NEW CLOTH MEDIA FILTER SYSTEM REPLACES 4 MGD TRAVELING BRIDGE FILTERS WITH 12 MGD CAPACITY IN EXISTING TANKAGE

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1 NEW CLOTH MEDIA FILTER SYSTEM REPLACES 4 MGD TRAVELING BRIDGE FILTERS WITH 12 MGD CAPACITY IN EXISTING TANKAGE Keith C. Dayton Department of Utilities Stafford County, Virginia P. O. Box 339 Stafford, Virginia ABSTRACT The Little Falls Run Wastewater Treatment Facility began experiencing operational difficulties with the dual-bay traveling bridge filter process as flows approached the rated capacity for the filters. Although initially the Utilities Department staff intended to duplicate the original filter process and thereby double the capacity, the recent introduction of a cloth media filter product designed to retrofit traveling bridge filters provided the opportunity to install filters with substantially greater capacity within the existing tankage and reserve valuable treatment facility space for future capacity increases. Following a successful pilot test and approval by the Virginia Department of Environmental Quality, the project was re-designed for the Aqua-Diamond filters. Replacement of the first filter cell was completed within six weeks and followed by a rigid performance and reliability test period. Results from this and subsequent testing have demonstrated the greatly improved efficiency of the cloth media filters over the previous traveling bridge sand media filters. KEYWORDS Cloth filter media, tertiary filters, traveling bridge filters, automatic backwash filters, Aqua- Diamond filters. INTRODUCTION Stafford County, Virginia is located approximately 40 miles south of Washington, D.C. and has experienced consistently high growth rates for over twenty years. As a result, the Department of Utilities has constructed and repeatedly expanded the two wastewater treatment facilities servicing the county. The Little Falls Run Wastewater Treatment Facility, located in Stafford County, Virginia was first placed in operation in The facility (Photo #1) was equipped with a headworks, lowload counter-current ( Schreiber ) aeration process, and circular clarifiers, all rated at 8 million gallons per day (mgd). Tertiary treatment consisted of two 44 by 16 traveling bridge dual media filters (Photo #2) rated at 4 mgd at a rate of 2 gallons per minute per square foot (gpm/ft 2 ). The original design 3363

2 Photo #1 - Little Falls Run Wastewater Treatment Facility Photo #2 Existing Traveling Bridge Filter (One of Two) 3364

3 anticipated the eventual addition of a duplicate filter system to bring the facility up to an 8 mgd facility rating. As flows to the filters approached 3 gpm/ft 2, operating staff were forced to bypass a portion of the flow directly to the ultraviolet disinfection system. Furthermore, backwash frequency increased markedly and treatment performance suffered. While the treatment facility was never at risk of violating the 9 mg/l TSS, 9 mg/l BOD and 2 mg/l total phosphorus limits, the certainty of higher flows and stricter permit limits in the near future meant that immediate tertiary filter improvements were necessary. INITIAL PROJECT DESCRIPTION The county proceeded with the filter improvements in accordance with the original concept for the facility construct two identical filter units in a space adjacent to the existing filters. This plan had the advantage of replicating equipment and processes very familiar to the operations staff. Design for these filter improvements were completed and approved by the Virginia Department of Environmental Quality (VDEQ) for a process rating of 6 mgd average flow and 12 mgd peak flow. This resulted in an increase in process rating of only 2 mgd for an estimated cost of $1.5 million. Additional concerns included difficulties presented by the limited construction area (Photo #3), the extended period of operational interference while constructing new concrete filter basins, and the use of the remaining area designated for tertiary treatment while obtaining only one-half of the 12 mgd projected build-out capacity needed for this facility. In light of these concerns, county staff decided to investigate other possible means to expand filter capacity at the Little Falls Run Wastewater Treatment Facility. AQUADIAMOND CLOTH MEDIA FILTERS One of the most promising alternatives was the AquaDiamond cloth media filters, a new product developed by Aqua-Aerobic Systems, Inc. for retrofit of existing traveling bridge filter equipment. These filters were designed for insertion in existing concrete tanks with a minimum of modification, thereby drastically reducing construction time and operations interference. The AquaDiamond cloth media filter system (Illustration #1) consists of deep pile cloth media (PA-13) stretched over a rigid frame in a diamond pattern to maximize the surface available for filtering. Average and peak design loadings for the cloth media are also somewhat higher than for the dual media at 3.25 gpm/ft 2 and 6.5 gpm/ft 2 respectively. Cleaning of the cloth is accomplished by means of tight fitting backwash shoes connected to a variable speed centrifugal pump mounted on a traveling bridge. During the backwash cycle, the bridge travels at around 40 feet per minute with the pumping rate of 450 gpm, cleaning one-half of the diamond assemblies on the pass down and the others on the return back. For our installation, it would require approximately 1,200 gallons per backwash cycle. 3365

4 Photo #3 Filter Expansion Area Filter Schematic Bridge Influent Filter Effluent Illustration #1 AquaDiamond Filters 3366

5 PILOT TEST At the time of this evaluation in early 2004, there were no operating installations of the AquaDiamond filter system. This made it essential to conduct a pilot study to determine if the product was suitable for this application. A test protocol was then developed to determine if the filters could achieve anticipated limits of 4.5 mg/l TSS and 0.5 mg/l TP. The test would include several days of extreme hydraulic and solids loading to assess the ability of the filter equipment to handle severe wet weather and process upset events. Bench scale testing was completed prior to shipment of the pilot filter to determine if any process modifications would be necessary. Those results indicated that the alum addition point must be moved ahead of the clarifiers to allow time to develop a larger and tougher floc. It was also noted that polymer may be required as a flocculating aid. Following arrival of the pilot filter (Photos 4 & 5), the three week test period began and was conducted by Aqua-Aerobics personnel with support from the plant operations staff. The pilot test results (Chart 1) clearly indicated the capability of the cloth filter media to meet anticipated tougher discharge limits, even under extreme operating conditions. Furthermore, the overall backwash volume of 0.56% of filter throughput indicated a very efficient use of backwash water. Finally, the pilot test determined that these results could be attained without the use of a polymer flocculating aid. Photo #4 Pilot Filter Unit 3367

