Bacterial Growth in DistributionSystems: Effect of AssimilableOrganic Carbon and BiodegradableDissolved Organic CarbonI S A B E L C . E S C O B A R , *A N D R E W A . R A N D A L L , A N DJ A M E S S . T A Y L O R

Civil and Environmental Engineering Department,University of Central Florida, P.O. Box 162450,Orlando, Florida 32816-2450

Two distribution systems, one treating water by ozonationand another treating water by nanofiltration in parallelwith lime softening, were monitored for bacterial growth.Both systems kept disinfectant residuals such as chlorineand chloramine in their respective distribution systems.Bacterial growth was assessed by heterotrophic plate counts(HPC) on R2A agar. In the distribution systems fed byozonated water, HPCs were correlated (R2 ) 0.96) usingan exponential model with the assimilable organic carbon(AOC) at each sampling site. Also, it was observed thatozonation caused a significant increase in the AOCconcentration of the distribution system (over 100%increase) as well as a significant increase in the bacterialcounts of the distribution system (average increase over100%). The HPCs from the distribution systems fed bynanofiltration in parallel with lime-softening water alsodisplayed an exponential correlation (R2 ) 0.73) with anexponential model based on AOC. No significant correlationwas found between bacteria growth on R2A agar andBDOC concentrations. Therefore, in agreement with previouswork, bacterial growth in the distribution systems wasfound to correlate with AOC concentrations.

IntroductionOrganic matter impacts the water quality of distributionsystems by generating color, undesired taste, and odors.When a chlorine residual is provided for disinfection, organiccompounds are responsible for high chlorine demands andthe formation of disinfection byproducts (DBPs). Also, thebiodegradable organic matter (BOM) that is not removedduring water treatment can potentially lead to the prolifera-tion of bacteria along the distribution system, which dete-riorates the water quality, accelerates corrosion rates of pipes,and can potentially increase the incident of bacteriologicaldiseases.

Since the promulgation of the THM Rule in 1979 and theSurface Water Treatment Rule (SWTR) in 1989, it has beenthe objective of water treatment to balance microbial removalwhile minimizing the formation of DBPs. One method toachieve this objective is to disinfect water with disinfectantsother than free chlorine, such as ozone, which do not produce

DBPs and also to decrease the chlorine residual concentrationin the distribution system. Widely utilized in Europe, ozoneoxidation is a common process in water treatment for theremoval of water pollutants, odors, color, and tastes; and itexhibits strong disinfectant properties. However, even whenlowering overall organic carbon levels, ozonation is knownto convert part of these large natural organic molecules intosmaller, more biodegradable organic molecules commonlyquantified as assimilable organic carbon (AOC) or biode-gradable dissolved organic carbon (BDOC) concentrations(1-5). These serve as “food” sources for heterotrophicbacteria present in the water and pipes throughout thedistribution system, and this can result in bacterial regrowth.

A treatment process that removes a wide majority ofpathogens, water pollutants, and organic carbon is nano-filtration, which is a type of membrane filtration. Whennanofiltration is used in the treatment of water, the subse-quent addition of free chlorine for residual is less likely toresult in THMs due to the very significant reduction in organiccarbon, one of the precursors of THMs. Some types ofmembranes may remove a large portion of the BOM presentin the water, which may translate into a correspondingreduction in AOC and BDOC concentrations in the finishedwater entering the distribution system. A significant fractionof the dissolved organic carbon (DOC) removed is also BOM,but since BDOC and AOC can still have significant impactson biostability at ppm or ppb levels, respectively, there is aneed to study the impacts of membrane treatment on theseparameters.

The impacts of ozonation and membrane filtration onAOC and BDOC concentrations in finished waters and thecorresponding potential impact it has on bacterial regrowthin distribution systems have been investigated in severalbench- and pilot-scale studies. These investigations wereconducted under controlled conditions. However, little full-scale data are available to quantify changes in the AOC andBDOC concentrations under dynamic field conditions, whichare potentially significantly different from bench- and pilot-scale conditions. Therefore, this project had the objectivesof conducting complete water quality and biological moni-toring of the process and distribution systems treated viaozonation or nanofiltration.

