Proper ph Adjustment Method Based on the Consideration of Suppression of Bromic Acid Formation in Drinking Water Treatment Processes

Transcription

1 Proper Adjustment Method Based on the Consideration of Suppression of Bromic Acid Formation in Drinking Water Treatment Processes Yoshihiro Kobayashi, Koichi Murata, Takashi Tanaka, Katsuhiro Tagawa, Koya Tanaka Osaka Municipal Waterworks Bureau Kunijima purification plant Kunijima Higashi-Yodogawa-ku Osaka city k- Abstract The final water quality depends largely on the of the water to be treated, which is well-known to affect the bromic acid (bromate) production during ozonation and the turbidity after coagulation-sedimentation. Therefore, it is necessary to properly control the of the water to be treated, according to the permissible standards of the treated water quality. We conducted an investigation on the surface water of the Yodo river, using two coagulants commonly used in Japan. In this investigation, we sought an effective method of control throughout the entire advanced water treatment process, considering the performance of water treatment in a coagulation-sedimentation basin, the suppression of residual aluminum, the suppression of bromate production, the maintenance of the effectiveness of ozonation, and the prevention of lead elution. As the result, we could formulate the method to control the of the water to be treated for each treatment process. Keywords adjustment; advanced water treatment; bromate; residual aluminum INTRODUCTION For the purpose of control during water treatment, the Osaka Municipal Waterworks Bureau (OMWB) has implemented strict control since prior to the introduction of advanced water treatment. For example, we conducted control for the coagulation process and a countermeasure to lead elution from water supply piping. Afterward, advanced water treatment for the purpose of removing mold odor and reducing trihalomethane was introduced. At that time, it built the ozone contact basins in front of and behind the rapid sand filtering basin and a GAC adsorption tank (Figures 1 and 2 show the water treatment flow before and after the introduction of advanced water treatment). The revision of the drinking water quality standards in ) added bromic acid, a by-product of ozonation, as an additional water quality standard item ("not to exceed 0.01 mg/l"). During ozonation, the bromate production increases as the water temperature and the increase. For this reason, water treatment facilities that use ozonation became subject to the requirement that the performance of water treatment in coagulation-sedimentation and filtering basins be properly maintained while the during ozonation be maintained at a proper level. We conducted jar tests using aluminum sulfate and polyaluminum chloride (PAC), coagulants commonly used in Japan 2) with regard to coagulation-sedimentation that needs control, to identify the s under which satisfactory performance of water treatment (i.e., the coagulation-sedimentation basin water turbidity should be 0.5 degrees or lower) can be achieved. Once having identified such s, we conducted simulation on the bromate production based on the thus identified and other s and could formulate an effective method for controlling the and the rate of ozone, as reported herein.

