ImprovingOperationalEfficiency ... FWRJ_tech2.pdf · ments were replaced with DOW Filmtec ... manual mode more often than automatic modebecauseoftheseinsufficiencies.When...

Constructed in the early 1980s, IndianRiver County Utilities’ Oslo RoadReverse Osmosis (RO) Water

Treatment Plant in Vero Beach is more than20 years old. At this age, improvements andreplacement of some key components werenot only necessary to maintain plant opera-tions and meet changing regulations, butwere also needed to improve the facility’soperating efficiency. This article evaluatessome of the key improvements to an agingmembrane treatment plant rated at 8.57 mil-lion gallons per day (MGD). Improvementsincluded:� Replacing aging fiberglass pipe with

HDPE and stainless steel pipe.� Replacing cartridge filter the vessel type

to improve pretreatment system.� Replacing aging membranes and improv-

ing treatment efficiency and operatingcosts.

� Installing an improved post-treatmentsystem.

� Upgrading high-service pumping systemto improve operations and operatingcosts.

� SCADA system improvements.� Upgrading electrical gear no longer avail-

able.Upgrades to the plant, including replace-

ment and rehabilitation of key equipment,have been ongoing for several years. This

article highlights some of the real-life experi-ences of maintaining plant operations whileimplementing these improvements and theeffects on treatment plant efficiency andoperations.

Replacing Fiberglass Piping

Raw water piping for many of the olderand larger RO plants was typically construct-ed of fiberglass (FRP) to minimize corrosion.Larger diameter piping for undergroundinstallations such as PVC typically was notavailable many years ago and could be con-structed more economically using FRP, com-pared to coated ductile iron piping.

Depending on the chloride levels of theraw water, FRP was the preferred pipe mate-rial many years ago when larger capacity ROplants were being constructed, but the brit-tleness of FRP and the inexperience of con-tractors with installing FRP piping systemsled to failures of the pipe, resulting in fre-quent shutdowns of treatment facilities.Many of these FRP piping systems weretherefore unreliable and warranted replace-ment with different materials, such as high-density polyethylene (HDPE) and stainlesssteel.

All the larger diameter underground rawwater and permeate water piping at the OsloRoad plant were originally constructed of

FRP. The trench piping within the processbuilding was also constructed of FRP, includ-ing the feedwater, permeate, and concentratepiping. Frequent failures of both piping sys-tems resulted in the decision by Indian RiverCounty Utilities to replace the FRP pipe withHDPE pipe. The difficulty in replacing thispipe came from the requirement that theplant stay fully operational with minimalshutdowns.

The facility operated at nearly 75 percentof its full capacity and is one of two regionaltreatment facilities providing potable water tothe system. The plant also had limited storagecapacity, which allowed no more than eight-

Improving Operational Efficiencyat an Aging RO Water Treatment Plantthrough Replacement & Rehabilitation

Jerry LeBeau and Mark D. Miller

Jerry LeBeau is an operations specialist inthe Maitland office of CDM, a consulting,engineering, construction, and operationsfirm. At the time this article was written, hewas water production superintendent atIndian River County Utilities. Mark. D.Miller, P.E., is a vice president with Port St.Lucie office of the engineering consultingfirm Kimley-Horn and Associates. This arti-cle was presented as a technical paper atthe Florida Section AWWA Fall Conferencein November 2007.

Figure 1. Removal of FRP Piping Figure 2. Temporary Cartridge Filter Header

20 • OCTOBER 2008 • FLORIDA WATER RESOURCES JOURNAL

hour shutdowns of treatment. The existingstorage capacity of 4 million gallons with only3 million gallons of useful storage, comparedto the plant rated capacity of 8.57MGD, bare-ly met Florida Department of EnvironmentalProtection requirements of 25 percent of thesystem’s maximum-day water demand, socritical piping tie-ins not only had to be lim-ited, but had to be carefully planned inadvance.

Temporary piping headers were con-structed to allow raw water to enter the feed-water pumps, while allowing removal of FRPpipe sections (see Figure 1 and Figure 2). Anadditional chemical injection assembly withstainless steel static mixer was constructed toallow removal of the existing FRP staticmixer assembly. Full sections of pipe that hadto be replaced in identical locations werephased in sections to minimize plant shut-down durations.

All of the underground FRP wasreplaced with HDPE pipe and the interiorprocess piping with 316 stainless steel (seeFigure 3). Grooved type couplings (Victaulicstyle) were used to help in expediting pipeinstallation, since they were much easier andquicker to install and allowed some flexibilityin pipe alignment.

