19. Rapid Disinfection Technique Using High- Concentration Ozone for Combined Sewer Overflow

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1 19.RapidDisinfectionTechniqueUsingHigh- Concentration for Combined Sewer Overflow. Takahashi 1, T.Kirihara 2, M.Koeda 3 Director, ChiefResearcher,Senior Researcher 3 Second Research Department Japan Institute of Wastewater Engineering Technology OUTLINE Disinfection properties A rapid disinfection technique using high concentration ozone makes it possible to disinfect a combined sewer overflow (CSO), such as untreated wastewater and primary treated at a coliforms concentration less than 3,000 cells/cm 3. High concentration ozone generated with a highly efficient ozone is dissolved immediately in the samples with a strong gassolution mixing pump. A rapid disinfection within 2 minutes can be achieved using this technique. Sterilization effect with ozone derives from the oxidation ability that directly destroys the cell wall of bacteria. On the other hand, in the case halogenous sterilizing agents such as chlorine, the disinfection mechanism is different from ozone. They destroy bacterial enzymatic systems when they pass through bacteria s cell wall and enter to the inside of the cell. Disinfection system Table 1 shows major components which construct a system proposed here. Figure 1 represents a schematic diagram of the rapid disinfection system. This system consists of ozone, ozone dissolving and independent power generation. Furthermore, the ozone consists of a raw gas supplying device and a high-concentration ozone (Mitsubishi Electric s original design with a continuous throughput of O 3, 180 g/m 3 (N)). As a safeguard, an exhaust ozone decomposer in exhaust gas is set up in the system. Table 1. List of main components constructing rapid disinfection system Component Description High-concentration ozone Using silent discharge to generate oxygen gas into high-concentration ozone Exhaust ozone Decomposing ozone in exhaust gas, which is left decomposer over from reaction, back into oxygen dissolving Concentration measuring instrument dissolving pump Independent power Measuring concentrations of ozone produced in the ozone, exhaust ozone and dissolved ozone in water Getting ozone dissolved in raw sewage water effectively A gas turbine-driven supplying various pieces of equipment included in the system

2 Pollution source Oxygen Independent power generation Waste ozone decomp oser River Lake dissolving pump Pump Waste water tube Sand Sedimentation Tank Pump well Proposed technology Waste Water Treatment Plant Figure 1. Schematic Diagram of High-Speed Sterilization System Table 2 shows the common objectives of a disinfection group in the sewage project SPIRIT21 promoted by the Ministry of Land Infrastructure and Transport. Table 3 represents ones which Mitsubishi Electric Corp. set forth in the hope of exceeding the requested performance described in Table 2. Table 2. Objectives of a disinfection group in the SPIRIT21 project Item of assessment Requested performance Treatment performance Achieving a concentration of coliforms of less than 3,000 cells/cm 3 in effluent Efficiency of disinfection Short time disinfection Safety of downstream Disinfection has small affect on organisms living downstream Others Decreasing the amount of chemicals and electric power Table 3. Objectives of Mitsubishi Electric Corp Item of assessment Requested performance Treatment performance Achieving a concentration of coliforms of less than 3,000 cells/cm 3 in effluent Efficiency of disinfection Accomplish adequate sterilization in a short period of time (within 2 minutes) Safety of downstream Restrain on disinfection s impact on organisms living downstream. Impact should be less than that from untreated water (as determined by umu test). Others Economic feasibility (Accomplish a significant reduction in running cost.) METHOD A demonstration test was run using a field test facility built in J wastewater treatment plant. 2

3 Samples of untreated water were obtained from a primary settlement pond. Table 4 shows the outline of the field test. Table 4. Field Test Specifications Test site J wastewater treatment plant Test period June, 2003 ~ August, 2004 Main features of test equipment Raw sewage water Untreated wastewater and primary treated water Processing flow rate 3 m 3 /h (maximum) O 3 injection rate 200 mg/l (maximum) O 3 consumption rate 120 mg/l (maximum) Field test equipment Figure 2 illustrates water treatment flow taking place in the field test equipment. Highconcentration ozone produced by the ozone is introduced from the dissolving pump s suction-side pipe, etc. in accordance with predetermined ozone sterilization conditions (ozone injection rate and reaction time). Powerful agitating action of the dissolving pump is utilized to dissolve ozone efficiently and sterilize coliforms in the untreated water. Sample tank injector sensor Cooler Exhaust Waste ozone decomposer reactor Sample tank B Sample output Oxygen Clear tube FI Window Reaction time 3min 2min Control Sample output Pump 1min Sample tank A Clear tube dissolving pump Air-water separator Sample tube Treated water tube Water pump Sand sedimentation tank Tap water tube & air tube Figure 2. Schematic Diagram of Field Test Equipment Labo-scale experiment For preliminary tests, beaker tests had been conducted to estimate roughly the ozone concentration which would be required to sterilize raw sewage water, i.e., untreated water obtained from J treatment plant to the target value (coliforms less than 3,000 cells/cm 3 ). In addition to that, it was checked whether the disinfection performance could be maintain under conditions causing a large variation in quality of untreated water at fixed reaction time of 1.7 minutes. Further, beaker tests had been carried out in regard to untreated water (obtained from a 3

