low doses. UV disinfection does not remove organisms from the water as filtration does and the disinfection mechanism is considerably different from chemical disinfectants such as chlorine and ozone. UV light lies on the electromagnetic spectrum between visible light and X-rays. The portion of the UV spectrum (100 nm to 400 nm) that is effective in disinfecting water is defined as the germicidal region and the wavelengths range from 200 nm to 300 nm. This is the range in which an organism s DNA and RNA absorbs UV photons (discrete packets of energy), with the peak germicidal effectiveness at a wavelength of 260 nm. When the UV photons are absorbed, the bonds are broken in the pyrimidine nucleotides damaging the organism s DNA and RNA; this inhibits the replication of the organism. Microorganisms that are unable to replicate cannot infect a host. There are some microorganisms, such as some viruses, that are more resistant to UV light than Cryptosporidium. Typically, free chlorine is used at facilities utilizing UV disinfection to achieve any required virus inactivation. Table 3 from the US EPA UV Disinfection Guidance Manual, contains the UV dose requirements for various target pathogens Table 3: UV Dose Requirements (mj/cm 2 ) Target Pathogens Log Inactivation Cryptosporidium Giardia Virus From US EPA UV Disinfection Guidance Manual, November 2006

6 UV units must be validated according to EPA guidelines. Commonly, UV units are prevalidated by a manufacturer, which consists of testing the unit in a specific configuration; this can then be applied to similar field applications of that UV unit. However, in order to use these prevalidated UV units, there must be a specified amount of straight approach pipe upstream from the UV unit. A number of configurations were examined, and an arrangement that the requirements for a prevalidated UV unit could be achieved. Comparison of LPHO and MPHO Reactors Typically, UV light is generated by applying a voltage across a gas mixture containing mercury vapor, resulting in a discharge of photons. The most widely used lamp technologies in the drinking water industry are low-pressure (LP) and medium-pressure (MP) UV lamps. LP lamps provide a monochromatic UV output primarily at 254 nm, which is nearly the most effective germicidal wavelength for inactivating pathogens of 260 nm whereas MP lamps generate polychromatic radiation including the germicidal UV range (200 nm to 300 nm). The polychromatic output of the MP lamps produce ten to twenty times more germicidal effective UV output than the LP lamps on a unit length basis. To make up for the lower lamp intensity output of the LP lamps, more LP lamps are needed per reactor than an MP system to provide an equivalent intensity, thus resulting in a larger UV unit. LP lamps operate at lower temperatures than MP lamps, thereby resulting in less thermal energy loss and longer lamp life. LP lamps operate at efficiencies ranging from 30 to 40 percent, which is higher than the MP lamps efficiency. Higher efficiencies result in lower required power input to the lamps which translates into lower energy consumption. Lower power input in LP lamp UV reactors requires significantly fewer or smaller electrical equipment including generators, Uninterruptible Power Supply (UPS), batteries, and switchgear compared to medium pressure systems. Further, the lower operating temperature of LP lamps may reduce the tendency for inorganics to oxidize on the quartz sleeve housing the UV lamp, thereby significantly reducing the frequency of sleeve cleaning. Overall, Low Pressure High Output (LPHO) units typically have lower operating and power costs. Some LP lamps are categorized as low pressure high output (LPHO). The mercury fill in these lamps are in the form of amalgam instead of liquid as seen in the regular LP lamps, which provides higher UV radiation at a low pressure in the lamp. Most of the water applications currently use the LPHO lamps or the MP lamps as the lamp technologies. Figure 1 illustrates a LPHO UV unit used for municipal applications and Figure 2 represents a MP UV reactor design commonly implemented in municipal water applications. Figure 1: Municipal LPHO UV Reactor (ITT Wedeco K Series)

7 Figure 2: Municipal MP UV Reactor (TrojanUVSwift ) Multiple factors must be considered when selecting between LPHO and MP units including unit space requirements, the lamp intensity output, lamp life, power use, power modulation capabilities, and sleeve cleaning. A summary of the advantages and disadvantages of each reactor type is provided below in Table 3-2. Table 4: UV Reactor Types Advantages/Disadvantages Reactor Type Advantages Disadvantages More efficient lamps Lower UV output per lamp Smaller electrical load More lamps required requirements Low Pressure High Output (LPHO) Larger footprint Less frequent lamp requirements sleeve cleaning Higher capital costs Lower O&M costs Medium Pressure (MP) Higher intensity UV output Fewer lamps Smaller footprint requirements Lower capital costs Less efficient lamps Larger electrical load requirements More frequent lamp sleeve cleaning Higher O&M costs Typically, higher headlosses DISCUSSION AND RESULTS Hydraulic Considerations A significant challenge in installing UV disinfection can be hydraulic constraints. The location of UV disinfection systems in the process scheme should be after filtration to ensure that solids, which can shield pathogens from UV light, are removed. Hydraulic considerations and constraints can vary widely from plant to plant, depending upon the available head between the filters and clearwells. For many water plants, particularly those on sites with flat topography, there may be limited head after filtration. Headloss through UV reactors typically varies from 6 inches to 24 inches, depending upon manufacturer and reactor design. There does not seem to be a direct link between headloss and UV lamp technology (low pressure, medium pressure), but rather, reactor size and design are more critical for determining headloss characteristics.

