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24 August 2016 • Florida Water Resources Journal

The City of West Palm Beach (City)plans to upgrade its 47-mil-gal-per-day (mgd) conventional surface water

treatment plant (WTP) treatment train byadding a new ultraviolet (UV) disinfectiontreatment system. A simplified process flowdiagram of the existing system is provided inFigure 1. In 2008, the local health departmentrequired the City to improve the treatmentfacility to include low-pressure membranesas an additional barrier to pathogens. TheUV disinfection was accepted as a more eco-nomical alternative treatment process thatfulfills the regulatory intent to install a sec-ondary barrier, while providing additionalprotection from pathogens.

The UV treatment system is planneddownstream of the existing rapid gravity fil-ters but upstream of any post-chemical addi-

tions. As part of the project, an additionalpowdered activated carbon (PAC) treatmentsystem is also planned directly downstreamfrom the raw water pump stations. The newUV system will be designed with a validatedUV dose to achieve 4-log Cryptosporidium in-activation. This UV dose will also providesome level of inactivation of other pathogens,such as Giardia lamblia, E. coli, viruses, andbacteria. High organics, with a typical totalorganic carbon (TOC) of 11 mg/L, were ex-pected to produce lower UV transmittance(UVT), posing a challenge for UV treatment.The UVT, a major UV design criterion, was inactuality much lower at 79 percent than typi-cal applications. Fouling is a well-known phe-nomenon that was not understood for thiswater quality, although known UV foulants,calcium, and iron are added during treatment.

The City decided to perform a pilotstudy to proactively determine the extent offouling and to make informed decisionsabout the full-scale UV installation. As thefirst large drinking water UV installation inthe state, the Florida Department of Healthalso wanted a pilot study to ensure that foul-ing could be overcome during full-scale treat-ment.

The specific objectives of the UV pilotstudy were the following:1) Characterize fouling/scaling of quartz

sleeves surrounding UV low- andmedium-pressure lamps withsoftened/filtered water from the City’s ex-isting WTP.

2) Quantify UV quartz sleeve fouling andcleaning requirements.

3) Compare effectiveness of two differentautomated mechanical cleaning systemswith external chemical cleaning from twodifferent manufacturers.

4) Quantify bacterial inactivation effective-ness of the UV systems as part of theoverall water treatment process.

Testing Methodology

Pilot OverviewTwo UV reactors of different classes,

medium-pressure and low-pressure high-output (LPHO), were run in parallel over 12weeks of testing (Table 1 and Figure 2). TheUV transmittance through the sleeve wasmeasured spectrophotometrically to deter-mine fouling using UV intensity sensormeasurements, with lamps running at a con-stant power level.

Pioneering Ultraviolet Treatment of Potable Water From High-Organic

Surface Water in FloridaGabe Maul, GJ Schers, Poonam Kalkat, and Scott Kelly

Gabe Maul is an environmental engineerwith MWH in West Palm Beach. GJ Schers issenior water treatment technologist withCH2M in Fort Lauderdale. Poonam Kalkat isdirector of public utilities and Scott Kelly isassistant city administrator with City of WestPalm Beach.

F W R J

Figure 1. Simplified Existing Treatment Plant Process Flow Diagram

Table 1. Summary of Ultraviolet Reactors Piloted in Parallel

Florida Water Resources Journal • August 2016 25

Feed water for the UV reactors waspumped from the filtered water flume usinga self-priming centrifugal pump and splitinto two parallel UV reactors (Figure 3). TheUV reactor effluents were pumped back intothe main process at the filter influent chan-nel. The chlorine injection point used inphase 2 was located before the UV influentpump and the first sample point.

Flow rates were set to match the 4-logCryptosporidium dose in accordance with thefull-scale design and assuming 80 percentUVT. The LPHO unit was only validated usingthe older German DVGW standard to the 3-log inactivation dose, and was assumed to beconservative enough to be equivalent to 4-loginactivation of the newer Ultraviolet Disin-fection Guidance Manual from the U.S. Envi-ronmental Protection Agency (EPA). The flowrate was maintained and the maximum powerlevel was set throughout the pilot operation.The UV dose was allowed to drop as foulingand lamp aging occurred in order to use theUV intensity decline to measure fouling.

This mode of operation is contrary tofull-scale operation where lamp power wouldbe increased to compensate for changes infouling, lamp aging, and UVT to maintainthe required UV dose. The UV operation wasdivided into two phases: phase 1 withoutcleaning, and phase 2 without cleaning butwith free chlorine pretreatment (Table 2).Phase 2 was originally planned to includeregular cleaning at different time intervals,but was changed when fouling was discov-ered to be low.

