6. Band Screen with Perforated Fine Mesh Panels

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1 6.Band Screen with Perforated Fine Mesh Panels N. Horie 1, M. Kabata 2, H.Sano 3 and S.Simozeki 4 Director 1, Chief Resercher 2, Senior Resercher 3 and Resercher 4 First Research Department Japan Institute of Wastewater Engineering Technology Nishi-ikebukuro, Toshimaku, Tokyo , Japan 1. TECHNICAL OUTLINE The screen described here is applicable to remove fine debris discharged to rivers and streams from combined sewer systems. Generally this screen is installed in the water channels in pump stations whereas the most of CSO screens are applied at over flow weirs. Raw combined sewer goes though screen with inside-out flow as shown in Figure 3. The screen is consisted with perforated fine mesh panels that are linked each other to form an endless band. Screenings, debris captured on the panels is moved upward with movement of the screen, and then is washed off by spray water to the trough located at the upper part of the screen. Depend on installation site conditions and raw combined sewer flow rate, we can offer two types of screen, band and drum. The band type is for screens with inlet width of 500 to 2500 mm, and the drum type for those with inlet width of 3000 to 8000 mm. Washing unit Drive unit Trough Mesh panel Trough Drive unit Frame Washing unit Mesh panel Frame Band Type Drum Type Figure 1. Sketch of the proposed band screen with perforated fine mesh panels 2. TECHNICAL FEATURES The mesh panel made of synthetic resin has thickness of 9-to 12-mm with tapered holes shown in Figure 2. Thanks to this configuration, the screen is free from severed blockage by debris and stable and continuous operation is realized. In addition, special shaped rubber seal between mesh panels and the screen frame is ensuring prevention of fine debris leakage from the screen. Furthermore, since the trough accepting mesh panel-washing drainage is located in upstream side of the screen, spilled drainage could be kept inside of the screen. These design features can keep debris from flowing out to the downstream. Debris removal (screen) No.6 1

Inflow D 1 (2~10mm) t(9~12mm) OUTFLOW Figure 2. Structure of the Mesh Panel SEWAGE INFLOW MESH PANELS SEAL THROUGH-THE-SCREEN OUTFLOW Figure 3. Flow pattern in the screen 3.

screen applied for improvement of combined sewer system must be capable of preventing discharge of scenically unpleasant foreign matters (i.e., toilet paper, excrements of human and animals, various sanitary items, food residue, wrappings and containers) into rivers and streams from combined sewer systems during storm events.

2 Inflow D 1 (2~10mm) t(9~12mm) OUTFLOW Figure 2. Structure of the Mesh Panel SEWAGE INFLOW MESH PANELS SEAL THROUGH-THE-SCREEN OUTFLOW Figure 3. Flow pattern in the screen 3. TARGETS TO BE ACHIEVED BY THE PROPOSED SCREEN The screen applied for improvement of combined sewer system must be capable of preventing discharge of scenically unpleasant foreign matters (i.e., toilet paper, excrements of human and animals, various sanitary items, food residue, wrappings and containers) into rivers and streams from combined sewer systems during storm events. 3.1 Required performance The screen should have the SRV (Screenings Retention Value) performance of 30% or more for debris of 5.6 mm or more in size. SRV (Screenings Retention Value) is the index that represents the removal rate of debris by the fine screen with perforated mesh panels proposed here. Shown below is the expression to calculate the index, where TSRE with is the removal rate of debris after installation of the screen, and TSRE the removal rate of debris before installation of the screen: TSREwith TSRE SRV (%) = TSRE TSRE with = DCSFwith + DCS DCSF + DCSO + DCS with with TSRE = DCSF DCSF + DCSO 2 Band Screen with Perforated Fine Mesh Panels

