fox metro water reclamation district solves filtration problems

FOX METRO WATER RECLAMATION DISTRICT SOLVES FILTRATION PROBLEMS WITH INNOVATIVE AQUADIAMOND® CLOTH MEDIA FILTER

P. Baumann, P.E.*, C. Kieffer**, T. Morrall**, R. Bauer, P.E.***

*Aqua-Aerobic Systems, Inc. 6306 N. Alpine Road Loves Park, IL 61111

**Fox Metro Water Reclamation District, Oswego, IL

***Walter Deuchler Associates, Aurora, IL ABSTRACT The Fox Metro Water Reclamation District evaluated, installed, and successfully tested cloth media filtration in a diamond configuration in existing traveling bridge sand filter tanks. The conversion to the cloth media diamond filters increased the flow capacity of three existing sand filters basins from 79,485 m3/day (21 MGD) to 272,520 m3/day (72 MGD). The diamond filter builds on cloth media filtration technology originally proven in disk filter configurations. The cloth media filter also demonstrated the ability to handle influent solids concentrations up to 140 mg/l under certain flow conditions. The ability to achieve the high flow and influent solids concentrations in the existing tanks saved site space and construction costs. KEYWORDS Filtration, Cloth Media Filtration, Traveling Bridge Sand Filter, Diamond Filter, Storm Flow INTRODUCTION The Fox Metro Water Reclamation District serves Aurora, Illinois and the surrounding area. When the plant started up in the 1920s, its trickling filter system was designed to treat an average daily flow of 30,280 m3/d (8 MGD). With the continuing growth of the Chicago suburbs the district serves, the population has grown to nearly 250,000 with no end in sight. The current plant processes include a bar screen, grit removal, primary clarifiers, aeration tanks, secondary clarifiers, traveling bridge sand filters, chlorine disinfection and dechlorination. The plant is designed for an average daily flow of 158,970 m3/d (42 MGD) and a peak flow of 321,725 m3/d (85 MGD), and discharges treated effluent to the Fox River.

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Copyright 2006 Water Environment Foundation. All Rights Reserved©

Figure 1 - Aerial View of Fox Metro Water Reclamation District

The plant’s filtration system consisted of nine 4.88 m (16 ft) wide by 33.55 m (110 ft) long traveling bridge sand filters. Eight of the filters were installed in 1978 and the ninth added in 1998. Based on a design flux of 2.04 m2 (3 gpm/sf), each filter was rated at an average flow of 28,766 m3/day (7.6 MGD). At peak flow conditions, using the state standard filter flux of 12.24 m3/hr/m2 (5 gpm/sf) each sand filter was rated for 47,956 m3/day (12.67 MGD). Theoretically, this per basin design flow capacity should have provided a peak plant capacity of 352,005 m3/d (93 MGD). Unfortunately, the nine sand filters were only able to pass around 238,455 m3/d or 26,495 m3/d/filter (63 MGD or 7 MGD/filter). The district tried to improve performance by replacing the sand in the beds, indexing the backwash and chlorinating upstream of the filters to retard bio-growth in the beds. While these actions improved performance, the peak design flow could not be achieved, and with age, continued to deteriorate. Solids loading also presented a problem. The sand filters had a design solids loading capacity of 3.5 kg/day m2 (0.72 #/sf/d), which adequately handled normal solid feed concentrations of 5 mg/l to 10 mg/l. During solids upsets in the secondary clarifiers, influent concentrations to the filter could be significantly higher, blinding the filter, which resulted in overflows. The filter blinding was exacerbated by the 70 minutes it took to complete a backwash cycle. It would take hours to clear a blinded filter after the solids concentration returned to normal. The inability to handle peak flows and solids conditions prompted the district to seek out a solution that would: • Provide a peak filter capacity of 352,005 m3/d (93 MGD). • Handle clarifier upsets effectively and efficiently. • Address projected future peak flow of 681,300 m3/d (180 MGD) in stages. • Eliminate bio-fouling in the filter media. • Allow the use of the existing sand filters during the installation of the new equipment.

