Water Conditioning & PurificationJ U L Y 2 0 0 8

IndustrialReverse Osmosis

System Design

By Jeff Tate

Saltsolution

Purewater

Pressure

Osmoticflow

Semi-permeablemembrane

Saltsolution

Purewater

Reverseosmotic

flow

Semi-permeablemembrane

Figure 1. Reverse osmosis process

Industrial system design

Figure 2. Spiral wound membrane cross section

IntroductionReverse osmosis (RO) is cur-

rently used in various applica-tions ranging from small,undersink drinking water unitsto large industrial systems. Thetechnology is widely used andaccepted as it removes both dis-solved ionic and organic impuri-ties. This article will focus on thereverse osmosis technology inindustrial applications and willdiscuss factors that differentiateindustrial RO from commercial-grade systems.

RO installations are some-times unsuccessful due to the sys-tem design and application. Theapproach and recommendations in thisarticle are based on 16 years of experi-ence in system design and application,as well as design experience and operat-ing observations of successful membranesystems. The goal of this paper is topresent factors that will contribute to re-liable operation for many years. Alternatedesigns are sometimes used.

A natural processOsmosis is the natural phenomenon

that occurs as pure water flows across asemi-permeable membrane to the sidethat contains a higher concentration ofdissolved solids. The higher the dis-solved solids concentration, the morepure water (dilute) will flow to the con-centrated side. The osmotic pressure thatoccurs results in a higher level of concen-trate water than dilute water (Figure 1).An externally applied pressure, higher

than the osmotic pressure, drives waterin the opposite direction, producing ahigher volume of pure water. This pro-cess is called reverse osmosis.

The most common RO mem-branes are spiral wound (Figure2). Several layers of flat-sheetmembranes, concentrate cham-bers (feed spacers) and dilutechambers (permeate carriers) arewound around a center permeatetube that collects the dilute wa-ter. The membrane element issealed around the outer edgeswith a glue line. The design ofspiral-wound membranes is suchthat feedwater enters on the feedside of the membrane at a highpressure. Reverse osmosis occursand low TDS water is collected inthe permeate tube, then exits thecenter of the membrane element.

The feedwater becomes more and moreconcentrated as it flows across the mem-brane and exits as the concentrate rejectat much higher TDS levels than the feed

Photo courtesy of Hydranautics

J U L Y 2 0 0 8Water Conditioning & Purification

flow. Since membranes are reported tohave pores as small as 0.005 microns, themajority of organics (dissolved and un-dissolved) and other suspended solidscannot pass through the membrane. Theyare therefore typically rejected with theconcentrate.

Commercial system designSmall commercial systems utilize

2.5- and four-inch diameter membranesare normally housed in PVC or stainlesssteel single membrane housings. Theremay be some PVC or stainless steel pip-ing, but hoses are typically used to con-nect the RO housings, reducing materialscost and assembly time to manufacture(Figure 3).

In commercial systems, instrumen-tation is basic. Depending on the manu-facturer, there are usually permeate andconcentrate flow rotameters with a con-centrate pressure gauge and a motorstarter

Usually, a preprogrammed micro-processor is used to start and stop the ROsystem based on product level in the per-meate storage tank. The microprocessorsalso feature pretreatment lockout to preventthe system from running if a media filteris in backwash or softener is in regenera-tion. Low- and high-pressure pump pro-tection is included for additional cost.

Commercial systempretreatment often includesa multimedia filter, softenerand activated carbon filter.Backwashable filters andsoftener vessels are fiber-glass reinforced plastic(FRP) constructed with atimer-based control valve.The systems include a plas-tic pre-filter rated at fivemicrons and a multistagecentrifugal pump. SS needlevalves are used to controlconcentrate flow with nocontrol of permeate flow.The result is a fluctuation ofproduct water flow rate.

Depending on themanufacturer, some partsmay be brass rather thanstainless steel to reducecost without concern of thelife the system. If the pur-chaser is educated aboutsystem benefits, concentrate recirculation,pre-filter inlet and outlet pressure gauges,pump throttling valves, soft motor start-ers or variable frequency drive (VFD) andFRP membrane vessels, cold-water mem-branes and permeate pressure gauges maybe included.