2 Surface water of the Yodo river Coagulationsedimentation basin Chlorine Rapid sand filtering basin Chlorine mixing basin Final water Figure-1: Treatment Flow Before the Instruction of Advanced Water Treatment Surface water of the Yodo river Coagulationsedimentation basin Ozone contact basin Rapid sand filtering basin Ozone contact basin GAC adsorption tank Figure-2: Treatment Flow After the Introduction of Advanced Water Treatment Chlorine mixing basin Final water MATERIALS AND METHODS The control in water treatment is largely dependent on the s of coagulationsedimentation. For this reason, we extracted the and rate s for each coagulant under which satisfactory performance of water treatment can be achieved. Based on the extracted s, we then conducted an investigation of the residual aluminum concentration and estimated the bromate production during ozonation. Performance Investigation by Coagulant Aluminum sulfate is the coagulant used by the Osaka city. Because raw water is 7.5 on an average, its is controlled by adding acid (sulfuric acid) so that the coagulated water has a value of 6.8. Therefore, we adjusted the raw water to 7.0 and conducted jar tests with the same rate as the actual facility to extract the s with aluminum sulfate. Also, we conducted PAC-based jar tests as a comparison without controlling the of the raw water because the range suitable for coagulation is relatively wide 2). In addition, we conducted an investigation of the PAC rate using three different scenarios: (1) the rate equivalent to the proper aluminum sulfate rate when calculated by converting to aluminum, (2) the rate equivalent to aluminum (1) minus 2 ml/m 3, and (3) the rate equivalent to aluminum (1) minus 4 ml/m 3. The details of the experiment s are as shown in Table-1. Since the focus is on the turbidity, what we measured after coagulation and sedimentation were the turbidity and UV absorbance (E260). Investigation of Residual Aluminum Concentration The residual aluminum concentration in the coagulation-sedimentation water is closely related to the of the water; as the changes from around 7.0 to a higher, the residual aluminum concentration increases 3). When the raw water has a of 7.0 or higher as in the Osaka city, using PAC without control poses a concern that the residual aluminum concentration may increase because the tends to increase above 7.0. Hence, we conducted jar tests under the s shown in Table-2 to Water under test Coagulant Acid Coagulant Table-1: Jar Test Conditions (Performance of Water Treatment) Surface water of the Yodo river Average turbidity of raw water: 2.1 to 19.6 degrees Coagulant rate, aluminum sulfate: 19.6 to 31.6 ml/l Aluminum sulfate, PAC For aluminum sulfate only, adjust raw water to 7.0 using sulfuric acid Aluminum sulfate: proper rate PAC: (1) Injection rate equivalent to that of aluminum sulfate when calculated by converting to aluminum (2) Injection rate (1) minus 2 ml/m 3 (3) Injection rate (1) minus 4 ml/m 3 * Conversion to aluminum: As 8% aluminum sulfate Rapid mixing 148 rpm x 5 minutes Slow mixing 48 rpm x 15 minutes Still standing time 20 minutes Measurement items Turbidity, E260, and after still standing Sample container Beaker, 1 L

3 Bromate (mg/l) investigate the residual aluminum concentration after put in the still standing state. In this investigation, we considered the characteristic of PAC that it allows a wider range of values suitable for coagulation. Therefore, we conducted jar tests using two different raw water scenarios: One is the use of normal raw water without control and the other is the use of raw water adjusted to 8.0 by use of sodium hydrate as a typical example Table-2: Jar Test Conditions (Residual Aluminum Concentration) Water under test (a) Surface water of the Yodo river (without control by acid) (b) Adjust the surface water of the Yodo river to 8.0 using caustic soda Coagulant PAC (1) Injection rate equivalent to the proper aluminum sulfate rate Coagulant when calculated by converting to aluminum -2 ml/m 3 (2) Injection rate equivalent to the proper aluminum sulfate rate when calculated by converting to aluminum +2 ml/m 3 Rapid mixing 148 rpm x 5 minutes Slow mixing 48 rpm x 15 minutes Still standing time 20 minutes Measurement items Residual aluminum concentration, turbidity, and, after still standing of high- raw water. The results were compared with the residual aluminum concentration in the actual basin. Simulation on Bromate Production (1) Simulation Items The bromate production during ozonation increases as the and temperature of the water ozonized rise 4). Thus it is deemed that by calculating the rate of the increase of the bromate production as the rises, it is possible to predict how the bromate production will change when the of the water ozonized rises. Considering the above, we simulated the bromate production with the rising, based on the current bromate production. From the results thereof, we made an estimation of the permissible upper limits of of the water that processed at the coagulationsedimentation basin when the bromate production does not exceed the water quality standard value, 0.01 mg/l, at low and high water temperatures. This estimation assumed that the coagulant should meet the rate and value obtained in preliminary investigation. How bromate is produced differs between high- and low- water Injection rate (ml/m 3 ) Table-3: Comparison Conditions Aluminum sulfate Osaka city's current rate PAC Equivalent to that of aluminum sulfate Ozonation (0.0 to 0.6) Bromide concentration (mg/l) (average) DOC (mg/l) (average) Table-4: Raw Water Quality Alkalinity (mg/l) (average) Water temperature ( C) (average) Water temperature ( C) (max) Measured value Quantitation limit (0.001) or lower 0 5/6 8/14 11/22 3/2 6/10 9/18 12/27 Figure-3: Annual Transition of Bromate Production temperature seasons, and the currently measured bromate production is up to approximately mg/l at high water temperatures and, at low water temperatures, within the range between a value equal to or lower than the quantitation limit and approximately mg/l. Hence we carried out provisional calculations on the assumption that control is made taking such seasonal variations into account. Before carrying out provisional calculations, we investigated the current status of bromate production during the period from August of 2013 to September of Also, given that the range for a floe forming zone is 6.5 to 7.0 when aluminum sulfate is used, the Osaka city is