4 primary settlement tank) for other 9 treatment plants ( A ~ I ) except the one ( J ), too, to elucidate the effect of water quality on the ozone concentration to reduce the number of coliforms lower than the target value. Finally, a genetic toxicity test was done in order to evaluate the safety of treated wastewater on ecosystem in downstream, as well. RESULTS AND DISCUSSION Preliminary test using labo-scale experimental apparatus Figure 3 shows the results of ozone treatment obtained by injecting ozone into raw sewage water sampled from a primary settlement tank of J plant in fine weather. The beaker test was done with ozone consumption rate ranging from 0 to 150 mg/l (ozone injection rate 0 ~ 450 mg/l). From this figure (indicating that sterilization effect begins to show when ozone consumption rate goes beyond 50 mg/l), it can be seen that the untreated water is of nature causing much ozone to be consumed without disinfection effects. Under the influence of this nature, ozone consumption rate which was necessary to bring down coliforms concentration to the target value of less than 3,000 cells/cm 3 was on the order of 100 mg/l. Reaction time for the test was set to within 2 minutes. Verification test using field test equipment Filed test on untreated water was carried out three times in rainy weather. Figure 4 shows the results of the test. The value of an ombrometer installed at the test site ( J treatment plant) was used to record the amount of rainfall. From these results it was determined that an ozone consumption rate of 100 mg/l would enable the test equipment to achieve the performance of disinfection (coliforms less than 3,000 cells/cm 3 ) constantly. Coliforms concentration (cells/cm 3 ) consumption rater (mg/l) Figure 3. Coliforms Concentration in Treated Water 4

5 Precipitation (mm) consumption rate (mg/l) Figure 4. Results of Disinfection in Field Tests Influence of water quality on the concentration of ozone to be injected The amount of invalid ozone consumed for other reactions except sterilization was considered as a factor affecting the effectiveness of ozone sterilization and relating to the water quality. In order to reveal the assumption, using untreated water sample at A ~ I treatment plants (a primary settlement pond), beaker tests were done to determine which kinds of substances cause the invalidity of ozone. Figure 5 shows relationship between ozone consumption rate and coliforms concentration through the beaker test on samples from the individual plants. Note that the tests were done on the basis of ozone injection rate in 0 to 700 mg/l. The result indicated that ozone consumption rate required to keep coliforms concentration below 3,000 cells/cm 3 would range from 45 to 130 mg/l and average 65 mg/l. Of the plants, ones distinguished by a higher ozone consumption rate is I and J plants which have high industrial wastewater loads and B plant which mainly accepts urban wastewater with little exhausting from homes. After the elimination of data relative to these 3 plants, mean ozone consumption rate required to reduce coliforms concentration within 3,000 cells/cm 3 is 50 mg/l, and there is presumably a strong factor that invalid ozone is heavily affected by the properties of raw sewage water (namely plant location). In order to examine the invalid consumption of ozone in more detail, the concentrations of COD Mn, iodine consumption and SS were selected as water quality parameters. The result of the analysis done on the basis of data obtained from the respective plants indicated that there is a relatively close correlation between ozone consumption rate and iodine consumption (R = 0.798, N = 9). On the other hand, COD Mn and SS showed moderate correlation. 5

