9-2. Wet-weather high-speed wastewater filtration system (primary treatment of untreated wastewater)

Size: px

Start display at page:

Download "9-2. Wet-weather high-speed wastewater filtration system (primary treatment of untreated wastewater)"

Wilfrid Hampton
6 years ago
Views:

1 9-2.Wet-weather high-speed wastewater filtration system (primary treatment of untreated wastewater) 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 This technology is based on utilization of a smaller version of the windmill-shaped filter media (measuring 9 x 9 x 5 mm) developed especially for the wet-weather high-speed wastewater filtration system (refinement of primary treatment) assessed by the SPIRIT 21 Committee in October It can be applied also for untreated wastewater in wastewater treatment plants as necessary, and also for primary treatment of untreated wastewater in pumping stations. 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 Base w ater Filtrate Upper screen Wash water drain tank media Existing public works structure Figure.1 Treatment flow 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. As shown in Figure 1, for washing, all of the filter tanks used the same filtrate stored above them, and this enables installation even on shallow tanks like those used for primary treatment. The technology can also remove debris and does not require equipment for advance removal of the same. Because it efficiently removes suspended solids (SS) as well, it enables a reduction in use of disinfectants as compared to the conventional treatment by primary settling tanks. Photo 1 media

2 media The filter media (see Photo 1) developed especially for the system are characterized by a highly uneven surface, windmill shape, and small size (they measure 7.5 x 7.5 x 4 mm). Due to their high void ratio, they are Filtration Washing effective for trapping SS, and Upper screen therefore enable a comparatively long duration of continuous Floating filter media Floating filter media Removal - SS (with a diameter filtration. The SS trapped on the in the range of µm or more) m ) uneven surface are easily expelled Debris in backwashing, when the -Hair - Vegetable waste - Leaves downflow widens the interstices - Oil balls, etc. Debris and SS between the media. Filtration does not require the addition of Figure.2 Filtration and washing coagulants. The small size of the filter media makes them effective for removal of biochemical oxygen demand (BOD) as well (except for dissolved BOD). Principle As shown in Figure 2, the principle in filtration is one of ordinary upflow high-speed filtration (No coagulants required). Inorganic substances and other matter settle to the bottom of the tank, and the system traps debris in the vicinity of the lower part of the filter bed and SS with the whole of the filter bed. As filtration proceeds, the filter media become clogged, and this increases the filtration resistance. When this resistance reaches 4 kpa (40 cm), the system automatically senses it and washes the filter. The rapid downflow of filtrate in washing expels the SS and debris from Base water Elutriate Upflow I. Start of filtration Base water Pressure-adjustment conduit media Filtrate Stormwater treatment effluent III. Backwash in wet weather Backwash using filtered stormwater Downflow Rise in the water level of the pressure-adjustment conduit Elutriate II. Filtration continuation Screen SS Debris IV. Washing after the rainfall Secondary treatment effluent Backwashing and replacement with secondary treatment effluent Figure.3 Equipment operation and water flow

3 the bottom of the tank. The notable point is that the filter media are not carried away with the elutriate. Equipment operation and water flow Figure 3 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. 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. Site of application The proposed sites of application are wastewater treatment plants and pumping stations. In wastewater treatment plants, the main use would be for refinement of primary treatment. The system could also be used to perform primary treatment of untreated wastewater as necessary. It could be installed by modification of idle primary settling tanks or other such facilities. In pumping stations, the system could be used for primary treatment of untreated wastewater. The filtrate could be released into public water areas, and the elutriate would be sent by pump to a wastewater treatment plant. In pumping stations, the facility would have to be newly installed. Because of the lack of secondary treatment effluent, application in nqsh Q Qsd q Refinement of primary treatment Stormwater treatment Primary settling tank nqshqsd q Reaction tank Final settling tank Wastewater treatment plant Qsd q Figure.4 Application for wastewater treatment plants Primary treatment of untreated wastewater Conventional direct discharge nqsh Figure. 5 Stormwater treatment Stormwater pump Wet-weather sanitary sewage pump Sanitary sewage pump q Pumping station To public water areas To wastewater treatment plants Application for pumping stations

