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DBP formation in breakpoint chlorination of wastewater

Water Research 39(2005)4755–4767

Xin Yang,Chii Shang ?,Ju-Chang Huang

Department of Civil Engineering,The Hong Kong University of Science and Technology,Clear Water Bay,Kowloon,Hong Kong

Received 11April 2005;accepted 2August 2005

Abstract

The formation of trihalomethanes (THMs)and haloacetic acids (HAAs),two major disinfection by-products (DBPs),from the breakpoint chlorination of three diluted yet buffered (pH 7.0)wastewater ef?uents was studied.The concentrations and distributions of THMs and HAAs species varied among different ef?uents at different zones of the breakpoint curves.Nevertheless,some common trends were observed.The formation of chloro-only THMs and HAAs,after normalization with the carbon contents of the ef?uents,increased with increasing the speci?c UV absorbance (SUVA)of the ef?uents but the dependency is not valid for bromo-or bromochloro-DBPs.The formation of THMs and HAAs showed no signi?cant inclination with increasing chlorine dosages up to the breakpoint,but increased sharply beyond the breakpoint dosing level.Bromine incorporations into THMs and HAAs increased with an increasing bromide to DOC molar ratio.In addition,the bromine incorporation was also found to be highly dependent on the chlorine dosage and the bromide to ammonia ratio.A longer reaction time increased the yields of THMs and HAAs and was found to favor the formation of dihalogenated HAAs.A two-stage correlation between the total THMs and the total HAAs was found for each wastewater ef?uent.r 2005Elsevier Ltd.All rights reserved.

Keywords:Breakpoint;By-products;Haloacetic acids (HAAs);Trihalomethanes (THMs);Bromide

1.Introduction

Chlorination is by far the most common method for disinfecting wastewater ef?uents before their discharge into receiving streams,rivers or oceans.Chlorination is still the predominant disinfection method due to its well-established practices and the broad-spectrum germicidal potency and low cost of chlorine.Chlorine reacts readily with a wide variety of organics,however,to form disinfection by-products (DBPs),of which trihalo-methanes (THMs)and haloacetic acids (HAAs)are the two major groups.THMs include four species:chloro-form (CHCl 3),bromodichloromethane (CHCl 2Br),di-bromochloromethane (CHClBr 2)and bromoform (CHBr 3).There are a total of nine chlorine and/or bromine containing HAA species:chloro-,dichloro-,and trichloroacetic acid (MCAA,DCAA and TCAA);bromo-,dibromo-,and tribromoacetic acid (MBAA,DBAA and TBAA);bromochloro-,bromodichloro-,and dibromochloroacetic acid (BCAA,BDCAA and DBCAA).Some of these DBPs are carcinogenic (Bryant et al.,1992)and,hence,strictly regulated.The current maximum allowable running annual average (RAA)concentrations of total THM (TTHM)and HAA5(the sum of the MCAA,MBAA,DCAA,TCAA,and DBAA)are 80and 60m g/L,respectively in the US Stage 1Disinfectants/Disinfection Byproducts Rule (D/DBPR)(USEPA,1998).Since some of these compounds (such as dihaloacetic acids (DXAA)and bromine-containing THMs and HAAs)pose a greater health

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concern than the others(Bull and Kop?er,1991;Bull and de Angelo,1995),ongoing research into THMs and HAAs is of particular interest(USEPA,1998).

In water supplies,the formation of HAAs and THMs upon chlorination has been found to be affected by many factors such as chlorine dosage,pH,ammonia nitrogen,bromide ion concentration,and contact time (Cowman and Singer,1996;Carlson and Hardy,1998). In breakpoint chlorination of natural water,the forma-tion and speciation of THM along the breakpoint curve are highly dependent on both bromide and ammonia concentrations(Luong et al.,1982;Amy et al.,1984; Rebhun et al.,1987;Duong et al.,1988).Yet,the formation of these DBPs during wastewater chlorination has so far received much less attention.Recently,Qi et al.(2004)studied HAA formation in the chlorination of wastewater within the monochloramination range and found that the HAA formation was affected by the monochloramination approaches,the wastewater qual-ity,and the treatment processes involved.However,the formation of THMs and HAAs along the breakpoint curves during wastewater chlorination still remains unclear.It is expected that chlorination of wastewater involves many reactions due to the presence of various chlorine-reacting species such as ammonia,organic carbon,organic nitrogen,and bromide at substantially high concentrations.These interactions may lead to or interfere with the formation of DBPs(White,1999).In addition,the compositions of the DBP precursors in wastewater are also expected to differ from those found in typical potable water,and this has a considerable effect on the DBP formation.Since there is no information available so far,investigation of the DBP formation during chlorination of wastewater is needed. Understanding the formation of DBPs in wastewater chlorination is important since many treated ef?uents are indirectly reclaimed as part of the water supply source downstream.

