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Validation method brings performance gains to UV disinfection

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By Steve Green and Kirsten Meyer

Unlike chlorine, ozone and other chemical disinfectants whose dose can be quantitatively determined by a CT value, determining the right ultraviolet (UV)  dose is not so simple. Until relatively recently, UV design engineers have faced a dilemma. Open channel UV disinfection system design has typically been based on either a calculated dose approach, which assumes ideal flow and UV intensity distributions (and not reflecting actual real system performance), or a conservative design approach, which ensures adequate performance but often results in significant overdesign.

The first approach risks noncompliance, while the second risks wasting capital and operating funds. Currently, the only way around this dilemma has been to conduct pilot testing. The design team determines the site-specific dose response characteristics of the pathogens of concern, together with appropriate UV system performance assessment, in order to develop the “best fit” design. Pilot testing an open channel UV system is very expensive and time-consuming.

Recently, improved understanding of how different pathogens respond to UV light has resulted in a major shift in the necessary sizing of UV systems for wastewater, allowing for best fit design without pilot testing. Qualified, extensive UV system validation testing that considers a large “validation envelope” and is in line with existing guidelines, now offers utilities and design engineers a more cost-effective and scientific approach for open channel UV system design.

UV Disinfection Guidance Manual 06

This positive shift in the necessary sizing of open channel UV systems was initiated after publication of the UV Disinfection Guidance Manual (UVDGM) by the United States Environmental Protection Agency (USEPA) in 2006. Different dose levels were determined, together with log inactivation credits by surrogates.

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Unlike alternative UV validation methods, such as the German DVGW W-294 and Austrian OENORM M 5873, which only test to a single UV dose set point of 40 mJ/cm2 Reduction Equivalent Dose (RED), the USEPA UVDGM validation allows for the selection of multiple data points. The result is a highly flexible UV performance prediction formula that can be applied to a broad range of disinfection targets. This way, dose response behaviours, which vary by type of organism (always specific to the range tested) are accounted for.

Cryptosporidium, an organism difficult to destroy with chlorine, is effectively inactivated by a relatively small UV dose, while viruses require a significantly larger UV dose to achieve the same log reduction.

In order to properly describe the performance of a tested UV system on different organisms, a new term ‘DL’ was established. It describes the dose required for 1 log reduction, also known as UV sensitivity. Incorporating such a term into a UV system’s validation formula, based on tests with surrogates exhibiting different UV sensitivities, allows one to determine how a UV system disinfects organisms of different UV sensitivity.

DL Approach

The DL approach allows UV system designers to determine a site-specific sensitivity via collimated beam tests for the project. This in turn allows for “best fit” design instead of more expensive pilot testing, provided design conditions are covered by the validation test conditions (validation envelope).

Based on the guidance provided in the UVDGM 06, the DL approach may allow for increased flow capacity of given UV systems for wastewater, compared to previous design approaches. This offers significant cost savings for upcoming wastewater projects. Also, many installed open channel systems in North America can be reviewed according to the latest guidelines and benefit from favorable validation results.

The DL approach allows for significant flexibility, depending on validation goals and the potential applications for full-scale UV systems. A biological surrogate reacts the same way to UV light as the critical bacteria, but does not have its harmful impact. Testing with multiple surrogates that bracket, or encompass, the range of UV sensitivities allows for tailoring the site design to the site-specific UV sensitivity of the target organisms (see Figure 1).

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Figure 1.

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