13. High Rate Filtration Process

Transcription

1 13.High Rate Filtration Process 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) Technical outline In this technique, cationic polymer coagulant is added to sewage influent, which is then fed to an upward stream filter basin. In the basin SS and other suspended organic substances are captured by floating filter media (specific gravity: 0.9) and removed. Figure 1 presents the treatment flow of this technique. The purpose of this technique is to reduce contaminant load due to the discharge under wet weather conditions. In addition, by reducing the filtration rate under dry weather conditions, the process can be operated as a substitute for the primary settling tank without chemicals addition. Lattice-shaped hollow Flocculation Filter cylindrical filter media basin basin Coagulant having high-porosity are used to trap SS over Raw water almost all layers of the Treated water filter installed in the tank. Even sewage of Screen Blower for backwashing high SS concentration Filter media can thus be treated, and superficial filtration, To drainage tank which may result in the case of sand filtration, Figure. 1 Treatment flow of high rate filter process can be prevented. By injecting cationic polymer coagulant into sewage influent, filtration ratio of 1,000m/day is assured. (Figure 2, picture of the filter media) The filter media undergo spiral flow air washing process first, then the drain is discharged. Sewage influent is used as washing water. Figures 3 and 4 show washing and draining processes. The average operation cycle of the filtration adopting this Figure. 2 Filter media technique is 159 minutes (filtration) + 21 minutes (washing), 180 minutes in total, under wet weather conditions.

2 Raw water Open Closed Treated water Closed Closed Closed Air Open Closed Open Closed Backwashing drain Filtration Washing Draining Figure. 3 Operation cycle Raw water valve Washing drain valve Air wash valve Air wash blower Duration (min.) Filtration Tank water draining Spiral flow washing Draining Raw water feeding Spiral flow washing Draining Filtration * The total washing time is 21 minutes. Figure. 4 Typical washing process (2) Development research Performance requirement and development goal [Development goal listed in the solicitation guidelines] The development goal listed in Solicitation guidelines of technical development on improvement of combined sewage system is to develop a technique, to be adopted to a combined sewerage system of a final treatment plant the purpose is to remove contaminants in the influent to the primary settling tank at higher removal efficiency than

3 that of a conventional technique (storm-water settling tank), whose BOD and SS removal efficiency is 30 %. Table 1 summarizes the concept. Table 1 Development goal listed in solicitation guidelines (performance requirement) Applicable range Influent to the primary settling tank of a final treatment plant Performance The developed technique should have contaminant removal requirement efficiency higher than that of a conventional technique (storm-water settling tank), whose BOD and SS removal efficiency is 30%. [Development goal established] The purpose of the development is to establish a technique that achieves the target set, which is listed in Table 2 and is higher than that listed in Table 1. Place of application Applicable range Target performance Table 2 Development goal we established Final treatment plant Influent to the primary settling tank of a final treatment plant Filtration rate 1000m/day BOD removal efficiency 50% or higher SS removal efficiency 70% or higher Procedure of development research [Place of experiment and period] Experiments were conducted during the period from December 1991 to March 1994 using as raw water the influent to the primary settling tank of East Line, Shibaura Treatment Plant, Bureau of Waterworks, Tokyo Metropolitan Government. The raw water flowed through a 25mm screen in a sand basin. The settling tank is not equipped with a coarse screen. The demonstration plant has average treatment capacity of 2,000m 3 /day for wet weather conditions, filtration area of 2m 2, filter media filling height of Figure. 5 Appearance of demonstration plant