Cartridge Filtration VesselReplacement

The original pretreatment filters werevertical-style cartridge filter vessels (seeFigure 4). The cartridges were not only diffi-cult to replace, since a ladder typically wasrequired to allow easier cartridge removal,but historically allowed sand or filtered mate-rial to settle in the discharge pipe during car-tridge element replacement.

The plant had a history of sand intrusionfrom a collapsed well to the membrane sys-tem.With the added disadvantage of vertical-style filter vessels allowing some of the settledsand within the vessels to migrate to themembrane system, sand intrusion to themembrane system was difficult to prevent.

In order to prevent any potential of sandintrusion, a “belt and suspenders” approachwas implemented by the utility. A sand sepa-rator was installed on the raw water main andthe vertical cartridge filter vessels werereplaced with horizontal vessels, allowingeasier access for cartridge replacement and amore efficient means of cartridge removal.

Membrane Replacement& RO Train

Operational Improvements

The existing membrane system was lim-ited in capacity because of membrane dam-age from scaling, fouling, and sand intrusion.The rated treatment capacity was limited to 5MGD, nearly 20 percent less than the designrated capacity of 6.0 MGD. Several othermechanical features limited the capacity,including leaking valves, lack of first-stagepermeate control valves, control philosophy,and reduced feedwater pump capacity.

Most of the mechanical issues were aresult of older technology and the age of theequipment. Some of the operational issuesthat needed to be changed were also philo-sophical, such as how control valves wereoperated.

Physical damage to the membranes,meant that replacing them was not only acritical element in restoring capacity, butimportant in improving the operating effi-ciency of the plant. Power consumption,chemicals, and finish water quality were all

directly affected by poor membrane perform-ance.

Membranes could not be replaced, how-ever, without correcting the other mechanicaland operational elements which allow themto maintain consistent performance. Severalof these include:

1st-Stage Permeate Control Valve

With newer technology membranes,which typically have higher flux capacities,the membrane system first stage typically hasthrottling valves that control the amount offlux in the first stage to prevent scaling poten-tial in the tailing elements. The membranesystem at the Oslo Road facility did not havethrottling valves.

Permeate header isolation valves hadbeen used for this purpose, but they providedvery little control, since they were butterflyvalves and there was no means of measuringtotal flow from each stage. V-port ball valveswere installed on the first-stage permeateheader of each of the RO trains in order tocontrol flux within the first-stage membranearray.

Interstage Cleaning System Valves

The cleaning system valves that wereconnected to the interstage piping werewafer-style butterfly valves, which are only asgood as the flanges they are sandwichedbetween. When additional membrane systemcapacity was desperately needed, the systemwas operated at elevated pressures, whichcaused leakage to increase through the inter-stage valves.

The interstage piping is constructed ofstainless steel, and the cleaning system pip-

Figure 3. New Stainless Steel Feedwater Pump Header Figure 4. Old Filter Vessels

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FLORIDA WATER RESOURCES JOURNAL • OCTOBER 2008 • 21

ing was constructed of PVC and Van Stonestyle PVC flanges. Although the PVC flangesare rated for 150 psi, it is difficult to preventleaking between these valves and the flangeswhen the system operates up to 130+ psi.The PVC flanges typically become over-stressed and crack when flange bolts aretightened where stainless steel flanges areinstalled on opposite sides of the butterflyvalves. These valves were replaced with lug-style butterfly valves, which allowed theflange bolts to be tightened to each of theirrespective pressure ratings.

Miscellaneous Improvements

Several operational and mechanicalmodifications improved the operating effi-ciency of the RO system. The feedwaterpumps were upsized with larger impellers tomaximize the use of the existing 150-horse-power motors. Some of the feedwater pumpimpellers were also worn and inefficient, soreplacement not only improved capacity, butincreased the design capacity of the entirefeedwater pump system.

The concentrate control valve operationwas originally designed with valve actuatorsthat failed closed upon lack of signal and airsupply and were forced closed during ROtrain shutdown. This contributed to a mem-brane scaling event in which feedwater valveswere not fully closed and created a conditionin which 100 percent recovery occurred. Theactuators were changed to operate fail openand remain open when the RO train is not inoperation.

Membrane Replacement

The membranes were not only scaledfrom a previous scaling event, but were fouledfrom sulfur fouling and had sand intrusion.These factors contributed to poor membraneperformance with elevated feed pressures andlower rejection. Feed pressures were elevatedby more than 40 psi in some cases and mem-brane rejection was non-existent in others.

Fouling occurred because of air enteringthe system through well pump intakes duringlow water levels and because of air releasevalves that allowed air to enter the well pumpcolumns during pump shutdown. Sandentered the system through the old vertical-style cartridge filter vessels and at one timewhen a well had collapsed. Most of theseoperational issues have been addressed toprevent future membrane damage.

