jar tests for evaluation of atrazine removal at drinking water treatment plants

ENVIRONMENTAL ENGINEERING SCIENCEVolume 16, Number 6, 1999Mary Ann Liebert, Inc.

Jar Tests for Evaluation of Atrazine Removal atDrinking Water Treatment Plants

TIAN CHENG ZHANG1 and STEVEN CHARLES EMARY2

'Department of Civil EngineeringUniversity ofNebraska-Lincoln at Omaha Campus

Omaha, NE 68182-01762Florence Treatment Plant

Metropolitan Utilities DistrictOmaha, NE 68112

ABSTRACT

The objective of this study was to evaluate the effects of major factors such as PAC dosage, pH,contact time, mixing energy, alum dosage, and enhanced coagulation on the effectiveness of atrazineremoval. Jar tests and response surface methodology were used to simulate conditions found in dif-ferent treatment facilities. The time course of atrazine concentration with an initial atrazine con-

centration of 12 /JLgfL and initial (Acticarb) PAC of 16 mg/L indicated that it took approximately5 days to reach equilibrium with the maximum atrazine removal of about 73%. Therefore, in treat-ment facilities, the adsorption of atrazine with this kind of PAC will be less than the removal achievedat equilibrium, due to the short retention time in a dynamic process. Mixing energy is a major fac-tor affecting atrazine absorption. With jar test times ranging from 30 to 60 min, increasing rpmsfrom 5 to 100 (G = 4 to 321 s_1) resulted in atrazine removals ranging from 34 to 59%. Withoutaddition of PAC, neither lime softening nor alum coagulation (conventional or enhanced dosagesranging from 6 to 18 mg/L) demonstrated atrazine removal. A synergistic relationship appears toexist between PAC dosage and enhanced coagulation (with pH adjusted to about 5.8); neither PACnor enhanced coagulation resulted in as high a removal rate of atrazine as the two did together(greater than 60%). The results of this study are useful for evaluation of different PAC applicationpoints in conventional drinking water treatment plants.

Key words: Atrazine; powdered activated carbon (PAC); jar tests

INTRODUCTION

Atrazine (2-chloro-4-ethylamino-6-isopropylamino-s-triazine) is a major herbicide of the s-triazine fam-

ily, which has been widely used throughout the UnitedStates, especially in the Midwestern States where corn isthe dominant crop (Lin et al, 1995). Atrazine (together

with other s-triazines) is often found in groundwater andsurface waters that are used as drinking water supplies(Food Chemical News, 1991a, 1991b; U.S.G.S., 1991;U.S. EPA, 1992; Schottler et al, 1994; Lin et al, 1995;Richards et al, 1995), which necessitates the implemen-tation of atrazine removal methodology at drinking-wa-ter treatment plants. One method for atrazine removal is

417

418 ZHANG AND EMARY

the application of powdered activated carbon (PAC) tothe treatment facilities (Miltner et al, 1989; Qi et al,1994; Adams and Watson, 1996).Conventionally, PAC is often used in water-supply

utilities worldwide for taste and odor control (U.S. EPA,1976; AWWA, 1977, 1986; Lettinga et al, 1978;Lalezary-Craig et al, 1986, 1988; Miltner et al, 1989).Since the late 1980s, considerable research has been con-

centrated on using PAC to control organic contaminantsin drinking water (Najm et al, 1991a, 1993; Qi et al,1994; Adams and Watson, 1996), with two major direc-tions: theoretical- and practical-oriented research. Fortheoretical-oriented research, most studies have been fo-cused on the predictive methods (e.g., equilibrium mod-els and mass transfer models) for evaluating the perfor-mance of a PAC adsorption system. The homogeneoussurface diffusion model (HSDM) is commonly used toevaluate adsorption kinetics (Crittenden and Weber,1978a, 1978b; Hand et al, 1983; Traegner and Suidan,1989, 1991). The procedure for using the equivalentbackground compound coupled with the ideal adsorbedsolution can be used as a method to qualify the compe-tition between trace organics and background organicmatter in water (Najm etal, 1990; 1991b; Qi, era/., 1994;Hopman et al, 1998).

For practical-oriented research, PAC has been used inthe Robert-Haberer process (Stukenberg and Hesby,1991; Harberer and Norman-Schmidt, 1991), the upflowfloc-blanket reactor (Najm et al, 1991c, 1993), and thesolids contact slurry recirculating clarifier (Kassam et al,1991), jet flocculation systems (Sobrinho et al, 1997),and other reactors (Najm et al, 1991b; Qi et al, 1994).Najm et al. (1991a) summarized the advantages and dis-advantages of different points for PAC application andthe important criteria for selecting these points. However,information still is not sufficient on how (1) to evaluatethe effects of different operating factors on PAC perfor-mance, and (2) to find the best location(s) for PAC ap-plication for the optimum removal of atrazine. Becausethe drinking water treatment train changes with differentwater supply sources, it is imperative to study the appli-cation of PAC for atrazine removal and to optimize thelocations of PAC application in treatment trains.

Traditionally, conducting jar tests is one of the mostcommon methods to evaluate coagulation efficiency. Re-cently, jar tests were used to evaluate criteria for en-

hanced coagulation compliance (U.S. EPA, 1993, 1994;Krasner and Amy, 1995; White et al, 1997; Vrijenhoeket al, 1998). Jar tests are often used to estimate the PACdose that is required to achieve the desired removal inconventional plants for test and odor control. In ajar test,contact time, mixing energy, operational conditions such

as pH, or PAC dose are the major factors that can be eval-uated. In this preliminary study, jar tests were introducedto study the effects of PAC on atrazine removal in drink-ing water treatment plants. Using jar tests to evaluate per-formance of PAC adsorption on organic compounds mayhave several advantages. (1) The jar test is well knownby workers in drinking-water treatment plants. Therefore,once the evaluation procedure is established, the tech-nique transfer is easier than other methods, such as theHSDM method, which needs a full understanding ofmathematical modeling and adsorption theory. (2) Mix-ing energy (e.g., the velocity gradient, G) can be intro-duced into PAC-adsorption jar tests. This is importantwhen optimization of PAC application points is needed.

