(521e) Minimization of RO Desalination Cost: Considerations of Membrane Permeability and Feed Salinity Variations
Reverse osmosis (RO) membrane water desalination is now well established as a mature water desalination technology. The cost of water production in a typical RO desalination plant generally consists of the cost of energy consumption, equipment, membranes, labor, maintenance and financial charges. Considerable efforts, dating back to the initial days of RO development in the early 1960s have been devoted to minimizing the specific energy consumption of water desalination. The introduction of highly permeable membranes in the mid 1990s with low salt passage has generated considerable interest, given their potential for reducing the energy required to attain a given permeate flow. Current low pressure RO membranes have made it economically and technically feasible to desalt brackish water and seawater on a large scale. Recently we developed a theoretical model for the minimization of energy consumption with and without energy recovery devices (ERDs), considering also membrane cost and the cost of brine management. The approach enables quantification of the optimal recovery which depends on various factors (e.g., use of energy recovery devices, brine disposal cost, membrane cost and the topological arrangement of membrane modules). Specifically, we found that a two-stage operation can decrease energy usage, while two-pass consumes more energy than single-pass operation.
The present study builds on previous work and addresses two major questions pertaining to the optimization of RO membrane desalting: (a) Ss there a measurable benefit in developing membranes that are more permeable than the current generation of high permeability RO membranes? and (b) Can one develop an optimization approach, to arrive at an operational strategy for minimizing the energy cost of RO desalting, when feed water salinity is temporally variable? For example, studies at a recent RO plant location in California have shown feed water salinity variations of up to 20% from the annual mean. With respect to the first question, the present analysis suggests that further significant improvements in RO membrane permeability are less likely to be the primary driver for reducing the cost of seawater desalination. The primary reason for the above conclusion is that with the availability of high permeability RO membranes, the net driving pressure of the exit brine stream for RO desalting now approaches the limit imposed by the thermodynamic restriction. Measurable reduction in RO water production cost can arise from a variety of other process improvements including, but not limited to, optimization of process configuration and control schemes (including optimization with respect to temporally fluctuating electrical energy costs), utilization of low cost renewable energy sources, as well as more effective and lower cost feed pretreatment. With respect to the second question, the present optimization approach has shown how effective operation of RO desalination plants, for feed water of temporally varying salinity, can be accomplished by employing an optimal time-varying operating policy to produce a constant permeate flow.. Analysis of extensive RO field data suggests that when am optimal operational strategy is employed, energy savings increase dramatically with the amplitude of feed salinity fluctuations. Finally, we studied the optimization of the brine management of single-stage RO desalting of a field water. In this case, the primary RO concentrate was treated by a chemical demineralization process and a secondary RO process is performed to decrease the concentrate volume.