A PARTNERSHIP BETWEEN GEOLOGISTS AND ENGINEERS: CONTROL OF BIOFOULING AT SEAWATER DESALINATION PLANTS

Seawater desalination is being used worldwide to produce potable water in regions with limited freshwater resources, especially in the Middle East, the Mediterranean, western coastal South America, the Caribbean, and in southern California. The most common and energy efficient desalination process is reverse osmosis. Seawater reverse osmosis (SWRO) operates by pumping seawater at a pressure of about 1000 psi (about 69 bar) through a polyamide membrane to remove the salt and produce a freshwater stream and a concentrate stream. The process is about 50% efficient in producing freshwater from average seawater with a TDS of 35,000 mg/L. The biochemistry of seawater is quite complex and a series of pretreatment processes must be used to remove organic matter before the seawater is allowed to enter the SWRO membranes. Despite extensive pretreatment of the raw seawater, the most vexing problem of the SWRO process is the biofouling of the membranes which reduces process efficiency, causes frequent chemical cleaning of the membranes, and increases cost of water treatment. Over the past decade geologists have been working with engineers and microbiologists to improve the quality of the seawater before it enters the treatment plant. The use of wells instead of surface intake systems has been demonstrated to remove all of the algae, up to 95% of the bacteria, about 50 % of the total organic carbon, and up to 95% of the biopolymer fraction of dissolved natural organic matter which contain acidic polysaccharides and other sugars. Well intake systems have reduced the number of necessary cleanings of the SWRO membranes by 50 to 75%, resulting in a treatment cost reduction of up to 20%. A new technology termed seabed filtration has been demonstrated to achieve high levels of organics removal. In the process an engineered filter is installed in the seabed and acts in a similar manner to slow sand filtration. Cooperative engineering and geological research has led to a better understanding of the marine chemistry of organic compounds and how to remove them using the natural system.