Using ozonation and chloramination to reduce the formation

ELSEVIER Desalination 176 (2005) 229-240

DESALINATION

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Using ozonation and chloramination to reduce the formation of trihalomethanes and haloacetic acids in drinking water

Cynthia Quay a, Manuel Rodriguez b*, Jean S6rodes a

aD~partement de G~nie Civil, Universit~ Laval, Quebec City, Canada b Ecole supdrieure d'amdnagement du territoire et d~veloppement r~gional (ESAD), Universit~ Laval,

Quebec City, G I K- 7P4 Canada, Tel. + 1 (418) 656-2131 ext. 8933; Fax +1 (418) 656-2018: email: [email protected]

Received 18 October 2004; accepted 28 October 2004

Abstract

Disinfection is a key treatment process for producing drinking water. However, it produces undesirable by- products that may cause adverse health effects. Disinfection by-products (DBP) such as trihalomethanes (THMs) and haloacetic acids (HAAs) are considered potentially carcinogenic and have been recently associated with reproduction problems. In the province of Quebec (Canada), the regulation respecting the quality of drinking water (RRQDW) published in June 2001 establishes a maximum average level for total THMs of 80 ~tg/L. This standard is difficult to meet by small municipalities served by surface water and which apply a limited treatment before disinfection. The purpose of this research is to develop a protocol that would lead to the identification of alternative water treatment and disinfection strategies to reduce THMs and HAAs. The case under study is a small utility that currently does not comply with the Quebec RRQDW. This protocol involves pilot studies and laboratory assays that simulate the formation of THMs and HAAs in the distribution system. Different water treatment options were investigated. Those options involve ozone addition followed by slow sand filtration, and the introduction of chloramines and/or chlorine. The results of this research showed that the highest THM and HAA reduction is reached using ozone and chloramines as a disinfection strategy.

Keywords: Drinking water; Trihalomethanes; Haioacetic acids; Chloramination; Ozonation; Chlorination; Pilot studies; Distribution system; Regulatory compliance; Slow sand filtration; Quebec; Canada

*Corresponding author.

Presented at the Seminar in Environmental Science and Technology: Evaluation of Alternative Water Treatment Systems for Obtaining Safe Water Organized by the University of Salerno with support of NATO Science Programme.

September 27, 2004, Fiseiano (SA), Italy.

0011-9164/05/$- See front matter © 2005 ELsevier B,V. All rights reserved

doi: 10.1016/j.desal.2004.10.015

230 C Guay et al. / Desalination 176 (2005) 229-240

1. Introduction

Disinfection is one of the most important processes for producing safe drinking water. Disinfection of drinking water by chlorine has been a strategy commonly applied for consumer protection against waterborne disease in Canada. However, chlorine reacts with natural organic matter (NOM) like humic substances to form disinfection by-products (DBPs) [1,2]. Trihalo- methanes (THMs) and haloacetic acids (HAAs) are the major DBPs detected in chlorinated water. THMs include chloroform (CHCI~), bromo- dichloromethane (CHCI2Br), chlorodibromo- methane (CHBr2CI) and bromoform (CHBr3). HAAs include nine substances, but the principal components are monochloroacetic acid (MCAA), dichloroacetic acid (DCAA), trichloroacetic acid (TCAA), monobromoacetic acid (MBAA), di- bromoacetic acid (DBAA) and bromochloroacetic acid (BCAA). DBPs are regulated because they are considered as potentially carcinogenic [3,4] and recently associated with reproductive problems [5-8].

In the Province of Quebec (Canada), the Regu- lation Respecting the Quality of Drinking Water (RRQDW), published in June 2001 establishes standards for treatment and disinfection of water delivered throughout distribution systems to 20 people or more [9]. The RRQDW establishes a maximum average level for total THMs of 80 gg/L (no standard exists for HAAs in the RRQDW). Utilities must achieve adequate treatment and disinfection to protect the consumers from patho- gens and simultaneously reduce DBP formation. Consequently, several small utilities in Quebec that use surface waters as their water source will have to update their treatment strategy in order to comply with standards of the RRQDW. Chlori- nated DBP precursors can be minimized by oxida- tion (the use of ozone, for example), by conven- tional treatment (coagulation-flocculation-sedi- mentation and filtration, but also using enhanced coagulation), by membrane filtration processes and by activated carbon adsorption [ ! 0]. Recently,

photo-degradation of NOM has been used to reduce DBP formation [ 11 ]. The use of alternative disinfectants that produce little chlorinated DBPs is also an interesting option. Alternative disinfectants are chlorine dioxide, ozone, UV and chloramines [12].

