Water - Osmosis and Desalination

Question 1

Define osmosis

Answer 1

the movement of water from an area of high water concentration to an area of low water concentration through a semi-permeable membrane

Question 2

During osmosis, do the solutes move across the semi-permeable membrane?

Answer 2

No as the solute molecules will not be able to pass through the membrane

Question 3

During osmosis, what happens to the water level is there are two solutions of different concentration with the same volume?

Answer 3

The water will move from the higher concentration into the lower concentration causing the water level to rise

Question 4

What is osmotic pressure?

Answer 4

the pressure that is needed to be applied to a solution to prevent the inward flow of water across a semi-permeable membrane wen it is in equilibrium with pure water (e.g. the ‘sucking’ force of a solution to take water)

Question 5

As the concentration of ions in the solution increase, what happens to the osmotic pressure?

Answer 5

it increases

Question 6

What is the osmotic pressure of pure water? Why?

Answer 6

0 because osmotic pressure is in reference to pure water

Question 7

What causes the osmotic pressure of a solution to change?

Answer 7

• Ion concentration - more ions ∴ more water needed to equalise concentration, less ion ∴ less water needed
• Temperature - Increases temp increase kinetic energy of the molecules in the solution ∴ higher osmotic pressure

Question 8

What is the osmotic pressure equation?

Answer 8

π = øViCiRT
π = osmotic pressure (kPa ie N/m2)
ø = osmotic co-efficient (normally 1 unless stated)
Vi = number of ions form disassociation 
Ci = concentration of ion in solution
R = universal gas constant (8.314)
T = temperature in KELVINS

Question 9

What is an isotonic, hypotonic and hypertonic solution?

Answer 9

• Isotonic: solutions seperate by membrane but have same osmotic pressure ∴ no net water movement
• Hypotonic: a solution separated by membrane which has a lower solute conc ∴ higher water conc ∴ lower π ∴ water LEAVES solution
• Hypertonic: a solution separated by membrane which has a higher solute conc ∴ lower water conc ∴ higher π ∴ water flows INTO solution

Question 10

What happens to a cell in isotonic, hypotonic and hypertonic solutions?

Answer 10

• Iso: nothing as net flow to water = 0
• hypo: cell will absorb water until isotonic/cell bursts
• hyper: cell will be drained of water causing it to shrink

Question 11

What happens when the pressure applied to a hypertonic solution is greater than the osmotic pressure of the solution?

Answer 11

Reverse osmosis will occur where water goes from hyper to hypo tonic solution (i.e. goes in the opposite direction)

Question 12

What is reverse osmosis commonly used in?

Answer 12

Desalination

Question 13

What is the practical use of desalination?

Answer 13

Remover self and other minerals from water to render it suitable for irrigation and/or human consumption

Question 14

Where is desalination most commonly used?

Answer 14

Middle east and carribean

Question 15

What are the three general types of water characterisation

Answer 15

• Potable water: drinkable water of less than 0.5g/L of salt
• Brackish water: typically ground water which has 1.5-5g/L of salt
• Seawater: water from the ocean/sea which has typically 35g/L of salt

Question 16

What are the two methods which can be used in desalination?

Answer 16

Reverse osmosis and distillation

Question 17

What is distillation?

Answer 17

Separation of components of a mixture on the basis of boiling point/volatility

Question 18

How does distillation work in the process of desalination?

Answer 18

Water has a boiling point of 100˚C and salt has a boiling point of ~1400˚C so the water evaporates before the salt and the water vapour can then be cooled, condensed and collected are pure water

Question 19

What is the plant precess flow diagram for reverse osmosis?

Answer 19

Feed water goes into pretreatment, a high pressure pump then forces this feed water through a membrane assembly create permeate (cleaner water) and retentate (dirty water), the permeate is sent for post-treatment and the retentate is discharged

Question 20

What is the purpose of pretreatment? What are some disadvantages of this step?

Answer 20

• to remove large particles which could block the membranes or cause fouling, kill any microbes in the water
• this process cost the most because of the added chemicals and subsequent removal of them

Question 21

What is the purpose of using the high pressure pumps? What is a disadvantage of using these pumps? What are the limitation of using these pumps?

Answer 21

• Needed to supply the driving force to overcome the osmotic pressure of the inlet water
• They are the most energy expensive component of the operation
• The pumps operate within the pressure limitations of the other equipment (e.g. pipes could burst, membrane could rip if pressure is too high)

Question 22

What processes can post-treatment involve?

Answer 22

For drinking water, pH adjustments and further disinfection are required, storage, deliver etc.

Question 23

Which kind of feed water requires a higher pump pressure and why: Seawater or brackish water?

