Characteristics of Electrolysis, Ozonation, and Their Combination Process on Treatment of Municipal Wastewater

By Kishimoto, Naoyuki Morita, Yukako; Tsuno, Hiroshi; Yasuda, Yuuji

ABSTRACT: The characteristics of municipal wastewater treatment by electrolysis, ozonation, and combination processes of electrolysis and aeration using three gaseous species (nitrogen [N^sub 2^], oxygen [O2], and ozone [O^sub 3^]) were discussed in this research using ruthenium oxide (RuO^sub 2^)-coated titanium anodes and stainless-steel (SUS304) cathodes. Electrolysis and electrolysis with nitrogen aeration were characterized by a rapid decrease in 5-day biochemical oxygen demand (BOD^sub 5^) and total nitrogen and a slow decrease in chemical oxygen demand (COD). In contrast, ozonation, electrolysis with oxygen aeration, and electrolysis with ozone aeration were characterized by transformation of persistent organic matter to biodegradable matter and preservation of total nitrogen. The best energy efficiency in removing BOD^sub 5^ and total nitrogen was demonstrated by electrolysis, as a result of direct anodic oxidation and indirect oxidation with free chlorine produced from the chloride ion (Cl^sup – ^) at the anodes. However, electrolysis with ozone aeration was found to be superior to the other processes, in terms of its energy efficiency in removing COD and its ability to remove COD completely, as a result of hydroxyl radical (*OH) production via cathodic reduction of ozone.

Water Environ. Res., 79, 1033 (2007).

KEYWORDS: electrolysis, ozonation, advanced oxidation process, hydroxyl radical, ozonide ion, mediator, wastewater treatment, chemical nitrification, chemical denitrification, energy efficiency.

doi: 10.2175/106143007X184023

(ProQuest: … denotes formulae omitted.)

Inroduction

Biological processes, such as the activated sludge process, are currently used for practical wastewater treatment. It is well- established that biological processes have a high 5-day biochemical oxygen demand (BOD^sub 5^) removal capability and are economical. However, these biological processes are unstable against fluctuations in pollutant load and are easily affected by toxic substances in the wastewater. Therefore, chemical processes, such as electrolysis and ozonation, have been developed for assisting them.

Electrolysis is one of the promising technologies for environmental applications, such as remediation of soils, conversion of gaseous pollutants, water treatment, metal recovery, and sensing pollutants. Many researchers have paid attention to electrochemical methods regarding their versatility, energy efficiency, amenability t0 automation, and environmental compatibility (i.e., Rajeshwar et al., 1994). Several reviews were pubiished in electrochemical methods for environmental applications (i.e., Bockris et al., 1994; Gutierrez and Crespi, 1999; Rajeshwar et al., 1994). Many researchers have tried to apply electrolysis to wastewater treatment, One of the main objectives for electrochemical wastewater treatment is disinfection; electrochemical disinfection has been described by a number of authors (i.e., Drees et al., 2003; Drogui et al., 2001; Matsunaga et al, 2000; Patermarakis and Fountoukidis, 1990). The mechanism of electrochemical disinfection is direct oxidation and indirect oxidation by electrochemically generated oxldants’ such as chlonne and hydrogen peroxide (H^sub 2^O^sub 2^). Nutnent removal is the other major target of electrochemical treatment. The electrochemical treatment of nitrogen is well- discussed, and two types of reactions are used for the treatment; one is oxidation of ammonia, and the other is reduction of the nitrate ion (NO^sub 3^^sup -^ ). The oxidation of ammonia was achieved by direct anodic oxidation and indirect oxidation by electrochemically generated oxidants, such as chlorine (Kim et al., 2006; Monica et al., 1980; Poon and Brueckner, 1975; Szpyrkowicz et al., 1995; Vanlangendonck et al., 2005), and the final products were nitrogen gas and nitrate ion. The reduction of nitrate ion was achieved by cathodic reduction, and the final products were nitrogen and ammonia (Dash and Chaudhari, 2005; Gender et al., 1996). Some research works focused on the electrochemical removal of phosphate by electro chemical precipitation of insoluble phosphates (Monica et al., 1980; Poon and Brueckner, 1975). The remaining major targets of electrochemical wastewater treatment are chemical oxygen demand (COD) and organic pollutants removal. Much research has been conducted on COD and organic pollutants removal. There are tw0 ways to degrade organic pollutants-direct anodic oxidation md indirect oxidation by electrochemically generated oxidants (Comninellis and Nerini, 1995). However, application of direct anodic oxidation for water treatment has encountered a particular difficulty-the vain consumption of energy by water discharge. The application of boron- doped diamond electrodes is one approach to promote direct anodic oxidation (Canizares et al., 2005; Panizza et &, 2001; Troster et al., 2002), because the electrodes have a wide potential window for water discharge. However, introduc tl0n of mediators, such as the chlonde ion, is still a very effective countermeasure to solve this difficulty (Iniesta et al., 2001). Electrochemically generated chlorine reacts with water to produce hypochlorous acid (HCIO) in acid solution or hypochlorite ion(ClO^sup -^) in basic solution. The HClO and ClO^sup -^ have high standard potentials of 1.482 and 0.81 V, respectively, versus the normal hydrogen electrode (vs. NHE) (Vonysek, 2004). They can oxidize organic pollutants, such as phenols, and denitrify ammonia-nitrogen through break-point chlorination (Tchobanoglous et al., 2003). However, it is difficult to achieve mineralization of organic pollutants by chlorination only, and the process produces chlorinated organic pollutants (Iniesta et al., 2001).

