1. BackgroundElectrochemical water disinfection can be

defined as the eradication of microorganisms byusing an electric current passed through the waterunder treatment by means of suitable electrodes.At the phase boundary between the electrodesand the water, the electric current leads to theelectrochemical production of disinfectingspecies from the water itself (for example,ozone), or from species dissolved in the water(for example, chloride is oxidised to freechlorine).

Attempts to clean or disinfect water by directelectrolysis had been reported as early as thenineteenth century (see, for example, Reference(1)). It has even been speculated that the electri-cal elements (the so-called ‘Baghdad battery’),which were discovered in 1936 in the ruins of aParthian city (inhabited from about 300 BC to300 AD) near Baghdad in Iraq, were in use forthe electrochemical preparation of germ-freewater (2). Since the end of the nineteenth centu-ry there have been frequent attempts to useelectrochemical disinfection (for example,References (3–6)). Until recently none have beensuccessful, at least not for long-term practical use.

Different terms are or have been in use todescribe this type of water treatment process orthe water produced by this process, such as ‘electrolytic disinfection’, ‘electrochemical

disinfection’, ‘anodic oxidation’, ‘functionalwater’ and ‘electrochemically activated water’among others.

There are three reasons why electrochemicalwater disinfection has arrived at technical matur-ity only recently, rather than earlier in the(possibly) 2000 years since its discovery:(a) Sufficiently stable and efficient electrode

materials for electrochemical water disinfec-tion have been developed and optimised onlyin the last forty years. These are titanium elec-trodes with mixed oxide coatings based oniridium and/or ruthenium oxide (7–9), anddoped diamond electrodes (10).

(b)The functional interrelationships betweenchloride concentration in the water, current,current density, electrode material, water qual-ity, electrochemical production of freechlorine and disinfecting action have beeninvestigated in detail only recently (11–15).

(c) Development work on electrochemical waterdisinfection has often been undertaken by amateurs in both electrochemistry and waterchemistry, and this remains somewhat truetoday. Only a few electrochemists have beeninterested in this topic, mostly only for a shortperiod in their career. This has resulted in mis-takes in device dimensioning and inunscientific explanations of the mechanism ofthe process.

Platinum Metals Rev., 2008, 52, (3), 177–185 177

Electrochemical Water Disinfection: A Short ReviewELECTRODES USING PLATINUM GROUP METAL OXIDES

By Alexander KraftGesimat GmbH, Köpenicker Str. 325, D-12555 Berlin, Germany; E-mail: alexander.kraft@gesimat.de

Electrochemical water disinfection is a rarely used but convenient and highly efficient way toproduce germ-free water. The technique works without the addition of chemical compoundsto the water to be treated, but is nevertheless based on the biocidal action of various chemicalsubstances. Electrodes with platinum group metals (pgms) or their oxides as active coatingsare generally the best suited to electrochemical water disinfection. In special cases, noveldoped diamond electrodes can be applied. A short historical and technical overview of theprocess is given, augmented by some application examples.

DOI: 10.1595/147106708X329273

2. General Processes for WaterDisinfection

Conventional disinfection methods may bedivided between chemical and physical processes.In chemical processes, disinfecting substancessuch as ozone, chlorine, sodium hypochlorite orchlorine dioxide are added to the water to be treat-ed. These processes are reliable, and have proventheir efficiency over many decades. They not onlykill microorganisms, but also provide a disinfectionreservoir which protects the water against recon-tamination for a certain time. A frequent drawbackof the chemical processes is unwanted side reac-tions of the disinfectants with substances presentin the water. These reactions lead to disinfectionbyproducts, some of which are considered danger-ous. There are also hazards in producing,transporting and handling large amounts of suchsubstances as chlorine and ozone.

In physical disinfection processes the micro-organisms are removed or killed by means ofirradiation with ultraviolet or ionising radiation,heating to elevated temperatures, ultrasound, orseparation through membrane filtration. The maindrawback of the physical disinfection methods isthe lack of a reservoir effect. These processes areonly effective in the immediate surroundings oftheir operating devices.

As compared with other chemical disinfectionmethods, the advantages of electrochemical waterdisinfection are obvious: no transport, storage anddosage of disinfectants are required. The disinfect-ing effect can be adjusted according to the on-sitedemand. Electrochemical water disinfection showsa reservoir effect and is often more cost-effectiveand requires less maintenance than other disinfec-tion methods. Photovoltaic power supply makes itpossible to use electrochemical water disinfectionfar from the electrical supply grid. This may beimportant for its application to drinking water indeveloping countries. Electrochemical water disin-fection can also be used in conjunction with otherdisinfection methods.

