Water - Osmosis and Desalination Flashcards
Define osmosis
the movement of water from an area of high water concentration to an area of low water concentration through a semi-permeable membrane
During osmosis, do the solutes move across the semi-permeable membrane?
No as the solute molecules will not be able to pass through the membrane
During osmosis, what happens to the water level is there are two solutions of different concentration with the same volume?
The water will move from the higher concentration into the lower concentration causing the water level to rise
What is osmotic pressure?
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)
As the concentration of ions in the solution increase, what happens to the osmotic pressure?
it increases
What is the osmotic pressure of pure water? Why?
0 because osmotic pressure is in reference to pure water
What causes the osmotic pressure of a solution to change?
- 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
What is the osmotic pressure equation?
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
What is an isotonic, hypotonic and hypertonic solution?
- 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
What happens to a cell in isotonic, hypotonic and hypertonic solutions?
- 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
What happens when the pressure applied to a hypertonic solution is greater than the osmotic pressure of the solution?
Reverse osmosis will occur where water goes from hyper to hypo tonic solution (i.e. goes in the opposite direction)
What is reverse osmosis commonly used in?
Desalination
What is the practical use of desalination?
Remover self and other minerals from water to render it suitable for irrigation and/or human consumption
Where is desalination most commonly used?
Middle east and carribean
What are the three general types of water characterisation
- 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
What are the two methods which can be used in desalination?
Reverse osmosis and distillation
What is distillation?
Separation of components of a mixture on the basis of boiling point/volatility
How does distillation work in the process of desalination?
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
What is the plant precess flow diagram for reverse osmosis?
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
What is the purpose of pretreatment? What are some disadvantages of this step?
- 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
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?
- 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)
What processes can post-treatment involve?
For drinking water, pH adjustments and further disinfection are required, storage, deliver etc.
Which kind of feed water requires a higher pump pressure and why: Seawater or brackish water?
Seawater because it has a higher osmotic pressure due to the higher concentration of salt requiring a higher driving force
What is a membrane?
A barrier which seperate two phases and restricts the transport of various chemical species either by size exclusion or by their chemical properties
What are the structures of reverse osmosis membranes typically?
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
What is the mode of filtration that reverse osmosis membranes use? How does this work
Cross-flow filtration, where the feed water passes across the top of the membrane splitting it into two streams
What are the two streams of the cross-flow filtration? Describe them
Permeate = water that has been filtered to have lower salt concentration Retentate = water that is retained (i.e. has higher salt concentration)
What is the rejection? How is it calculated?
The percent of salt filtered out of the permeate
(1- Cp/Cf) * 100%
Cp = concentration of permeate
Cf = concentration of feed
What does a rejection of 0% and 100% indicate? are they possible?
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)
What is the recovery? How is it calculated? What is a typical recovery of a membrane?
-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%
What law allows us to balance the amount of salt that flows into and out of the feed? What is the equation for this?
-Law of conservation of mass (e.g. all mass is conserved)
-CfVf = CpVp + CrVr
f = feed
p = permeate
r = retentate
What is an assumption that needs to be made in order for the mass conservation equation to be valid? Why must it be made?
Density of the water doesn’t change, it does slightly but it is a valid assumption to make
How can membrane assembly be arranged differently?
In a two pass system or a concentrate-staged array
Describe a two pass system. What advantage does it give? When would it be used?
- 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
Describe a concentrate-staged array. What advantage does it give? When would it be used?
- 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
What impacts can insoluble salts and organic compounds have on membranes?
Insoluble salts: form scales on membrane surfaces
Organic compounds: cause fouling
How can feed water vary?
Depends on the source type, time of the year can have variations in salinity and biological activity
What are some common feed water contaminants?
Total dissolved solids, total organic organics, pH and temperature and silica content, dissolved ions (i.e. Mg2+, Ca2+, CO32-, Cl- etc)
What are the scaling problems? How are they treated? What are some problems with these treatments?
-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
What causes fouling problem?
Bacterial and Algal fouling
What is silica precipitation dependent on? How can this be advantageous/disadvantageous?
very pH dependent so can control silica compound build up by controlling pH BUT membranes are also pH sensitive which can affect overall process
How is bacterial and algal fouling treated? What problems can this cause?
- 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
What are the environmental impacts of delineation?
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