Ozone is a strong oxidant, with standard potentials of 2.07 V in acid solution and 1.25 V in basic solution (vs. NHE) (Hoare, 1985). As ozonation produces fewer chlorinated organic pollutants than chlorination (Richardson et al., 1999), ozonation has been used for the disinfection process in water works. There have been attempts to apply ozonation to the treatment of organic pollutants, but it has proved difficult to mineralize organic pollutants by ozonation alone, although ozone can transform the organic pollutants into smaller molecules. Furthermore, the toxicity of aldehydes, one of the final products of ozonation, is a disadvantage (Can and Gurol, 2003; Nawrocki et al., 2003).

The aim of this study was to investigate the effects of three gaseous species (oxygen [O2], ozone [O^sub 3^], and nitrogen [N^sub 2^]) used as mediators assisting electrolysis in the treatment of municipal wastewater. We have summarized the treatment characteristics of electrolysis, ozonation, and combination processes of electrolysis and aeration using the three gaseous species and discussed the possibilities of each process, in terms of assisting biological processes.

Material and Methods

Experimental Apparatus. Figure 1 shows the experimental setup. The apparatus consists of a reactor with six electrodes, a tube pump for circulating water, an ozone generator, an injector of ozone gas, two ozone gas monitors (Ebara Jitsugyo PG-320, Tokyo, Japan), an ozone gas trap, a direct current (DC) power supply (A&D AD-8735, Tokyo, Japan), and a voltmeter (Advantest R6441A, Tokyo, Japan). The reactor is a glass box with a width of 7 cm, depth of 7 cm, and height of 143 cm. Three pairs of anodes and cathodes are installed in the reactor, with 5 mm between each electrode. As the ruthenium oxide (RuO^sub 2^) electrode is known to have high electrocatalytic activity on the promotion of Cl- to Cl2 (Kim et al., 2006), titanium coated with RuO2 (DAISO MD-52, Osaka, Japan) is used for the anodes. The cathodes are made of stainless steel (SUS304), which is economical. The effective total surface area is 0.3 m2 for both anodes and cathodes. Water is circulated from the top to the bottom of the reactor by the tube pump, at a flowrate of 1 .9 L/min. Water temperature in the reactor is regulated to 20 +- 1[degrees]C by circulating water at 20[degrees]C through a water jacket installed around the reactor. Pure nitrogen or pure oxygen gases (analytical- grade), supplied from high-pressure gas cylinders, is injected to the reactor through the injector installed in the circulating pump tube line. Ozone gas is produced from pure oxygen by the ozone generator and is then injected to the reactor through the injector. In these experiments, the concentration of ozone gas produced was set at 20 mgO^sub 3^/L. All gases were supplied at a flowrate of 0.5 L/min. The DC power supply was set at galvanostatic mode, with an electric current of 3.0 A (current density of 10 A m^sup -2^). All experimental runs were not repeated, because of the limitation of wastewater preparation. The observed electrode potentials were 5.2 V for electrolysis only, 5.1 V for electrolysis with N^sub 2^ aeration (N^sub 2^ electrolysis), 5.0 V for electrolysis with O2 aeration (O2 electrolysis), and 4.8 V for electrolysis with O^sub 3^ aeration (O^sub 3^ electrolysis).