In electrochemical water disinfection, elec-trodes (at least one cathode and one anode) areinserted either directly into the volume of water tobe disinfected, or into a bypass pipe. A DC voltage

is applied between the electrodes, leading to theelectrolysis of the water. At the anode the mainproduct is oxygen (Equation (i)):

2H2O → O2 + 4H+ + 4e– (i)

accompanied by an acidification of the water in thevicinity of the anode. At the cathode, hydrogen isformed (Equation (ii)):

2H2O + 2e– → H2 + 2OH– (ii)

and the water near the cathode becomes alkaline.Since the evolved hydrogen is generally unwanted,it must be separated from the water stream.Because only small amounts are formed at normalcurrents (about 0.4 litres of hydrogen is producedper amp-hour), this is possible without problemsin most cases.

In most practical applications, simple undividedelectrochemical reactors employing parallel-plate,monopolar electrode stacks are inserted into thereactor pipe. The electrode plates may be config-ured as unperforated or perforated plates, or asexpanded metal.

Recently, an electrochemical disinfectionprocess which completely avoids hydrogen pro-duction has been developed. Atmospheric oxygenis reduced to hydroxyl ions at a gas diffusion cath-ode (16) (Equation (iii)):

O2 + 2H2O + 4e– → 4OH– (iii)

Here the cathodic reaction (Equation (iii)) replacesthe hydrogen producing reaction (Equation (ii)).The gas diffusion electrodes are composed of aporous graphite-polytetrafluoroethylene (PTFE)layer, in contact with a metal mesh as current collector, and backed by an oxygen-permeablePTFE layer to prevent water leakage. The graphitecarries a manganese oxide catalyst which eliminatesunwanted hydrogen peroxide.

3. Production of Free Chlorinefrom the Chloride Content ofthe Water

If electrochemical disinfection is applied todrinking water, industrial water, seawater or othersolute-containing water, its effect is mainly basedon the electrochemical production of hypochlorite

Platinum Metals Rev., 2008, 52, (3) 178

and/or hypochlorous acid from the chloride content of the water. The effectiveness of thismethod has always been accepted for water whichcontains higher concentrations of chloride ions (17),such as seawater with about 19 g l–1 chloride (18), orwhere large amounts of sodium chloride have beenadded, for instance to swimming pool water(chloride concentrations here are usually about 2–5g l–1). For the disinfection of drinking water andother waters with much lower chloride content, theeffectiveness of the method was not clear for a longtime (19). It was eventually demonstrated that evenat very low chloride concentrations (less than 100 mg l–1) sufficient free chlorine can be producedto efficiently disinfect water (11–15).

The disinfectant hypochlorous acid/hypochloriteis produced at the anode in a side reaction to oxygenevolution. The following simplified reaction mecha-nism is proposed. First, chlorine is producedelectrochemically from chloride ions dissolved in thewater (Equation (iv)):

2Cl– → Cl2 + 2e– (iv)

Chlorine hydrolyses in water and hypochlorous acid(HClO) is formed (Equation (v)):

Cl2 + H2O → HClO + HCl (v)

Hypochlorous acid and the hypochlorite anion forma pH-dependent equilibrium (Equation (vi)):

HClO ClO– + H+ (vi)

In the nomenclature of water disinfection, thesum of hypochlorous acid and hypochlorite concen-trations is usually termed ‘free chlorine’ or ‘activechlorine’. The disinfecting effect of free chlorine isbased on the release of atomic oxygen according toEquations (vii) and (viii):

HClO → O + Cl– + H+ (vii)

ClO– → O + Cl– (viii)During the disinfection, chloride ions which have

been consumed by electrochemical free chlorineproduction are reformed. Thus there is no overallchange in the chemical composition of the waterduring electrochemical water disinfection.

