9-1. Wet-weather high-speed wastewater filtration system

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1 9-1.Wet-weather high-speed wastewater filtration system N. Horie 1, M.Kabata 2, K.Sano 3, S.Kanamori 4 Director, Chief Researcher,Senior Researcher 3, Researcher 4 First Research Department Japan Institute of Wastewater Engineering Technology 1 Outline of the technology System profile The system is based on the principle of upflow filtration utilizing floating filter media. As shown in Figure 1, it consists of a plural number of filter bed filters and a wash water drain tank. Base water fed by means of a lift pump flows over from the inflow conduit at the top, and an equal amount of filtrate overflows from the filtration system. As a result, the system is capable of handling wet-weather wastewater, which fluctuates in terms of quality and quantity. In addition, the filtrate stored above the filter is used to wash the filter media by downflow based on this difference of water level. Base w ater Filtrate Upper screen Wash water drain tank media media The special floating filter media (see Photo 1) are characterized by a highly uneven surface, windmill Existing public works structure Figure. 1 Treatment flow shape, and small size. Because of the high void ratio, they are effective for trapping suspended solids (SS), and offer a comparatively long duration of continuous filtration. The SS trapped on the uneven surface are easily expelled in backwashing, when the downflow widens the interstices between the media. Due to the small size, filtration does not require the addition of coagulants. Photo 1 Special filter media

2 Principle As shown in Photo 2, in filtration, the principle is one of ordinary upflow high-speed filtration. Inorganic solids and other such matter precipitate to the bottom of the tank, debris are kept below the filter bed, and SS are trapped throughout the filter layer. Filtration Washing Upper screen Base water Upflow Floating filter media Removal - SS (with a diameter in the range of µm or more) m ) Debris - Hair - Vegetable waste - Leaves - Oil balls, etc. Figure. 2 Filtration Filtrate Floating filter media Downflow Debris and SS Figure. 3 Washing As shown in Figure 3, during washing, the SS and debris are expelled from the bottom of the tank by the rapid downflow, but the filter media are not carried away. Operation process Filtration Washing Filtration Lift pump To simplify the system, the lift pump is valve operated continuously during both Requisite time filtration and washing, as shown in (minute) Figure 4. Although base water therefore enters even during washing, reduction of the filtrate recovery Figure. 4 Operation process ratio is suppressed by shortening the washing duration to about one minute. Equipment operation and water flow Figure 5 shows the equipment operation and water flow. As shown in Item I, right after the start of filtration, the water level in the pressure-adjustment conduit and the filter tank is the same, and the amount of filtrate overflow from the filter tank is equal to the inflow of base water into the pressure-adjustment conduit. As the filter media become clogged, the water level in the influent pressure-adjustment conduit rises, as shown in Item II. When it reaches the prescribed level (in the range of cm), the sequence proceeds to Item III and washing begins. As shown in Item III, washing is performed by opening the elutriate valve on the bottom of the tank.

3 After the end of the rainfall, the system is completely washed and replaced with secondary treatment effluent as shown in Item IV, in preparation for the next rainfall. I. Start of filtration Base water Pressure-adjustment conduit II. Filtration continuation 2 Development and research 2.1 Requisite performance and development targets Development targets (requisite performance) noted in the rules for preparation of papers The development target noted in the rules for preparation of papers for technology development related to improvement of combined sewerage was a performance surpassing that of the conventional technology (stormwater settling tanks). This area is shown in Table 1. media Stormwater treatment effluent III. Backwash in wet weather Backwash using filtered stormwater Rise in the water level of the pressure-adjustment conduit Figure. 5 Equipment operation and water flow Screen SS Debris IV. Washing after the rainfall Secondary treatment effluent Backwashing and replacement with secondary treatment effluent Table. 1 Development targets (requisite performance) noted in the application guideline Range of application influent into primary settling tanks in wastewater treatment plants technology with a performance surpassing the pollutant Requisite performance removal performance (removal ratios of 30% for BOD and 30% for SS) of the conventional technology (stormwater settling tanks) Development targets presented by the technology proposer Table 2 shows the development target presented by the technology proposer. The proposer set targets that are higher than the requisite performance noted in Table 1.

Filtration rate 1,000 m/day Development targets SS removal ratio At least 70% Debris removal ratio 100 2.

