12. Super-High-Speed Fiber Filtration for Untreated Combined Sewage Water Overflow on Rainy Days

Transcription

1 12.Super-High-Speed Fiber Filtration for Untreated Combined Sewage Water Overflow on Rainy Days N. Horie 1, M.Kabata 2, K.Sano 3, S.Kanamori 4 1. Technology Overview (System overview) This wastewater treatment system consists of a pretreatment process to remove debris and a high-rate fiber filtration process. Figure 1 shows the treatment flow of this system. The pretreatment can be either settling or super-fine screening, which should be selected according to the condition of installation site. System: Pretreatment + high-rate fiber filtration Figure 1 Diagram of treatment flow Photograph 1 Filter media The settling treatment performs gravity settling for about 15 minutes with a overflow rate of 240 to 300 m 3 /(m 2 day). The super-fine screening uses a super-fine screen that can remove debris with a diameter larger than 6mm. (This experiment incorporated a rotary screen that was researched and developed by the First Development Research Subcommittee and subsequently proven for its performance.) The high-rate fiber filtration uses floating fiber filter media, and performs upflow filtration at a maximum filtration rate of 3,000 m/day. Photograph 1 shows the filter media. Having a high porosity, the floating fiber filter media can ensure a higher filtration rate than those of conventional filters, prevent surface filtration, and capture suspended solids (SS) in the entire filter layer. Therefore, the filter is featured by high SS capture capacity and an increased resistance to high load.

2 The combination of settling and high-rate fiber filtration is used and installed when an existing primary sedimentation basin is modified. Figure 2 shows an example of application to a primary sedimentation basin. The combination of super-fine screening and high-rate fiber filtration applies to a small wastewater treatment site or pumping station. Figure 3 shows an image of application to a wastewater treatment plant. Figure 4 shows an image of application to a pumping station. Figure 2 Figure 3 Example of application to a primary sedimentation basin Image of application to a wastewater treatment plant Figure 4 Image of application to a pumping station (Cycle of operation) Figure 5 shows the cycle of operation (filtration and washing processes) during wet weather. Table 1 lists the steps of the washing process. As filter media clogging progresses, the water level in the pressure balancing tank rises. When the water reaches a predetermined level, the washing process starts. The washing process consists of such steps as drainage, agitation, and agitation/drainage. All these steps are performed automatically.

3 After rainfall stops, a full washing operation (see Table 2) is performed to replace the whole content of the filtration tank with clean water to keep the filter in clean status. In a wastewater treatment plant, the secondary effluent is used as the clean water for this purpose. In a pumping station, the filtrate stored in the storage tank or industrial water is used as the clean water. If the secondary effluent is treated by filtration during dry weather for the beneficial use of facilities, the full washing operation is not necessary because the plant operation switches to the filtration of the secondary effluent after rainfall stops. Table 1 Washing method (during rainfall) Table 2 Washing method (after rainfall)

4 Figure 5 Cycle of filtration and washing process 2. Development and Research (1) Requisite performance and development goals (Development goals (requisite performance) specified in the Application Guidelines) Table 3 lists the development goals specified in the Application Guidelines for Development of the Technology Concerning the Improvement of Combined Sewer System. The guidelines specified that the technology must exceed the pollutant removal performance of conventional technology (stormwater settling basin). Table 3 Development goals (requisite performance) specified in the Application Guidelines Sewage discharged from the pumping station during wet weather in a combined Scope sewer system, or the wastewater flowing into the primary sedimentation basin in a wastewater treatment plant The technology must exceed the pollutant removal performance (BOD removal Development goal efficiency : 30%, SS removal efficiency : 30%) of conventional technology requisite performance(stormwater settling basin).

5 (Development goals presented by the technology proposer) Table 4 lists the development goals presented by the technology proposer. The technology proposer set development goals that exceed the requisite performance shown in Table 3. Table 4 Development goals presented by technology proposer Pumping station Application site Wastewater treatment plant Sewage discharged from a pumping station during wet weather Scope Wastewater flowing into the primary sedimentation basin in a wastewater treatment plant Pretreat-ment Super-fine screening or settling Development goal Filtration rate SS removal efficiency 2,0003,000 m/day 60% or more (by whole system) (2) Development and research methods (Site and period of experiment) A series of tests were conducted at the Kyokusai Wastewater Treatment Center of the Sewerage Bureau of Okayama City during the period from March 2003 to June The influent wastewater flowing into the primary sedimentation basin of the Center was used as the raw wastewater. The influent wastewater had passed through a coarse screen with an aperture size of 150 mm and a fine screen with an aperture size of 50 mm. The experiment involved two high-rate fiber filtration systems (with a filtration area of 0.5m 2 ) as main demonstration facilities. One system used a horizontal-flow settling tank for pretreatment; the other system used a superfine screen for pretreatment. The horizontal-flow settling Photograph2 Demonstration test system tank was operated for a settling time of 12 minutes (with of high-rate fiber filtration overflow rate of 300 m 3 /(m 2 day)), and the aperture size of the super-fine screen was 5 mm in diameter. Photograph 2 shows the demonstration facilities for high-rate fiber filtration. (Start of operation during wet weather and operating conditions) During wet weather, the test system started operation when the stormwater pump started, and stopped operation when the stormwater pump stopped. The test system operated during each period of rainfall at a predetermined filtration rate of 1,000, 1,500, 2,000, 2,500, or 3,000 m/day. (3) Results of development and research (Definitions of removal efficiencies as pollutant removal performance)

