10. High-rate Filtration (Synthetic Media)

Transcription

1 10.High-rate Filtration (Synthetic Media) N. Horie 1, M.Kabata 2, K.Sano 3, S.Kanamori 4 1. Outlines of Technology 1-1. System Outline This technology, as shown in the Figure 1 below, is an upflow filtration system, wherein the debris and SS contained in the wastewater influent are removed by the filter bed of synthetic media. For more effective use of the system, this system is mainly for a CSO control in rainy weather but can also be used. P Figure.1 An outline Flow 1.2 Filtering Media The filtering media, as shown in the Photo 1, is made by synthetic fiber specially processed and has a high void ratio* (95% and above), which enables the trapping of debris and SS in and out of the media simultaneously. It is flexible and therefore easily to free the captured SS off the system during the air washing. This filtration can be operated without chemical addition. * spatial ratio of non-fiber volume against filtering media volume 1.3 Principle Table1.Specificati-ons As shown in the Figure 2, this of Media technology system is a Spherical Shape combination of fine screen of 6 media mm mesh and a high-rate Diameter 33 filtration with synthetic fiber Vinylidene Fiber media, which removes the Polychlorid quality e debris, SS and BOD. Specific This system starts in rainy below 1.7 Photo 1. gravity weather, when a wastewater Appearance of Media influent is over and above the capacity of a wastewater treatment plant or pumping station. Filtration cycle is interrupted for air washing when filter resistance through the filter media reaches the terminal filtration B

2 resistance. B B Figure 2. Filtration Flow 1.4 Operational Procedures Filtration is started after the filtering media is retained by move the plate. Wastewater influent is flown upflow. Washing is started when the water achieves the terminal filtration resistance. Washing of the filtering media is made when the movable plate, which retaines filter bed, is shifted to keep an ample space for filtering media to rotate. The SS captured by the filtering media is removed by the air washing. The removed SS is flown to outside for discharge by raw wastewater from the upper part of the filter. After the washing, water in the high-rate filtration is drained and the movable plate is returned to its original filtering position. The movable plate can also be equipped underneath. One tank shall be 25 m 2 or less. For a rough idea, a tank of 9 m 2 or more shall have the moving plate underneath, and a tank of less than 9 m 2 shall have the moving plate on the top. Table 2 shows the time schedule of the washing and Figure 3 shows the operational cycle of the equipment. Table2. Washing Time Schedule B Figure 3. Cycle of Filtration and Washing B

3 2. Research and Development 2.1 Requisite Performance and Development Goal (1) Development Goal (requisite performance) specified in the Application Guidelines. Table 3 shows the technical development goal (requisite performance) specified in the Application Guidelines for the Improvement of Sewage of Combined System. Table3. Development Goal requisite performance specified in the Application Guidelines. Scope Sewage discharged from the pumping station on a rainy day in a combined sewerage system, or inflow wastewater going into primary sedimentation tank in the wastewater treatment plant Development goals (Requisite performance) Performance for removing pollutant to be over and above the conventional one (by stormwater sedimentation tank) that is BOD removal efficiency being 30% and SS removal efficiency being 30%. (2) Development Goal proposed by Designer) Table 4 shows the development goal presented by the technical proponent. Table 4. Development Goal presented by the technical proponent. Scope Sewage discharged from the pumping station on a rainy day in a combined sewerage system, or inflow wastewater going into primary sedimentation tank in the wastewater treatment plant Development goals Pretreatment by a fine screen of 6 mm mesh shall be made. Target rate of the high-rate filtration at the maximum filtration speed of 1,000m/day shall be 50% or more for SS removal and 30% or more for BOD removal. 2.2 How Research and Development were made. (1) Place, Period, Raw Wastewater and Equipment As a raw wastewater, the wastewater flowing into the primary sedimentation tank of Iriezaki (East Area) wastewater treatment plant, Kawasaki City, Kanagawa Prefecture was used for the experiment from August 2003 to January Photo 2 shows the external view of the demonstration plant and Table 5 shows its specifications. Furthermore, the flow of the experiment is illustrated in the Figure 4. Photo 2. External View of the Experimental Demonstration Plant

4 Item Condition of experiment Maximum allowable level Filtration Area 1m 2 1m 2 Height of Filter bed 0.5m 1.25m Filtration Speed 1,000m/d 2,400 m/d Filtration Resistance 25kPa 30kPa (2) Operation in Rainy Weather and the Conditions for Operation The operation of the demonstration plant was usually made for the filtration of secondary effluent of the treatment plant in fine weather, and was changed into a filtration of wastewater influent discharged by the plant, when such primary wastewater discharge was started in rainy weather. The filtration in rainy weather was made at a constant filtration speed of 1,000 m/day or 1,500 m/day. 2.3 Results of Research and Development (1) Definition of Removal Efficiency as a Capacity for Removing Pollutants Load calculation is based upon Figure 5, in which the integration value (area) during every rain hour (maximum 5 hours) is assumed to be the load. The removal efficiency, according to the load, is calculated in the following formula: Removal efficiency = [1 Effluent Load/Influent Load] x 100 (%) (2) Performance for SS Removal Figure 6 shows the influent and effluent SS loads per hour at every rain. Removal efficiency was within 50 70% and the average removal efficiency was 60.3%. (3) Rate of SS Removal according to Figure 4. Flow of the Experimental Demonstration Plant Figure 5. Fundamental Idea for Load Calculation Figure 6. Performance for SS Removal