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METHODOLOGY In 2000, the district retained the services of Walter E. Deuchler Associates, Inc to review expansion options. The first option considered was to simply add more sand filters and rehabilitate the existing units. The district discounted this option because the footprint required to handle peak flows or high solids was too significant. The district and their engineer then considered cloth media filters in a disk configuration. Six 12-disk filters would fit into each of the nine existing basins. Each 12-disk unit would handle 22,710 m3/d (6 MGD) peak flow at higher solids loadings. Retrofitting three basins with a total of eighteen 12-disk filters would provide a peak capacity of 408,780 m3/d (108 MGD). Retrofitting all nine basins would provide 1,226,340 m3/d (324 MGD) capacity for future expansion. The cloth media filters in the disk configuration provided a design solids loading capability that was nearly three times that of the current sand filters allowing effective operation through a broader range of influent solids concentrations. The cloth media disk filter completes a backwash cycle in just over three minutes, significantly reducing the time required to recover from a solids upset. Figure 2 - Cloth Media Disk Filter Operation

The cloth media filter utilizes a unique pile cloth specifically engineered for particulate removal. The cloth is mounted to a hollow frame. The pile fibers are laid flat forming a filtration mat approximately 3 mm to 5 mm deep (Figure 4). Solids are captured on the surface and within the depth of the mat (Figure 5). As solids accumulate, the head loss through the mat increases, initiating the backwash cycle.

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Figure 3 - Natural State of Pile Media

Figure 4 - Active Filter Depth of Pile Media

During the backwash cycle, the cloth rotates past a backwash shoe that draws clean, filtered water back through the cloth. This reversed flow fluidizes the pile fibers from the flat mat orientation and stands the fibers upright momentarily (Figure 6). The disruption of the mat and movement of the fibers in the presence of the reversed flow of water cleans the entire mat depth, removing the accumulated solids. The trailing edge of the backwash shoe lays the fibers back down, reforming the filtration mat. Since only a fraction of the filtration area is in backwash at a given moment, the remaining area is available and filtration continues through the backwash cycle.

3-5 mm

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Figure 5 - Filtering/Solids Accumulation on Pile Cloth Media

Figure 6 - Backwash View of Pile Cloth Media

The height of the disk filter complicated the retrofit of the existing shallow sand filter basins. The existing basins are 1.9 m (6.33 ft) deep and the disk filters require a basin depth of around 3.5 m (11.5 ft) (Figure 7). This required raising the basin walls above the existing 80.67 elevation. The raised tanks would impact the influent hydraulics. While the head loss through the disk filter is similar to the sand filter, raising the walls increases the influent elevation. The plant had the available head but the district’s intent was to gradually phase in the new filters while continuing to utilize the existing filters during peak events. The higher inlet elevation complicated the split between the disk and sand filters and would have required the construction of an upstream splitter box. As a result of these tank depth issues, the district abandoned the cloth media disk filter concept.

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Figure 7 - Disk Filter Retrofit

Diamond Cloth Media Configuration The Fox Metro Water Reclamation district was not alone in their need to upgrade existing sand filters. Plants across the country were experiencing similar growth requirements and limited space to expand. In response to this need, Aqua-Aerobic Systems developed a cloth media filter configuration that would retrofit into existing traveling bridge sand filters and provide additional capacity from the existing sand filter civil structures. The diamond configuration was developed and tested in 2003 and presented to the district for consideration in early 2004. The AquaDiamond® filter utilizes the same engineered pile media successfully used on the AquaDisk® filter. The pile cloth is mounted to a plastic frame with a diamond shaped cross section. The frame serves to support the cloth and the central opening that is formed by the frame conducts the filtered water to the effluent chamber. Figure 8 - Inside View of AquaDiamond Filter Frame

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Platform

1 of 8 Diamonds

Effluent Wall

End of Diamond

Backwash Channel

The design criteria used to develop the diamond configuration of cloth media are identical to the criteria used for the well established disk configuration which has over 500 units in operation. Filtration rates are directly tied to the cloth surface area. Since the cloth depth and construction are identical, filter performance and backwash usage were projected to be identical. Even though confidence was high that the diamond would match the performance of the disk, the district included testing to confirm the performance in the contract. Figure 9 - Retrofitted AquaDiamond Filter Basin

