8.Rotary Screen 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 1-22-8 Nishi-ikebukuro, Toshimaku, Tokyo 171-0021, Japan 1. TECHNOLOGY OVERVIEW This rotary screen is installed in a pumping station of a combined sewerage system to remove suspended solids from raw wastewater. When installed in a channel of the pumping station, the rotary screen traps suspended solids by perforated panels. The raw wastewater flows into the rotating screen and is filtered when leaving the screen from the inside to the outside. Suspended solids are trapped on the inside surface of the screen, and raked upward as the screen rotates. The solids collected at the top of the inside of the screen are flushed out with flushing water down into a discharge trough for collection. The rotary screen is available in two types, the drum-rotating and panel-running types, which should be selected according to the condition of the installation site. Figure 1. A schematic of rotary screen 2. MECHANISM OF ROTARY SCREEN The rotary screen has a mechanism to scrape the solids (screenings) trapped inside the screen upward and collect them at the top of the inside of the screen. As the scraping device, the drum-rotating type uses a scraper, and the panel-running type uses solid-lifting shelves. As the standard screen material, the drum-rotating type uses a punching metal plate (with round openings) made by SUS, and the panel-running type uses a polypropylene resin panel (with hexagonal openings). The principle of operation is common to the both types. Debris removal (screen) No.8 1
As shown in Figure 2, "Flow of automatic operation with stormwater," the rotary screen is operated automatically according to the water level in the channel where the rotary screen is installed in the pumping station. In the experiment described later, automatic operation was set up so that the rotary screen starts when the water level in the stormwater inflow channel exceeds a high (H) level (900 mm) because of rainfall and stops when the water level in the stormwater inflow channel drops below a low (L) level (500 mm). Figure 2. Flow of automatic operation with stormwater 2.1 Features of the drum-rotating type Since the drum-rotating type of rotary screen has a circular shape at cross section, it cannot easily accommodate high water levels. Therefore, the drum-rotating type suits the installation in a position at the spout of a stormwater discharge pump or before a stormwater balancing reservoir or retention tank, where the water level does not change much. The influent flows inside the cylindrical drum, and is filtered through the rotating screen. Solids are trapped on the inside surface of the screen, and raked upward by the scraper that rotates at a higher speed than the rotational speed of the screen. The solids scraped upward at the top of the inside of the screen are flushed out with flushing water down into a discharge trough. The fibrous matter entwining through the openings in the screen is scraped by the scraper to prevent clogging of the screen. The drum rotates in a circular motion, which is different from the sliding movement of the panel-running type. 2.2 Features of the panel-running type Since the panel-running type of rotary screen has an elliptical shape at cross section, it can easily accommodate the changes in water level. Therefore, the panel-running type suits the installation in a stormwater inflow channel of a pumping station, where the influent water level may change largely. 2 Rotary Screen
The panel screen consists of the perforated resin panels concatenated to form an elliptical structure. The influent flows inside the panel screen structure, and is filtered through the panels. The solids (screenings) trapped on the panels are lifted by the solid-lifting shelves mounted at panel joints up to the top of the rotary screen, and flushed out with flushing water down into a discharge trough. Each panel is designed to be as thick as 15 mm to nearly eliminate the entwining of fibrous matter. Even if fibrous matter is caught on panels, it can be easily flushed out under pressure with flushing water sprayed from the outside of the panels. Thus, the clogging of the screen can be prevented. The panel-running type is irrelevant to functional problems even if the water level rises. The drive motor, power transmission device, discharge trough, and flusher can be installed on an upper floor to avoid submersion under water. Figure 3. Screen mechanisms 3. DEVELOPMENT GOAL 3.1 Development goal specified in the application guidelines Technology to remove solid matter from the wastewater discharged from gravity outfalls The technology must be able to prevent visually unpleasant solid matter (e.g., toilet paper, human and animal excrements, sanitary items, garbage, and wastes, such as containers and wrapping materials) from being left in the sewage that is discharged in rainy weather from the gravity outfalls or pumping stations in a combined sewerage system. 