Membranes

Question 1

definition of membranes

Answer 1

A membrane (also known as semipermeable synthetic membrane) is a group of thin layers of materials ( polymers or ceramic) that is capable of separating substances from  liquids when a driving force is applied across the memberane.
Historically it was used for desalination of water however they are increasingly used for removal of bacteria , microorganisms.

Question 2

Exam question
Name the different types of membrane processes and briefly dicsuss the modes under which they operate and mention the type of impurities that each is capable of removing.

Answer 2

1-) Microfiltration, operates by straining and it removes suspended solids and bacteria.
2-)Ultrafiltration, operates by straining and removes suspended solid, bacteria and part of viruses.
3-)Nano filtration, operates by ionic diffusion and removes suspended solids, bacteria, viruses and multivalent ions.
4-)Reverse osmosis; operates by ionic diffusion and removes suspended solids, bacteria, viruses, multivalent ions, and monovalent ions.

Question 3

Exam question:
Name and explain briefly the different flow configuration that the membranes operates under and explain their physical configuration?

Answer 3

1-) cross flow configuration
the flow is tangentional to the surface of the membrane( retentate is removed from the same side as flow,,
permeate is removed from the perpendicular side of flow).
physical configuration: tubular-spiral
2-)Dead end configuration
flow is perpendicular to the membrane,permeate passes and retentate stays
physical configuration:
Hollow fiber.

Question 4

Mention and briefly discuss the different types of fouling that could affect membranes and discuss the methods for preventing this fouling from taking place.

Answer 4

1-) colloidal fouling apply pretreatment ( coagulation-flocculation and filtration)
2-) biological fouling we apply pretreatment by disinfection.
3-) organic apply preatment for the organic substance present for example through flotation we can remove oils.
4-)Scaling apply pretreatment by softening of water to remove metal deposits such as magnesium and clacium we can also lower the pH.

Question 5

What are the materials used in membranes ?

Answer 5

Materials used in membranes include cellulose, acetate.

Also ceramic membranes such as alumina and titania.

Question 6

What does the retention of organic and inorganic solution depend on?

Answer 6

the retention of organic and inorganic solution depend on the solubility of the solution and diffusivity in the membrane and on the net charge on the surface of the membrane.

Question 7

What are the properties of the selected membrane and what is the main modeling equation for dead end filtration

Answer 7

Selected membrane must have the following properties:
1-Must be able to process large amounts of feed streams.
2-Must have high selectivity for certain particles.
3-Must have high mechanical stability.
4-Must be reproducible and to have low manufacturing cost.
The equation is in the book see it.

Question 8

What is membrane fouling and what does it can result in?

Answer 8

Membrane fouling is a process where the solute deposit on the membrane or inside the membrane causing a severe decrease in the flux and affecting the quality of water.

Question 9

How can the fouling be

Answer 9

The fouling can be either reversible or irreversible.
Reversible fouling means that fouling can be a strong shear force or by backwashing. If no treatment is applied a strong matrix of fouling layer will be formed and the fouling get irreversible, meaning the membrane needs replacement .

Question 10

What are the indicators of fouling and how a membrane ca be treated?

Answer 10

Indicators of fouling:
Flux (under constant pressure the flux will decrease due to fouling)
Transmembrane pressure (TMP) (TMP pressure increases to compensate for the fouling).
A membrane when fouled can be treated:
phsically (backflushing using permeate)
chemically (using acids and bases)
biologically (by using biocides to remove bacteria)

Question 11

Mention and briefly discuss the physical configurations of the membranes

Answer 11

The three physical configuration of membranes are:
1)Hollow fiber membranes
based on ultrafiltraion
they operate with flows from outside to inside or from inside outside.
fluid flow through the center and permeate passes through the fiber wall to the outside of the membrane
Highly flexible design and can handles large volumes for circulation, dead end, and single pass operations.

Spiral membranes
based on RO or NF technology.
used for water with high concentration of dissolved solids such as seawater
water enter through one end of the membrane and flow into permeate channels, spiral to the center core where it exist as permeate

Tabular membranes
designed for the treatment of fluids with large amounts of solids but it doesnt exclude their application to water with low concentration of suspended solids.
they operate in tangentional or cross flow design where water is pumped along the membrane surface in a sweep action.