Reverse Osmosis System Operation

Presented by:Sujit Dahal

Chapter: 4

Key RO system Components

• Filtered water transfer pumps• High-pressure pumps• RO membranes• RO skids (trains)• Energy recovery system• Membrane flushing system• Clean-in-place (CIP) membrane cleaning system• Instrumentation and control system

Fig 1. General RO membrane filter configuration

• Components may differ from one desalination plant to another depending on type of intake, source water quality, energy recovery system, and design configurations

• Entire volume of pretreated water is used in RO system

• Energy is recovered and reused to drive high pressure RO pump

• Cartridge filter and chemical feed system are considered as pretreatment and not as RO system components

Purpose and Function

• Filtered Water Transfer Pumps Typically vertical turbine or horizontal centrifugal pumps

are used to convey water to RO system Based on flow pattern:1. Direct flow-through desalination system• Intake pump station is designed to deliver suction pressure needed

for efficient operation of high pressure pump• Pretreatment is designed in such a way that: it does not break pressure by use pressure gravity media filters or pressure-

driven membrane pretreatment filters and withstand the extra pressure needed for suction of high pressure RO pump

• Suction pressure in SWRO: 2-6 bars

BWRO: below 1 bar

• Filtered Water Transfer Pumps

2. Desalination system with interim pumping•Separate pump is installed to booster the filtered source water flow to suction pressure needed for high pressure RO pump•In state-of-art design of SWRO, transfer pumps are equipped with variable frequency drives (VFDs) to cost effectively control feed pressure of RO pumps.•This is required because of impact of source water temperature and salinity on osmotic pressure and net driving pressure (NDP)

•Temperature AND/OR Salinity Required NDP and feed pressure

• Filtered Water Transfer Pumps

• To maintain HPPs at their maximum performance efficiency and constant feed flow at all the times, VFDs are installed which reduces overall plant energy use.

• VFDs are used either on high-pressure RO pumps motors or on filter effluent transfer pump motors

• Installation of VFDs on filter effluent transfer pumps is usually more cost effective.

• If the difference of maximum and minimum operational pressure is (2-5 bars) VFDs are installed on transfer pumps

• If the salinity of source water varies in wider range (more than 30% of average annual level), VFDs are installed on high-pressure RO pumps.

• Filtered Water Transfer Pumps

Fig: Direct flow through desalination system

• High-Pressure Feed Pumps

• Designed to deliver adequate pressure to separate fresh water from salts.

• Typically, the pressure is 5-25 bars for BWRO and 55-70 bars for SWRO

• Required pressure depends on source water salinity, temperature, target product water quality, and configuration of the RO system.

• These pumps are sized based on performance curves provided by pump manufacturers

• Wetted pump materials should be of adequate quality stainless steel. (316L or greater for BWRO and Duplex and Super duplex for higher salinity BWRO and SWRO)

• Spiral-Wound Polyamide Membrane Elements

• Classified into 3 based on type of water they are configured and designed to desalinate

1. NF elements

2. BWRO elements

3. SWRO elements• Have similar configuration

but differ by type of membrane material, salt rejection, permeability, and operating pressure range.

• Pressure vessels

• Inside pressure vessels (housings), six to eight membrane elements are installed in series

• Elements are interconnected with short plastic, spool pipe segments with sliding rubber seals (O-rings) or via specially designed interlocking interconnectors.

• One pressure vessel houses 6-8 RO membrane elements; 8 is more beneficial especially for medium and large plants because of smaller number of pressure vessel housing

• Could also yield lower overall concentration polarization factor due to higher feed, beneficial in terms of fouling too.

• Need slightly high feed pressure

• Minimum feed flow= 10 and maximum flow= 17 CMH

• Minimum recommended flow of concentrate per vessel= 2.7CMH

• Pressure vessels

• Alternative Membrane Configuration within the Vessels

• Standard configuration All membrane elements in same vessel are identical and flux

decreases in the direction of flow First two elements produce over 35-40% of total plant flow In actual conventional SWRO systems, flow distribution is uneven

and first membrane element produce over 25% of total permeate flow (uses over 25% of pressure available for desalination)

Last element yield 6-8% of total permeate Decrease in permeate production id due to increase in feed salinity

and associated osmotic pressure The downstream RO elements are underworked, hence efficiency of

conventional SWRO systems in not at optimum level

• Alternative Membrane Configuration within the Vessels

• Internally staged Configuration with Different membranes

• Hybrid membrane configuration with SWRO elements of different productivity and rejection within same vessel makes optimal use of energy

• Most energy-efficient desalination process with lowest fouling can be achieved by redistributing and evening out the feed pressure and flux in near equal level.