The filter basin has eight diamond laterals that run the length of the basin. The influent end of each diamond is closed. The effluent end of the diamond lateral attaches to the effluent wall which has portals (Figure 10) to allow the filtered water to leave the diamond and the filter basin. Each basin provides 71 m2 (768 sq ft) more filtration area compared to each of the existing sand filters. Figure 10 - AquaDiamond Filter Effluent Wall

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Platform

Backwash Shoes

Diamonds

In addition to providing additional filtration area in the same foot print, the higher flux rate provided by the cloth media further increased the flow though the existing structures. The pile cloth media has a design flux rate that is 2/3 more than the existing sand filters. The combined effect of the additional surface and higher flux allowed the district to filter on average 45,420 m3/day (12 MGD) and peak at 90,840 m3/day (24 MGD) reliably though a single basin that on good days could treat 26,495 m3/day (7.0 MGD) with the existing traveling bridge sand filters. With the potential of tripling the flow to each basin, the hydraulics in the existing feed and effluent channels and associated piping to the filter building must be considered. Under the current plant conditions, the existing inlet and outlet structures are adequate. Once the flow capability of the building exceeds 378,500 m3/day (100 MGD) an additional feed and effluent line will be required. The current inlet and outlet are located at opposite corners of the building. The new pipes will enter/exit midway along the existing channels to properly distribute the flow to all nine basins. Figure 11 - AquaDiamond Filter Backwash Shoe

The cloth is cleaned by backwash shoes (Figure 11) mounted to a platform that traverses the length of the tank during backwash. A platform mounted pump provides the backwash flow necessary to clean the pile cloth. Valves activate half the backwash shoes in the outbound direction cleaning every other diamond. The remaining diamonds are cleaned during the return of the platform. The platform travels at 9.15 m (30 ft) per minute, completing the entire backwash process takes less than six minutes. The existing effluent channel that ran the length of the filter was converted into the waste channel. This channel carries backwash water and water from the floating solids trough to an existing waste pit. The waste channel has a 1.75% slope to prevent pooling between backwashes, reducing the breeding grounds for insects. At the design flow rate of 45,420 m3/day (12 MGD), solids settling in the basin are of little concern. At design flow, the basin retention time is 5.3 minutes so little if any settling occurs. Due to diurnal flows, however, flows late in the evening can result in solids settling in the filter

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Floor Suction Headers

Platform

Diamonds

basin. To address these settled solids, suction headers run between the diamonds to collect the material (Figure 12). The suction headers use the same platform mounted pump as the backwash function. The floor cleaning frequency is adjustable and typically occurs once a day.

Figure 12 - AquaDiamond Filter Floor Suction Headers

Floating solids accumulate over time in filtration basins. While the existing sand filters were equipped with a floating skimming device, they were ineffective. The district manually removed floating material from the existing filters using swimming pool skimmer nets and requested a more effective alternative. The AquaDiamond filter design takes advantage of the higher velocity influent flow characteristics of the basin, which tend to drive floating material to the effluent end of the basin without mechanical assistance. A trough, integral with the effluent wall, collects scum as the basin level rises (Figure 13). The trough is connected to a motorized valve that opens during a floating solids removal cycle. With the valve open, floating solids flow over the trough edge, into the trough, and out to the waste channel. The valve opens automatically just before the tank level reaches the top of the trough weir and remains open for an operator-adjustable time period. The frequency of the floating solids sequence is operator- adjustable and typically activates every eight to ten backwashes.

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Effluent Wall

Floating Solids Collection Trough

Guide Wheels

Guide Angle

Figure 13 - AquaDiamond Filter Floating Solids Removal Arrangement

Flow enters the filter through two 762 mm (30 in) diameter pipes. Each influent pipe has a flow meter and slide gate to control flow into each filter. The desired flow is set at the control panel. The gates modulate as required to achieve the set point flow. While most installations could use a simple influent weir to distribute flow between filters, the gates were necessary in this case to force a portion of the flow to the remaining sand filters if desired. The district had experienced "crabbing" problems with the existing sand filter bridge as the bridge traveled the length of the tank. The district thus wanted to prevent this problem in the new design. The platform associated with the AquaDiamond filter is guided by an angle mounted to the top of the tank wall. A pair of wheels mounted to the front and rear of one side of the platform engages the vertical leg of the angle (Figure 14). Forces that would drive the platform out of alignment are transferred to the angle through these wheels, insuring the platform remains perpendicular to the diamonds through its travel. Figure 14 - AquaDiamond Filter Guide Wheels