3.2 Performance requirement The minimum goal of performance requirement was set to the screenings retention value (SRV) of 30% for the solids 5.6mm or larger in size. SRV is an index that indicates the ratio of the solids removed by the screen to the whole solids contained in the influent. The formula to calculate SRV is as follows: Debris removal (screen) No.8 3
In the formula, TSRE with represents the efficiency of solid removal with the screen installed, and TSRE without represents the efficiency of solid removal by a weir without the screen installed. TSREwith TSREwithout SRV (%) = 100 1 TSRE without TSRE with = Volume of Volume of solids removed from sewage + Volume of solids trapped by screen solids removed from sewage + Volume of overflowed with + Volume of solids trapped by screen with solids with TSRE without = Volume of Volume of solids removed solids removed from sewage without from sewagewithout + Volume of overflowed solids without Here, Volume of solids removed from sewage with : Dry weight of the solids removed when the screen is installed Volume of solids trapped by screen : Dry weight of the solids trapped by the screen Volume of overflow solids with : Dry weight of the solids overflowing into discharge channel when the screen is installed Volume of solids removed from sewage without : Dry weight of the solids removed when the screen is not installed Volume of overflow solids without : Dry weight of the solids overflowing into discharge channel when the screen is not installed 3.3 Basic experimental conditions and important confirmation items The basic conditions for experiments and the key points to be confirmed were as follows: 3.3.1 Screen operation performance (1) Continuous operation test The rotary screen was operated continuously for six hours a day in five consecutive days to test its capacity and operation. The test was conducted on condition that the rotary screen was operated in fine weather for screening of sewage and the influent flow rate was adjusted to the nominal capacity of the rotary screen. (2) Hampering-solid screening test A test was conducted to confirm the removal of the solids that might hamper the function of the rotary screen. The solids that might hamper the function of the rotary screen and were 4 Rotary Screen
selected for the test included square timbers, waste cans, plastic bottles, disposable chopsticks, plastic bags, plastic foam trays, waste cloths, hairs, and fibers. 3.3.2 Influence on sewage discharge in rainy weather (1) Head loss with the rotary screen operated Head loss was measured while the rotary screen was operated with the flow rate set to 50% and 100% of the nominal capacity of the rotary screen. (2) Head loss with the rotary screen function stopped Head loss was measured while the rotary screen was stopped or blocked under the same flow rate conditions as described above. (3) Marginal processing capacity of the rotary screen The marginal processing capacity of the rotary screen was confirmed with the flow rate set to 150% of the nominal capacity. 3.3.3 Possibility of rotary screen installation in the combined sewerage facilities to be improved A research was conducted on the actual conditions of the existing facilities (pumping stations) in the 191 cities throughout Japan that were using combined sewerage systems. Based on the research data, an average pumping station was configured and an installation model was designed. Table 1. Pumping station specifications for model design Flow rate Stormwater grit chamber Volume of influent 608.976m 3 /day 7.05m 3 /s Volume of effluent 552,960m 3 /day 6.40m 3 /s Volume of blocked and collected wastewater 56,016m 3 /day 0.65m 3 /s Length (inner size) 22.0m Width 4.0m Height 4.2m Distance from water level to chamber bottom 2.2m Distance from water level to chamber roof 2.0m Water area load 3,600m 3 /m 2 d Pore size of coarse screen 50mm Pore size of fine screen 30mm 4. DEVELOPMENT AND RESEARCH METHODS 4.1 Site and period of experiment Installation site: Takaida Pump Station of Higashi-Osaka City in Osaka Prefecture, Japan Experimental periods: March to August, 2003 (drum-rotating type) and September, 2003, to February, 2004 (panel-running type) Debris removal (screen) No.8 5