Material. Municipal wastewater collected from a wastewater treatment plant in Shiga Prefecture, Japan, was filtered with a glass fiber filter (Whatman GF/B, pore size of 1 .0 [mu]t?, Whatman, Florham Park, New Jersey) before the experiments. The objectives of the filtration were to simulate an effluent from a primary sedimentation tank and avoid the deposition of solid on the electrode surface. The wastewater used was not sterilized. The water quality of the filtered wastewater is summarized in Table 1. The filtered wastewater was poured into the reactor and circulated by the tube pump for equalization. Then, the gas valve was opened, and the DC power supply and the ozone generator, if necessary, were turned on simultaneously. The switch-on time is defined as the beginning of an experiment. The filtered wastewater in the reactor was sampled at regular time intervals through a sampling tap. Chemical Analysis. The COD, BOD5, hydrogen peroxide (H^sub 2^O^sub 2^), total nitrogen, nitrate-nitrogen and nitrite-nitrogen (NO^sub x^-N), ammonia-nitrogen, chloride ion (Cl^sup -^), chlorine residual, dissolved ozone (DO^sub 3^), electrical conductivity (EC), and pH of samples were measured by the methods shown in Table 2.

Results and Discussion

pH Changes. The solution pH often affects treatment performance of electrochemical reactors (Iniesta et al., 2001; Vlyssides et al., 2002). Slight pH changes were observed over time during our experiments. The observed pH before and after treatment was as follows: ozonation only (7.3 – > 6.9), electrolysis only (7.1 – ? 7.5), N^sub 2^ electrolysis (7.1 [arrow right] 7.2), O2 electrolysis (7.6 [arrow right] 7.9), and O^sub 3^ electrolysis (7.3 [arrow right] 7.1). Thus, as all processes used here run under approximately neutral pH, we can discuss the difference in mediator effect of each process, without the discussion of pH effect.

Behavior of COD and BOD^sub 5^. Figure 2 shows time-course changes in COD and BOD^sub 5^. Using O^sub 3^ electrolysis, almost all COD and BOD^sub 5^ were removed within 8 hours of treatment. Using electrolysis only and N^sub 2^ electrolysis, BOD^sub 5^ was almost completely removed after 8 hours of treatment, although 57 to 58% of the initial amount of COD remained. Ozonation only and O2 electrolysis resulted in approximately 20 mg/L of COD and BOD^sub 5^ remaining. Thus, the processes applied were categorized into two groups based on the transition patterns of COD and BOD^sub 5^ with time. Electrolysis only and N^sub 2^ electrolysis were classified as group I, and ozonation only, O2 electrolysis, and O^sub 3^ electrolysis were classified as group II. In the processes of group I, amounts of COD and BOD^sub 5^ decreased simultaneously, and residuals after 8 hours treatment were mainly composed of nonbiodegradable organic matter. This means that the removal of BOD^sub 5^ was mainly successful. In contrast, BOD^sub 5^ temporarily increased during the first 1 or 2 hours when undergoing group II processes and then decreased simultaneously with COD. Enhancement of biodegradability (BOD5/COD ratio) by ozone and hydroxyl radicals was reported by a number of researchers (i.e., Chamarro et al., 2001; Contreras et al., 2005; Oh et al., 2003; Poznyak and Araiza, 2005; Wang et al., 2004) and is considered to be associated with the partial oxidation of organic matter to give low- molecular-weight oxygenated compounds (Beltran, 2004). In the group ? processes, except O2 electrolysis, ozone was fed into the reactor. Accordingly, we inferred that a certain amount of nonbiodegradable COD was transformed to BOD^sub 5^ at the beginning of treatment by ozone and/or hydroxyl radicals produced in the reactor. The mechanism of O2 electrolysis is discussed in the Treatment Mechanisms section.

Behaviors of Nitrogenous Compounds. Figure 3 shows time-course changes in total nitrogen, NOx-N, and ammonia-nitrogen. When the samples underwent group I processes, which include electrolysis only and N^sub 2^ electrolysis, ammonia-nitrogen was found to decrease, and NO^sub x^-N was found to increase slightly with the passage of time. The total nitrogen removal ratios after 8 hours of treatment by electrolysis only and N^sub 2^ electrolysis reached 65 and 43%, respectively.