6 As the pollutant removal performance, the efficiencies of removing suspended solids (SS) and biological oxygen demand (BOD) are calculated based on the total loads each period of rainfall (up to 5 hours) as follows: Total influent Removal efficiency% load total effluent Total influent load load 100 (SS removal performance) Figure 6 shows the relations between the influent and effluent loads per filtration area during each period of rainfall when settling (for 12 minutes) was used for pretreatment. The average SS removal efficiency at a filtration rate of 2,000 to 3,000 m/day (set for the development goal) was 70.5%, and the overall average was 71.4%. Figure 7 shows the relations between the influent and effluent loads per filtration area during each period of rainfall when super-fine screening was used for pretreatment. The average SS removal efficiency at a filtration rate of 2,000 to 3,000 m/day (set for the development goal) was 72.3%, and the overall average was 75.2%. Figure 6 SS removal performance of the whole system (Pretreatment: settling) Figure 7 SS removal performance of the wholesystem (Pretreatment: super-fine screening)

7 (BOD removal performance) Figure 8 shows the relations between the influent and effluent loads per filtration area during each period of rainfall when settling (for 12 minutes) was used for pretreatment. The average BOD removal efficiency at a filtration rate of 2,000 to 3,000 m/day (set for the development goal) was 60.5%, and the overall average was 60.4%. Figure 9 shows the relations between the influent and effluent loads per filtration area during each period of rainfall when super-fine screening was used for pretreatment. The average BOD removal efficiency at a filtration rate of 2,000 to 3,000 m/day (set for the development goal) was 59.2%, and the overall average was 58.3%. Figure 8 BOD removal performance of the wholesystem (Pretreatment: settling) Figure 9 BOD removal performance of the wholesystem (Pretreatment: super-fine screening)

8 (Relation between SS capture weight and head loss) Figure 10 shows the relation between the SS capture weight and the filtration head loss. Table 5 lists the approximate expressions obtained from the data shown in Figure 10. Generally, the filtration head loss increases exponentially as the SS capture weight increases. Although there is a variation of data depending on the quality of influent, the SS capture weight globally tended to become lower as the filtration rate becomes higher. Figure 10 Relation between SS capture weight and filtration head loss at each filtration rate Table 5 Approximate expressions to calculate SS capture weight from filtration head loss Filtration rate Approximate expression YFiltration head loss kpa XSS capture weightkg/m 2 ln y ,000 m/day X ln y ,500 m/day X ,000 m/day X ln y ,500 m/day X y ln R 2 ln y ,000 m/day X (Filtrate recovery ratio) Figure 11 shows the relations of the influents and effluents in this system. The filtrate recovery ratio is the ratio of recovered filtrate to the raw wastewater (when the ratio is larger,

9 the percentage of washing wastewater volume is smaller). The filtrate recovery ratios of the two test systems are expressed by expressions (1) and (2) below. Table 6 lists the filtrate recovery ratios calculated from the SS removal performance and the standard SS concentration (180 mg/l) of the combined sewer overflow. In the test system combining settling with high-rate fiber filtration, the filtrate recovery ratio was 94.7% at a filtration rate of 2,000 m/day, 92.4% at 2,500 m/day, or 90.3% at 3,000 m/day. In the test system combining super-fine screening with high-rate fiber filtration, the filtrate recovery ratio was 94.3% at a filtration rate of 2,000 m/day, 91.7% at 2,500 m/day, or 89.2% at 3,000 m/day. Settling + high-rate fiber filtration Super-fine screening + high-rate fiber filtration odvolume of the influent into settling basinm 3 osvolume of the influent into screening facilitym 3 **Volume of the influent into filtration processm 3 1*Effective volume of treated effluentm 3 **Volume of the influent required to raise the filtrate level after washingm 3 **Volume of the treated effluent for washingm 3 **Volume of washing wastewaterm 3 **Volume of wastewater of drainage stepm 3 **Volume of wastewater of agitation/drainage stepm 3 **Volume of screen washing waterm 3 **Volume of screen washing wastewaterm 3 Figure 11 Relations of influents and effluents in the test systems Settling + high-rate fiber filtration 1 (Filtrate recovery ratio) 100 od -6 ( 1 S 10 ) (1) Here, the filter run time (T 1 ) is defined by the expression below