4 Table. 2 Development targets presented by the technology proposer Site of application wastewater treatment plants Range of application influent into primary settling tanks in wastewater treatment plants Filtration rate 1,000 m/day Development targets SS removal ratio At least 70% Debris removal ratio Development and research method Test place, period, base water, and proving equipment As base water, the test used primary settling tank influent at the Kita-Tama No. 2 Wastewater Treatment Plant operated by the River-Basin Sewerage Division, Bureau of Sewerage, Tokyo Metropolitan Government. It was conducted from September 2001 to February The base water had been passed through a coarse (150 mm) screen and fine (50 mm) screen at the grit chamber. Name specifications number High-speed filters 0.5 m 2 x effective depth of 4 m 3 media Thickness of 1 m 1.5 m 3 total Lift pump 0.5 m 3 /min, m 3 /min 1 1 per system Table. 3 Specifications of proving test equipment Height of 5 m (1/1); same as actual equipment As shown in Table 3 and Figure 6, the proving test equipment consisted of three filter tanks, each with an area of 0.5 m 2. They were as high as the actual equipment (see Photo 2). Photo 2 Proving test equipment Specifications of proving test equipment Common equipment base Filtration rate Treatment rate up to 1,000 m/day up to 1,500 m3/day Base water receiving tank Secondary treatment effluent (from existing facilities, if needed) Base water media Joint-use treatment effluent tank media media Treated effluent 0.5 m2 x 3 tanks; filter media height: 1 m Existing drainage conduit Sanitary wastewater receiving well valve 1 valve 2 valve 3 tank transportation pump Figure. 6 Proving test equipment flow

5 Start of wet-weather operation and operating conditions The operation of the proving test equipment was timed so that the treatment plant carried out primary treatment (1Q overflow). The operation was conducted at a constant filtration rate for each rainfall. The testing applied speeds of 200, 400, 600, 800, and 1,000 m/day. 2.3 Results of development and research Definition of removal ratios as indicators of performance for removal of pollutants As indicators of pollutant removal performance, removal ratios for SS and BOD were calculated by the following formula Removal (Total influent load - total effluent load) = 100 based on the total load per rainfall. ratio(%) Total influent load SS removal performance Figure 7 shows the correlation between the influent and effluent loads per square meter of filtration area and hour in each of the total of eight rainfalls. The removal ratio is influenced by filtration rate and base water concentration. Through multiple regression analysis, the SS removal ratio was estimated by the formula shown in this figure. With this formula, the SS removal ratio is 73.3 % at a filtration rate of 1,000 m/day and base water SS concentration of 200 mg/l. BOD removal performance Figure 8 shows the correlation between the influent and effluent loads per square meter of filtration area and hour in each of the total of seven rainfalls. The removal ratio is influenced by filtration rate and base water Total effluent load (kg/m 2 /hr) Filtration rate m/day (base water concentration mg/l) Removal ratio 0% Removal ratio 30% Removal ratio 50% Removal ratio 70% Total influent load (kg/m 2 /hr) SS removal ratio (%) 0.002A0.164B A: filtration rate (m/day) B: base water concentration (mg/l) Figure. 7 Total effluent load (kg/m 2 /hr) Summary of SS removal performance Filtration rate m/day (base water concentration mg/l) Removal ratio 0% Removal ratio 30% Removal ratio 50% Removal ratio 70% Total influent load (kg/m 2 /hr) BOD removal ratio (%) * 0.030A0.122B A: filtration rate (m/day) B: base water concentration (mg/l) * Variation depending on the ratio of dissolved BOD present in the base water Figure. 8 Summary of BOD removal performance