The objectives of this study are,therefore,to investigate the formation of THMs and HAAs and their distributions along the breakpoint curves during chlorination of three wastewater ef?uent obtained from different sewage treatment works in Hong Kong.The roles of bromide,contact time,and wastewater char-acteristics on the DBP formation were investigated and the correlations between the formation of THMs and HAAs were established.

2.Materials and methods

2.1.Chemicals

All chemical solutions were prepared from reagent-grade chemicals or stock solutions.Dilution to target aqueous concentrations was accomplished with double distilled,deionized(DDDI)water.Solutions were stored at41C and brought to room temperature before use.A free chlorine(HOCl)stock solution(2500mg/L)was prepared from5%sodium hypochlorite(NaOCl)(from Allied Signal)and it was periodically standardized using DPD/FAS titration(Standard Methods,1998).

2.2.Wastewater sampling and characterization Undisinfected wastewater ef?uent samples were col-lected from the Shek Wu Hui(SWH)Sewage Treatment Works(STWs),the Sha Tin(ST)STWs and the Stonecutters Island(SI)STWs in Hong Kong,which represent a fresh water nitri?ed secondary ef?uent,a saline nitri?ed secondary ef?uent,and a saline chemi-cally enhanced primary treatment(CEPT)ef?uent, respectively.Samples were carefully collected and transferred to the laboratory in an ice cooler and stored at41C to minimize changes in the constituents.They were brought to room temperature in a water bath prior to examination.Dissolved organic carbon(DOC), ammonia and bromide ion concentrations of each sample were measured with a TOC analyzer(Shimadzu TOC5000A),a?ow injection analyzer(QuickChem FIA+,8000Series),and an ionic chromatograph (Dionex DX500)equipped with an anionic column (Dionex ICE-ASI),respectively.UV absorbance was measured with a UV–vis spectrophotometer(MultiSpec-1501,Shimadzu)at254nm with a10mm quartz cuvette. The wastewater ef?uent samples were further diluted with a phosphate buffer solution(0.01M)at a1:1.5 volume ratio to buffer the pH at7.

2.3.Chlorination of wastewater

Chlorination experiments were carried out in a well-mixed batch reactor at pH7.Each test run was initiated by adding a pre-set chlorine dosage to the test solution. The mixed solution was quickly portioned into two65-mL aluminum-foil-covered glass vials,which were later sealed without head space.The vials were kept at room temperature(21711C)in the dark.Samples were withdrawn from the vials after2and24h,dechlorinated and extracted for HAA and THM analyses.In addition, samples were withdrawn and subjected to chlorine residual determination using the DPD/FAS titration (Standard Methods,1998).The extracted samples,if not examined immediately after extraction,were stored at à101C for a period of less than three days before measurements were taken.

2.4.THM and HAA measurements

The concentrations of THMs and HAAs were determined using a gas chromatograph(Finnigan Trace GC)with an electron capture detector(ECD)by

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USEPA method551.1and552.2(USEPA,1995a,b), respectively,with minor modi?cations of the GC column and the temperature ramping program.A DB-5MS fused silica capillary column(30m?0.25mm I.D. with0.25m m?lm thickness;J&W Scienti?c)was used for the analysis.THMs and HAAs were measured under the following temperature program:(1)THM:hold at 351C for5min,ramp to701C at101C/min,hold 3.5min,then ramp to1801C at201C/min and hold for 2min;(2)HAA:351C for10min,ramp to701C at51C/ min,hold for10min,ramp to1001C at51C/min,hold for5min,then ramp to1351C at51C/min and hold for 10min.The speciation and quantities of the formed THMs and HAAs were obtained by comparing the chromatograms of the samples with the calibration curves developed with internal standards for THMs and HAAs(from Supelco).