Answer 23

Seawater because it has a higher osmotic pressure due to the higher concentration of salt requiring a higher driving force

Question 24

What is a membrane?

Answer 24

A barrier which seperate two phases and restricts the transport of various chemical species either by size exclusion or by their chemical properties

Question 25

What are the structures of reverse osmosis membranes typically?

Answer 25

Nano-porous polymer sheets or tubes that consist of a dense film through which small molecules are transported by diffusion under the driving force of f pressure difference

Question 26

What is the mode of filtration that reverse osmosis membranes use? How does this work

Answer 26

Cross-flow filtration, where the feed water passes across the top of the membrane splitting it into two streams

Question 27

What are the two streams of the cross-flow filtration? Describe them

Answer 27

Permeate = water that has been filtered to have lower salt concentration
Retentate = water that is retained (i.e. has higher salt concentration)

Question 28

What is the rejection? How is it calculated?

Answer 28

The percent of salt filtered out of the permeate
(1- Cp/Cf) * 100%
Cp = concentration of permeate
Cf = concentration of feed

Question 29

What does a rejection of 0% and 100% indicate? are they possible?

Answer 29

0% = no salt is filtered out of permeated (e.g. membrane is just a large hole) so it is possible
100% = all salt is removed from permeate which is pretty much impossible (we can get close with most membranes operating at 99% FYI)

Question 30

What is the recovery? How is it calculated? What is a typical recovery of a membrane?

Answer 30

-The amount of permeate that is produced relative to the amount of water coming into the system
-Recovery = Vp/Vf
Vp = volume of permeate
Vf = volume of feed
-Typically recoveries are at 10%

Question 31

What law allows us to balance the amount of salt that flows into and out of the feed? What is the equation for this?

Answer 31

-Law of conservation of mass (e.g. all mass is conserved)
-CfVf = CpVp + CrVr
f = feed
p = permeate
r = retentate

Question 32

What is an assumption that needs to be made in order for the mass conservation equation to be valid? Why must it be made?

Answer 32

Density of the water doesn’t change, it does slightly but it is a valid assumption to make

Question 33

How can membrane assembly be arranged differently?

Answer 33

In a two pass system or a concentrate-staged array

Question 34

Describe a two pass system. What advantage does it give? When would it be used?

Answer 34

• Where feed water is passed through multiple membrane arrays and the permeate is then passed through this multiple times
• Increases the rejection (i.e. more salt removed from feed)
• When water is really salty and needs to be thoroughly filtered before consumption

Question 35

Describe a concentrate-staged array. What advantage does it give? When would it be used?

Answer 35

• Where feed water is passed through multiple membrane arrays and the permeated is removed and retentate is passed through multiple stages of membrane arrays (of decreasing amounts, e.g. 4-2-1 array)
• Increase the recovery (i.e. more water is extracted from feed)
• When the feed is not very salt and maximum water want to be extracted from the feed

Question 36

What impacts can insoluble salts and organic compounds have on membranes?

Answer 36

Insoluble salts: form scales on membrane surfaces

Organic compounds: cause fouling

Question 37

How can feed water vary?

Answer 37

Depends on the source type, time of the year can have variations in salinity and biological activity

Question 38

What are some common feed water contaminants?

Answer 38

Total dissolved solids, total organic organics, pH and temperature and silica content, dissolved ions (i.e. Mg2+, Ca2+, CO32-, Cl- etc)

Question 39

What are the scaling problems? How are they treated? What are some problems with these treatments?

Answer 39

-Precipitation of Mg2+/Ca2+ with CO32-, sulphuric acid dosing, increases potential of sulphate scaling
-Precipitation of Ca2+ and SO42-, dosing with crystallisation inhibitor, use of inhibitor is detrimental to membrane
Silica precipitation, use of lime, but it is expensive

Question 40

What causes fouling problem?

Answer 40

Bacterial and Algal fouling

Question 41

What is silica precipitation dependent on? How can this be advantageous/disadvantageous?

Answer 41

very pH dependent so can control silica compound build up by controlling pH BUT membranes are also pH sensitive which can affect overall process

Question 42

How is bacterial and algal fouling treated? What problems can this cause?

Answer 42

• Treated by using chlorination or ozone
• using chlorination can degrade the membrane so water needs to be dechlorinated before it is filtered, ozone is hard to handle and must be produced on site

Question 43

What are the environmental impacts of delineation?

Answer 43

It is very energy intensive which may have indirect enviro consequence (e.g. use of coal plants to produce electricity), it also create hyper saline brine which can be a pollutant if not release back into the enviro properly