Harold Wright, David Gaithuma, Trinia Dzurny, Chris Schulz, Kevin McCurdy, Travis Bogan, Alexander Cabaj, Alois Schmalweiser, Yoshi Ohno, and Thomas Larason

OBJECTIVES:

The project objectives were to (1) develop a comprehensive understanding of current guidelines for UV measurement, dose monitoring, and sensor technologies, (2) quantify the capabilities and limitations of commercial sensors in use today, (3) identify and evaluate the impacts of sensor properties on dose monitoring with a focus on the differences between commercial technologies and current guidance, and (4) develop and obtain stakeholder buying on guidelines for UV sensors and dose monitoring.

BACKGROUND:

The Long-Term 2 Enhanced Surface Water Treatment Rule (LT2ESWTR) lists ultraviolet disinfection as a technology for achieving Cryptosporidium, Giardia, and virus inactivation credits. Disinfection credit is based on performance indicated by a validated online monitoring system that uses UV intensity measured using UV sensors. UV sensors should provide accurate and reproducible dose monitoring. Issues with UV sensors include guidelines not agreeing on sensor properties, commercial UV sensors being used that do not meet guidelines, and poor sensor performance observed with UV installations.

HIGHLIGHTS:

Field and laboratory studies showed significant variability with UV sensors, especially with UV sensors used with medium-pressure (MP) UV systems. Spectral and angular response criteria specified by current guidance can cause significant variability. UV sensor accuracy can be significantly improved by controlling the angular orientation of sensors within the sensor port, field calibration of duty sensors using the average reading of three reference sensors, and calibrating MP reference sensors using polychromatic light. Computational fluid dynamics (CFD)-based analysis of dose monitoring shows that Reduction Equivalent Dose (RED) bias impacts on dose delivery can be predicted using biodosimetry data measured using one or two test microbes.

APPROACH:

To meet the project objectives, the research team (1) conducted a literature review and vendor survey, (2) developed instrumentation for measuring UV sensor properties with different UV sensor designs, (3) measured the performance of current UV sensors under field and laboratory conditions, (4) analyzed the impacts of sensor properties on dose delivery monitoring using the latest in CFD-based dose and UV intensity modeling, and (5) obtained feedback on draft guidance from stakeholders.

RESULTS/FINDINGS:

The literature review and vendor survey provides comprehensive information on UV sensor components and properties. Advances included photodetectors that respond only to germicidal UV light. The European and North American field studies showed significant duty and reference sensor variability with MP UV sensors (> 10 percent), but much less variability with LP sensors (< 10 percent). Many utilities were not conducting regular UV sensor and online UVT monitor checks. Duty sensor checks using the average of three reference sensors and field calibration of duty sensors significantly improved dose monitoring accuracy. Only one UV sensor type showed significant long-term degradation. The field studies demonstrated the importance of transparent dose delivery monitoring. Lab study quantified measurement properties of nine UV sensor types from six manufacturers. All sensors showed significant non-linearity at low UV intensities that causes calibration errors when UV sensors designed to measure high irradiances are calibrated using low irradiance from 254 nm sources. UV sensor calibration reports should specify sensor's reference plane. Significant variability was observed with spectral and angular responses that can be minimized using polychromatic calibration and field calibration, respectively. Four sensor types showed significant long-term degradation with UV exposure.

IMPACT:

The report proposes UV sensor guidance endorsed by the UV system manufacturers. Field calibration of duty sensors is being used by at least one UV vendor. UV vendors have adopted calibration of MP UV sensors using polychromatic light. UV sensor guidance has been proposed to the AWWA Standards Committee. At least one UV vendor is using validated dose monitoring algorithms that accounts for RED bias impacts of dose delivery. Adoption of guidance and dose monitoring algorithms proposed in this report will significantly improve the accuracy of dose monitoring by UV reactors thereby providing significant public health and financial benefits.

MULTIMEDIA:

The project deliverables include CFD-based software for calculating log inactivation and RED with four UV reactors representative of commercial drinking water technologies, all as a function of flowrate, UVT, lamp on/off status and power setting, and microbe UV sensitivity.

RESEARCH PARTNER:

Capital Regional District Water Department

PARTICIPANTS:

Participants included the National Institute of Standards and Technologies (NIST), nine North American and nine European utilities, and eight UV system and UV sensor manufacturers.