3.Results and discussion

3.1.Breakpoint chlorination

The wastewater samples from the SWH STWs,ST STWs and SI STWs were?rst characterized by measuring the ammonia,organic carbon,UV254,and bromide concentrations.Speci?c UV absorbance (SUVA,UV254/DOC),which was a common indication parameter of the aromatic contents of the samples,was also calculated.The result showed a large variation in water quality with respect to these constituents as displayed in Table1.This was due to different treatment schemes and the use of seawater for toilet?ushing in some areas covered by the treatment plants.In general, the wastewater samples from the SWH and ST STWs, which employed secondary treatment,showed similar DOC and ammonia concentrations.The bromide concentration,however,varied due to the use of seawater for toilet?ushing in the service area of the ST STWs while no seawater was used in the area covered by the SWH STWs.In addition to a high bromide concentration,the samples from the SI STWs showed much higher concentrations of ammonia and DOC since no secondary treatment was provided.However,it should be noted that the SUVA value of the sample from the SI STWs was much lower than that of the samples from SWH and ST STWs.A signi?cant difference in the SUVA values of the samples from SWH and ST STWs was also observed,although the treatment processes involved in and the DOC concen-trations of these two treatment works were similar.The difference may be attributable to the composition of the wastewater from the SWH STWs,which treats not only municipal wastewater but also the land?ll leachate and wastewater from slaughterhouses.As a result,the breakpoint chlorine dosages and the patterns of the breakpoint curves are varied among the samples from the three STWs(Fig.1).The chlorine dosages to achieve 2-and24hour breakpoint chlorination of the waste-water samples collected from the SWH,ST and SI STWs were found to be9.5,16,and71mg/L,respectively.The breakpoint curves of SWH STWs and SI STWs displayed a typical curved shape,but that of ST STWs showed distinctly different features.It is worth noting that free bromine or bromamines potentially formed from oxidizing bromide ions may interfere with the measurement of chlorine residuals using DPD/FAS titration method(Standard Methods,1998).Therefore, residuals shown in Fig.1are the‘‘apparent chlorine residuals’’.

3.2.TTHM and THAA formation

Large variations in the patterns and the yields of TTHM and total HAAs(THAA)per mmole DOC were observed when the different wastewater ef?uents were chlorinated for2-hour(Fig.1).This may be attributable to a combined effect of competition among haloamination,oxidation of bromide to bromine,and halogenation/haloamination of precursors,dominating

Table1

Characteristics of the wastewater ef?uent samples from three different treatment schemes

Treatment scheme SWH STWs ST STWs SI STWs

Secondary+nitri?cation Secondary+nitri?cation Enhanced primary Seawater blend-in No E25%E25%

NH4+(mg-N/L) 2.5 2.821.2

DOC(mg-C/L)7.07.247.2

UV254(cmà1)0.1750.1120.174

SUVA(L mgà1mà1) 2.48 1.560.37

Brà(mg/L)0.9622.031.5

Note:In the chlorination experiments,the wastewater samples were further diluted with phosphate buffer(0.01M,pH7)at a1:1.5 volume ratio.

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T T H M , T H A A (μm o l D B P /m m o l D O C )

0.0

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0.2

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0.40.5

0.6

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C h l o r i n e r e s i d u a l s (m g /L a s C l 2)

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Chlorine dosage (mg/L as Cl 2)

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Fig.1.2-hour breakpoint chlorination curves and TTHM and THAA formation curves.

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differently at various sections of the breakpoint curves (Rebhun et al.,1987).The actual products appeared to depend on the relative rates of the respective reactions and this is discussed in more detail in later sections.