Where there is a low chloride concentration inthe water to be treated (as in drinking water) the cur-rent efficiency of the electrode material for theproduction of free chlorine is crucial; it should be ashigh as possible. Very great differences have beenfound in the efficiency of free chlorine productionbetween different electrode materials at low chlorideconcentrations (11–13, 15). Figure 1 shows thedependence of the free chlorine production efficien-cy on chloride concentration for two dimensionallystable anode (DSA®) type electrodes (using activecoatings of IrO2 or IrO2/RuO2), platinum andboron-doped diamond electrodes. It can be seenthat DSA® type electrode materials clearly outper-form diamond and Pt electrodes, which aretherefore not generally applicable as anodes forwater disinfection based on the electrochemical

Platinum Metals Rev., 2008, 52, (3) 179

Doped-diamond Pt

IrO2/RuO2

IrO2

Free

chl

orin

e pr

oduc

tion

effic

ienc

y, m

g A

h–1

125

100

75

50

25

0 20 40 60 80 100 120 140 160 180Chloride concentration, mg l–1

Current efficiency, %

10

8

6

4

2

0

Fig. 1 Dependence of theelectrochemical free chlorineproduction efficiency on thechloride content of theelectrolysed water understandard conditions using fourdifferent anode materials(iridium oxide, mixediridium/ruthenium oxides,platinum, doped-diamond)

production of free chlorine. Diamond electrodeshave another disadvantage in this application area.Because of their high overvoltage for both oxygenand chlorine evolution, they may further oxidisehypochlorite to chlorate and perchlorate (20). Onlyvery low levels of these disinfection byproducts arepermissible in potable water. Chlorate and per-chlorate are not formed at DSA® type and Ptelectrodes (11, 13). Earlier devices for electro-chemical water disinfection employed anodematerials such as carbon (1) or Pt (3, 4) with verylow production efficiency for free chlorine.

Another important aspect in choosing theappropriate electrode material is electrode lifetime.Although electrode lifetime has been improvedwhere electrode polarity remains constant, fre-quent polarity change between anode and cathodeis still problematic in this regard. Because of theformation of calcareous deposits at the cathodeduring electrolysis in water containing calcium andmagnesium ions, polarity reversal is necessary toclean the cathode surface of these deposits at reg-ular intervals (12, 13). The alkaline pH in thevicinity of the cathode (see Equation (ii)) leads tothe precipitation of calcium carbonate (CaCO3)and magnesium hydroxide (Mg(OH)2). On revers-ing the polarity, the former cathode acts as an

anode and the scaling is removed due to the acidicanodic pH (see Equation (i)). But polarity reversalreduces electrode lifetime. This is especially truefor IrO2 or mixed IrO2/RuO2 electrodes (12, 13).Figure 2 shows the results of a long-term experi-ment (still running) on the lifetime of electrodes ofvarious materials under periodic reversal of polari-ty. A steep rise in cell voltage indicates the totalstripping of the active coating at the end of theelectrode lifetime. The shortest lifetime of the elec-trodes tested was observed for the RuO2-coated Tielectrodes, followed by IrO2-coated electrodeswith a lifetime of about three months. MixedIrO2/RuO2-coated electrodes had a lifetime ofnearly one year under our experimental conditions.These materials are all clearly outperformed byplatinised Ti electrodes, which are still runningseemingly unaffected after nearly eight years.

Other means for cathode cleaning have beeninvestigated, such as ultrasonication (21), and con-tinuously rotating brushes (22) or vanes (23) whichmove over the cathode surface. Another method isthe imposition of a current pulse, which increasesthe formation of gas bubbles (24). None of thesetechniques have been successful in long-termoperation, so that polarity reversal remains the bestautomatic cathode cleaning technique to date.

Platinum Metals Rev., 2008, 52, (3) 180

IrO2/RuO2IrO2

PtRuO2

Cel

l vol

tage

, V

30

25

20

15

10

5

0 100 200 300 400 500 600 700 2250 2500 2750Electrolysis time, days

Electrolysis time, years0 1 6 7 8

Fig. 2 Cell voltageversus electrolysistime in Berlin tapwater (conductivityabout 0.8 mS cm–1)for titaniumelectrodes coatedwith different activeelectrode materials(ruthenium oxide,iridium oxide,mixediridium/rutheniumoxides, platinum):polarity reversalevery 30 minutes,current density 20mA cm–2, electrodedistance 4 mm

An example of an application of electrochemi-cal water disinfection based on free chlorineproduction is the ‘AQUADES-EL’ device. This isoptimised for the cyclic operation of hot waterrecirculating systems in larger residential and com-mercial buildings. Potable water, especially in suchsystems, is especially prone to microbial contami-nation. This is due to high water temperatureswhich favour the growth of microorganisms suchas Legionella. From time to time the media reporton legionellosis outbreaks due to contaminateddrinking water systems in hotels, hospitals etc.However, Legionella is only the most notoriousgenus of germs. Others are also dangerous, such asPseudomonas aeruginosa, Escherichia coli andStaphylococcus aureus, to name but a few.