• Recent SWRO plants use internally staged design (ISD) with three different models of membrane with different permeability within same vessel.

• Disadvantage is RO membrane elements cannot be used interchangeably at any position which hinders membrane rotation making more inconvenient and costly.

• Hence, ISD vessel industrial use is limited.

• Internally staged Configuration with Different membranes

• RO System Piping• High quality stainless steel piping is typically used for high-pressure feed.• Higher the source water salinity and brine concentration, higher the quality of

steel required to prevent from corrosion• Copper-nickel alloys are also used for brackish and seawater intake screens and

other facilities• Fiberglass-reinforced plastic (FRP) and HDPE piping are also used for low-

pressure applications• Quality of steel is measured as a function of %content of Cr, Mo, and N

contained in steel termed as pitting resistance which is given by:

PREN = %Cr + 3.3ⅹ%Mo+16ⅹ%N• Reinforced flexible tubing could be used for NF and low-salinity BWRO

systems, but its useful life is shorter and has higher replacement costs.

• PVC (schedule 80), is commonly used for low-pressure permeate piping and valves.

• Flexible tubing should be covered with UV resistant coatings and need to be replaced every 24-48 months.

• RO System Piping

• General velocity recommendation for different pipe materials: Stainless steel: 2.5-3.5 m/s Schedule 80 PVC piping: 1.5-2 m/s Schedule 40 PVC piping: 1-1.5 m/s FRP: 1.5-2 m/s

• Pressure piping are typically rated at minimum of 150% of its design maximum operating pressure and fully restrained.

• Pipe coating and cathode protection is used to protect buried material from corrosion and dissimilar metals are isolated to protect from electrolysis

• Buried gravity pipes are sloped uniformly with minimum cover of 1m and equipped with vents and drains at pipe high and low points.

• RO System Piping

• RO Skids• Skids are support

structures for arrangement of multiple pressure vessels.

• They are made of powder coated structural steel, plastic coated steel or plastic.

• RO train is combination of RO feed pumps, pressure vessels, feed, concentrate and permeate piping, valves, couplings and other fittings, energy recovery system, and instrumentation that can function independently.

• Energy recovery systems

• Remaining energy from RO concentrate can be recovered and reused using specifically designed equipment called energy recovery device (ERD)

• Payback time for installation in SWRO plants is usually less than 5 years

• For BWRO plants, savings are significantly low, hence not frequently used.

• Can be divided into centrifugal and isobaric ERDs based on principle of operation.

• Most widely used centrifugal energy recovery devices used in SWRO and BWRO plants are pelton wheel and hydraulic turbocharger.

• Pelton Wheel

• Concentrate pressure is converted to rotational kinetic energy which is applied to shaft of wheel

• Concentrate discharge is done by gravity

• Energy conversion rate is about 80-90%

• Max. size of RO train is equal to size of pelton wheels and max. size available commercially is 21000 CMD

• Simple to operate, more compact and less costly than isobaric ERDs.

• But are more costly and less efficient than turbochargers for small size installations.

• Turbochargers

• Hydraulic turbo booster (HTB) consists of turbine and centrifugal pump on same shaft.

• Installed in series with a single-stage, medium-pressure centrifugal pump driven by electric motor.

• Has concentrate bypass to reduce flow when it is more than required to boost the feed pressure target level.

• Medium pressure pump:- 50-75% of total RO feed pressure (35-46 bars)

• HTB:- remaining pressure (25-37 bars) up to 56-70 bars

• Total energy efficiency of system = 80-90%

• Low equipment cost, minimum space, and simple O&M.

• Low efficiency for large plants

• Isobaric Chamber –Type ERDs

• Transfer concentrate pressure directly into RO feed pressure via pressure exchange by use of piston.

• Energy-bearing concentrated stream is applied to back side of piston of isobaric chambers known as pressure exchangers.

• Conveys 45-50% of total volume of feed water and the rest is handled by high pressure centrifugal pumps.

• Has widespread application and has reduced desalination power costs by aprox. 10-15% compared to other technologies.

• Efficiency of pressure exchangers about 93-95%• Two most commonly used isobaric chamber-type ERDs are:

Pressure exchangers (PX) and Dual work exchanger energy recovery (DWEER) system

• Isobaric Chamber-Type ERDs

• PE are most widely used in medium and large SWRO plants

• High energy recovery efficiency and reliability

• More compact compared to DWEER system

• Has fewer moving parts than DWEER• Key components made of fiberglass

which is low cost and non-corrosive.

• Lower concentrate/filtered water mixing than PE

• Latest DWEER system are cost effective compared to older ones

• Have more moving parts, take longer to commission, and are more maintenance intensive.

