JIWET technical report. 1. Brush Screen

Transcription

1 1. Brush 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 Nishi-ikebukuro, Toshimaku, Tokyo , Japan 1. OUTLINE OF TECHNOLOGY The brush screen system is a non-powered screen system to remove impurities installed on the overflow of the stormwater outlet chamber in the combined sewer system. This system traps impurities a cylindrical polypropylene brush installed on the overflow weir. The brush rotates any external power supply by utilizing the force of a waterwheel driven by overflow water. The rake on the front of the brush collects the trapped impurities. The impurities then accumulate inside the float type baffle plate, and, when the water level falls, drop into the stormwater outlet chamber from which they are carried through the intercepting sewer to the sewerage treatment plant. Figure 1. Diagrammatic illustration of the brush screen system Debris removal (screen) No.1 1

2 2. MECHANISM OF TECHNOLOGY The brush screen system comprises a cylindrical polypropylene brush, a waterwheel, a driving chain, a rake, a float-type baffle plate, and a cover. Both sides of the brush screen system are covered bypass plates to half the height of the waterwheel so that water can be discharged through the bypass plates in case of abnormal water level. Rake removing trapped impurities Brush Brash capturing impurities contained in overflow water Rake Brash rotating by force of waterwheel Impurities are accumulated inside the float type-baffle plate. Float type baffle plate Weir Waterwheel Overflow water Waterwheel rotating by means of partial flow of overflow water Intercepting sewer Discharged When water level falls, the float type baffle plate opens, and let impurities accumulated inside drop onto the wet weather overflow chamber. Impurities are carried to the sewerage treatment plant through an intercepting sewer. Intercepting sewer Weir Figure 2. Mechanism of brush screen system 2 Brush Screen

3 3. DEVELOPMENT TARGET 3.1 Development target of invitation outline Technology to remove screenings contained in outflow from outfall in gravity system. Technology capable of preventing the discharge of foreign matters scenically unpleasant (i.e. toilet paper, excrements of men and animals, various sanitary items, matters such as food residue, wastes such as wrappings and containers) in sewage discharged at gravity outfalls and/or pump stations on wet weather in combined sewerage system. 3.2 Required performance target The minimum performance target that should be achieved is set at a 30 percent SRV (Screening Retention Value) of impurities sized larger than 5.6mm. The SRV is the index of the rate of impurities removal by the screen system. The calculation expression is as shown below. In this expression, means impurities removal rate the screen system and means impurities removal rate by the weir the screen system. SRV (%) Volume Volume of intercepted impurities Volume of trapped impurities by screen system of intercepted impurities + Volume of overflowed impurities + Volume of trapped impurities by screen system + Volume of Volume of intercepted impurities intercepted impurities + Volume of overflowed impurities Volume of intercepted impurities : Dry weight of impurities intercepted when the screen system is installed Volume of trapped impurities by screen system : Dry weight of impurities trapped by the screen system Volume of overflowed impurities : Dry weight of impurities flowed out to the discharge side when the screen system is installed Volume of intercepted impurities : Dry weight of impurities intercepted when the screen system is not installed Volume of overflowed impurities : Dry weight of impurities flowed out to the discharge side when the screen system is not installed Debris removal (screen) No.1 3

4 3.3 Important points to be checked Experiments were carried out under the basic conditions below as important points to be checked Operation performance of screen (1) Continuous operation test Efficiency and operation checks were made for five 6-hour days in a row. The object of the experiment was waste water in fine weather. The flow rate was set based on the nominal capacity of the screen system. (2) Function-inhibiting experiment The efficiency of removal of impurities that may inhibit the function of the screen system was checked. Impurities inhibiting the function include square timbers, empty cans, PET bottles, disposable chopsticks, plastic carrier bags, styrofoam trays, waste cloths, hairs, waste textiles, and others Influence on wet-weather sewer discharge (1) Head loss caused by installed screen system Head loss caused by installed screening system was checked under the condition of the flow rate equivalent to 50 and 100 percent of nominal screen capacity respectively. (2) Head loss when screen system is suspended Under the flow condition described in (1) above, head loss was checked when the screen system is suspended (due to blockage or breakdown). (3) Treatment capacity limit of screen system The treatment limit capacity was checked under the condition of 150 percent nominal treatment capacity of the screen Examination of applicability to combined sewer facilities in need of improvement Based on actual condition surveys on the stormwater outlet chambers of the existing sewage facilities using the combined sewer system in 191 cities in Japan, model designs thereof were created by setting an average stormwater outlet chamber. 4 Brush Screen