4 maintaining the coagulation at 6.8 throughout a year. There is little reduction during the phase from the coagulation-sedimentation process to the ozonation process. Therefore, our simulation assumed that the ozonation process took place at 6.8, the same value as the actual facility. (2) Simulation Method Since PAC has a wide coagulation range as described above, the performance of water treatment gained with PAC is assumed equivalent to that with aluminum sulfate as far as the turbidities of sediment water and filtered water are concerned, even without adjustment using acid. Also, the Osaka city is re-adjusting the of purified water to 7.5 as a countermeasure to lead elution from water supply piping. When PAC is used, however, less reduction is caused by the coagulant itself and therefore such re-adjustment is expected to require less caustic soda. Hence, as the s for ensuring the performance of water treatment equivalent to the current performance, we assumed the maximum when using PAC would be approximately 7.4 ( ) in comparison with the (=6.8) with aluminum sulfate, as shown in Table-3. Also, Table-4 shows the Osaka city's raw water quality. Figure-3 shows the results of the investigation of bromate production in the Osaka city's actual purification plants. Bromate is below mg/l when water temperature is low but it exhibits a relatively high value, approximately mg/l, during high water temperature seasons. Hence, we carried out provisional calculations on the dependency and investigated the impact on bromate production, considering the route where bromate was produced. PH Control Method Taking into Account the Entire Water Treatment Process Based on the proper identified as described above, we sought to determine a control method taking into account the entire water treatment process, separately for each of the conventional and advanced water treatment processes. RESULTS AND DISCUSSION Performance Investigation by Coagulant We conducted jar tests during low, ordinary, and high water temperature seasons, and Figure-4 shows the relationship between the turbidity and the coagulant rate along with the raw water s (during ordinary water temperature seasons). These results indicate that the PAC rate required to ensure that the turbidity of final water is stably maintained at 0.5 degrees or lower is equivalent to the proper aluminum sulfate rate when calculated by converting to aluminum. This applies to other seasons as well. For both aluminum sulfate and PAC, the turbidity of sediment water is 0.2 to 0.3 degrees.

5 Turbidity of final water (in degrees) E260(abs/cm) PAC (3) ((1)-4 ml/m³) PAC (1) (equivalent to aluminum) PAC(2) ((1)-2 ml/m³) Aluminum sulfate n=30 (raw water average) Turbidity: 4.7 degrees E260: : Coagulant rate (mg/l) Figure-4: Relationship between Turbidity of Final Water and Coagulant Injection Rate (Ordinary Water Temperature Seasons) PAC (3) ((1)-4 ml/m³) PAC (1) (equivalent to aluminum) PAC(2) ((1)-2 ml/m³) Aluminum sulfate n=30 (raw water average) Turbidity: 4.7 degrees E260: : Coagulant rate (mg/l) Figure-5: Relationship between E260 and Coagulant Injection Rate (Ordinary Water Temperature Seasons) Next, Figure-5 shows the relationship between E260 and the coagulant rate (during ordinary water temperature seasons). Similarly to turbidity, E260 is stably around 0.02 abs/cm if the aluminum sulfate and PAC rates are equivalent when calculated by converting to aluminum. These results indicate that the performance is stable throughout a year whichever coagulant is used and that PAC can provide performance almost equivalent to aluminum sulfate as long as its rate is equivalent to that of aluminum sulfate when calculated by converting to aluminum. Investigation of Residual Aluminum Concentration Table-5 shows the relationship between the residual aluminum concentration and. When treating ordinary raw water using PAC as the coagulant without control of raw water, the residual aluminum concentration was higher than when using aluminum sulfate. Also, under the assuming high- raw water, the residual aluminum concentration was within the water quality standard value of 0.2 mg/l but beyond the target value of 0.1 mg/l 1). Thus, from the viewpoint of controlling the residual aluminum concentration, control of raw water is deemed necessary when treating high- raw water. Coagulant Raw water Table-5: Relationship between Residual Aluminum Concentration and Jar test results PAC 7.42 Actual basin Aluminum sulfate after control by caustic soda - 8 (high ) - 8 (high ) - Coagulant rate (ml/m 3 ) ((1) Injection rate equivalent to the proper aluminum sulfate rate when calculated by converting to aluminum -2 ml/m 3 ) ((2) Injection rate equivalent to the proper aluminum sulfate rate when calculated by converting to aluminum +2 ml/m 3 ) 20.3 after control by acid Measured water (in degrees) Measured water turbidity (in degrees) Measured water residual aluminum concentration (mg/l)

