1. Reverse Osmosis Desalination
Is ocean water a viable source of drinking water?
** General Note ** - Times for each slide are estimates. If there’s extra time remaining, give more time for groups to complete the group activity (slide 19)
7 minutes – Go around the classroom asking students if they think ocean water could be a viable source of drinking water and why or why not. They just need to give an educated guess
Jenny Meyer

2. What is desalination?
A process that removes dissolved minerals (including salt) from seawater, brackish water (water that has more salinity than freshwater but less than seawater), and wastewater
Ocean water = 35,000 ppm of salt
Brackish water = 1,000 – 10,000 ppm of salt
Fresh water = less than 1,000 ppm of salt
300 million people worldwide rely on desalination for their fresh water needs
1 minute to cover slide material

3. What is osmosis?
A process in which a solute (ex. salt) is dissolved in a solvent (water) and the solvent moves through a semipermeable membrane into a solution of higher solute concentration (water with higher amounts of dissolved salt). The concentration of the solute on the two sides of the membrane tends to equalize.
2 minutes to cover and explain slide material

4. What is osmotic pressure?
The pressure that must be applied to the solution to prevent the flow of water across the semipermeable membrane
Equation: π = MRT
π = osmotic pressure (atm)
M = molar concentration of dissolved species (mol/L)
R = ideal gas constant ( L atm mol-1 K-1 )
T = temperature (K)
2 minutes to cover and explain slide material

5. Given that the average molarity of seawater is 0
Given that the average molarity of seawater is M and a temperature of 293 K, calculate the osmotic pressure.
Equation: π = MRT
π = osmotic pressure (atm)
M = molar concentration of dissolved species (mol/L)
R = ideal gas constant ( L atm mol-1 K-1 )
T = temperature (K)
5 minute for students to complete question

6. Answer π = MRT π = 0.479 mol/L * 0.082057 L atm mol-1 K-1 * 293 K
2 minutes to go over answer and answer any questions

7. What is reverse osmosis?
Unsurprisingly, reverse osmosis is the opposite of osmosis
The solvent (water) filters out of a high concentrate solution (saltwater) into a lower concentrate solution (freshwater)
Pressure must be applied to the saltwater side of the tank to push the water through the membrane.
Therefore, reverse osmosis (RO) makes desalination possible
60% of desalinated water was treated by RO in 2008
2 minute to cover and explain slide material

8. How much pressure must be applied in RO?
Review: Pressure =
Must apply enough pressure on the saltwater side of the tank to counteract the natural osmotic pressure as well as enough pressure to push the water through the filter
psi for fresh and brackish water
psi for seawater
350 psi of natural osmotic pressure
2 minute to cover and explain slide material

9. When was reverse osmosis developed?
1950s - Researchers began to look for ways that ocean water could be desalinated
Was a high priority technology goal during the Kennedy administration
1959 – Researchers at UCLA created a functional synthetic RO membrane from cellulose acetate polymer. It could reject salt and pass fresh water at realistic flow rates and pressures
1965 – First RO desalination plant in Coalinga, CA
1 minute to cover slide material
Cellulose acetate polymer filter

10. What does the reverse osmosis process look like with today’s technology?
A video describing the reverse osmosis process at a desalination plant in Sydney, Australia
4 minutes – run time of YouTube clip

11. RO Today (cont’d)
A video giving a closer look at a RO membrane assembly
3 minutes – run time of YouTube clip

12. Where is reverse osmosis most frequently used?
The Middle East
Only receives cm of water per year
Oil rich which keeps energy prices low
70% of Saudi Arabia’s water comes from desalination but only 50% of that amount uses RO
California and Florida
11 minutes – 1 minute to cover slide material, 10 minute runtime of YouTube clip

13. Given that 200 million KWH/day are used in desalination plants worldwide and 300 million people use that water, how much natural gas would be consumed per person through desalination?
10 minutes for students to complete problem

14. Answer 1 KWH = 3.311 cubic feet of natural gas
200 million KWH = 662,200,000 cubic feet of natural gas
cubic feet of natural gas per desalination customer per day
Average annual natural gas consumption per customer = 528 thousand cubic feet (California, 2015)
cubic feet per day
If an entire community served by desalinated seawater, the annual power usage for a household would increase by 10%
2 minutes to go over answer and answer any questions

15. What are the main disadvantages of RO desalination?
High energy needs
Plants uses 200 million KWH/day worldwide
3-10 KWH are needed to produce 1 cubic meter of freshwater compared to 1 KWH for conventional freshwater
Roughly 2/3 of the energy is used to supply the necessary water pressure
Cost
1,000 gallons of RO desalinated water costs $2.5 - $5 per 1000 gallons compared to $2 for conventional fresh water
2 minute to cover slide material

16. Disadvantages (cont’d)
Disposal of brine
Brine – the wastewater from the RO process that has a high salt concentration (generally twice as much as the intake water)
Brine is disposed back into the ocean. Long term effects on marine life are not understood
Can be mixed back in the open seawater with high speed jets so salinity levels are back to normal within 100 feet of the release point.
Inefficiency of the RO process
55-75% of the intake water is discharged as brine
1 minute to cover slide material

17. What maintenance is performed on a RO plant?
Preventive maintenance
Instrument calibration, pump adjustment, chemical feed inspection and adjustment, leak detection and repair, and structural system repair
Fouling
Caused when membrane pores are clogged by salts or other particulates
Membranes must be cleaned or backwashed about every 4 months
Cartridge filter elements must be replaced about every 8 weeks
Membrane lifespan is about 2-3 years
1 minute to cover slide material

18. Could a RO desalination be powered by renewable energy?
Since the largest obstacle for RO desalination is high energy needs, could this be avoided by using renewable energy?
A RO desalination system designed to be powered by wind or solar energy
Read the following article: m/s/601419/to-make-fresh-water- without-warming-the-planet- countries-eye-solar-power/
12 minutes – 1 minute to cover slide material, 4.5 minute YouTube clip, 6.5 minutes to read article

19. Group activity: Research a new technology that could improve RO desalination or desalination in general and present findings
The technology could be one that improves efficiency or decreases negative environmental impacts
45 minutes – 30 minutes of group work, 15 minutes to present results

20. Examples of new technologies
Shock electrodialysis
cleans-and-disinfects-water/
Microbial capacitive desalination
treatment-and-reuse-oil-and-gas-wastewater-simpler
Desalination pipe in Santa Monica, CA
5 minutes – have students click on one of the links and read the corresponding article