6 concentration. Through multiple regression analysis, the BOD removal ratio was estimated by the formula shown in this figure. With this formula, as shown in Table 4, the BOD removal ratio is 53.9 % at a filtration rate of 1,000 m/day and base water BOD concentration of 200 mg/ L. In addition, it is 72.6 % at a filtration rate of 365 m/day, which is average in the operation. Table. 4 BOD removal ratio Filtration rate BOD removal ratio (%) 365 m/day (average filtration rate in actual operation) ,000 m/day (maximum filtration rate) 53.9 SS and BOD treatment performance cases Figures 9 and 10 present cases of change over time in the treatment performance. In both rainfalls, the initial pollution peak came between 45 minutes and 2 hours after the start of the rain, but the filtration proceeded smoothly thereafter. 600 m/day Base water Treated effluent 800 m/day Base water Treated effluent Time (hours) Time (hours) Change in SS concentration over time Change in SS concentration over time 600 m/day Base water Treated effluent 800 m/day Base water Treated effluent Time (hours) Time (hours) Change in BOD concentration over time Change in BOD concentration over time Figure. 9 SS and BOD removal (600 m/day) Figure 10 SS and BOD removal (800 m/day) Removal of debris The removal ratio for debris (with a size of at least 2 mm) was 100 % in the nine rainfalls. Photo 3 shows the results of treatment at a filtration rate of 1,000 m/day. The filter media were also able to remove oil balls.

7 Filtrate recovery ratio The filtrate recovery ratio was defined as the amount of filtrate as percentage of the amount of base water. The formula shown below was applied to estimate this ratio from the results of the test. The filtrate recovery ratio is a function of the washing speed and washing duration (which are set as conditions of the operation) as well as the filtration rate and base water concentration, which vary with the water flow and quality. Filtrate recovery ratio (%) 100elutriate ratio (%) B C D 1000 ( ) A: filtration rate (m/day) B: base water concentration (mg/l) C: washing speed(m/ min) Paper 12g(4%) Metal 0.2g(0.1%) Wood, leaves, grass 47g(17%) Hair, etc. 21g(8%) Kitchen waste/garbage 21g(8%) Oil balls 21g(8%) Plastic 24g(9%) Other debris 147g(53%) Debris in 100 m 3 of wet-weather wastewater on 22 October 2001 Photo 3 Debris removed At a filtration rate of 365 m/day (the average) and base water SS concentration of 180 mg/l (the average for all combined sewerage wastewater nationwide* 1 ), the filtrate recovery ratio was 90.3 v/v %. *1 : From a report on the findings of a study concerning measures for improvement of combined sewer systems released by the Ministry of Land, Infrastructure and Transport in March Technical assessment Table 5 shows the development targets noted in the rules for preparation of papers and the results of assessment Table. 5 Development targets noted in the rules and assessment influent into primary settling tanks in wastewater treatment plants in Range of application combined sewer systems Development targets technology with a performance surpassing the pollutant removal (Requisite performance (removal ratios of 30% for BOD and 30% for SS) of the performance) conventional technology (stormwater settling tanks) BOD and SS removal ratios of at least 30 % each; confirmation of Assessment results ability to deliver the requisite performance Table 6 shows the development targets presented by the technology proposer and assessment results.

8 Range of application Development targets Assessment results Table. 6 Development targets by the technology proposer influent into primary settling tanks in wastewater treatment plants in combined sewer systems [Removal ratios] removal ratios on the following levels at a filtration rate of 1,000 m/day - SS: at least 70% (at a base water concentration of 200 mg/l) - Debris: 100 [Average filtrate recovery ratio] - Ratio of recovery of filtrate as percentage of actual wet-weather wastewater: at least 90% [Removal ratios] It was confirmed that the system basically attained the development targets for removal ratios at a filtration rate of 1,000 m/day (effective filtration rate of 800 m/day) and base water SS concentration of at least 200 mg/l. [Average filtrate recovery ratio] Based on the test results, it was confirmed that the system basically attained the development target for the average filtrate recovery ratio at a filtration rate of 365 m/day (effective filtration rate of 330 m/day), which is thought to be the average one in actual operation, and base water SS concentration of 180 mg/l. 3 Features of the technology 3.1 Effective use of existing primary settling tanks, etc. Use of a thinner (less than 1 m in thickness) filter media layer enables installation on tanks (primary settling tanks and stormwater settling tanks) with an effective depth of more than 2.5 m. In this (Before modification of the primary settling tank) case, there is no need for additional installation space. As shown in Figure 11, the existing facility can be modified into a filter by partitioning it. There are only three filter components: the special (After modification into the high-speed filter) upper screen, special filter media, Special upper screen and high-speed washing equipment. Special filter media The system can also be newly Distribution tank pipe installed from civil engineering (Partition panels) facilities. The construction is High-speed washing equipment low-cost because it requires only a Washing wastewater pump shallow tank (depth of no more than Figure. 11 Installation on a primary settling 5 m).