Chloramines have been used in the United States for several years as an alternative to chlorine during secondary disinfection in order to reduce the occurrence of chlorinated DBPs in distribution systems. This disinfectant is not commonly used in the Province of Quebec. The other advantages of chtoramines are that they are stable in the distribution system, efficient for taste and odour control, and they favour biofilm control [13-15]. However, as yet, limited information is available on DBP formation associated with chloramines [16].

The goal of this research project is to assess the use of ozonation in pre-disinfection and the use of chloramines in secondary disinfection as treatment alternatives for reducing chlorinated DBP formation in a small Quebec water utility. The research comprises two steps. First, a portrait of the spatial and temporal occurrence of THMs and HAAs in the distribution system is drawn. Second, pilot studies and laboratory assays that simulate the formation of THMs and HAAs in the distribution system are performed. Pilot studies include variable alternatives of water treatment and disinfection, including ozone followed by slow sand filtration, and the addition of chloramines and/or chlorine.

2. Methodology

The case under study is the water utility delivering water to of the municipalities of Saint- Aubert and Saint-Jean-Port-Joli located in the Bas- Saint-Laurent region (Province of Quebec). The utility supplies water to approximately 3,000 people (produced flow rate of about 1350 mVd). The utility draws its raw water (highly eoloured) from the Trois-Saumons Lake (Fig. 1). Water

C. Guay et al. / Desalination 176 (2005) 22~240 231

l ' ro is-S aunlons Lake

Ch lo r ine disinfect ion Ch lo r ina t i nn l ) is l r i lmtion s~stem protection

I I

Fig. 1. Actual treatment process.

treatment currently consists of slow sand filtration and chlorination for primary disinfection before water storage. Chlorine is also used for post- disinfection (following water storage) ensuring that a detectable level of residual chlorine is main- mined at the extremities of the distribution system. The current water treatment method does, however, produce high THMs and HAAs concentrations in the distribution system.

A field study helped to establish a spatial and temporal portrait of THMs and HAAs in the plant and within the distribution system. The non- chlorinated filtered effluent from the water plant and four sites along the distribution system - - from the entrance to the extremity - - were iden- tified for the study. Various tracer studies based on fluoride and calcium carbonate were carried out to estimate the approximate residence time of water at the following locations: the reservoir outlet (RI) located on the water plant site; the beginning of the distribution system (R2), the middle of the distribution system (R3) and the end of the distribution system (R4). Water residence times from the chlorination point are approximately 10, 18, 60 and 90 h for R1, R2, R3 and R4 points, respectively. A sampling campaign at these sites was carried out between March and June 2003 (six times for each point approximately every three weeks). Campaigns were carried out in March (I), in April (2), in May (1) and in June (2). The measured parameters were pH, tempera-

ture, free and total chlorine, ultraviolet absorbance at 254 nm (UV-254), total organic carbon (TOC), THMs and HAAs.

Pilot studies were performed to compare different treatment alternatives to reduce THM and HAA formation. Two identical pilot units were constructed in PVC (PI: slow sand filtration and P2: ozone followed by slow sand filtration) (Fig. 2). Two distribution boxes set the flow rate at 0.2 m/h. The slow sand filtration pilot unit (Pl) reproduces biological filtration as applied currently at the water plant. The ozone + slow sand filtration pilot unit (P2) ensures colour removal and pre-disinfection before biological filtration. Ozone was applied to obtain a residual concentration at the ozonation contact column and to achieve levels of true colour under 5 TCU. The mean ozone dosage was 3.55 mg/L and the mean residual ozone concentration was 0.58 mg/L. The ozone contact column measured 6 m long and i 5 cm in diameter, the flow rate was 0.6 m3/h and the theoretical contact time was 10 min. For both pilot units, the media used in filters was the same as in the actual filter of the treatment plant. Sampling campaigns for monitoring raw water, filtered water from the treatment plant, ozonated raw water, and effluents of P I and P2 units were carried out two or three times a week from March to July 2003. Water pH, temperature, turbidity, residual ozone, dissolved oxygen and true colour were measured in situ. UV-254 and TOC were measured as organic matter