5 Particle Size Table 6 shows the SS removal efficiency according to the size of particles. The table shows that the smaller the particle size, the lower the SS removal efficiency is, which means the performance for SS removal depends on the distribution of SS particles in the raw wastewater. Table 6. SS Removal efficiency according to Particle Size (4) Performance for BOD Removal Figure 7 shows the influent and effluent BOD load per hour at every rain. By measuring the BOD after filtering of the influent by a paper filter of 1 m mesh, it has also been confirmed that the BOD removal efficiency has a positive effect on the BOD concentration attributable to SS. Removal efficiency was within 30 60% and the average removal efficiency was 49.4%. (5) Removal of Debris Table 7 shows the concentration of debris at each demonstrative run and Photo 3 shows the sample pictures of the debris. The debris mainly Figure 7. Performance for BOD Removal is composed of weeds and oil balls as well as vinyl and plastic products, which are in small number but big volume. Such debris was successfully removed by this system. Table 7. Debris concentration before and after Filtration Influent[mg/L] Effluent[mg/L] RUN RUN RUN RUN RUN RUN RUN (6) SS Capture and Filtration Resistance

6 The filtering operation of this equipment was halted when the terminal filtration resistance is attained. Relations between SS capture and filtration resistance at each SS concentration (less than 150mg/L, mg/l and over and above 300 mg/l respectively) are shown in the Figure 8. Appropriate expressions (wherein, f means SS capture and p means filtration resistance) described in Table 8 are obtained out of the results in Figure 8. It is can be learned from the expression that the higher the concentration, the more Figure 8. Relations between SS Capture SS are captured. and Filtration Resistance (7) Filtered water Recovery Based upon the above results, influent and effluent water balance is defined as shown in the Figure 9 and Table 9. Filtered water ratio against wastewater influent in percentage is defined as the filtered water recovery. (Bigger recovery means smaller amount of filtered water. Elutriation quantity ratio (= elutriation amount/influent amount), filtration Condition Filtration Speed Filtration Resistance A guide expression SS concentration Table 9. Standard overflow in rainy weather with SS concentration of 180mg/L is used for Less than 150mg/L mg/L Over and above 300mg/L Table 8. Approximate Expression of SS capture and Filtration resistance 1000m/d 25kPa pfiltration resistance[kpa] fss capture[kg/m 2 ] recovery ratio (effective treatment amount/influent amount) and effective filtration rate (effective treatment amount/filter ALL Fig.9 Filtration amount for Influent and Effluent Washing.

7 area/one cycle filtration time including washing time) are calculated and listed in the Table 10 for comparison with average ratios. It has been confirmed that the calculated values are approximately the as for the results of experiments. * Report on the Improvement of Combined Sewage System by issued by Ministry of Land, Infrastructure and Transport in March Tab.10 Elutriate quantity ratio, Filtration recovery ratio, Effective filtration speed Item Average value for test Calculated value Designed Filtration Speed m/d 1,000 1,000 Elutriate Amount Ratio Filtration Recovery Ratio Effective Filtration Speed m/d SS Removal Efficiency BOD Removal Efficiency Assessment Result Development Goal specified in the Application Guidelines, the Development Goal presented by the technical proponent and their assessment results are shown in Table 11 and Table 12. Table 11. Technical Development Goal (Requisite Performance) specified n the Application Guidelines and its Results Scope Sewage discharged from the pumping station on a rainy day in a combined sewerage system, or inflow wastewater going into primary sedimentation tank in the wastewater treatment plant Development goals Performance for removing pollutant to be over and above the conventional (Requisite one (by the stormwater sedimentation tank) that is BOD removal efficiency performance) being 30% and SS removal efficiency being 30%. Result We achieved necessary ability. SS removal efficiency achieved the development mark. BOD removal efficiency achieved the development mark. Tab12. Development goals presented by the technical proponent and assessment results Scope Development mark presented by the technical proponent Assessment result Sewage discharged from the pumping station on a rainy day in a combined sewerage system, or inflow wastewater going into primary sedimentation tank in the wastewater treatment plant Pretreatment by a fine screen of 6 mm mesh shall be made. Target rate of the high-rate filtration at the maximum filtration speed of 1,000m/day shall be 50% or more for SS removal and 30% or more for BOD removal. We gained the following result at 1,000m/d of the filtration rate(the effective filtration rate was 795m/d) SS removal efficiency achieved the presentation development mark. BOD removal efficiency achieved the presentation development mark. * effective filtration rate means a treated amount (excluding elutriation) divided by total processing time (including filtration time and washing time)

