The Metropolitan Water District of Southern California (Metropolitan), in conjunction with Koch Membrane Systems (KMS), evaluated three generations of a 16"-diameter by 60"-long reverse osmosis (RO) element in parallel with a commercially available 8"-diameter element. Design inefficiencies in the first-generation 16" element resulted in a 20% lower specific flux when compared to an 8" element. After making improvements in element design, the overall specific flux for the last-generation 16" element was shown to be comparable to that of the 8" element. However, a slightly higher fouling rate was observed for the 16" element. An economic evaluation comparing costs using both 16" and 8" diameter elements for a 185-million gallon per day (mgd) [700 million l/d (ML/d]] RO plant showed that a design using 16" elements may save as much as 12.4% ($4.2 million per year) in combined capital and operations and maintenance costs. The present value of capital-cost savings alone was estimated at $44.3 million - a 27% reduction. The majority of these cost savings were attributed to reductions in train piping, support frame, miscellaneous train equipment (i.e., instrumentation, cleaning skid, and prefilters), and footprint.