Reverse Osmosis module design and engineering emerged with membrane technology
evolution. In order to understand module design, first membrane configuration needs to be
explored, since the module design is always tailored according to the membrane
characteristics. There is a significant difference between membrane chemistries (most
important ones being cellulose acetate and thin film composite with polyamide barrier
layer), and more importantly, between the different membrane configurations (hollow fine
fiber and flat sheet). Therefore, before looking into detail on the module configuration, the
membrane development needs to be considered.
3. • Dramatic decrease in RO membrane prices (50% - 8yrs).
• Increasing costs of chemicals (esp. NaOH) for IX systems.
• Increased return on investment for reverse osmosis as pretreatment
to an ion-exchange system.
• Increased manufacturer awareness in properly designing
pretreatment systems.
• Increase in comfort level for operation of customer-owned RO
systems.
• Consistent water quality
REASONS FOR INCREASED POPULARITY WITH
REVERSE OSMOSIS
29. What is RO Permeate and % Recovery?
• Permeate is water recovered as product.
• % Recovery = Permeate Flow Rate x100
– % Recovery calculates percent of feedwater that
becomes product.
– % Recovery describes performance of the system
– Greater recovery=less waste=cost savings.
– Recovery typically ranges from 50% to 75% (can go
as high as 85%)
– % Recovery and permeate quality are inversely
related.
REVERSE OSMOSIS
Make-up Flow Rate
31. What is RO Concentrate and % Rejection?
• Concentrate (or Brine) is the waste from the RO.
• Reject is a calculation of the percentage of
solids/solutes in feedwater rejected by the
membrane.
– Typically ranges from 95% to 99+% for most ionic
solutes and set by membrane manufacturer.
– Greater % reject means better permeate quality.
– Species dependant
• Multi-valent ions (Ca2+, Mg2+) higher rejection
• Monovalent ions (Na+, Cl-) lower rejection
• Gases (O2, CO2) no rejection
REVERSE OSMOSIS
32. RO Systems
– Reject Staging
• Multi-stages for reject
• Increased utilization of water
– Incremental increase in investment
– Minimal decrease in water quality
– Multi - Pass
• Product staging
• Improves water quality
– May eliminate the need for downstream
polishing
38. What Are the Advantages of RO ?
• Removes nonionic impurities and dissolved solids
(i.e. organics, silica, bacteria)
• Reduction of hazardous chemical storage and
handling associated with ion exchange
• Economic advantages increase with increasing
feed TDS
39. What Are the Disadvantages of RO?
• Concentrate is rejected and this can be a significant volume of
water.
• RO membranes reject a fixed percentage of feedwater ions
– Further treatment is required for many applications.
• Ultimate filter which is easily fouled:
– Increasing operating costs
– Reducing membrane life
43. REVERSE OSMOSIS
•80% to 90% of problems are
related to pretreatment of RO
Feedwater
•The purpose of pretreatment is
to prevent
• Membrane Fouling
• Membrane Scaling
• Membrane Degradation
52. REVERSE OSMOSIS
•RO
For:
• DOSAGE CONTROL
• LEAK ANALYSIS DIAGNOSTICS
• TRUE SYSTEM RECOVERY
• MEMBRANE INTEGRITY
• COST CONTROL
SCALE CONTROL
53. Components of RO TRASAR®
Trasar 8000 Handheld Fluorometer
for Monitoring and Diagnostics
Trasar 3000 Fluorometer for
On-line Monitor & Control
54. 120 MW CCGT Cogeneration
Plant, Florida
• Description of demineralization system
– Water source: city water
– Pre-treatment: feed water dechlorination
– Reverse osmosis: 2X100 gpm systems
– Post-treatment: mixed-bed ion exchange column
• Performance issues
– Fouling of membrane elements resulting in frequent
cleanings
– Poor permeate quality resulting in frequent regeneration of
polishing ion exchange bed.
55. Active control of
scale inhibitor dosage
0
5
10
15
20
25
30
0
50
100
150
200
250
Hours of Continuous Operation
PPMasAntiscalant
Before Control After Control
Reduced membrane fouling.
0
5
10
15
20
25
30
0
20
40
60
80
100
120
140
160
180
200
Hours of Continuous Operation
ppmantiscalant
Actual Dose
Target Dose & Control
Limits
56. RO TRASAR® Benefits: 120 MW
CCGT Cogeneration Plant
> $37,600/yrTotal Savings
TBD88%Every 800K galEvery 100K galPolishing IX
regeneration
$6,00050%4 yrs life2 yr lifeMembrane
replacement
$26,00085%4 per year26 per yearCleaning
$5,60025%9 ppm12 ppmAntiscalant
$/yr%AfterBefore
Savings
57. Colloids
• Cause:
– Surface water
– Corrosion in system piping - (Line
all vessels)
• Prevention:
– Coagulation & filtration
– Zeolite softening
– Chemical Antifoulant
63. Results of Membrane Problems
• Reduced water quality
– Shorter run lengths on downstream IX
• Premature membrane replacement
• Higher operating costs
65. Monitoring
• Pretreatment
– 90% of operational
problems are found here
• System
– 10% of operational
problems are found here
66. RO System Monitoring
• Pretreatment monitoring
- Silt Density (SDI), Turbidity, pH,
Oxidants Particle Size and Counts
- Temperature, Pressure, TDS
- Foulants (bacteria, metals,
hardness, silica)
67. RO System Monitoring
• Performance monitoring
- Percent salt rejection
- Normalized permeate flowrate
- Differential pressure
68. Pretreatment
• Silt Density Index (SDI)
• Langlier Saturation Index (LSI)
• Stiff Davis Index (TDS >4,000 mg/L)
• Feedwater Analysis
69. Silt Density Index (SDI)
• Empirical indication of potential fouling
• Based on rate of plugging a 0.45m filter
• Hollow Fiber SDI < 3.0
• Spiral Wound SDI < 5.0
• Typical Well Water SDI < 3
• Typical Surface Water SDI > 6
73. Feedwater Analysis
• Minimal water test includes
– Ca, Mg, Fe, Al, Silica
– SO4, Alkalinity, pH, Conductivity
– SDI
– TOC
– Color
• Full water analysis should include
analysis scaling/fouling contaminants
94. Trouble Shooting Changes
• Check Instrument Calibrations
– Compare Percent Recovery by Conductivity Vs.