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Installation Installation was relatively easy and was conducted through a 2.9 m (9.5 foot) wide garage door, which all equipment and debris moved through. Once the old bridge and wiring was removed, the sand was removed from the basin using a vacuum truck (Figure 15). A small end loader was lowered into the basin to remove the filter plates, cell dividers and floor grout. The existing effluent wall was removed and replaced with a stainless steel wall that incorporated portals for the diamonds and the scum collection trough. Figure 15 - Retrofit in Progress - Removing Sand Filter Underdrain

Circular openings were cut into the influent end wall to accommodate the two 762 mm (30 in) diameter influent pipes. An additional wall was added at the influent end to provide access to the control gates. The abandoned existing influent channel that ran the length of the basin was filled to avoid the collection of water and to provide a smooth access surface to the platform. The influent and effluent ports to the former sand filter were filled flush with grout. The tank floor received a 50 mm (2 in) layer of grout to smooth and level the surface. With the civil work completed, the basin was ready for equipment installation. The guide rail was installed on top of the existing wall. The diamond sections were partially assembled outside the basin and joined together in the tank. The cloth was mounted to the frames and the assembled diamonds attached to the effluent wall and the floor (Figure 16).

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Figure 16 - Cloth Media Installation

The platform was rolled into place. The backwash shoes and suction headers for floor cleaning were mounted to the platform and aligned between the diamonds. A track supported festoon system brought power and control to the bridge. RESULTS In January 2005, the first converted basin was brought on-line. For the initial four weeks of operation, the filter ran successfully at the design flow of 45,420 m3/day (12 MGD) with the remaining flow directed to the existing sand filters. The filter consistently produced an effluent less than 2 NTU (2-3 mg/l TSS) through this period. Following the initial startup period, the filter ran at a peak flow of 90,840 m3/day (24 MGD) for seven consecutive hours to evaluate performance. During the test period, the flow averaged 86,767 m3/day (22.9 MGD) with the highest recorded flow of 109,765 m3/day (29 MGD). Influent and effluent TSS composites were taken every 15 minutes during the peak flow test period, as well as grab samples. A turbidity meter provided constant effluent readings throughout the period as well.

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Figure 17 - TSS and NTU Results at Peak Flow Fox Metro 24 MGD Flow

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Figure 17 graphs the results of the peak flow test, as well as the performance prior to, and after, the test. While few TSS samples were taken outside the peak flow test period, turbidity logging continued. The graph shows that the increase in flow had little impact on effluent performance. The effluent turbidity plot in Figure 17 displayed periodic peaks that coincide with filter backwashes, which is consistent with all types of filters. These peaks typically last less than 5 minutes and never exceed 2 NTU. Since they represent such a small percentage of the total time during the day, the impact on the average effluent performance is negligible. The district was also interested in backwash usage. Backwash as a percentage of forward flow is directly related to the incoming solids concentration. While flow volumes for both forward flow and backwash were available on a continuous basis, TSS information was not. A test was conducted to determine if the actual backwash water used as a percentage of forward flow met the contract requirements based on the TSS value experienced during the test. The testing was conducted under both design and peak flow conditions. The design flow portion ran for a 24-hour period. Influent TSS values were taken every 5 minutes for a one-hour composite. The average of the 24-hour influent samples was 4.33 mg/l. At this concentration, the specification allowed 0.6% as a percentage of forward flow. The actual backwash used was 0.3%.