5 Table 1. Factors of model design Flow rate Structure of stormwater outlet chamber Structure of weir Item Value Drainage district area 37.86ha Designed inflow water volume 0.84m 3 /s Designed effluent water volume 0.74m 3 /s Planned inflow water volume 1.87m 3 /s Planned effluent water volume 1.77m 3 /s Interception quantity 0.10m 3 /s Interception ratio 3.00 times Length (inside dimension) 3.10m Width (inside dimension) 2.30m Height (inside dimension) 1.56m Inflow pipe diameter 1.35m Discharging pipe diameter 1.35m Intercepting pipe diameter 0.30m Opening diameter of manhole cover 0.60m Manhole depth (ground inflow side manhole bottom) 2.84m Weir length 2.60m Weir height (inflow) 0.31m Weir height (outflow) 0.87m Weir crest top 1.25m Weir width 0.15m 4. METHOD OF RESEARCH AND DEVELOPMENT 4.1 Experiment place and period Research at the on-site experiment plant Location: The wet-weather grit chamber on the west of Imafuku Sewagetreatment Plant, Osaka City Treatment district: Entire Asahi-ku and Miyakojima-ku; and some parts of Joto-ku and Tsurumi-ku, Osaka City Covered area: 1,616ha Treatment volume: 320,000m3/day Experimental period: May 2003 July Verification research at stormwater outlet chamber Location: Storm water outlet chamber Sumiyoshi 8, Oriono 2, Sumiyoshi-ku, Osaka City Experimental period: Sept. to Dec Debris removal (screen) No.1 5

6 4.2 Specifications of the experimental facility Research at the on-site experiment plant Table 2 shows the specifications of the screen system and the experimental equipment. Table 2. Specifications of screen system and experimental facility Item Brush screen Experimental tank Submerged pump Sampling pump Specifications 500mm L500mm (waterwheel size: 400mm) Nominal capacity 3.8m3/min Steel plate tank 2.2mL 1.0mW 1.7mH m3/min 8m 11kW 2 pumps m3/min 3m 1 pump Verification research at stormwater outlet chamber Table 3 shows the specifications of the screen and the experimental facility. Table 3. Specifications of the screen and the experimental facility Item Brush screen Stormwater chamber outlet Specifications 500mm L500mm (water wheel size: φ400mm) Nominal capacity: 3.8m3/min Size: 6.7mL 3.6mW Weir length: approx. 7m Weir height: approx. 300mm (inflow side) 4.3 Impurities sampling method Research at the on-site experiment plant Figure 3 shows the sampling method. All impurities carried in through the inflow side were sampled. Deposited impurities and impurities caught by the brush screen were collected 2mm wire mesh. The float type baffle plate was fixed so that the impurities caught by the brush screen may be collected. Some impurities on the outflow side were sampled using a sampling pump and separated impurities were sampled using a 2mm wire mesh. The sampling time was 30 minutes. 6 Brush Screen

7 When water level falls, the float type baffle plate opens, and let impurities accumulated inside drop onto the wet weather overflow chamber. Impurities are carried to the sewerage treatment plant through an intercepting sewer. Intercepting sewer Weir Figure 3. Impurities sampling method Verification research in stormwater outlet chamber The sampling method is shown in Figure 4. Impurities caught by the brush screen are collected the float type baffle plate being fixed. All impurities flowed out of the brush screen were all trapped into the trapping cage (2mm wire mesh) set behind the waterwheel. Such impurities were collected after a rain. Volume of captured impurities when a screen is mounted A2 (g) Volume of over flow impurities, a2 (g) Capture cage (wire net a 2 mm mesh size) Figure 4. Impurities sampling method Debris removal (screen) No.1 7

8 4.4 Analysis method Collected impurities were sifted 2mm-, 5.6 mm-, and 9.5mm-mesh screens (JIS Z :2000). Sampled impurities were washed clean water and poured over layered screens for classification. After classification by size, such impurities were classified into 9 categories below, and dry weight of each was measured: 1) papers 2) human excrements 3) kitchen wastes, 4) vegetations 5) hairs 6) vinyl/plastic wastes 7) oil balls 8) metals/glasses 9) others Adding impurities a bucket or other method Purities washed ith t are 9.5 mm screen 5.6 mm screen 2 mm screen Figure 5. Screening method 4.5 Experimental method for the required performance target Research at the on-site experiment plant The calculation formula for the SRV is shown below. Since the screen in this experimental facility is irremovable, it is impossible to measure the amount of impurities trapped in the stormwater outlet chamber which is necessary for the calculation of. Therefore, the volume of sedimented impurities was assumed to be the volume of impurities trapped in the stormwater outlet chamber. Further, the SRV calculation was done, targeting impurities sized larger than 5.6mm. SRV (%) Brush Screen