4 2m, and filter basin dimensions of 1m (W) 2m (L) 4m (H). Figure 5 shows the appearance of the demonstration plant. [Start of operation in wet weather and operating conditions] Under dry weather conditions, treatment tests were conducted in the demonstration plant at the filtration rate of 400m/day. When start of rainfall was confirmed, the operation for wet weather conditions was started at filtration rate of 1000m/day with cationic polymer added at the rate of 2.0mg/L. Interlocking with the operation of the treatment plant was not provided. The total time required for the experiment was 388 hours, and the total amount of water fed was 28,167m 3. Result of development research Evaluation method The following points were taken into consideration when we confirmed that the performance requirements and objectives of the development of this technique had been achieved. [Definition of contaminant removal efficiency] The contaminant removal efficiency was calculated using the following expression based on the total load by rainfall. Removal efficiency (%) = (Total influent load Total effluent load) / Total influent load 100 The total influent load was calculated per rainfall by multiplying the inflow at the time of measurement by the water quality of the influent. Linear interpolation was performed between each measurement to smooth out the fluctuation, and the area was calculated as an integration of the values obtained in 5 hours from the start of sampling. Since the number of data obtained on BOD concentration was few, BOD concentration was calculated based on the correlation between the BOD and the turbidity of raw water and treated water as well as on the actual measurements of spot samples, and the results were used as data. Result of evaluation [Test result of treatment performance under wet weather conditions] Figure 6 shows the amount of precipitation, turbidity, SS concentration, BOD concentration of raw water and treated water, and filtration head loss.

5 Figure. 6 Test result of treatment performance under wet weather conditions [SS removal performance] Figure 7 presents the correlation between the influent load and the effluent load per filtration area and time for each of 40 rainfalls. The average removal efficiency was

6 77.9%. Removal efficiency: 50% Removal efficiency: 70% Removal efficiency: 80% [BOD removal performance] Figure. 7 Summary of SS removal efficiency Figure 8 presents the BOD concentration of spot samples of raw water and treated water. Almost all data show that the BOD removal efficiency of those samples was 30% or higher. The average BOD removal efficiency was 64.6%. Removal efficiency: 30% Removal efficiency: 50% Removal efficiency: 70% Figure. 8 BOD removal performance of spot samples Figure 9 presents the correlation between the influent BOD load and effluent load per filtration area and time for 40 rainfalls. The load was calculated in the same way as the calculation of SS removal performance. Since the number of data for analysis of BOD

7 concentration was small, the BOD concentration was calculated based on turbidity, using the correlation between the turbidity and actual BOD measurement values. The average removal efficiency was 51.8%. Removal efficiency: 30% Removal efficiency: 50% Removal efficiency: 70% Figure. 9 Summary of BOD removal performance [Amount of trapped SS and maximum filtration head loss] Figure 10 presents the relation between the head loss during the experiment and the amount of trapped SS per filtration area. The maximum amount of trapped SS was about 25.7kg/m 2. The head loss tended to increase with the increase of the amount trapped SS. Based on the maximum head loss value, about 15kPa (1.5m) is considered to be the upper limit of practical head loss value in design. Figure. 10 Relation between amount of trapped SS and head loss

8 [Height of filter layer and amount of trapped SS] The standard height of the filter layer in this technique is 2m. By reducing the height of the filter layer, the required system height can be reduced, which facilitates introducing the system into an existing primary settling tank by modifying it. However, if the height of the filter layer is reduced and water is fed at the same filtration rate, a breakthrough of filter layer occurs with smaller amount of trapped SS. To prevent this from occurring, washing must be performed before the amount of trapped SS reaches the specified level. With this in view, we examined, changing the height of the filter layer, and identified the amount of trapped SS that causes a breakthrough. Figure 11 presents the amount of trapped SS per filtration area when the height of filter layer is changed to 1.01m and to 1.66m. When the height of the filter layer is made to be 1.01m, the maximum amount of trapped SS was 9.34kg/m 2 -filter media, and when the height is made to be 1.66m, it was 23.8kg/m 2 -filter media. Figure. 11 Amount of trapped SS by height of filter layer [Filtration rate and filtrate recovery ratio] Fig. 12 and Table 3 present the definition of water balance in filtration process used to evaluate the filtration rate and the filtrate recovery ratio. Filtration area of filter (m 2 ) Figure. 12 Inflow and outflow of the filter Table 3 Inflow and outflow of the filter