DWEER

• Routine RO System Operation• Either VFDs or pressure

control valves are installed to adjust feed flow and pressure of centrifugal high-pressure pump.

• These valves are throttled to achieve target recovery, feed flow and pressure

• Two main goals that drive RO system operation are to:1. Produce desalinated water of a target flow rate2. Meet target permeate water quality specifications defined by the final users of

desalinated water• Both goals are achieved simultaneously and continuously• Present desalination plants have at least 96% annual capacity availability (350days)• Well designed and well operated plants can achiece over 98% availabilty

• Routine RO System Operation• Over time, RO membrane has to be operated at higher feed pressure to

compensate for loss of permeability and produce same quality of water.• Water with high content of fine solid and colloidal particulates, NOM,

biodegradable dissolved organics, aquatic microorganisms and mineral scale-forming compounds are responsible for higher fouling.

• Fouling capacity of water may change with seasons and extreme weather events

• Feed pressure is limited by max. capacity of feed pumps and max. pressure the RO membranes can withstand

• Increase in salinity and temperature = negative impact on product water quality and vice versa

• RO membrane permeability along with their productivity decrease irreversibly with time.

• Typically , an RO system loses 8-15% of its initial productivity over a period of 3-5 years.

• Maintaining RO System Fresh Water Production Rate• Compensation for loss of productivity is done by increasing feed pressure of

high pressure pumps via increase pump speed using VFDs or by one of the following actions:

1. Closing concentrate flow control valve which increase source flow through membrane

2. Opening pressure control valve which increases feed pressure within limit of pump capacity

• Maintaining RO system Permeate Quality• Product water quality can controlled by increasing feed pressure and by operation at

high recovery (increasing membrane flux)

• This accelerates membrane fouling and hence there is need to increase pressure or flux to overcome accelerated fouling

• This leads to permanent loss of long term production capacity of RO membranes.

• Alternative way is to adjust permeate backpressure valve and closing it reduces concentrate polarization.

• During periods of RO shutdown this may lead to thin film delamination if permeate backpressure exceeds 0.3 bar.

• Alternative Approaches for Ro system operation

• Running the RO system at constant near max. pressure of RO feed all the

times

At 25° C and 1 year operation permanent loss of membrane productivity for:•55 bars less than 2%•69 bars 19%

For actual case in SWRO desalination plant with source temperature between 28-35 ° C:•In 2 years 25% and in 4 years 50% production capacity lost

• RO system startup

• Key system components to be set before starting RO trains are:

1. Calibration of all instruments

2. Verification of pressure relief protection is in place

3. Open permeate drain line

4. Open concentrate control valve

5. Set the pressure control valve to partially (<50%) open position.

6. Increase gradually the feed pressure of the high pressure pumps until target pressure is reached

7. Gradually close concentrate valve to achieve desired Ro train recovery

• Key Routine Operational Tasks

• Involves frequent checking RO train feed and permeate flow rates, conductivities, feed and concentrate pressures and adjusting RO feed pressure/ concentrate flow.

• Change in more than 10% source water quality and quantity require identification of reasons for changes and predict adjustments required to maintain target levels of all parameters.

• Includes daily visual inspection of RO train equipment and identify deviations from normal operations (unusual vibrations, noise, overheating, corrosion, leaks, failures, and out of range sensor readings)

• Performance Monitoring• Performance Data Collection Typical Performance Parameters of RO systems1. Source water pH, conductivity, turbidity, SDI, ORP, and Temp.

2. Permeate and concentrate conductivity and pH

3. Source water flow and pressure

4. Concentrate flow and pressure

5. Differential pressure

6. Permeate flow (difference of feed and concentrate flow)

7. Recovery of individual RO trains and entire RO system (ratio of RO permeate and feed flows)

8. Membrane control valve status

9. High-pressure pump and ERD status

10.Membrane feed, interstage, and concentrate pressures

11.Product water clear well/ storage tank level

12.Degasifier effluent pH (for BWRO plants)

13.Product water pump flow and discharge pressure

• Performance Monitoring• Performance Data Collection Typical conditions for shutdown of Individual RO TrainsInitiated automatically or manually under these conditions:

1. Membrane feed pump low suction or high discharge pressure

2. ERD system is shut down/malfunctioning

3. Train recovery rate is out of range

4. Train concentrate flow is low

5. Train permeate flow is out of range

6. Train permeate pressure is too high

7. Water level in product water clear well or finished water storage tank reaches preset high level

8. Loss of individual RO train power

• Performance Monitoring• Performance Data Collection Typical conditions for shutdown of Entire RO TrainsInitiated automatically or manually under these conditions:

