Hazen and Sawyer 545 Mainstream Drive, Suite 320 Nashville, TN

Transcription

1 Hazen and Sawyer 545 Mainstream Drive, Suite 320 Nashville, TN Central WWTP Capacity Improvements and CSO Reduction Technical Memorandum No. 2: New Headworks Facility Hazen No February 12, 2018

2 Table of Contents 1. Introduction... 1 Background Design Criteria Flows Process / Operations Rapid, Heavy Storm Flows First Flush Conditions and Heavy Grit Loads Seasonal Leaf Loads Large Debris Operational Flexibility Automatic Flow Control Passive Hydraulic Bypass Discharge to Flow Equalization Ability to Transfer Flow Between North and South Areas of Plant Existing Conditions & Constraints Existing South Headworks Facilities East and West Screens South Aerated Grit Chambers Abandoned Climber Screens Existing North Grit Facility Screening Grit Removal New Headworks Facility Site Selection Facility Hydraulic Constraints Headworks Unit Processes and Equipment Screening Equipment Coarse Screens Coarse Screen Design...16 Table of Contents i

3 4.1.4 Fine Screens Fine Screen Design Fine Screenings Washing/Compacting Screening Bypass Screenings Conveyance and Handling Grit Removal Equipment Grit Separation Grit Washing and Classifying Odor Control Foul Air Under Covers Coarse and Fine Screen Buildings Mechanical / HVAC System Selection Basis System Description Coarse and Fine Screen Buildings System Description Electrical Building: Controls: Materials of Construction: Ductwork Roof Top Units Fans Plumbing System Selection Basis Domestic Water Services Sanitary Waste and Vent Systems Storm Water System Natural Gas Systems Re-Use Water Wash down water (non-potable) Plumbing Fixtures Fire Protection Systems System Selection Basis: Fire Water Services...33 Table of Contents ii

4 Table of Contents iii

5 List of Tables Table 4-1: Coarse Screen Design Criteria Table 4-2: Fine Screen Design Criteria Table 4-3: Washer/Compactor Design Criteria Table 4-4: Quantity of 12-ft Diameter Stacked Tray Units to Treat 200 MGD* Table 4-5: Quantity of 12-ft Diameter Stacked Tray Units to Treat 240 MGD* Table 4-6: Grit Removal Design Criteria Table 4-7: Odor Control Ventilation Rates Table 4-8: Odor Control Design Criteria Table 4-9: Rooftop Outdoor Air Gas Heating and Ventilating Air Handling Units Table 4-10: Roof Mounted Exhaust Fan Flows and Air Changes Table 4-11: Rooftop Unit Size and Supply Flow List of Figures Figure 1-1: Option 3 Flow Diagram... 2 Figure 2-1: Leaves Causing Overflow at Existing West Screen Compactor in South Headworks 5 Figure 2-2. Existing CPS Coarse Grit Well Clamshell... 6 Figure 3-1: East (left photo) and West (right photo) Screens at South Grit Facility... 8 Figure 3-2. Plan Drawing of East and West Screens... 8 Figure 3-3. Plan Drawing of Existing Aerated Grit Chambers... 9 Figure 3-4. Photo of Abandoned Climber Screens at South Grit Facility Figure 3-5. Plan Drawing of Abandoned Climber Screens at the South Grit Facility Figure 3-6. Plan Drawing of the North Grit Facility Figure 3-7. Site Layout of Proposed Headworks Facility Figure 4-1: Proposed Headworks Facility Figure 4-2: Conventional Multi-Rake Bar Screen Lower Sprocket (courtesy Huber) Figure 4-3: Duperon FlexLink Elements Figure 4-4: Perforated Plate Screen (courtesy Enviro-Care) Figure 4-5: Center Flow Screens (courtesy of Hydro-Dyne) Figure 4-6: Troughed Belt Conveyor (courtesy of Custom Conveyor Corporation) Figure 4-7: Cleated Belt Conveyor (courtesy of Serpentix) Figure 4-8: Screenings Diverter to Dumpsters Figure 4-9: Example Stacked Tray Grit Removal System (Courtesy Hydro International) Figure 4-10: Hydro International Washer/Classifier Systems Figure 4-11: Wemco Washer/Classifier System Table of Contents iv