4.2 Specifications of test equipment Table 2 lists the specifications of the rotary screen and test equipment. Table 2. Specification of the rotary screen and test equipment No. Name of equipment Drum-rotating type Panel-running type 1 Rotary screen Nominal capacity: Inner diameter: φ600 2 Raw wastewater tank 1.2 m 3 3 Untreated-effluent feed pump for rainy weather Submergible pump: φ300 300 m 3 /h 4 River water feed pump for fine weather Submergible pump: φ150 150 m 3 /h 5 Filter Floating granular filter:3m 3 /h Floating granular filter:10m 3 /h 6 Flushing water tank 0.5m 3 2.7m 3 7 Flushing water pump Centrifugal pump: 8 Raw wastewater sampler In-line centrifugal pump Net-mounting cage with automatic valve (5-sample collection) 9 Effluent sampler 10 Solid collector Net-mounting cage (1-sample collection) 11 Discharged-water tank 2.7m 3 12 Flowmeter Electromagnetic flowmeter (bore diameter: 250 mm) 13 Water level gauge Supersonic type (2 units) 14 SS sampler 19-sample collection (2 units) 4.3 Solid sampling method A submergible pump (300 mm diameter) (or submergible pump [300 mm diameter] for the marginal capacity test in fine weather) was installed before a coarse screen in a channel where stormwater (or sewage for the marginal capacity test in fine weather) flows in from the combined sewerage system. The influent was fed into the raw wastewater tank by the submergible pump. For the sampling of solids in the influent, raw wastewater is fed to the raw wastewater sampler through a channel branched from the channel to the rotary screen. For the sampling of solids in the effluent from the rotary screen, the effluent channel from the rotary screen is branched within the test facility to the effluent sampler. The effluent fed through the branch channel is fed to the five cages that are equipped with 2mm-mesh polyethylene net and are automatically switched sequentially at intervals of 20 minutes to collect samples. 4.4 Analysis method Each 2mm-mesh polyethylene net containing the solids sampled by the sampler were rinsed with warm water in a bowl to remove extraneous solids from the periphery of the net. Then, only the solids caught in the net were collected and put into a container. The sieves with pore sizes of 9.5 mm, 5.6 mm, and 2 mm conforming to JIS standard were placed in piles with the 9.5 mm sieve positioned at the top. The collected solids were put on the 9.5 mm sieve, and sifted through while being flushed. Fragile solids (e.g., human excrements) were treated in advance with a slow flow of water. The solids caught on each sieve were picked up using a 6 Rotary Screen
spoon and tweezers, and dried at a constant temperature of 105. Dried solids were cooled off in a desiccator, and then weighed to know their dry weight. Sifted solids were divided with tweezers into 9 types (such as paper, human excrement, garbage, plants, hairs, plastics, oil ball, metal and glass, and other). Each type of solids was put in an aluminum cup, and dried at a constant temperature of 105. Dried solids were cooled off in a desiccator, and then weighed to know their dry weight. After weighing, the ratio of each type of solids was calculated. Photo 1. Sifting of solids 4.5 Performance testing method To test the screen performance to treat wastewater in fine weather, the influent flow rate was set to 50% and 100% of the nominal capacity of the rotary screen. To test the screen performance to treat the sewage discharged from the combined sewerage system in rainy weather, the drum-rotating type was operated 13 times, and the panel-running type was operated 8 times. The following formula was used to calculate SRV. TSREwith TSREwithout SRV (%) = 100 1 TSRE TSRE with Here, C + = b a ( A C) A without 100 A: Volume of 5.6 mm or larger solids flowing into the facility (g/m 3 ) C: Volume of 5.6 mm or larger solids flowing into blocked and collected wastewater basin (g/m 3 ) Debris removal (screen) No.8 7
