The objective of the AwwaRF Tailored Collaboration Optimzation of UV Reactor Validation is to improve the existing tools for UV reactor validation, thereby optimizing system sizing (reducing costs) while maintaining public health protection. This is being achieved through 3 main initiatives. The first involves finding a better challenge microorganism. The organisms most frequently selected for use in these tests are MS-2 coliphage (in the United States) and Bacillus subtilis spores (in Europe). Because these microorganisms are much more resistant to UV light than many waterborne pathogens, an uncertainty factor, termed the "RED bias", is required (USEPA, 2003b). Use of a RED bias safety factor can lead to over-sizing of hydraulically well-designed UV systems. The goal of this work was to identify and evaluate microbial surrogates whose inactivation characteristics better match the target pathogen(s), thereby reducing the required RED bias value. The second initiative involves finding a better UV-absorbing compound. During reactor validation, low UV transmittance at 254 nm (UVT) water conditions are simulated by the addition of UV-absorbing chemicals to the feed water. The most commonly used UV-absorbing compounds are coffee (NWRI/AwwaRF, 2000), flourescein (ÖNORM, 2002) and lignin sulphonate (DVGW, 1997). For a given UVT, the UV absorbance spectra of coffee, flourescien and lignin sulphonate differ substantially from that of water treatment plant (WTP) waters. This difference impacts both dose delivery and monitoring with UV systems equipped with medium-pressure (MP) lamps. This necessitates the use of an uncertainty factor to account for this difference in conditions between the test and the WTP. This safety factor is termed the "polychromatic bias." The second goal of this project is to identify and validate sources of NOM for lowering UVT during validation to reduce or, if possible, eliminate, the polychromatic bias factor. The third initiative involves investigating the impact of lamp and sleeve aging on dose delivery. Spectral shifts in the UV output of MP lamps (Phillips, 1983) and the UV transmittance of lamp sleeves (Kawar, 1998) with aging has been reported. Very significant visual changes have been observed in drinking water pilot studies (Mackey, 2004) and at many wastewater treatment plants. The degree of spectral shifts in lamp output and sleeve UV transmittance with commercial UV reactors used in drinking water applications is not known. If significant, there will be a need to restrict sensor response or use appropriate safety factors to account for these effects. For this reason, this third task involves the characterization of the UV output of new and aged (during standard operation at a water or wastewater plant) UV lamps and sleeves. This paper summarizes the interim results of this on-going study and the implications for the adoption of new validation tools in full-scale applications. Includes 10 references, tables, figure.