Reference Work Entry

Encyclopedia of Applied Electrochemistry

pp 335-342

Date:

Disinfection of Water, Electrochemical

Introduction

Disinfection is the killing of vegetative microorganisms. Accepted methods must be able to reduce microorganisms by at least five orders of magnitude. Electrochemical water disinfection (ECD) is one of these methods [1]. ECD acts by means of electrochemical disinfectant generation onsite or inline (Fig. 1). Besides the disinfection effect, benefit of ECD arises from relatively simple cell construction and easy automation.
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Disinfection of Water, Electrochemical, Fig. 1

Onsite (a) and inline addition (b) of disinfectants to water

The method must be distinguished from the killing or inactivation of microorganisms previously adhered to a surface (biofilm at different stages) by applying electrochemical surface polarization and from electroporation technologies working with field strength up to 10 MV m−1. Whereas direct electron transfer seems to exist for adhered microorganisms [2], oxidant production is the most probable mechanism in electrochemical disinfection [3]. There were many speculations on the killing mechanisms in the past, including electrical field and pH effects. In flow-through treatment disinfection, however, immediate killing of all cells is not possible. The chemical reaction of disinfectants with microorganisms may proceed within a timescale from seconds to hours as well as outside the treatment unit.

Typical disinfectants that can be electrochemically produced are free active chlorine/free available chlorine (FAC) species as the sum of dissolved Cl2, HOCl, and OCl. If these components react with selected water constituents, by-product formation occurs. In some cases, these by-products may also act as disinfectants (chloramines [4]). Other possible disinfectants obtained, depending on the electrolysis conditions, are ozone, hydrogen peroxide, chlorine dioxide, heavy metal ions, and bromine active species.

The participation of radicals, such as singlet and triplet oxygen radicals, hydroxyl radicals, or radicals resulting from consecutive reactions of radicals with water constituents, is discussed very controversially. Radical lifetimes in the range of nanoseconds in a μm-reaction zone near the anode let one conclude that direct reactions with microorganisms are not decisive for their killing. Obviously, under these conditions, the majority of microorganisms are killed in reaction with more stable products (H2O2, O3, FAC). Oxidant generation is mostly related to anodic reactions. However, cathodic reactions such as the reduction of dissolved oxygen to H2O2 may also contribute to generation of an oxidant:
$$ {{\mathrm{ O}}_2}+2{{\mathrm{ H}}_2}\mathrm{ O}+4{{\mathrm{ e}}^{-}}\to 2{{\mathrm{ H}}_2}{{\mathrm{ O}}_2} $$
(1)
A large variety of cell constructions, often using gas diffusion electrodes, have been developed or tested [57]. Widespread technologies using natural salt matrices for oxidant production in a flow-through regime (Fig. 1b) are known as inline electrolysis, tube electrolysis, anodic oxidation, low-amperic electrolysis, electrochemical water activation, or by brand names. In addition, immersion constructions have been reported. Fixed installations and mobile systems are in use (Fig. 2).
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Disinfection of Water, Electrochemical, Fig. 2

(a) Immersion stack cell and (b) transportable disinfection cell with water container pump and electrolysis unit, both for discontinuous outdoor ECD; voltage sources are not shown

Inline methods are mostly related to the treatment of relatively low amounts of water per unit of time. Typical applications include the disinfection of pool and drinking water, for example, in hospitals, submarines, airplanes, senior residences, and other facilities. Potential nondrinking water applications include urban wastewater treatment, cooling water disinfection, saline wastewater disinfection, fish farming, vegetable washing, cleaning of pipe systems and containers, algae removal, tumor research, lens washing, dental implants, medical equipment, and cleaning of filters and permeates, often combined with odor abatement.