BackgroundThe water industry uses “regrowth” and “aftergrowth”synonymously to describe the unexplained occurrence ofcoliforms in the distribution system. However, more specificdefinitions of these terms have been provided (6). Regrowthis the recovery of disinfectant-injured cells that have enteredthe distribution system from the water source or treatmentplant, while aftergrowth is the growth of microorganismsnative to a water distribution system. Breakthrough is theincrease in bacterial numbers in the distribution systemresulting from viable bacteria passing through the disinfectionprocess, and growth is the increase in viable bacterialnumbers in the distribution system resulting from bacterialgrowth downstream of the disinfection process (7). Mech-anisms responsible for the presence of coliform bacteria indrinking water supplies are often unknown or poorlyunderstood, so the term “episode” or “occurrence” is moreappropriate to describe the presence of coliform bacteria(8). Increased counts of chemoheterotrophic bacteria colo-nies in distribution system water are the result of bacterialcontamination from outside the distribution system ormultiplication of bacteria within the system or regrowth (9).The major concern involving regrowth is the multiplication

* Corresponding author’s present address: Chemical and Envi-ronmental Engineering Department, University of Toledo, 3055Nitschke HallsMail Stop 305, Toledo, OH 43606-3390; phone: (419)-530-8267; fax: (419)530-8086; e-mail: Isabel.Escobar@utoledo.edu.

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of potentially pathogenic or opportunistically pathogenicbacteria.

Organic compounds, either dissolved or particulate (whichcan be hydrolyzed to a soluble form), provide energy sourcesfor heterotrophic bacteria, which use organic carbon for theproduction of new cellular materials or as energy. As a result,BOM is gradually consumed as the water travels along atypical distribution system. The potential consequences ofthis are bacterial proliferation, degradation of the distributionwater quality, acceleration of pipe corrosion, and otherundesirable effects. Bacterial regrowth is possible even withminute amounts of BOM entering the distribution system(some bacteria, such as Pseudomonas, can even grow indistilled water lines). The concentration and nature ofcompounds promoting growth of bacteria (i.e., electrondonors, acceptors, and nutrient sources) are generallyconsidered to be the major growth-limiting factors andprovide a potential means of control of aftergrowth andregrowth.

AOC. AOC refers to a fraction of the total organic carbon(TOC) that can be utilized by specific strains of definedmixtures of bacteria, resulting in an increase in biomassconcentration that is quantified. AOC typically comprisesjust a small fraction (0.1-9.0%) of the TOC (10). AOCrepresents the most readily degradable fraction of BDOC/BOM. The inoculum for the AOC bioassay can be composedof a mixture of pure bacterial strains cultivated in laboratoryconditions (Pseudomonas fluorescens P17 and SpirillumNOX), mixtures of environmental bacteria (11) characterizedby a great nutritional versatility, or mixtures of bacteria thatutilize groups of specific compounds (9). Bacterial growth ismonitored in the water samples by colony counts, and theaverage growth (Navg) observed during the incubation isconverted into AOC units (as µg/L as acetate-carbon). Thisis done by using the growth yield of the bacteria derivedfrom calibration curves obtained using standard concentra-tions of organic compounds (e.g., acetate or oxalate). Asignificant correlation exists between the AOC concentrationand the density of heterotrophic bacteria in distribution watersupplies (9, 12). Van der Kooij (9) showed that heterotrophicbacteria in a nonchlorinated system did not increase whenAOC was lower than 10 µg/L, while LeChevallier et al. (12)suggested that regrowth may be limited by AOC levels lessthan 50-100 µg/L in systems maintaining a chlorine residual.

BDOC. The BDOC content represents the fraction of DOCthat has been assimilated and/or mineralized by a het-erotrophic flora (13). The inoculum for the test consists ofenvironmental bacteria suspended or alternately fixed on asupport, such as sand or porous beads. BDOC is the differencebetween the initial DOC of the water sample and theminimum DOC observed during the incubation period of 28d for suspended indigenous bacteria or 5-7 d for bacteriaattached to sand (14). Joret et al. (15) suggested that BDOCvalues represent 10-30% of the total DOC content of drinkingwater. An absence of biodegradable organics after watertreatment to limit bacterial regrowth has been recommendedin the literature (10, 16). Servais et al. (17) have associatedbiological stability, which corresponds to no BDOC con-sumption within the distribution system, with a BDOCconcentration of 0.16 mg/L or less in the finished water.More recently, Volk et al. (14) determined values of 0.15 mg/Lat 20 °C and 0.30 mg/L at 15 °C for achieving biologicalstability in distribution systems of Paris suburbs. Finally,coliform occurrences were related to the existence of BDOCcontent greater than 0.10-0.15 mg/L (14).