3 here, DCSF with : Amount of debris in combined sewer fed to WWTP after installation of the screen DCS : Amount of debris captured by the screen DCSO : Amount of debris in CSO after installation of the screen with DCSF : Amount of debris in combined sewer fed to WWTP before installation of the screen DCSO : Amount of debris in CSO before installation of the screen 3.2 Other subjects to be certified. The performance of the proposed screen was evaluated by head loss, operability and steadiness under the inferior operation. (1) Operability during continuous operation (2) Removal performance for large materials (3) Head loss at normal operation (4) Head loss during inferior operation 3.3 Feasibility study To demonstrate concrete figures of the screen, size selection of the screen was done under the possible design conditions which was based on a fact-finding survey on existing pump stations located in 191 cities across the country, in which combined sewer systems are employed. Flow rate Stormwater chamber Table 1. Characteristics of Pump Station for Feasibility study Inflow 608,976m 3 /day 7.05m 3 /sec Runoff outflow 552,960m 3 /day 6.40m 3 /sec Combined sewer flow fed to WWTP 56,016m 3 /day 0.65m 3 /sec Length 22.0m Width 4.0m Height 4.2m Height from bottom of chamber to water level Height from water level to ceiling of chamber Hydraulic surface loard 2.2m 2.0m 3,600m 3 /m 2 day Convention Opening width of the coarse screen 5.0cm al screen Opening width of the fine screen 3.0cm Debris removal (screen) No.6 3

4 4. METHOD OF EVALUATION 4.1 Test site and period The field test was conducted at Edagawa WWTP in Nishinomiya city, Hyogo, Japan. Experiment period: June 2003 to February Description of test facilities Table 2. Description of test facilities Items Screen Intake pump Solid-liquid separator Washing water pump Collector of screenings in treated water Water channel Level gauge Description Band screen with perforated fine mesh panels Inlet width: 500mm Panel width: 500mm Nominal flow capacity: 300m 3 /hr Aperture: 6 mm Vortex type; 250m 3 /hr (6 inch) x 2 units Fine bar screen (2-mm openings),for captured screenings 0.25m 3 /min (2 inch) x 1 units Punched metal basket (2-mm dia.) with light duty nylon net 1.5m (W) x 2.0m (D) x 7.0m (L) Ultrasonic level gauge x 2 units 4.3 Sampling method Sample screenings were collected by the following ways. During each test run, screen panel washing drainage was introduced to a solid-liquid separator (the 2-mm opening bar screen) to collect sampled screenings. Duration time for collection was 20 minutes. 4.4 Composition analysis Size distribution Sampled screenings were sieved by four JIS- conforming 200-mm diameter sieves, each having openings of 2.0 mm, 4.0 mm, 5.6 mm and 9.5 mm, were laid one over another in a pan (see Figure 4). In order to make sieving easy, sieving procedure was done by wet method shown in Figure Classification After sieving, each components in four sieves was classified into 9 categories as bellow: 1) paper 2) excrements of human 3) garbage 4 Band Screen with Perforated Fine Mesh Panels

5 4) herbage 5) hairs 6) vinyl and plastic materials 7) metals and refuse glass 8) oil balls 9) others After classification, all components were dried up by an oven (100 to 150 C) for two hours to weigh them. SCREENINGS (APPROX. 100G) Figure 4. Method of sieving screenings by size 4.5 SRV performance calculation method The SRV (Screenings Retention Value) performance of the proposed screen was calculated using the expression below: TSREwith TSRE SRV (%) = TSRE where TSRE = 0 is established, hence comes the following: TSREwith 0 Y SRV (%) = 100 = TSREwith 100 = Y + Z Screen Y (g/m 3 ): Amount of screenings collected by the screen Z (g/m 3 ): Amount of screenings that passed through the screen Figure 5. Material balance before/after the screen Debris removal (screen) No.6 5

