Projects
Membrane Distillation (MD)
For the western states of the U.S. where untapped brackish groundwater and geothermal resources are abundant, membrane distillation (MD) offers a unique opportunity to desalinate geothermal brackish groundwater with innate heat to supplement freshwater demand. In contrast to RO, MD is not a pressure-driven process and relies mainly on the vapor pressure gradient across hydrophobic membranes to drive the production of distilled water. Membrane distillation is especially attractive to horticulture and aquaculture operations that utilize geothermal brackish water as heating sources and where the demand of quality water for irrigation or cultivation is high.
Direct contact membrane distillation (DCMD) is the simplest and the most commonly used configuration of MD for water desalination. In DCMD, both the hot brackish feed and cold distilled permeate are in direct contact with a micro-porous membrane. Liquid water is prevented from wetting the membrane pores by controlling the pore size and hydrophobicity of the membrane. Water vapor generated from the hot feed is transported across the membrane via diffusion induced by the trans-membrane vapor pressure gradient, and condenses on the cold permeate side. High hydrophobicity is one of the key parameters in polymer selection to prevent pore wetting and subsequent performance deterioration in desalination.
In this project, two types of hollow fiber membranes (HFMs), polyvinylidene fluoride (PVDF) and polytetrafluoroethylene (PTFE), are evaluated for their desalination performance under the DCMD and vacuum membrane distillation (VMD) modes. The PVDF HFMs were fabricated using a dry jet-wet spinning process and the PTFE HFMs were acquired commercially. Membrane modules with both types of fibers were fabricated with varying packing densities and module geometries for lab-scale testing under different operating conditions. A pilot-scale system is under construction for deployment to Masson Farms, the 2nd largest geothermally-heated commercial greenhouse, for long-term field evaluation. The intent is to develop a techno-economic model for large-scale MD applications in desalination.
Direct Contact Membrane Distillation (DCMD)
Fiber Spinning/Module making
The PVDF fibers were fabricated using a dry jet-wet spinning process via a spinneret. After extrusion from the spinneret, the nascent polymer fibers traveled through an air gap before entering the coagulation bath containing a non-solvent for phase inversion. The membrane pore structure can be manipulated by adjusting the compositions of the non-solvents in the bore fluid and in the coagulation bath, respectively. The newly spun hollow fibers were immersed in de-ionized water for 24 hours to remove any residual solvent. Thereafter, the wet fibers were freeze-dried before characterization and module-making.
Lab-Scale Experimental Setup
Hot-Water Loop at Masson Farms
P&ID Diagram for the Pilot-Scale DCMD System
Lab and Pilot-Scale DCMD Systems
The lab-scale membrane modules were evaluated for their performance in terms of water flux and salt rejection. A pilot-scale DCMD system equipped with pilot-scale modules is under construction and will be deployed to Masson Farms located in Radium Springs, NM for long-term field testing. Masson Farms is a large-scale indoor greenhouse that employs geothermal brackish water for space heating as well as freshwater from on-site wells and the Rio Grande River for irrigation. In order to satisfy the water quality requirements for irrigation, reverse osmosis (RO) is employed for desalination of the irrigation source water. Addition of anti-scalant is needed for the RO process due to the elevated concentrations of calcium and magnesium ions in the source water. The chemical treatment process increases the energy footprint and operating cost of the greenhouse. With the existing energy of the brackish geothermal fluid, MD offers a unique opportunity to meet the demand for irrigation at a potentially lower cost.