4 pumping stations would require a supply of industrial-use water for complete washing in dry weather after the rainfall. Treatment features The system is capable of comparatively stable removal of BOD and SS at filtration rates of up to 1,200 m/day, but removal ratios decline substantially at higher speeds. It is therefore considered effective for refinement of primary treatment at filtration rates of no more than 1,200 m/day. For debris, the system delivers a removal ratio of 100 % at filtration rates of up to 1,500 m/day. At these speeds, the removal ratios for BOD and SS are on the same level as in the conventional treatment by primary settlement. As such, it can be applied for primary treatment of untreated wastewater at speeds of up to 1,500 m/day. 2 Development and research 2.1 Requisite performance and development targets Tables 1 and 2 show the development targets (requisite performance) noted in the rules for preparation of papers and those presented by the proposer of the technology, respectively. Table.1 Development targets (requisite performance) noted in the rules for preparation of papers Range of influent into primary settling tanks in wastewater treatment plants and/or application wastewater discharged from pumping stations in combined sewer systems Development technology with a performance surpassing the pollutant removal targets (requisite performance (removal ratios of 30% for BOD and 30% for SS) of the performance) conventional technology (stormwater settling tanks) Range of application Development targets Table.2 Development targets presented by the technology proposer influent into primary settling tanks in wastewater treatment plants and/or wastewater discharged from pumping stations in combined sewer systems [Removal ratios] removal ratios on the following levels at a filtration rate of 1,500 m/day - SS & BOD: at least 30% - Debris: 100% [Average filtrate recovery ratio] - Average ratio of recovery of filtrate as percentage of the real wet-weather wastewater volume: at least 80% 2.2 Development and research method Test site, period, base water, and proving test equipment As base water, the test used influent at the Sakae No. 2 Wastewater Treatment Plant operated by the Yokohama Sewage Works Bureau. It was conducted for all of 15 rainfalls from June 2002 to March 2003, at filtration rates that ranged from 200 to 1,600 m/day (but were

constant for each rainfall). Table. 3 Specifications of proving test equipment Name Specifications Number High-speed filters 0.5 m 2 x effective depth of 4 m 1 media Thickness of 0.8 m 0.

5 constant for each rainfall). Table. 3 Specifications of proving test equipment Name Specifications Number High-speed filters 0.5 m 2 x effective depth of 4 m 1 media Thickness of 0.8 m 0.4 m 3 total Lift pump 0.5 m 3 /min, 1m 3 /min 1 per system Existing secondary treatment effluent When necessary Photo 2 Proving test equipment 2.3 Test results Definition of removal ratios as indicators of pollutant removal performance As indicators of pollutant removal performance, removal ratios for SS and BOD were calculated by the following formula based on the total load per rainfall (up to five hours). Removal ratio(%) = (Total influent load - total effluent load) 100 Total influent load Pump Test-use grit chamber Figure m2 media bed thickness 0.8m Base water Elutriate valve Treated effluent tank media Elutriate Treated effluent Pumping for elutriate transportation Elutriate tank Existing distribution tank Proving test equipment flow SS removal performance SS effluent load (kg/m 2 /hr) 40% removal ratio 50% removal ratio 30% removal ratio 70% removal ratio SS influent load (kg/m 2 /hr) Figure. 7 Graph of correlation between influent load and effluent load (SS)

6 Figure 7 shows the correlation between data for influent and effluent loads for the run in each rainfall. It can be seen that the SS removal ratios Primary treatment of untreated wastewater were at least 40 % in all runs including Refinement of primary treatment runs 9 and 15, when the filtration rate was 1,500 m/day, the subject of 100 Base water concentration 180 mg/l assessment as to attainment of the development targets. Removal ratios are influenced by filtration rate and base water concentration. A linear approximation mg/l 50 mg/l formula was obtained by using a filtration rate of 1,200 m/day as the point Filtration rate (m/day) 2000 of inflection and varying the influence of (Filtration rate A: 0A1200m/day) the speed (i.e., using it as a coefficient). Removal ratio (%)= A0.140B58.4 With this formula, the SS removal ratio (Filtration rate A: 1200A1600m/day) is 77.9 % at a filtration rate of 1,000 m/day and a base water SS concentration of 180mg/L. Removal ratio (%)= A0.140B104.1 A: filtration rate [m/day] B: base water concentration [mg/l] BOD effluent load (kg/m 2 /hr) BOD removal performance SS removal ratio (actual measured value) Figure. 8 SS removal performance 40% removal ratio 50% removal ratio 30% removal ratio 70% removal ratio BOD influent load (kg/m 2 /hr) Figure. 9 Graph of correlation between influent load and effluent load (BOD)