The objectives of this study were to: (1) evaluate theeffects of major factors, such as PAC dosage, pH, con-tact time, mixing energy, alum coagulant, enhanced co-

agulation, on the effectiveness of atrazine removal; and(2) investigate the interrelationship(s) among these fac-tors through conducting jar tests. The experimental re-sults were then used to evaluate the best PAC applica-tion location(s) of conventional (surface) drinking-watertreatment plants, with a drinking-water treatment plantbeing used for a case study.

MATERIALS AND METHODSChemicals and Waters

The atrazine (2-chloro-4-ethyamino-6-isopropylamino-1,3,5-triazine) used in this study was obtained from ChemService (West Chester, PA). Purity and lot number were99.7% and 25-116A, respectively. The adsorbent used inthis study was Acticarb PAC (Calgon Carbon Corpora-tion, Pittsburgh, PA). The characteristics of this carbonare as follows: iodine number, 1199 mg/g; moisture as

packed, 3%; apparent density, 0.54 g/cm3; ash content,6%; and 98% of the carbon passing through #325 mesh.Aluminum sulfate obtained from General Chemical Co.(St. Louis, MO) was used as a coagulant in the corre-

sponding jar tests, and is also used in the upflow floc-blanket reactors (primaries) for clarification in FTP (seeCase Study). NaOH (8 N) was used in jar tests to demon-strate the impact of pH adjustment on PAC adsorptivityin lime-softening basins.

Three different kinds of water were used in this study.Organic-free water was distilled water followed by treat-

ment of GAC adsorption, ion exchange, and microfiltra-tion; (2) influent of pre-sedimentation tank; and (3) influ-ent of upflow flow-blanket reactors (primary basins in Fig.8), originally sampled from Florence Treatment Plant (FTP)located on the Missouri River, Omaha, NE (see Table 1).

JAR TESTS FOR ATRAZINE REMOVAL 419

Table 1. Influent Characteristics of Pre-sedimentation and Upflow Floc-Blanket Reactors

Component Expressed asInfluent of pre-sedimentation tank

concentrationInfluent of upflow floc-blanket reactors

concentration

IronManganeseTurbidityCalciumMagnesiumNitrateChlorideSulfateTOCPHAlkalinityHardnessAtrazine

Fe, mg/LMn, mg/L

NTUCa, mg/LMg, mg/LN03, mg/LCl, mg/LS04, mg/L

mg/Lunits

CaC03, mg/LCaC03, mg/L

Mg/L

<0.02<0.0213474304.4014

2485.48.25

195323

0.00

<0.02<0.02

874304.3514

2505.18.25

191311

0.00

Equilibrium Isotherm TestTo determine the equilibrium time for PAC to adsorb

atrazine, 12 4.5-L amber glass bottles were used. Vol-umes (1,000 mL) of organic-free water were spiked with12 pg/L atrazine and treated with 16 mg/L PAC. Thesebottles were mixed at a constant 100 rpm using a Lablinemodel 208 magnetic stirrer and a 3" Teflon-coated stir-ring bar. The mixing was then stopped at different times(ranging from 15 min to 20 days) for sampling and analy-sis of the amount of atrazine remaining in the solution.When atrazine adsorption became constant, the equilib-rium was achieved. Five days were needed to reach theequilibrium.

The second step was to conduct isotherm testing, us-ing either 4.5-L glass bottles containing 1,000-mL vol-umes of organic-free water. Four of these bottles were

spiked with 16 pg/L atrazine and treated with 2, 4, 8,and 16 mg/L PAC, respectively. The other four bottleswere spiked with 3 pg/L atrazine and treated with 2,4, 8, and 16 mg/L PAC, respectively. The bottles were

sealed with Teflon-lined caps and stirred at 100 rpmand at room temperature (25°C). After the 5-day equi-librium time was reached, samples were withdrawn,centrifuged, and filtered through 0.22-¿im nylon mem-

brane filters (Millipore, Bedford, MA) for atrazineanalyses.

Mixing EnergyIn this study, two series of tests—one with a specific

contact time of 30 min and the other 60 min—were con-

ducted with ajar-test apparatus (model #3461 made byPhipps & Bird, Inc., Richmond, VA). Organic-free wa-

ter, spiked with atrazine of 12 pg/L and PAC of 16 mg/L,was used for these tests. For two different contact times,the atrazine residual in the test bottle was analyzed un-

der different velocity gradients (G values, calculated as

follows):G = VCdAu3/2vV (1)

where C<¡ = 1.8, the drag coefficient of the paddle; A =

0.001875 m2 (=0.025 X 0.075 m), the area of the pad-dle; u = rpm*0.236 m/60 s, the velocity of the paddlesrelative to that of the water, the circumference of the jartest container was 0.236 m; v = 10~6 m2/s, the kinematicviscosity of water; and V = 1,000 mL, the water volumein the beaker.

