ABSTRACT
Sea water desalination by reverse osmosis (SWRO) has been in refinement since its inception some 25 years ago. Major efforts have been directed at improving membrane life, productivity and salt rejection. In the early 1980s, plants were operated, enerally, at recoveries of 25% and pressures up to 1000 psig, producing water meeting WHO standards of less than 500 mg/L TDS and chloride under 250 mg/L. The manufacturer warranted these facilities for 2 or 3 years. Salt rejections were in the 98.5% to 99% range. Plant energy consumption was about 45 Kwh/Kgal (12 Kwh/m3).

Over the past decades, many changes have taken place:membrane life has expanded such that suppliers will warrant a 10 or more year life at a fixed replacement rate. Product TDS can be 300 mg/L, at recoveries of 40% to 50% with system pressures raised to 1200 psig. Importantly, energy consumption has been halved to about 21 Kwh/Kgal (5.5Kwh/m3) as a result of higher conversions and the use of energy recovery devices. Still it is apparent that, in today's market, 75% to 85% of the total cost of water (TCW) is energy use and capital amortization. The remaining costs; membrane replacements, chemicals, labor and supervision, and maintenance parts, together, amount to only 20% to 25% of the TCW. Thus, energy reduction becomes the main focus to improving desalting economics.

A significant way to lower energy is to further raise system conversion. However, in doing this, the brine osmotic pressure begins to approach the applied pressure and this, in turn dversely affects product flow and quality. SWRO facilities are not designed efficiently; that is, they are limited by this mechanically derived differential between the osmotic and applied pressures. In brackish water, designs are based on the solubility product of the least sparingly soluble salt, as modified by antiscalants. Up to now, commercial membranes are not permitted to operate above 1200 psig (82.7 barg). Thus, in sea water, the osmotic pressure limitations are controlling versus the desired thermodynamic water chemistry.

Recently, membranes have been commercialized, which can operate at very high recoveries (55% to 65%), thus allowing SWRO plants to be at their most efficient level. The modules are capable of performing at high pressure [up to 1400 psig (96.6 barg)] with a salt rejection of 99.7% (58,00 mg/L feed). This paper discusses this innovation and its effect on energy consumption and capital in a Caribbean Sea plant. Data obtained from other areas of the world are also reviewed.