9 Inflow Outflow During filtration During washing Increase of water level of filter basin after washing Total Time Amount of raw water (m 3 ) Amount of treated water for washing (m 3 ) Amount of washing drain (m 3 ) Amount of treated water (m 3 ) Table 4 summarizes the inflow and outflow in the operation cycle of this process. Table 4 Amount of inflow and discharge in 1 cycle Amount of inflow and effluence in 1 cycle (1) Increase of water level (2) Filtration process (3) Draining of water within basin (4) Draining The water level is increased by 4m from the state where the filter basin is empty. Increase of water level in filter basin after washing (C) = Filtration area (m 2 ) 4 (m) Amount of water that undergoes filtration for 159 minutes out of 180 minutes of 1 cycle, excluding the increase of water in (1) Inflow of raw water for filtration (A) = Filtration area (m 2 ) 1000 (m/day) 159 (min.) / 1440 (min./day) Filtration area (m 2 ) 4 (m) Water within the basin is drained to the water level 1m below the drainage level. Washing drain (E) = Filtration area (m 2 ) 1 (m) Following the draining of water within the basin in (2), water is drained to the level 3m lower. Washing drain (E) (addition) = Filtration area (m 2 ) 3 (m)

10 Raw water is fed to the water level 3m higher. (5) Inflow Amount of washing water (B) = of raw Filtration area (m 2 ) 3 water (m) The water fed in (4) is drained. (6) Draining Washing drain (E) (addition) = Filtration area (m 2 ) 3(m) A: Inflow of raw water for filtration = Filtration area (m 2 ) (1000 [m/day] 159 [min.] / 1440 [min/day] 4 [m]) = Filtration area (m 2 ) (m) B: Amount of washing water (B) = Filtration area (m 2 ) 3 (m) C: Increase of water level after washing = Filtration area (m 2 ) 4 (m) D: Amount of treated water for washing = 0 E: Amount of washing drain = Filtration area (m 2 ) 7 (m) Amount of effective treated water = (A + B + C E) = Filtration area (m 2 ) (m) From the above, the washing water volume ratio is calculated as follows: Washing water volume ratio = (D) / (A + B + C) 100 = 7 / ( ) 100 = 6.17 (%) The filtrate recovery ratio is calculated as follows: 100 (%) 6.17 (%) = 93.8 (%) Furthermore, the effective filtration rate is calculated as follows: Effective filtration rate = (Amount of effective treated water) / (Filtration area) Duration of 1 cycle (day) = (Filtration area) / (Filtration area) 180 (min.) / 1440 (min./day) = (m/day) Technical evaluation [Result of development compared to the target of development (performance requirement) listed in the solicitation guidelines] Table 5 summarizes the result of evaluation compared to the development target (performance requirement) listed in the solicitation guidelines.

11 Table 5 Target of development listed in solicitation guidelines and evaluation Applicable range Influent to primary settling tank of a final treatment plant in combined sewerage system Target of development The developed technique should have contaminant (performance requirement) removal efficiency higher than that of conventional technique (storm-water settling tank), whose BOD and SS removal efficiency is 30%. Result of evaluation BOD removal efficiency of higher than 30% and SS removal efficiency of higher than 30% were obtained. The target performance requirements have thus been satisfied. [Target performance we established and result] Table 6 lists the result of evaluation compared to the target performance we established. Table 6 Target performance we established and evaluation Applicable range Influent to the primary settling tank of a final treatment plant in combined sewerage system Target of development The target removal efficiency at filtration rate of 1,000m/day was set as follows: SS removal efficiency: 70% or higher BOD removal efficiency: 50% or higher Result of evaluation At filtration rate of 1,000m/day (effective filtration rate: 851m/day), the following results were obtained. The average SS removal efficiency was 77.9%, which satisfies the target value. The average BOD removal efficiency was 51.8%, which satisfies the target value. Treatment performance under dry weather conditions (outside the evaluation range) The system adopting this technique is to be operated as a substitute for the primary settling tank under dry weather conditions. The treatment performance of the system under dry weather conditions is shown below. Table 7 lists the target and operating conditions in dry weather we presented. Table 7 Target and operation conditions in dry weather Item Value Filtration rate 400m/day BOD removal efficiency 40% or higher SS removal efficiency 60% or higher Chemical dose None Washing cycle 6 hours