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The peak flow testing lasted three hours. Influent samples were taken every 15 minutes and averaged 5.6 mg/l TSS for the period. This related to an allowable backwash use of 0.8%. The actual consumption was 0.7%. The AquaDiamond filter met the specified backwash usage under both design and peak conditions. With these tests completed, the performance of the diamond configuration cloth media filter was confirmed to match the performance of the well-established disk configuration. While it was evident that the diamond filter used significantly less water than the existing sand filters, there was no easy means to quantify the sand filter backwash. The sand filters were not equipped with influent metering and so the results from a continuous measurement of flow across the 6.1 m (20 ft) long weir would be suspect. One can draw a relative conclusion for the difference between the two filters by looking at the solids concentration of the backwash water. The higher the solids concentration in the backwash, the greater amount of solids removed with each volume of backwash water use and therefore, the greater the efficiency. The average concentration of the sand filter backwash is 178 mg/l. The cloth media diamond filter averages 650 mg/l. Since the cloth media diamond filter carries over three times the solids, one could assume a reciprocal reduction in backwash usage. The ability to handle elevated solids concentrations to the filter was of significant interest to the district. The sand filters typically had trouble dealing with solids over 20 mg/l. The influent concentration to the sand filters could exceed 20 mg/l for short periods if some of the clarifiers were out of service during a storm event, or the mixed liquor settling characteristics were poor. A second critical consideration was the future application of the filters. A significant portion of the area serviced by the district has combined sewers. The District continues to evaluate methods of treating flows under storm conditions. Physical-chemical processes, or a combination of physical-chemical/biological processes, are under review to determine the most cost-effective alternatives. The capability to handle higher solids loading to these filters will inevitably be part of this evaluation. Testing was conducted to evaluate the cloth media diamond filter’s ability to handle elevated solids concentration. Return activated sludge was metered and mixed into the influent flow fed to the filter to achieve the desired feed concentration. The influent solids concentration was increased in 10 mg/l increments to determine the solids limit of the filter. Each solids level was maintained for approximately 30 minutes to insure that a steady state had been reached. Activated sludge was transported to the filter building by a septic hauler truck. The activated sludge was transferred to a nurse tank equipped with a mixer to prevent solids settling in the tank. The sludge was pumped from the nurse tank and metered into the influent channel using a distribution manifold. Activated sludge samples were taken from the feed line approximately every 5 minutes for lab analysis. Since the concentration of the activated sludge varied between truck loads, the influent

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solids concentration varied from the desired set points as seen in Figure 18. At an influent flow rate of 34,065 m3/day (9 MGD), the filter operated successfully at a high average influent solids concentration of around 140 mg/l. Figure 18 - Influent Solids Concentration Testing

DISCUSSION As result of the successful performance testing, the district installed two additional basins that went on line in October 2005 bringing the total cloth media AquaDiamond filter capacity to a peak of 272,520 m3/day (72 MGD). The three filters handle all of the plant flow up to the 272,520 m3/day (72 MGD) at which time the sand filters come on-line. The district plans to add additional AquaDiamond filters as budgets allow. The success at the Fox Metro Water Reclamation District facility has encouraged several other treatment facilities to retrofit existing sand filter basins to increase throughput and reduce filter related maintenance. CONCLUSIONS • The diamond configuration of the cloth media filter matches the performance of the well-

established disk configuration. • The low profile of the diamond configuration easily retrofits into existing traveling bridge

sand filter structures without significant tank modifications. The existing building housing the filters will not require special construction access, provided a 2.9 m (9.5 ft) wide opening is available.

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• The diamond filter will significantly increase the flow through the existing sand filter structures eliminating construction of additional tanks and buildings.

• Cloth media demonstrated lower backwash usage compared to the sand filters at similar solids loading rates.

• Cloth media filtration can accommodate solids loading rates that are significantly higher than sand filters. The higher solids capability results in compliant effluent over a wider range of influent solids conditions.

ACKNOWLEDGEMENTS The authors would like to acknowledge the Fox Metro Water Reclamation District for their participation, cooperation and access to the plant site, which enabled the verification of this innovative technology. REFERENCES Bourgeous, K.N., J. Riess, G. Tchobanoglous, J. Darby, and L. Johnson (2001) Performance Evaluation of a Cloth Media Disk Filter For Wastewater Reclamation, Proceedings 74th Annual Conference & Exposition of WEF, Atlanta, Ga.

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fox metro water reclamation district solves filtration problems

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