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Urban Water Security Research Alliance Electrochemical Treatment of Reverse Osmosis Concentrate: Strategies to Minimise the Formation of Halogenated By-products Arseto Yekti Bagastyo Electrochemical Treatment of Problematic Water Recycle Waste

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Electrochemical Treatment of Reverse Osmosis Concentrate: Strategies to Minimise the Formation of Halogenated By-products Arseto Yekti Bagastyo

Electrochemical Treatment of Problematic Water Recycle Waste Streams

Science Forum, 19-20 June 2012

Urban Water Security Research Alliance

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SLIDE 2

2

  • Sent to power stations, industry and agriculture.
  • Supplement for drinking water supplies (emergency

drought response)

RO spiral wound modules

Reverse osmosis Concentrate (ROC)

  • 15-25% of the feed water.
  • Rejected contaminants are concentrated up to 7 times!

Electrochemical oxidation of ROC

High conductivity of ROC lowers the energy consumption. No use of chemicals!. Using appropriate electrode material and anode potential, a series of oxidant species is generated at the anode (e.g. OH▪, H2 O2, O3 ).

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SLIDE 3

O2 / H2O Cl2 / Cl- O3 / H2O H2O / OH▪ S2O8

2- / SO4 2-

C2O6

2- / CO3 2-

H2O2 / H2O

Boron doped diamond (BDD)

M+H2 O M (OH▪) + H+ + e- OH▪ are generated by water electrolysis at the electrode surface (M):

M + mCO2 + nH2 O + H++ e- M (OH▪)+R

Since (BDD)OH▪ are quasi-free, i.e. not adsorbed by the anode and similar to aqueous OH▪,

  • xidative degradation of organic matter will be enhanced (Bejan et al., 2012):

(Comninellis, 1994)

(Rychen et al., 2010)

Any ions present in the solution (e.g. Cl-, SO4

2-, CO3 2-) will also be oxidized at the electrode

surface or by the generated OH▪ :

Cl2 +2e- Cl- S2 O8

2-+2e-

2SO4

2-

C2 O6

2-+2e-

2CO3

2-

ROC has typically ≥1g/L of Cl-, and intense electrochemical hypochlorination may lead to the formation of toxic, chlorinated by- products!!!

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SLIDE 4

4

Relative mass distribution

  • f

chloride radicals and hydroxyl radicals (De Laat et al, 2004) Relative distribution

  • f

active chlorine species (Deborde and Von Gunten, 2008)

pH < 3: HOCl/Cl2 pH 6-10: HOCl/OCl- pH < 4: Cl2

▪-

pH > 6: OH▪

/ HOCl▪-

Competition between OH▪ and HOCl/OCl- will be affected by the pH, with pH≥6 favouring the participation of OH▪.

BDD electrochemical oxidation at acidic and neutral pH

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SLIDE 5

5

  • Results. Removal of COD and DOC

 Faster DOC removal at pH 6-7  enhanced participation of OH• and possibly other oxidants (e.g. S2 O8

2- and HC2

O8

  • ) in oxidative degradation of organics.

 Incomplete DOC removal at both pH  remaining DOC 32-36% (persistent organic fraction, not oxidisable by the COD test kit).  Faster COD removal at pH 2  intense electro-chlorination by the dominant HOCl species.

pH 2: 48% DOC pH 6-7: 54% DOC removal pH 2: 64% DOC removal pH 6-7: 68% DOC removal DOC COD pH 2 pH 6-7

  • t

: 96 h

  • vol : 5 L ROC
  • I

: 0.5 A

  • EAN

: 3.4-3.7 V

DOC0 : 42 mg/L (3.5 mM) COD0 : 136 mg/L

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SLIDE 6

6

Chloride FAC pH 2 pH 6-7

Results. Chloride oxidation and measured free available chlorine (FAC)

7.2 mM Cl- 2.3 mM Cl-

 Faster oxidation of Cl- to Cl2 at pH 2.  Lower FAC in acidic pH is observed due to Cl2 stripping to the gas phase..

Both Cl- and HOCl/OCl- can act as scavengers of OH• and generate less reactive chloro-radicals (e.g. OCl•, Cl• and Cl2

  • ).
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SLIDE 7

7

Results. Formed trihalomethanes (THMs) and haloacetic acid (HAAs)

  • Higher concentrations of THMs/HAAs

measured at pH 6-7.