In general,at a same concentration and form of chorine residuals(either free or combined chlorine), samples of the SWH and ST STWs with higher SUVA values produced higher yields of TTHM and THAA than that of the SI STWs with a much lower SUVA value.However,the dependency between DBP yields and SUVA values cannot be held when we compare the data obtained with the ef?uent samples of the SWH and ST STWs.The discrepancy can be attributable to the large difference in the bromide concentrations of the two samples.The high bromide concentration of the sample of the ST STWs signi?ed the effect of bromide on the DBP formation to rapidly form more bromine-contain-ing species(refer to later discussion in Sections3.3and 3.4)thereby weakening the SUVA versus TTHM and THAA dependencies among these two samples. Although the concentrations of TTHM and THAA and their distribution varied among the wastewater samples from different sources,general trends were observed along the breakpoint curves.Before the breakpoint,both TTHM and THAA showed slight or moderate increases with increasing chlorine dosages. Once the chlorine dosages reached or slightly exceeded the breakpoint dosage,substantial increases in the concentrations of TTHM and THAA were observed. In addition,the relative distribution of TTHM and THAA showed different patterns for different ef?uent samples.In the cases of chlorinating the wastewater ef?uents obtained from the ST and SI STWs,TTHM levels exceeded THAA levels.This was in agreement with the general trend in potable water chlorination reported in the literature(Roberts et al.,2002;Villa-nueva et al.,2003).Nevertheless,it is not applicable to the SWH STWs at chloramination dosages where THAA levels exceed TTHM levels.Occasionally, THAA has also been observed at a concentration higher than TTHM(Singer et al.,1995;Arora et al.,1997). Comparing their formation at2-and24-hour reaction time,similar trends were observed for both TTHM and THAA,but their yields were enhanced at a prolonged reaction time(data not shown).

3.3.HAA concentration and speciation

Fig.2illustrates the formation of different HAA species after2-hour chlorination of wastewater samples from the three STWs at various chlorine dosages.The arrows in the abscissa represent the breakpoint dosages. As shown in Fig.2(a)representing HAA formation from the SWH STWs samples,?ve HAA species including DCAA,TCAA,DBAA,BCAA and BDCAA were detected.Among them,TCAA was the dominant species at all the chlorine dosages and occupied about50%of the sum of the molar concentrations of the HAA species. Its yield slowly increased at low chlorine dosages but the rate of increase in the yield was considerably high at chlorine dosages beyond the breakpoint.Due to the low bromide concentration(0.96mg/L)in the SWH STWs ef?uent,brominated species,such as DBAA,BCAA and BDCAA,occupied only a small fraction of the total HAAs and their yields remained relatively constant at all tested chlorine dosages.It was interesting to note that TCAA was the dominant species even at low chlorine dosages,since it is generally believed that the formation of trihalogenated HAAs(TXAA)is greatly suppressed either by adding monochloramine or after converting free chlorine to monochloramine(Cowman and Singer, 1996;Qi et al.,2004;Hwang et al.,2000).The difference may be again attributable to the distinct composition of the wastewater from the SWH STWs,which treats also the land?ll leachate and wastewater from slaughter-houses.As such,the precursors contributing to the TCAA formation are likely to be different from those in most water.Therefore,the general rule of the suppres-sion of TCAA formation by chloramination may not be true in all cases.Whether this is applicable to wastewater chlorination needs to be further veri?ed,however. Fig.2(b)illustrates the formation of HAA species from the chlorination of wastewater samples from the ST STWs after2hours at various chlorine dosages. DCAA,TCAA,DBAA,TBAA and BCAA were detected at chlorine dosages below15mg/L as Cl2.In addition to these?ve species,BDCAA and DBCAA were detected at high chlorine dosages(X15mg/L as Cl2).Before the breakpoint dosage(16mg/L as Cl2),the concentrations of all the species showed little change with increasing chlorine dosages.DBAA was the predominant species due to the high bromide concentra-tion in the samples.The concentration of TCAA was also considerably high,which again contradicted the ?ndings in the literature discussed earlier.When the chlorine dosages just exceeded the breakpoint dosage, the yields of most species(except DCAA and BCAA) showed a considerable increase,with DBAA,TBAA and DBCAA showing higher increases than other HAA species.With further increases of chlorine dosages beyond20mg/L as Cl2,DBCAA concentration con-tinued to increase,but DBAA and TBAA concentra-tions decreased.This result indicates that the forma-tion of these HAA species depends on a chlorine-to-bromide ratio.This can be explained by the chlor-ine–ammonia–bromide chemistry.At low chlorine dosages(before the breakpoint),free chlorine preferen-tially reacts with ammonia to form monochloramine, since the second-order reaction rate constant of ammo-nia and chlorine[k ammonia2chlorine?2:9?106Mà1sà1 (Margerum et al.,1978)]is around three orders of magnitude larger than that of bromide and chlorine

