Electrochemical disinfection is an efficient method used for treatment of drinking water. It has great environmental compatibility as compared to conventional disinfection methods. In this study, the effects of the electrode materials and working conditions were investigated. The experimental results show that the type 1 (iridium oxide-coated anode and cathode) electrode system generated a high concentration of free available chlorines (FACs) because iridium has higher electrocatalytic activity than ruthenium. When the applied voltage increases, the acceleration of oxidation reactions in the electrochemical cell resulted in the increased generation of FACs. The solution inflow rate is approximately inversely proportional to the residence time of the salt solution in the electrochemical cell. A long residence time can induce a higher FAC generation. In addition, the production of FACs is increased with the decreasing electrode open ratio (a/A). With a/A > 0 and a lower inflow rate, the FAC concentration tends to approach a maximum value because of by-product generation. The ozone species generated in the electrochemical cell were determined by the maximum voltage. The electrode open ratio affected the ozone generation rate due to the mixing effect of cathode products.

INTRODUCTION

Water pollution has become a critical problem due to the rapid development of industries in past decades. The scarcity of clean and safe drinking water has reduced life expectancy and increased infant mortality rates, particularly in developing countries (Gadgil 1998). The most common water pollution is of biological origin (Crittenden et al. 2005; Tang et al. 2014), which includes the mixing of various micro-organisms (e.g. viruses, pathogenic bacteria, protozoa) and parasites in water. To prevent waterborne diseases, both physical and chemical water disinfection methods are frequently used (Gadgil 1998; Vasudevan & Oturan 2014). The physical methods of water disinfection include boiling, filtration, and UV disinfection. Boiling and filtration are traditional and very effective methods for water disinfection, however they require a lot of energy. The UV disinfection method is also very effective for various types of pathogenic bacteria and viruses, yet it requires periodic maintenance, e.g. UV lamp replacement. The chemical water disinfection technologies usually involve the addition of oxidation species into raw water for oxidization of any harmful micro-organisms present. The major oxidation species used for water disinfection are ozone and chlorine. Ozone (O3) is a potent oxidation species that can be decomposed into an oxygen molecule and a nascent oxygen atom. Although ozone can exhibit a strong disinfection effect, it cannot provide residual protection against recontamination in a water distribution system (Gadgil 1998; Crittenden et al. 2005; Loo et al. 2012; Amin et al. 2013; Tang et al. 2014).

Chlorine is currently the most widely used water disinfectant. Traditionally, liquid chlorine is injected into raw water, producing HOCl (hypochlorous acid), OCl (hypochlorite ion), H+, and Cl. The hypochlorous acid is a stronger bacterial disinfectant than the hypochlorite ion among these species (Gadgil 1998; Crittenden et al. 2005). Although the liquid chlorine method provides a greater residual disinfection concentration than other chemical methods (e.g. ozone), it has a critical disadvantage in that liquid chlorine needs an appropriate supply chain and there are associated safety issues. It also shows a relatively low residual concentration compared to the other chlorine-based disinfection method, i.e. an electrochemical disinfection process (Gadgil 1998; Diao et al. 2004).

The electrochemical disinfection process is based on electrical decomposition of highly concentrated sodium chloride solution into a variety of oxidants in the electrochemical cell (Patermarakis & Fountoukidis 1990; Gadgil 1998). The major products, Cl2, HOCl, and OCl, from this process are called free available chlorines (FACs). Recently, advancements have been made in the electrochemical disinfection method to generate mixed oxidants including ozone (Jeong et al. 2006; Jung et al. 2006). Researchers at Los Alamos Technical Associates, Inc. (Gram et al. 1988; Baker & Bradford 1994) have demonstrated a mixed oxidant electrochemical cell and have tested mixed oxidants (FAC and ozone species) production performance. They found that the mixed oxidant solution can result in strong inactivation of specific micro-organisms. The possible applications of electrochemical disinfection in water treatment include small water treatment utilities, remote communities, emergency and disaster areas (Venczel et al. 1997). The mixed oxidants can be generated on-site in the field and it requires little time and cost to maintain the system. Moreover, this kind of system merely needs water and salt or seawater to generate mixed oxidants without special chemicals. Jeong et al. (2006) studied the role of reactive oxygen species (e.g. O3, H2O2) compared with FACs in the electrochemical inactivation of micro-organisms, and their results showed that the reactive oxygen species have a higher disinfecting activity than FAC species. Although a number of studies have been conducted to investigate the exact mechanism of mixed oxidant production, there is still no clear mechanism due to the limitation of detection techniques for the mixed species (Gordon et al. 1998, 2002). The following mechanism is widely accepted for mixed oxidant production (Martínez-Huitle & Brillas 2008; Jeong et al. 2009).