In contrast, the total nitrogen in samples subjected to group ? processes, which include ozonation only, O2 electrolysis, and O^sub 3^ electrolysis, was not removed, while ammonia-nitrogen was found to decrease. For ozonation and O^sub 3^ electrolysis, NO^sub x^-N increased by almost the same amount as ammonia-nitrogen decreased, which indicates that chemical nitrification took place. However, the amount by which NO^sub x^-N increased was less than the amount of ammonianitrogen removed by O2 electrolysis. The missing nitrogen was transformed into chloramines, as shown in Figure 4.

Treatment Mechanisms. For electrolysis only, both free chlorine and combined chlorine were detected during treatment (Figure 4), and removal of total nitrogen and ammonia-nitrogen was observed. At an anode, the chloride ion present in municipal wastewater is oxidized to chlorine gas (Cl2), which partly reacts with water molecules to produce HClO, as follows (Monica et al., 1980):

… (1)

The mechanism for the removal of ammonia-nitrogen seems to involve the formation of chloramine and its subsequent degradation through the following reactions:

… (2)

… (3)

… (4)

… (5)

… (6)

The above mechanism is known as break-point chlorination. This explains why both free chlorine and combined chlorine were detected during treatment in the present study. We concluded that COD was removed by direct anodic oxidation and indirect oxidation by free chlorine. The same reactions were inferred to occur during N^sub 2^ electrolysis, because a high concentration of combined chlorine was observed (Figure 4). Thus, both electrolysis only and N2 electrolysis showed similar profiles, in terms of BOD^sub 5^-to-COD ratio and amounts of nitrogenous compounds.

The COD removal during ozonation was mainly caused by direct oxidation by ozone and, to some extent, by indirect oxidation by active oxygen species, such as the hydroxyl radical (*OH), which was produced through radical chain reactions (Tomiyasu et al., 1985). Ozone and *OH are known to react with ammonia (NH^sub 3^) and transform it to NO^sub 3^^sup -^ (Hoigne and Bader, 1978; Kuo et al., 1997; Singer and Zilli, 1975). However, the second-order reaction rate constant of the reaction of ozone and NH^sub 3^ is 20 M^sup -1^ *s^sup -1^, which is much smaller than that of *OH and NH^sub 3^, 8.7 x 10^sup 7^ M^sup -1^ *s^sup -1^ (Hoigne and Bader, 1978). In this research, the pH of municipal wastewater during ozonation was found to range from 6.9 to 7.3. Accordingly, most ammonia-nitrogen existed as ammonium (NH^sub 4^^sup +^), which does not react with ozone or *OH (Hoigne and Bader, 1978). The relatively small rate constant of the reaction with ozone and the low concentration of NH^sub 3^ result in slow chemical nitrification by ozone. In addition, as the amount of *OH produced by ozonation was small, the contribution of *OH to nitrification was limited. Thus, only a small amount of ammonia-nitrogen was nitrified during ozonation.

The profiles of COD and BOD^sub 5^ in O2 electrolysis were similar with those of ozonation only, although the ammonia-nitrogen depletion rate in O2 electrolysis was higher. Oxygen is known to form ozone in an anodic reaction (Amadelli et al., 2000), and the profiles of COD and BOD^sub 5^ indicate that ozone is one of the main oxidants in O2 electrolysis. This inference is supported by the detection of dissolved ozone during treatment (Figure 4). In addition, the presence of chloramines demonstrates the production of free chlorine from Cl^sup -^ at the anodes. As the pH in the anodic diffusion layer was low, most ammonia-nitrogen existed as NH^sub 4^^sup +^. This indicates that direct oxidation of ammonia-nitrogen by ozone produced at the anodes was limited. However, ozone can react with monochloramine, as follows (Haag and Hoigne, 1983):

… (7)

We concluded that nitrification of ammonia-nitrogen in O2 electrolysis proceeds through the reaction between ozone produced at anodes and monochloramine from the reaction between ammonia- nitrogen and free chlorine.