Experimental SectionOzone WTP. The Orlando Utilities Commission (OUC) PineHills Plant initially treated high quality Floridan aquifer waterfor the oxidation of hydrogen sulfide gas, corrosion control,

and disinfection. The treatment did not involve aeration sincethe source water has relatively low sulfide levels (average0.44 mg/L as total sulfide). Until May 1998, the treatment forhydrogen sulfide consisted solely of chlorination; in addition,caustic (pH control) fluoride and hydroflorosilicic acid wereadded to the process stream. Raw water from five wells waspumped to a common raw water main, which passed througha chemical feed vault where the raw water was chlorinated(2.5 mg/L Cl2) to oxidize the hydrogen sulfide gas to elementalsulfur. Fluoride was also added in the chemical feed vault.The prechlorinated water then flowed through two waterstorage reservoirs in series where the precipitated sulfur couldsettle and collect. Water from the second water storagereservoir then flowed through a second chemical vault whereadditional chlorine (2.0 mg/L Cl2) was added to provide theresidual required in the water distribution system. Sodiumhydroxide was also added in the chemical feed vault to raisethe pH to approximately 8.5 for corrosion control.

After May 1998, the raw water was diverted to two baffled,multi-zone, contact tanks where ozone was diffused into thefirst zone to oxidize the hydrogen sulfide gas to sulfate. Theozone contact tanks were sized to provide 10 min of totaldetention time at the maximum daily flow. Air was diffusedinto the water near the outlet end of the contact tanks toraise dissolved oxygen concentration close to the saturationlimit and to assist in stripping any remaining ozone. Thecontact tanks were closed and operated at a slightly negativepressure. The ozone-treated water then flowed through achemical feed vault where chlorine (2.0 mg/L) was added.The ozone dose averaged 4.7 mg/L during the study. At thishigh ozone dose, along with the lack of biological filtrationafter ozonation, a significant increase in the AOC levelsentering the distribution system was expected. Figure 1 is aschematic diagram of the ozone oxidation process at PineHills. As shown, ozone was introduced into raw water forsulfide oxidation followed by chlorination for residualdisinfection. Sampling points were located in the part of thedistribution system that contained water from just the PineHills plant. The plant was on line at a capacity of 9.4 × 104

m3/d (25 MGD) and averaged 6.9 × 104 m3/d (18.3 MGD)during the study.

No difference was observed between the raw watertemperature before (24.9 ( 0.74 °C) and after (24.8 ( 1.08°C) the introduction of ozonation as a treatment process. Ingeneral, the water temperature in the plant effluent variedfrom a minimum of 21 °C to a maximum of 27 °C (i.e., averageof 25 ( 1.5 °C before and 26 ( 0.7 °C after the introductionof ozonation). Finally, the distribution system water tem-perature, which averaged 25 ( 2.6 and 25 ( 2.3 °C before andafter the introduction of ozonation as a treatment process.Therefore, the AOC and BDOC incubation temperatures (25and 22-25 °C, respectively) were indicative of distributionsystem temperatures.

Sample collection was performed by following the guide-line outlined in Standard Methods 9060A (18). In additionto the plant effluent, there were five distribution systemssampling points: two with average hydraulic residence times(4 and 6.5 h) and three with maximum hydraulic residence

FIGURE 1. OUC Pine Hills process diagram for ozone oxidation.

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times (10.5, 18.5, and 27 h). A total of 12 monthly samplingcampaigns were conducted during which temperature, pH,turbidity, and chlorine residual were measured at the timeof sampling. Total coliforms, HPCs, UV-254, DOC, BDOC,and AOC were measured within 24 h of sampling at theUniversity of Central Florida.