9 3.2 No need for base water pretreatment Because the system accepts and can treat base water that has merely passed through the screen on the grit chamber (with a gauge in the range of mm)* 2 as is, it does not require pretreatment (a front screen). This feature eliminates the need for cleaning of screens after a rainfall and facilitates maintenance. * 2: According to the 2001 edition of the sewerage facility planning/design guidelines and commentary 3.3 Ease of operation in wet weather The biggest feature of this technology is the ease of operation in wet weather. Because there is no need for preparation and application of chemicals, there is no manual labor regardless of the rainfall timing and fluctuation in the amount of water. After rainfalls, routine checks are made to confirm that the filtration proceeded well (by reference to data on filtration pressure loss, etc.). Table. 7 Work in wet weather Manual operation work Reference: equipment operation Start of rainfall None - Base water pump startup filtration = start of During rainfall None - Operation following influent fluctuation (fluctuation in the amount of base water influent = fluctuation in the filtration rate) - Sequential automatic washing of the high-speed filters at a prescribed pressure loss value Peak rainfall None - Direct discharge from the overflow weir of the distribution tank for flow in excess of the prescribed value End of the rainfall None - Base water pump shutdown = filter shutdown After the rainfall (dry weather) Complete washing (as appropriate) - Forced washing of the filter media in all filters using secondary treatment effluent 3.4 Treatment without coagulants Another major feature of the system is that it does not require the use of chemicals. The running cost consists solely of the charge for the electricity mainly for the lift pump, and comes to about 1.10 yen/m 3 of treated effluent. This is about the same as the corresponding figure of 1.03 yen for the lift pump in conventional treatment by primary settling (see Figure 12). Table. 8 Conditions applied in calculation of maintenance cost Facility capacity 0.2 million m 3 /day Yearly rainfall duration hr Number of rainfalls times Total treatment volume 1.77 million m 3

10 3.5 Reduction of use of disinfectants (item outside the scope of assessment) The system has a lower maintenance cost than the conventional treatment by a primary settling tank in the case of disinfection of the effluent from the latter. This is because the high-speed filtration effectively removes SS. As shown in Figure 13, at a treatment scale of 0.2 million m 3 /day, the yearly cost reduction relative to the conventional treatment is estimated at 1.68 million yen (24 %). 4 Method of application Site for application of the technology (1) Refinement of primary treatment in wastewater treatment plants (2) Primary treatment of untreated discharge at pumping stations, etc. The first application was assessed by the SPIRIT 21 Committee in October The second is scheduled to be assessed in fiscal Tens of thousands of yen/year Reduction Plan for installation on a primary settling tank Figure. 13 Maintenance cost (disinfectant cost) Due to the spread of advanced treatment and other developments in recent years, a rate of 50 m/day* 3 has come to be considered advisable as the standard surface loading of primary settling tanks. In contrast, existing primary settling 35 m/day 50 m/day tanks were generally designed for 35 surface loading of about 35 m/day. As m/day 50 m/day such, high-speed filter tanks can be 35 m/day 50 m/day planned for the purpose noted in the first 35 m/day High-speed filter application above by revising the design Conventional (before installation) After installation values as shown in Figure 14. Figure. 14 Change in surface loading *3: According to the draft version of the before and after installation Same calculation conditions as in Table 8 Yearly maintenance cost Proposed system Conventional primary settling tank treatment - Use of sodium hypochlorite (70 yen/kg) - Addition in amounts controlled so that the residue is no more than 0.1 mg (Cl) /L

11 standard activated sludge process design guidelines published in 1995 by the Japan Sewage Works Agency. Example of installation on a primary settling tank As shown in Figure 15, the facility for stormwater treatment can be installed for the amount of Q overflow in wet weather. Figure 16 shows the placement of the filters and washing wastewater tank in the case of a rectangular settling tank. To assure efficient use of the filter surface, modification into one filter with three conduits is advisable. The system can also be installed on circular settling tanks. Q overflow Stormwater treatment Reaction tank Primary settling Q tank Figure. 15 Partial modification of a primary settling tank for transformation into a stormwater filtration facility DEVELOPED COMPANY NGK Insulators, Ltd. TEL : +81-(0) FAX : +81-(0) Final settling In the case of modification of the primary settling tank into 3 conduits and 1 filter tank Figure. 16 Layout of filters in a primary settling tank

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