a: Volume of 5.6 mm or larger solids flowing into the test rotary screen (g/m 3 ) b: Volume of 5.6 mm or larger solids trapped on the rotary screen (g/m 3 ) Figure 4. Pattern fiagram for SRV calculation 4.6 Test methods to confirm important items 4.6.1 Screen operation performance (1) Continuous operation test The drum-rotating type was operated continuously for six hours a day in five consecutive days (a total of 30 hours) to test its capacity and operation. The influent flow rate was set to 100% (165 m 3 /h) of the nominal capacity of the rotary screen. The panel-running type was operated continuously for six hours a day in five consecutive days (a total of 30 hours) to test its capacity and operation. The influent flow rate was set to 100% (128 m 3 /h) of the nominal capacity of the rotary screen. (2) Hampering-solid screening test The test was conducted under the conditions described in section 3.3, and oil ball, hairs, paper, and fibers were added as the solids that might hamper the function of the rotary screen. 4.6.2 Influence on sewage discharge in rainy weather (1) Head loss with the rotary screen operated Head loss was measured while the rotary screen was operated with the flow rate set to the nominal capacity of the rotary screen (165 m 3 /h for the drum-rotating type or 128 m 3 /h for the panel-running type). (In the panel-running type, one half of screen openings were sealed to adjust for the pump capacity.) (2) Head loss with the rotary screen stopped Head loss was measured under an abnormal condition while the rotary screen was blocked or the drive motor was stopped with the flow rate set to the nominal capacity of the rotary screen (165 m 3 /h for the drum-rotating type or 128 m 3 /h for the panel-running type). 8 Rotary Screen
(3) Marginal processing capacity of the rotary screen The marginal processing capacity of the rotary screen was checked while the rotary screen was operated with the flow rate set to more than the nominal capacity of the rotary screen (165 m 3 /h for the drum-rotating type or 128 m 3 /h for the panel-running type). (In the panelrunning type, one half of screen openings were sealed to adjust for the pump capacity.) The operating load was set to 1.5 times as high as the nominal capacity (261 m 3 /h for the drumrotating type or 192 m 3 /h for the panel-running type). 4.6.3 Possibility of rotary screen installation in the combined sewerage facilities to be improved The possibility of the rotary screen installation in existing combined sewerage facilities was studied under the conditions described in section 3.3. 5. RESULTS OF DEVELOPMENT AND RESEARCH 5.1 Performance requirement SRV calculation results showed that the SRV of the drum-rotating type was 96% to 98% when treating wastewater in fine weather or 91% to 99% when screening sewage in rainy weather. The SRV of the panel-running type was 95% to 98% when treating wastewater in fine weather or 93% to 99% when screening sewage in rainy weather. In the case of sewage treatment in rainy weather with the flow rate set to 100% of nominal capacity, the average SRV was 97.6% for both drum-rotating and panel-running types. Figure 5. Relation between flow rate and SRV(drum-rotating type) Debris removal (screen) No.8 9
Figure 6. Relation between flow rate and SRV(panel-runnning type) 5.2 Confirmation of important items 5.2.1 Screen operation performance (1) Continuous operation test Figure 7 shows the changes of influent flow rate, head loss and SRV observed during the continuous operation test on the drum-rotating type rotary screen. When the influent flow rate was constant, the head loss was about 60 mm without showing remarkable changes. There was no problem (e.g., clogging of the screen) concerning screen maintainability, and the automatic operation of the rotary screen could be performed normally. Figure 8 shows the changes of influent flow rate, head loss and SRV observed during the continuous operation test on the panel-running type rotary screen. When the influent flow rate was constant, the head loss was 60 mm or less without showing remarkable changes. There was no problem (e.g., clogging of the screen) concerning screen maintainability, and the automatic operation of the rotary screen could be performed normally. 10 Rotary Screen
JIWET technical report Figure 7. Results of continuous operation test (on the drum-rotating type) 1st day 2nd day 3rd day 4th day 5th day Figure 8. Results of continuous operation test (on the panel-runnning type) Debris removal (screen) No.8 11