Disinfectants, if generated continuously in an electrochemical cell, may be added to a side or a mainstream of water. In special applications (i.e., drinking water), lower and upper limiting concentrations for FAC exist for the resulting concentration (often between 0.1 and 6 mg dm−3). In comparison with ozone and chlorine dioxide, the main advantage of chlorine species as disinfectants is their reservoir effect in a system over increased time. The main disadvantage is the formation of halogenated by-products. Because of this, and to avoid microorganism adaptation effects, continuous use of disinfection cells without periodic interruption is not recommended.

Inline flow-through cells dominate the market. Divided and undivided cells are known. Cells have been commercialized to a remarkable extent since the 1970s, especially in Eastern Europe and Germany. Selected schematic constructions and technologies are shown in Figs. 3 and 4.
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Disinfection of Water, Electrochemical, Fig. 3

Typical undivided flow-through constructions for ECD: tube electrolyzer (a), bipolar cell with nonoptimal current distribution (b), and bipolar parallel plate reactor (c)

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Disinfection of Water, Electrochemical, Fig. 4

Typical technologies using divided flow-through cells for ECD: with special catholyte recirculation (a), with addition of catholyte to the main water stream (b), and removal of by-products in a final fixed bed reactor (c)

Mostly, so-called mixed oxide (MIO) electrodes (typically TiO2 doped with IrO2, RuO2, or VaOx on Ti carrier material) are used in practical cases. TaOx may be part of the oxide layers. Many researchers have studied other surface modifications. RuO2-based electrodes seem to have better chlorine production compared with IrO2 or Pt layers, but specific composition and methods of producing the activated layers are decisive for chlorine production efficiency [8, 9]. Untreated titanium, graphite, carbon cloth and glassy carbon, carbides, and nitrides were also studied. A relatively new, but sometimes problematic, material is boron-doped diamond (BDD, [10]). Applications of BDD anodes are accepted as so-called advanced oxidation methods. Summaries and disinfection-related papers can be found [8, 1114]. Most applications and studies use direct current for disinfectant production. A few papers deal with alternating and pulsating current.

A large variety of technological schemes, electrode constructions, electrode materials, and water matrices have often resulted in confusion of health administrations in assessing applicability. There are big differences worldwide in practice and legislation with respect to the usability of several disinfectants in special water systems. For example, contrary to North America or Australia, chloramine- or bromine-based disinfection is not widely used in Europe. In Germany, H2O2 may be used for pipe cleaning but not for drinking water disinfection, etc. Because in most cases (ECD) chlorine species are responsible for disinfection effects, the treated water must be classified as chloride containing or as water free of chloride.

Electrochemical onsite ClO2 generation at ppm levels is a challenging new technology. Technologies dealing with the targeted electrochemical evolution of oxygen or the cathodic generation of H2O2, ferrates [15], chloramines, percarbonate, peroxodisulfate [16], or bromine for disinfecting waters are not included in the scope of this paper.