Nanofiltration and Lime-Softening WTP. The raw waterto WTP 3, operated by Palm Beach County Water UtilitiesDepartment (PBCWUD), was from the Biscayne Aquifer, ashallow, surficial aquifer, with significant surface influenceand some saltwater intrusion. WTP 3 divided the raw waterinto two streams (one treated via lime softening and onetreated via nanofiltration) and combined the two effluentsjust prior to the distribution system. WTP 3 with an ultimateprojected design capacity of 36 MGD and current designcapacity of 6.0 × 104 m3/d (15.8 MGD) averaged 4.4 × 104

m3/d (11.5 MGD) of actual flow during the 1-yr study. WTP3 used a 32:16 nanofiltration array made of thin-filmcomposite spiral-wound elements (TFCS 8929ULP-mem-brane element; Fluid Systems, San Diego, CA) designed toreject hardness at 95% and chloride ion at 85%. Thenanofiltration train was designed for 3.5 × 104 m3/d (9.3MGD), and averaged 3.0 × 104 m3/d (7.8 MGD) during thestudy. Before the water was passed through the cartridgefilters, 140 mg/L of sulfuric acid and 2 mg/L of antiscalant(proprietary) were added. After the water passed throughthe cartridge filters and the membranes, 1.5 mg/L of chlorinewas added. The water was then aerated, and 45 mg/L ofsodium hydroxide was added to the finished water from thenanofiltration train.

The lime-softening train in parallel with the nanofiltrationtrain from WTP 3 was designed to treat 2.5 × 104 m3/d (6.5MGD) and averaged 1.4 × 104 m3/d (3.7 MGD) during thestudy. Before flocculation, 8 mg/L chlorine was added to theinfluent water. Then, 201 mg/L of lime, 0.05 mg/L OPC, ananionic polymer, and 13 mg/L of ammonia were added toit during flocculation. After sedimentation, 15 mg/L chlorineand 0.50 mg/L sodium hexametaphosphate (SOX), to keepfilters from solidifying, were added to the water. The effluentsfrom both the nanofiltration and lime-softening trains wereblended and had 4 mg/L of chlorine and 1.3 mg/L of ammoniaadded to the blended water, which was then sent to a 5 milliongallon storage tank. Figure 2 shows a schematic of WTP 3.All chemical additions to the nanofiltration train remainedconstant during a 3-month period in which the lime-softeningtrain was off line.

The water temperature ranged from a minimum of 21.3°C to a maximum of 28.8 °C (average of 25.6 °C). Therefore,the AOC and BDOC incubation temperatures (25 and 22-25°C, respectively) were indicative of distribution systemtemperatures.

Sample collection was performed by following the guide-line outlined in Standard Methods 9060A (18). The permeatesampling point was located immediately after the membranesbut prior to post-membrane chemical additions. There werefive distribution system sampling points: two with averagehydraulic residence times (6 and 10 h), and three withmaximum hydraulic residence times (22, 28, and 30 h). Atotal of 12 monthly sampling campaigns was conductedduring which temperature, pH, turbidity, and chlorineresidual were measured at the time of sampling. Totalcoliforms, HPC, UV-254, DOC, BDOC, and AOC weremeasured within 24 h of sampling at the University of CentralFlorida. Sampling extended from September 1997 throughAugust 1998. For a 3-month period (November 1997-January1998) during sampling, the lime train was shut down.

HPCs. HPCs were performed by spread plating on R2Aagar incubated at 22 °C for 7 d, according to Standard Method9215B (18). Results were expressed in colony-forming unitsper milliliter (cfu/mL). The procedure was performed entirely

inside a laminar flow hood (model 62674, Envirco Coorpo-ration, Albuquerque, NM) equipped with a HEPA filter.

AOC. AOC was measured using the rapid method ofLeChevallier et al. (19), except that plate counts were usedto enumerate bacteria rather that ATP fluorescence, inconjunction with Standard Methods 9217 (18) and themethod of van der Kooij (20). The procedure used atemperature of 25 °C for sample incubation and is outlinedin great detail in ref 21. Quality control for the AOC bioassaywas performed using blank controls, 100 µg/L sodium acetatestandards, and duplicate samples. The 100 µg/L sodiumacetate standards inoculated with P17 produced an averageAOC of 93.80 ( 20.00 µg/L as acetate-C, while for NOX, theyproduced an average AOC of 77.20 ( 12.53 µg/L as acetate-C. Experimental yield values from acetate standards for P17(4.08 ( 0.81 × 106 cfu/µg of acetate-C) and NOX (9.26 ( 1.50× 106 cfu/µg of acetate-C) compared reasonably well withthe literature values as specified in Standard Methods (4.1× 106 and 1.2 × 107 cfu/µg of acetate-C for P17 and NOX,respectively). Literature yield values were used for the AOCcalculations to conform to the Standard Method (18). Nocontrols were made to assess the effect of the thiosulfate(used to neutralize chlorine residual) since it has beendetermined not to affect AOC concentration (22) and isincluded in the Standard Method.

