Reverse Osmosis Fundamentals

1
Reverse Osmosis Fundamentals

• Bill Loyd
• Technical Service Development Specialist
• 31st Annual Water Quality Association Convention
  and Exhibition
• Las Vegas, Nevada
• March 29, 2005

2
Welcome to RO Fundamentals

• How temperature, pressure and salt concentration
  affect RO performance.
• How to determine Flux and Recovery Rates.
• Boundary Layer and Concentration Polarization

3
Factors that Impact on RO Performance

• Basic Effects of
• Temperature
• Pressure
• Salt Concentration

Feed
Permeate
Pump
Concentrate
4
Temperature Effects

• RO permeate flow is strongly dependent on the
  temperature of the feed water
• The higher the temperature the higher the
  permeate flow
• Why? Lower viscosity makes it easier for the
  water to permeate through the membrane barrier
• RULE OF THUMB for every 1oC the permeate flow
  will increase 3

5
Temperature Variations on Water Flux
Temperature Correction Factor (TCF)

• Membrane manufacturers provide temperature
  correction tables for more accurate conversions.

6
TCF Factors

• Example if a membrane produced 100 gpd at 21oC
  how much would it produce at 25oC?
• Answer 114.8 gpd (1001.148 114.8 gpd)
• 43 12
• (114.8-100/100)100 14.8

7
Temperature Variations on Salt Flux

• Rule of Thumb salt flux increases 6 for 1oC
  increase.
• Increasing temperature increases salt passage
  more than water passage.
• Generally you will get better rejections at lower
  temperatures.

8
Factors that Impact on RO Performance

• Basic Effects of
• Temperature
• Pressure
• Salt Concentration

Feed
Permeate
Pump
Concentrate
9
Pressure Effects
PSI

• If you double net driving pressure (NDP) to an RO
  unit you will double your permeate flow.
• NDP is the sum of all forces acting on the
  membrane

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What is Osmosis?

• Movement of water through a semi-permeable
  membrane from an area of lower concentration to
  an area of higher concentration

Semi-permeable membrane
11
What is Osmotic Pressure?
2.3 Feet
100 mg/l
0 mg/l
100 mg/l
0 mg/l

• If we add salt to one side of the vessel the
  water level will rise.
• How far will the water level rise?
• Rule of thumb is 1 psi of pressure for every 100
  ppm TDS (1 psi equals 2.3 feet of water head.

12
Why does Osmosis Occur?
There are a greater number of water molecules
colliding with the membrane on the pure water
side.
100 mg/l
0 mg/l
13
What is Reverse Osmosis?
100 mg/l
0 mg/l
100 mg/l
0 mg/l

• Osmotic Pressure must be overcome
• In this case how much pressure would have to be
  applied to overcome the osmotic pressure?
• Answer 1 psi

14
What is Net Driving Pressure?

• Sum of all forces acting on the membrane
• Pump or Feed pressure
• Back pressure from line restrictions and storage
  tank.
• Osmotic Pressure of the feed and permeate water

15
NDP Examples
150 psi
50 psi
Pump
0 mg/l
100 mg/l
0 mg/l
1000 mg/l
0 psi
0
10 psi
1 psi
150 psi
50 psi
0
0
NDP 49 psi
NDP 140 psi
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What is Average NDP?
Salt concentration is increasing continually as
it passes over the membrane.
Feed
Permeate
100 mg/l
Pump
200 mg/l
Concentrate
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What is Average NDP?
Feed pressure is decreasing continually as it
passes over the membrane
Feed
Permeate
50 psi
Pump
45 psi
Concentrate
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Average NDP
NDP Inlet 50 psi 1 psi 49 psi NDP Outlet 45
psi 2 psi 43 psi
50 psi 100 mg/l
Feed
Permeate
Pump
49 psi 43 psi
45 psi 200 mg/l
NDP
2
Concentrate
46 psi
NDP
19
Factors that Impact on RO Performance

• Basic Effects of
• Temperature
• Pressure
• Salt Concentration

Feed
Permeate
Pump
Concentrate
20
Affect of Salt Concentration

• The rate of salt passage through a membrane is
  independent of pressure.
• Rate of salt passage is determined by the salt
  concentration gradient.