The approach to membrane replacementincluded:� Select the worst performing RO train with

the least capacity and poorest water quali-

ty and replace those membranes first.� Install profile valves to determine if some ofthe existing membranes were salvageable.

� Order new membranes once the numberwas defined and schedule delivery withreplacement.

� Remove existing membranes, keep sal-vageable ones, and perform correctivework on RO trains.

� Flush membranes for 24 hours, collectbact’s, and place into operation.The original membrane softening ele-

ments were replaced with DOW FilmtecNF90-400. Membrane projections were per-formed for the proposed NF90-400 elementsand predicted initial feed pressures below 90psi and permeate water quality of 130 mg/ltotal dissolved solids.

Once the membranes were replaced forthe first RO train, the remaining trains werereplaced one at a time to minimize impact tothe plant’s capacity. Table 1 illustrates theresults of the membrane replacement.

The feed pressure, delta p, permeate, andconcentrate flows were restored to optimallevels consistent with the membrane projec-tions. For instance, the average feed pressurefor Train 4 dropped significantly after mem-brane replacement from 115 psi to 80 psi,respectively. Besides the feed pressure signifi-cantly decreasing after replacement, the Stage1 pressure drop had a similar trend, experi-encing a reduction from 40 psi to 14 psi.

Train Capacity Increase to1.8 mgd (1.5 mgd Design)

The existing RO train arrays were some-what oversized and on average operated at aconservative flux rate of 13 gallons per square

foot per day (GFD). Higher average flux ratesup to 15 GFD could be achieved safely withconsistent raw water quality and careful con-trol of the first-stage element flux. Operatingat higher flux rate could also be achievedwhen one of the RO trains was down withoutjeopardizing overall treatment plant capacity.It is also another means of providing standbycapacity and improving redundancy of thetreatment system. In order to confirm theability of the membrane system to operate ata higher flux rate, RO train 4 was operated atan elevated capacity of 1.8 MGD.

Three membrane projections were per-formed that included having each train pro-duce 1.6, 1.7, and 1.8 MGD. The average sys-tem flux was 15.64 GFD, resulting in a leadelement flux rate of 22.4 GFD, which isacceptable and within flux rates that havebeen pilot tested before. Typical elevated fluxrates have been tested up to 27 GFD formembranes of this type.

One important operational condition ofoperating at higher flux rates is to insure thatthe first-stage elements don’t over flux.Installation of the first-stage permeate con-trol valve allowed control of first-stage per-meate flow to prevent over fluxing the leadmembranes.

AlarmsThe original alarms were also outdated

and did not adequately protect the mem-brane system. None of the alarms triggered atrain shutdown. Prior to membrane replace-ment, the RO train operating logic wasstreamlined and shutdown alarms wereadded for key operating parameters, whichincluded high feed pressure, high first-stagepermeate flow, and low concentrate flow.These parameters have appropriate setpointsthat will trigger alarm or shutdown, with agiven time delay, depending on the parame-ter type.

Normalized Trends & Historical DataA custom normalizing spreadsheet was

prepared and installed on the SCADA sys-tem, which automatically collects raw datafrom each of the RO train field devices dur-ing each operator shift (eight hours) andstores the data within the spreadsheet toprovide historical and real-time trending ofeach RO train.

With all of these improvements to theRO trains, the reduction in operating pres-sure saved Indian River County approximate-ly $5,000 per month in electrical costs. Notonly did operating costs decrease, but waterquality improved significantly, allowing theplant to blend more raw water with the per-meate stream.

Figure 5. Typical RO Train Array

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Improvements to thePost-treatment System

The post-treatment system consisted oftwo forced-draft degasifiers which removeddissolved hydrogen sulfide and carbon diox-ide gas from the permeate stream. Thedegasifiers and blowers were elevated abovethe ground storage tanks, allowing the treat-ed water to flow by gravity into the center ofthe tanks. Although the only advantage ofthis installation was to eliminate repumpingthe treated product water, several disadvan-tages included:� Maintenance of the system was more dif-

ficult and routine inspection of the equip-ment was less frequent.

� The water quality of the effluent, such aspH and chlorine residual, was difficult tomonitor because these parameters couldbe measured only at the POE, giving littletime to react and make changes to thetreatment process.

� No odor control system existed, whichcaused heavy corrosion of the equipmentsurrounding the degasifiers and odorcomplaints from neighboring properties.Since the degasifiers had reached the

end of their useful life, a new post-treatmentsystem was installed to replace them. Thesystem included a clearwell, transfer pumps,forced-draft degasifiers, two-stage off-gasscrubber system, chemical injection, andmonitoring. Key advantages to the new sys-tem are the ability to maintain consistenttreatment of the product water, the ability toimprove mixing and blending of raw water,and the ability to improve monitoring of the

treated water prior to transferring it to theground storage tanks.