7 Figure 9 shows the correlation between data for influent and effluent loads for the run in each rainfall. It can be seen that, when the base water concentration was at least 50 Primary treatment of untreated wastewater Refinement of primary treatment mg/l (i.e., excluding runs 6, 7, and 9, 100 Base water concentration 180 mg/l when the concentrations were 34.9, 31.4, and 23.5 mg/l, respectively), the mg/l BOD removal ratios were at least 30 % in all runs. Removal ratios are influenced by filtration rate and base water mg/l 0 concentration. A linear approximation Filtration rate (m/day) formula was obtained by using a 2000 filtration rate of 1,200 m/day as the point of inflection and varying the (Filtration rate A: 0A1200m/day) BOD removal ratio (%)*= A0.182B56.9 (Filtration rate A: 1200A1600m/day) influence of the speed (i.e., using it as BOD removal ratio (%)*= A0.182B138.9 a coefficient). With this formula, the A: filtration rate [m/day] BOD removal ratio was 77.6 % at a B: base water concentration [mg/l] filtration rate of 1,000 m/day and base water BOD concentration of 180 mg/l. Figure. 10 BOD removal performance BOD removal ratio (actual measured value) Removal of debris A test of debris removal was conducted in seven rainfalls at filtration rates ranging from 600 to 1,500 m/day. The system removed all solid debris with a size of at least 1 mm, for a removal ratio of 100 %. Filtrate recovery ratio The filtrate recovery ratio is defined as the ratio of recovery of filtrate as percentage of the base water. (The proportion of elutriate declines as this ratio rises.) The formula noted below was used to estimate the filtrate recovery ratio Filtrate recovery ratio (%)100elutriate ratio (%) based on the test results. 100 B C D ( ) The filtrate recovery ratio is a A: filtration rate (m/day) function of the washing speed and B: base water concentration (mg/l) washing duration (which are set as operation conditions) as well as the C: washing speed(m/ minute) D: washing duration (minute) filtration rate and base water Figure. 11 Calculation of the filtrate recovery ratio

8 concentration, which vary with the water flow and quality. The filtrate recovery ratio was 91.6 v/v % at the average filtration rate* 1 of 245 m/day and base water SS concentration of 180 mg/l* 2. *1: The average filtration rate postulated in operation of the facility; see page 40 of the "Details" section of the technical assessment sheet for the wet-weather high-speed wastewater filtration system (primary treatment of untreated wastewater). *2: The average value in the results of monitoring of Cabinet-designated cities noted in 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 4 shows the development targets noted in the rules for preparation of papers and assessment results. Table. 4 Development targets noted in the rules for preparation of papers and assessment results influent into primary settling tanks in wastewater treatment plants Range of application and/or wastewater discharged from pumping stations in combined sewer systems Development targets technology with a performance surpassing the pollutant removal (requisite performance) performance (removal ratios of 30% for BOD and 30% for SS) of the conventional technology (stormwater settling tanks) BOD and SS removal ratios of at least 30 % each; confirmation Assessment results of ability to deliver the requisite performance Table 5 shows the development targets presented by the technology proposer and the assessment results.

9 Table. 5 Development targets presented by the technology proposer and assessment. Range of influent into primary settling tanks in wastewater treatment plants and/or application wastewater discharged from pumping stations in combined sewer systems Removal ratios removal ratios on the following levels at a filtration rate of 1,500 m/day Developme - SS & BOD: at least 30% nt targets - Debris: 100% Average filtrate recovery ratio - Average ratio of recovery of filtrate as percentage of the real wet-weather wastewater volume: at least 80% Assessment results [Removal ratios] The following items were confirmed for removal ratios at a filtration rate of 1,500 m/day (effective filtration rate of 1,242 m/day* 3 ). - The target for the SS removal ratio was attained. - The target for the BOD removal ratio was attained when the average base water BOD concentration was at least 50 mg/l. - The target for the debris removal ratio was attained. [Average filtrate recovery ratio] Based on the test results, it was confirmed that the system basically attained* 4 the development target for the average filtrate recovery ratio (with an estimated value of 91.6 %) at a filtration rate of 246 m/day (effective filtration rate of 225 m/day), which is thought to be the average one in actual operation, and base water SS concentration of 180 mg/l. *3: Effective filtration rate is the quotient of division of the amount of effluent minus elutriate by the total treatment duration inclusive of the filtration area and washing time. *4: The term "basically attained" means that the results for removal ratios and other items obtained by methods other than load data (i.e., calculation utilizing spot data, data converted from turbidity, and correlation formulas) surpassed the targeted values. 3 Features of the technology 3.1 Effective use of existing primary settling tanks, etc. The system does not require pretreatment facilities, and can be installed on tanks (primary settling tanks and stormwater settling tanks) with an effective depth of at least 2.5 m because of the thinness of the filter media bed (0.8 m) and curtailment of the filtration loss head to 4 kpa. In this case, additional installation space is not required. As shown in Figure 12, existing tanks can be modified into filter (Before modification of the primary settling tank) (After modification into the high-speed filter) Distribution tank Special upper screen Special filter media Elutriate pipe (Partition panels) High-speed washing equipment Washing wastewater pump Figure. 12 Installation on a primary settling t k