Anode reactions: 
formula
Cathode reaction: 
formula
The reaction mechanism shows that the most important point of the mixed oxidant process is to balance the production of FACs and reactive oxygen species. To improve the performance of production of mixed oxidants, the electrochemical cell must be optimized.

In this study, the effects of electrode type and working conditions of the electrochemical cell on the generation of mixed oxidant disinfectants have been investigated. The different combinations of anode and cathode materials in the mixed oxidant electrochemical cell consisting of two electrodes have been examined. With regard to electrochemical cell working conditions, voltage-current characteristics, effect of the electrode open ratio, and the rate of salt solution inflows have been investigated in detail.

EXPERIMENTAL

Figure 1 shows the design of the mixed oxidant electrochemical cell used in this study. Two electrodes are embedded in each polymethylmethacrylate (PMMA) plate and these plates are assembled with a PMMA spacer between them. The cell has one flow inlet (for salt solutions) and two outlets (for a mixed oxidant stream and a waste stream including hydrogen gas and sodium hydroxide). The mixed oxidant flow outlet is located at the anode side because the reactions for mixed oxidants occur there. The PMMA spacer here partially separates the facing side of the anode and the cathode in order to suppress the reaction between the mixed oxidants and the cathode side products. The electrode open ratio (a/A) of the spacer area (a) over the electrode area (A) is varied between 0 and 0.5 to investigate the separation effect on mixed oxidant generation. The 3.5% sodium chloride solution, a model of seawater, has the flow rate (Q) ranging from 100 to 500 mL/min. In this study, the cell volume was measured to be 16.2 mL (9 × 9 × 0.2 cm). The averaged residence time of the salt solution was calculated to be 9.72 s, 29.16 s and 48.6 s for each inflow rate, respectively. The voltage applied across the two electrodes ranges from 2.5 to 4.5 V. Figure 2(a) shows an image of the experimental setup. A gear pump (ISM 901B, ISMATEC, Switzerland) supplies stirred solutions into the electrochemical cell and a DC power supply (DRP-305DN, Digital, Korea) provides the electrical voltage.
Figure 1

Schematic illustration of the mixed oxidant disinfectants generator.

Figure 1

Schematic illustration of the mixed oxidant disinfectants generator.

Figure 2

(a) Experimental setup for the generation of mixed oxidant solutions; (b) cross section SEM image of the ruthenium oxide-coated DSA electrode.

Figure 2

(a) Experimental setup for the generation of mixed oxidant solutions; (b) cross section SEM image of the ruthenium oxide-coated DSA electrode.

The key component of the mixed oxidant electrochemical cell is two electrodes. The anode part of the cell is called a dimensionally stable anode (DSA). The DSA (Samsung DSA Co., Korea) is made of titanium and various metal coatings on it (e.g. platinum, ruthenium oxide, iridium oxide) (Lacasa et al. 2013; Pan et al. 2014; Rajab et al. 2015). Figure 2(b) shows the cross section scanning electron microscope (SEM) image of the ruthenium oxide-coated DSA. In this study, two kinds of DSA and two kinds of cathodes are used. Table 1 lists the combination of these types along with the specification of the anode (iridium oxide and ruthenium oxide-coated anode) and the cathode (iridium oxide-coated cathode and SUS304 (stainless steel type 1.4301, C: Cr: Ni = 0.08%: 18.5%: 9%)).

Table 1

Electrode types and specifications

 Type 1
 
Type 2
 
Type 3
 
Electrode type Anode Cathode Anode Cathode Anode Cathode 
Base material Ti Ti Ti SUS304 Ti Ti 
Coating material IrO2 IrO2 IrO2 – RuO2 IrO2 
Coating thickness 2 μm 2 μm 2 μm – 2 μm 2 μm 
 Type 1
 
Type 2
 
Type 3
 
Electrode type Anode Cathode Anode Cathode Anode Cathode 
Base material Ti Ti Ti SUS304 Ti Ti 
Coating material IrO2 IrO2 IrO2 – RuO2 IrO2 
Coating thickness 2 μm 2 μm 2 μm – 2 μm 2 μm 

Electrode dimension: 70 × 70 mm.

Distance between anode and cathode: 6 mm.

