A new forward osmosis (FO) technology, dubbed Aquapod©, was developed at the Illinois Sustainable Technology Center by researchers Kishore Rajagopalan and Vinod Patel. It 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 or contaminants 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.

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 freshwater sources, however, has focused attention on harnessing nonconventional sources for water production. But, these sources require additional energy input for treatment to render them useful.

Desalination - water extraction from saline water 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.

However, reliance on electrical energy for water production - such as in 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.

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 semi-permeable 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 water 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. The Aquapod© process meets both of these requirements; and the end result is a safe, simple process that has a lower carbon footprint than current FO and RO processes.

The next goal of ISTC's 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.

Applications in Coal-Fired Power Plants

Coal-fired power plants use tremendous amounts of water - 60 billion-170 billion gallons annually - for cooling and pollutant removal, then release the wastewater to wastewater treatment plants where it is expensive to treat. ISTC researchers Kishore Rajagopalan and Kevin OBrien are examining the scalability of Rajagopalan’s patented waste heat coupled forward osmosis (FO)-based water treatment system, Aquapod©, to improve wastewater quality and reduce water usage in a coal-fired +500 megawatt power plant.

Conventional membrane technologies require electricity and are not as efficient. The Aquapod© process works on waste heat from power plants rather than direct energy inputs. By mapping both wastewater and waste heat availability at a power plant, the researchers will match the available heat sources to the wastewater being treated. They will evaluate the Aquapod© FO technology when processing degraded water streams, including flue gas desulfurization and cooling tower blow down, ash pond effluent, plant wastewater sources, and treated municipal water.

“The target is to enable the recovery of at least 50% of the water in highly degraded water sources cost-effectively without extensive pretreatment,” Rajagopalan said.

The researchers maintain that the Aquapod© FO process can double to quadruple the amount of water recovery from power plant effluents per unit of input energy compared to the current state-of-the-art processes. The patented process is also safer because it does not use gaseous ammonia like in other wastewater treatment methods.

Trimeric Corporation, an engineering firm, will provide guidance on system design, engineering, and scale-up.