Commercial RO permeate flow ofthe system is usually rated close to themaximum allowed by the membranemanufacturer, rather than based on theapplication and membrane flux. Recov-ery is defined as the permeate flow ratedivided by the feed flow rate, and ex-pressed as a percentage. Recoveries canbe as low as 15 percent without concen-trate recirculation or as high as 75 per-cent with concentrate recirculation. Sincethe feedwater flow rate drops as perme-ate passes across the membrane, thefeedwater becomes concentrated withhigh scaling and fouling containmentsthe further across the membrane surfaceit flows.

In order to minimize fouling or scal-ing of the concentrate contaminants, aminimum flow rate is required to main-tain high velocity and turbulence on themembrane surface. The concentrate recir-culation option allows higher recoveriesand less wastewater by mixing feedwaterwith rejected concentrate water.

Industrial system designHigher end, heavy industrial RO

systems should have a different designphilosophy than their commercial coun-terparts. Some system manufacturersimplement commercial features into theindustrial design; however, large RO

plants should providemore reliable performanceand membrane life thantheir commercial counter-parts. Although systemcost is important, long-term operation, membranelife and performance arealso critical as the mem-brane elements represent asignificant investment toindustrial RO users.

In commercial sys-tems, membrane cost for2.5- and four-inch diameterelements may range from$100-$350 per element.Most consider small mem-branes disposable due tothe low-cost of membranereplacement and smallnumber of membrane ele-ments in a system. Indus-trial systems generallyfeature multiple eight-inch

membranes, usually ranging in numberfrom 18 to 180 per skid (see photo onpage 38). Each eight-inch membrane typi-cally costs $600-$700, so the membranesrepresent tens and even hundreds ofthousands of dollars.

The recommended pressure vessel

holds six elements or less per vessel. Somesystem providers may use vessels capableof holding up to seven or eight elements,but the concentrate flow and velocitydrops as flow passes across each mem-brane element inside the vessel. As con-centrate velocity drops, scaling increases.Therefore, a maximum of six elements pervessel is the recommended design to pre-vent scaling.

Reliable performance in salt rejectionand flow are critical for heavy industrialusers. To achieve even better purity, per-meate can be further polished byelectrodeionization (EDI). By combiningRO and EDI, the system can produce con-tinuous ultra-pure water without chem-ical regeneration.

Feedwater sourcesFeedwater source is the first concern

when designing an RO system. Forbrackish water reverse osmosis (BWRO)membranes, the water source is typicallysurface or well water, but can also be in-dustrial or municipal wastewater. Evenif the source is municipally treated, it isimperative to review where the munici-pality draws their water. This allows foroptimal system design, as the character-istics of the water source will affect theRO membrane operation.

The water source is one factor indetermining the potential for fouling andscaling. Fouling is the accumulation ofsolids on the membrane surface and/orfeed spacer. Scaling is a chemical reac-tion where dissolved solids are precipi-tated out from the feedwater on theconcentrate side of the membrane. Themost common forms of scaling are cal-cium carbonate, barium sulfate, calciumsulfate, strontium sulfate, calcium fluo-ride and silica.

Surface water may be from lakes, riv-ers, reservoirs, etc. By its very nature, sur-face water is prone to fouling due toseasonal fluctuations in suspended solids,biological contaminants and total organiccarbon (TOC). It tends to contain low to-tal dissolved solids (TDS), heavy metalsand hardness. Surface water is usuallydisinfected to kill bacteria, most com-monly with chlorine, ultraviolet light (UV)or ozone. While this disinfection removesthe living microorganisms, the remainingbyproducts are organic matter and foodfor bacteria. Disinfection therefore can re-sult in higher organic and/or biofoulingpotential on the membranes downstream.During rain, the suspended solids mayincrease which increases the potential forplugging of the membranes.

The two most meaningful methodsof measuring suspended solids are tur-

Figure 3. Commercialsystem designPhoto courtesy of Hydranautics

Water Conditioning & PurificationJ U L Y 2 0 0 8

bidity and silt density index (SDI). Tur-bidity is most commonly measured innephelometric turbidity units (NTU) andincreases as the water ability to transmitlight (transparency) decreases. SDI is acalculation of fouling potential accordingto test standard ASTM D-4189. It is cal-culated by flowing water through a 0.45-micron filter at 30 psi in a 500 mL jarbefore and after a standard 15-minute runtime through the filter. The percentage ofplugging is calculated by comparing thetime to fill a 500 mL jar with the ROfeedwater before the test (ti) with the timerequired to fill after the test (tf).

