Mitigation of Bromate during Ozonation

Transcription

1 Mitigation of Bromate during Ozonation John C. Crittenden, Ph.D., P.E., N.A.E. Daisuke Minakata Arizona State University Zaid K. Chowdhury A Malcolm Pirnie Perspective 1

2 Outline Application of ozone to water treatment Concern about Bromate Mechanisms of bromate formation Bromate formation control ph depression NH 3 addition Cl 2 -NH 3 process Case study - City of Peoria, Arizona - An Innovative and Economic Approach to Bromate Mitigation 2

3 Application of ozone to water treatment Direct reactions with O 3 Indirect reactions with HO produced by O 3 with NOM O 3 + NOM HO + byproducts HO is quenched by the reaction with NOM HO + NOM byproducts Factors to reduce stability of aqueous ozone residuals High ph Low alkalinity High TOC High temperature 3

4 Concern about bromate Formation of bromate (BrO 3- ) during ozonation in the presence of bromide ion (Br-) A nationwide survey of Br- in drinking water sources: approximately 80 μg/l (Amy et al., 1994) Br- in costal area is expected higher 10 μg/l of BrO 3- standard of MCL associated with cancer risk, Stage 1 of the Disinfectant/Disinfection By-Product (D/DBP) Rule (EPA, 1998) When ozone is applied to disinfection, tradeoff between inactivation of cryptosporidium and bromate formation should be concerned. 4

5 Mechanisms of bromate formation Simplified reaction scheme for bromate formation during ozonation Disproportionation pka = 8.8 Ozone involving pathway O 3 + Br - OBr - + O OBr- + O 3 2O 2 + Br OBr- + O 3 O 2 + BrO k (M -1 s -1 ) HOBr + O 3 O 2 + BrO H+ <0.013 BrO O 3 BrO O O 3 + Br BrO + O HO involving pathway HO + HOBr BrO + H 2 O HO + OBr - BrO + OH HO + Br- Br + OH BrO + BrO + H 2 O BrO OBr- +2H BrO + BrO 2 - OBr- + BrO At lower ph (ph<pka=8.8), less BrO 3- is produced via ozone pathway since HOBr is dominant. As a result, O 3 decay is slower. 5

6 Contribution of O 3 and HO to the reactions with Br- f HO Typical range for water treatment f O E E E-07 [HO ]/[O3] More than 90% of oxidation of Br- occurs with O 3. Only at high [HO ]/[O 3 ], HO oxidizes Br-. 6

7 f HO 1.0 Contribution of O 3 and HO to the reactions with HOBr/OBr- HOBr/OBrpH= ph=6.5 ph=7.0 ph=7.5 ph=8.0 ph= f O E E E-07 [HO ]/[O3] At [HO ]/[O 3 ]=10-8, ph=7.0, the fraction of [HOBr tot ] oxidized by HO is 70%. HOBr is mostly oxidized by HO. 7

8 Control of bromate formation 1 ph depression- ph depression resulting in more HOBr due to pka=8.8 of HOBr reduces BrO 3- formation. However, in drinking water treatment at around neutral ph, HOBr is mainly oxidized by HO. Therefore, ph depression does not reduce BrO 3 - formation drastically. At reduced ph, O 3 decay is slower and the [HO ]/[O 3 ] is decreased during ozonation. As a result, reduction of BrO 3 - at the steady state is distinctive. It is noted that it cannot be expected the proportional relationship between [HO ]/[O 3 ] and BrO 3- formation. 8

9 O 3 (mg/l) BrO3- (ug/l) Model simulation - ph effect - HO + NOM products k = (mgc/l) -1 s -1 Westerhoff et al 2007 Br + NOM Br- + products k = (mgc/l) -1 s -1 Pinkernell and von Gunten 2001 Reactor type CMBR Init O3 (mg/l) 1.0 ph 6, 6.5, 7, 7.5, 8, 8.5 Br- (ug/l) 300 NOM (mgc/l) ph= ph= ph= ph= ph= ph=8.5 ph=6.0 ph=6.5 ph=7.0 ph=7.5 ph=8.0 ph= Time (min) Time (min) 9