X.Yang et al./Water Research39(2005)4755–47674759

C o n c e n t r a t i o n (μm o l

D B P /m m o l D O C )

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0.25

0.30

C o n c e n t r a t i o n (μm o l

D B P /m m o l D O C )

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0.10

0.15

(a)(b)(c)

Chlorine dosage (mg/L as Cl 2)

C o n c e n t r a t i o n (μm o l

D B P /m m o l D O C )

0.00

0.02

0.04

0.06

Fig.2.HAA formation from breakpoint chlorination of wastewater from the three STWs after a 2-hour contact time (black arrows show the breakpoint dosages).

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[k bromide2chlorine?3:8?103Mà1sà1(Rebhun et al., 1987)].Also,monochloramine can further react with bromide to form bromochloramine and bromamines (Bousher et al.,1989).The reactions of chloramines and bromamines with organic matter yielded small quanti-ties of the bromo-and/or chloro-HAAs in our tests.At chlorine dosages just exceeding the breakpoint,it was likely that the available free chlorine primarily reacted with bromide ions thereby yielding the free bromine residuals.At chlorine dosages much higher than the breakpoint level(420mg/L as Cl2in this case), however,both free chlorine and free bromine residuals were expected to be in equilibrium,thereby favoring the formation of bromo–chloro species rather than the bromo-only species.

Fig.2(c)illustrates the formation of HAA species from the chlorination of wastewater samples from the SI STWs after2hours at various chlorine dosages.Seven of the nine HAA species excluding MBAA and MCAA were detected in most of the cases.Among those,TCAA and DCAA were the predominant species at chlorine dosages below the breakpoint.The yields of the individual HAA species generally increased with in-creasing chlorine dosage notably after the breakpoint and the increases were more signi?cant for TXAA.This observation was in agreement with the suggestion that free chlorine enhanced the formation of TXAA (Cowman and Singer,1996).

Comparing the data among Figs.2(a)–2(c)where ef?uent samples with different SUVA values were chlorinated,it can be found that the formation of chloro-only HAAs,after normalization with the carbon content,increases with increasing the SUVA values of the ef?uents.However,the correspondence is not held for bromo-and bromochloro-DBPs.It can be explained by the DBP formation chemistry involved.It has been well established in the literature that both SUVA values and bromide concentrations affect the formation of THMs and HAAs in drinking water chlorination (Chellam and Krasner,2001;Croue et al.,2000).The predominance between these two parameters therefore depends on the relative importance of these two factors. For the formation of chloro-only HAAs under the tested conditions,although it is expected that the presence of bromide ions will reduce the formation by forming bromine-containing species,it is likely that the SUVA values are yet the dominating factor.On the other hand, the difference in bromide concentrations overrides the difference in the SUVA values to govern the formation of bromo-and bromochloro-HAAs under the tested conditions.Therefore,the correlations between the SUVA values and the formation of bromine-containing HAAs no longer exist.This?nding suggests that the role of bromide ion may not be signi?cant in the formation of chloro-only HAAs but shall be carefully examined in assessing the formation of bromine-containing HAAs.

Regulations concerning HAAs for chlorination of wastewater have not been promulgated,though HAA5 (including MCAA,DCAA,TCAA,MBAA and DBAA) are regulated in the D/DBP Rule and HAA6(including HAA5and BCAA)must be reported in the ICR for drinking water(USEPA,1998;USEPA,1996).It is evident from the result of this study that the monitoring of only HAA5or HAA6is not good enough for all types of wastewater chlorination.Before the breakpoint,the concentration of HAA6in this study contributed to 70–90%(by mass)of HAA9(including HAA6,DBCAA, DCBAA and TBAA)of each wastewater sample.Thus, monitoring HAA6should be fairly acceptable as long as the chlorine dosages remain below the breakpoint dosage. After the breakpoint,however,the sum concentration of DBCAA,DCBAA and TBAA occupied30–60%(by mass)of that of HAA9in chlorinating wastewater containing high concentrations of bromide ions(ef?uent samples of ST and SI STWs).In this situation,regulation and monitoring of nine HAA species becomes necessary.