Turbidity should be less than 0.5NTU for optimal performance and maxi-mum of 1.0 NTU. Acceptable SDI levelsat the RO inlet are less than 5.0 (15-minute test), but SDI should be less than3.0 for optimal performance.

Well water (also calledground water) is taken from un-derground supplies. It usuallycontains very little suspendedsolids, as the earth acts as a natu-ral filter when water drains un-derground. Well water typicallyhas higher dissolved solids andfrequently is high in hardness,heavy metals and possibly silica.

PretreatmentRO pretreatment is optimized based

on feedwater characteristics. Suspendedsolids are removed by filtration. Mediafiltration is limited to filtering between10 to 20 microns, while membrane filtra-tion (such as ultrafiltration) can filter be-low 0.03 microns.

Scaling potential is reduced either byion exchange softening or chemical in-jection. Ion exchange (IX) softening is areliable technology if regenerated prop-erly; however, industrial ion exchangesystems are more expensive than chemi-cal injection. Industrial IX systems areusually dual trains. For reliability, thevessels are recommended to be ASMEcode carbon steel with epoxy or rubberlining and external painted surface. Thevalves are automatically PLC-controlled.

Acid injection and antiscalant injec-tion are two methods for reducing scal-ing on membrane surfaces. For acidinjection, hydrochloric acid is preferredover sulfuric acid, as the latter can in-crease the sulfate scaling potential. Theprocess of acid injection will increase car-bon dioxide levels that load polishing ionexchange or EDI.

Antiscalant chemical injection simplyrequires a small storage tank filled withRO permeate and suitable antiscalants,level switches, manual isolation valves

and a chemical metering pump. The ap-propriate antiscalant chemical is deter-mined by the specific scaling ions, pH andrecovery. Antiscalant chemicals are gen-erally considered safer than acid. Chemi-cal selection and dosage are provided bythe chemical supplier. However, a reliableand experienced manufacturer with a fullrange of antiscalant products designed formany specific feedwater and RO require-ments should be used.

Scaling potential is measured inLangelier saturation index (LSI) forbrackish water. LSI is calculated by sub-tracting the calculated pH of saturationof calcium carbonate from the actual feedpH. LSI should be less than negative 0.2if antiscalant injection is not used.Antiscalant injection permits LSI as highas 1.8 depending on the antiscalant used.

RO membranes cannot handle oxi-dizers, including free chlorine. If chlo-rine is in the feedwater, it must beremoved either by activated carbonadsorption or sodium metabisulfite in-jection. Activated carbon filters are morecapital intensive than bisulfite injectionand carbon media is a known breedingground for bacteria. Bisulfite can bemanually controlled, but automatic con-trol and proper set up will ensure chlo-rine is entirely scavenged without overinjection of the bisulfite, which can causesulfate-reducing bacteria (SRB) growth.

Control instrumentation can be areliable online titration chlorine monitoror oxidation reduction potential (ORP).ORP set points will vary from site to site,so an experienced technician should runtitration tests to determine the optimalset point. ORP is set 30 to 50 millivolts(mv) lower than at which zero ppm freechlorine is achieved. Manual tests for freechlorine should be performed duringroutine maintenance to confirm thesetpoint over time.

Staging and selection of RO mem-branes is based on feedwater and prod-uct water requirements. As the systemages, an increase in salt passage and adecrease in flux is expected. Membranesshould have a three-year warranty, so athree-year projection is typically used fordesign. Salt passage increases as tem-perature increases; the warmest tempera-

ture is used to confirm permeate quality.The pressure required to produce thesame permeate flow increases as tem-perature decreases.

In commercial systems, a smallpump is typically sized to deliver the re-quired flow at 77°F (25°C) with 500 or1,000 ppm TDS. When colder water en-ters the system, the user must settle fordecreased flow. Industrial users requirethat the minimum permeate flow bemaintained at all times. The pump mustbe sized based on the pressure and flowat the coldest temperature after a mini-mum of three years of operation withspecific water analysis of the plant. Thepump will be oversized at warmer tem-peratures, so a pump throttling valve orvariable frequency drive is utilized toachieve desired flow.