10 Super-fine screening + high-rate fiber filtration (Filtrate recovery ratio) (2) os Here, the filter run time (T 1 ) is defined by the expression below b Filtration ratem/ T 1 Filter run timeh S 0 SS concentration of the influent into settling basing/m 3 k 1 SS removal efficiency of settling basin (% g 1 SS capture weight of high-rate fiber filtration (kg/m 2 ) f 11 SS removal efficiency of high-rate fiber filtration (%) S 1 SS concentration of the influent into high-rate fiber filtrationfacility (g/m 3 ) Table 6 Filtrate recovery ratio at each filtration rate Filtration ratem/day 2,000 2,500 3,000 Filtrate recovery Settling + high-rate fiber filtration ratio (%) Super-fine screening + high-rate fiber filtration (Filter media durability) The filter was examined by an accelerated abrasion test using polypropylene pellets as imitated debris. The result of the test indicated that the mass and exterior view of the filter were normal even after 12,720 times of washing, which is equivalent to 40 years of use (on the assumption that the safety factor is 3 and washing is performed 318 times a year). (see Figure 12) The filter is also tested to examine the influence of fat and oil through filtration of the raw wastewater that was made by adding salad oil in tap water until the concentration of normal hexane extracts becomes 200 mg/l (10 times as high as the concentration of sewage influent). The result of the test indicated that the adhesion of fat and oil became the equilibrium state and the SS removal efficiency was lowered to 70% of that of the new filter after the period of use equivalent to 10 years (on the assumption that the safety factor is 3 and the total filtering time per year is 72 hours). However, the SS removal efficiency of the tested filter was restored by cleaning up to 90% of the new filter. Figure Mass change of filter media

11 (4) Technological assessment (Development goals (requisite performance) specified in the Application Guidelines) Table 7 shows the results of the assessment of the high-rate fiber filtration technology on the basis of the development goals (requisite performance) specified in the Application Guidelines. Table 7 Scope Development goals (requisite performance) specified in the Application Guidelines and the results of assessment Sewage discharged from the pumping station during wet weather in a combined sewer system, or the wastewater flowing into the primary sedimentation basin in a wastewater treatment plant Development goal requisite performance Assessment result The technology must exceed the pollutant removal performance (BOD removal efficiency : 30%, SS removal efficiency : 30%) of conventional technology (stormwater settling basin). The following is approved: The filtration technology ensured a BOD removal efficiency of 30% or more and an SS removal efficiency of 30% or more, and was determined to meet the specified requisite performance. (Development goals presented by the technology proposer) Table 8 shows the results of the assessment of the high-rate fiber filtration technology on the basis of the development goals presented by the technology proposer. Table 8 Development goals presented by the proposer and the results of assessment Sewage discharged from the pumping station during wet weather in a combined sewer Scope system, or the wastewater flowing into the primary sedimentation basin in a wastewater treatment plant With the pretreatment by super-fine screening or settling, the removal efficiency at a Development filtration rate of 2,000 to 3,000 m/day is determined as follows: goal SS removal efficiency: 60% or more (by the whole system) Assessment result The following is approved: With the pretreatment by either super-fine screening or settling, the high-rate fiber filtration technology ensured an SS removal efficiency of 60% or more at a filtration rate of 2,000 to 3,000 m/day, and was determined to have achieved the presented development goals. Effective filtration rates(*) are as follows: Settling + high-rate fiber filtration 1,692 to 1,995 m/day Super-fine screening + high-rate fiber filtration 1,652 to 1,887 m/day * The effective filtration rate is the value obtained by dividing the volume of treated effluent (excluding washing wastewater) by the filtration area and the total treating time (including washing time).