  • Release by hydrolysis of other DBPs which

were not measured in this study, e.g. haloacetonitriles, haloacetaldehydes and haloacetamides (Chen, 2011).

pH 2 pH 6-7 pH 2 pH 6-7

  • Polychlorinated species were dominant.
  • Trichloromethane (TCM): 70-80% of THMs
  • Trichloroacetic acid (TCAA): 40-50% of HAAs
  • Decrease in THMs and HAAs was observed

with the increasing the electrolysis time.

  • chloro-THMs+HAAs (as molar conc. Cl-):

16-28% of AOCl (at 5.2 Ah L-1) was decreased to 4-8% of AOCl (at 10.9 Ah L-1).

THMs

TCM TCAA

HAAs

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8

Results. Formation of halogenated organics (AOX)

 Decrease in AOBr at longer oxidation time  brominated organics are further oxidised.

AOCl AOBr pH 2 pH 6-7 AOCl AOBr

  • AOCl and AOBr formation

was higher at pH 2.  3% of initial [Cl-], i.e. 39.5 mM after 11 Ah L-1 was incorporated into the remaining organics. However, the ratio DOC:AOCl in the final sample was 1.25:0.9 (pH 2) and 1.1:0.8 (pH 6-7), i.e. the remaining organics were highly chlorinated.

  • As expected,

adsorbable organic chlorine (AOCl) was the dominant AOX species.

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SLIDE 9

Novel strategy. Electrodialysis

  • f ROC (ROCED

) for Cl‐ separation prior to electro‐oxidation

Electrodialysed ROC (ROCED ):

  • 91% Cl- separated

(lowered from 37.5 to 4 mM)

  • 17% COD permeated
  • 12% DOC permeated
  • Conductivity decreased from 5.2 to 1.4 mS cm-1
  • pH: 6.8

Electrochemical oxidation of ROCED :

  • In order to maintain the conductivity and investigate the effects of electro-generated

OH▪/ROS and/or S2 O8

2-/SO4 ▪-/OH▪

species on the oxidative degradation of organics for the same initial ROCED (at pH 6-7)  addition of NaNO3 and Na2 SO4 to ROCED

  • Re-addition of NaCl was done for control experiments.
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Results. Removal of COD and DOC, and formed THMs and HAAs

 COD removal was lowered from 100% ([Cl-]=37 mM) to 60-74% ([Cl-]=4 mM).  DOC removal was increased from 38% ([Cl-]=37 mM) to 51% ([Cl-]=4 mM), particularly in the presence of SO4

2- electrolyte, due to the contribution of S2

O8

2-/SO4 ▪-.

 Increased formed THMs and HAAs (in sulfate): oxidation of Cl- to reactive chloro-species in the bulk by S2 O8

2- ions, and/or in the vicinity of the electrode surface by SO4 ▪- radicals.

 The formed THMs and HAAs was significantly decreased when [Cl-] was lowered from 37 to 4 mM. (5.6 Ah L-1) (5.6 Ah L-1)

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SLIDE 11

Conclusions Conclusions

  • The most efficient COD removal

is observed in the presence of high concentrations of Cl‐

  • ions. It is mainly achieved by electro‐chlorination,

which is favoured at pH 2 .

  • Faster DOC removal was observed at pH 6‐7, likely due to the enhanced

participation of OH• in the indirect oxidation mechanism.

  • At both acidic and neutral pH the formation of THMs, HAAs and AOX

was observed, with AOCl being the dominant species. While THMs and HAAs are degraded by prolonging the electrolysis time, AOCl is continuously formed. The toxicity of the remaining organic fraction remains to be determined.

  • Separation of chloride ions

prior to electrochemical oxidation seems to be the only option for an application of this process for the treatment of highly saline waste streams such as ROC.

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Acknowledgements Acknowledgements

  • Australian Research Council

(grants LP0989159)

A/Prof Damien Batstone (UQ) Dr Jelena Radjenovic (UQ) Prof Korneel Rabaey (UQ/UGent) Curtin Water Quality Research Centre Dr Ina Kristiana and A/Prof Cynthia Joll

QLD Health and Forensic Analytical Service

  • UQ International and APAI

Scholarships

  • Advisors:
  • Collaborators:
  • Project team:

Prof Jurg Keller (UQ-Project Leader) Dr Wolfgang Gernjak (UQ)

  • Co-authors:

Damien Batstone, Wolfgang Gernjak, Korneel Rabaey and Jelena Radjenovic

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Urban Water Security Research Alliance THANK YOU www.urbanwateralliance.org.au