Questions and Answers
What is the primary function of a membrane in membrane-based desalination technologies?
To separate substances when a driving force is applied across the membrane
Which of the following membrane-based desalination techniques is not suitable for highly concentrated salt solutions?
Electrodialysis (ED)
What is the main difference between microfiltration, ultrafiltration, nanofiltration, and reverse osmosis?
The nature of the driving force and the molecules to be separated
Which membrane-based desalination technique involves the use of an electric potential to pass saltwater through a series of stacked anionic and cationic membranes?
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What is the primary characteristic of a membrane that allows it to separate substances?
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Which of the following is NOT a common application of membrane-based desalination technologies?
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What is the common point among all membrane-based desalination techniques?
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Which membrane-based desalination technique is used for removal of bacteria, microorganisms, particulates, and natural organic material?
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What is the purpose of the thin dense layer on top of a membrane?
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What is the primary advantage of using membrane-based desalination technologies?
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Study Notes
High Pressure Membrane Technology
- Has taken over thermal desalination for drinking water production since 1984-1995 due to innovations in energy recovery systems and high-efficiency permeable membrane materials.
Reverse Osmosis (RO)
- A membrane-based demineralization technique used to separate dissolved solids, such as ions, from solution.
- Can effectively remove radium, natural organic substances, pesticides, cysts, bacteria, and viruses.
- Membranes act as perm-selective barriers, allowing certain species (like water) to permeate through while retaining other dissolved species (like ions).
Osmosis & Reverse Osmosis
- Osmosis: a natural phenomenon where water flows across a semi-permeable membrane from low-concentrated to high-concentrated solutions due to chemical potential.
- Reverse Osmosis: applying additional pressure to reverse the flow from high salt to low salt side, allowing water to flow in the opposite direction.
Advantages of RO
- Removes nearly all contaminant ions and most dissolved non-ions.
- Operates immediately, without a minimum break-in period.
- Bacteria and particles are also removed.
- Operational simplicity and automation allow for less operator attention and make RO suitable for small system applications.
- Compact construction of systems provides lower investment for buildings and space than conventional treatment.
Limitations of RO
- High energy consumption mainly due to membrane fouling, which reduces efficiency and limits its application.
Nanofiltration (NF)
- Membranes have a nominal pore size of approximately 0.001 μm.
- Require higher operation pressure than MF or UF.
- Can remove virtually all cysts, bacteria, viruses, and humic materials.
- Also removes hardness from water, sometimes called “softening membranes.”
- Hard water treated by NF requires pretreatment to avoid precipitation of hardness ions on the membrane.
Membrane-Based Desalination Technologies
- Membrane processes (RO, ED, MD, and FO) are increasingly used for removal of bacteria, microorganisms, particulates, and natural organic material from water.
- These processes can be grouped under the common head of membrane-based desalination techniques.
Other Desalination Techniques
- Electrodialysis (ED): uses an electric potential to pass saltwater through a series of stacked anionic and cationic membranes.
- Other techniques include microfiltration (MF), ultrafiltration (UF), and forward osmosis (FO).
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