8 However, overall headloss through a UV disinfection system can be as much as 3 to 4 feet once yard piping, fittings, valves, and flowmeters are considered. Therefore, a significant portion of the total headloss is a function of the UV system layout, and its location on the WTP site. This is what makes hydraulic considerations for UV systems such a site-specific issue. If sufficient hydraulic head is not available at the WTP, then some water systems have either installed intermediate pumps, or installed UV disinfection after the finished water pumps. Construction of an intermediate pump station adds significant capital and O&M costs to the UV facility. Also, pump control is a critical issue if the UV system is controlled using the Calculated Dose Approach and is dose-pacing based on flow and UVT. Flow changes should be made slowly to ensure the UV control system has sufficient response time to adjust and avoid under-dosing. Quartz sleeves usually have a fairly high pressure rating and can be suitable for use after intermediate pumps, or high service pumps. However, quartz sleeves can be sensitive to pressure transients or surge, which further emphasizes the need for effective pump control. This can be achieved through either variable frequency drives, or pump control valves. Another consideration is that high service pumps are typically sized to pump a larger flow than the rated capacity of the WTP. Some high service pump stations may be designed to pump 120 to 150 percent of WTP capacity to satisfy a portion or all of the peak hour demand. This would significantly affect UV system design since UV reactors would be sized to treat the high service pump flows rather than filtered water flow. Site Constraints As discussed before, the recommended location of UV disinfection systems in the process scheme is post-filtration. Therefore, the ideal location of UV facilities is along the route of the filtered water piping from the filters to the clearwell to minimize yard piping and headloss. Another consideration is elevation of the UV facility. UV reactors must be installed with the top of the reactor below the hydraulic grade line to ensure the reactors are flooded at all times. If air is entrained in the top of the reactor, then UV lamps could over-heat and break. Therefore, the UV room is usually belowgrade and the facility design must consider access for operators and equipment removal. Water Quality and UV Transmittance UV transmittance (UVT) is a key design criteria for UV disinfection systems since it directly affects the penetration of UV light through the UV reactor. Typical design values for UVT in filtered water at water plants range from 88 to 95 percent. It is recommended that the design UVT be based on the 95 th percentile of historical UVT data to ensure the UV system is designed with sufficient conservatism. Figure 3 represents a UVT trend for a typical conventional water plant in North Carolina.

9 Figure 3: UV Transmittance for a Conventional WTP Filtered Water UVT (%) Average = 93.2% 95th Percen4le = 89% 80 Jan Apr Jul Oct Feb May Aug Date Electrical Considerations The electrical service requirements for UV disinfection systems depends upon the lamp technology selected. Medium-pressure, high output systems typically have higher total connected loads than lowpressure, high-output (LPHO) systems. For systems designed to achieve 3-log inactivation of Cryptosproidium and Giardia, the total connected load can be: LPHO Systems: 0.5 to 1.4 kw per mgd MPHO systems: 2.0 to 2.4 kw per mgd We typically recommend that a water system pre-bid the UV equipment early in design so the final, detailed design be completed based on the selected technology. Another key consideration is power quality. UV lamp ballasts currently used in the market are solid-state and can be sensitive to minor fluctations in the incoming electrical supply. Therefore, uninterruptible power supplies (UPS) are often recommended to provide a more consistent incoming power supply. Another advantage of a UPS is that it will maintain the UV disinfection in operation during power outages, and ensure continued disinfection while standby generators startup. This is critical since flow through the water plant does not stop instantaneously upon power loss. Also, the UPS can provide power to other appurtenances such as effluent valves and provide for a controlled UV system shutdown in the event of an extended power outage.

10 Trends in UV Regulatory Approval With the promulgation of the LT2ESWTR, the number of UV installations has increased as water systems with water supplies deemed at risk look for new tools to provide additional Cryptosporidium inactivation. Based on a survey of utilities in the UV Disinfection Knowledge Base (Water Research Foundation Report No. 3117), the primary reason for installing UV disinfection was for Cryptosporidium inactivation (over 50 respondents). However, about 50 respondents indicated that Giardia inactivation credit was a goal for UV disinfection, and close to 30 respondents indicated that reduction of chemical CT was a goal. This points to a growing number of water systems using UV disinfection for DBP compliance as well. Based on a survey that we have conducted, UV disinfection has been approved for Cryptosporidium and Giardia inactivation credit in Arizona, California, Tennessee, Utah, and Washington. Many other states are currently in the process of developing guidelines or considering requests for inactivation credit such as New York and North Carolina. Currently, there are 4 water plants in North Carolina that have installed UV disinfection. At the time of this article, NC PWS has not officially granted inactivation credit for UV disinfection. However, the City of Raleigh has requested 3-log credit for both Cryptosporidium and Giardia, and has been working with NC PWS over last few years to review operating protocols, system controls, and safety measures to ensure continued disinfection. The City of Raleigh does intend to use UV disinfection as a DBP compliance tool as well as provide for multiple barrier disinfection. UV Lamp Break Issues One of the key issues and concerns for water system operators, regulators, and the public regarding UV disinfection is the potential for UV lamp breaks. Low and medium pressure UV lamps contain small amounts of mercury, which can be released into potable water during a lamp breakage. On-line lamp breakage is a rare occurrence and will occur only if both the protective quartz sleeve and the lamp tube are broken. The potential for lamp breakage can be caused by any one of the following elements: Temperature variations that occur when lamps operate in air or stagnant water, Excessive positive or negative pressures within the disinfection unit, Major electrical surges, or Improper maintenance and handling. An evaluation of the risk of mercury contamination conducted by the International UV Association (IUVA) identifies the following three possibilities for release of mercury: Lamp breakage during initial installation, cleaning, or replacement, Improper disposal/recycling practices for spent UV lamps, or Mercury release due to catastrophic lamp failure during operation. Water Research Foundation (WRF) Report 3117 included some research to estimate mercury releases from UV lamp breakage events. The WRF report found that mercury release would be well below the MCL for low pressure lamps, but did find situations where higher levels of mercury was released during a medium pressure lamp breakage. The key recommendation from the WRF report was for each WTP tp develop a Lamp Breakage Response plan in order for WTP staff to respond quickly to lamp breakage events to limit mercury release into the potable water.