Main Systems and Reactions

Electrolytes Free of Chloride

In chloride-free water, the generation of ozone is the reaction of choice. Buffer or supporting electrolyte components must be added to the anolyte. Only anode materials with high oxygen overvoltage (Pt, glassy carbon, PbO2, BDD) are able to generate ozone in water electrolysis. An overall reaction is shown in Eq. 2:
$$ 3{{\mathrm{ H}}_2}\mathrm{ O}\to {{\mathrm{ O}}_3}+6{{\mathrm{ H}}^{+}}+6{{\mathrm{ e}}^{-}} $$
(2)
Splitting of water is the main parasitic reaction (Eq. 3):
$$ 2{{\mathrm{ H}}_2}\mathrm{ O}\to {{\mathrm{ O}}_2}+4{{\mathrm{ H}}^{+}}+4{{\mathrm{ e}}^{-}} $$
(3)
To avoid side reactions and to isolate hydrogen evolution (Eq. 4 for acidic conditions), the anolyte and catholyte must be separated each from other:
$$ 2{{\mathrm{ H}}^{+}}+2{{\mathrm{ e}}^{-}}\to {{\mathrm{ H}}_2} $$
(4)
Reaction mechanisms vary with different electrode materials. Ozone formation was first shown for Pt anodes [17, 18]. Initial commercial cells were equipped with PbO2 layers on porous Ti carriers (Membrel technology). BDD is a promising commercial material despite its high price. The formation of hydroxyl radicals is accepted for many electrode materials in a more or less adsorbed state and in connection with more or less density of active sites. It is also accepted that on BDD, ozone is formed as the result of stepwise radical formation in a μm reaction zone (Eqs. 5, 6, 7, and 8):
$$ {{\mathrm{ H}}_2}\mathrm{ O}\ \to {{}^{.}}\mathrm{ O}\mathrm{ H} + {{\mathrm{ H}}^{+}} + {{\mathrm{ e}}^{-}} $$
(5)
$$ ^{.}\mathrm{ OH}\to {{\mathrm{ O}}^{.}}+{{\mathrm{ H}}^{+}}+{{\mathrm{ e}}^{-}} $$
(6)
$$ 2{{\mathrm{ O}}^{.}}\to {{\mathrm{ O}}_2} $$
(7)
$$ {{\mathrm{ O}}_2}+{{\mathrm{ O}}^{.}}\to {{\mathrm{ O}}_3} $$
(8)
Hydroxyl radicals may quickly form H2O2: [19]
$$ {2^{\ .}}\mathrm{ OH}\to {{\mathrm{ H}}_2}{{\mathrm{ O}}_2} $$
(9)

Other radical reactions are possible. In most cases, the reaction scheme is not well understood.

Current efficiency calculated from the concentration of dissolved ozone is a function of current density [20]. Technical cells are equipped with expanded mesh anodes to increase current densities. Ozone is added onsite or inline. Immersion constructions were also reported. As compared with silent discharge, electrochemical ozone generation is characterized by the high specific energy demand of about 2 kWh per kg ozone. Much higher ozone concentrations can be adjusted, however.

In electrolytes that are extremely poor in chloride ions, formation of ozone must be considered as a side reaction. Often, the standard DPD test for chlorine analysis shows higher values than expected [21]. Further research is still necessary. Application of MIO anodes under these conditions is highly risky because the obtained by-product spectrum can be contrary to rules for drinking water. Some researchers have studied the disinfection effect in waters without chloride ions, mostly using sulfate electrolytes and Pt, stainless steel, and other electrode materials [2225]. Discussions are sometimes questionable due to difficulties in relating the disinfection effect with species clearly responsible for it.

Electrolytes with Chloride Concentrations in the Range of Grams per Liter

MIO anodes are in widespread use for this technology. The anodic product is chlorine, with current efficiencies higher than 90 % (Eq. 10):
$$ 2\mathrm{ C}{{\mathrm{ l}}^{-}}\to \mathrm{ C}{{\mathrm{ l}}_2} + \mathrm{ 2}{{\mathrm{ e}}^{-}}$$
(10)
The electrochemical reactor must be divided because hydrogen evolution is the main cathodic reaction (Eq. 4, avoiding explosive mixtures). As in industrial chlorine-alkaline electrolysis, chlorine can be directly contacted with the water to be disinfected, or it can be used for preparation of sodium hypochlorite by reacting with NaOH (Eq. 11) for subsequent addition:
$$ \mathrm{ C}{{\mathrm{ l}}_2} + \mathrm{ NaOH}\to\ \mathrm{ NaOCl} + {{\mathrm{ H}}^{+}} + \mathrm{ C}{{\mathrm{ l}}^{-}} $$
(11)

Because this technology does not resolve problems with chlorine dosage from gas balloons (uncontrolled setting free of chlorine gas), it is not widely used in practice.