A longer contact time enhanced the yields of each HAA species,but did not affect the formation trend of the species along the breakpoint.The extent of enhancement was dependent on the chlorine dosages. Before the breakpoint,the enhancement was not signi?cant,but after the breakpoint,there was a considerable increase in the yields of HAAs.In addition, it was found that DXAA occupied greater fractions in total HAAs(see Fig.3)and the TXAA fractions became smaller with longer contact time.This observation is in agreement with the?ndings of Qi et al.(2004)wherein the formation of TCAA(the only TXAA species)was relatively constant but DXAA showed a gradual increase in its concentration when the contact time increased from1-minute to5-hour in chloraminating wastewater.In light of the concept of‘‘fast formers’’(organics reacting quickly with halogen)and‘‘slow formers’’(organics reacting slowly with halogen)pro-posed by Reckhow and Singer(1984),the results of the current study suggest that the fast formers and slow formers may preferentially form TXAA and DXAA, respectively,in wastewater chlorination.

High concentrations of bromide ions tend to shift HAA species to bromine-containing ones.This can be clearly demonstrated by comparing HAA formation in wastewater samples from the ST STWs and SWH STWs.In these two wastewater samples,the concentra-tions of ammonia nitrogen and DOC were comparable, but the concentration of bromide ions was greatly different.The brominated HAA species dominated in samples from the ST STWs,while the chlorinated species,such as TCAA,were the largest contributor to the total HAA yields for samples from the SWH STWs. To access bromine substitution in HAA quantita-tively,bromine incorporation factor n0(Br)is often used. It can be calculated from the following equation

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M o l e f r a c t i o n o f D X A A

0.0

0.2

0.4

0.6

0.8

1.0

(a)0.0

0.2

0.4

0.6

0.8

(b)(c)Chlorine dosage (mg/L as Cl 2)

0204060

100

0.0

0.2

0.4

0.6

0.8

Fig.3.Mole fraction of DXAA into total HAAs as functions of chlorine dosages.

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(Shukairy et al.,1994):

where the concentration of each HAA species is expressed in m mol/L and THAA refers to the total micromolar concentration of HAAs.An increase in the n 0(Br)value indicates the formation of more bromine-substituted species.

Fig.4displays the bromine incorporation into HAAs as functions of chlorine dosages in wastewater samples from the three STWs after 2-and 24-hour chlorination.As shown,n 0(Br)was the highest for samples from the ST STWs and the lowest for samples from the SWH STWs.This was correlated with the Br à/DOC ratios of 458,100and 20m M/mM for samples from the ST STWs,SI STWs and SWH STWs,respectively.The result was in agreement with a study by Chellam and Krasner (2001)in which an increase in n 0(Br)with an increasing Br à/DOC ratio was shown.The bromine incorporation into HAAs as functions of chlorine dosages and contact time showed different patterns for wastewater samples from different sources.In general,the n 0(Br)increased with increasing chlorine dosages for samples from the ST STWs,while it decreased with increasing chlorine dosages for samples from the SWH STWs.The n 0(Br)remained relatively constant at chlorine dosages below the breakpoint but sharply increased after the break-point for samples from the SI STWs.This can be explained by the variation in the bromide to ammonia (Br à/NH 3)ratio.The lowest Br à/NH 3ratio (0.4)for the samples from SWH STWs limited the available bromide ions to react with chlorine but allowed the added chlorine to remain in the form of free chlorine or react with ammonia to form chloramines.Increasing the

residuals in either form by adding more chlorine led to the formation of chloro-species thereby decreasing the n 0(Br)values.Conversely,at the highest Br à/NH 3ratio (7.9),the bromide ions in the samples from ST STWs outcompeted ammonia (at a low concentration)to react with chlorine.As a result,the n 0(Br)values generally increased with increasing chlorine dosages.At a very high chlorine dosage (420mg/L as Cl 2),however,the n 0(Br)value of the ST STWs only decreased.This was possibly due to high free chlorine residuals at such a high dosage.Longer contact time increased the n 0(Br)values in samples from the ST STWs and SI STWs,but the n 0(Br)values in wastewater from the SWH STWs remained relatively constant.This observation contra-dicted the data reported in Symons et al.(1996),in which n 0(Br)values decreased with increasing reaction time because bromine reacted faster with organic matter than did chlorine.The discrepancy may be due to the different time scale and much higher bromide ion concentrations in the current study.3.4.THM concentration and speciation