10 HOBr (mole/l) OBr- (mole/l) HOBr and OBr- concentration profiles at different ph 4.0E E E E E E-07 ph=6.0 ph= E-07 ph=7.0 ph=6.0 ph=6.5 ph= E E-07 ph=7.5 ph= E E E-07 ph= E-07 ph= E-07 ph=8.0 ph= E E Time (min) Time (min) It is observed that ph depression lowers OBr- concentration, which reacts with O 3 at k=330 and 100 M -1 s -1 to produce Brand BrO 2 -, respectively, whereas HOBr does not. 10

11 Control of bromate formation 2 - NH 3 addition - Simplified reaction scheme for controlling bromate formation k (M -1 s -1 ) HOBr + NH 3 NH 2 Br + H 2 O OBr- + NH 3 NH 2 Br + OH NH 2 Br + OH- OBr- + NH The maximum effect is at high NH 3 as HOBr becomes very small. However, it is observed that excess NH 3 decreases the efficiency of bromate control. Not efficient in water containing medium to high NH 3 11

12 O 3 (mg/l) BrO3- (ug/l) Model simulation - NH 4+ addition - Reactor type CMBR Init O3 (mg/l) 1.0 ph 8.0 Br- (ug/l) 300 NOM (mgc/l) 1.0 NH4+ (ug/l) 0, 50, 100, 200, 400, Time (min) 120NH4+ = 0 ug/l 100 NH4+ = 50 80ug/L NH4+ = ug/L 40NH4+ = 200 ug/l Time (min) NH4+ = 0 ug/l NH4+ = 50 ug/l NH4+ = 100 ug/l NH4+ = 200 ug/l NH4+ = 400 ug/l NH4+ = 800 ug/l 12

13 Control of bromate formation 3 Cl 2 -NH 3 process - Source water Cl 2 addition NH 3 addition HOCl hinders Br- oxidation to Br by HO HOCl + Br- HOBr + Cl OCl- + Br- OBr- + Cl HOCl OCl- + H+ pka = 7.5 NH 3 reacts with both HOBr and HOCl HOBr + NH 3 NH 2 Br + H 2 O OBr- + NH 3 NH 2 Br + OH HOCl + NH 3 NH 2 Cl + H 2 O Ozonation Effective to hinder HO during ozonation to reduce BrO 3 - HOCl and NH 2 Cl oxidize specific moieties of NOM and reduces their reactivities toward O 3, and also scavenge HO. * HOCl, NH 2 Br, and HOBr react with NOM to produce THMs and TOX 13

14 Br- (ug/l) HOBr tot (M) Model simulation Cl 2 -NH 3 process 1 - HOCl (5 min.): Bromide ion is converted into HOBr by HOCl. HOBr produced reacts with NOM. 300 Chlorination 5 min. HOCl (um) 0, 5, 10, 15 Br- (ug/l) 300 TOC (mgc/l) 1.0 ph 8.0 phosphate (M) Ammonia addition 1 min. NH4+ (ug/l) 400 Ozonation 60 min. reactor type CMBR O3 (mg/l) E E-06 HOCl = 5 um 1.5E-06 HOCl = 10 um 1.0E-06 HOCl = 15 um 5.0E-07 HOCl = 5 um HOCl = 10 um HOCl = 15 um Time (min.) 0.0E Time (min.) 14

15 O 3 (mg/l) BrO 3 - (ug/l) HOBr (M) Model simulation Cl 2 -NH 3 process 2 - NH 3 addition (1 min.): HOBr is significantly masked by NH E E E-07 HOCl = 5 um HOCl = 10 um HOCl = 15 um 0.0E Time (min.) O 3 (60 min.): Pre-addition of HOCl followed by NH 3 decreased BrO 3 - by factor of 2~7 as compared with the case of only NH 3 addition Time (min.) HOCl = 0 um HOCl = 5 um HOCl = 10 um HOCl = 15 um Time (min.) HOCl = 0 um HOCl = 5 um HOCl = 10 um HOCl = 15 um 15