1. Pump run failure at intake, critical chemical feed pumps or product transfer pumps

2. Source water pressure below a preset minimum

3. Feed water pH, ORP, chlorine residual, temperature, or turbidity are above maximum acceptable preset design levels.

4. Water level in product water clear well reaches preset high level

5. Loss of power

• Performance Monitoring• Impacts of Source water and operational parameters on

RO system Performance

Factors affecting RO system performance in terms of desalinated water quality and quantity:

1. Changes in source water quality

2. Normal wear and tear of Ro membranes, cartridge filters, equipment, and instrumentation

3. Membrane and equipment failures

4. Operator errors

5. Changes in regulatory requirements

• Source Water Salinity

Higher feed water salinity reduces net driving pressure because of increased osmotic pressure, which in turn decreases permeate flux

Water salinity Salt concentration gradient = accelerated salt transport through membraneLower salt rejection

• Source Water Temperature

Warmer water reduces viscosity and helps to increase membrane permeability

But osmotic pressure is increased at same time which is not desirable Rule of thumb: Permeate flux increases by 3% for every 1°C increase in

temperature High temperature loosens the membrane structure leading to increase in

salt passage as RO membrane are made of plastic material.

Temperature above 40°C accelerates compaction of membrane support layer and results in premature irreversible loss of membrane permeability.

Recommended to operate below 45°C.

• Source water fouling potential

• Depends mainly on following WQ parameters: Suspended solids Silt and colloids Organics Oil and grease Mineral scaling foulants

• Source water Oxidation potential Strong oxidants (Cl, O3, permagnate, bromamine, etc) cause irreversible

damage to membrane structure and can permanently increase membrane salt passage

Measured by ORP of source water If ORP > 250mV, membrane would be damaged irreversibly

• RO system recovery

• Increase in recovery causes slow decrease in permeate flux till osmotic pressure exceeds the applied pressure.

• NDP is not enough and fresh water production is stopped.

• RO feed pressureFeed pressure

NDP Membrane flux

• At high pressure more water is produced with constant amount of salts, hence permeate salinity decreases (salt rejection increases with pressure)

• Net Driving Pressure (NDP)

• NDP = pressure available to produce fresh water at given time• Determined by RO feed pressure, osmotic pressure, and

membrane permeability• RO system production is proportional to available NDP.• NDP = f(membrane type, source water salinity and temp.)

• To counter this problem, high-pressure feed pumps should be designed with built-in-size and flexibility to control feed pressure

• This is required especially when new generation membrane or higher fresh water productivity is installed.

• Performance Data analysis and Normalization

• Data normalization is standard procedure to adjust key water quality parameters (permeate flow, salt passage) in order to eliminate effect of temperature and evaluate the effect of membrane fouling and/or oxidation separately.

• These parameters are recalculated for TCF and MCF and the standard formula is given by:

NPF = Qpⅹ TCF25 (NDPinitial /NDPpresent)ⅹ MCFMembrane compaction factor

NDP at time of measurement

NDP during initial operation period of RO membrane system (48h after installation)

Temperature correction factor provided by supplier of RO membrane: If not known, can be calculated as:TCF25 = 1/(1.03(T-25))

Permeate flow measured at ambient temperature

• RO system maintenance1. Membrane Flushing and preservation• Typically equipped with a permanently piped membrane flushing system

to automatically or manually every time RO membrane trains are taken off service

• If not flushed causes gradual fouling and deterioration of membrane permeability due to microorganisms growth, silt and SS settling, etc.

• Either flushed using RO permeate water or chemically conditioned filtered water

• Flushing is completed in direction of feed flow and flush water is drained through the concentrate line

• Permeate line should be open to avoid backpressure and consequently avoid membrane thin film caused by delamination.

• Flushing can be carried out by following methods: Flushing before Standby or Short term shutdown Flush before or after Long-Term RO train Shutdown

• RO system maintenance1. Membrane Flushing and preservation• Membrane Preservation for Long-Term Shutdown It is recommended to fill the TO train with sodium bisulfate (SBS

concentration of 500-1000 mg/l) in case RO membrane has to be shut down for long period of time (> 1 week),.

If there is heavy biofouling, cleaning should be done using CIP procedure.

At the end of flushing, RO membranes should be filled up with nonconditioned RO permeate.

Addition of SBS consumes Oxygen in flush water and will suppress biogrowth on membrane surface.

pH of the solution should be checked weekly (If pH < 3.2, the solution should be replaced with fresh solution)

SBS should be replaced once per 2 weeks – 1 month If ambient temperature < 27°C, solution should be replaced in every

month and it the temperature increases the threshold replacement should be made in every 2 weeks.

• RO Membrane Cleaning