Chlorine DemandThe effect of PAC on chlorine demand was evaluated

by adding PAC to some samples and analyzed total chlo-rine using the Standard Methods, procedure 2350 B(American Public Health Association, 1995). One-litersamples of the influent of upflow floc-blanket reactorswere placed in a water bath held at the temperature of16°C (the water temperature at FTP's intakes). Each sam-

ple was dosed with a known amount of chlorine and PACand placed in the dark for 1 h. After 1 h each sample wastitrated with an amperemetric titrimeter (Wallace & Tier-nan model 311 ) to determine free and combined chlorineconcentration. When added together, these parameters are

referred to as the total chlorine in the solution. When thetotal chlorine value is subtracted from the original dos-ing amount of chlorine, the difference is the chlorine de-mand of the solution.

ENVIRON ENG SCI, VOL. 16, NO. 6, 1999

420 ZHANG AND EMARY

Jar Tests and Experimental DesignsJar tests were designed to evaluate atrazine removal

corresponding to different PAC application points andoperating conditions. The following factors were tested:alum dose, PAC dose, and pH. Alum was 6 to 18 mg/L(midpoint 12 mg/L); PAC, 8 to 16 mg/L (midpoint 12mg/L); pH was adjusted to 11.20 to 11.60 (midpoint11.40) for lime softening and 5.80 to 6.60 (midpoint 6.2)for enhanced coagulation in the upflow floc-blanket re-actors. These ranges and midpoints were selected basedon conventional drinking-water treatment plants and in-corporated into the following experimental designs forjar tests.

Jar test procedures. The procedures are: (1) rapid mixat 100 rpm for 3 min; (2) slow mix at 30 rpm for 12 min;(3) allow to settle for 15 min, decant, and filter; and (4)collect eluent for analysis of atrazine with EPA method

507 (U.S. EPA, 1990). All of the jar tests were performedin random order to eliminate errors arising from a pre-determined sequence.

Jar tests investigating individual factors. Two sets ofjar tests were conducted to investigate independent im-pact of each individual factor alone such as alum, PAC,and pH on atrazine removal (Table 2). Influent of pre-sedimentation tanks and upflow floc-blanket reactors,spiked with atrazine of 12 pg/L, were used in these jartests, respectively.

Jar tests investigating interrelationships among dif-ferent factors. In this study, a rotatable central compos-ite design based on the response surface methodology(Montgomery, 1991) was used to design the jar tests toinvestigate the interrelationships among different factors(Tables 2 to 4). After accomplishment of each set of those

Table 2. Rotatable Central Composite Design and Experimental Results of Jar Tests for

Evaluation of Alum and PAC on Atrazine Removal in Pre-sedimentation Reactor"

Variable(XJ

Centerpoints Semirange -1.414

Factorial as star points1 0 1 1.414

Alum (X,)PAC (X2)

12 mg/L12 mg/L

3.56.3

1212 16

20.517.6

Run/randomnn b

Experimental design Influent of pre-sedimentationAlum(X,)

(mg/L)

PAC(X2)(mg/L)

Atrazineremaining

cone. (pg/L)

Atrazineremoved

(%)

Influent of upflow reactor

Atrazineremaining

cone. (pg/L)

Atrazineremoved

(%)

1/92/63/54/15/76/37/128/109/410/1111/212/8

6186

18121220.53.5

12121212

88

16161212121217.66.3

1212

7.27.06.16.67.06.76.66.46.07.56.56.8

40.041.749.245.041.744.245.046.750.037.545.843.3

7.37.06.36.16.96.56.66.75.97.56.66.8

39.241.747.549.242.545.845.044.250.837.545.043.3

aInitial atrazine concentration was 12 /¿g/L. Two sets of jar tests were conducted using influent of pre-sedimentation tank andupflow floc-blanket reactors, respectively. In this experimental design, 0 means central point; "—1" can be obtained using semi-range

—

central point (6-12 = —6); "+1" can be obtained using central point + semirange (12 + 6 = 18); "—1.414" can be ob-tained using (semirange) * 1.414

—

central point (6*1.414—

12 = —3.5); and "+1.414" can be obtained using central point +(semirange)* 1.414 (12 + 6*1.414 = 20.5).

bRun number is the design number, while the random number is the actual experimental number.


Table 3. Rotatable Central Composite Design and Experimental Results of Jar Tests for Evaluationof pH and Alum on Atrazine Removal in the Upflow Floc-Blanket Reactors3

VariablePU

Centerpoints Semirange 1.414

Factorial as star points1 0 1.414

Alum (Xi)PAC (X2)

12 mg/L6.2

60.4

3.55.6

126.2

186.6

20.56.8

Runnumber

Randomnumber

Alum(Xi)(mg/L)

pH(X2)(-)

Atrazineremaining

cone. (pg/L)

Atrazineremoved(%)

123456789101112

11102457139

126

6186

18121220.53.5

12121212

5.85.86.66.66.26.26.26.26.85.66.26.2

11.411.611712.111.611.711.511.711.811.611.711.5

5.03.32.50.03.32.54.22.51.73.32.54.2

"Initial atrazine concentration was 12 pg/L, and influent of upflow floc-blanket reactors was used.

jar tests, Statgraphics (STSC, Inc., 1988) was used to (1)conduct multiple regression using least squares to esti-mate the regression models, which were approximatedusing second-order polynomial functions, (2) producemultiple X-Y-Z plots, and (3) produce response surfaceplots (Emary, 1996).