Data Collection and Analytical MethodsThe UV sleeve fouling, water quality, UV

intensity readings, metal concentrations inacid used to clean fouled sleeves, and micro-bial activity before and after the UV reactorswere recorded during the pilot operation.Sleeve fouling was measured with offline UVtransmittance measurements using an Agi-lent Cary 60 UV-Vis spectrophotometer op-tics bench (Figure 3b). The three sleeves fromeach UV reactor were removed, rinsed withdeionized (DI) water and left to air dry, thenmeasured along with a spare reference sleeveof both types on the optics bench on a bi-weekly basis. A fouling factor (FF) was calcu-lated for each sleeve measurement using thefollowing equation:

FF = (Tmeasurement /Treference)0.5

Where Tmeasurement is the UV trans-mittance (percent) through the sleeve andTreference is the UV transmittance (percent)

(a) (b)(a) Low-Pressure High-Output and (b) Medium-Pressure

(a) (b)(a) Ultraviolet Pilot Setup and (b) Cary 60 Optics Bench for Measurement of the Ultraviolet Sleeve

Table 2. Schedule of Testing Phases

Figure 4. Ultraviolet Intensity Sensor Measurements for Low-Pressure High-Output ReactorContinued on page 26

Figure 2. Photos of Installed Ultraviolet Pilot Reactors

Figure 3. Pilot Testing Photos


through a spare reference sleeve as measuredon the optics bench.

Bacterial activity was tested at five dif-ferent locations at the water treatment plantusing three tests: 1. Colilert - total coliform and E. coli pres-

ence/absence2. MI Agar - colony counts of total coliform

and E. coli3. Heterotrophic plate counts (HPC) - colony

counts of total heterotrophic bacteria

At the end of phases 1 and 2, one sleevefrom each reactor from each UV system wasrinsed with 12 percent phosphoric acid andanalyzed for metals. The impact of PAC pre-treatment of raw water on the UVT of settledand filtered water was simulated using jar tests.

Results: Ultraviolet Intensity Sensor Measurements

The UV intensity sensor measurementsover the course of the UV pilot operation forthe LPHO reactor and medium-pressure(MP) reactor are provided in Figures 4 and 5,respectively. Both LPHO and MP reactor UVintensity decreased steadily during phase 1,and decreased at a greater rate during phase2. At the beginning of phase 2, there was asharp increase in UV intensity of both reac-tors. One possible explanation for the in-crease in UV intensity could be biofilmremoval of the pilot with the free chlorineclean and subsequent free chlorine residualthrough the reactors (biogrowth in the pumpand piping was suspected as detailed in thediscussion section under phase 1).

Results: Phase 1 Sleeve Fouling – No Cleaning

Average values of fouling factors of thesleeves from MP and LPHO reactors are sum-marized over phase 1 in Figure 6.

The average fouling factors of both sys-tems remained above 97 percent over 1000lamp hours during phase-1 operation, whichindicates low fouling. Lamp hours in thegraph have been adjusted by subtracting theburn-in period hours for each reactor of ap-proximately 85 hours. At the end of the burn-in period, all sleeves were to be cleanedaccording to vendor recommendation withLime-A-Way and denatured alcohol; however,the cleaning left a residue that was recordedas a drop in UV transmittance on the opticsbench as shown on the first data point. Thismethod of cleaning was discontinued after

Figure 5. Ultraviolet Intensity Sensor Measurements for Medium-Pressure Reactor

Figure 6. Ultraviolet Reactor Sleeve Fouling for Phase 1 – No Cleaning

Note: Error bars represent the standard deviation of the three sleeve fouling factors. After the measurement at540 lamp hours without cleaning, one sleeve from each reactor broke and was replaced. Therefore, the sleevemeasurements after 540 hours are shown as the average of the remaining two sleeves and do not show stan-dard deviations.

Figure 7. Ultraviolet Reactor Sleeve Fouling for Phase 2 – No Cleaning

Continued from page 25


the initial cleaning at the end of the burn-inperiod; instead, the sleeves were completelyreplaced after phase 1. Flow decreased slowly,day to day, on the LPHO unit, and valves wereadjusted twice to compensate, indicating apossible increase in headloss in the pipes.

Results: Phase 2 Sleeve Fouling – No Cleaning with Free Chlorine Pretreatment

Average values of fouling factors of thethree sleeves of MP and LPHO reactors aresummarized over the course of phase 2 inFigure 7.

The average fouling factor of the MP sys-tem remained above 98 percent after 540hours, while the average fouling factor of theLPHO system decreased to approximately 86percent after 540 hours without cleaning. TheLPHO sleeves were more fouled toward theend that was closer to the reactor inlet asmeasured using the optics bench.