Fig.5illustrates the formation of each THM species after 2-hour chlorination of wastewater samples at various dosages.As shown in Fig.5(a),in the case of the SWH STWs,two THM species including CHCl 3and CHCl 2Br were observed before the breakpoint (9.5mg/L as Cl 2),while CHCl 3,CHCl 2Br and CHClBr 2were observed after the breakpoint.Among these,CHCl 3was the most abundant THM species and occupied over 80%of the total molar concentration and its concentra-tion elevated gradually with an increase in the chlorine dosage.CHCl 2Br and CHClBr 2concentrations were low at chlorine dosages below the breakpoint and rose to higher levels after the breakpoint.The formation trends were generally consistent with those for TXAA forma-tion shown in Fig.2(a)and hence,can be explained using the same principles.

Fig.5(b)illustrates the formation of each THM species after 2-hour chlorination of wastewater samples from the ST STWs.All four THM species were detected in this case.CHBr 3occupied a fraction of around 50–60%of the total THM molar concentrations and its concentration elevated gradually with increasing chlor-ine dosage before the breakpoint and showed a signi?cant jump at the breakpoint and a slight decrease thereafter.The other THM species including CHCl 3,CHCl 2Br and CHClBr 2remained at relatively low levels

n 0eBr T?

?MBAA t?BCAA t?BDCAA t2e?DBAA t?DBCAA Tt3?TBAA

THAA

(1)

Chlorine dosage (mg/L as Cl 2)

100

B r o m i n e i n c o r p o r a t i o n i n H A A -- n '(B r )

Fig. 4.Bromine incorporation in HAAs as functions of chlorine dosages.

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C o n c e n t r a t i o n (μm o l

D B P /m m o l D O C )

C o n c e n t r a t i o n (μm o l

D B P /m m o l D O C )

C o n c e n t r a t i o n (μm o l

D B P /m m o l D O C )

0.0

0.1

0.2

0.3

0.4

0.5

0.0

0.1

0.2

0.3

0.4

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Chlorine dosage (mg/L as Cl 2)

0.00

0.02

0.04

0.06

0.08

0.10

Fig.5.THM formation from breakpoint chlorination of wastewater from the three STWs after a 2-hour contact time (black arrows show the breakpoint dosages).

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before the breakpoint.After the breakpoint,CHClBr 2concentration rose to higher levels,while the concentra-tions of CHCl 2Br showed slight changes with chlorine dosage.Again,the trends of the formation generally agreed with those for TXAA formation shown in Fig.2(b).

All four THM species were detected at similar molar concentrations after 2-hour chlorination of wastewater samples from the SI STWs as shown in Fig.5(c).At chlorine dosages exceeding the breakpoint,the yields of most species showed greater increases than those before the breakpoint.THM formation trends agreed with those for TXAA formation shown in Fig.2(c).

Increasing the reaction time also increased the yields of each THM species (data not shown).The extent of enhancement was small before the breakpoint and large after it.

Similar to the HAA formation,the formation of chloroform increased with increasing the SUVA values of the sample but the tendency does not exist between the SUVA values and the formation of bromo-and bromo-chloro-THMs.High concentrations of bromide ions tended to shift THM species to bromine-containing ones.The bromine incorporation into THMs,n (Br),was calculated from the following expression (Symons et al.,1993):n eBr T?

?CHCl 2Br t2?CHClBr 2 t3?CHBr 3

TTHM

(2)

where the concentration of each THM species is expressed in m mol/L and TTHM refers to the total micromolar concentration of THMs.Fig.6shows the bromine incorporation factor for THM for different samples as functions of chlorine dosages at 2-and 24-hour reaction times.