The AQUADES-EL system has been pro-duced and distributed since 1998 by the German

companies AquaRotter GmbH and G.E.R.U.SmbH. More than 400 systems have been sold andinstalled in hotels, hospitals, barracks, retirementhomes and other buildings. Figure 3 shows theAQUADES-EL device, which is usually located ina bypass to the water recirculation system. Itemploys Ti electrodes coated with mixedIrO2/RuO2. An advanced control system mea-sures the free chlorine concentration in the waterat several locations in the distribution system atregular time intervals by amperometric detection.

In many industrial areas, large amounts ofwater are used for cooling purposes, as evidencedby the cooling towers dominating the landscape.Microbial contamination of cooling water,whether in cooling towers or air conditioningsystems, currently poses a major problem. Largeconcentrations of disinfectant are usually added to

Platinum Metals Rev., 2008, 52, (3) 181

Fig. 3 Electrochemical disinfectiondevice AQUADES-EL (AquaRotter GmbHand G.E.R.U.S mbH, Germany) equippedwith mixed iridium/ruthenium oxide-coated titanium electrodes:right-hand side: reactor pipe withinserted electrode stack; upper middle:power supply and control unit; upper left:amperometric free chlorine sensorequipment

the cooling water. The two commonest biocidesfor cooling towers are free chlorine and quartenaryammonium compounds. New biocides continue tobe developed, but electrochemical water disinfec-tion is a promising alternative for this application.

Electrochemical disinfection has been success-fully implemented in the cooling waterrecirculation system of a German paper mill(Figure 4). The ‘Hypocell® B4’ device fromG.E.R.U.S mbH employs four parallel reactorpipes. The titanium electrodes carry a mixedIrO2/RuO2 coating. About 130 m3 of water istreated. The water has a typical pH of 8.3, a con-ductivity of 1.9 mS cm–1 and a chlorideconcentration of about 280 mg l–1. The flow rate inthe treatment bypass is 1000 l h–1.

4. Ozone ProductionIf water with low or zero chloride concentra-

tion is required, the addition of sodium chloride isnot acceptable and free chlorine cannot be pro-duced in situ. Disinfection must therefore be basedon other electrogenerated species. By using anodeswith a high oxygen overvoltage, a high current

density and a low water temperature it is possibleto produce ozone directly from the water accord-ing to Equation (ix):

3H2O → O3 + 6e– + 6H+ (ix)

Electrochemical ozone production has beenknown since the nineteenth century (25).Electrolysis was the first production method forozone (25), but for most applications ozone is nowproduced by corona discharge. The disadvantagesof electrolytic production include too low a currentefficiency, complicated production systems, un-stable electrode materials (such as lead oxide(PbO2) anodes) and/or difficult-to-handle elec-trolytes. Electrolytic production may become moreattractive using a new simple electrode assembly(26) of a ‘sandwich’ configuration: diamondanode/solid polymer electrolyte (SPE)/cathodesandwich.

By using such a diamond-SPE sandwich or sim-ilar device in deionised water, ozone can beproduced with a high current efficiency (up to47%) (27, 28). These devices can easily be insertedinto water pipes or reservoirs to produce therequired quantity of ozone directly from the waterto be treated. The Nafion® ion exchange mem-brane may be employed as the SPE material. If thedoped diamond anode is replaced by a Pt- orIrO2-coated anode, a much lower ozone produc-tion rate is measured. On the other hand, if Pt isused as the cathode material, the overvoltage forhydrogen production can be lowered, minimisingthe cell voltage.

For the electrochemical disinfection of ultra-pure water, or other waters with very lowconductivity (such as rain water), electrochemicalozone production with diamond-SPE sandwichelectrodes is the method of choice. Figure 5 showsa small stack of two diamond electrode/SPE/dia-mond electrode sandwiches. With this technology,water volume rates from 60 l h–1 up to 5 m3 h–1 canbe treated using different sized electrode stacks. Inrecent years several pilot projects have beenequipped with this technology. An improvedozone generating device is in development. Thisproduces no hydrogen, and the cathode consumesoxygen.

Platinum Metals Rev., 2008, 52, (3) 182

Fig. 4 Hypocell® B4 electrochemical disinfection device(G.E.R.U.S. mbH, Germany), with four pipe reactorsequipped with mixed iridium/ruthenium oxide-coatedtitanium electrodes in a cooling water system, installed ata German paper mill

5. Disinfection or Germ Minimis-ation by ElectrochemicallyProduced Oxygen

In some applications, electrolytically producedoxygen, the main anodic reaction product, showssome germicidal activity. This is especially true ifanaerobic bacteria are the disinfection target.