Analytical Methods and QualityAssurance/Quality Control

Atrazine was analyzed using EPA method 507 (U.S.EPA, 1990). A 1-L sample was fortified with 50 pL ofsurrogate standard solution and extracted in a 2-L sepa-rately funnel. Samples were adjusted to pH 7 with a phos-phate buffer. NaCl was added to improve the partitioncoefficient, and the sample was extracted three times with60 mL méthylène chloride. The méthylène chloride was

then concentrated and exchanged with methyl ierr-butylether using a Kuderna-Danish apparatus. The sample vol-ume was then adjusted to 1-mL and analyzed using a Tra-cor 540 gas Chromatograph (Tremetric, Austin, TX) hav-ing a nitrogen-phosphorous detector with a DB-5capillary column.

Standard solutions of atrazine covering a range of 1 to20 pg/L were prepared in the stock solutions being testedand extracted using EPA method 507. The standard so-

lutions were refrigerated. At least three standard solutionswere used each day in the operation of the gas Chro-matograph (GC) to confirm the standard curve of fivestandard points. If the daily variation in the slope of thestandard curve was higher than 10%, all standard solu-tions were injected into the GC, and a new standard curve

was determined. All of the data points presented in thispaper are the results of duplicate samples. Precision was

assessed by analyzing duplicate samples taken from eachexperiment and comparing the results. The objective was

a relative error of less than 10% between duplicate sam-

ples. If the difference was greater than 10%, the experi-ment was repeated, and the cause of the discrepancy was

investigated.

RESULTS AND DISCUSSION

Isotherm DataIt took approximately 5 days to reach the equilibrium,

and the corresponding maximum atrazine removal wasabout 73% (Fig. 1). During the tests, there was no pHchange in the batch reactors, which may be due to thelow concentration of atrazine. Based on 5 days as theequilibrium time, the isotherm of atrazine adsorption on


422 ZHANG AND EMARY

Table 4. Rotatable Central Composite Design and Experimental Results of JarTests for Evaluation of Alum, PAC, and pH (PAC Plus Enhanced

Coagulation) on Atrazine Removal in the Upflow Floc-Blanket Reactors3

Variable (X,) Center points Semirange -1.682Factorial as star points

1 0 1 1.682

Alum (X¡)PAC (X2)pH (X3)

12 mg/L12 mg/L

6.2

640.4

1.95.35.5

685.8

12126.2

18166.6

22.718.76.9

Runnumber

Randomnumber

Alum(X,)

(mg/L)

PAC(X2)

(mg/L)

pH(X3)(-)

Atrazineremaining

cone. (pg/L)

Atrazineremoved

(%)

1234567891011121314151617181920

186

12721

138935

161519171114104

20

6666

181818181.9

22.112121212121212121212

8161688

161612125.318.71212121212121212

5.86.65.86.65.86.65.86.66.26.26.26.25.56.96.26.26.26.26.26.2

5.76.95.36.75.67.05.16.66.46.67.16.56.17.37.07.26.96.66.97.1

52.542.555.844.253.341.757.545.046.745.040.845.849.239.241.740.042.545.042.540.8

"Initial atrazine concentration was 12 pg/L, and influent of upflow floc-blanket reactors was used.

Atrazine adsorbed

Atrazine removal

Atrazine remaining

100 150Time (hr)

200

14

12

10

8

6

4

2

250

100

60 TO

OE<u<Dc

420

-10

FIG. 1. Equilibrium test results of atrazine adsorption on PAC. The initial atrazine concentration was 12 pg/L, while PAC con-

centration was 16 mg/L. Experiment was conducted using 12 batch reactors.


PAC was accomplished. The Freundlich isotherm para-meters, Un and K, were 0.852 and 162.39 (mg/g)(L/mg)1/n, respectively. A Langmuir isotherm also was

tried, but no reasonable parameters were found.Adams and Watson (1996) used Calgon Water Pow-

dered Low Activity, a bituminous coal carbon similar topulverized Calgon F-200, and found that Un = 0.44 to0.56, K = 140 to 572 (mg/g) (L/mg)l/n, depending on theinitial pH in the batch reactor. Qi et al (1994) used WPHpowdered activated carbon, and found Un = 0.335, K =

287.33 (mg/g) (L/mg)1/n (796.9 (/¿m/g) (L/pm)l,n). Najmet al. (1991a) reported that Un = 0.29, K = 283.18 forDDW water and Un = 0.36, K = 293.35 (mg/g)(L/mg)1/n for groundwater.

The K value found in this study is comparable to theK values found in previous studies, but the Un valueseems higher. The reasons for these differences may bedue to the different PAC and the very low initial atrazineconcentrations (3 to 12 pg/L) used in this study. Thelower initial atrazine concentration provides a lower sur-face loading of atrazine (Qi et al, 1994), resulting in a

lower isotherm, which is the case in this study. There-fore, the atrazine isotherm at a given initial concentrationin water cannot be represented accurately by one Freund-lich equation (Qi etal, 1994). The isotherm data obtainedin this study suggests that, in treatment facilities, atrazineremoval by PAC be less than 73% (the removal achievedat equilibrium) if Acticarb PAC is used, due to the shortretention time of treatment facilities.

Mixing EnergyIncreasing the contact time from 30 to 60 min only re-

sulted in approximately 5% additional atrazine adsorp-tion (Fig. 2). However, increasing the G value from ap-proximately 4 to 321 s"1 (the corresponding rpms from

5 to 100) resulted in an increase in atrazine removals from37 to 59%. It seems that, if the G value continuously in-creases (in this study, this was beyond the capability ofthe jar test apparatus), the atrazine removal will be reach-ing its maximum removal. Therefore, mixing energy (theG value), instead of the hydraulic retention time (HRT),is more important when the supplied HRT in a PAC con-tactor is not sufficient for equilibrium to be reached.