232 C. Guay et al. /Desalination 176 (2005) 229-240

. . . . . . . . G a s exff

R a w w a t e r II I": 1 1 , . I D~s~rib~lloR bo×

•• i ozonato"-----~" I

0, ; bo~t e 125 m ;~

15 c m

O z o n a t i o n contain;{

columr'~ 3 .3 rn

/ \

O v e r flow

Fig. 2. Pilot filtration units and their dimensions.

• ,4 ~ x>? .¸ '%<~/,.x ~ . ,

Grave~ (30 c m

Ozonated. S l o w s a n d S l o w s a n d f i l t r a t i o n

filtration pilot pilot (P2) (Pq

indicators. Other parameters were measured once a week at the raw water and at the pilot filter effluents: heterotrophic plate count (HPC), total and fecal coliforms, ammoniacal nitrogen (NH 3 + NHj), total Kedjahl nitrogen (TKN), nitrite (NO~) and nitrate (NOj).

Laboratory-scale experiments were conducted to compare different treatment and disinfection scenarios, including the current treatment and disinfection strategy (Fig. 3). The experiments were performed with water samples collected at the plant filter effluent and the pilot unit effluents. DBP formation potential (DBPFP) was evaluated six times for each scenario between March and June 2003. Samples were collected at the same time as samples collected for the field study. For DBPFP protocols for scenarios A, B and E, chlorine dosages were chosen according to the doses applied in the plant. Contact times used in experiments (RT1 to RT4) aimed at representing residence time of water in locations RI to R4. For

each campaign, contact times were adjusted according to the distribution flow rate at the time of sampling. Incubation temperature was adjusted in accordance with the temperature measured in the distribution system at the time of sampling. For scenario A, the optimal chlorine doses were determined from eleven chlorination experiments (from 0 to 5 mg/L). To determine the optimal dosage, free chlorine was used as an indicator. Sodium hypochlorite solution was used for laboratory chlorination. Optimal chlorination doses for the six experiment dates were 1.5, 4.0, 4.0, 3.0, 3.0 and 2.5 mg/L, respectively, and optimal doses for post-chlorination were 0.7, 0.3, 0.6, 0.6, 0.8 and 0.4 mg/L, respectively. These dose levels were also used for scenarios B and E.

The protocol for scenario C used the same chlorination doses, contact time (RT) and water temperature as the protocol for scenarios A, B and E. However, in this case, post-disinfection was ensured by chloramination. For scenario D, only

C. Guay et al. / Desalination 176 (2005) 229-240 233

1 t N 4

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. . . . . ii . . . . . . . . . . . . . . R: i " ~, =

, . % e # a r ~ o ~

0 ZO NA'nO N / !

• . . ~ : . . . . . . . . . . . . . . . . . . . . . . ............. { : ~ ; . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ~ / : : , ~

" .............. # ............. i ~ t Y

. . . . . . . . . . . 1 . . . . . . . . . . . . I . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . RT . . . . . . . . . . . . . . . . . . . . . . . .

Fig. 3. Description of the scenarios under study.

SLOW SAND IFIUIRA'IION

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

I . . . . .

........................... ~:i:,~ ........................... : . . . . . . . . . . . . . . . . . I . . . . . . . . . . . . . . .......................... ~,~ ...................... !

i ................................. ~:i~4 .....................

chloramines were used for disinfection and post- disinfection (same contact time and temperature conditions as the other scenarios). For both scenarios C and D, chioramines dosage was 2.5 mg/L. According to the water temperature measured in the field, incubation temperatures were 4°C for a campaign in March and both campaigns in April (denoted April 1 and April 2), 8°C for the campaign in May and the first campaign in June (denoted June 1 ) and 14°C for the second campaign on June 2 (denoted June 2). For laboratory experiments carried out and for all scenarios, THMs, HAAs, water pH, UV-254, free and/or total chlorine were measured at four different contact times: -10 h, -18 h, -60 h and -90 h. Table 1 presents the analytical methods used for the measured parameters.