6 Photo #5 Pilot Filter Unit Pilot Filter Results Applied Solids Effluent TSS Design Limit Applied Flow TSS (mg/l) Flow (g/sf) Days 0.00 Chart #1 Pilot Filter Test Results 3368

7 PROJECT IMPLEMENTATION Following review and approval of the pilot test results and revised preliminary engineering report by the VDEQ, the project was re-designed to incorporate the diamond pattern cloth media filters. Whereas the project schedule slipped six months investigating alternatives, it was decided to expedite completion by issuing a bid for process equipment only while final design details for the installation were developed. The final design (Illustration #2) required only minor modifications to the concrete tanks and would raise the unit process capacity to 12 mgd average and 24 mgd peak with both units online. Illustration #2 Single Filter Bay Design Once the process equipment arrived at the site, assembly and installation required only six basic steps as discussed below. Each step identified below took approximately one week to complete. To limit interference with facility operations, only one unit was removed from service for modification at a time. Assembly (Photo #6): erection of the polyethylene cloth media frames. Demolition (Photo #7) : removal of the fiberglass reinforced baffles and washwater trough, along with removal of the existing filter media and cleaning of the concrete walls and floors. 3369

8 Pour New Floors (Photo #8): Prepare existing floors and pour 10 of concrete to adjust filter cell height and seal the previous effluent penetrations. Install Effluent Plate and Filter Base Plates (Photo #9): The end plate is necessary to establish an effluent chamber and overflow weir. The base plates provide intermediate support for the filter frames. Install Filter Frames (Photo #10): The cloth media is placed loosely over the frames, then stretched and fastened after the frames are installed in their final position. Install Bridge, Backwash Trough, Electrical & Controls (Photo #11): Although technically the most challenging task, the extensive preparation resulted in a very smooth installation. Photo #6 Assemble Frames 3370

9 Photo #7 Demolish Existing Filters Photo #8 Pour New Concrete Floors 3371

10 Photo #9 Effluent Plate & Filter Base Plates Photo #10 Filter Frames & Cloth Media 3372

11 Photo #11 Bridge & Controls CONSTRUCTION SUMMARY The first filter was started up six weeks after removal of the existing unit from service. We then conducted an intensive eight week performance and reliability demonstration period during which all initial mechanical and control glitches were corrected. Following that period, the second filter was removed from service and the new filter equipment installed within a four week period. Equipment costs for the project were $965,000, with another $200,000 expended for the installation contract and electrical upgrades completed independently. Total costs for the project came out at under $100,000 per million gallons per day of filtering capacity. The first unit was placed in service approximately one year after the decision was made to proceed with the AquaDiamond filter product. The Utility Department was able to recover most of the lost time caused by evaluating a new technology by avoiding the lengthy construction period that would have been necessary to build new filter tanks. 3373

12 PERFORMANCE RESULTS The new filters were tested for operational performance as part of the testing protocol. This data is shown in Charts 2 & 3. This data is compared with performance data (Charts 4 & 5) collected from the existing traveling bridge filters prior to their removal from service. Effluent results for the cloth filters were well below the design target of 4.5 mg/l TSS. Significantly, the backwash flows generated by the filter operation were reduced to 0.1% of filter throughput. These flows were therefore reduced by a factor of 10, and greatly improved efficiency at our facility. Flow vs TSS Effluent Flow Effluent TSS 3.5 Design Target MGD Mg/l /2/05 9/3/05 9/4/05 9/5/05 9/6/05 9/7/05 9/8/05 9/9/05 Days 9/10/05 9/11/05 9/12/05 9/13/05 9/14/05 9/15/05 9/16/05 9/17/05 Chart #2 New Filter Performance (Flow vs. Effluent TSS) 3374

13 Backwash Volumes Vs Flow Effluent Flow Backwash % % % % /17/05 8/18/05 8/19/05 8/20/05 8/21/05 8/22/05 8/23/05 8/24/05 8/25/05 MGD 0.7% 0.6% 0.5% 0.4% 0.3% % Backwash Flow 0.2% 0.1% 0.0% Days Chart #3 New Filter Performance (Flow vs. Backwash Volumes) 3375

14 Flow vs Effluent Solids Effluent Flow Effluent TSS Design Target MGD 4 10 Mg/l Days 0 Chart #4 Original Dual Media Filter Performance (Flow vs. TSS) 3376

15 Backwash Volumes Vs Flow Effluent Flow Backwash % /28/05 3/29/05 3/30/05 3/31/05 4/1/05 4/2/05 4/3/05 4/4/05 4/5/05 4/6/05 4/7/05 4/8/05 4/9/05 4/10/05 MGD 9% 8% 7% 6% 5% 4% 3% 2% 1% 0% % Backwash Flow Days Chart #5 Original Dual Media Filters (Flow vs. Backwash Volumes) CONCLUSION The selection of the AquaDiamond cloth media filters for retrofitting the existing traveling bridge dual media filters provided an economical, highly efficient solution to upgrading the filters at our Little Falls Run WWTF. The new filters have proven very dependable even under extreme operating conditions. We will be able to meet the more stringent discharge limits soon to be in place without additional filter modifications and preserved valuable expansion space for the future. 3377