6 4.6 Field test method Continuous running test In order to confirm steady operability, continuous running test of 6 hours a day for 5 days was executed using dry-weather sewer whose flow rate was the nominal flow capacity of 300 m 3 /hr. Raw sewer was taken in from a combined sewer channel in the The removal debris was weighed. And head loss caused by the screen was measured during this test Removal performance for large materials As raw sewer was taken from down stream of 50mm coarse screen in WWTP, it would be possible that relatively large size of debris could be contained in raw sewer. In order to confirm a capability on removal of large materials, removal test was executed by means of adding foreign materials in raw sewer such as wood bars, empty cans, PET bottles, chopsticks, plastic bags, foam trays, waste cloths, hairs, fibrous materials, and so on. Flow rate was kept at nominal flow rate of 300 m 3 /hr Head loss at normal operation Raw Sewer taken from combined sewer channel or combined storm water channel in WWTP was fed to the screen installed in the test water channel. The flow rate was adjusted at 30, 60, 100 and 150% of the nominal flow capacity of 300m 3 /day, respectively. Throughout the operation ultrasonic level sensors measured water level differential, that is head loss, between the upstream and downstream of the screen Head loss screen movement Considering possible shutdown of the screen due to power interruption or other causes, the screen drive motor was shut down during operation. Difference of water level between upstream and downstream of the screen was measured during shutdown. The flow rate was regulated at the nominal flow capacity of 300 m 3 /hr Head loss washing water To check the impact of washing water upon the screen operation, after thoroughly cleaning the screen mesh panels at the rated flow of washing water, the pump was stopped during operation and difference in water level between upstream and downstream of the screen was measured. The flow rate was regulated at the nominal flow capacity of 300 m 3 /hr Head loss with blinded mesh panels To check the impact of mesh panel blockage upon the screen operation, head loss was measured when a given number of the screen mesh panels were covered by plastic film to simulate blockage of some meshes. Covering rate was set as 50 and 75% of whole apertures. The flow rate was regulated at nominal flow capacity of 300 m 3 /hr. 6 Band Screen with Perforated Fine Mesh Panels

7 5. RESULTS 5.1 Required performance The SRV (Screenings Retention Value) performance in dry weather has been found to be 100% for screenings of 5.6 mm or more when flow is 30%, 60%, 100% and 150% of the nominal flow capacity of the equipment. Figure 6. SRV measurement results 5.2 Other subjects Continuous running test It was confirmed to work stable any unusual phenomenon like overloading of drive unit, unusual head loss. Figure 7. Continuous running test results Removal performance for large materials It has been verified that the proposed screen could remove the most of larger foreign materials except wood blocks. Unusual phenomenon like overloading of drive unit and unusual head loss could not be observed during test. Remained wood blocks were drifted within the screen. Therefore it would need longer time or higher flow rate to remove such a large and heavy foreign materials Debris removal (screen) No.6 7

8 Table 3. Results of removal capability for large foreign materials Foreign debris PET bottles PET bottles PET bottles Empty cans Empty cans Empty cans Empty cans Plastic bags Plastic bags Chopsti cks Wood blocks Condition Standard size / Dimensions Qty used for experiment (pcs.) Qty collected (pcs.) Collection rate (%) Impediment (Yes/No) Dry 500mL No Wet 500mL No Wet Dry Wet Wet Wet Dry Wet Dry Dry 500mL L L No 2.0L mL mL No 500mL mL mL No 500mL mL mL No 500mL mL mL No 500mL mm x 200mm mm x 300mm No 400mm x 400mm mm x 200mm mm x 300mm No 400mm x 400mm mm x 3~6mm x 4mm No 150mm x 90mm x 40mm mm x 45mm 4 3 x 45mm 150mm x 40mm x 15mm 100mm x 45mm x 45mm 100mm x 40mm x 15mm Note1 No Note 1. Remains were kept in upstream side of the screen. 8 Band Screen with Perforated Fine Mesh Panels

9 5.2.3 Head loss at normal operation The mean head loss has been 10 mm or less under the various test flows ranged 30 to 150 % of nominal flow capacity. No adverse effect, such as unusual rise of head loss, has been observed even when flow is 150% of the nominal capacity Head loss (mm) Flow rate (m3/h) Figure 8. Results of experiment on head loss Remarks: In order to simulate head loss during possible large debris loading, additional test was conducted as follow: Adding debris in raw sewer at rate of 300 g/m 3 continuously by manual during test. Head loss was measured during 120 minutes. Head loss measured during the test has been 100 mm or less. It has been observed that head loss decreased promptly after stoppage of debris adding Head loss screen movement During stoppage of the screen movement head loss increased gradually and reached over 300 mm in about 30 minutes. The head loss continued to further rise Head loss mm Elapsed time min Figure 9. Head loss performance during stoppage of screen movement Debris removal (screen) No.6 9