3. Results and discussion

3.1. Influence o f water treatment

Table 2 presents the results collected during the monitoring of the water quality parameters conducted during the 4 months of operation of the pilot units. Water quality from pilot unit P1 was similar to that of the water quality produced by the plant. A slight reduction of TOC averaging 0.2 mg/L (6%) was observed between the raw water and the effluent of pilot unit P1. The reduc-

Table 1 Parameters and methods for water quality measurement

Parameters Methods

pH Stand. Meth. 4500-H+ Temperature Stand.Meth. 2550 UV-254 Stand.Meth 5910 TOC Stand. Meth. 5310B True colour Stand. Meth. 2120B Turbidity Stand. Meth. 214A Free chlorine Stand. Meth. 4500-CI-F HPC SM 921513 Fecal coliform SM 9222D Total coliform SM9222B NH3 + NH4 + TECHNICON 98-70 W/A or

QCO19-96 TKN SM 420B or TECHNICON 98-70

W/A or QCOI 8-95 NO2- - NO3- S M 4500 - NO3q-I + or Q C O 2 8 - 9 5

tion of TOC by pilot unit P2 was in average 0.6 mg/L (15%). Similar TOC reductions were observed by Lykins Jr. et al. [17]. Based on the UV-254 measurements, pilot unit P2 demonstrated better removal of DBP precursors than pilot unit PI. On average, UV-254 reductions of 13% and 38% were observed for pilot units P1 and P2, respectively. This study also showed that pilot unit P2 was efficient for removal of colour, turbidity, HPC and coliforms (Table 2).

234 C Guay et al. / Desalination 176 (2005) 229-240

Table 2 Characteristics of raw and treated water quality during the period under study (average values are shown except when indicated)

Parameters Raw water Treated water P 1 ~ P22

pH 7.15 7.25 7.39 7.25 Temperature min-max, °C 0.2-16.0 1.0-15.5 3.0-16.0 3.0-16.0 UV 254 nm, cm ~ 0.138 0.113 0.12 0.085 TOC, mg/L 3.9 3.6 3.7 3.3 True colour, TCU 14 10 10 3 Turbidity, NTU 0.54 0.14 0.2 0.23 HPC, MPN/ml 134 n.a. 105 32 Fecal coliform, MPN/100 ml 4 n.a. I 0 Total coliform, MPN/100 ml 19 n.a. 9 0 NH3 + NH4 ÷, mg/L <0.02 n.a. <0.02 <0.02 TKN, mg/L <1.0 n.a. <1.0 <1.0 NO2-NO3-, mg/L 0.58 n.a 0.29 0.29

~slow sand filtration pilot filter effluent 2ozone-slow sand filtration pilot filter effluent

3.2. Occurrence o f THMs and HAAs

The levels of chlorinated DBPs formed following each scenario varied considerably. How- ever, speciation of THMs and HAAs was com- parable. Non-brominated DBP species dominated in the waters under study. In the actual distribution system and in experimental waters of scenarios A and E, chloroform was the predominant THM compound, it represented from 94% to 98% of total THMs. The second THM species detected was CHBrCI 2, a brominated THM with some toxicological implications [18]. However, levels for this compound were very low: the highest concentration measured during the period under study was 4 ~tg/L (for scenario A). Amongst the species of HAAs, only three were found: TCAA, DCAA and MCAA. TCAA and DCAA were the predominant species. TCAA represented in average 61%, 53%, 63%, 50% 42% and 58% of the HAAs in waters of the distribution system and for scenarios A, B, C, D, E, respectively. DCAA represented in average 38%, 46%, 35%, 50%, 58% and 42% of the HAAs in waters of the distribution system and for scenarios A, B, C, D, E, respectively. Such results are similar to those observed in other Canadian distribution systems [16].