High-Service Pump UpgradesThe high-service pumps were originally

designed for a 7.5-MGD plant rated capacity.With the addition of elevated storage withinthe system and an uprating of the treatmentcapacity to 8.57 MGD, no increase in high-service pumping capacity was implemented atthat time. It was assumed that the elevated stor-age wouldmeet peak flows and the north waterplant would provide the necessary peak pump-ing requirements. With the new August 2003amendments to Chapter 62-555 permittingrequirements for public water systems and anincrease in water system demands, upgrades tothe high-service pumps were necessary.

With the upgrades, three new 3,200-gal-lons-per-minute pumps were designed,which provide 9.2 MGD of pumping capaci-ty, slightly more than the plant rated capacitywith the largest pump out of service. Thehigher-capacity pumps were also more effi-cient at the design operating point. Largerimpellers were selected for the same modelpump, which also provided flexibility in pro-viding a shelf spare pump.

Due to normal plant operation at nearly75 percent of capacity, replacement of thehigh-service pumps had to be phased one at atime in order to maintain adequate pumpingcapacity during replacement.

SCADA& ElectricalSystem Improvements

The programmable logic controller(PLC) and supervisory computer and dataacquisition (SCADA) system were antiquated

and in need of optimization. Several deficien-cies included: poor documentation withinthe ladder logic programming software, out-dated drivers for the PLC hardware, alarmsthat were disabled because instruments wereuncalibrated, loops that were disabled orinoperable, and outdated HMI software.

In general, the plant was operated inmanual mode more often than automaticmode because of these insufficiencies. Wheninstrumentation was consistently out of cali-bration or was inoperable, it was much easierto operate in manual mode or force valueswithin the control logic to maintain opera-tion of the plant.

A significant portion of the SCADA sys-tem optimization could be implemented onlyonce an instrumentation technician wassecured and calibration of the majority ofequipment was complete. Also, calibration ofsome of the instruments uncovered moreissues than were initially observed. Given theage of some of the equipment and the poorcondition of some of the valve actuators,instruments, equipment, etc., the resultingfailures prevented staff from being moreproactive in fixing problems, since much oftheir time was used in reacting to failures andproblems. This perpetuated operating thefacility in manual mode, rather than the pre-ferred automatic mode.

Several of the key benefits of optimizingthe control system recently include:� Improved automation of the system

allows more time to focus on complianceissues and enables staff to be proactive inmaintaining equipment.

� Optimizing alarms has renewed staff ’sconfidence in significant alarms andawareness of critical alarms.

Figure 6. Existing Degasifier Figure 7. New Post-treatment System

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FLORIDA WATER RESOURCES JOURNAL • OCTOBER 2008 • 25

� Implemented contract services for a part-time SCADA system integrator andinstrument technician has helped main-tain confidence in operating the systemin automatic mode.

� Normalized data from the RO systemindicates current performance is consis-tent and stable.

� There is less wasted water during shut-downs and startups.

� There are reduced chemical demandswith optimized chemical feed loops.Some of the electrical gear and motor

control centers were also in need of replace-ment, since they were more than 25 yearsold. There were no replacement componentsavailable for this equipment, and corrosionof some of the bus bars was severe enough towarrant replacement due to the potential forsudden failure. All of the high-servicepumps were fed off these components, sothis was a critical item that needed immedi-ate attention.

The approach was to develop a phasingplan that allowed for the demolition andreplacement of the motor control centers tooccur within a single 48-hour (maximum)shutdown. The plan also included transition-ing the high-service pumps from the existingstarters to the proposed variable frequencydrives with no loss of pumping capacity.

Summary

Given the age of the Oslo Road plant,taking advantage of a replacement and reha-bilitation program is an effective vehicle toimprove operations and the efficiency of thetreatment process. With improvements intechnology, stricter regulations, and operat-ing efficiency becoming more important anddrawing more attention, implementating sev-eral key plant improvements can result inmultiple benefits. ����

www.fwrj.com

BENEF ITImproved reliability, prevents intrusion of sand from pipe failures

Easier access for element replacement and prevents sand intrusion

Lower operating costs and improved water quality

Easier maintenance, improved water quality, reduced corrosion and odor complaints

Increase capacity and improve operating costs

Improve automation, reduce staff effort, less wasted water, and reduced chemicaldemands

IIMMPPRROOVVEEMMEENNTT

• Replacement of FRP Piping

• Cartridge Filter Vessel Replacement

• Membrane Replacement

• Improved Post-treatment System

• Upgrade High Service Pumps

• SCADA and Electrical SystemImprovements