DHARMARAJ.N

CENTRE FOR EDUCATION (CFE)

CENTRAL ELECTROCHEMICAL RESEARCH INSTITUTE (CSIR-CECRI)

KARAIKUDI - 630006

A review on electrochemical technologies for water

disinfection

ERADICATION OF MICRO-ORGANISMS USING ELECTRIC CURRENT PASSED THROUGH WATER UNDER TREATMENT BY MEANS OF SUITABLE

ELECTRODES.

Electrochemical disinfection

Why “Electrochemical” Disinfection?

Disinfecting substances added to water in the case of chemical disinfection.

May form dangerous disinfection products.Electrochemical methods are highly localized

and controlled; higher efficiency with lesser by-products and lower disinfectant concentration

No transport, storage anddoage of disinfectants required

Low maintanence

AT THE PHASE BOUNDARY BETWEEN THE ELECTRODES AND WATER, DISINFECTION MAY OCCUR BY EITHER OF THE TWO MECHANISMS:

1.ELECTRIC CURRENT LEADS TO PRODUCTION OF DISINFECTING SPECIES FROM WATER (AS IN THE CASE OF OZONE)

2.FROM SPECIES DISSOLVED IN WATER (AS IN THE CASE OF CHLORIDE OXIDIZED TO FREE CHLORINE)

General mechanisms of disinfection

Electrochemical production of free chlorine

Disinfecting action: due to the production of molecular oxygen from free chlorine

Free chlorine: sum of hypochlorous acid and hypochlorite concentrations.

Electrochemical free chlorine production at lower chloride concentrations reported[1-3]

[1] J.Appl. Electrochem., 1999, 29, (7), 859[2] J.Appl. Electrochem., 1999, 29, (8), 895[3] Recent.Res. Dev. Electrochem., 2003, 6, 27

Reaction mechanism

Chlorine produced electrochemically from chloride ions dissolved in water

2Cl– → Cl2 + 2e–

Hydrolysis of chlorine to form hypochlorous ion Cl2 + H2O → HClO + HClpH dependent equilibrium between hypochlorite and

hypochlorous acid. HClO → ClO– + H+

Production of molecular oxygen form free chlorine HClO → O + Cl– + H+

ClO– → O + Cl–

Electrode materials

Different efficiencies have been reported for different electrode materials [1]

Dimensionally stable type anodes (DSA) found to be most efficient [4]

Carbon electrodes have high oxygen and chlorine evolution overpotential; may oxidize hypochlorite further to chlorate and perchlorate [5]

No such reactions in DSA [6]The Titanium substrate Insoluble anode (TSIA) :

CECRI’s contribution to India’s chlor alkali industry

[4] Wasserwishaft Wassertechnik. 2006, (9), 36[5] Recent res.dev. Electrochem.,2003, 6, 27[6]J.Appl Electrochem, 2007, 35, 1357

Comparison of electrodes

Figure (a) dependence of the free chlorine production efficiency on chloride concentration for two dimensionally stable anode (DSA) type electrodes (4)

Fig (b): Lifetime of electrodes of various materials under periodic reversal of polarity [4]

Electrode Life

Regular polarity reversal done for cleaning cathodes [7]

Periodic reversal of polarity decreases electrode life

Ultrsonication [8], rotor brushes [9], vanes [10] are cleaning methods investigated

Current pulse cleaning [11] also investigatedLack of satisfactory optimization in any of

these makes polarity reversal used till date[7] J. Appl. Electrochem., 1999, 29, (8), 895[8]J. Appl. Electrochem., 2002, 32, (6), 597[9] German Patent Appl. 19,836,431; 1999[10] U.S. Patent 5,885,426; 1999[11] European Patent 0,862,538; 1999

Electrochemical disinfection device AQUADEL, with Ruthenium-iridium oxide coated Ti electrodes, reactor pipe with inserted electrode stack on the right hand side

Electrochemical Ozone production

Electrolysis- first known method for ozone production[12]

Possible to produce ozone form water directly at high overpotential, high current density and low water temperature

3H2O → O3 + 6e– + 6H+ Very low current efficiency, complicated production

systems, and unstable electrodes hinder processDiamond anode/solid polymer electrolyte

(SPE)/cathode. Sandwiched configuration improves efficiency [13]

[12] M.B. Rubin, Bull. Hist. Chem, 2001, 26, (1), 40[13] Stadelman et al European patent appln. 1,730,080; 2006

Hypocell B4, an electrochemical disinfection device which operates by ozone production

SPE based systems

High current efficiencies (upto 47%) [14,15]Easily inserted into water pipes or reservoirs

produce required quantity ozone directly form water to be treated

Nafion exchange membrane usually employed

Carbon anodes usually employedThe use of platinum as anode reduces

hydrogen overpotential; increases efficiency[14] Kraft et al Electrochem commun, 2006, 8, (5), 883[15] K. Arihara et al J.Electrochem. Soc., 2007, 154 (4), E71

Disinfection by anodically produced oxygen

Electrochemically produced oxygen- the main germicidal product

Used in cases where disinfection targets are anaerobic bacteria

Usually used in wash stations, where anaerobic digestion leads to a bad odor.

Electrolytically produced oxygen in the water dispersed as fine bubbles

Pt coated anodes most suitableCathodes are usually carbon based

Disinfection by cathodically produced hydrogen peroxide

Disinfectant: Hydrogen peroxideCathodic production of Hydrogen peroxide in water

electrolyte reported [16-18] O2 + 2H2O +2e- → H2O2 +2OH-

Peroxide produced from dissolved oxygen in waterAnodic production of oxygen can be coupled with

this process, leading to higher oxygen concentrations.

Common anode materials: Dimensionally stable anodes (DSA)

Common cathode materials: Graphite or other carbon based electrodes[16] J.Appl. Electrochem., 2001, 31, (8), 877

[17] Water. Res., 2001, 35, (13), 3235[18] Intl. J.Env.Health.,2002, 204, (5-6), 303

Summary

Pt- most favored for oxygen production from natural waters

Carbon based electrodes lead the way for ozone and hydrogen peroxide production

Dimensionally stable anodes (DSA) best suited for hypochlorite generation

Conclusion

Electrochemical technology: several advantages when compared to conventional techniques

Reliability proven in many practical applications.

Only few products currently marketed.Research is on to bring about higher

efficiencies at low cost for such systems.Future prospects for this technology are

brilliant with a wide spectrum of applications.