Fresh water is a major resource constraint in the world today. Changing rainfall patterns, low levels in traditional water reservoirs, increased deforestation and increased water demands from population growth and improved sanitary needs are leading to water scarcity in almost all developed and developing countries. Energy efficient seawater desalination is the best solution to meet the growing fresh water needs of the world’s population and its increased practice of water-intensive agriculture. Wastewater treatment has also come of age; most cities and municipalities perform some form of water treatment, prior to groundwater replenishment, discharge or runoff. The market for seawater desalination and for water purification from other water sources is of significant size, and is increasing annually. According to Lux Research, the world’s water needs are expected to grow 40 percent by 2030, and today’s stressed water resources and systems are unsustainable. Water cultivation systems are expected to see market growth from $522 billion in 2007 to nearly $1 trillion in 2020.
Challenge
Two main directions have conventionally been investigated for efficient desalination: evaporation techniques and membrane processes. Evaporation techniques have used multi-stage flash distillation (MSF), multi-effect distillation (MED) or vapor compression technologies. The MSF technology’s worldwide capacity currently adds up to greater than 48% of installed desalination plants. Membrane processes, mainly reverse osmosis (RO), are presently the fastest growing technology, approximating around 22% of world desalination capacity. RO uses dynamic pressure to overcome the osmotic pressure of salt solutions, allowing water-selective permeation of salt-free water from the saline side of a membrane to the freshwater side. However, the capital and operating costs of these processes remain inordinately higher than the cost of fresh water. The cost barrier of desalination has steadily been decreasing, and has come down to the level of 50-80 cents/m3 ($617-987 per acre-foot) of desalinated water. Desalination of brackish water by RO processes is even cheaper, ranging from 20-35 cents/m3 ($247-432 per acre-foot). However, these costs are still inordinately higher than the cost of fresh water, sometimes by a factor of five or more. Thermal desalination requires inordinate amounts of heat to overcome the latent heat of vaporization of water (540 kcal/l or 663,331,689 kcal per acre-foot), though use of waste heat and steam from adjoining power plants allows improved system economics. Similarly, RO systems need high pressures and extensive pre-treatment of seawater to allow sufficient permeation through the membrane, leading to a seawater conversion rate of around 35-50%. Initial membrane costs and replacement issues lead to high capital investments and operating costs for RO systems. For seawater desalination to be of practical use, the cost of desalinated water needs to be on parity with conventional fresh water costs; if this can be achieved, cost-efficient desalination plants can be used for all coastal fresh water consumer and irrigation needs.
Solution
QuantumSphere Inc. (QSI) has developed an alternative technology for a low-cost seawater desalination system. The technology is based on the principle of forward osmosis (FO) to separate water from seawater, in a low-energy, ambient temperature and pressure process, using novel, organic solutes, capable of solute-solvent separation downstream of the FO process. Forward osmosis has been a relatively new process technology used for desalination of seawater. Unlike reverse osmosis (RO) processes, which employ high pressures ranging from 1000-1500 psi to drive fresh water through the membrane, forward osmosis uses the natural osmotic pressures of aqueous solutions to effect fresh water separation. A ‘draw-down’ solution, having a significantly higher osmotic pressure than the saline feed-water, flows along the permeate side of the membrane, and water naturally transports itself across the membrane by osmosis. Osmotic driving forces in FO can be significantly greater than hydraulic driving forces in RO, leading to higher water flux rates and recoveries. Thus, it is a non-pressurized system, allowing design with lighter, compact, less expensive materials. These factors translate in savings both in capital and operational costs. Energy represents about 40% of the costs of RO desalination (and around 80% of the costs of thermal desalination) and is projected to increase with the upward trend in energy prices. In addition, the lower amount of more highly concentrated by-product brine is also more easily managed. Specific advantages of the QSI technology over thermal distillation processes include the ability to purify water from the seawater feed at ambient temperatures and pressures, and the negligible corrosion rates expected in the operational environment, allowing use of common alloys like copper, aluminum and stainless steels for the processor, due to non-existent scaling behavior at the temperatures of operation. Specific advantages of the proposed technology over conventional RO processes include much lower pressures of operation, lack of extensive pretreatment, and the lowered total dissolved solids (TDS) levels (5-50 ppm as compared to 300-500 ppm for RO). In addition, the FO process can also be used upstream of conventional RO process plants, allowing for increased energy efficiency at much lower capital costs.