9 Volume of intercepted impurities Volume of trapped impurities by screen system Volume of intercepted impurities + Volume of overflowed impurities + Volume of trapped impurities by screen system Volume of impurities trapped by weir and screen A1 Volume of inflow impurities A1' A1 + a1' Q1 q1 + Volume of Volume of Volume Volume of Volume of intercepted impurities intercepted impurities + Volume of overflowed impurities impurities trapped by weir of inflow impurities A1' sedimented impurities A A1 + a1' Q1 q1 ( 1 2) Verification research at stormwater outlet chamber The formula for the calculation of SRV is shown below. In this research, all impurities flowed into the brush screen system over the overflow weir were trapped. Impurities carried out were also trapped by a trapping cage. Since the subject is only impurities flowing from the weir, the weir removal rate, there is no need to take into consideration. Thus the value of is 0. Therefore SRV is the proportion of the Volume of impurities trapped by the screen in impurities flowed in. Calculation of SRV was completed using impurities larger than 5.6mm. SRV (%) 1 A2 100 A2 + a2 100 Volume of intercepted impurities Volume of trapped impurities when the screen system is used Volume of intercepted impurities + Volume of overflowed impurities + Volume of trapped impurities when the screen system is used Volume of trapped impurities when screen system is used Volume of inflow impurities Volume of trapped impurities when screen system is used Volume of trapped impurities by screen system + Volume of overflowed impurities A2 A2 + a2 + Debris removal (screen) No.1 9

10 Volume 0 of Volume of intercepted impurities intercepted impurities + Volume of overflowed impurities 4.6 Experimental method for checking important items Performance of screen system In the on-site experiment plant, function-inhibiting experiments and operations were conducted under the basic conditions described in section 3.3. (1) Continuous operation experiment Table 4 shows the specifications of the experiment. Flow volume was 3.1m 3 /min (82% of the nominal capacity). Table 4. Experimental data Date July 7 (Mon) 8 (Tue) 9 (Wed) 10 (Thu) 11 (Fri) Operation hours 10:00-10:00-10:00-10:00-10:00-16:00 16:00 16:00 16:00 16:00 Weather Fine Fine Fine Fine Fine Flow volume (m 3 /min) (2) Function-inhibiting experiment Impurities were directly added from the top of the experimental tank. Situation after a 30 minutes operation was examined Influence on wet weather sewer discharge Experiments were performed under the basic conditions described in section 3.3. (1) Head loss when screen system is installed The target flow rate was set at m 3 /min, which are equivalent to percent of nominal capacity. (2) Head loss when screen system is suspended While blocking off inflow to simulate conditions of system suspension arising from a complete blockage of the brush inflow area, the overflow water level was measured. However, in order to minimize rises in water levels due to installation of the brush screening system, the upper part of the brush cover was left open to the value of half of the diameter of the brush. Experiments were conducted under the basic conditions described in section 3.3. The flow volume was 2.1m 3 /min, equivalent to 55 percent of nominal capacity. 10 Brush Screen

11 (3) Treatment capacity limit of screen system In order to minimize rises in water levels due to installation of the brush screening system, the upper part of the brush cover was left open to the value of half of the diameter of the brush. The experimental target flow rate was set at 5.4m 3 /min, equivalent to 142 percent of the nominal capacity Examination on applicability to facilities in need of improving combination system Applicable possibility was examined under the basic conditions described in section RESEARCH AND DEVELOPMENT RESULTS 5.1 Required performance target Research made at the on-site experiment plant Table 5 shows the SRV measurement results obtained in the on-site experiment plant. The SRV therein was approx percent. It is considered that such large gap between SRV measurements derives from: (i) small volume of inflow impurities respect to the volume of treatment water, and, consequently, (ii) selective inflow of impurities some of which are difficult to trap the brash and others easy. Table 5. Measurements of experiment plant SRV No. Date weather Addition of impurities Flow volume (m 3 /min) SRV (%) 1 June 26 Fine No June 27 Rain No July 7 Fine No July 8 Fine Yes July 9 Fine Yes July 10 Fine No July 14 Fine No July 14 Fine No July 8 Fine Yes July 8 Fine Yes July 9 Fine Yes July 10 Fine No Verification research at stormwater outlet chamber Table 6 shows the measured results of SRV made in the stormwater outlet chamber. According to the results of the verification research executed in the stormwater outlet chamber, the system exhibited stable impurity-removal-performance of percent. Figure Debris removal (screen) No.1 11