Qi et al. (1994) reported that the atrazine removal ef-ficiencies were between 30 to 65% for a reaction timebetween 15 and 100 min in a batch reactor with an ini-tial atrazine concentration of 22.5 pg/L and a PAC doseof 3.325 mg/L. Similar results were obtained in a batchreactor with an initial atrazine concentration of 252.4/xg/L and a PAC dose of 2.63 mg/L. The results of thisstudy are consistent with these previous results. How-ever, Qi et al (1994) also reported that the atrazine re-

moval efficiencies were between 60 to 93%, for a reac-

tion time of 15 to 100 min in a batch reactor with an

initial atrazine concentration of 22.5 pg/L and a PACdose of 8.4 mg/L, or 222 pg atrazine/L and a PAC doseof 12.88 mg/L. As one can see from Fig. 1, the PAC usedin this study can never achieve an atrazine removal effi-ciency higher than 73%. Therefore, in some cases, the re-

sults are not consistent with what Qi et al (1994) re-

ported.Traditionally, the dynamic processes of PAC adsorp-

tion of organic compounds are studied using completelymixed batch reactors or CSTRs. Most of these previousstudies did not pay enough attention to mixing energy be-cause G values were not mentioned in these studies. Fora conventional flocculation basin with its G values be-tween 70 s"1 and 10 s_1, the atrazine removal of PACapplication should range from 34 to 44%. Because, basedon tracer studies, sludge residence times in the floccula-

100 150 200 250Velocity gradient (1/sec.)

FIG. 2. Effects of velocity gradients, G values, of jar tests on atrazine removal.


424 ZHANG AND EMARY

tion basin range from 15 to 25 min (Lettinga et al, 1978),although the contact time of floes associated with an op-timized flocculation basin can range from 30 to 60 min.For pre-sedimentation basins, very little agitation (G <10 s_1) is available; therefore, contact is minimal. In ad-dition, settling can be accentuated with the addition ofalum. Therefore, the atrazine removal in a pre-sedimen-tation basin will be very poor (less than 34%), if there isno rapid mixing tanks or CSTRs prior to the pre-sedi-mentation basin. A CSTR with high G values (approxi-mately 100 rpms) and a retention time of 30 prior to theflocculation basin or pre-sedimentation basin will pro-vide more than 50% atrazine removal with PAC. In ad-dition, atrazine removal over the typical mixing energy(10-50 s"1; Pontius, 1990) found in upflow floc-blanketreactors will be around 39.2 to 48.3%. Actual atrazine re-

moval may be higher, as contact time in the upflow floc-blanket reactors is approximately 24 h.

Jar Tests

To evaluate the best locations for PAC addition points,two groups of jar tests were tested, and the results are re-

ported as follows.

Jar tests investigating individual factors. Virtually no

removal was seen with the alum coagulant alone usinginfluent of the pre-sedimentation and upflow floc-blan-ket reactors (Fig. 3a). When PAC alone was used,atrazine removal ranged from 40.8 to 49.2% for pre-sed-imentation water and 40.0 to 46.7% for upflow floc-blan-ket reactors influent water and increased as PAC dosagesincreased (Fig. 3b). When pH effect was evaluated, pHadjustment was accomplished through the addition of 8N NaOH during the rapid-mix segment of the jar test se-quence. Elevated pH adjustment (pH = 11.2 to 11.6), asseen in the lime-softening process, resulted in negligibleatrazine removal ranging from 0.5 to 2.6% (Fig. 3c). Be-cause of the poor relationship between elevated pH andatrazine adsorptivity, it was deemed unnecessary to ex-

amine the process using slaked lime. The softeningprocess would further inhibit the atrazine adsorption bypossibly encapsulating the PAC and thus, reducing thesites available for adsorption.Jar tests investigating interrelationships among dif-

ferentfactors. Four additional series of jar tests were con-

ducted. One series investigated the effect of alum andPAC together on atrazine removal using the influent ofthe pre-sedimentation tanks. The remaining three serieswere conducted using the influent of upflow floc-blanketreactors. The objectives of these three series jar tests were

to: (1) determine the impact of PAC and alum used to-

gether on atrazine removal; (2) evaluate atrazine removal

Alum dosage (mg/L)

60

48

S 36oE

24

12

| Presed water fj Floc-blanket basin

PAC dosage (mg/L)

FIG. 3. Effects of each individual factor on atrazine removal.(a) Alum coagulation only without addition of PAC; (b) PAConly; and (c) pH adjustment only without PAC or alum.

under conditions of enhanced coagulation with alum andpH adjustment (but without PAC addition); and (3) ex-

amine the impact of PAC, alum, and pH adjustment usedtogether on atrazine adsorption efficiency.

Table 2 shows the rotatable central composite designand results of jar tests for evaluation of the effects of


alum and PAC together on atrazine adsorption. Based onthe results shown in Table 2, multiple regression usingStatgraphics was conducted. Two polynomial equationswere obtained to represent the response surface ofatrazine concentration remained (pg/L) after jar tests vs.

alum concentration (mg/L) and PAC dosage (mg/L).Equations (2) and (3) are for influent of pre-sedimenta-tion tanks and upflow floc-blanket reactors, respectively.Atz. cone, remained = 8.56

-

0.20 *(PAC)-0.0030*(Alum)2 + 0.0070*(PAC)*(Alum) (2)

Atz. cone, remained = 8.34-

0.13*(PAC)-0.00056*(Alum)2 (3)

The coefficient of determination, R2 is 0.87 for Equa-tion (2) and 0.93 for Equation (3). The response surfacesare shown in Fig. 4. The statistical model fitting results,ANOVA analysis, and other detailed information on mul-tiple regression can be found in Emary's thesis (1996).