6 Bromate (mg/l) Simulation on Bromate Production Bromate can be produced not only from bromide via hypobromite ion but also from radical species. For simplification purposes, however, we decided to make a rough estimate of the dependency of bromate production without considering the production by radical reaction 5). Formula (1) below expresses that the sum of Hypobromite ion [OBr-] and Hypobromite [HBrO] is C (constant): C = [OBr-] + [HBrO] Formula (1) Also, Formula (2) is based on the definition of the acid dissociation constant: Ka=[OBr-][H+]/[HBrO]=10 (-pka) Formula (2) From Formula (1) and (2), the Hypobromite ion concentration is given by Formula (3) (pka=8.59) 6). [OBr-] 10 -pka+ / ( pka+ ) Formula (3) In water treatment, the quantity of bromate produced from bromide is extremely small. Under such ozonation s, the bromate production can be estimated on the assumption that it is proportional to Formula (3). Table-6 shows the results of calculating the bromate production scale factor associated with the increase of during ozonation, as described above. Indeed it is a simple calculation taking into account only a particular route of the production of bromate. These results, however, effectively indicate that the bromate production increase rate associated with the increase of during Table-6: Bromate Production Increase Rate Associated with Increase Initial Table-7: Bromate Production Increase Rate Associated with Increase (From actual data expressions of the Osaka Water Supply Authority) Initial increase rate that assures water quality standard ozonation becomes increasingly higher as the rises. This finding well matches the actual data at the Osaka Water Supply Authority's purification plant that uses the surface water of the Yodo river as the water resource 7), similarly to the Osaka city (Table-7). Hence we made a verification of the control during ozonation, based on Table We assumed that the bromate production and increase actually measured at the Osaka city, respectively, slightly increased to +0.1 and increased more than two-fold to Also, we assumed a value of +0.6 to describe what would occur without raw water control by acid. We calculated the production amount expected in such cases based on Table-5 and the results 0.01 (Water quality standard) Current /6 8/14 11/22 3/2 6/10 9/18 12/27 Figure-6: Annual Transition of Bromate Production (Provisional Calculations)