Flow
• Identify the Location of the Decline
– Front end Vs. Back end, Stage 1 Vs. Stage 2
• Investigate Potential Causes of the Problem
– Use both visual and analytical data
• Correct the Potential Cause of the Problem
95. Troubleshooting
Instrument Calibration
• Compare Recovery Calculations
– Conductivity Vs. Flow
• Pressure Meter Change Out
– Quick Disconnects
• Hand Held Vs. On-line Instrumentation
• Thermometers
96. Trouble Shooting / Changes in Salt Rejection
• Check Individual Pressure Vessel
Performance
• Probe the Pressure Vessel (Spiral
Wound)
• Individual Membrane Testing
– Single Element Test Skid
Identify the Location of the Decline
98. When to Probe
• High salt passage
• Individual pressure vessels have
high conductivity
99. Individual Element Performance
• Probe the Pressure Vessel (Spiral Wound)
– 1/4” Tubing into Permeate Line
– Conductivity versus Penetration
• Location of Problem
– Check front / mid / end each element
– Note direction of feed water flow
101. Membrane Element # Permeate TDS (ppm)
1 Lead End 25
2 23
3 25
4 21
5 Tail End 54
Probing a Pressure Vessel
Remember to Note Feedwater Direction
102. Troubleshooting Individual Membrane Elements
• Individual Membrane Testing
– In House
• Non-destructive
– Single Element Test Stand
– Autopsy
• Potentially Destructive
• More detailed Information`
113. When do I clean?
• When any ONE of the following changes:
NPF by 10% - 15%
Differential Pressure by 10% - 15%
Salt Rejection by 10% - 15%
• Start planning your strategy at the first
indication of a minimum change!
114. Proper Cleaning Maintenance
NormalizedNormalized
PermeatePermeate
FlowrateFlowrate
TimeTime
Cleaning after 10Cleaning after 10--15% decline15% decline
NormalizedNormalized
PermeatePermeate
FlowrateFlowrate
TimeTime
Improper Cleaning MaintenanceImproper Cleaning Maintenance
Cleaning after >15% declineCleaning after >15% declineCleaning after >15% declineCleaning after >15% decline
Waiting tooWaiting too
longlong
to cleanto clean
reducesreduces
RORO
performanceperformance
Cleaning after aCleaning after a
1010--15% decline15% decline
maximizes ROmaximizes RO
performanceperformance
116. Cleaning
• Do Not Exceed Mftr. Specs!
– pH
– Pressure Drop
– Temperature
– Flow Rate
117. Cleaning
• Cleaning CF size < Operating CF size
• Use permeate as Make-up
• Mix Chemical according to instructions
• Utilize “maximum” conditions
• Dump “first system volume” (i.e., flush)
118. Cleaning
• Return permeate & concentrate to tank
• Make as little permeate as possible
(Open concentrate valve wide open)
• Pump Size is critical
119. How To Choose a Chemical Cleaner
• Cleaner Selected for:
– Membrane Type
– Characteristics of Foulant
– Convenience
• Optimum Service
• Acid Cleaners First
• Followed by Caustic Cleaners
121. Things to Remember
• Start planning to clean when:
– Differential Pressure changes 10%
– NPF changes 10%
– Salt Rejection changes 10%
122. Things to Remember
• Order of cleaning chemicals:
– Caustic/Acid (can vary with contamination)
– Acid/Caustic (can vary with contamination)
– Sanitization
• Waiting too long will cause irreversible
damage!
124. Membrane Cleaning Frequency
Cleaning Frequency
Quarterly or less
Every 1-3 months
Every month or more
Adequacy Estimate
Adequate
Marginal
Not adequate
125. Clean Until
• pH Doesn’t Change
• Color Doesn’t Change
• Flow Doesn’t Change
• Pressure Doesn’t Change
126. RO Element Test & Cleaning Stand
Feed
Pressure
Gauge
Permeate
Flowrate
Globe
Valve
Reject
Flowrate
Needle
Valve
Reject
Pressure
Gauge
Permeate
% Salt Rejection
Monitor
Differential
Pressure
127. Benefits of Maintaining an RO
• Reduced operating costs
• Reduced maintenance costs
• Reduced downtime
• Extended membrane life
• Improved water quality and output
130. REVERSE OSMOSIS SUMMARY
• Understanding RO terms is important for successful
unit operation.
• Initial design is critical and will determine long term
permeate quality.
• Data collection and normalization is vital to
maintenance and trouble shooting. (RO Eye)
• Pretreatment key to keeping membranes
performing well. (Permacare, RO Trasar, Permafloc
and/or Ultrasoft, Ultrasand)
• Cleaning based on trends in normalized data.
(Permaclean)