Some remarkable features of O^sub 3^ electrolysis were the complete removal of COD and the complete transformation of ammonianitrogen to NO^sub x^-N. In general, complete removal of COD is not achieved by ozonation and electrolysis around neutral pH (Beltran, 2004; Israilides et al., 1997). However, the complete removal of COD was observed in O^sub 3^ electrolysis. When ozone is injected to the electrolytic reactor, the following electrochemical reactions are expected (Kishimoto et al., 2005):

… (8)

… (9)

The standard potentials of reactions 8 and 9 are 1 .25 V (vs. NHE) (Hoare, 1985) and 1.23 V (vs. NHE) (Kishimoto et al., 2005), respectively. These potentials are much higher than those of the competitive cathodic reactions, such as hydrogen (H^sub 2^) formation from hydrogen ion (H+) (O V), OH^sup -^ formation from O2 (0.401 V), hydroperoxide ion (HO^sub 2^^sup -^) formation from O2 (- 0.0649 V), and superoxide ion (*O^sub 2^^sup -^) formation from O2 (- 0.33 V). Therefore, reactions 8 and 9 proceed simultaneously at the cathode. The ozonide ion (*O^sub 3^^sup -^) produced in reaction 9 sequentially reacts with water (H2O) and produces *OH, as follows (Tomiyasu et al., 1985):

…(10)