7 thereof are shown in Figure-6. It turned out that the must be controlled within the current plus approximately +0.1 in order to ensure that the bromate production concentration is within the Osaka city's target range. In addition, the must be controlled within +0.4 to ensure that the said concentration is within the water quality standard value of mg/l. These results suggest that it is not preferable to allow the to increase significantly above the current value. Hence, maintaining the within the proper range (approximately 6.8) considering the bromate suppression during ozonation can ensure that the during the coagulation with aluminum sulfate meets the required coagulation. This also ensures that the is within the proper range ( 7.0 or lower) where there is no concern of the leak of residual aluminum. PH Control Method Taking into Account the Entire Water Treatment Process Figure-7 shows an example of control after ozonation is introduced, taking into account the proper at each water treatment process step. Figure-8 shows an example of the conventional control for comparison purposes Raw water Acid Suppression by acid When maintaining current water quality (PAC) Current treatment (aluminum sulfate) Coagulant Ozonation Ozonation Caustic soda Water supply Treatment process Raw water Acid When maintaining current water quality (PAC) Current treatment (aluminum sulfate) Coagulant Coagulation Caustic soda Water supply Treatment process Figure-7: An Example of Control During Water Treatment (Advanced Water Treatment) Figure-8: An Example of Control During Water Treatment (Conventional Water Treatment) At the Osaka city's purification plants, where the average of raw water is 7.5, it is preferable to make the during the coagulation-sedimentation and ozonation processes lower than that of raw water. Therefore, it is effective to perform control by acid prior to the coagulation-sedimentation process. On the other hand, it is necessary to make the water supply higher than the phase from coagulation-sedimentation to ozonation as a countermeasure to lead elution from water supply piping. This requires control by adding caustic soda. Also, the lowering effect of aluminum sulfate itself contributes to the reduction of acid (sulfuric acid) required for raw water control. However, this is not applicable when the is extremely low due to low alkalinity when turbidity is high. One of the methods to reduce bromate production is by reducing the ozone rate to offset the increase of bromate production caused by the increase of. However, this may render it impossible to implement control based on the dissolved ozone concentration. And, such reduction of the ozone rate is not realistic because it may degrade the performance of water treatment and may adversely affect the effect of suppressing the biological proliferation such as of chironomids 8). Thus it can be said that as far as the performance of removing turbidity loads and E260 is concerned, PAC allows treatment across a range wider than aluminum sulfate. Water treatment with high is, however, difficult from the viewpoint of suppressing the residual aluminum and bromate production concentrations. Therefore, proper control is necessary even if PAC is used.

8 CONCLUSIONS We conducted investigation of the proper of the water to be treated in the respective treatment processes using the surface water of the Yodo river, from the viewpoints of suppressing residual aluminum, suppressing bromate production, maintaining the effect of ozonation, and taking countermeasures to lead elution. As the result, we obtained the following findings regarding the control in advanced water treatment: (1) We compared aluminum sulfate versus PAC using jar tests and experiment facilities. As the result, it turned out that from the viewpoint of countermeasures to residual aluminum, proper control is required for the coagulation-sedimentation process whichever coagulant is used. (2) We conducted simulation on the and bromate production during ozonation. As the result, it turned out that the during ozonation must be maintained at approximately 6.8 to maintain the performance of ozonation and suppress the bromate production. (3) For advanced water treatment using ozonation, it is important to control the for each treatment process. It is necessary to maintain the coagulation within the proper range during coagulation-sedimentation, suppress bromate production during ozonation, and control the during water supply as a countermeasure to lead elution. Considering the above findings, we formulated the control method for the entire water treatment process according to the coagulant used. As described above, with the Osaka city's raw water quality, the relatively low and narrow coagulation range, one of the characteristics of aluminum sulfate, is effective on the suppression of bromate during ozonation. Also, the lowering effect of the coagulant itself is deemed to contribute to the raw water control. REFERENCES 1) Ministerial Ordinances on the Drinking Water Quality Standards (Ordinances of the Ministry of Health, Labour and Welfare No.101, May 30, 2003) 2) Technical Guide on the Use of Coagulants for Drinking Water Funded by Commission Fees from the Ministry of Health, Labour and Welfare, pp March, 2001, Japan Water Research Center 3) I Hara, et al "Relationship Between Water Temperature and Residual Aluminum Concentration During the Coagulation Process Using Aluminum Sulfate, 1998", OMWB Investigation Research and Testing Results, pp ) Y Kato, "Research on Efficient Application of Ozonation to Water Treatment and the Control of By-products", Yokohama National University, pp.9-10 (2015), Doctoral Dissertation 5) W. R. Haag, et al, "Ozonation of Bromide-Containinng Waters: Kinetics of Formation of Hypobromous Acid and Bromate", J. Environ. Sci. Technol., Vol.17, pp ) Handbook of Chemicals - Basics II, 5th Revision, Maruzen, pp.332 7) K Ueno, et al., "Current Status Regarding Bromate at Murano Purification Plant and Formulation of a New Ozone Control Method", pp , Osaka Water Supply Authority, Technical Research Presentation, 5th Collection of Papers 8) (accessible as of 2016/11/18)