As PAC dosage increases, the remaining atrazine con-

centration after the test decreases (Fig. 4). Application ofalum to pre-sedimentation tanks has almost no effect onatrazine removal (Fig. 4a). However, increase in alumconcentration in upflow floc-blanket water will decreaseatrazine removal (if pH is not a variable), which may bedue to the encapsulation of PAC by coagulation and floc-culation (Fig. 4b). The difference in water quality para-meters between the two locations does not impact greatlythe relationship among alum, PAC dosages, and atrazineadsorptivity. The best atrazine removal is about 50%,with the PAC dosage of about 16 mg/L.

The impact of enhanced coagulation alone on atrazineremoval was evaluated by adjusting the pH and varyingthe alum dosage. This series of jar tests did not include theaddition of PAC. Influent of upflow floc-blanket reactorswas spiked with 12 pg/L atrazine, and jar test procedureswere the same as those described before. Table 3 showsthe rotatable central composite design and results of jartests. Based on Statgraphics analysis, the best coefficientof determination, R2 found is 0.32. Therefore, there is no

good relationship among atrazine removal, alum, and pH.As pH is adjusted and alum dosage is varied using upflowfloc-blanket reactors influent water, atrazine removal is notenhanced. The highest atrazine removal value is 5.0%. Thisoccurred at a pH of 5.8 and an alum dosage of 6 mg/L(Table 3). Therefore, atrazine removal is negligible usingenhanced coagulation without PAC.

Although coagulation did not demonstrate any appre-ciable atrazine removal, literature (U.S. EPA, 1991) in-dicates the stability of atrazine will be reduced at lowerpH values. Miltner et al (1989) indicates that a syner-getic relationship exists between PAC and enhanced co-

agulation. Therefore, the fourth series of jar tests were

conducted to examine the effect of alum, pH, and PACon atrazine adsorptivity (Table 4). Jar test procedureswere the same as those described before. Based on theresults in Table 4 and the same procedure aforementionedfor multiple regression using Statgraphics, a polynomialequation to represent the response surface of atrazine con-

centration remained after jar tests (pm/L) vs. alum con-centration (mg/L), PAC dosage (mg/L), and pH (units)can be obtained:

Atz. cone, remained = 1.03-

0.027*(Alum)-0.21*(PAC) + 1.43*pH

-

0.007*(Alum)2-0.009*(PAC)2

-

0.10 (pH)2-

0.0012*(Alum)*(PAC) +0.034*(Alum)*(pH) + 0.064*(PAC)*(pH) (4)

The coefficient of determination, R2 is 0.95. The inabil-ity to plot a four-dimensional graph necessitates the useof box graphs denoting rotatable design representing thethree independent variables examined in this jar test se-ries (Fig. 5).

Lower pH (with H2S04) in conjunction with elevatedPAC application dosages results in higher atrazine re-moval (adsorptivity) (Fig. 5a and b). The lower pH val-ues seem to increase the hydrophilic characteristics of thesoluble atrazine (U.S. EPA, 1991), resulting in higher ad-sorption on the PAC. The higher PAC dosages makemore adsorption sties available. There is a net effect ofincrease in atrazine removal (adsorptivity) with enhancedcoagulation (adjusting pH with H2SO4) (Fig. 5c).Atrazine removal can be higher than 60%, with a PACdosage of 16 mg/L and a pH adjustment to 5.8 (Fig. 5c).This synergistic relationship between the PAC dosageand enhanced coagulation can be explained according tothe previous studies. The presence of natural organic mat-ter (NOM) significantly decreases the PAC capacity andrate of adsorption of synthetic organic compounds (SOC)due to the competition between NOM and SOC (Najmet al, 1990). Enhanced coagulation at pH about 5 to 6significantly increases the removal of NOM comparedwith coagulation at ambient pH (U.S. EPA, 1993; Kras-ner and Amy, 1995; Vrijenhoek et al, 1998). Generally,aluminum (or iron) coagulation can remove NOM ac-

cording to two mechanisms: (1) adsorption onto alu-minum hydroxide or ferric hydroxide floe, and (2) for-mation of insoluble complexes (aluminum or ironhumâtes or fulvates) in a manner analogous to chargeneutralization (Krasner and Amy, 1995). Lower pH re-duces the charge density of humic and fulvic acids, mak-ing them more hydrophocic and adsorbable (Krasner andAmy, 1995; Kranser et al, 1996). The second mecha-nism is relatively more dominant at lower dosages andpH conditions. Therefore, the synergy effect of the PACdosage and enhanced coagulation is attributable to the in-


426 ZHANG AND EMARY

PflC (ng/L)

fllun (ng/L)

PAC (ng/L)12 tó

AI un (ng/L)FIG. 4. Response surface for evaluation of the interrelationship among PAC, alum, and atrazine removal using influent of (a)pre-sedimentation tanks; and (b) upflow floc-blanket reactors.

hibition of NOM competition with atrazine for adsorp- flow floc-blanket reactors could be a much better PACtion sites. This inhibition is presumably due to the for- application point than pre-sedimentation basins if en-

mation of insoluble alum-humate complexes (pH at 5.8 hanced coagulation (with pH adjustment) is coupled withand low alum dosage) (Hubel and Edzwald, 1987; Vri- PAC application. These results are consistent with thejenhoek et al, 1998). These results indicate that the up- previous modeling prediction reported by Qi etal (1994).


nhiM (ag/L)

FIG. 5. Response surface for evaluation of the interrelationship among PAC, enhanced coagulation (alum and pH), and atrazineremoval in upflow floc-blanket reactors. Atrazine concentration remained in the test solution vs. PAC and pH at alum dosage of(a) 12 mg/L and (b) 3 mg/L; and (c) atrazine removal vs. PAC and alum at pH = 6.2.


