Water Environment Federation (WEF)

Sensor Setpoint Validation of Reclaimed Water UV Reactors


Reclaimed water ultraviolet light (UV) disinfection system can be a critical component of a multiple barrier approach for public health protection. Current control methodologies rely upon assumed end of lamp life factors and sleeve fouling factors, which often result in overdosing when the lamps are new and the sleeves are clean and underdosing as the lamps age and the sleeves foul. The use of calibrated UV intensity sensors to monitor real time delivered UV dose results in energy savings and dose delivery confidence, as detailed here.

Drinking water ultraviolet light (UV) disinfection systems have pioneered the effective method of real-time UV dose monitoring through the use of calibrated and reliable sensor technologies. Reclaimed water UV reactors have traditionally not followed this efficient method of dose monitoring and reactor control for two main reasons. First, reclaimed water UV systems are substantially larger than their drinking water cousins on a per gallon basis because of the substantial difference in ultraviolet light transmittance (UVT) between various water and reclaimed water supplies. Second, reclaimed water UV systems have traditionally had higher dose targets, but less stringent equipment monitoring requirements, than drinking water reactors. The combination of these two factors, coupled with the expense and intricacy of incorporating numerous calibrated and reliable UV sensors, has kept the reclaimed water UV industry away from real-time dose monitoring. Further, the reclaimed water UV industry has strayed far from accurate and reliable UV intensity sensors, with the majority of UV sensors providing little to no real value in reclaimed water UV operation and control. Conventionally used UV sensors for reclaimed water applications can only be relied upon to indicate catastrophic dose failure.

A need for a true understanding of real-time delivered UV dose is emerging for high dose UV disinfection for sensitive wastewater reclamation applications. The Ultraviolet Disinfection Guidelines for Drinking Water and Water Reuse (National Water Research Institute/American Water Works Research Foundation, May 2003), hereafter called the 2003 UV Guidelines, has laid a solid foundation for proper research and design of UV reactors for wastewater reuse applications. The conventional application of the 2003 UV Guidelines includes a rigorous bioassay followed by a derating of measured reactor dose by appropriate aging and fouling factors. An example follows:

Reactor XYZ has been tested and approved over a range of flow, power settings, and UVT. The end result is a dose equation for the reactor with new lamps and clean sleeves as shown in Equation 1.

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