18 Rapid Disinfection of Combined Sewer Overflow Using Dioxide

Ｒ. Takahashi1 , T.Kirihara2 , M.Koeda3

1 Director, 2 Chief Researcher, 3 Senior Researcher Japan Institute of Wastewater Engineering Technology

ABSTRACT This is a technology in which disinfection of untreated sewage from pump stations in combined sewer systems and primary effluent from final treatment plants is performed by oxidization using . As a distinctive feature of the technology, the disinfectant is rapidly and uniformly injected and diffused in the water channel cross-section, resulting in substantially improved disinfection efficiency in comparison with conventional methods using chlorine disinfectants such as sodium .

KEYWORDS Disinfection, chlorine dioxide, ClO2 generation system, ClO2 injection system

1. OUTLINE OF TECHNOLOGY 1.1 Principle of technology Chlorine dioxide is expressed by the chemical formula ClO2, and is a reddish-yellow gas with a pungent odor resembling chlorine or ozone at room temperature. Its physical constants are shown in Table 1. Chlorine oxide is water soluble and exists in aqueous solutions in the form of ClO2. Because it is strongly oxidizing and has the following distinctive properties, it is considered suitable for use as a disinfectant in combined sewers: 1) Rapid disinfection effect in comparison with chlorine agents (sodium , etc.), 2) In the range of pH 6-10, the effect of pH on disinfection effectiveness is not as great as with chlorine, 3) Does not react with ammonia, 4) Does not form tri-halide-methane, and 5) Does not display persistence, and therefore has little adverse effect on public waters. Furthermore, contact with coliform groups is achieved within a short time as a result of rapid and uniform injection/diffusion of ClO2 in the water channel cross-section. By increasing the disinfection speed, this makes it possible to reduce disinfectant consumption. Table 1 Physical constants of chlorine dioxide Molecular weight 67.46 Boiling point 11°C (101kPa) Melting point –59°C (101kPa) Specific gravity 2.33 (gas, 11°C, relative value when air = 1) Approx. 130°C Flash point (Partial pressure of ClO2 in air: 13-40kPa) Liquid: Reddish-brown Gas: Reddish-yellow Appearance Crystal (anhydride): Yellowish-orange (hydride): Yellow

This disinfection technology comprises a ClO2 generation system and a ClO2 injection system which ensures rapid diffusion of the ClO2 in the water channel.

Takahashi et al. 1 1.2 ClO2 generation system Because ClO2 decomposes easily, on-site production is necessary. Figure 1 shows a schematic flowchart of the ClO2 generation system (case of 3-solution suction method). The 3-solution suction Figure 1 Schematic flowchart of ClO2 generation system method is a method in which (case of 3-solution suction method) influent sewage or secondary effluent is Controller Control valve supplied to the ClO (solenoid valve) 2 generation system, creating Reactor a suction force in the reactor Feed water Hydrochloric pump which lifts the chemicals, Ｐ acid: 15% HCl resulting in reaction of the Strainer chemicals and formation of Sodium chlorite Sodium hypochlorite ClO2 in the device. Three 25% NaClO2 12% NaOCl chemical solutions are used, Pump these being sodium chlorite (25wt%), sodium Sand Pump well Primary sedimentation Secondary hypochlorite (12wt%), and screen pond screen hydrochloric acid (15wt%). The reaction equation is shown in Eq. – 1. 2Na ClO2 + NaOCl + 2HCl Æ 2 ClO2 + 3NaCl + H2O Eq. – 1

1.3 ClO2 injection system (equipment for promoting ClO2 diffusion) This is a system for promoting rapid and uniform diffusion of the injected ClO2 in the water channel. It comprises injection nozzles installed a multiple locations and a baffle plate (installed downstream from the nozzles).

2. DEVELOPMENTAL RESEARCH 2.1 Performance requirements and judgments standards for technical evaluation The performance requirements and judgment standards for the technical evaluation in SPIRIT 21 are shown in Table 2. Table 2 Performance requirements and judgments standards for technical evaluation Technical Performance requirement Judgment standard evaluation item Treatment Shall reduce total coliform in Confirm that total coliform in treated water is 3000/cm³ performance effluent to 3000/cm³ or less. or less in demonstration experiments. Time for obtaining disinfection Confirm necessary reaction time quantitatively, and Disinfection effect shall be short. verify that high disinfection efficiency is achieved in efficiency comparison with the conventional technology. Safety of Shall be little effect on aquatic life Confirm that the effect of treated water on aquatic life downstream in downstream drainage basin as a in downstream basin is slight from genotoxicity tests, water basin result of disinfection. ecotoxicity tests, and tri-halide-methane formation. Shall reduce chemical and power Prepare model design based on results of consumption. demonstration experiments, and confirm reduction of Others chemical/power consumption by study of running costs, including chemical/power consumption.