Electrochemical cells with MIO anodes for hypochlorite solution production as stock solutions for onsite addition of HOCl/OCl mixtures have found increasing application worldwide (cell units producing more than 100 kg FAC per day). Divided cells and NaCl concentrations in the range of seawater (3.5 %) are typical, but undivided cells are also offered by some manufacturers. Chlorate formation inside the stock solution at longer storage times is a problem with this technology:
$$ \begin{array}{l}\mathrm{ OC}{{\mathrm{ l}}^{-}} + 2\mathrm{ HOCl}\to\ \mathrm{ Cl}{{\mathrm{ O}}_3}^{-} + 2{{\mathrm{ H}}^{+}} + 2\mathrm{ C}{{\mathrm{ l}}^{-}} \end{array}$$
(12)

Some plant manufacturers prefer the use of water with a chloride concentration close to that of drinking water with optional addition of NaCl salt. The production of anolyte solution for disinfection has the advantage that the resulting pH is relatively low, and HOCl may be the predominant species. Its disinfection ability is much higher than that of OCl.

Electrolytes with Chloride Concentrations Typical for Drinking Water

This technology for direct drinking water electrolysis has been the focus of interest for 60 years [26], but detailed kinetic studies began only at the end of 1990s. Many drinking water regulations recommend a maximum concentration of 250 mg dm−3 for chloride ions. This is sufficient to produce FAC with typical current efficiency amounts of 5–15 % on MIO anodes and in undivided cells. Nearly linear curve behavior for FAC production and current efficiency versus chloride concentration (Fig. 5) exist [8, 9]. Often, current efficiency maxima are observed between 100 and 200 A m−2 due to the competing oxygen evolution reaction. Specific electroenergy demands (for electrolysis current) of 50 kWh per kg FAC can be measured.
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Disinfection of Water, Electrochemical, Fig. 5

Quasi-linear behavior of chlorine (FAC) production versus chloride concentration (left) and FAC current efficiencies versus chloride concentration at different current densities (right)

At very low chloride concentrations (normally lower than 10 mg[Cl] dm−3), no FAC can be found. Two general mechanisms are responsible for this behavior:
  1. (i)

    Formed FAC is converted to chloride by oxidants such as H2O2.

     
  2. (ii)

    Formed FAC is oxidized to products such as chlorate and perchlorate in electrochemical reactions. Finally, only traces of chloride can be found in the treated water.

     

In long-term electrolysis, all prior existing Cl is finally distributed between the species FAC, ClO3 , and ClO4 [27]. Reaction rates are higher for BDD anodes as compared to MIO anodes. Intermediates are probable but not exactly known. Participation of radicals is also possible on MIO and Pt electrodes [28].

Considering electrode kinetics, the most probable first step is the oxidation of chloride to the chloride radical by electron transfer or by reaction with hydroxyl radicals:
$$ \mathrm{ C}{{\mathrm{ l}}^{-}}\to \mathrm{ C}\mathrm{ l}+{{\mathrm{ e}}^{-}} $$
(13)
$$ \mathrm{ C}{{\mathrm{ l}}^{-}} + {^{.}}\mathrm{ OH}\to \mathrm{ C}\mathrm{ l} + \mathrm{ O}{{\mathrm{ H}}^{-}} $$
(14)

Chlorine molecules formed from the chlorine radical quickly react with water and hydroxide ions to form hypochlorous acid and hypochlorite ions with a strong pH dependence of species distribution [4].

Chlorine Dioxide Generation from Chlorite

Chlorine dioxide is an effective disinfectant that reduces the formation of organic disinfection by-products. There are different chemical and electrochemical technologies for producing and adding ClO2 onsite, initiating the reaction from chlorite or chlorate [5]. It is noteworthy here that even traces of chlorite added to a water stream may quickly react to form chlorine dioxide at low ppm levels [29]:
$$ \mathrm{ Cl}{{\mathrm{ O}}_2}^{-}\to \mathrm{ Cl}{{\mathrm{ O}}_2} + {{\mathrm{ e}}^{-}} $$
(15)
When chloride ions are present during electrolysis, chlorine dioxide formation is accelerated [29], obviously as the result of the quick reaction with hypochlorous acid in the acidic electrode layer:
$$ \begin{array}{l}2\mathrm{ HCl}{{\mathrm{ O}}_2}+\mathrm{ HClO}\to 2\mathrm{ Cl}{{\mathrm{ O}}_2} + \mathrm{ C}{{\mathrm{ l}}^{-}}+{{\mathrm{ H}}_2}\mathrm{ O}+{{\mathrm{ H}}^{+}}\end{array}$$
(16)

Semi-inline technologies with chlorite additions are still under research.

