Membrane Processes
Membrane processes are divided into two classes:
- Micro- and Ultra- filtration (MF and UF) membrane separates through sieving particles (pores)
- Reverse Osmosis, Nanofiltration, and Forward Osmosis (RO, NF, and FO) membrane separate through solution and diffusion of molecules and ions.
The difference between the two types in each category is the degree of separation. Click here for a link to a good industrial filtration spectrum. For a detailed description of the membrane processes please click to download DWPR Report #29, "The Desalting and Water Treatment Membrane Manual: A Guide to Membranes for Municipal Water Treatment (2nd Edition)," which includes specifications for membranes available at the time of publication (1998). Table 1 summarizes the various levels of membrane treatment processes and conventional processes that they can replace.
Table 1. Membrane Process Levels of Separation
Membrane Process |
Contaminants |
Conventional Process |
Microfiltration |
Turbidity |
Settling, |
Ultrafiltration |
Above + Virus, color and odor causing organics |
Above + powdered activated carbon filtration (difficult) |
Nanofiltration |
Hardness, sulfate, iron, manganese |
Lime softening, coagulation/ floccultation, media filtration |
Reverse Osmosis and Forward Osmosis |
Dissolved salts |
Distillation |
Microfiltration. Microfiltration filters water through mediums with tiny pore sizes (from slightly below 0.1 micron to slightly above 1 micron). The pore size refers to the medium--not anything that might acculate on the medium. This is smaller than the size range for bacteria, algae and cysts. However, the pore size is larger than viruses, so microfiltration does not remove viruses well.
Ultrafiltration. Ultrafiltration filters water through mediums with pore sizes smaller than microfiltration and larger than nanofiltration. UF is a relatively new process for municipal water treatment. It has been used more extensively in industrial processes for concentrating milk, protein, and cells; recovering process materials such as paint, colloidal metals, and dyes; and recovering oily waste water. In the last 15 years, the application of UF in drinking water treatment has expanded profoundly due to changes in drinking water quality regulations.
Nanofiltration and Reverse Osmosis require hydraulic pressure to overcome the osmotic pressure of the feed solution. Osmosis is a natural process whereby water is transported through a semi-permeable membrane from a solution of low concentration to one of high concentration. Plants use this phenomena to draw in water from soil. Reverse osmosis occurs when pressure in excess of the osmotic pressure is exerted on the high concentration side of the membrane, inducing pure water to diffuse through the membrane to the low concentration side. Nanofiltration membranes have more open structures than reverse osmosis membranes, and they reject large ions like calcium and sulfates at higher rate than small ions like sodium and chloride.
Forward Osmosis uses a draw solution such as sugar and nutrient solutions as a driving force. The diving solution on one side of the membrane draws fresh water from water on the other side that is less saline, but may be turbid and contaminated with pathogenic bacteria and virus. The draw solution in concentrated form is inside a bag formed from semi-permeable membrane. The bag is left in the water source until it is full of diluted nutrient/sugar solution. These are sometimes provided for baby formula in developing countries and for backpackers and military use. Other draw solutions under investigation are Ammonia-carbon dioxide, nano-magnetic ferritin particles, and seawater. Forward osmosis has not been used for large applications in the past. In recent years, forward osmosis has gained more attention as a method for producing power from the energy released as fresh water rivers enter the ocean. The energy of mixing can be harvested through drawing the fresh water through a membrane into the seawater and using the increase in volume to drive a turbine to generate electricity. Statkraft, an energy supply company based in Oslo, Norway, built an osmotic energy plant near Oslo.
Reclamation has carried out and funded the research of others into many areas of membrane processes including:
- General Membrane Information
- Pilot Studies
- Membrane Materials and Module Development
- Membrane Characterization
- Fouling and Cleaning of Membrane Systems
Efficient and effective operation of filtration membrane requires knowledge of a water preparation and membrane sustainment. Specific areas of importance for membrane filtration operation include:
- General Pretreatment
- Pretreatment for Specific Contaminants
- Membrane Fouling and Cleaning
- Membrane Development
Research in these categories include a variety of pretreatment techniques, techniques for removing specific compounds, mechanisms causing membrane fouling and techniques to remove fouling agents during cleaning.
Research Priorities
- Develop technology and techniques to reduce the fouling of membrane systems.
- Implement bacterial signaling compounds and biofilm-inhibiting compounds in the desalination process.
- Develop new feed spacer designs to enhance RO element efficiencies.
- Increase recovery of RO systems by evaluating and developing antiscalants.
- Improve the performance of reverse osmosis membranes with MF membrane pretreatment.
Tampa Bay side ported vessel configuration
New flow distributor design on left, the old design on the right.