2.2 Location and periods of experiments Table 3 shows an outline of the location where the demonstration experiments were conducted, together with the periods of the experiments. Figure 3 shows a flowsheet of the experimental system at the Kanazawa Water Treatment Plant.

2 Rapid Disinfection of Combined Sewer Overflow Using Chlorine Dioxide

Figure 3 Flowsheet of system used in experiment

ClO2 generator Dilution sodium ※1 To dilution sodium hypochlorite hypochlorite tanks tank. P (switching) Miscellaneous ※１ ※1 water M M F P

P Miscellaneous ClO2 Sodium Hydro- Sodium feed chlorite hypo- water tank pump chloric P tank acid chlorite (25%) tank tank (15%) (12%) Sodium Baffle plate hypochlorite S S S Sampling position pump

Distribution tank Experimental water tank (experimental system)

Possible to installS baffle plate P

Automatic feed Experimental water tank water pump (conventional system) (for water intake) Sodium P Secondary screen hypochlorite stock Primary screen solution Kanazawa Water Treatment Plant combined sewage sand sedimentation Pump well pond

Table 3 Location and period of experiments Clear weather experiment Rainy weather experiment Location of experiment Kanazawa Water Treatment Plant (Yokohama) Period of experiment February 2004 to April 2004 June 2004 to July 2004 Kanazawa Plant sand sedimentation pond Kanazawa Plant sand sedimentation pond Raw water influent (clear weather) influent (rainy weather) Comparative study of disinfection speed Demonstration of disinfection effect Outline of experiment with ClO2 and conventional technology using sewage in rainy weather. (sodium hypochlorite). Remarks Installed on slab at B3F sand sedimentation pond.

2.3 Results of developmental research 2.3.1 Results of clear weather experiment A comparison of the disinfection results with ClO2 injection and the conventional technology (injection of sodium hypochlorite in injection pipe) is shown in Figure 5. With the conventional technology, the total coliform in the treated water was 2.2 x 103 to 2.7 x 3 3 10 /cm with an injection rate of 21 mg/L. In contrast, with ClO2 injection, the total coliform in the treated water was less than 3000/cm3 with an injection rate of 15-16mg/L. 2.3.2 Results of rainy weather experiment An experiment with untreated sewage during rainy weather (influent at Kanazawa Water Treatment Plant sand sedimentation pond) was performed three times during rainfall when the maximum hourly precipitation rate was 5 mm/h or higher.

Takahashi et al. 3 The results of ClO2 treatment under conditions of ClO2 injection rates of 5, 7.5, and 15 mg/L and a contact time of 3 min showed that the total coliform and fecal coliform counts were less than 3000/cm3 in all treated water samples. An outline of the results of these demonstration experiments is shown in Table 4.

