| Current desalination methods, while effective, require significant energy consumption and expense. This study proposes an alternate means of producing fresh water from saline and challenging feed water sources. This new method, ammonia - carbon dioxide forward osmosis (FO) desalination, allows for high recovery of highly saline and fouling feedwaters with significantly reduced prime energy consumption and expense. Initial investigations into the feasibility of the process for the desalination of a seawater salinity feed showed that high membrane fluxes (up to 15 GFD), high salt rejections (>98%), and high feedwater recoveries (>85%) were achievable using forward osmosis. Further modeling of the recycling of the draw solutes indicated that the energy use of the NH3-CO2 FO process would be significantly reduced (∼/10 the electrical energy of RO and 1/3-1/2 the thermal energy of MSF and MED) compared to the conventional desalination processes of RO, MSF, and MED. Energy use was further reduced in modeling of the use of multiple distillation columns to act as "stages" in the solute recycling. A lab-scale pilot system was constructed and tested in continuous, steady state desalination of a synthetic brackish water feed stream consisting of 3500 ppm NaCl in deionized water. The process produced water with <550 ppm NaCl and <1 ppm NH3 in the manner predicted in the studies leading up to its construction. Several proposals for future research to improve modeling accuracy were additionally proposed. In the course of the research on FO desalination, the novel draw solution was found to be applicable to the generation of power in a closed cycle pressure retarded osmosis system, referred to as an osmotic heat engine (OHE). Membrane power densities of up to 250 W/m2 were predicted, as were engine efficiencies of up to 16% of the efficiency of a Carnot engine over the temperature range of 40-160°C. The OHE appears to hold great promise for the production of carbon neutral power from low temperate heat sources, including extended geothermal resources, but its potential will be primarily determined by the cost and value of the power it may produce. |