6 Coliforms concentraiotn Cells/cm 3 1.E E E E E E E consumption rate mg/l) Figure 5. Consumption rate for Disinfection at Some Plants Genetic Toxicity using umu test With the purpose of investigating into genetic toxicity, a total of 4 water samples untreated water, ozone treated water (O 3 injection rate - 50 mg/l and 100 mg/l, reaction time minutes), and sodium hypochlorite-treated water (injection rate - 20 mg/l, reaction time - 2 hours) were prepared, and applied them to a umu test in two modes--one utilizing metabolic activation method (S9 (+)) and the other is not utilizing it (S9 (-)). Figure 6 shows the results of the test. All of 1000, 500, 250, 125, 62.5, 31.3 and 15.6-times concentrated samples were found to be negative. On the other hand, in the case of sodium hypoclorite-treated sewer sample, the genetic toxicity judgment values roughly the same as those of untreated water were obtained in regard to sodium hypochlorite-treated water. Samples which had been treated by ozone were evaluated for two concentration case (O 3 injection rates were 50 mg/l & 100 mg/l). The result was that as ozone injection rate increases, the samples showed a tendency to give less impact on the ecosystem than untreated water though in the range of negativity for genetic toxicity. Next, the samples were analyzed for bromate concentration formed as a byproduct of the ozone sterilization by means of ion chromatography. Two different sets of data were obtained: one showing no evidence of bromate being produced even when ozone consumption rate exceeded 100 mg/l (bromide ion was not detected) and the other showing the evidence of bromate being produced when ozone consumption rate exceeded 40 mg/l (bromide ion was determined to be 2.6~2.8 mg/l). In the latter case, bromate concentration produced at the maximum ozone consumption of 109 mg/l in rainy weather is approximately 0.08 mg/l which is in excess of the water quality standards in Japan of 0.01 mg/l. However considering dilution with river water, and also taking account of the fact that ozone-treated effluent is allowed to exhaust to rivers or streams only on a limited number of heavy-raindays throughout the year, it may be supposed that bromate production is not a serious problem. Because of the possibility that the concentration of bromate can reach closely to the regulated value of water quality standards, careful studies should be required in the case when a water supply or sewage facility is located in the downstream part of a river on which the 6

7 disinfection system will be installed. Criterion (A-B)/B (-) Concentrate rate (- Figure 6. Dose-Response Curves of Samples Economic feasibility Table 5 shows the results of a rough estimation of running cost for the disinfection system with a capacity of 10,000 m 3 /h between the proposed system (only for yearly combined sewer over flow) using O 3 and the conventional system using sodium hypochlorite (for yearly treated water). The calculation shows that when the proposed technology is applied to the disinfection of untreated water with an independent power included, its yearly running cost will account for 38.1% of that of the conventional disinfection process. Table 5. Comparison of Running Costs Technology utilizing Item high-speed ozone sterilization Raw sewage water Sterilizing agent injection rate Unit price of sterilizing agent Running cost per amount of treated water Quantity of water treated in a year Yearly running cost Untreated water 110 mg/l Chlorine sterilization in secondary treatment process Water from secondary treatment process Sodium hypochlorite (average) 3 mg/l (effective chlorine content - 12%) - Estimated at 30 yen/kg 62.7 yen/m yen/m 3 40 Qm 3 /year (2 x Qm 3 /h x 3 h x 20 days/year x 1/3) 25,080,000 yen/year (62.7 yen/m 3 x 40 x Qm 3 /year) 8,760 Qm 3 /year (Qm 3 /h x 24 hours x 365 days/year) 65,700,000 yen/year (0.75 yen/m 3 x 8,760 x Qm 3 /year) Yearly running cost ratio

8 Table 6. Summary of the Project Results Item of Development goal assessment (required performance) Attain a performance level which permits coliforms Treatment concentration in treated capability effluent to be reduced below 3,000 cells/cm 3. Increased sterilization efficiency Safety in downstream waters Others(economic feasibility/practic ality) Accomplish adequate sterilization in a short period of time. Restrain on disinfection s impact on ecosystem in downstream waters. Cut down chemicals/power requirements. Result of assessment It is determined that the proposed technology can reduce coliforms concentration in treated effluent below 3,000 cells/cm 3 within 2 minutes and has required performance capability. It is verified by a genetic toxicity (umu) test that both untreated water and ozone-treated effluent are negative. Also, it is proved that there is the tendency for treated effluent to give even less impact on the ecosystem when it was treated within the range of negativity. Though, there were some cases that the treatment produced bromate, this byproduct is assumed to pose no problem considering its concentration and dilution by river water. Running cost at a treatment flow of 10,000 m 3 /h was 62.7 yen/m 3 provided that an independent power is installed. Technical Assessment Table 6 shows the results of technical assessments done on the proposed technology. ADVANTAGES OF PROPOSED TECHNOLOGY The proposed technology provides the following advantages: High speed Use of high-concentration ozone enables sterilization process for combined sewer overflow to be completed within 2 minutes. Safety Little or no residue is confirmed. Limited production of byproduct harmful to the ecosystem makes it possible to accomplish safe sewage disinfection. Other water quality improvement tasks than sterilization Promising possibility includes removal of colors and odor. Wide applicability Applicable not only to the sterilization of combined sewer overflow from pump stations or wastewater treatment plants but also to that of primary settled overflow from wastewater treatment plants. DEVELOPED COMPANY Mitsubishi Electric Corporation Kobe Works Public Projects & Systems Department Wadamisaki-cho Hyogo-ku, Kobe , Japan zn_isida@pic.melco.co.jp 8