Several types of separation techniques are used to separate either dissolved or undissolved particles in water. These are briefly described below:
´ Filtration. Filtration is the physical process whereby particles suspended in water are separated by forcing the fluid through a porous medium (i.e., a filter). The suspended particles are trapped in the filter. Filtration relies on the pore size of the membrane, which can be varied to remove particles and molecules of various sizes. Microfiltration processes generally work best for separating very fine particles (0.1-0.001 microns) from the liquid. Sampling the effluent can monitor the effectiveness of these processes.
´ Distillation. Simple distillation is a process in which heat is applied to a liquid, causing a portion of the liquid to vaporize. Contaminants with different boiling points from water are thereby separated. The vapors are condensed, producing a liquid product called distillate.
´ Freeze Crystallization. Freeze crystallization freezes contaminated water. When the solution is slowly frozen, water crystals form on the surface, from which they are separated from the remaining solution.
´ Membrane Pervaporation. Membrane pervaporation uses several permeable membranes to preferentially sorb volatile organic compounds (VOCs) from contaminated water. Contaminated water is heated, causing the VOCs to change into the vapor phase. With the assistance of vacuum pressure, VOCs pass through the membrane wall to a condenser where they return to the liquid phase. (Note that Vapor-Phase Separation, described as a separate technology, is designed to separate vapors from off-gas. However, it uses a similar concept to membrane pervaporation.)
´ Reverse Osmosis. Reverse osmosis uses a selectively permeable membrane that allows water to pass through it, but which traps heavy metals and radionuclide ions on the other side of the membrane. If used for removing radionuclides from water, the size and charge of the ion being treated affect reverse osmosis. Because radium and uranium ions are large and highly charged, reverse osmosis is particularly effective at removing these dissolved radionuclides from contaminated solutions.
Limitations and Concerns
Reverse osmosis generates a concentrated waste stream containing radionuclides that must be treated further or disposed. Removal efficiencies for microfiltration have been shown to be greater than 99 percent for uranium, plutonium, and americium.
Depending on what is fed into the system, the microfiltration process generates three waste streams: a filter cake of solid material, a filtrate of treated effluent, and a liquid concentrate that contains the dissolved contaminants. The filter cake and/or liquid concentrate require further treatment or disposal.
Membrane pervaporation and freeze crystallization are limited to aqueous waste streams.
The feed stream should be dilute to make the freeze crystallization process cost effective.
The presence of oil and grease may interfere with water separation processes by decreasing flow rate.
Space must be adequate for distillation and freeze crystallization treatment systems. Distillation units are comparatively tall.
Explosive compounds should not be used in the distillation process.
These ex-situ separation processes are for groundwater treatment. Most liquid-phase separation processes are used mainly as a pretreatment or post-treatment process to remove contaminants from wastewater. The target contaminant groups for ex-situ separation processes are VOCs, semi-volatile organic compounds (SVOCs), pesticides, metals, most radionuclides, and suspended particles. At Department of Energy (DOE) sites, these processes are used to separate tank wastes. However, tritium cannot be removed easily because of its chemical characteristics.
Technology Development Status
Membrane processes for removing contaminants from liquid-phase are commercially available for most contaminants. More studies are needed to assess how effectively radionuclides can be removed from liquid media. Performance data on distillation and freeze crystallization are not available. Reverse osmosis is commercially available.
Other Resources and Demonstrations
See descriptions of Membrane Separation (designed for vapor-phase separation) and Solid-Phase Separation.
Through membrane processes, uranium concentrations of 300 µg/L were reduced by 99 percent in Florida ground water. See U.S. DOE, ñCost and Performance Report, PerVapTM Membrane Separation Groundwater Treatment, Pinellas Northeast Site,î October 1997.