Membrane flux also needs tobe considered. Flux is the rate ofpermeate flow per membrane area,usually measured in gallons persquare foot per day (gfd). Commer-cial systems usually rate the sys-tem around 20 gfd, which causesrapid decrease in product flow andquality. Industrial systems shouldbe designed for no more than 16

gfd for well water, 12 gfd for surfacewater and 25 gfd for second pass (RO per-meate feedwater). Manufacturers specifya minimum concentrate flow and maxi-mum feed flow for each membranemodel. Membrane selection, staging andsystem recovery are designed to meet theminimum requirements and not exceedmaximum requirements (Table 1).

Salt passage design usually increasesat an average of seven to 10 percent peryear.

Industrial RO systemcomponents

Cartridge filters, also called ROprefilters, protect RO membranes by cap-turing suspended solids particles as wa-ter passes through the cartridges.Cartridge filter housings should be 316Lstainless steel for industrial grade sys-tems; the number and length of cartridgeelements should be sized for 3.5 gpm per10-inch length. Filters should have anominal rating of no more than five mi-crons.

High-pressure pumps increase thefeedwater pressure to that which is re-quired for RO membrane operation.Pumps should be specifically sized andselected based on the temperature rangeand the three-year operating conditionsestablished for the membranes. An indus-trial safety factor defined by the pumpperformance curve should be included in

Table 1. Average flux rates and expectedpercent decrease in flux

Water type SDI Average flux Flux decline/year (%)

Surface water 3 - 5 8 - 12 gfd 7.3 - 9.9

Well water < 3 14 - 16 gfd 4.4 - 7.3

RO permeate <1 20 - 25 gfd 2.3 - 4.4

J U L Y 2 0 0 8Water Conditioning & Purification

system inlet, after chemical injection. AnSDI test kit should also be included withthe system.

The control system is comprised of aprogrammable logic controller (PLC) witha recommended local 10-inch minimumcolor touch screen HMI and an optionalremote Windows®-based operator stationwith remote monitoring capability. In au-tomatic mode, the system is designed tooperate based on a call for water and vari-ous other system parameters, such as thequality of water, water flow rate, etc. ROunits usually start and stop based on stor-age tank levels. Controllers display inletand permeate conductivity, water tem-perature, ORP, pH, flows and operatingstatus of RO and EDI pretreatment. Con-trollers have an input option to automati-cally start/stop, based on product watertank levels or other relay input.

Upon shutdown and after 24-hoursof idle operation, the controller auto-matically flushes the membranes forseveral minutes using permeate water.This requires automatic valves that arededicated for membrane flushing. Thefast-flush feature common with commer-cial systems is not recommended in theindustrial design, as the inlet water con-tains the scaling and fouling contami-nants that are removed with permeateautoflush.

In the semi-automatic mode, PLCscontrol and monitor all automatic trainfunctions, while manual adjustment andmonitoring is necessary for manual sys-tem devices, such as valves and indica-tors. Bisulfite injection rate is controlledvia bisulfite pump speed, which in turnis controlled by a PID with ORP signalas the process variable; set point for theORP signal is determined in the field bytitration.

A membrane clean-in-place (CIP)system is necessary for on-site chemicalcleaning and sanitization and to preparethe RO membranes for long-term storage.RO CIP is a manual process of connect-ing the selected stage to the CIP system,mixing the proper chemicals in the CIPtank and allowing the solution to recir-culate through the selected stage. Manualvalves are provided at each stage inletand outlet and to isolate the previous andfollowing stages in order to clean eachstage individually.

Hoses usually connect CIP systemsto the RO skid, but hard piping providesfor convenient cleaning. CIP systemsinclude conical-bottom gallon, high-den-sity polyethylene tanks designed to holdthe necessary chemicals. Tanks shouldhave a low-level switch for pump and

heater protection. Some manufacturersdo not include a heater with the CIP sys-tem, but a heater provides much moreeffective cleaning, especially for biologi-cal, organic and silica fouling. The heateris sized to heat the tank water to 120°F(48.8°C) in four hours.