BDOC. The procedure for BDOC determination followedthe technique using sand fixed bacteria (21, 23). In thismethod, there were two incubation flasks per sample plustwo controls used in the BDOC method: sample flasks 1 and2 (duplicates with 300 mL of sample each), activity controlflask (300 mL of 2 mg/L sodium acetate solution), andinhibition control flask (300 mL of sample plus 3 mL of 200mg/L sodium acetate stock solution). Approximately 100 (10 g of drained biological sand was placed in each incubationflask, and 300 mL of water/sample was poured into eachincubation flask containing sand. The initial TOC of the watersample was measured and labeled as DOCo. The sample flasks

FIGURE 2. Process schematic for PBCWUD’s WTP 3.

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were then incubated at room temperature (22-25 °C), andon days 3-6, the DOC concentration of each water samplewas measured. The incubation continued until a minimumDOC value was reached.

Statistical Analysis. The monitoring data (2 yr for theozone plant and 12 month for the nanofiltration/lime-softening plant) was regressed using SPSS version 9.0(Chicago, IL) and interpreted based on a 95% confidencelevel.

Results and DiscussionOzone Distribution System. The Pine Hills WTP, operatedby OUC, was monitored from May 1997 to May 1999. UntilMay 1998, the raw water was treated via the addition ofchlorine for hydrogen sulfide (2.5 mg/L Cl2) and for residual(2.0 mg/L Cl2). In May 1998, the treatment process wasswitched to ozonation for hydrogen sulfide removal (4.7 mg/LO3) and chlorination for residual disinfectant (2.0 mg/L). Ineach campaign there were seven sampling sites, the rawwater, the plant finished water, and in the distribution systemtwo average hydraulic retention time sites (HRTs of 4 and 6.5h) and three maximum HRT sites (10.5, 18.5, and 27 h). Theraw water to the plant was the deep Floridan aquifer, whichdisplayed very stable and low organic carbon concentrations,averaging an AOC of 75 ( 35.0 µg/L as acetate-C, a BDOCof 0.11 ( 0.16 mg/L, and an average DOC concentration of1.12 ( 0.43 mg/L prior to ozone. HPCs for the raw wateraveraged 178 ( 93 cfu/mL, and 2 mg/L of chlorine, as Cl2 wasadded to the treated water.

Prior to ozone, the plant effluent and distribution systemfrom the Pine Hills plant displayed an AOC from 62 to 81µg/L as acetate-C and a BDOC of 0.12-0.23 mg/L. The δAOC(i.e., sampling site AOC - raw water AOC) or the increase inAOC caused by raw water treatment was approximately zerobefore ozone, and HPCs ranged from 3 to 7 cfu/mL. After theozone was added, the plant effluent and distribution systemAOC significantly increased (p-value < 0.0001) to 128-161µg/L as acetate-C, while the δAOC increased to an averageof 60 µg/L as acetate-C. Therefore, AOC after ozoneapproximately doubled, which agreed with previous studies(1, 3, 24). As expected, the AOC of the treated watersignificantly increased due to ozonation, probably becauseof the partial oxidation of larger organic carbon compoundsto carboxylic acids facilitated by ozone oxidation (1, 4, 16,17). The BDOC ranged from 0.19 to 0.40; however, the BDOCwas highly variable and thus not significantly different (p-value ) 0.707) from before ozone. Finally, like AOC, the HPCsincreased to 8-15 cfu/mL, approximately double, which wassignificantly different from before ozone (p-value < 0.0001).