Results: Fouling Characterization

Concentrations of some known foulantsin the feed water to the UV reactors were: • Iron ~220 µg/L • Total hardness 115 mg/L as calcium car-

bonate (CaCO3)• TOC 6.5 mg/L

In the acid rinsate of LPHO and MPsleeves, absolute concentrations were meas-ured at the end of phase 1 and phase 2, aswell as the blank, which are presented in Fig-ure 8. The exact absolute concentrations werenot important, but the relative magnitude ofconcentrations show that the acid rinsatemeasured from the sleeves was significantlygreater than the blank.

The major metal foulants measured inthe acid rinsate of fouled sleeves in phase 1were primarily iron (~40 percent) and zinc,followed by aluminum, and copper to a lesserextent (Figure 9). The major metal foulantmeasured in the acid rinsate of fouled sleevesin phase 2 was iron (~75 percent), and the re-mainder foulants were zinc and copper onthe MP sleeves, and calcium on the LPHOsleeves (Figure 10).

Results: Powdered Activated Carbon Jar Testing

The UVT of settled water after pretreat-ment with PAC ranging from 0 to 40 mg/L ispresented in Figure 11. It increased about 1

Figure 8. Concentrations of Metals in the Acid Rinsate

Note: The blank was measured in phase 2 only. The 6 percent phosphoric acid was used as acid rinsate in phase1, and 12 percent phosphoric acid was used in phase 2.

Continued on page 28

Figure 10. Foulant Characterization on the Sleeve Acid Rinsate – End of Phase 2

(a) MP (b) LPHO

(a) MP (b) LPHO

Figure 9. Foulant Characterization on the Sleeve Acid Rinsate – End of Phase 1


percent with 10 mg/L PAC and about 2.5 per-cent with 40 mg/L PAC on unfiltered water.

Results: Cleaning Method Comparison

The UV transmittance measurementstaken before and after the sleeves werecleaned are presented in Figure 12. The‘Fouled’ column shows fouling factors ofsleeves operated without cleaning for ap-proximately 1000 hours and 600 hours forphases 1 and 2, respectively. The mechanicalclean for the LPHO unit malfunctioned dur-ing phase 1, so only approximately one-thirdof the sleeve was cleaned. Fouling factors of‘fouled’ sleeves were similar between LPHOsleeves (e.g., T1 and T2) and similar betweenMP sleeves (e.g., C1 and C2).

Sleeve UV transmittance was improvedby acid rinsing between 0 and 10.7 percentand changed by mechanical cleaning by -0.6to 6.9 percent. In all cases, except the phase-1 MP mechanical clean, acid rinsing im-proved sleeve UV transmittance more thanmechanical cleaning. None of the cleaningsimproved sleeve UV transmittance to thesame UV transmittance of the referencesleeve (100 percent fouling factor).

Results: Microbiological Testing

The MI agar testing results at variousplaces in the WTP process train are graphedin Figures 13 and 14 for E. coli and total col-iform, respectively.

In general, the number of colony form-ing units (CFU) of total coliform and E. colidecreased as the water progressed through thetreatment process. E. coli was absent in allsamples measured after the UV reactor, andundefined (where a colony was not foundduring a test that tested <100 mL of sample)in all samples after the raw water. Total col-iform was present in measurable quantities inthe raw water, decreased in the flume sample,and then increased in the UV influent pumpsample. Total coliform was measured after theUV reactors at 3 CFU/100mL or less on threeseparate days at the end of phase 1 and the be-ginning of phase 2. The HPC results at thevarious sampling locations in the plant areprovided in Figure 15.

The HPC results from MP and LPHOUV reactor effluent water were significantlyless than the samples obtained from other lo-cations in the full-scale plant. The datashowed higher HPC in the UV influent pump

Figure 12. Comparison of Ultraviolet Sleeve Cleaning Methods

Figure 13. MI E. coli Test Results

Note: Undefined values varied depending upon the volume of the sample tested: 7/28: <10 CFU/100mL; 8/4- 8/25 : <5 CFU/100mL: 9/8 - 9/22: <2 CFU/100mL.

Figure 11. Settled Water Turbidity at Various Powdered Activated Carbon Pretreatment Doses

Continued on page 30



location than the flume during phase 1 and astep decrease in HPC at the UV influent afterchlorination began in phase 2 to below 1000CFU/mL. Log reduction over the UV reactorswas 1-log to 2-log of plate counts of bacteria,which are not the same as true log inactiva-tion. The HPC results were well within theEPA-recommended limit of 500 CFU/ml forpotable water. During microbiological testing,E. coli was not detected in any sample afterboth UV reactors, total coliform colonies wereless than 3 CFU/100 mL after the UV reactors,and HPC results showed significant reductionof total bacteria after both UV reactors.