12 3. Features of This Technology (1) Possible conversion of existing facilities When the pretreatment incorporates settling, the former part of existing primary sedimentation basin or stormwater settling basin can be used as the settling basin, and the latter part of the existing basin can be converted into the high-rate filtration basin. In this case, the baffle walls and sludge scraper, etc. of the existing influent conduits or settling basin can be diverted for the high-rate fiber filtration process. When the pretreatment incorporates super-fine screening, the super-fine screen can be installed at the influent part of existing primary sedimentation basin or stormwater settling basin, and the latter part of the existing settling basin can be converted into the high-rate filtration basin. Thus, the system does not require additional space, and can be installed quickly at a low cost. Note that this system can also be installed as a new facility. (2) Space saving due to high filtration rate High-rate fiber filtration provides a high filtration rate at up to 3,000 m/day, and super-fine screening provides high treating capacity per unit installation area. The system combining these processes accordingly contributes to space saving. Therefore, the system can be easily introduced in a small site of wastewater treatment plant. (3) Easy, low-cost operation and maintenance due to non-chemicals treatment This system treats untreated wastewater without addition of chemicals during wet weather, and does not require the preparation for chemical solution, selection of chemicals, and their management. The system performs washing automatically during operation, reducing the load of operation and maintenance. The operating cost, including the power cost for lifting pumps, is as low as about 1.1 yen per 1m 3 of treated water. 4. Application of This Technology (Application site) In a wastewater treatment plant, this system can be installed through conversion of an existing primary sedimentation basin. In a pumping station or a site where a sufficient size of extra basin is not available, this system can be installed as a new facility. (Configuring a high-rate filtration facility) Configuring a high-rate filtration facility requires the approximation of the total filtration area based on the volume of wastewater to be treated and the design filtration rate. When an existing sedimentation basin is to be used for the high-rate filtration facility, the configuring procedure shown in Figure 13 must be followed.

13 Select the basin to be modified. The effective water depth of the basin to be used must be 2.5 m or more. The shape of the basin to be used is rectangular or circular. Select the pretreatment method. <When modifying a primary sedimentation basin or stormwater settling basin> If the basin is rectangular, preferentially select settling, which allows the use of the existing facility. If the basin is circular, select super- fine screening because the existing sedimentation basin cannot be used. <When installing the system as a new facility> Preferentially select super-fine screening, which allows reduction of installation space. <When pretreatment is settling> Approximate total filtration area = area of primary sedimentation basin to be modified β Approximate the total filtration area. Modification of 1 basin of 3 channels: β0.08 Modification of 1 basin of 2 channels: β0.09 (see Figure 16) <When pretreatment is super-fine screening> Approximate total filtration area = area of primary sedimentation basin to be modified β Modification of 1 basin of 3 channels: β0.24 Modification of 1 basin of 2 channels: β0.17 (see Figure 17) Set the design filtration rate. Approximate the total volume of wastewater to be treated. See Table 9.1,0002,500 m/day Approximate total filtration area design filtration rate As shown in Figures 16 and 17, when a primary sedimentation basin is modified, the approximate volume of wastewater to be treated is larger when super-fine screening is used as pretreatment (in comparison with the case using settling as pretreatment). Figure-13 Procedure for configuring a high-rate fiber filtration facility (by conversion of an existing sedimentation basin)

14 Figure 14 Example of approximating total filtration area (settling + high-rate fiber filtration) Figure 15 Example of approximating total filtration area (super-fine screening + high-rate fiber filtration) Table 9 Examples of design filtration rates according to the number of high-rate fiber filtration tanks Number of high-rate fiber filtration tanks of 1 line When maximam filtration head loss is 2 m Design filtration rate Filtration rate during washing of 1 tank When maximam filtration head loss is 1 m Design filtration rate Filtration rate during washing of 1 tank m/day m/day m/day m/day 2 1,500 3,000 1,000 2, ,000 3,000 1,300 2, ,250 3,000 1,500 2, ,400 3,000 1,600 2, ,500 3,000 1,660 2, Development and Research: Items Not Subjected to Assessment (Reference) (Performance of the operation during dry weather) This technology enables the filtration of secondary effluent during dry weather for the purpose of using facilities effectively. Performance of the filtration was examined by sampling the influent flowing into the filtration process during dry weather (secondary effluent) and the treated effluent (filtrate) every 2 hours in 24 hours. Figures 16 and 17 show the results of the analysis on the SS and BOD

15 concentrations of the samples. The results indicated that the SS removal efficiency was 52% to 63%, and the BOD removal efficiency was 23% to 32%. Figure 16 SS concentrations of treated effluent and secondary effluent Figure 17 BOD concentrations of treated effluent and secondary effluent DEVELOPED COMPANY Ishigaki Co., Ltd. TEL : +81-(0) FAX : +81-(0) Nippon Steel Corp. TEL : +81-(0) FAX : +81-(0) Kurita Water Industries, Ltd. TEL : +81-(0) FAX : +81-(0) Sumitomo Heavy Industries, Ltd. TEL : +81-(0) FAX : +81-(0) Kobelco Eco-Solutions Co., Ltd. TEL : +81-(0) FAX : +81-(0) Hitachi Plant Engineering & Construction Co., Ltd. TEL : +81-(0) FAX : +81-(0) Sanki Engineering Co., Ltd. TEL : +81-(0) FAX : +81-(0) Maezawa Industries, Inc. TEL : +81-(0) FAX : +81-(0)