10 5.2.5 Head loss washing water In about 90 minutes after shutdown of the washing water pump, head loss commenced to increase, reaching around 300 mm in about 150 minutes. And also congestion of screenings in the trough had been observed Head loss mm Elapsed time min Figure 10. Head loss performance washing water Head loss with blinded mesh panels Higher head loss had been observed when blockage mesh panels increased. At the nominal flow capacity of the screen, head loss was 100 mm or less when 50% of panel meshes were blinded, and it increased to nearly 250 mm when 75% were blinded. 30flow 60flow 100flow 150flow 300 Head loss mm Elapsed time min Figure 11. Results of experiment with 50% blinded mesh panels 30flow 60flow 100flow 150flow 300 Head loss mm Elapsed time min Figure 12. Results of experiment with 75% blinded mesh panels 10 Band Screen with Perforated Fine Mesh Panels

11 Figure 13 shows relation between head loss and a square of channel flow rate in term of (m/sec) 2. We found that head loss was correlated both with flow rate and blockage ratio on mesh panel. Figure 13. Relationship of head loss with a square of channel flow velocity and panel mesh blinding percentage 5.3 Result of feasibility study Table 4 and Figure 14 show the result of feasibility study. Table 4. Summaries of Feasibility study Modifications Head loss Advantages and disadvantages * No modifications of pump station. * The screen should be installed as the substitute for an existing fine screen. 110mmWater level at which the screen is installed * Existing screenings conveying facility can be utilized because the screen is installed as the substitute for an existing one. * Washing water is required. If the water discharged from the screen is made use of, installation of a filter is required. * The screen can be flexibly increased/decreased in size according to the throughput requirement. Debris removal (screen) No.6 11

12 Figure 14. Sample layout for feasibility study 6. EVALUATION ON SRV PERFORMANCE The evaluation result is summarized in Table 5: Table 5.Evaluation result Application Evaluation result The proposed screen should be installed in a pump station. SRV has been found to be 100% when flow is 100% of the nominal capacity of the equipment; thus, the equipment is considered to have the required performance. (Method: the mean data from the 8 samples in dry weather and 2 samples in wet weather) 12 Band Screen with Perforated Fine Mesh Panels

13 7. NOTICE The following considerations are required when installing the proposed equipment: The expected head loss on the proposed screen will be 300 mm in consideration of the unusual conditions such as large debris loading, malfunction of washing water supply, blockage of mesh panels and so on. Recommended countermeasures for unusual rising of head loss will be installation of overflow weir and/or emergency discharge gate. Required quality for washing water is filtered water grade or at least solid particle free water treated by 20-mesh strainer will be preferable. In case that the screen must be installed in pump stations grit chambers, to minimize the wears and tears on the screen caused by sand, water channels should be cleaned after storm events and inspections of the screen may be recommended. Possible impact of the screen head loss upon backwater must be checked and verified before installation of the screen. DEVELOPED COMPANY Ataka Construction & Engineering Co., Ltd. Address: 6-6, Motoasakusa 2-Chome, Taito-ku, Tokyo, , Japan TEL: +81-(0) FAX: +81-(0) Hitachi Plant Engineering & Construction Co., Ltd. Address:13-2 Kita-Otsuka 1-Chome, Toshima-ku, Tokyo, , Japan TEL: +81-(0) FAX: +81-(0) Kobelco Eco-Solutions Co.,Ltd. Address: ON Bldg.9-12,5-Chome, Kitashinagawa, Shinagawa-ku, Tokyo, , Japan TEL: +81-(0) FAX: +81-(0) Maezawa Industries,Inc Address: No.7-2,2-Chome, Yaesu, Chuo-ku, Tokyo, , Japan. TEL: +81-(0) FAX: +81-(0) Debris removal (screen) No.6 13

Screening, Definition: The unit involved is called a screen.

Screening, Definition: The unit involved is called a screen. Screening, Definition: Screening is a unit operation that separates materials in and/or on water (found in different sizes) from water and from entering water treatment facilities and mains. The unit involved