New sea water intake filtration system

Abstract

The presence of matter in suspension form in the free water is becoming a major problem for installations withdrawing water from water bodies such as lakes, seas, and oceans. The problem is growing fast as the demand for clean water is steadily increasing. Many remedial systems have been used such as extending intakes far offshore to subturbulent suspended matter-free depths or by constructing invloved screening systems onshore which require skilled personnel for operation and maintenance. Elaborate pretreatment systems and filtration netrworks are employed in the fast growing reverse osmosis industry. To keep clean raw water flowing some highly automated systems have been used. These require highly skilled personnel to operate and maintain them. A new type of screening system have been developed by Elarbash Systems of Libya and M&S Systems International of Malta. The system can be located onshore. It utilizes physically and chemically stable, non-bioegradable and non-erodable materials in its screening system. It is concealed under the sea floor and hidden away from wave action. Gravity is the only force required to drive water flow through the filtration bed to the pump intake basin from which the water is pumped to the installation concerned. Cylindrical or box screens can be used to transport the water from the filter bed via a number of pipes to the pump basin. The sizes of the filter bed area and of the screens and pipes vary with the amount of clean water required and with the soil type of the site. The mixture of organic matter and soil present in suspension form in the water is moved around by rocking motion caused by waves is the principle cause for the cloging of intake systems, and filter facilities in reverse osmosis systems, damage to pumps, fouling and erosion of heat exchangers. This suspension therefore must be prevented from flowing into the intake pipes or channels and into the pump intake basin. If the screens were laid on the seafloor, the suspended organic matter and fine sand grains may flow into the pipes and block them. If the screens were made sufficiently fine to eliminate fine matter from flowing into them, a larger filtration field may be required thus raising the costs of the intake system. Haveing the filter bed buried under the sea floor inhibits the rocking motion of the organic matter-soil mixture suspended in the water and prevents the suspension from working its way through the screens. It also eliminates wave effects on the bed since the bed cover is at the same topographic level of the surrounding sea floor. Flow capacities actually increase during stormy weather. The filter bed system is constructed about 5 to 10 meters into the water from the shoreline and covered with ten to ninty centimeters of the site sand. The filter is manufactured in modular form to provide transportations and construction flexibilities. The filter bed can be expanded to cope with the increase of water demand. Costs of the system based on turn-kay basis is estimated to be less than 30% of that of conventional systems. In addition, it reduces the need for pretreatment for reverse osmosis plants to a large degree. The system is maintenance-free and requires no back wash. Costs are presently at around 300 US Dollars per square meter. 10m × 16m bed would provide enough raw water for a 5000 cubic-meter-per day RO plant constracted on the Arabian Gulf, where the beach sand is fine and mixed with silt.