8 3 Methods of Use and Special Features of the Technology 3.1 Process Flow Process flow when this technology is applied taking water amount into consideration is shown in the Figure 10. when it is applied to a pumping station: The flow shows the example where a stormwater pump is used as a lifting pump for filtration. when it is applied to wastewater treatment plant The flow shows the example where a stormwater pump is used as a lifting pump. Study may be made in order that the water can be obtained from the stormwater pump (if it has surplus capacity). When the water inflow exceeds the designed water amount, design has to be made to let the water flow through a by-path pipe. Note: Qsd: Quantity of Maximum Designed Sewage per day (m 3 /day) Qsh: Quantity of Maximum Designed Sewage per hour (m 3 /day) nqsh: Quantity of Designed Sewage on Rainy Day (m 3 /day) 3.2 System Flow Figure 11 shows the system flow of this technology. In this technology system, raw water lifted by a lift pump is put into high-rate filtration to go through pretreatment equipment, filtering equipment and elutriation equipment by gravity flow. In rainy weather, raw water is removed of its debris content using a screen of 6 mm or more, and is distributed to plural high-rate filter by way of Figure 10. Example for Process Flow Figure 11. System Flow P distribution tank. Elutriation after washing is made flowing from underneath and outward to the outside of the system. For an improvement of the primary sedimentation, hopper part can be changed into elutriation tank, which can eventually secure the storage capacity.

9 Total thickness of layers of lower part filter bed, filter bed (including cleansing space) and effluent layer must be 2.5 meters. The bottom of the tank shall have an inclination of about 1/100 for easy removal of sediment. Figure12. Height of High-Rate Filter Therefore, an effective water depth of at least 2.5 meters is required. 3.4 Small space with high-rate filtration Since the filtration is made in high speed (1,000m/day), Table 13. Rainy Day Work and Function of Equipment big space is not necessary. Furthermore, the plural filter system, whose every system can perform its own washing, enables a continuous filtration in high speed. 3.5 Easy work in rainy weather Since no chemical addition is required and the filtration is made by a natural gravity flow, no particular work is necessary on a rainy day. Usual washing maintenance after rain is made upon the discretion of the staff. Table 13 shows the work in the rainy weather and the function of the equipment. 3.6 Utilization of existing primary sedimentation tank As mentioned in the Figure 13, this filtration technology can be applied by installation of filter bed in the inside of an existing primary sedimentation tank for natural flow of raw wastewater by gravity. Utilization of the existing tank Distribution tank Screen tank for filtration Influent Treatment water layer Filter bed Washing area Elutriation tank For adjustment of inflow pressure Filter Effluent cannel Figure13. Example of Modification of Primary Sedimentation tank into High-Rate Filter

10 makes it unnecessary to secure space for a new building in the plant. 4 Research and Development: Items out covered by Evaluation (for Reference purpose only) Demonstrational experiment was carried out for upgrading technology for primary treatment water. 4.1 Outline of process Filtration experiment was carried out in the wastewater treatment plant upon the primary effluent in rainy weather. Conditions of the experiment are shown in the Table 14, where no pretreatment was installed and the raw water was filtered directly. Table 14. Specifications of the Experimental Demonstration Plant Item Condition of experiment Maximum allowable level Filtration Area 1m 2 1m 2 Height of Filter bed 1.0m 1.25m Filtration Speed 2,000m/d 2,400 m/d Filtration Resistance 30kPa 30kPa 4.2 Performance of SS Removal SS removal performance is shown in the Figure 14. Removal efficiency was within 50 80% and the average removal efficiency was 60.9%. 4.3 Performance of BOD Removal BOD removal performance is shown in the Figure 15. Removal efficiency was within 30 70% and the average removal efficiency was 52.1%. BOD removal efficiency, same as for the substitute technology of primary treatment, has a positive effect on the concentration of BOD attributable to SS. 4.4 Water Balance Based upon the results of the above demonstrational experiment, elutriate quantity ratio, filtration recovery ratio and effective filtration speed were found out and shown in the Table 15. The upgrading up of the effluent was Figure.14 Performance of SS Removal well achieved. It has been confirmed that this technology is quite feasible in terms of water balance. Figure.15 Performance of BOD Removal

11 Table.15 Elutriate quantity ratio, Filtration recovery ratio, Effective filtration speed Item Upgrading up of the primary effluent (Average value for test) Designed Filtration Speed m/d 2,000 Elutriate Amount Ratio 9.2 Filtration Recovery Ratio 90.8 Effective Filtration Speed m/d 1,802 SS Removal Efficiency 60.9 BOD Removal Efficiency 52.1 DEVELOPED COMPANY Mitsui Engineering & Shipbuilding Co., Ltd. TEL : +81-(0) FAX : +81-(0)