428 ZHANG AND EMARY

RiverIntake&

Screen

CSTR

PAC ApplicationPoint Ü 2 „.„ ,.

.PAC ApplicationPoint ft 3

RapidMix

PAC ApplicationPoint # 1

FloeBasin

SettlingBasin

PAC ApplicationPoint # 4

CI, ApplicationPoint H 2

UpflowFloc-

BlanketReactor

SettlingBasin

Cl2 ApplicationPoint U 1

DistributionSystem

Filter

Clear well

FIG. 6. A hypothetical drinking water treatment scheme for discussion of selecting the best PAC application points.

Selection ofPAC Application Points

Najm et al (1991a) reported that, in a conventionaltreatment plant, the common PAC application points arethe plant intake, rapid mix, and filter. They further rec-ommended that a continuous-flow slurry contractor thatprecedes the rapid mix should also be considered. Qi etal (1994) suggested that efficient use of PAC can beachieved using an upflow floc-blanket reactor or aPAC/UF process. To discuss the best point(s) for PACaddition, a hypothetical treatment train with four poten-tial PAC application points (Fig. 6) is used for the fol-lowing discussions. Note that a conventional drinking-water treatment plant may not contain some of thereactors.

Application of PAC at the river intake may providelong contact time and good mixing. However, some or-

ganic compounds may be adsorbed that would otherwiseprobably be removed by the treatment processes followedup (Najm et al, 1991a), and thus the PAC usage rate in-creases. In addition, some treatment plants apply a poly-mer at the river station at the same time PAC would beapplied. Addition of the polymer would cause the PACto drop out of solution on the way to the rapid mix basin.

PAC application point before the filter is not recom-

mended, because the proximity to the filtration processwould likely lead to breakthrough of the filter beds. Inaddition, PAC application will increase CI2 demand. Fig-ure 7 shows chlorine demand resulting from PAC addi-tion, indicating that the impact of PAC on chlorine de-mand is very obvious, and depends on PAC dosages.

The advantage of PAC application point #1 is the com-

bined retention time (about 60 min) resulting from theCSTR in series with the flocculation basin. A CSTR typ-

-o 3

<D 2

O1 1

8 12PAC dosage (mg/L)

16

FIG. 7. Chlorine demand resulting from PAC addition.


ically has a 30-min retention time, while a rapid mixingtank (point #2) may have a contact time less than a fewminutes. Higher mixing energies could be achievedthrough this arrangement, resulting in higher atrazine re-

moval (Fig. 2). The carbon is completely mixed with thewater in the CSTR before the water flows to the rapidmixing basin, where it is coagulated, and then to the floc-culation and sedimentation basins.

Another advantage of the PAC application point #1 isthe distance in the treatment train from the first CI2 ap-plication point. The discovery that trihalomethanes(THMs) can be formed during drinking-water treatment

(Rook, 1974) has caused major concern due to the po-tential health hazard associated with these compounds.An increase in chlorine demand results in higher THMproduction. For this reason the proximity of the PAC ap-plication to the chlorination application is very impor-tant. To minimize chlorine demand as a result of PACapplication, the PAC should be maintained as far fromthe CI2 application point #1 as possible. Therefore, PACapplication point #1 is advantageous over PAC point #2.However, if the treatment plant does not have a CSTR,PAC application point #2 can also be used.If a treatment plant does not have a CSTR prior to the

rapid mixing basin but has upflow floc-blanket reactors,as FTP (see Case Study), point #4 may be the best loca-tion for PAC application due to the following reasons.

(1) Based on the results obtained in this study, a syner-gistic relationship was demonstrated between PAC andenhanced coagulation (with a pH adjusting to around 5.8),and the best atrazine removal (greater than 60%, Fig. 5)could be achieved. (2) The maximum adsorptive capac-ity of the carbon in the floc-blanket reactor would beachieved due to a longer carbon resident time (e.g., 9 hto several days, Najm et al, 1991a; Qi et al, 1994) com-

pared to 30 to 60 min carbon residence times in the pre-sedimentation basins. Mixing energy should be as highas the floc-blanket will tolerate (Fig. 2) to obtain thehighest atrazine removal. (3) PAC adsorption of atrazineor other organic compounds can be enhanced through a

pH adjustment. (4) The need for a major capital expen-diture required to build a PAC contact facility (e.g.,rapid mix contactor, CSTR, etc.) can be avoided. (5)There is a potential to regulate the amount of water incontact with PAC by adjusting the split in the upflowfloc-blanket reactors (see Case Study). However, more

studies are needed to evaluate whether point #1 is bet-ter than point #4.

For a treatment plant that has no CSTR reactor and no

upflow floc-blanket reactor followed by a settling basin,points #1 and #4 do not exist, and point #2 will be thebest PAC application point. Application of PAC prior tothe rapid mix basin would have the advantage of addi-

tional contact time, and the rapid mix process would en-

courage more contact between the atrazine and the PAC.Because coagulant is added in the conventional treatmentscheme at the rapid mix chamber, the opportunity for en-hanced coagulation (adjustment of the pH to 5.6 to 6.8)may exist.