21
Affect of Salt Concentration (cont)

• Higher Salt Concentration will decrease the NDP
• Why? Because Higher Osmotic Pressure must be
  overcome.
• Higher Salt Concentration will decrease the
  Quality of the Product Water (Permeate)
• Why? Because of less dilution of the salt

22
Salt Passage is Independent of Pressure
NDP 100 psi
NDP 200 psi
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Salt Passage vs. Salt Rejection
NDP 100 psi
Salt Rejection Rejection Feed TDS Product
TDS
x 100
Feed TDS
Rejection 100 ppm Product 5 ppm
x 100
100 ppm
Rejection 95
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Salt Passage vs. Salt Rejection
NDP 200 psi
Salt Rejection Rejection Feed TDS Product
TDS
x 100
Feed TDS
Rejection 100 ppm Product 2.5 ppm
x 100
100 ppm
Rejection 97.5
25

• Questions?

26
Review Questions

• As water temperature increases what will happen
  to permeate flow?
• What will happen to salt passage?
• What will happen to overall permeate quality?

27
Review Questions

• As NDP increases what will happen to permeate
  flow?
• What will happen to salt passage?
• What will happen to overall permeate quality?

28
Review Questions

• As Salt Gradient increases what will happen to
  permeate flow?
• What will happen to salt passage?
• What will happen to overall permeate quality?

29
Flux Recovery Rates

• Flux Rate means the amount of permeate passing
  through a section of membrane
• Typically given in GFD or gallons per square foot
  per day

12
Membrane
1 ft2
12
GPD
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Flux Rate Sample Exercise

• A home drinking water element is producing 25 gpd
  and has 5 square feet of active membrane area.
• 25/55 gfd

12
Membrane
1 ft2
12
5 GFD
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Notes on Flux

• Lower flux rates help reduce the membrane fouling
  rates.
• Flux rate is usually determined by quality of the
  feedwater and manufacturer guidelines.

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Recovery - What is it?

• Percentage of the feed water that is turned into
  product water.
• Examples A system may have a Recovery of 5,
  30, 75 etc.

Permeate Flow
x 100
Recovery
Feed Flow
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How to Determine Recovery

• Divide permeate flow by the feed flow (permeate
  concentrate) and multiply by 100.

Permeate Flow
x 100
Recovery
Feed Flow
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Example How to Determine Recovery

• A four inch system has a feed flow rate of 4 gpm
  and a permeate flow of 1 gpm. What is the
  recovery rate?
• (1/4) x 100 25

Permeate Flow
Recovery
x 100
Feed Flow
1 gpm
Recovery
x 100
4 gpm
Recovery 25
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Notes on Recovery

• The higher the recovery the lower the permeate
  quality
• The higher the recovery the greater chance of
  scaling

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Boundary Layer

• Water close to the membrane surface does not flow

Boundary Layer
Membrane
37
Boundary Layer

• Creates an area for colloids to collect and foul
  the membrane.
• Water flux through the membrane helps hold
  foulants in place.

Boundary Layer
Membrane
Boundary Layer
Membrane
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Concentration Polarization

• Concentration polarization is a function of the
  boundary layer.
• It results in an increased salt concentration at
  the membrane surface.

Boundary Layer
Membrane
100 mg/l
105 mg/l
Boundary Layer
110 mg/l
115 mg/l
Membrane
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Concentration Polarization
Bulk Stream

• The higher the flux rate through the membrane the
  higher the salt concentration at the membrane
  surface.
• Typically the TDS is 1015 higher than the
  concentration in the bulk stream.

Boundary Layer
Membrane
100 mg/l
105 mg/l
Boundary Layer
110 mg/l
115 mg/l
Membrane
10-15 Increase is typical
40

• Questions?