(2) Hampering-solid screening test Table 3 lists the results of the test using the drum-rotating type. The drum-rotating type tended to have difficulty in collecting plastic bags, plastic foam trays, and disposable chopsticks, which are floatable and easily caught on parts of the discharge trough. The square timbers of 5 cm or less in length, 30cm-square waste cloths, and waste cans were collected at comparatively high collection rates of 90% to 100%. Since those solids difficult to be collected may remain and be accumulated in the facility, the facility will require inspection after rainfall and periodical cleaning. Table 4 lists the results of the test using the panel-running type. The panel-running type showed 100% collection of square timbers, plastic bags, plastic foam trays, plastic bottles, waste cans, and disposable chopsticks. The overall collection rate of the panel-running type was higher than that of the drum-rotating type. Waste cloths were trapped at a rate of 100%. However, 40% of the trapped waste cloths were caught on parts of the discharge trough. This problem was caused by dimensions of the test equipment. Hairs, paper, oil balls were collected at collection rates of 85% to 90%. The panel-running type rotary screen could be operated normally without its functions being hampered. Table 3. Hampering-solid screening test (on the drum-rotating type) Solid that may hamper screen functions Size Square timber 50 40 20, 50 45 45, 85 55 55 Input Quantity Collected Quantity Collection rate(%) 150(100) 40 20 4 pcs 1 pcs 28 8 pcs 8 pcs 100 Plastic bag 250 300 3 pcs 0 pcs 0 Plastic form tray 100 100(150) 8 pcs 4 pcs 50 Waste cloth 700 300 1 pcs 0 pcs 0 300 300 2 pcs 2 pcs 100 Disposable chopsticks 18090 30 pcs 10 pcs 30 Oil ball 20 30150 150 2,400g 1,698g 71 Hair, paper, and fiber 1,060g 710g 67 Waste can 350mL 3 pcs 3 pcs 100 Plastic bottle 500mL 6 pcs 4 pcs 66 12 Rotary Screen
Table 4. Hampering-solid screening test (on the panel-running type) Solid that may hamper screen functions Size Input Quantity Collected Quantity Collection rate(%) 30 (1216) (150 Square timber 170), 40 12 50, 30 20 100(150), 70 70 70 11 pcs 11 pcs 100 Plastic bag 200 300 7 pcs 7 pcs 100 Plastic form tray 120 100 6 pcs 6 pcs 100 (Cut piece of form tray) 120 100 1 pcs 1 pcs 100 Waste cloth 700(400) 300 5 pcs 5 pcs 100 Disposable chopsticks 180 18 pcs 18 pcs 100 Oil ball 1,600g 1,390g 86.9 Hair 266g 238g 56.4 Paper, and fiber 200g 174g 86.8 Waste can 350mL 4 pcs 4 pcs 100 Plastic bottle 500mL 5 pcs 5 pcs 100 2L 1 pcs 1 pcs 100 5.2.2 Influence on sewage discharge in rainy weather (1) Head loss with the rotary screen operated Figure 9 shows the relation between flow rate and head loss observed while the drum-rotating type rotary screen was operated to treat wastewater in fine weather. While the drum-rotating type was operated to treat sewage in rainy weather, the head loss was 60 to 70 mm, and the operation was normal without any problem although the test conditions varied depending on the number of fine days preceding the operation, precipitation, and operation time zone among 13 times of automatic operation. Figure 10 shows the relation between flow rate and head loss observed while the panelrunning type rotary screen was operated to treat wastewater in fine weather. Since the filtration area of the panel-running type changed according to the water level, the head loss was less with the influent water level set to the designed value of 650 mm in comparison with the head loss measured with the influent water level of 500 mm. For the operation to treat sewage in rainy weather, the influent water level was set to about 500 mm, and the head losses of 16 to 25 mm measured during 8 times of automatic operation. When the panelrunning type was operated with the flow rate set to 100% of the nominal capacity of the rotary screen, the average head loss was 21 mm, and the operation was normal. Debris removal (screen) No.8 13
Head loss (mm) Over flow 60 Nominal capacity () Figure 9. Head loss with the drum-rotating type rotary screen operated Head loss (mm) 21 Nominal capacity () Figure 10. Head loss with the panel-runnning type rotary screen operated (2) Head loss with the rotary screen function stopped When the drum-rotating type rotary screen was stopped and influent was kept being fed at the flow rate set to 100% of the nominal capacity of the rotary screen, the screen was clogged, and the influent flowed over the influent weir into the effluent channel. However, no mechanical problems were found. At that time, the influent water level was 710 mm, which was 210 mm higher than the normal influent water level. The overflow occurred about 1 minute after the rotary screen started operation. When the panel-running type rotary screen was stopped with the screen fully blocked and influent was kept being fed at the flow rate set to 100% of the nominal capacity of the rotary screen, the influent flowed over the influent weir into the effluent channel. However, no 14 Rotary Screen