Case StudyThe results obtained from the aforementioned tests can

be applied to analyze the best PAC application point(s)at FTP. The FTP is a split treatment lime-softening/alumcoagulation facility (Fig. 8). Cationic polymer coagulantis adding to the river intake. Water then enters pre-sedi-mentation tanks with an HRT of 18 to 24 h. The mixingbasins are baffled basins without rapid mixing function.The basin #6 is for contact and settling. The mixing basinsand basin #6 were built for the old treatment plant be-fore the primaries were built, and do not function specif-ically. Water leaves basins #6 and is divided into the fourprimaries (four upflow floc-blanket reactors). Primaries#1 and #4 are the alum coagulation basins, while pri-maries #2 and #3 are lime softening. The water is thenrecombined in a center flume and enters settling basin #3for further blending. The "D" basin, a baffled tank, isused for adding anionic polymer for the second stage ofcoagulation during runoff time. Basins #2 and #1 are forsecondary sedimentation and stabilization of alkalinity.The filters and clearwells are conventional.

Four potential PAC application points exist: (1) mix-ing basin, (2) "D" basin, (3) the pre-sedimentation basins,and (4) the upflow floc-blanket reactors upflow basins(primaries). The current application point for PAC is thepre-sedimentation basins.

Based on results of this study, the mixing basin is notrecommended as a PAC application point due to the mix-ing energy not being high enough to keep the PAC sus-

pended. The "D" basin is not recommended for a PACaddition because carbon applied at this location eventu-ally breaks through the filter beds and can be detected inthe clear well (Fig. 8).

Comparison of jar tests results shows that the bestatrazine removal at pre-sedimentation tanks can be about50%, while that at upflow floc-blanket reactors can behigher than 60%, with a PAC dosage of 16 mg/L and a

pH adjustment to 5.8. Therefore, for FTP, the upflowfloc-blanket reactor would be the best PAC applicationpoint.

More studies are needed if the upflow floc-blanket re-actor with chlorine addition at its effluent is used as aPAC application point. Currently, it is not very clear howmuch PAC will escape for the effluent of the upflow floc-blanket reactor. In FTP, additional jar tests and a field-


430 ZHANG AND EMARY

CI. Addition

PolymerAlum

RiverWater

IntakeScreens

LowServicePumps

PAC

MixingBasins

Basin # 6Contact &Settling

PAC

CI. Addition

Basin tt 3H Blending TTÎ1

PAC

D' BasinMixing

Basin # 2Settling

Basin # ISettling

Filters

Fluoride

Basin # 1Storage

(Clearwell)rn

PumpStation

PumpStation

Soda Ash &Chlorine

To DistributionSvstem

FIG. 8. Schematic of the Florence Treatment Plant. Three potential PAC application points shown in the figure. In this study,it was hypothesized that primary basins 1 and 4 (upflow floc-blanket reactors) would be a better PAC application point.

scale test conducted in the upflow floc-blanket reactorsshowed that some PAC did escape from the reactors. Ox-idizing agents (such as CI2) can greatly reduce the ad-sorbing potential of PAC and increase the disinfectantdemand. If the oxidizing disinfectant is chlorine, tri-halomethane production may be increased, risking thenoncompliance of another U.S. EPA regulation (FederalRegister, 1979). Therefore, the location of chlorine ap-plication point after the upflow floc-blanket reactor atFTP may need to be moved.

SUMMARY

The results obtained in this study demonstrate that jartests can be used to evaluate PAC performance foratrazine removal. The efficiency of PAC application foratrazine removal depends on many factors. In treatmentfacilities, the adsorption of atrazine with PAC is usuallya dynamic process, with an atrazine removal less than theremoval achieved at equilibrium, due to the short reten-tion time. Specifically, this study demonstrates that mix-ing energy is a major factor affecting atrazine adsorption.Traditional studies on PAC performance for adsorption

of organic compounds using batch or CSTRs did not fo-cus on mixing energy.

Based on jar tests and response surface technology, itwas found that (1) lime-softening processes alone willnot enhance atrazine removal; (2) alum coagulant aloneor enhanced alum coagulation without the addition ofPAC will result in a minimal increase of atrazine removal(less than 5%); and (3) a synergistic relationship appearsto exist between PAC dosage and pH adjustment occur-ring with enhanced coagulation; neither PAC nor en-

hanced coagulation (with a pH adjusted to 5.8) resultedin as high a removal rate of atrazine as the two did to-

gether (greater than 60%). Therefore, the two most im-portant variables impacting atrazine removal are PACand enhanced coagulation (with a pH around 5.8).

Selection of the best point for PAC application dependson treatment trains and chlorination points. For a con-

ventional treatment plant with a CSTR or rapid mixingtank before flocculation and sedimentation basins, appli-cation of PAC to the CSTR or rapid mixing would be a

good selection. For a treatment plant with upflow floc-blanket reactors, as in the Florence Treatment Plant, thebest point for PAC application could be at the upflowfloc-blanket reactors. In addition, the distance that a PAC


application point is upstream from a chlorine applicationpoint should be maximized to avoid an increase in chlo-rine dosage.

ACKNOWLEDGMENT

The authors acknowledge the following people fortheir assistance in this study: Li Li, and Jim and PattyTrebbien for providing computer assistance and statis-tical calculation; Drs. J. Stansbury, M. Moussavi, andMs. A. Blakey of the University of Nebraska-Lincolnfor their useful suggestions and comments about theproject and the manuscript. The authors appreciate theFlorence Treatment Plant for providing laboratoryequipment and chemicals, and the National ScienceFoundation/Experimental Program to Stimulate Com-petitive Research (NSF/EPSCoR) for partially fundingthe project.

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Corresponding author:Tian Cheng Zhang

Department of Civil Engineering205D PKI

University of Nebraska-Lincoln at Omaha CampusOmaha, NE 68182-0176

phone: 402-554-3784fax: 402-554-3288

E-mail: [email protected]

jar tests for evaluation of atrazine removal at drinking water treatment plants

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