Figures 3 and 4 show the AOC and BDOC vs HPC,respectively. There are 12 points in each plot because of sixbefore ozone (plant effluent and five distribution system

sampling sites) and six after ozone. Each sampling point isthe average of 1 yr of sampling either before or after ozone.Figure 3 shows that the AOC increased significantly after theintroduction of ozone and that there was a significant relationbetween HPCs and the AOC at plant effluent and distributionsystem both before and after ozone. The disinfectant residualwas not included as a variable in the prediction model sinceits addition remained constant at 2.0 mg/L. Before ozone,the distribution system averaged 1.04 ( 0.34 mg/L whileafter ozone it was 1.17 ( 0.24 mg/L in the distribution system;thus, not significantly different. A series of linear as well asnonlinear regression models were tested in order to try allpossible combinations and to determine the best predictionmodel based on 95% confidence. Of the models tested, HPCas a function of AOC (eq 1), displayed the most significantprediction model:

This relation produced an adjusted R2 of 0.96 and MSE of3.37. On the other hand, all BDOC models showed weakrelations with HPCs as seen from Figure 4. When BDOC andHPC were regressed, the best R2 obtained was 0.12, whichindicates a weak relation. Figure 5 shows the actual averageHPCs vs the AOC model (i.e., eq 1) predicted counts. It wasdetermined that there was no significant difference betweenthe actual and the predicted HPCs (p-value ) 0.997).

Nanofiltration in Parallel with Lime Softening. TheBiscayne Aquifer supplies all municipal water systems fromPalm Beach County southward, including the pipeline systemto the Florida Keys. It is a highly permeable wedge-shapedunconfined aquifer that is more than 61 m (200 ft) thick incoastal Broward County and thins to an edge 56.3-64.4 km(35-40 mi) inland in the Everglades. The Biscayne Aquifer

FIGURE 3. HPC vs AOC for the OUC Pine Hills WTP effluent andfive distribution system sites before and after ozone.

FIGURE 4. HPC vs BDOC for the OUC Pine Hills WTP effluent andfive distribution system sites before and after ozone.

FIGURE 5. AOC model predicted vs actual HPCs for the OUC PineHills WTP effluent and five distribution system sites before andafter ozone.

HPC ) 2.00 × e(0.011×AOC) (1)

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is composed primarily of quartz sand, which permits rapidinfiltration of rainfall. Unusual weather patterns wereexperienced during the 12-month sampling period. Heavyrainfall events occurred during the months of Februarythrough the beginning of May, and a strong drought followeduntil the initial days of July, when heavy rainfall happenedagain. This unstable weather was associated with record hightemperatures recorded in the PBCWUD raw water anddistribution system. During the period of monitoring, theraw water AOC averaged 141 ( 140.00 µg/L as acetate-C,BDOC was 2.8 ( 1.48 mg/L, DOC averaged 11.8 ( 3.09 mg/L,and HPC was 231 ( 74.68 cfu/mL. During each samplingcampaign, there were nine sampling sites: raw water,nanofiltrate, lime-softening effluent, plant effluent, twoaverage HRT sites (8 and 10 h), and three maximum HRTsites (22, 28, and 30 h).

The AOC of the plant effluent (blended water betweennanofiltration and lime softening) and five distribution systemsampling sites ranged from 20 to 440 µg/L as acetate-C,while the δAOC ranged from -121 to 124 µg/L as acetate-C.The BDOC ranged from 0.00 to 1.39 mg/L, and the DOCranged from 1.83 to 5.16 mg/L. Finally, the HPC of the planteffluent and distribution system ranged between 0 and 20cfu/mL, and the chloramine residual ranged from 3.0 to 5.0mg/L. Therefore, the data were highly variable, probably dueto the high variability of the raw water. As with the ozoneplant, several linear and nonlinear regression models weretested where HPC was the y-variable and AOC and BDOCwere tried as x-variables. HPC was best predicted as anexponential function of AOC (eq 2) with a R2 of 0.73 and MSEof 14.76:

Figure 6 shows the AOC concentrations vs HPC, whichsomewhat supports the finding that a relation exists betweenHPCs and AOC for the plant effluent and distribution systems.Figure 7, BDOC vs HPC, also supports the findings that BDOCwas not significant in the prediction of HPCs with the bestregression model showing a R2 of 0.11. Finally, Figure 8 showsthe model-predicted HPCs vs the actual HPCs, and it wasdetermined that there was no significant difference betweenthem (p-value ) 0.763).