The concentration of FAC in the mixed oxidant solution was measured by the N,N-diethyl-p-phenylenediamine (DPD) colorimetric method. Upon mixing of DPD FAC powder with a sample solution, a red-violet color appeared. This mixture was treated with a colorimeter (Pocket Colorimeter II, HACH Co., Loveland, USA) to measure the FAC concentration. Because the measurement range of the colorimeter device is from 0.1 to 8 mg/L FAC (usual range of the DPD method), the original sample was diluted with deionized water with a 1:100 volume ratio (Vsample: VDI = 0.1 mL: 10 mL). The value of the diluted sample was analyzed by the colorimeter and the original FAC concentration was calculated considering the dilution factor. For measuring reactive oxygen species present in the mixed oxidant solution, the ozone species were detected by the 4-aminoantiphrine color comparison method using an enzyme (WAK-O3, Kyoritsu, Japan). The mixed oxidant sample solution was put in a test tube and allowed to react with the reagent for 1 minute. After the reaction, the solution color was compared with a standard color chart. All ozone measurement experiments were conducted three times at room temperature.

Other experimental measurements were performed four times for each data point. The errors were estimated by student's t-distribution with a 95% interval, given by: 
formula
where t, σ, and N are the average value for each measurement, the t-distribution value, the standard deviation, and the number of experiments, respectively.

RESULTS AND DISCUSSION

Characteristics of iridium oxide-coated electrode cell

In characterizing the working conditions, iridium oxide-coated anodes and cathodes (type 1 in Table 1) were used for the mixed oxidant electrochemical cell. The working conditions such as the applied voltage, open ratio of the electrode (a/A), and inflow rate of the salt solution were considered as variable parameters. As important performance indicators, the total current generated through the electrochemical cell and the FAC concentration of the mixed oxidant solution were also measured.

Figure 3 shows the current-voltage characteristics of the electrochemical cell. The total current linearly increases with the increasing voltage in all cases. This result is well matched with Ohm's law. The total current was found to increase with the increasing open ratio of electrodes (for the cases of a/A =0, it was impossible to increase the voltage beyond 4 V due to the current limitation of our DC power supply). The results however show that the inflow rate of the salt solution has a negligible effect on the total current (Figure 3(a)). As the inflow rate has almost no effect on current, the error bars were added for the 300 mL/min case only.
Figure 3

Voltage-current characteristics of the mixed oxidant electrochemical cell with the type 1 electrode combination against the applied voltage, the salt solution inflow rate and the electrode open ratio of (a) a/A = 0.5, (b) a/A = 0.25, (c) a/A = 0.

Figure 3

Voltage-current characteristics of the mixed oxidant electrochemical cell with the type 1 electrode combination against the applied voltage, the salt solution inflow rate and the electrode open ratio of (a) a/A = 0.5, (b) a/A = 0.25, (c) a/A = 0.

Figure 4 shows the FAC generation as the function of various working conditions. The concentration of FAC increases with increasing voltage due to the acceleration of oxidation reactions in the electrochemical cell. The FAC concentration however decreases with the increasing salt inflow rate. This can be explained by the decreased residence time of the salt solution in the electrochemical cell (Hsu 2005). This result suggests that there must be an optimal operating condition between the applied voltage and the inflow rate for efficient FAC generation. One interesting aspect of the result is that the concentration of FAC seems to approach a maximum value when the applied voltage is higher than 4 V and a/A > 0 in the low inflow rate (e.g. 100 mL/min) condition. This phenomenon is mainly due to the generation of by-products (e.g. oxygen gas) at a high voltage (>4 V) and the partially separated facing electrodes (a/A > 0). When the flow rate is relatively low, the by-products cannot be removed and they would also hinder the ionic current generation. This result is consistent with previous studies by Kraft et al. (1999), Ghernaout et al. (2011) and Schaefer et al. (2015).
Figure 4

FAC generation characteristics of the mixed oxidant electrochemical cell with the type 1 electrode combination against the applied voltage, the salt solution inflow rate and the electrode open ratio of (a) a/A = 0.5, (b) a/A = 0.25, (c) a/A = 0.

Figure 4

FAC generation characteristics of the mixed oxidant electrochemical cell with the type 1 electrode combination against the applied voltage, the salt solution inflow rate and the electrode open ratio of (a) a/A = 0.5, (b) a/A = 0.25, (c) a/A = 0.