16 BrO 3 - (ug/l) BrO 3 - (ug/l) Model simulation Cl 2 -NH 3 process 3 Effect of reactions of NOM with NH 2 Br, HOCl, HOBr With reactions Time (min.) HOCl = 0 um 8.0 HOCl = 5 um 6.0 HOCl = 10 um HOCl 4.0 = 15 um Time (min.) Without reactions HOCl = 0 um Series1 HOCl = 10 um HOCl = 15 um The reactions of NOM with HOBr and NH 2 Br significantly reduces HOBr/OBr concentrations which are the key intermediates for the subsequent BrO 3 - formation. 16

17 TTHM (ug/l) Model simulation Cl 2 -NH 3 process 4 TTHM formation Improved EPA 1998 empirical model (Sohn et al., 2004) [TTHM] = [Cl 2 ]{A TTHM (1-exp(-kt)} ln(k) = ln ln([nh 3 -N]) 1.12 ln(temp) ln([br-]) ln(ph) ln(a TTHM )= ln ln([nh 3 -N]) ln([cl 2 ]) ln(ph) [TTHM]=predicted trihalomethane conc. in initial phase (~5h), µg/l Chlorination 5 min. HOCl (um) 0, 5, 10, 15 Br- (ug/l) 300 TOC (mgc/l) 1.0 ph 8.0 phosphate (M) Ammonia addition 1 min. NH4+ (ug/l) 400 [Cl 2 ]=applied chlorine dose, mg/l [DOC] = dissolved organic carbon, mgc/l [NH 3 -N] = ammonia-nitrogen conc., mg/l as N [Br-]= bromide concentration, µg/l Temp = temperature, C t = reaction time, h Time (min) HOCl = 5 um HOCl = 10 um HOCl = 15 um 17

18 Control of bromate formation -optional (O 3 /H 2 O 2 process)- Advanced oxidation process: O 3 /H 2 O 2 process: HO O 3 HO + O O 2 k = M -1 s -1 H 2 O 2 + O 3 O 2 + H 2 O k = M -1 s -1 produce more HO at higher ph due to pka = 11.6 (H 2 O 2 ) H 2 O 2 also scavenges HO H 2 O 2 + HO HO 2 + H 2 O k = M -1 s -1 => optimum O 3 /H 2 O 2 ratio (typically 2 ~ 3) H 2 O 2 addition keeps the O 3 concentration low. In the presence of Br-, H 2 O 2 /HO 2 - reacts with HOBr/OBr- to produce Br- resulting in BrO 3 - formation. In addition, there is significant contribution of the reaction of O 3 with Br in regard with BrO 3 - formation. 18

19 An Innovative and Economic Approach to Bromate Mitigation: City of Peoria, AZ Evaluation Zaid K. Chowdhury A Malcolm Pirnie Perspective 19

20 Raw water O 3 Polymer Coagulant Chlorine Caustic Arizona Canal Recycle Stream Pre - Sed GWTP is a 12 mgd conventional WTP Uses surface water Ozone Contactor Applies O 3 prior to coagulation followed by deep bed GAC filters Biological GAC filters reduce T&O, DBP precursors Elevated level of bromide (Br - ) in source water O 3 dosage to its minimum Floc/ Sed. Basin Biologically Active GAC Filters Reservoir Distribution System Zaid K. Chowdhury, A Malcolm Pirnie Perspective 20