Future Directions

Address Problems in Electrochemical Water Disinfection

By-Product Formation

Species produced by ECD must be classified as disinfection products (e.g., FAC), disinfection by-products (named as DBPs, e.g., chloramines and THMs) resulting from disinfectant reactions with organic matter or from inorganic reaction schemes (chlorate), and electrolysis by-products (e.g., nitrite and ammonium ions from cathodic processes or organic intermediates on BDD anodes). Ozone may be an electrolysis product or by-product depending on the target of electrolysis. Due to health risks, some of these by-products are regulated by legislation. Many by-products are not yet anchored in national or international rules. Overall, the subject has not yet been researched enough. Concerning reactions of FAC with organic matter, by-products similar to those observed in chlorination processes can be expected [30].

Electrochemical reaction behavior of organic compounds is difficult to assess. Research works sometimes show that intermediates may be formed that are more toxic than the initial system. The formation of chlorate [31], bromate on mixed oxide electrodes [32], peroxodisulfate, and H2O2 [19] is well known as well as the formation of DPDs such as THMs and AOX.

In studies of Bergmann and co-workers, the following by-products of inline drinking water electrolysis were identified for the first time: chlorate, perchlorate, bromate and perbromate on BDD, and other anodes; chlorine dioxide as a by-product in disturbed chloride electrolysis; nitrite, ammonia/ammonium as cathodic products on mixed oxide electrodes; and hydrogen peroxide on BDD anodes and MIO cathodes [8, 3336]. Many results were confirmed by other researchers [37, 38]. Risks for halogenate and perhalogenate formation increase in the order of dosage of chlorine as Cl2 – dosage as a hypochlorite solution – inline electrolysis of FAC species.

Electrochemical methods for ammonia and ammonium reduction were suggested by Kim et al., Kapalka et al., and others [39, 40]. Modification of reaction conditions and clarification of mechanisms are the subject of current research.

The state of legislation and control concerning electrochemical disinfection devices in environmentally oriented applications is not satisfactory. Only in the USA, first attempts for perchlorate restrictions in drinking water can be observed. In Germany, a first complex project on inline electrolysis was finished in 2010. Currently, the method is still outside legislation but tolerated. The project included cell producers, water authorities, and research groups.

Formation of Deposits on Electrodes and System Surfaces

Especially in cells without a separator between anode and cathode and in processes using natural water matrices, cathodes show a tendency to be coated with deposits. Due to the increased pH in the cathodic electrode layer, mainly calcareous and hydroxidic scaling with earth alkali ions is observed (CaCO3, Mg(OH)2, etc.). Several antiscaling methods have been suggested, such as periodic dissolution in acids, change of polarity, rotating brushes or cleaning vanes, ultrasonic treatment, and current pulsation. Many methods for scaling quantification were suggested (potential control, quartz mass balance, etc. [41, 42]). In practical application, cell voltage and cell current monitoring are the most widespread methods used. In some cases, deposition of iron hydroxide on water pipe walls was observed when electrolysis shifted the pH to a more basic pH region.

Gas Inside the System

If the chloride concentration is in the range of milligrams per liter, the primary electrochemical reaction is the splitting mechanism of water, with oxygen and hydrogen formation. Use of separators is the best way to keep hydrogen gas out; however, oxygen still remains in the system and should be allowed to escape in a sophistic way. Increased corrosion is sometimes reported as the result of changing water decomposition and pH during electrolysis.

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