Reducing costs for large-scale desalting plants using large-diameter, reverse osmosis membranes

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.

Evaluation of three generations of 16×60 elements showed improved performance, in terms of specific flux, with each successive generation. The 16″-diameter RO elements required modifications in leaf lengths, permeate carrier, and numbers of leaves to improve performance. The specific flux of the final 16×60 element when compared to a commercially available 8×40 or 8×60 element was 0.31 and 0.30 gfd/psi, respectively. Salt rejection for both 16″ and 8″ elements was generally greater than 98.5%. Although specific flux and salt rejection data showed that the third-generation 16×60 element was equivalent in performance to an 8×40 or 8×60 element, a slightly higher fouling rate was observed for the 16×60 element. With additional testing and modifications to the RO element, the fouling rate is expected to decrease. Preliminary cost estimates for an RO plant producing 185 mgd permeate flow showed cost savings when incorporating the 16×60 elements into the design. Compared to commercially available 8×60 elements, savings of $44.2 million (27%) in capital costs or $4.1 million (12%) per year for annualized capital and O&M costs are projected. The majority of cost savings using large-diameter RO elements came from reduced skid piping, support frame, other appurtenances, and project contingencies. Further research and development of a large-diameter RO element to improve the overall efficiency and fouling rate will continue to reduce the cost of large-scale (>100 mgd) RO plants.