The n (Br)values were the highest for samples from the ST STWs and the lowest for samples from the SWH STWs.Again,this correlated with the Br à/DOC ratio.The bromine incorporation factor was also found to

increase with a longer contact time for most of the samples from the ST STWs and SI https://www.doczj.com/doc/d1533098.html,pared to the n 0(Br),however,the dependence of n (Br)on chlorine dosages and the Br à/NH 3ratios was smaller.A clear jump in only the n (Br)values was observed in the SWH STWs case.In addition,for wastewater samples from the ST STWs and SI STWs,the n (Br)values were larger than the n 0(Br)values,which indicated that the brominated species comprised higher molar fractions in THMs than in HAAs.The n (Br)values were found to be smaller than the n 0(Br)values of wastewater samples from the SWH STWs,however.These ?ndings indicate that the bromine incorporation into THMs and HAAs depends on the characteristics of the precursors in wastewater and the formation mechanisms of THMs and HAAs involving bromide are unlikely to be similar.3.5.DBP relationships

Although the formation mechanisms of THMs and HAAs were unclear,linear regression analyses were performed to evaluate the relationships among the concentrations of THAA,TTHM,and the chlorine consumption.The concentrations of TTHM and THAA here are expressed in mass concentration (m g/L)since the development of the correlations are mainly for prediction and regulation purposes and DBP control in wastewater treatment practices.Chlorine consumption and the concentration of THAA and TTHM did not show good correlation results for each wastewater source (not shown)(0.52o R 2o 0.86),if all the data points were considered.This was because the chlorine residuals were in different forms (either combined chlorine or free chlorine)in different zones of the breakpoint curve,and hence,their effects on the formation of THMs and HAAs were not expected to be the same.

Fig.7shows the correlations between THAA and TTHM yields of wastewater samples from the three STWs.The correlations were established in two stages based on the chlorine dosage below or above the breakpoint dosage.As shown,the correlation coef?-cients at the two stages are high.The results suggest that the concentration of total THMs may be used as a surrogate for predicting the total concentration of HAAs since HAA analysis is more complicated and time-consuming.More investigations are needed to further prove this assertion however,since better correlations can only be achieved if more data are available.

4.Conclusions

The concentrations and distribution of THM and HAA species varied among different wastewater ef?uents

Chlorine dosage (mg/L as Cl 2)

100

B r o m i n e i n c o r p o r a t i o n i n T H M -- n (B r )

0.0

0.5

1.0

1.5

2.0

2.5

3.0

Fig. 6.Bromine incorporation in THMs as functions of chlorine dosages.

X.Yang et al./Water Research 39(2005)4755–4767

4765

and in different zones of the breakpoint curves of the same wastewater source.Despite the differences,com-mon characteristics exist.The formation of chloro-only THMs and HAAs,after normalization with the carbon contents of the samples,increases with increasing the SUVA values of the ef?uents.However,the correspon-dence does not exist for bromo-or bromochloro-DBPs,since the concentration of bromide ions becomes the dominating factor.The formation of THMs and HAAs

increased sharply at chlorine dosages slightly above the breakpoint dosage.

Bromide concentrations had great in?uence on the THM and HAA distribution.Bromine incorporation into THMs and HAAs increased with increasing Br à/DOC molar ratios.The bromide incorporation was also found to be related to the chlorine dosages and the Br à/NH 3ratios.In addition,in the case of the chlorination of bromide-rich wastewaters,the predominant HAAs gradually shifted from DXAA to TXAA at chlorine dosages above the breakpoint.The general rule of suppression of TCAA formation by chloramination may not be always applicable to wastewater chlorination however.A long contact time enhanced the formation of THMs and HAAs and favored the formation of DXAA.Chlorine consumption was found not to be a good surrogate for the formation of total THM and HAA.Nevertheless,a two-stage correlation between total concentrations of HAAs and THMs for each wastewater source suggested that the concentration of total THMs may be used as a surrogate for predicting the concentration of total HAAs.

Acknowledgements

This study was supported in part by the Hong Kong Research Grants Council under Grant HKUST6035/01E.

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