Membrane Fouling

Prevention

By: Jason Bailey

Plant Operators Conference May 2016

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Five Fundamental Causes of Fouling

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Scale Formation

Colloidal Fouling

Chemical Fouling

Inadequate Membrane Cleaning Techniques

Biological Fouling

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First Membrane, First Stage:Bio, Iron, Silt, etc.

Sand & Paint Chips

AntiscalantSoftener Resin

Other Foulants

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Last Membrane, Second Stage:Scale-Silica, Calcium Carbonate, etc.

Key Instrumentation to Identify Fouling

• Pressures

– Feed, Interstage and Concentrate

• Conductivity

– Feed, Product and Concentrate

• Flows

– Product and Concentrate

• Temperature

RO Troubleshooting - RO System Monitoring

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Cartridge

Filter

PSI

1st Array 2nd Array

Concentrate

Permeate

pH

PSI

SDI

NTU

Temp

PSI

COND

GPM

PSICOND

GPM PSI

COND

PSI

SDI

Cl2

NTU

Feed water

High Pressure

Pump

Data Normalization

• Normalization calculations are necessary to correct changes in temperature, TDS feed flow, pressure, and other factors that are not directly related to fouling.

• Example: A 10-degree decrease in temperature causes feeding 30% loss in the permeate flux.

• As temperature increases, the permeate flux increases.

• When the temperature increases, the salt rejection decreases.

Performance vs. Temperature

Temperature

RO Troubleshooting Temperature Effect

• When the pressure increases, the salt rejection increases.

• When the pressure increases, the permeate flux increases.

Performance vs. Pressure

Pressure

RO Troubleshooting Pressure Effect

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20

40

60

80

100

120

140

160

180

200

Temperature

Flow

Pressure

RO Troubleshooting Data Interpretation

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85

90

95

100

105

Normailized Flow

RO Troubleshooting Data Interpretation

Membrane Cleaning

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Data Collection is the Key

RO Troubleshooting

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Biological and Colloidal Fouling

RO Troubleshooting

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Biological and Colloidal Fouling

Factors Influencing Biogrowth

Biogrowth

Bacteria

Temp.

Oxygen

Level

Nutrients

RO Troubleshooting - Biological Fouling

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1st array 2nd array

Concentrate

Permeate

Pre Filter

Slime in Vessels or Pipe. Noticeable odor.

High Pressure Differential in 1st

Array

RO Troubleshooting – Biological Fouling

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First array exhibits a high dPFront end biological

slime with a strong odor

1st Array 2nd Array

Concentrate

Permeate

Feed Water

Into pre filter

PSI PSI

RO Troubleshooting - Colloidal Fouling

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1st array 2nd array

Concentrate

Permeate

Pre Filter

Indicators of colloidal fouling would be:•High pressure differential on first array.•Membrane telescoping.•High turbidity in feed water.•Reduced permeate flow.

RO Troubleshooting – Colloidal Fouling

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Colloidal particulate materials: called "Colloidal

silica, fine clays, oxides particles and bacteria which are usually deposited in the

first bank of RO system

Concentrate

Permeate

• The value of SDI (Silt Density Index) is an indicator of the potential of colloidal fouling. Although sometimes can not predict how quickly fouling will occur.

• SDI <5 is recommended

• The value of turbidity is a good indicator to evaluate the performance of filtration methods that feed the RO system. Ideal turbidity is <0.2 NTU

• Particle counts are also a good indicator to evaluate the pretreatment.

• Particle counts of <100 / ml in the range of 2-50 microns is ideal.

Colloidal Fouling Control

RO Troubleshooting – Humic/Organic Fouling

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Humic / Organic fouling can occur in 1st or 2nd

array and is usually more common in surface

waters.

1st Array 2nd Array

Concentrate

Permeate

RO Troubleshooting – Chemical Fouling

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1st array 2nd array

Concentrate

Permeate

Pre Filter

Incompatible pretreatment chemicals can cause front end fouling that is difficult to remove.

When incompatible chemistries are mixed together there will be a measurable increase in turbidity.

RO Troubleshooting – High Front End dP

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Concentrate

Permeate

The symptoms are:Increased Feed PressuresReduced Permeate FlowMembrane Telescoping

Turbidity vs. RO Cleaning Frequency

Turbidity vs. Cleaning Frequency

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1

2

3

4

5

6

7

0 0.2 0.4 0.6 0.8 1

Turbidity, NTU

Cle

an

ing

Fre

qu

en

cy,

Mo

nth

s

Particle Count vs. RO Cleaning Frequency

Particle Count vs. Cleaning Frequency

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1

2

3

4

5

6

7

0 500 1000 1500 2000 2500

Particle Count, Number/mL

Cle

an

ing

Fre

qu

en

cy,

Mo

nth

s

SDI vs. Cleaning Frequency

SDI vs. Cleaning Frequency

0

1

2

3

4

5

6

7

0 1 2 3 4 5 6 7

SDI

Cle

an

ing

Fre

qu

en

cy,

Mo

nth

s

Most Important Factors for Effective MMF Performance

• Proper media selection and installation.

• Proper service & backwash flow rates.

• Slow moving valves.

• Proper lateral designs.

• Backwash on turbidity breakthrough or pressure differential.

• Proper chemical selection.

RO Troubleshooting - Scale Formation

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1st array 2nd array

Concentrate

Permeate

Pre Filter

Scaling occurs when soluble minerals become concentrated in the RO and exceed saturation limits.

Symptoms of scale would be poor salt rejection, low product flow and high pressure differential in last array.

RO Troubleshooting - Scale Formation

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1st array 2nd array

Concentrate

Permeate

Pre Filter

Scale will often be seen on the downstream end of the last membrane in the system or on the inside of the last pressure vessel.

Scale can be abrasive and scratch the membrane surface causing permanent damage.

RO Troubleshooting – Scale Fouling

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Scale forms from the back of the system in the downstream

arrays and fouls backwards toward the front resulting in

high dP, loss of permeate flow, loss of permeate quality.

Concentrate

Permeate

RO Troubleshooting - Calcium Carbonate Scale

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1st array 2nd array

Concentrate

Permeate

Pre Filter

This is an example of severe calcium carbonate scale.

Calcium carbonate scale is not always white. The weight of a scaled membrane can be much greater than a new membrane.

RO Troubleshooting - Calcium Carbonate Scale

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1st array 2nd array

Concentrate

Permeate

Pre Filter

Mix a solution containing ½ DI water

and ½ Hydrochloric acid. Drop a

small sample of the foulant in the

solution. If the foulant contains

calcium carbonate, it will bubble. If

there is residual material after

bubbling has stopped, then the

foulant consists of more than just

calcium carbonate.

.

RO Troubleshooting - Sulfate Scale

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1st array 2nd array

Concentrate

Permeate

Pre Filter

Symptoms of sulfate scale would be the same as other scales. Poor salt rejection, low product flow and high pressure differential in last array.

RO Troubleshooting - Sulfate Scale

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1st array 2nd array

Concentrate

Permeate

Pre Filter

Symptoms of sulfate scale would be the same as other scales. Poor salt rejection, low product flow and high pressure differential in last array.

Barium Sulfate scale can be the most abrasive and scratch the membrane surface causing permanent damage. See the SEM photo.

4000 PPM10000 PPM

Concentration Factor

50% Recovery2x Concentration

75% Recovery4x Concentration

90% Recovery10x Concentration

Feed Water

Product Water

Concentrate Water

1000 PPM

2000 PPM

Antiscalant

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Proper Chemical

Proper DosageDilution Strength

Proper Recovery

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