21 Current condition Br - ~ 100 μg/l O 3 /TOC ~ 0.3 mg/mg (i.e., for TOC 3.5 mg/l, O 3 ~ 1.0 mg/l) No O 3 residuals after first chamber, No disinfections credit BrO 3 level ~ 3.0 μg/l Effective ozonation strategies Allow higher ozone (O 3 /TOC > 0.5 to 1.0) (O 3 ~ 1.7 to 3.5mg/L) Ensure reliable BrO 3 compliance (< 8 μg/l) Improve T&O, TOC removal and DBPs reduction Combine effect of : effective O 3 strategy, Enhanced Coagulation, GAC replacement for improving WQ Zaid K. Chowdhury, A Malcolm Pirnie Perspective 21

22 Bromate (mg/l) Full-scale Bromate Formation ug/l O3/TOC Bromate Full Scale Data Full-Scale WTP Simulations WTP Simulations Source: Process Monitoring O 3 /TOC Zaid K. Chowdhury, A Malcolm Pirnie Perspective 22

23 Bromate (mg/l) (mg/l) 25.0 Full-scale WTP simulation O 3 /TOC = O 3 /TOC = O 3 /TOC = 0.3 Effluent Target = 8.0 µg/l ph Zaid K. Chowdhury, A Malcolm Pirnie Perspective 23

24 Bromate Reduction BROMATE MITIGATION Conventional ph Depression Quench O 3 with H 2 O 2 Addition Innovative Cl 2 / NH 3 Process Zaid K. Chowdhury, A Malcolm Pirnie Perspective 24

25 ANALYTICAL SETUPS Bench-scale Ozonation Unit Bench-scale Ozone Reactor Zaid K. Chowdhury, A Malcolm Pirnie Perspective 25

26 Bromate Yield [BrO 3 /Br] RESULTS (No Mitigation) 0.24 Bench-Scale Test WTP Simulation BrO 3 ~ mg/l BrO 3 ~ 8 mg/l 0.04 BrO 3 ~ 4 mg/l O3/TOC Ratio Zaid K. Chowdhury, A Malcolm Pirnie Perspective 26

27 Bromate Yield RESULTS (ph Depression) 0.24 O3/TOC = BrO 3 ~ 18 μg/l Effluent Target = 8 mg/l BrO 3 ~ 5 µg/l 70% Reduction No Mitigation Technique ph Depression Zaid K. Chowdhury, A Malcolm Pirnie Perspective 27

28 Bromate Yield 0.24 RESULTS (Chlorine/Ammonia Process) 0.20 BrO 3 ~ 19 mg/l BrO 3 5 mg/l 79% Reduction Effluent Target = 8 mg/l 95% Reduction 0.00 No Mitigation Technique 1.0 mg/l Cl min free Cl mg/l NH min before O mg/l Cl min free Cl mg/l NH min before O mg/l Cl min free Cl mg/l NH min before O mg/l Cl min free Cl mg/l NH min before O 3 Zaid K. Chowdhury, A Malcolm Pirnie Perspective 28

29 TTHM (mg/l) 24 RESULTS (THM Formation) mg/l Cl 2 Residuals 1.0 mg/l Cl 2 Residuals TTHM ~ 16 µg/l 16 TTHM ~ 12 μg/l mg/l Cl 2 Residuals mg/l Cl min free Cl mg/l NH mg/l Cl min free Cl mg/l NH mg/l Cl 2 98 min free Cl mg/l NH mg/l Cl min free Cl mg/l NH mg/l Cl min free Cl mg/l NH mg/l Cl min free Cl mg/l NH mg/l Cl min free Cl mg/l NH mg/l Cl min free Cl mg/l NH mg/l Cl min free Cl mg/l NH 3 Zaid K. Chowdhury, A Malcolm Pirnie Perspective 29

30 CONCLUSIONS Depressing ph to 6.5 or lower will control bromate and allow higher ozone dose H 2 O 2 addition did not mitigate bromate formation Maintaining Cl 2 residuals of mg/l with a contact time > 4 minutes followed by mg/l of NH 3 addition will control bromate and allow higher ozone dose Cl 2 /NH 3 process will contribute some DBPs Zaid K. Chowdhury, A Malcolm Pirnie Perspective 30