Table 4 Outline of results of demonstration experiments Experiment No. R-1 R-2 R-3 June 6, 2004; June 25, 2004 July 29, 2004 Date/time of experiment 9:00-17:00 11:00-16:00 9:00-17:00 No. of rain-free days prior to experiment 4 days 3 days 2 days※１ 35.0(Kanazawa Water 9.0(Kanazawa Water 5.5(Kanazawa Water Treatment Plant) Treatment Plant) Treatment Plant) 15.5(Kanazawa Pump 9.0(Kanazawa Pump 22.0(Kanazawa Maximum hourly precipitation (mm/h) Station) Station) Pump Station) 16.0(Mutsu-ura Pump 10.0(Mutsu-ura Pump 11.0(Mutsu-ura Station) Station) Pump Station) 23.0(Kanazawa 62.0(Kanazawa Water 20.0(Kanazawa Water Water Treatment Treatment Plant) Treatment Plant) Plant) 50.0(Kanazawa Pump 26.5(Kanazawa Pump Total precipitation (mm) 37.0(Kanazawa Station) Station) Pump Station) 48.0(Mutsu-ura Pump 26.5(Mutsu-ura Pump 22.0(Mutsu-ura Station) Station) Pump Station) Raw water for disinfection Untreated sewage Untreated sewage Untreated sewage Contact time (min) 3 3 3 Duration of experiment (h) 5 4 6 Disinfectant injection rate (mg/L) 5.0, 7.5, 15 7.5, 15 7.5, 15 Raw 1.9×104-3.9×105 4.3×105-2.4×106 3.2×104-3.4×106 water Total coliform (coliform/cm3) Treated 1.0×102※2-3.0×102 1.5×102-2.5×103 1.0×101※3-3.8×102 water Total coliform inactivation Treated 2.3-3.4 2.5-4.2 2.6-4.6 ratio water Residual ClO concentration Treated 2 0.5-12.3 1.0-7.2 2.7-13.1 (mg/L) water Residual sodium chlorite ion Treated 4.2-12.0 4.6-9.9 4.7-10.2 concentration (mg/L) water Residual ion Treated 1.3-3.0 1.6-3.0 2.5-4.5 concentration (mg/L) water Figure 5 Comparison of disinfection with ClO2 and conventional technology

2.25分後（二酸化塩素） After3分後（二酸化塩素） 3 min (ClO ) After 2.25 min (ClO2) 2 After2.25分後（次亜塩素酸ﾅﾄﾘｳﾑ） 2.25 min (NaOCl ) After3分後（次亜塩素酸ﾅﾄﾘｳﾑ） 3 min (NaOCl ) 3000/cm³3000個/cm3

Results of2/25 experiment実験結果 on Feb. 25 Results of experiment on March 3 3/3実験結果 107 107

]

] 3 3 106 106 ] ] 3 3 105 105 /cm /cm 個 個 [ [ 104 104

103 103 大腸菌群数 大腸菌群数 102 102

101 101 Total coliform[count/cm Total coliform[count/cm 0 5 10 15 20 25 0 5 10 15 20 25 Injection注入率 rate[mg/L] [mg/L] Injection注入率 rate[mg/L] [mg/L]

4 Rapid Disinfection of Combined Sewer Overflow Using Chlorine Dioxide

2.3.3 Disinfection effect of ClO2 Figure 6 Dependence of total coliform (i) Effect of operational factors on disinfection inactivation rate on ClO2 injection ate and Operational factors in disinfection include contact time (batch experiment) the ClO injection rate and the contact time. 2 0 Figure 6 shows the total coliform inactivation rate obtained with ClO2 injection rates of 5- 1 ClO二酸化塩素2 injection 20mg/L and contact times of 0.5-5 min plotted rate注入率 2 against the contact time. From this figure, the 5.0mg/L 7.5mg/L increase in the total coliform inactivation rate 3 10mg/L is large until a contact time of 1 min, and the 20mg/L 4

subsequent increase is gradual. It is therefore 大腸菌群数不活化率 [Log] considered that an adequate value for

5 disinfection using ClO2 was obtained with a 0123456 接触時間 [min] contact time of 3 min, which was the time set Total coliform inactivationrate [Log] Contact time [min] in the demonstration experiments. (ii) Relationship between residual ClO 2 Figure 7 Relationship between residual ClO2 concentration and total coliform inactivation concentration and total coliform inactivation rate rate (demonstration experiments) The relationship between the residual ClO2 concentration and total coliform inactivation 0 rate obtained from the demonstration 1 experiment results is shown in Figure 7. 2 Because the ClO2 injection rate was intentionally reduced, here, the data in the 3 figure include data in which the total coliform 4 in the treated water exceeded 3000/cm3. As 5 can be seen in Figure 7, the total coliform 大腸菌群数不活化率 [Log] inactivation rate tends to increase as the 6 residual ClO concentration increases. 0.1 1.0 10.0 100.0

2 Total coliform inactivationrate [Log] Residual残留二酸化塩素濃度 [mg/L] ClO2 concentration In Figure 8, the total coliform inactivation [mg/L] rates obtained in the batch experiments and demonstration experiments are arranged in terms of the product of the residual ClO2 concentration and contact time. The straight Figure 8 Relationship between (residual line in the figure shows the results of an ClO2 concentration x contact time) and total empirical equation (y = 0.69Ln(x) + 1.55, R = coliform inactivation rate 0.78) obtained from the experimental data. 0 実証実験Demonstration バッチ実験Batch (iii) Relationship between raw water quality 1 and ClO2 consumption 2