n.a.: non available MPN: most probable number

3.3. Spatio-temporal portrait o f DBPs in the distribution system

Levels of THMs and HAAs were the highest at the extremity of the distribution system (R4), which is the location corresponding to the highest residence time (Fig. 4). This type of portrait was also observed by Williams et al. [18] and by Rodriguez et al. [20]. An interesting result is that about 75% of both THMs and HAAs were formed at the reservoir outlet (RI). Temporal evolution of chlorinated DBPs, chlorine demand, TOC and UV-254 are presented in Fig. 5. For the first five campaigns, HAA concentrations were higher than THM concentrations. However, for the second campaign carried out in June (June 2), THM concentrations were greater than HAA concentrations. This result could be explained by a possible biological degradation of HAAs favoured by relative high water temperatures. Biological degradation of HAAs has been suggested before by others [21,22]. As observed in Fig. 5, TOC, UV-254 and ch lor ine demand were good indicators for chlorinated DBP occurrence. Thus, temporal variations of water quality parameters appear to influence temporal variations of DBP levels.

C. Guay et al. / Desalination 176 (2005) 229-240 235

220 ,~

200

180

1 6 0

1 4 0

1 2 0

100-

~ 8 0

z 60

40.

2 0

0

EITHMs G H A A s

T I

Rt R2 R3 R4

Fig. 4. Spatial evolution o f THMs and HAAs in the distribution system (bar represents average values; standard deviations are also shown).

22O

A 180 _1

160-

120- I "o loo. c

~ 60. I I-- 4o.

2 0

04

r'ITHMs I~IHAAs OTOC • UV-254 I I chtorine demand

e

! ÷

r-I,

,i

March April (1) April (2) May June (1)

• 5.50

- 5.00

4.50

4.00

-3.50

3.00

• 2.50

• 2,00

150

1.00

0.50

- - - 0 . 0 0

June (2)

g ~ A

(a--n

=g g u

Fig. 5. Temporal evolution of THMs and HAAs, TOC, UV-254 and chlorine demand in the distribution system (bar represents average values; standard deviations are also shown).

.3.4. Laboratory-scale study

Laboratory-scale experiments allowed one to estimate the chlorinated DBP reduction associated with the use of ozone before slow sand filtration (comparison between scenarios B and E). For scenarios B and E, chlorine dose and contact time were identical. Pilot unit P2 effluent was used for scenario B, while pilot unit PI effluent was used for scenario E. The use of ozone in scenario B

allowed an average reduction of THMs and HAAs of 44% and 37%, respectively (Fig. 6a). Average THMs in scenario B were lower than the maximum contaminant levels of the RRQDW. However, levels of HAAs remained higher than 60 ~g/L, which is the maximum contaminant level of the United States disinfectants and disinfection by- products stage I rule [23]. Laboratory-scale experiments also allowed one to estimate of chlorinated

236 C. Guay et al. / Desalination 176 (2005) 229-240

2O0

180

160

140

120

100

80

80

40

20

0

(a) E]Scenado B

THMs HAAs

200

180-

160

140 ]

120 ! -J ~ , 100

(b)

8° i 80

40

20

0 ~ .................. THMs

DScenano B D Scenario D

.................................. i L . . . . . . . .

HAAs

Fig. 6. Reduction of THMs and HAAs by (a) ozone treatment and (b) chloramine disinfection (bar represents average values; standard deviations are also shown).

DBP reduction associated with the use ofchloramines instead of chlorine for disinfection and post- disinfection of the ozonated/filtrated effluent (pilot unit P2). To achieve this, chlorinated DBPs formed in scenario B were compared to those formed in scenario D. The use ofchloramines in scenario D allowed an average reduction of THMs and HAAs of 98% and 93%, respectively (Fig. 6b). This con- firms the results recently obtained by Singer et al. [24].

Fig. 7 compares the average concentrations of THMs and HAAs at the contact time corresponding to the extremity ( -90 h) for the different scenarios. Error bars indicate the minimum and the maximum values of these compounds, respectively, measured during the six experiments carried out between March and June 2003. As observed, laboratory-scale experiments (Scenario A) efficiently reproduced the formation o f DBPs in the distribution system (Dist. Sys.). Mean THM con-

22O

20O

180

~ 1 6 0 4 ~ 14o

~ 12O ~

~N 4o. N

E]THMs ~HAAs

Dist. Sys, A B C D E

Fig. 7. Concentrations of THMs and HAAs in the distribution system and for scenarios A to E (bar represents the average of six campaigns and four different residence times; minimum and maximum values are also shown).