Dependence on electrode materials

Figure 5(a) shows the results with SUS304 as the cathode (type 2 in Table 1) and Figure 5(b) shows the results with ruthenium oxide as an electrode coating material for the anode (type 3 in Table 1). The salt flow rate and electrode open ratio were fixed at 100 mL/min and zero, respectively, because the highest FAC concentration was found in this conditions. The trend for the FAC generation and total current is basically the same as described for type 1 electrodes.
Figure 5

Voltage-current characteristics and FAC generation characteristic for (a) type 2 and (b) type 3 electrode combinations.

Figure 5

Voltage-current characteristics and FAC generation characteristic for (a) type 2 and (b) type 3 electrode combinations.

The experimental results show that the type 1 electrode system, which has iridium oxide as a coating material for both anode and cathode, generates a greater FAC concentration compared to the others. Two basic requirements of electrode materials for efficient mixed oxidant generation are high oxygen over-potential and higher electrocatalytic activity. Iridium has a higher electrocatalytic activity than ruthenium which resulted in higher FAC generation efficiency in the experiments (Kraft et al. 1999; Jeong et al. 2009).

Characteristics of ozone species generation

A possible reaction mechanism of generating ozone species is , as discussed in the introduction. This reaction is not stable, because ozone species are maintained in the mixed oxidant solution for rather a short time. In addition, as applied voltage increases, the by-products generation rate increases simultaneously (Kraft et al. 1999; Ghernaout et al. 2011; Schaefer et al. 2015). To investigate the characteristics of ozone species generation in the electrochemical cell, the maximum voltage to produce ozone species was determined. The maximum voltage to produce ozone species is the limiting voltage where ozone species can be detected in the electrochemical cell and measurement system. Figure 6 shows the maximum voltage against electrode type for different electrode open ratios. As the main product in the mixed oxidants is FAC and the highest FAC generation was found with the type 1 electrode system (see Figures 4 and 5), ozone generation was investigated in more detail with the type 1 electrode system. The plot shows that the maximum voltage increases with the lower electrode open ratio but there is little change with the electrode type (no spacer case). This observation can be attributed to the mixing effect of ozone species with other products on the cathode surface. As the electrode open ratio increases (separation area increases), fewer ozone species can mix with cathode side products. It suggests that the change in the open ratio may enable control over the reaction between the anode side products and the cathode side products.
Figure 6

Maximum voltage of ozone species versus electrode type for different electrode open ratios.

Figure 6

Maximum voltage of ozone species versus electrode type for different electrode open ratios.

An inactivation study for a specific micro-organism such as Cryptosporidium is needed (Venczel et al. 1997). Recently, Venczel et al. (1997) reported that the mixture oxidants exhibit a disinfection efficacy on Cryptosporidium. They however did not explain the inactivation mechanism clearly. For the ozone species disinfection method, Finch et al. (1993) demonstrated that a relatively high concentration of ozone species can inactivate Cryptosporidium. The inactivation study may strongly confirm the existence of reactive oxygen species such as ozone.

CONCLUSIONS

This experimental study was conducted on the effect of electrode materials and some key working conditions on efficient generation of mixed oxidant disinfectants. Three types of electrode combinations were examined while varying working conditions such as applied voltages, salt solution inflow rates, and electrode open ratio. The experimental results showed that the type 1 (iridium oxide-coated anode and cathode) electrode system generates more FACs as compared to others under similar working conditions. This was ascribed to the higher electrocatalytic activity of iridium. The effect of the operating conditions on generation of mixed oxidant disinfectants are summarized as follows: when the applied voltage increases, the acceleration of oxidation reactions in the electrochemical cell can result in increased generation of FACs. The solution inflow rate relates to the residence time of the salt solution in the electrochemical cell, and a long residence time could result in high FAC generation. In addition the production of FACs is increased with decreasing electrode open ratio (a/A). When a/A > 0 and the inflow rate is low, the FACs concentration tends to approach a maximum value because of the generation of by-products. The ozone species generation in the electrochemical cell was determined by the maximum voltage that can produce ozone species. The electrode open ratio affects the ozone generation due to the mixing effect with cathode side products. These all important operating conditions can be utilized in designing an optimal mixed oxidant disinfectant generator. For future work, studies on sodium chloride solution concentration and inactivation of micro-organisms (e.g. Cryptosporidium) are underway.

ACKNOWLEDGEMENTS

This work is supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MEST) (No. NRF-2014R1A2A2A01003618).