The species *OH has high standard potentials of 2.38 V in acid solution and 1.55 V in basic solution (vs. NHE) (Hoare, 1985) and is believed to be responsible for most decomposition in organic molecules (Andreozzi, 1999). We inferred that the complete removal of COD in O^sub 3^ electrolysis was achieved by *OH produced in reactions 9 and 10. Reactions 9 and 10 proceed near the cathodes, where the pH is high enough for ammonia-nitrogen to exist as NH^sub 3^. As a result, nitrification of ammonia-nitrogen was enhanced in the cathodic diffusion layer by *OH and ozone. The reaction between ozone and monochloramine also contributed to the enhanced nitrification of ammonia-nitrogen, in a similar fashion to O2 electrolysis. Energy Efficiency. In Figure 5, cumulative amounts of removed COD, BOD^sub 5^, and ammonia-nitrogen were plotted against energy consumed. The energy consumed was calculated using the sum of the electric power consumed by the DC power supply (power factor: 0.60) and the ozone generator. The electric power consumed by the ozone generator is strongly influenced by the oxygen source (pure oxygen or air). Although the maximum ozone generation efficiency using pure oxygen was reported to be approximately 220 g/kWh (Kitayama and Kuzumoto, 1997), it did not include the energy for producing pure oxygen. A relatively low concentration of ozone gas (20 mg/L) was used in this study. In the low concentration range, ozone generation from air is generally used for the practical treatment. The ozone generation efficiency using air was reported approximately 80 g/kWh at the ozone gas concentration of 20 mg/L (Kitayama and Kuzumoto, 1999). When ozone gas is generated from air, nitrogen oxides contaminate the ozone gas (Braun et al., 1990). Accordingly, a nitrogen oxide remover from the ozone gas should be used before water treatment. The ozone generation efficiency of 80 g/ kWh is unable to be applied for calculation of the energy efficiency in our experiments. In the actual ozone facilities for the drinking water treatment plant, ozone generation efficiency from air, including the energy for the air dryer and the nitrogen oxide remover, was reported to be 67 g/kWh (Oka et al., 2006). Thus, the energy consumed by ozone generation was calculated from 67 g/kWh in Figure 5. Although our estimated energy consumption is highly biased because of lower ozone utilization efficiency comparison with practical treatment plants, we can use it to compare the relative energy efficiencies of the processes discussed. The characteristics of each treatment may be seen in Figure 5. In terms of energy efficiency in COD removal, O^sub 3^ electrolysis was found to be superior to the other treatments, though the energy efficiency of electrolysis was nearly equal to that of O^sub 3^ electrolysis at the beginning of treatment. However, electrolysis was found to be the most effective in removing BOD^sub 5^ and ammonia-nitrogen. The O2 electrolysis and N^sub 2^ electrolysis were not efficient in removing COD, BOD^sub 5^, and ammonia-nitrogen. Ozonation was superior to O2 electrolysis and N^sub 2^ electrolysis in COD removal efficiency, but was the worst in ammonia-nitrogen removal efficiency. Consequently, different treatment technologies were found to be favorable, depending on the objective of water treatment and possible combinations of pre- and/or post-treatment. For the purpose of removing BOD^sub 5^ and ammonia-nitrogen, electrolysis was found to be the best process because of its high energy efficiency and its ability to transform ammonia-nitrogen to nitrogen gas. When COD removal is the main purpose of water treatment, however, O^sub 3^ electrolysis was found to be the most favorable, because of its ability to remove COD completely and relatively high energy efficiency in this research. Much research has been conducted on the electrochemical treatment of practical wastewater. Szpyrkowicz et al. (1995) studied electrochemical treatment of tannery wastewater and reported energy efficiency ranging from 38.0 to 69.0 kWh/kgCOD in raw wastewaer (initial COD = 1774 mgO/L, Cl^sup -^ concentration = 2562 mgCl/L) and ranging from 68.0 to 459 kWh/ kgCOD in treatment of secondary effluent (initial COD = 176 to 458 mgO/L, Cl^sup -^ concentration = 1935 to 2560 mgCl/L). Israilides et al. (1997) reported that the energy efficiency of electrochemical treatment of olive oil wastewater with the addition of sodium chloride (NaCl) at 4 w/v% reached 4.73 kWh/kgCOD at the COD removal of 76% (initial COD = 178 000 mgO/L). Panizza and Cerisola (2006) also treated olive mill wastewater with the addition of 5 g/L NaCl, and energy efficiency of 3 1 .4 kWh/kgCOD was observed for 96% removal of COD (initial COD = 26.5 g/L). In the electrolysis of a secondary effluent of domestic wastewater (EC = 500 [mu]S/cm, Cl^sup -^ concentration = 57 mgCl/L) by Miller and Knipe (1965), electric power of 1331 kWh/kgCOD (calculated from their experimental data) was required to remove 80% of initial COD (60 mgO/L). Vlyssides et al. (2002) obtained energy efficiency of 38.54 kWh/ kgCOD in electrolysis of domestic wastewater at pH 7 with addition of NaCl at 0.8 w/v% (initial COD = 1047 mgO/L). In our O^sub 3^ electrolysis experiment, the COD removal ratio reached 84% (56.0 [arrow right] 9.0 mgO/L) after 6 hours of treatment, and the electric power required amounted to 52.4 kWh/m^sup 3^. This indicates energy efficiency of 1117 kWh/kgCOD, which was much higher than the results by Israilides et al. (1997), Szpyrkowicz et al. (1995), Panizza and Cerisola (2006), and Vlyssides et al. (2002), and smaller than the result by Miller and Knipe (1965). Judging from these results, the addition of a large amount of NaCl seems to be very effective. However, this is not necessarily developed by the mediator effect of Cl^sup -^. The initial COD of wastewater used by Israilides et al. (1997) and Panizza and Cerisola (2006) was very high. The high concentration of reactants generally shows high current efficiency in electrochemical treatment. Furthermore, a large amount of NaCl gives a high EC to wastewater and reduces the electric power per electric current. Therefore, we cannot directly compare the mediator effect of ozone with the mediator effect of Cl^sup -^ obtained by the research mentioned above. Focusing the mediator, residual COD after electrolysis with addition of NaCl may cause another problem. In the study by Israilides et al. (1997), COD almost reached a plateau of 11 000 mgO/L after 10 hours electrolysis. Accordingly, a post-treatment will be required to adapt the treated water quality for the criteria. Furthermore, as chlorination of wastewater often forms hazardous chlorinated compounds (Freuze et al., 2005; Zhang and Minear, 2006; Yang et al., 2005), chlorination misleads us about the effect of the chlorinated compounds to human health and natural environment. Although Cl^sup -^ is an excellent mediator, the addition of a large amount of NaCl causes another problem in water reclamation and reuse-high salinity in treated water. From this point-of-view, O^sub 3^ electrolysis seems to be superior to simple electrolysis, because O^sub 3^ electrolysis does not add any residual matter, and the complete removal of COD reduces the possibility of hazardous material accumulation in the treated water.

In general, chemical processes are relatively expensive compared with biological processes. Accordingly, the practical application of chemical processes is focused on certain cases as follows:

(I) When an objective of water treatment cannot be achieved by any other economical process;

(?) When a chemical process assists other economical processes; and

(III) When a biological process is difficult to apply because of wastewater toxicity, a lack of space, the absence of an operator, and an odor problem.