12 Figure 13 presents an example of treatment result of operation under dry weather conditions. The average SS removal efficiency was 67.6%, and BOD removal efficiency was 40.4% in operation throughout the year. Figure. 13 SS removal performance under dry weather conditions

13 (3) Features of this process In comparison with existing technical measures against overflow of combined sewerage system, this process features the following; High filtration rate assures space saving. Since the influent to the primary settling tank can be treated at the filtration rate as high as 1,000m/day under wet weather conditions, measures against overflow of combined sewerage system can be taken in minimal space. High SS and BOD removal efficiency can be obtained by feeding polymer coagulant. By feeding cationic polymer to sewage inflow at the ratio of about 2.0mg/L, SS removal efficiency of 70% and BOD removal efficiency of 50% can be assured at the filtration rate of 1,000m/day (effective filtration rate: 851m/day). The use of high porosity filter media assures long washing intervals and high filtrate recovery ratio. The use of filter media having porosity as high as 90% assures high SS trapping efficiency, allowing washing intervals to be 3 hours or longer. Consequently the filtrate recovery ratio increases, and the drainage volume decreases. The system can be operated as a substitute for the primary settling tank under dry weather conditions. The system can be operated as a substitute for the primary settling tank under dry weather conditions, which eliminates the need to be on standby when weather is fine and facilitates switching to the operation under wet weather conditions. In addition, since the system does not have to be on standby, there is no need for measures against corrosion such as filling the tank with secondary treated water. Since the system is operated at the filtration rate of 400m/day under dry weather conditions, primary treatment can be performed within an area smaller than that required by a conventional primary settling tank, whose filtration speed is 30 to 70m/day. In addition to new installations, a part of the primary settling tank can be modified to introduce the process. The system can be newly installed or introduced to an existing system low in height, using the main unit or the inlet channel. (4) Method of incorporation [Place of incorporation] As a substitute for the primary settling tank of a final treatment plant Since the system can be operated as a substitute for the primary settling tank, it is not required to be on standby when weather is fine, it facilitates switching to the operation under wet weather conditions. Modification of an existing plant is also allowed in addition to new installations. [Operation flow]

14 To adopt this technique to a large-scale combined sewerage system, it is desirable that a system consist of 8 tanks, and filtration area be allowed where treatment can be performed even while washing is in progress in one system, with standard filtration duration of 3 hours and washing time (21 min./basin) taken into consideration. Figure 14 shows the operation flow of the system having 8 basins. Washing drain Polymer Raw water Lift pump Sand basin Polymer mixing basin High rate filtration basin Flocculation basin blower Treated water To biological reaction tank Disinfection device Figure. 14 Operation flow [New installation] Settings of high rate filtration facilities are made, following the procedure shown below. Set of maximum treated water volume Design storm-water flow Setting of filtration rate Filtration rate = 1,000m/day Setting of number of filter basins The number of basins is set with spare washing basin taken into consideration. Setting of filtration area Filtration areatreated water volume (m 3 /day) Number of basins/ 1,000 (m/day) (Number of basins 1) Figure. 15 High rate filtration facility setting procedure (new installation) [Modification of existing facilities]

15 The system can be installed to existing facilities, following the procedure shown below. Select a basin to be modified (1)The difference of water level between the influent and the effluent should be about 1.5m. Installation of inflow facilities where raw water is fed to a high rate filter should be allowed. (2)System height of about 5.5m must be assured. (3)The basin should be able to hold drain of about 6.2% of raw water, or installation of concentration facilities near the basin should be allowed. Calculate total filtration area Total filtration area = Area of primary settling tank to be modified 0.7 Actual filtration rate = 1000m/day Set of actual filtration rate (Number of basins 1) / Number of basins Set of maximum treatment volume Total filtration area Actual filtration rate Figure. 16 Setting procedure for high rate filtration facilities (modification of existing facilities) Figure. 17 Calculation example of total filtration area

16 DEVELOPED COMPANY Hitachi Plant Engineering & Construction Co., Ltd. TEL : +81-(0) FAX : +81-(0)