The Critical Role of UV in Meeting Pathogen and Pollutant Targets for IPR and DPR Applications
Austa Parker and Andy Salveson
Carollo Engineers, Inc.
2700 Ygnacio Valley Road #300, Walnut Creek, CA 94597
With increasing uncertainty in regional water supplies and the prolonged drought in the West, it is important for communities to diversify their water portfolios. Wastewater is a reliable supply that can be repurposed to provide another option for a community’s water source. Non-potable and potable wastewater reuses are two means of repurposing treated wastewater. Non-potable water reuse is valuable but has limitations, including the seasonal demand for non-potable water, the costs and complexity of extensive “purple pipe” systems and the common lack of irrigation or industrial customers to maximize water reuse. Indirect potable reuse (IPR) and direct potable reuse (DPR) provide an additional category in a municipal water treatment source portfolio, allowing the capture and use of all treated water through every season.
In IPR scenarios, secondary treated wastewater undergoes further treatment through an advanced water purification facility (AWPF) before being supplied to a groundwater basin or surface water supply, providing an environmental buffer. The environmental buffer is able to provide additional treatment through filtration, photolysis, etc. and provides “response retention time,” which is a set time value that allows for water quality monitoring and response. Currently in California, Title 22 regulatory requirements set by the California Division of Drinking Water (DDW) allow for groundwater recharge via surface spreading or groundwater injection of treated water (CDPH 2014). For groundwater injection, full advanced treatment (FAT) through an AWPF must be employed. FAT includes microfiltration/ultrafiltration (MF/UF), reverse osmosis (RO) and UV advanced oxidation (UV AOP).
DPR projects do not use the environmental buffer, instead relying upon advanced treatment and advanced monitoring systems to provide confidence in water quality. Engineered storage buffers provide an added measure of safety to the final water quality. DPR treats secondary wastewater effluent with an AWPF, and the purified water is either delivered straight to the consumer or first blended into a drinking water treatment plant for delivery (Figure 1). There are no current regulations for DPR in California or nationally. However, the state of Texas is approving DPR projects on a case by case basis, and the Colorado River Municipal Water District’s facility in Big Spring, Texas, has been successfully producing potable water through DPR for three years.
Meeting treatment requirements with UV processes
Treatment technologies and monitoring techniques are used to protect public health in potable reuse scenarios. Risk mitigation is the foundation for public health protection and setting technology treatment target limits. Research studies have shown the goal of potable reuse systems should be to eliminate the acute risk and mitigate the chronic risk through potable reuse treatment (WRRF 2014). Pathogens are the main, but not only, concern in acute risk scenarios, NDMA and other emerging contaminants represent the chronic risk. The UV process is beneficial for (1) disinfecting virus, protozoa and bacteria, (2) NDMA removal and (3) removal of a wide-range of emerging contaminants through advanced oxidation. Details of how UV processes provide treatment to meet these regulatory requirements are detailed below, with supporting data from a study conducted by Carollo Engineers at the Silicon Valley Advanced Water Purification Center (SVAWPC) with the Santa Clara Valley Water District (SCVWD) in Santa Clara, California.
Title 22 recycled water regulations require an IPR facility to demonstrate and meet 12-log virus, 10-log Giardia and 10-log Cryptosporidium (12/10/10) removal across the facility (CDPH 2014). Each individual treatment process in the facility cannot be granted with a removal credit greater than 6-log for each virus, Giardia and Cryptosporidium (6/6/6). Therefore, multiple processes must be used (MF/RO/UV AOP) to receive the necessary log removal credits. The UV process provides, by far, the most significant pathogen barrier, being the only process in the treatment train capable of being awarded all 6/6/6 log removal credits.
No measurable concentrations of these pathogens are typically found in RO permeate (UV influent); however, an additional barrier is necessary in the event of off-spec water passing through the RO system and for meeting the 12/10/10 requirements. For UV disinfection, the most resistant known pathogen to low-pressure (LP) UV inactivation is Adenovirus. The UV dose response of Adenovirus type 2 (Ad2) shows a 6-log reduction of Ad2 can be obtained at a UV dose of 235 mJ/cm2 (Gerba et al. 2002). This provides a conservative UV dose setpoint for pathogen inactivation in potable reuse systems.
A typical UV dose for UV AOP systems following RO in potable reuse facilities is >800 mJ/cm2. This provides a much higher dose than necessary to achieve the 6/6/6 log-removal requirement for pathogen disinfection. Demonstration of pathogen disinfection via seeded MS-2 removal, with simulated UV process failures was conducted at the SVAWPC. Simulated failures included modulating power settings (50%, 75% and 100%) in parallel with intentional lamp outages (1 and 2 lamps out), with the MS-2 log-removal response being measured. Results from this challenge testing show even with lamp outages and significant power reduction (50%), the UV system was able to provide 6-log removal of MS-2 virus (Figure 2). This challenge testing demonstrates the robust nature of the UV process to provide a high level of performance in RO permeate, even in failure scenarios.