In an example of case I, micropollutants, such as 1,4-dioxane, are not removed by the conventional activated sludge process (Abe, 1999). On the contrary, the authors have confirmed that O^sub 3^ electrolysis can degrade 1,4-dioxane (Kishimoto et al., 2007). Therefore, the application of O^sub 3^ electrolysis as a post- treatment of the activated sludge process may be effective in the sanitization of treated water through removal of micro-organic pollutants. In an example of case II, electrolysis may be applicable in the treatment of residual BOD5 in secondary effluents. In particular, when electrolysis is applied after an activated sludge process in which nitrification is inhibited by aeration control, simultaneous removal of residual BOD5 and total nitrogen may be expected. As an example of case III, biological processes are unstable towards toxic substances, and fluctuations in BOD^sub 5^ load are difficult to automate and require vast areas for operation. Accordingly, chemical processes are suitable for small-scale wastewater treatment plants.

Conclusions

In the present study, the treatment characteristics of electrolysis, ozonation, and combination processes of electrolysis and aeration using three gaseous species (N^sub 2^, O2, and O^sub 3^) were experimentally discussed. The conclusions obtained are as follows:

(1) The processes applied were categorized into two groups according to the resulting transition patterns of COD, BOD^sub 5^, and total nitrogen. Electrolysis and N^sub 2^ electrolysis constitute group I; these are characterized by rapid decreases in BOD^sub 5^ and total nitrogen and a slow decrease in COD. In contrast, ozonation, O2 electrolysis, and O^sub 3^ electrolysis, which are members of group II, result in the transformation of persistent organic matter to biodegradable matter and the preservation of total nitrogen.

(2) The COD and BOD^sub 5^ removal mechanism in electrolysis and N^sub 2^ electrolysis consists of anodic oxidation and oxidation by free chlorine formed from Cl^sup -^ at the anodes. Biodegradable organic pollutants were degraded before nonbiodegradable pollutants by these processes. Ammonia-nitrogen was completely transformed to nitrogen gas by the free chlorine produced from Cl^sup -^ at the anodes.

(3) One of the main oxidants in ozonation and O2 electrolysis was ozone. In addition to ozone, free chlorine produced at the anodes and anodic oxidation contributed to the nitrification of ammonia- nitrogen and the removal of BOD^sub 5^ and COD in O2 electrolysis.

(4) Hydroxyl radicals produced through cathodic reduction of ozone resulted in complete removal of COD in O^sub 3^ electrolysis. A high local pH in the cathodic diffusion layer transformed NH^sub 4^^sup +^ to NH^sub 3^. Consequently, nitrification of ammonia- nitrogen was enhanced in the cathodic diffusion layer by *OH and ozone, and complete nitrification was observed in O^sub 3^ electrolysis. (5) Electrolysis was found to be the most energy- efficient process for removing BOD^sub 5^ and ammonia-nitrogen. However, O^sub 3^ electrolysis was the best of five processes discussed, in terms of its capability of removing COD completely, its energy efficiency in removing COD, and less possibility of formation of chlorinated organic compounds.

Credits

Part of this work was financially supported by the Grants-in-Aid for Scientific Research (No. 16760445), Ministry of Education, Culture, Sports, Science and Technology, Tokyo, Japan; the Research Foundation for the Electrotechnology of Chubu (No. R- 14234), Japan; and Joint Research Center for Science and Technology, Ryukoku University, Otsu, Japan.

Submitted for publication November 18, 2005; revised manuscript submitted September 13, 2006; accepted for publication January 23, 2007.

The deadline to submit Discussions of this paper is December 15, 2007.

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Naoyuki Kishimoto1*, Yukako Morita2, Hiroshi Tsuno3, Yuuji Yasuda4

1 Inroduction Department of Environmental Solution Technology, Faculty of Science and Technology, Ryukoku University, Otsu, Japan.

2 Kubota Co Ltd Hyoeo Japan.

3 Department of Urban Environment Engtneenng, Graduate School of Engineering, Kyoto University, Kyoto, Japan.

4 Yokohama Research & Development Center, Mitsubishi Heavy Industries, Ltd., Japan.

* Department of Environmental Solution Technology, Faculty of Science and Technology, Ryukoku University, 1-5 Yokotani, Setaoe- cho, Otsu 520-2194, Japan; e-mail: naoyuki@rins.ryukoku.ac.jp.

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