An example of this type of application is thewash water cycle of car wash stations. Here, theformation of anaerobic digestion products oftenleads to bad odours. Anaerobic conditions areeliminated via the fine dispersion of bubbles ofelectrolytically produced oxygen in the water. Thisis a highly effective mode of dissolution. For thisapplication, Pt-coated electrodes are the most suit-able anodes, because the main germicidal effect isbased on electrolytically produced oxygen and noton free chlorine. Figure 6 shows the Hypocell® B4device for germ minimisation in car wash stations,based on this technology. About twenty-five systems have been successfully installed.

6. Disinfection by CathodicallyProduced Hydrogen Peroxide

While most of the possible disinfectants in electrochemical water treatment are produced atthe anode, hydrogen peroxide may also be pro-duced at the cathode. This process has been usedby Dhar et al. (29) and Drogui et al. (30, 31) forwater disinfection (Equation (x)):

O2 + 2H2O + 2e– → H2O2 + 2OH– (x)

Oxygen dissolved in the water may serve asthe reactant in Equation (x). The maximum

concentration of oxygen in water which is in equi-librium with air at 25ºC is about 10 mg l–1

(0.3 mmol l–1). The oxygen produced by the anod-ic half reaction according to Equation (i) can alsobe used for the cathodic production of hydrogenperoxide. In this case, higher concentrations ofdissolved oxygen are possible, because the water isin contact with pure oxygen, and not merely withair. It is also possible to use a gas diffusion cath-ode on which the oxygen from the surrounding airis reduced to H2O2. In terms of energy efficiency,the electrode material best suited to H2O2 produc-tion is graphite. This material (without additionalcatalysts) is also the core component in gasdiffusion electrodes for H2O2 production.

Because of its lower oxidation potential, H2O2is a less effective disinfectant than free chlorine orozone. Therefore, higher concentrations and/orlonger disinfection times are necessary, limiting itsapplicability. Hydrogen peroxide has the advan-tage that its disinfectant action produces neitherbyproducts nor residues.

7. ConclusionElectrochemical water disinfection has many

advantages compared with conventional disinfec-tion technologies. It has proven its reliability inseveral practical applications, mainly for the dis-infection of drinking water, swimming pool waterand industrial cooling water. Electrochemicalwater disinfection has also been used or testedfor the reduction of bacterial contamination indental water supplies (32), and for the disinfec-tion of contact lenses (33) and ion exchange

Platinum Metals Rev., 2008, 52, (3) 183

Fig. 5 Electrode stack with twodiamond-SPE-electrode sandwiches forozone production in deionised water(Gerozon device by G.E.R.U.S. mbH,Germany)

resins (34) etc. However, only a few electrochem-ical water disinfection products are currentlyavailable on the market. This is due to the relativeunfamiliarity of the technology, and to fierce market competition with other technologies.Eventually, the cost and performance advantagesof electrochemical technology should lead to itswider use.

RuO2 and/or IrO2-coated electrodes are thebest suited to disinfection based on hypochloritegeneration. This is due to their high productionefficiency for hypochlorite from water with a verylow chloride content. Pt is the favoured electrodematerial for oxygen production from naturalwaters. Pt electrodes are also the most stable.

For the production of ozone and hydrogen per-oxide, pgm electrodes are not the first choice,being outperformed by carbon electrodes, i.e.doped diamond for ozone, and graphite for hydro-gen peroxide production.

This article is a translated and revised version ofReference (35).

Platinum Metals Rev., 2008, 52, (3) 184

Fig. 6 Hypocell® B4 electrochemical disinfection device(G.E.R.U.S. mbH, Germany) equipped with platinisedtitanium electrodes, mounted in a car wash station

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Platinum Metals Rev., 2008, 52, (3) 185

The AuthorAlexander Kraft is co-founder and Managing Director ofGesimat GmbH, Berlin, Germany, a company which develops‘smart’ switchable electrochromic glazing. Before startingGesimat in 1998, he developed electrochemical waterdisinfection technologies and devices at G.E.R.U.S. mbH,Germany, from 1994. He continued to work for G.E.R.U.S. as ascientific adviser until 2006. Alexander Kraft received a Ph.D. inPhysical Chemistry (semiconductor electrochemistry) fromHumboldt University, Berlin, in 1994.