Industrial RO systems are designedto clean each stage individually. CIPpumps should be 316SS with TEFC mo-tor starters designed to deliver chemicalsto the selected RO stage. Flow metersmeasure the cleaning solution flow rate.A one-micron cartridge filter on the out-let piping from the feed pump protectsRO membranes from any debris that mayhave been collected during the clean upprocess. A pH probe and transmittermonitors the pH of the cleaning solution.The necessary manual valves and PVCpipe and fittings are pre-mounted, wiredand installed on a chemical-resistant,epoxy-coated steel skid.

References1. ASTM International. D 4189-95 (Reapproved2002). Standard Test Method for Silt Density In-dex (SDI) of Water.

2. Bates, Wayne. RO Water Chemistry.Hydranautics, Inc.

3. Dey, Avijit and Thomas, Gareth. ElectronicsGrade Water Preparation. 1st Edition 2003, ISBN0-927188-10-4.

4. FILMTEC Reverse Osmosis Membranes Tech-nical Manual. Dow Liquid Separations. Janu-ary 2004

5. Hydranautics Design Limits. Hydranautics,Inc. Jan 23, 2001.

About the author� Agape Water Solutions, Inc. PresidentJeff Tate specializes in the design and engi-neering of RO and EDI systems, filtrationand ion exchange. Previously, he was SalesManager for E-Cell Corporation (now partof GE Water) and President of Omexell, Inc.(now part of Dow Filmtec). Tate developedthe EDI market for North and South Americaand has engineered hundreds of RO and EDIsystems worldwide. He earned a BS Degreein mechanical engineering with high honorsfrom Drexel University and an MBA fromTemple University. He can be reached at:jtate@agapewater.com or www.agapewater.com.

About the company� Agape Water Solutions, Inc. manufacturescustom-engineered, membrane-based systemsfor water treatment companies and providesOEMs, dealers and distributors with compo-nents, systems and technical support. Thefirm is the Master Service Provider in NorthAmerica for Ionpure Technologies, a leadingmanufacturer of EDI modules.

determining pump size. The motorshould be total enclosed fan cooled(TEFC). Pumps should be operated by asoft-start motor starter, located in the con-trol panel or motor control center (MCC)for membrane protection.

RO pressure vessels are recom-mended to be FRP and should be nolonger than six membranes per vessel toallow optimal concentrate flow. Theyshould be designed to meet AmericanSociety of Mechanical Engineers (ASME)Boiler and Pressure Vessel Code, Section Xstandards. For an additional cost, vesselscan be inspected during fabrication by anASME authorized inspector and code-stamped.

High-pressure piping in the RO sys-tem should be 316LSS for high-corrosionresistance, fully fabricated and welded byqualified welders. All stainless steel fit-tings should be welded with threadedconnections wherever possible to preventleaks. Low-pressure piping is usuallyschedule 80 PVC. Piping should be hy-drostatically tested with deionized wa-ter, with quality level and pressure set bya quality control engineer. In all cases,tests must be done hydrostatically.

Automatic valves should be installedat RO inlets, permeate, permeate divert,flush inlets and flush outlets. Butterflyor diaphragm valves are common for lowpressure; 316SS ball valves are used forhigh pressure. Pneumatic actuators areeither spring-return fail-closed (product)or fail-open (divert), with two-positionlimit switches and 80-psi spring set-up.Pneumatic tubing should also be 316SS.

Instrumentation and controlsInstrumentation is a critical compo-

nent to the system’s successful operation,so only reliable manufacturers and in-struments that can be calibrated shouldbe considered. All instrumentationshould be monitored by the PLC. Instru-mentation requirements include manyelements.

Flow sensors are installed in perme-ate and concentrate lines. Stainless steel,liquid-filled pressure gauges are installedin the combined permeate outlet, the con-centrate outlet and at each stage. Pres-sure gauges at the inlet and outlet of eachstage allow the user to trend the pressuredrop across each stage, which assists withtroubleshooting and determining clean-ing requirements. Pressure switches areinstalled at the pump suction, pump out-let and across the cartridge filter. Con-ductivity probes are installed at thesystem inlet and permeate. ORP or freechlorine and pH probes are installed at

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