mechanical problems were found. At that time, the influent water level was 900 mm, which was 250 mm higher than the normal influent water level. The overflow occurred about 1 minute after the rotary screen started operation. (3) Marginal processing capacity of the rotary screen The drum-rotating type caused overflow when it was operated with the flow rate set to 150% of the nominal capacity of the rotary screen. The panel-running type did not cause overflow but only caused a rise of influent water level even when it was operated with the flow rate set to 150% of the nominal capacity of the rotary screen. Since the filtration area of the panel-running type increased as the water level rose, the marginal processing capacity of this type could not be confirmed. 5.2.3 Possibility of rotary screen installation in the combined sewerage facilities to be improved Table 5 and Figures 11 to 13 show the results of the study on installation models. Table 5. Results of the study on installation models Drum-rotating type Panel-running type Scheme 1 Scheme 2 Figure No. Figure 11 Figure 12 Figure 13 Installation location Outlet of pump discharge tank Location of existing fine screen Location of existing fine screen Modification - Modification of building frame of pump discharge tank - Addition of machine hatch - Addition of machine hatch - Addition of machine hatch - Modification of screen bottom panel Head loss 280mm 200mm 100mm Advantage and - Installable in discharge tank - Subject to influence - No major modification of building frame - Head loss of 200 mm disadvantage of charge in effluent water level and substantial rise of influent water level - Less head loss - Easy inflow of grits into the screen (Grits can be discharged into pump well.) Debris removal (screen) No.8 15
6. TECHNOLOGICAL ASSESSMENT Table 6 lists the results of technological assessment. Table 6. Assessment results Scope Assessment results Pumping station When the flow rate was 100% of the nominal capacity, the rotary screen (both drum-rotating and panel-running types) ensured the average SRV of 97%. Therefore, the rotary screen was determined to meet the performance requirement. (Assessment method: Assessment based on the average of the data obtained in 13 test operations of drum-rotating type in rainy weather and the average of the data obtained in 8 test operations of panel-running type in rainy weather) 7. NOTES The following points must be noted when the rotary screen is installed: The allowable maximum head loss due to the rotary screen must be set to 300 mm in consideration of the influence on the rotary screen itself and on the upstream side of the rotary screen. It is assumed that the head loss may exceed the allowable maximum limit because of a screen malfunction, blocking of mesh panels, or water inflow at over the designed flow rate. To prepare for such trouble, an emergency discharge gate must be installed. In addition to clean water, the effluent from a filter can be used as flushing water. The required volume of flushing water depends on the width of screen panel. Before installation of the rotary screen, it is necessary to confirm the influence of the head loss due to the rotary screen on side channels. If the rotary screen is installed in a pumping station that has no grit chamber, the influent is assumed to include more grits as solids when compared with the influent into the pumping station having a grit chamber. To prevent grits from intruding into the sliding parts of the screen and causing damage to sealing, the channel where the rotary screen is installed must be cleaned to remove grits by sweeping or vacuum cleaning periodically. Also, the frequency of screen inspection must be adjusted according to the volumes and types of the solids included in the influent. 16 Rotary Screen
TECHNOLOGY PROPONENT Ishigaki Company, Ltd. For more information about this proposed project, please contact: Proposed person;yoshiaki Kawana / Manager International Marketing Dept. Ishigaki Company, LTD. Address ;1-1-1,Kyobashi, Chuo-ku, Tokyo, Japan Phone ;+81-3-3274-3518 Fax ;+81-3-3274-3557 E-mail ;yoshiaki.kawana@ishigaki.co.jp Debris removal (screen) No.8 17