3

4 大腸菌群数不活化率 [Log] 5

6 Total coliform inactivationrate [Log] 110100 Residual残留二酸化塩素濃度×接触時間 [mg・min/L] ClO2 concentration x contact time [mg・min/L]

Takahashi et al. 5 The ClO2 injection rate necessary to Figure 9 Relationship between turbidity obtain the targeted inactivation rate for total and ClO2 consumption colifom in the raw water is greatly affected 6月6日 20 June 6 by the content of ClO2-consuming substances 6月25日June 25 7月29日 in the raw water. The results of a study of the

July 29 16 raw water quality items which contribute to ClO2 consumption showed high correlation 12 factors for BOD and turbidity and for

8 CODMn and turbidity. The solid fraction was therefore thought to account for a large 4 percentage of total organic matter. Figure 9 二酸化塩素消費量 [mg/L] consumption [mg/L] consumption

2 shows the relationship between turbidity and 0 ClO2 consumption. Because the relationship ClO 0 100 200 300 400 Turbidity濁度 [度] [degree] between turbidity and ClO2 consumption was substantially linear regardless of the day when the measurements were made, to a certain extent, it was considered possible to estimate ClO2 consumption from turbidity alone. Equation 2 is an empirical equation which was obtained from the relationship between turbidity and ClO2 consumption.

(ClO2 consumption) = 0.055 x (turbidity degree) – 2.618 (Eq. – 2)

2.3.4 Verification of safety of downstream drainage basin The effect of the disinfection technology in which ClO2 is added to sewage on aquatic life in the downstream drainage basin was investigated. Table 5 shows the results of a safety assessment of sewage to which disinfectants were added. From this, it was found that the toxicity of water treated with ClO2 is on approximately the same level as that of raw water and water treated with sodium hypochlorite.

Table 5 Results of safety assessment Dis- Umu test Microtox test infectant Specimen addition Metabolic activity (S9 mix) EC50 [%] with no concentration rate [mg/L] Yes No After 5 min After 15 min

Raw water － Negative Negative 1.81 2.42

Water treated with ClO2 25 Negative Negative 2.13 2.46

Water treated with NaOCl 30 Negative Negative 1.82 2.31

Table 6 shows the results of measurements of tri-halide-methane when ClO2 and sodium hypochlorite were added to untreated sewage sampled in rainy weather and the solutions were allowed to stand at room temperature for 24 hours. Unlike sodium hypochlorite, no increase in tri-halide-methane was observed with ClO2.

Table 6 Results of measurement of tri-halide-methane formation Tri-halide-methane formation rate (mg/L) Type of disinfectant Injection rate (mg/L) CHCl3 CHBrCl2 CHBr2Cl CHBr3 T-THM (Raw water) － 0.002 <0.001 <0.001 <0.001 <0.005

6 Rapid Disinfection of Combined Sewer Overflow Using Chlorine Dioxide

7.5 0.002 <0.001 <0.001 <0.001 <0.005 ClO2 25 0.002 <0.001 <0.001 <0.001 <0.005

NaOCl 30 0.022 0.010 0.007 0.005 0.044

Standard for drinking water quality (Japan) ≦0.06 ≦0.03 ≦0.1 ≦0.09 ≦0.1

WHO standard ≦0.2 ≦0.06 ≦0.1 ≦0.1 (*) (*) The sum total of the ratios of individual measured values to the guideline values shall not exceed 1. Note: Specimens were tested 24 hours after addition of the disinfectant.

2.4 Economic comparison Table 7 shows a comparison of the annual running cost when this disinfection technology is applied to disinfection of the primary effluent in water treatment plants and the annual running cost of sodium hypochlorite disinfection in advanced (tertiary) sewage treatment.