REFERENCES

REFERENCES
Amin
M. M.
Hashemi
H.
Bovini
A. M.
Hung
Y. T.
2013
A review on wastewater disinfection
.
International Journal of Environmental Health Engineering
2
,
22
.
Baker
F. A.
Bradford
W. L.
1994
Electrolytic cell for generating sterilization solutions having increased ozone content. US Patent No. 5,316,740
.
Crittenden
J. C.
Trussell
R. R.
Hand
D. W.
Howe
K. J.
Tchobanoglous
G.
2005
Water Treatment: Principles and Design
.
2nd edn
,
John Wiley & Sons
,
NJ
.
Finch
G. R.
Black
E. K.
Gyurek
L.
Belosevic
M.
1993
Ozone inactivation of Cryptosporidium parvum in demand-free phosphate buffer determined by in vitro excystation and animal infectivity
.
Applied and Environmental Microbiology
63
(
4
),
1598
1601
.
Gadgil
A.
1998
Drinking water in developing countries
.
Annual Review of Energy and Environment
23
,
253
286
.
Gordon
G.
Evans
F. L.
Goodrich
J. A.
1998
Electrochemical Mixed Oxidant Treatment: Chemical Detail of Electrolyzed Salt Brine Technology
.
IT corporation
,
Cincinnati, Ohio
.
Gordon
G.
Evans
F. L.
Goodrich
J. A.
2002
Measuring oxidant species in electrolyzed salt brine solutions
.
Journal American Water Works Association
94
(
10
),
111
120
.
Gram
H. F.
Muller
M. E.
Pendergrass
A. M.
Rink
P. A.
1988
Electrolytic method and cell for sterilizing water. US Patent No. 4,761,208
.
Jeong
J.
Kim
J. Y.
Yoon
J.
2006
The role of reactive oxygen species in the electrochemical inactivation of microorganisms
.
Environment Science and Technology
40
(
19
),
6117
6122
.
Jung
J. J.
Baek
K. W.
Park
S. Y.
Oh
B. S.
Kang
A.
Kang
J. W.
2006
Water disinfection and control strategy of disinfection by-products in electrochemical process
. In:
Proceedings of the AWWA Water Quality Technology Conference
,
Denver, CO, Curran Associates, Inc
.
Kraft
A.
Stadelmann
M.
Blaschke
M.
Kreysig
D.
Sandt
B.
Schröder
F.
Rennau
J.
1999
Electrochemical water disinfection Part I: hypochlorite production from very dilute chloride solutions
.
Journal of Applied Electrochemistry
29
(
7
),
861
868
.
Lacasa
E.
Tsolaki
E.
Sbokou
Z.
Rodrigo
A. M.
Mantzavinos
D.
Diamadopoulos
E.
2013
Electrochemical disinfection of simulated ballast water on conductive diamond electrodes
.
Chemical Engineering Journal
223
,
516
523
.
Loo
S. L.
Fane
A. G.
Krantz
W. B.
Lim
T. T.
2012
Emergency water supply: a review of potential technologies and selection criteria
.
Water Research
46
(
10
),
3125
3151
.
Martínez-Huitle
C. A.
Brillas
E.
2008
Electrochemical alternatives for drinking water disinfection
.
Angewandte Chemie International Edition
47
(
11
),
1998
2005
.
Pan
C. J.
Hou
Y. H.
Zhang
B. B.
Zhang
L. C.
2014
Fabrication of anticoagulation layer on titanium surface by sequential immobilization of poly(ethylene glycol) and albumin
.
Bio-Medical Materials and Engineering
24
(
1
),
781
787
.
Patermarakis
G.
Fountoukidis
E.
1990
Disinfection of water by electrochemical treatment
.
Water Research
24
(
12
),
1491
1496
.
Tang
W. W.
Xia
J.
Zeng
X. P.
Wu
L. J.
Ye
G. Q.
2014
Biological characteristics and oxidation mechanism of a new manganese-oxidizing bacteria FM-2
.
Bio-Medical Materials and Engineering
24
(
1
),
703
709
.
Vasudevan
S.
Oturan
A. M.
2014
Electrochemistry: as cause and cure in water pollution – an overview
.
Environmental Chemistry Letters
12
(
1
),
97
108
.
Venczel
V. L.
Arrowood
M.
Hurd
M.
Sobsey
D. M.
1997
Inactivation of Cryptosporidium parvum oocysts and Clostridium perfringens spores by a mixed-oxidant disinfectant and by free chlorine
.
Applied and Environmental Microbiology
63
(
4
),
1598
1601
.