12 6 shows the relationship between the average treatment flow rate and SRV obtained through the verification research. Figure 6 shows that SRVs conform to the requirements of the experiment, and are approx. 60% of total nominal capacity (3.8m 3 /min). No. Date Table 6. Measurements of wet weather overflow SRV Average treatment flow rate (m 3 /min) SRV (%) 1 Sept Nov Nov Nov Nov Figure 6. SRV Measurements in stormwater outlet chamber 5.2 Important points to be checked Operation performance of screen system (1) Continuous operation experiment In a continuous run, stable operations were observed any abnormality in the moving parts including the brush, waterwheel, and driving chain. The brush showed no abrasion after operation. During the operation almost no deposit of impurities was found on the brush core. Total operating hour for the verification research in the stormwater outlet chamber was about 18 hours. During the research period no abnormality was found, and stable operation was confirmed. 12 Brush Screen

13 (2) Function-inhibiting experiment Table 7 shows the results of the function-inhibiting experiment. The brush screen system completely prevented the impurities tested in this experiment from flowing out. About 10% of cast disposable chopsticks were caught by the brush screen and flowed into the float type baffle plate. Floatable wastes such as square timbers and empty cans floated on water surface flowing into the brush screen, and did not affect the screen system. Materials such as plastic carrier bags, nylon ropes, and empty cans flowed into the brush section, but, after hitting the brush, observed to get caught in water flow again, drifting in the tank. No nylon ropes were found to be tangled in the brush. Impurities type Square timber Table 7. Result of function-inhibiting experiment Size 80mm 600mm long 50mm 300mm long Added amount Trapped amount Flowed amount 1 each 0 0 Empty cans 350mL can, 500mL can 1 each 0 0 PET bottles 500mL Disposable chopsticks 200mm long Plastic carrier bags Approx. 200mm 250mm Approx. 300mm 400mm 1 each 0 0 Nylon rope φ6mm 1m long Waste cloth Approx. 200mm 200mm Approx. 400mm 300mm 1 each Influence on wet-weather sewer discharge (1) Head loss when screen system is installed Figure 10 shows the result of head loss measurement. The head loss increased about 43%, compared to that the installation of the brush screen system. Figure 7. Head loss the screen system installed Debris removal (screen) No.1 13

14 (2) Head loss the screen system shut down If the inflow is blocked, water flowed over the bypass plates on the both sides of the brush and the brush cover. The depth of the overflow water was approximately 320mm. The normal depth of 2.1m 3 /min overflow water in weir is as follows: H Q 1.8B 2 3 Q Treatment flow rate (m 3 /s) B Weir width (m) brush width + bypass plate width m H 10 84mm This is equivalent to the observed value (70mm). Therefore, the weir height can be deemed to have increased to the value of half of the diameter of the brush (250mm) by blocking off the inflow. (3) Treatment capacity limit of screen system The treatment capacity of the brush screen system reaches the limit when the depth of overflowed water reaches the height of the semidiameter of the brush (250mm). When the capacity is exceeded, water overflows over the bypass plates located on the both sides of the brush and over the brush cover. The depth of the overflowed water was about 310mm Examination of applicability to combined sewer facilities in need of improvement The result is shown in Figure 8 and Table 8. Table 8. Results of model design examination Changes The shorter side of the overflow chamber (inside dimension) changed from 2,300mm to 2,600mm and the longer side from 3,100mm to 3,600mm (inside dimension) 600mm manhole added on the discharge side Reasons To secure 400mm clearances from the walls for access space for the inspection of the brush screen and the inflow of intercepting sewer For waterwheel inspection. The waterwheel size in this system is 590mm and cannot be carried in through the existing 600mm manhole due to the existence of the ladder; it should be carried in through a new manhole to which a ladder will be installed afterward. 14 Brush Screen

15 Figure 8. Review results of model design Debris removal (screen) No.1 15

16 6. TECHNOLOGY ASSESSMENT Table 9 shows the assessment result. Table 9. Assessment result Scope of application Assessment result Stormwater outlet chamber SRV is approximately 60 when the flow volume is 100% of the nominal capacity; the necessary performance found to be achieved. (Assessment method: inference from five wet weather data) 7. POINTS OF CONCERN In introducing this technology, the following points should be noted: Since waterwheel is not activated if it is submerged to about 100mm, the water level of the discharge side should be reviewed. Detection of abnormality in the screen system basically relies on inspections to be conducted after heavy rain, in addition to normal inspections performed once several months. When needed, devices such as a water-level gauge must be considered to be added. With the amount of the head loss due to the installation of the screen system added to the pre-installment running water level, backwater should be examined regard to the longest upstream conduit route. The head loss of this technology is half the height of the brush diameter at maximum. TECHNOLOGY PROPONENT KUBOTA Corporation Address 1-3,Nihonbashi-Muromachi 3-chome, Chuo-ku, Tokyo , JAPAN Tel Fax Person in charge Yuji Otsuka (Water & Sewage Engineering Dept.) E. mail sewage@kubota.co.jp 16 Brush Screen