(403a) Advances in Seawater Ro Desalination

The economics of seawater desalting using reverse osmosis technology has been continuously improving with a reduction of product water cost as a result of lower investment costs and decreased power consumption. The practical examples of the decreasing desalted water trend are recently built or awarded large, 55,000 ? 330,000 m³/day (14.5 ? 87 mgd) RO seawater projects: Larnaca, Cyprus and Ahskelon, Israel. In these projects the cost of desalted water produced is significantly below $1.0/m³ ($3.8/kgalon). The reduction of capital and operating cost has been achieved by more efficient system configuration and better selection of construction materials. Energy consumption of the RO process has decreased due mainly to two technology developments. One of the significant improvements was development and commercial introduction of high permeability, low salt passage (high rejection) seawater RO membranes. Another important contributor to reduction of energy consumption was the improvement of efficiency of pumping equipment and utilization in seawater RO applications of new energy recovery devices (e.g., pressure exchangers). The energy consumption strongly depends on feed water salinity and the operating parameters of the RO unit. It is an aggregate value of the energy used in the pretreatment system, energy to pump feed water to high pressure, energy required to treat and pump permeate and energy to run auxiliary equipment. The combined energy usage in seawater desalination in large desalination plants, built recently, is in the range of 3 ? 4 kwhr/m³ (11 ? 15 kwhr/kgallon). The fraction of energy utilized by high pressure pumping system is about 80% of the total enegry usage. It is well recognized that the oeprational reliability of seawater membrane system depends on the quality of feed water. Feed seawater to large RO systems is supplied from open intake structures located off shore. This type of water source requires effective pretreatment due to high concentration of colloidal matter. Until recently pretreatment systems were mainly based on a ?conventional? water treatment technology: coagulation, flocculation and media filtration followed by cartridge filtration. The conventional pretreatment is not always effective in producing feed water of sufficient quality. Therefore, use of membrane pretreatment, based on microfitration (MF) or ultrafiltraion (UF) technology, is being evaluated and tested as a potential pretreatment method to improve feed water quality. Field test results indicate ability of MF and UF to treat seawater and produce high quality RO feed water. It must be recognized, hwoever, that the major impediment of the wide spread application of membrane pretreatment in desalination plants is relatively high cost of this pretreatment option as compared to the conventional filtration solution. Nevertheless, a number of medium size RO seawater desalination systems with membrane pretreatment are being built and should start commercial operation soon. In addition to description of recent experience with large scale RO desalination, an overview of modern RO seawater desalination technology will be provided with indication of current and future drivers for RO process development. Equipment performance and process parameters that affect process economics and oeprational reliability will also be discussed.