10 tanks merely by partitioning. There are only three filter components: the special upper screen, special filter media, and high-speed washing equipment. The system can also be newly installed from the civil engineering facilities. The construction is low-cost because it requires only a shallow tank (with a depth of no more than 5 m). 3.2 No need for pretreatment (for advance removal of debris) Because the system accepts and can treat base water that has merely passed through the grit chamber screen (with a gauge in the range of 25-50mm)* 5 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. *5: According to the 2001 edition of the sewerage facility planning/design policy and commentary 3.3 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 primary settling treatment (see Figure 13). Table. 6 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 Yen/m3 Basically equal Tens of thousands of yen/year Treatment units Yearly maintenance cost Basically equal Proposed system Conventional primary - At an electricity tariff of 15 yen/kwh settling tank treatment Figure. 13 Maintenance cost (electricity charge) 3.4 Less work in wet weather, from the beginning to the end of the rainfall The biggest feature of the technology is the ease of work 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 as shown in Table 7. After rainfalls, routine checks are made to confirm that the filtration proceeded well (by reference to data on filtration pressure loss, etc.). In addition, the system is completely washed using secondary treatment effluent or the

11 equivalent. Start of rainfall Manual operation work Table. 7 Work in wet weather Reference: equipment operation None - Base water pump startup = start of filtration During rainfall Peak rainfall End of the rainfall After the rainfall (dry weather) None None None Complete washing (as appropriate) - 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 - Direct discharge from the overflow weir of the distribution tank for flow in excess of the prescribed value - Base water pump shutdown = filter shutdown - Forced washing of the filter media in all filters using secondary treatment effluent 3.5 Reduction of use of disinfectants (item outside the scope of assessment) Disinfection of primary treatment effluent is about 40 %* 6 less expensive than in the case of the conventional treatment by primary settlement, and 60 %* 7 less expensive than in that of disinfection of untreated wastewater. This lower expense derives from the decrease in the requisite amount of chlorine because the filtrate has less SS than the supernatant from primary settlement. * 6: Subsection 8 (Item outside the scope of assessment) of the "Details" section of the technical assessment sheet for the wet-weather high-speed wastewater filtration system (refinement of primary treatment) *7: Subsection 8 (Item outside the scope of assessment) of the "Details" section of the technical assessment sheet for the wet-weather high-speed wastewater filtration system (primary treatment of untreated wastewater) 4 Method of application of the assessed technology 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. Example of plan for installation through partial modification of a primary settling tank Due to the spread of advanced treatment and other developments in recent years, a rate of 50 m/day* 8 has come to be considered advisable as the standard surface loading of primary

12 settling tanks. In contrast, existing primary settling tanks were generally designed for surface loading of about 35 m/day. As such, high-speed filter tanks can be planned for the Primary settling tank: Primary settling tank: 35 m/day 50 m/day purpose noted in the first application Primary settling tank: Primary settling tank: 35 m/day 50 m/day above by revising the design values Primary settling tank: Primary settling tank: 35 m/day 50 m/day as shown in Figure 14. Primary settling tank: High-speed filter 35 *8: According to the draft version of m/day Conventional the standard activated sludge (before installation) After installation process design guidelines Figure. 14 Change in surface loading published in 1995 by the Japan before and after installation Sewage Works Agency. Example of layout in filter installation through partial modification of a primary settling tank Figure 15 shows the placement of the filters and elutriate 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. In the case of modification of the primary settling tank into 3 conduits and 1 filter Elutriate tank Figure. 15 Layout of filters in a primary settling tank DEVELOPED COMPANY NGK Insulators, Ltd. TEL : +81-(0) FAX : +81-(0) URL :

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

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