Development of a New Forward Osmosis System for Desalination

One route to cope with the rising demand for fresh water— as well as to mitigate the vulnerability of industrial and agricultural infrastructure to water scarcity— is to expand water resources. The limited availability of traditional sources, however, has focused attention on harnessing nonconventional sources for water production. However, these sources require additional energy input for treatment to render them useful.

Desalination— water extraction from saline sources such as seawater, inland brine aquifers, produced water, and the like— is one pathway to ensure a reliable source of fresh water for the foreseeable future. Desalination by reverse osmosis (RO), an electrically driven process, is a proven method and considered a critical element of future water management options in conjunction with water conservation and reuse.

Reliance on electrical energy— such as in reverse osmosis (RO)— comes with two significant drawbacks: (a) the lost opportunity to use the power elsewhere; and (b) the increased carbon footprint of the treated water. To minimize the adverse impacts of desalination while expanding water availability, it is important to couple desalination with renewable energy, use low-grade heat to drive thermal desalination, and increase operational efficiency.

Forward osmosis (FO) has been investigated for a number of years as a desalination method to solve the freshwater shortage. In particular, instead of applying 50+ atmospheres of pressure to force water through a filter as in RO, FO takes advantage of natural osmotic pressure differentials to move liquid through a semipermeable membrane. The principle of FO uses the ability of water solutions containing high concentrations of inorganic or organic substance (called draw solutions) to draw water from brackish or seawater and in some instances even saturated salt solutions. Two inherent challenges are being addressed to make the use of FO more widely applicable: (a) the development of appropriate membranes; and (b) a method to reconstitute the draw solution using low-grade thermal energy in a safe, simple configuration.

A new FO technology, dubbed Aquapod©, developed at the Illinois Sustainable Technology Center, uses a noncorrosive, nontoxic, stable osmotic solution (aqueous magnesium sulfate) to raise the osmotic pressure on one side of the salt-filtering membrane. The increased osmotic pressure pulls water through the membrane, leaving much of the salt behind. In a departure from established FO practices, the diluted draw solution is recovered for reuse using a combination of a thermo-reversible polymer and heat energy. The end result is a safe, simple process that results in a lower carbon footprint than current FO processes.

The goal of ISTC’s Kishore Rajagopalan is to scale up the Aquapod (FO) process to achieve lower capital and operating costs. At this stage in its development, the method appears most applicable to small- or medium-sized desalination units and for thermally sensitive operations such as food and pharmaceutical processing. The Aquapod process has been issued Patent No. US8852436. Trevi Systems (CA) has procured an option to license.