Impact of COVID-19 and Viruses On A UV Disinfection System Design Dose

INTRODUCTION
Waterborne transmission is known to be a key exposure pathway for enteric viruses and human norovirus (hNoV) is the number one cause of acute gastroenteritis globally (Ahmed, 2014). While the presence of enteric viruses in waterways and associated health effects are by no means emerging, monitoring and regulation of viruses in our water systems is of emerging interest for the protection of public health and the environment. The EPA and other regional regulatory agencies have been investigating the need for, and potential consequences of, regulating viruses in wastewater effluent – either in lieu of or in addition to current microorganism criteria for the fecal bacteria indicator organisms Escherichia coli (E.coi) and enterococci. Several considerable drivers exist in favor of enteric virus regulations including potentially higher environmental persistence than bacteria, higher resistance to treatment, and higher infectivity; all of which point to virus-based control being more protective of public health. However, significant hurdles have thus far prevented the successful deployment of virus regulations including: - variability in mechanisms and effectiveness of disinfection technologies, - knowledge gaps around health-based thresholds, - lack of feasible enumeration and quantification methods, - consensus on the most appropriate indicator organism, and - the perceived cost burden to an infrastructure system that is already substantially underfunded. Perhaps most importantly, 'there is no clear-cut epidemiological evidence linking viral GE (gastroenteritis) outbreaks from exposure to bathing waters that do meet criteria based on fecal indicator bacteria'' (Dorevitch 2016). While virus effluent limits are yet to be promulgated through wastewater discharge permits, consideration for regulations and their resulting impact on current and future wastewater disinfection systems have long proceeded the SARS-CoV-2 pandemic (Pandemic). Further, the pandemic has prompted significant pubic interested in the presence and threat of pathogens in waterways. The wastewater industry has proven its adaptability through implementing influent monitoring to predict coronavirus outbreaks and the Pandemic has ushered in a new perspective on both virus monitoring and potential regulation. As a result, disinfection projects occurring during the pandemic have been privy to the influence of virus-based control considerations, irrespective of official discharge regulations.
South Platte Renew (SPR) owns and operates the 189 megaliter per day (MLD) (50 million gallons per day [MGD]) SPR Water Resource Recovery Facility (WRRF) in Englewood, Colorado. Increasingly stringent nutrient limits being promulgated by the state of Colorado are driving the implementation of a new chemical phosphorus removal system and the conversion from chloramine to UV disinfection under the Chemical Phosphorus Removal and UV Disinfection Project (Project). The purpose of this paper is to present a case study highlighting how both heightened disinfection awareness resulting from the Pandemic and consideration for other non-regulated enteric pathogens influenced the Project design criteria, and therefore capital and operating costs, for the new UV disinfection system.
DISCUSSION
The state of Colorado's (State) design criteria guidance document, WPC-DR-1, requires a UV design dose of 30 millijoules per centimeter squared (mJ/cm2) assuming a UV transmittance of 65 percent. However, the Project conservatively selected a design UVT of 60 percent based on historical operating data. Collimated beam tests (CBT) were then performed by two UV equipment vendors (Trojan and Wedeco) to validate the UV dose necessary to achieve effluent E.coli limits. The results of the tests are presented in Figure 1 and show that at doses greater than 10 mJ/cm2, the effluent E. coli concentrations are reduced below SPR's effluent limit. Therefore, the state-required design dose of 30 mJ/cm2, based on validation using MS2 as the surrogate organism, is overly conservative by a factor of 2 to 3. Figure 1: Collimated Beam Test Results The State allows projects to request a design dose variance given sufficient seasonal operating data and/ or collimated beam testing. Given the CBT results presented above, the Project engaged the State regulators in the preliminary design phase to request a lower design dose of 20 mJ/cm2. The Project team compiled and presented information to support the argument for a lower UV design dose as follows.
1. The use of MS2 as a surrogate organism for UV system validation is not representative of E.coli-based disinfection control. The dose response curve presented in Figure 2 shows MS2 is an overly conservative surrogate organism and the same log inactivation is achieved for E.coli and T1 at less than 10 mJ/cm2.
2. The State provides tiered disinfection dose criteria for oxidant-based disinfection (such as chlorine) with increasing levels of upstream treatment (Figure 3). The SPR WRRF includes trickling filters and solids contact, nitrifying trickling filters, and tertiary denitrification filters and therefore the disinfection design dose should consider the impacts of increased degrees of upstream treatment.
3. There is no significant difference in the inactivation of enteric pathogens achieved for a dose of 20 compared to that of 30 mJ/cm2. Figures 4 and 5 summarize literature reporting the dose required to achieve 1, 2, and 3 log-inactivation of waterborne enteric bacteria, protozoa, and viruses of potential concern. For bacteria and protozoa, both doses easily achieve 3-log inactivation for all pathogens. Adenovirus, MS2, and rotavirus show similar resistance against both doses and therefore higher design doses do not provide a significant increase in disinfection projection. Figure 2: Dose Response Curve for Different Surrogate Organisms Figure 3: WPC-DR-1 Tiered Disinfection Dose Requirements for Chlorine Figure 4: Proposed and Required Dose Efficacy Against Various Pathogens
RESULTS AND CONCLUSIONS
While regulators and the Project team agreed the data presented supported the argument that a design dose of 20 mJ/cm2 would be protective of public health based on both existing bacterial permit criteria as well as other enteric pathogens of concern, the regulator denied the requested variance. The primary factor influencing their decision was the SARS-CoV-2 pandemic and associated public interest and trust in public water systems. The Project's final design criteria are summarized in Table 1. The new UV disinfection system will be retrofitted into the existing chlorine contact tanks and consists of two UV disinfection channels with four UV banks per channel (Figure 5).
The higher design dose requirement resulted a system that is approximately 50 percent larger than that of the requested dose increasing both capital and annual operating cost. In the near term and during low flow conditions, turndown limitations may result in the system having to overdose thereby increasing operational costs. Long term O&M cost impacts are also expected as the larger system requires additional maintenance and parts replacement for bulbs, ballasts, and wipers. The Projects is closing out the final design phase at this time and will be in construction at the time of final manuscript submission. Table 1: SPR Disinfection Design Criteria Figure 5: Rendering of SPR's UV System