In summary, this study determined that a switch fromchlorination to ozonation significantly increased (p-value <0.0001) the AOC of the plant effluent and distribution systemsampling sites probably due to the breakdown of largerorganic carbon compounds, such as humic and fulvic acids,into compounds quantifiable as AOC. Also, a switch to ozonealso caused a significant increase in HPCs (p-value < 0.0001),which agreed with the increase in AOC concentration. Thisincrease resulted in HPCs that were still fairly low, probably

because of the chlorine residual that was maintained in thedistribution system. A significant relation (R2 ) 0.96) wasdetermined to exist between HPCs and the AOC concentra-tions in the distribution system fed by chlorinated water andlater by ozonated chlorinated water. Likewise, in a systemtreating raw water via nanofiltration and lime softening, arelation (R2 ) 0.73) was found to exist between HPCs andAOC concentrations in the plant effluent and distributionsystem even in the presence of a significant chloraminedisinfectant residual. Finally, for both distribution systemsmonitored, BDOC was not related to HPCs probably becauseBDOC measures large molecular weight organic carboncompounds that are not as readily available for bacterialdegradation as AOC compounds are.

Impact of StudyA significant number of studies have been conducted atbench- or pilot-scale to evaluate the dynamics of distributionsystems, with specific emphasis on the impact of water qualityon the microbiological quality of these systems. Such studies,while quite informative, are limited in their ability to simulatethe complex dynamics of operating full-scale distributionsystems. Specifically, the impact of the concentrations ofBOM on the microbiological quality of full-scale systems inthe presence of all the other compounding factors is still indispute. One of the main unanswered questions about thisissue is the following: If a water supplier undertakes actionsat the treatment plant that significantly change the con-centration of BOM in its distribution system, should thesupplier expect a correspondingly significant change in themicrobiological quality of the distribution system?

This project included close monitoring of two full-scaledistribution systems that underwent significant changes inthe concentration and form of BOM entering them. Themonitoring programs were designed to allow for the collection

FIGURE 6. HPC vs AOC for the Palm Beach County WTP effluentand five distribution system sites.

HPC ) 1.86 × e(0.0049×AOC) (2)

FIGURE 7. HPC vs BDOC for the Palm Beach County WTP effluentand five distribution system sites.

FIGURE 8. Model predicted vs actual HPCs for the Palm BeachCounty WTP effluent and five distribution system sites.

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of data before and after the implementation of the treatmentchange.

This study highlighted the importance of maintaining aminimum chlorine residual in a distribution system. In fact,the results suggest that the bulk-water bacterial counts inany system are more sensitive to chlorine residual than theyare to BOM concentrations. Even waters containing very lowBOM concentrations (less than 50 µg/L AOC and less than0.1 mg/L BDOC) still required the presence of a chlorineresidual to maintain the bulk-water bacterial counts atacceptable levels. This is not to say that the biological stabilityof a distribution system is not affected by BOM concentration.However, this does emphasize the notion that the mainte-nance of a chlorine residual throughout the distributionsystem is probably the strongest barrier against high plaktonicbacterial counts in the system.

As a final note, this study shows that the reliance on BOMlevels alone, from either a regulatory standpoint or anoperational standpoint, is not appropriate without a com-prehensive assessment of other controlling factors.

AcknowledgmentsThis study was funded by the American Water WorksAssociation Research Foundation grant awarded for RFP 361.The authors thank Mark LeChevallier, Cheryl Norton, IssamNajm, Lina Boulos, Laurent Kiene, and Carlos Campos, whoare also involved with this AWWARF grant. Christian Volk isthanked for his help early in the project. In addition, EugeniaCarey and Jaya Navani’s team from PBCWUD and RichardDunham, Rick Coleman, Cliff Russell, and Mike Malone fromOUC were instrumental to the success of the project. TheEPA STAR fellowship program is thanked for providingadditional monetary support.

NomenclatureAOC assimilable organic carbon

BDOC biodegradable dissolved organic carbon

BOM biodegradable organic matter

DBP disinfection byproducts

DOC dissolved organic carbon

HPC heterotrophic plate count

HRT hydraulic retention time

MGD million gallons per day

MSE mean square of errors

OPC anionic polymer

OUC Orlando Utilities Commission

p-value observed significance level

PBCWUD Palm Beach County Water Utility Depart-ment

R2 coefficient of determination

SOX sodium hexametaphosphate

SWTR surface water treatment rule

THM trihalomethane

TOC total organic carbon

WTP water treatment plant

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Received for review February 20, 2001. Revised manuscriptreceived June 25, 2001. Accepted June 28, 2001.

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