Discussion of Results

Phase 1Sleeve fouling measurements declined

less than 3 percent over 1000 hours duringphase 1. This represents low fouling; for com-parison, the fouling factors in the HetchHetchy pilot study of MP systems decreasedto 80 percent in less than 100 hours (Kim etal, 2009). The UV intensity decreased by 9and 18 percent for LPHO and MP systems,respectively, in that period. Based on sleeveUV transmittance and UV intensity meas-urements, fouling is considered low duringPhase 1. Low fouling has positive implica-

tions on maintenance efforts and cost of thefull-scale UV plant.

Biogrowth during phase 1 was suspecteddue to several indicators:S Increase in TOC and color a week after

Phase 1 began.S Increase in headloss to the LPHO unit

characterized by decreasing flow and re-quired adjustments of valves.

S Observed odor when pump and upstreampiping was opened.

S Step increase in UV intensity after initialchlorination.

S Greater bacterial presence via MI total co-liform and HPC in the UV influent pumpthan in the filtered water flume.

S On the second day of operation withprechlorination, total coliform up to 3CFU/mL were detected in the LPHO andMP UV reactors, even though total col-iform was not detected in the UV influentpump. Possible biogrowth in piping couldbe the source for total coliform hits.

Phase 2In Phase 2, with free chlorine addition,

sleeve fouling increased greatly for the LPHOunit, but not for the MP unit. Fouling char-acterization showed that the increased foul-ing was ~75 percent iron, and ~ 20 percentcalcium. The increased oxidation potential ofchlorine may have oxidized Fe2+ to Fe3+,which could have contributed to increasedfouling. Surprisingly, sleeve fouling measure-ments showed greater fouling in the LPHOsystem, but UV intensity showed a greaterrate of decline in the MP system. It appearsthat temperature was not the strongest indi-cator of fouling as previously hypothesizedbecause MP fouling would have been consis-tently greater. The difference in velocity mayhave led to the different results betweensleeve measurements and UV intensity read-ings. The UVT did not correlate with anyother water quality parameter tested (datanow shown because of space constraints).

Conclusions

A UV disinfection pilot study was con-ducted to evaluate preliminary operationaldata on a small-scale UV unit in order to ver-ify the effectiveness of UV disinfection andto make decisions about the full-scale UV in-stallation. Two UV reactors of differentclasses, one medium-pressure and one low-pressure high-output, were run in parallelover 12 weeks of testing. The UV sleeve foul-ing, water quality of the feed water, UV in-tensity readings, and microbial activityFigure 15. Heterotrophic Plate Count Results

Figure 14. MI Total Coliform Test Results

Note: The UV Influent samples on 8/25 and 9/8 were undefined because they were tested using less than100mL. Therefore, the results of these tests were <5 and <2 CFU/100mL, respectively.



before and after the UV reactors were meas-ured. The major findings of the study were:S The fouling rate of filtered water without

online cleaning was low as measured bysleeve fouling (<3 percent fouling factordecline) and UV intensity sensor readings(percent decline of 9 and 18 percent forLPHO and MP, respectively) over 1000hours, even with biogrowth present up-stream (phase 1).

S The fouling rate of filtered water withoutonline cleaning was increased by free chlo-rine pretreatment as measured by sleevefouling (fouling factor decline of 16 per-cent and 4 percent for LPHO and MP, re-spectively) and UV intensity sensorreadings (percent decline of 18 and 49percent for LPHO and MP, respectively)over ~600 hours (phase 2).

S The major metal foulants were primarilyiron, and to a lesser extent, copper, zinc,and aluminum during phase 1, primarilyiron during phase 2, and to a lesser extentcalcium (LPHO) or zinc and copper (MP).Relative to the overall foulant amount, thepercentage of iron and calcium increasedduring phase 2 when a prechlorine dosewas applied to the reactors.

S Offline acid cleaning improved sleeve UVtransmittance more than online mechani-cal cleaning, but up to 3 percent of thefouling factor was irreversible. Periodiconline cleaning is suggested to proactivelyprevent irreversible fouling.

S The UV reactors were effective in inacti-vating a significant portion of bacteria, asshown in the total coliform, E. coli, andHPC tests. The HPC results were wellwithin the EPA-recommended limits of500 CFU/mL for potable water.

S In jar tests, PAC addition prior to coagu-lation improved the settled water UVT.The UVT increased about 1 percent with10 mg/L PAC and about 2.5 percent with40 mg/L PAC.

S For full-scale, low fouling is expected foreither MP- or LPHO-type UV reactors,and both types provided adequate clean-ing solutions without need for additionalpretreatment. Prechlorination is expectedto increase fouling rates significantly ifused in full-scale. SS

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