N-Nitrosodimethylamine (NDMA) is a disinfection byproduct formed in water and wastewater treatment. The US Environmental Protection Agency (EPA) defines NDMA as a human carcinogen with a carcinogenic risk level of 0.69 ng/L in drinking water (EPA 2016). NDMA is poorly removed by RO membranes (Plumlee et al. 2008). UV is proven to destroy NDMA through photolysis, with 90% removal based on a UV dose of ~900 mJ/cm2 (Sharpless and Linden 2003). Title 22 regulations require potable reuse facilities to meet a limit of 10 ng/L following the UV process, which is achieved in FAT facilities using a high UV dose, typically between 700 and 1,100 mJ/cm2. New methods of tracking NDMA removal through UV treatment are needed, as the analytical turn-around time for the samples does not allow for online monitoring. During the study performed at the SVAWPC, NDMA removal and total chlorine destruction were measured simultaneously with the lamp failures from the MS-2 removal study in the previous section (Figure 2). Results from the study are shown in Figure 3. NDMA removal was impacted by UV lamp and power failures, with the total chlorine destruction corresponding to the NDMA removal changes. Thus the UV process provides both NDMA destruction and the ability to indirectly monitor final effluent quality through periodic chloramine destruction testing. Note: Work from this same study indicates that hydrogen peroxide will interfere with the accuracy of chloramine monitoring.
Emerging contaminant removal
Emerging contaminants not only pose a potential risk to human health, but are also an integral part of public perception and acceptance of potable reuse projects. RO removes most emerging contaminants found in secondary treated wastewater effluent, leaving few contaminants to be treated by UV processes, aside from NDMA. An additional barrier is needed following RO, in case of passage of contaminants through RO. UV with the addition of an oxidant (NaOCl/H2O2) generates non-selective hydroxyl radicals that are fast-acting and effective for a wide-array of emerging contaminant destruction.
To validate UV AOP systems for emerging contaminant removal, Title 22 regulations require the demonstration of 0.5-log removal of 1,4-dioxane through an advanced oxidation system. This requirement is based on 1,4-dioxane being a conservative surrogate for AOP performance, as it is not amenable to UV degradation. Figure 4 shows the hydroxyl radical reaction rate for NDMA and 1,4-dioxane relative to several other emerging contaminants commonly found in secondary treated wastewater effluent (Hokanson et. al. 2011). 1,4-dioxane is shown to be a conservative indicator for the removal of many other pollutants. Demonstrating 0.5-log removal of 1,4 dioxane provides confidence in the UV AOP system functionally destroying emerging contaminants from RO permeate.
UV monitoring and control strategies
The success of potable reuse systems is crucially dependent on reliable treatment and monitoring tools, with the definition of reliable in potable reuse systems meeting the four “Rs”: Reliable, Robust, Redundant and Resilient (Pecson et al. 2015).
Monitoring techniques, such as a peroxide weighted dose, are being explored due to the ability to monitor these parameters online. Hydrogen peroxide dose, UV sensor and UVT monitoring allows for a correlation between UV dose and peroxide to be found, and for a given system with 1,4-dioxane removal demonstration, the optimized UV and hydrogen peroxide dose can be determined, providing confidence and potential cost savings for the facility. Figure 5 shows an example of data demonstrating the effectiveness of a peroxide weighted dose and correlation of 1,4-dioxane at the SCVWD during pilot testing. A peroxide weighted dose correlated well with the measured removal of 1,4-dioxane.
Future of UV and UV AOP in potable reuse
Monitoring and sensor technologies are important for the viability of UV processes for potable reuse in the future. Providing reliable sensor technologies that are designed for high UVT water (>99%) from RO permeate is paramount for future system design. Additionally, optimizing oxidant and UV dose for a given AOP system and a given oxidant will provide utilities with reliable operation and potential cost savings. This optimization is leading to further pilot testing, and more importantly data collection, across the industry.
Large data sets from several municipalities are aiding in driving the industry forward for setting baseline performance of treatment technologies in potable reuse. A baseline performance will allow for online monitoring tools developed for UV processes to more accurately detect when normal operation is not being met. This information will provide further confidence in public health protection via UV processes in potable reuse applications.
The authors would like to acknowledge the Santa Clara Valley Water District and their engineering and treatment plant staff for their support and providing data from the SVAWPC for this paper.
CDPH. 2014. Groundwater Replenishment Using Recycled Water (Water Recycling Criteria. Title 22, Division 4, Chapter 3, California Code of Regulations). California State Water Resources Control Board Division of Drinking Water. http://www.waterboards.ca.gov/drinking_water/ertlic/drinkingwater/documents/lawbook/RWregulations_20140618.pdf. Published 6/18/14. Final.
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