C Guay et al. / Desalination 176 (2005) 229-240 237

centrations formed by laboratory-scale experiments were overestimated by 5%, while HAA concentrations were underestimated by 7%. These results suggest that protocols applied in scenarios B, C, D and E reproduce well the chlorinated DBP levels that would be obtained in full-scale scale conditions at the distribution system.

Results suggest that scenario E cannot be considered as an option, because average THMs and HAAs concentrations remained higher than the maximum contaminant levels of RRQDW and the United States disinfectants and disinfection by- products stage I rule. Average THMs and HAAs concentrations were 113 pg/L and 130 ~g/L, respectively. Scenario B would allow compliance with the RRQDW THM standard (mean of 63 ~tg/L). However, average HAAs remained higher than the maximum contaminant level of the United States disinfectants and disinfection by-products stage 1 rule (mean concentration of 93 pg/L). Scenarios C and D are the two treatment/disinfection options that would allow simultaneous compliance with the RRQDW THM standard and HAA maximum contaminant level of the United States disinfectants and disinfection by-products stage I rule. Mean concentrations of THMs for scenarios C and D were 37 and I ~tg/L, respectively, whereas mean concentrations of HAAs were 36 and 8 pg/L, respectively.

3.5. Regulatory implications of the results

As mentioned earlier, sampling campaigns and laboratory-scale experiments were conducted during four months (March to June). For this reason, the mean value of THMs and HAAs calculated for this period cannot be used as an annual average value as required by the RRQDW and the United States Stage I Rule on disinfectants and disinfection by-products. That is, the average values for the period under study cannot be used directly to determine if the scenarios that were investigated comply with the standards. The average

THM and HAA values for the period under study can be considered as being representative of spring only. To overcome to this limitation, two recent databases resulting from recent field research projects conducted in Canada were used [16,20]. These databases allowed for the generation of information about THMs and HAAs in several distribution systems (the database for Rodriguez et al. [20] comprised only THMs). For each of the databases, seasonal values of THMs and HAAs were calculated first. Those mean values were subsequently used to establish the running mean annual value for THMs and HAAs. For each compound, a comparison between the annual mean value and the mean value for spring allowed for a correction factor to be established for this specific season (Table 3). The correction factors calculated for THMs were 1.55 and 1.03 according to the Health Canada database and the Rodriguez et al. database [20], respectively. The correction factor for HAAs was calculated at 1.03 according to the Health Canada database. The correction factors were then applied to the data obtained from the sampling campaigns and the laboratory-scale experiments, as shown in Table 3. In accordance with these results, scenarios A, B and E would not be in compliance with the annual standards for THMs and HAAs of the RRQDW and the United States Stage I Rule for disinfectants and disinfection by-products. Only scenarios C and D would be in compliance with these standards.

The levels of THMs and HAAs in scenario C were, however, very different from scenario D. Fig. 8 compares the formation of THMs and HAAs for each campaign of these two scenarios (for both extended and limited experimental contact times). For all the campaigns conducted in this investigation, scenario D produced fewer DBPs than scenario C. For scenario C, the mean concentration of THMs over the extended experimental contact time was 37 lag/L (range between 20 and 60 lag/L). For scenario D, the mean concentrations of THMs for the extended experimental contact time was 2 tig/L (range between 0


Table 3 THMs and HAAs generated from the laboratory-scale study for spring and estimated annual concentration using databases from Health Canada [16] and Rodriguez et al. [20]

Laboratory scale study Estimated concentration

TI-IMs HAAs THMs (1.03) THMs(1.55) HAAs (I.03) (lag/L) (!agg'L) (lag/L) (lag/L) (t~L)

Distribution system 140 171 144 217 176 Scenario A 150 149 155 233 153 Scenario B 75 ii1 77 116 114 Scenario C 45 41 46 70 42 Scenario D 3 10 3 5 10 Scenario E 132 158 136 205 163

70 IIIRT1 THMs [ ] RT2 THMs r'l RT1 HAAs [ ] RT2 HAAs

6 0

5" 50

~ 40 "T"

~ 30

~ 20 T 1--

1 0

Fig. 8. Levels of THMs and HAAs for Scenarios C and D for all the campaigns; limited (RTI -10 h) and extended experimental contact times (RT2 -90 h) are illustrated.

and 3 lag/L). On average, scenario D allowed a THM reduction of 95% in comparison to scenario C.

