This paper provides a rationale for key assumptions and decisions made to develop the design for the Ultraviolet (UV) system for the in-plant recycle stream treatment system at the East Bay Municipal Utility District (EBMUD) Walnut Creek Filtration Water Treatment Plant (WTP). Results of the analysis are as follows: Cryptosporidium is the target pathogen for UV inactivation in the recycle stream; free-swimming pathogens are the basis of UV dose selection; design dose for the system is 40 mJ/cm2; acceptable UV systems have validated UV dose and UV sensor response, closed-vessel configuration, and mechanical cleaning of quartz sleeves and UV sensor probes; UV transmittance of 85% (254 nm and 10 mm pathlength); approximate Capital Cost of $1.1 million for current conditions; and, approximate O&M Cost of $0.06 per 1,000 gallon for current conditions. Includes tables, figures.