Table 7 Comparison of annual running costs (water treatment rate: 10,000 m3/h) Item ClO2 Sodium hypochlorite Raw water to be disinfected Untreated sewage Tertiary effluent Planned water quality Total coliform ※1 3.4 x 106/cm3 – Planned injection rate 15 mg/L – Average injection rate 8 mg/L 3 mg/L ※3

Na ClO2 ¥160/kg Unit cost of chemicals NaOCl ¥30/kg ¥30/kg (assumed) HCl ¥30/kg Running cost per unit of treated water ¥9.32/m3 ¥0.75/m3 Annual amount of water treated 40 Qm3/y 8760 Qm3/y ※2※4 (2 x Qm3/h x 3h x 20d /y x 1/3) (Qm3/h x 24h x 365d/y) Annual running cost not including ¥3,728,000/y ¥65,700,000/y power base-rate cost (¥9.32/m3 x 40 Qm3/y) (¥0.75/m3 x 8760 Qm3/y) Annual power base-rate cost ¥420,000/y Negligible. Annual running cost ratio 6.3 100 Remarks ((1) + (2)) /65,700 x 100 (Notes) ※1 The value of total coliform in “Planned water quality” was set referring to the results of demonstration experiments (including those by other companies). ※2 “Q” means the designed water treatment rate (m³/h) of the water treatment plant. ※3 The injection rate of sodium hypochlorite was set hypothetically at 3 mg/L based on the value of 2-4 mg/L in “Sewage Facility Planning/Design Guidelines and Commentary” (Japan Sewage Works Association, 2001 Ed.). ※4 The amount of untreated sewage generation was assumed to be 1/3 of the amount of primary effluent (Japan institute of Wastewater Engineering Technology, Sewerage and Wastewater Management Dept., City and Regional Development Bureau, Ministry of Land, Infrastructure and Transport, March 2002). As the frequency of occurrence of primary effluent, release of primary effluent corresponding to 2Q for a continuous period of 3 hours was assumed to occur 20 times/year.

2.5 Technical evaluation The results of an evaluation of this technology are shown in Table 8. Table 8 Results of evaluation of technology Technical Development target Results of evaluation evaluation items (performance requirement) Treatment Shall reduce total coliform in Was confirmed to meet performance requirement. performance effluent to 3000/cm3 or less. Total coliform in treated water can be reduced to Treatment Time for obtaining disinfection less than 3000/cm³ in a contact time of less than 3 efficiency effect shall be short. min when treating untreated sewage.

Takahashi et al. 7 Shall be little effect on aquatic life in Was confirmed to meet performance requirement. the downstream drainage basin as a When disinfectant was injected in untreated sewage Safety of result of disinfection. under addition conditions where disinfection can be downstream sufficiently achieved, safety was judged to be on drainage basin the same level or higher than with disinfection by the conventional technology. Shall reduce chemical/power Chemical and power consumption are within the consumption. Equipment shall be practical range. Equipment is compact and can be compact, and shall be possible to retrofitted easily in existing facilities. Others install in storm water pump stations and combined sewage treatment plants.

3. FEATURES OF TECHNOLOGY The distinctive features of this disinfection technology are as follows. 1) Rapid disinfection effect in comparison with chlorine disinfectants (sodium hypochlorite, etc.). 2) In the range of pH6-10, the effect of pH on disinfection effectiveness is not as great as with chlorine. 3) Does not react with ammonia. 4) Does not form tri-halide-methane. 5) Does not display persistence, and therefore has little adverse effect on public waters. 6 The ClO2 injection system (equipment for promoting diffusion of ClO2) enables rapid and homogenous injection and diffusion of ClO2 in the water channel cross-section. 7) The equipment is compact and can be retrofitted easily in existing facilities.

4. APPLICABLE SCALE AND LOCATIONS 1) Applicable scale There are no particular limitations on the applicable scale of this technology. 2) Applicable locations (i) Pump stations: Storm water sand sedimentation ponds, discharge channels and discharge pipes for release of untreated sewage (ii) Water treatment plants: Discharge channels and discharge pipes for release of primary effluent In particular, this is a superior technology for locations without chlorine contact tanks where disinfection by conventional techniques is inadequate. Furthermore, because existing discharge channels/pipes can be used without modification to perform the function of a chlorine contact tank, retrofitting of existing plants is extremely easy.

Please contact for the further information of this paper: 2-1, Suehiro-cho, Tsurumi-ku, Yokohama-shi, Kanagawa, Japan, Water Purification & Waste Treatment Plant Sec. Water Treatment Plant Engineering Department, Water and Waste Water Engineering Division JFE Engineering Corporation e-mail: [email protected]

8 Rapid Disinfection of Combined Sewer Overflow Using Chlorine Dioxide