For all campaigns, Scenario C (with extended contact time) complied with the United States Stage 1 Rule on disinfectants and disinfection by- products for HAAs. The mean concentration for HAAs for this scenario (extended contact time) was 41 lag/L (range between 12 and 63 ~tg/L). For the scenario D (also for extended contact time), the mean concentration of HAAs was at 9 lag/L (range between 2 and 20 pg/L). On average, Scenario D allowed a HAA reduction of 78% in comparison

to Scenario C. Even if the Quebec RRQDW does not consider standards for HAAs, Scenario D would have to be favoured over all the other scenarios to ensure low concentrations of chlorinated DBP levels in the distribution system.

In the practice, the choice of the type of treatment will not only be based on the capability of minimizing the occurrence of chlorinated DBP and reaching adequate microbial de-activation levels to comply with standards. Other factors such as the infrastructure cost, the operational cost, the potential odour and taste problems must also

C. Guay et al. / Desalination 176 (2005) 22~240 239

Table 4 Advantages and drawbacks of the different scenarios

Scenarios Desinfection Standard compliance Operational Cost Operational Distribution Taste and flexibility difficulty system odour

THM HAA* protection problems

B O3 ÷ SSF +CI2 no no yes medium medium C12 medium C 03 + SSF + CI2 yes yes yes medium medium NH2CI low D 03 + SSF yes yes no high medium NH2C1 low E SSF + CI2 no no no low low CI2 high

* United States Stage I Rule for disinfectants and disinfection by-products; SSF: slow sand filtration

be considered. Table 4 presents some of the critical points to be evaluated in selecting treatments for each scenario studied.

4. Conclusions

Conclusions of this research are as follows: • Concentrations of THMs and HAAs in the dis-

tribution system under study were the highest at the extremity. Laboratory-scale experiments showed similar behaviour of chlorinated DBPs for the highest contact times.

• Two species of THMs (chloroform and CHBrCI2) and three species of HAAs (DCAA, TCAA and MCAA) were detected in the distribution system and during the laboratory-scale experiments.

• Laboratory-scale experiments efficiently reproduced the levels of THMs and HAAs in the distribution system. They allowed one to compare of different treatment/disinfection strategies.

• The laboratory-scale experiments on the pilot unit effluents demonstrated that the use of ozonation prior to sand filtration reduces sig- nificantly the concentrations of THMs and HAAs when chlorine is used as a disinfectant. However, the reduction of these compounds is much higher when chloramines are used as a secondary disinfectant.

• The treatment/disinfection scenario consisting ofozonation/slow sand filtration + chlorine +

chloramines and the scenario consisting of ozonation/slow sand filtration + chloramines would allow compliance with the RRQDW for THMs The scenario consisting ofozonation/slow sand filtration + chloramines must be favoured in order to allow compliance for chlorinated DBPs and microbial inactivation efficien@ standards. However, infrastructure and operational factors must also be considered. In the future, when the water plant under study is updated, full-scale studies must be carried out to identify the optimal operational conditions that will simultaneously ensure adequate microbial inactivation levels, reduce chlorinated DBP occurrence and reduce operational costs.

Acknowledgements

This project was conducted through a research contract from BPR Groupe Conseii, Quebec, Canada; special thanks are due to Mr Pierre Coulombe and Ms Edith Laflamme. The authors also thank Michel Bisping, Christine Beaulieu and Jessica Egan for field and laboratory help. The contribution of the referees (Marc Anderson and Huseyin Selcuk) is appreciated.

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Using ozonation and chloramination to reduce the formation