6 1. Introduction This technical memorandum (TM) provides an update to the Central WWTP project and replaces the previously prepared TM No. 2: South Headworks Facility Final Basis of Design Report (BODR), dated December 28, 2016; TM No. 3A: North and South Fine Screening, dated December 21, 2016; and TM No. 3B: North and South Grit Removal, dated December 21, The previously separate North Headworks and South Headworks projects have been merged into a single facility, and the intent of this TM is to document the latest design decisions for the CWWTP Headworks. Background Nashville Metro Water Services (MWS) commissioned Hazen and Sawyer to prepare a BODR for a new Headworks Facility for the Central WWTP (CWWTP). The Headworks Facility detailed in TM No. 2 was to serve the combined flows entering the CWWTP and was to be called the South Headworks Facility. A similar and parallel BODR effort was also completed by Brown and Caldwell for a new North Headworks Facility, which serves the separate sanitary collection system. The North Headworks Facility work is documented in the Central Wastewater Treatment Plant Capacity Improvements and CSO Reduction BODR, dated December 2016, prepared by Brown and Caldwell. North screening and grit improvements are documented in Technical Memorandum Nos. 3A and 3B as presented in the original BODR. MWS has contracted with a Construction Manager at Risk (CMAR) for delivery of the CWWTP upgrades work. After the individual North and South Headworks Facility TMs were completed, MWS, the CMAR, Hazen and Sawyer, and Brown and Caldwell engaged in several discussions regarding the separate headworks facilities. The outcome of the headworks facility discussions is that combining the separate North and South Headworks Facilities into one unified facility will better serve the CWWTP operation. Combining the two headworks will allow for easier management of flows between dry and wet weather and will provide one location on the plant site to handle screening and grit material. Hazen and Sawyer has been retained by MWS to design the new unified Headworks Facility. Subsequent to the revised headworks approach decision, (2) two workshops were held between MWS, Hazen and Sawyer, Brown and Caldwell, and the CMAR. Three treatment schemes were prepared and discussed in the first workshop on June 29, The (3) three flow schemes were as follows: Option 1 Combine separate North and South facilities as independently laid out into one facility. South (combined system) flow up to 240 MGD would pass through coarse screens, followed by stacked tray grit removal, then through fine screens, and then discharged to the South primary tanks. North (separate system) flow up to 200 MGD would pass through fine screens, followed by stacked tray grit removal, and then discharged to the North primary tanks. North and South flows would remain separate through the facility. Option 2 North and South flows would be comingled at the front of the Headworks Facility, and all flow up to 350 MGD would pass through coarse screens, followed by stacked tray grit removal, followed by fine screens, and then discharged to the North and South primary tanks. Up C-1

7 to 100 MGD of the south flow would be diverted upstream of the Headworks Facility to Excess Flow Treatment Unit (EFTU). Option 3 Similar to Option 2 with all flow passing through coarse screens, stacked tray grit removal tanks, and fine screens (layout shown in Figure 1-1). However, the headworks capacity would be 440 MGD, and the flow diversion to EFTU would occur downstream of the headworks facility. Figure 1-1: Option 3 Flow Diagram Option 3, which maximizes screening and grit removal of the wastewater, was selected to carry forward into the next step in the design process. The Option 3 facility plan was then evaluated in more detail and modified to include certain features for flow separation, redundancy, and equipment protection. Two more detailed facility layouts were presented and discussed during the second workshop, held on July 28, The primary difference between the two detailed layouts is the location of the screenings and grit dumpster rooms. One option situated the dumpster rooms at the east end of the buildings, whereas the other option placed the dumpster rooms at the center of the building. The layout with the dumpsters at the east end of the buildings was selected to carry forward into detailed design. Situating the dumpster rooms at the east end of the building enables placement of the roll-up doors away from the public and farthest from 3 rd avenue, and it also allows for the dumpsters to be handled on one side of the facility instead of two. Having the dumpster rooms on the end of the building also allows for the most accessibility by the dumpster trucks. C-2

8 2. Design Criteria 2.1 Flows The new Headworks Facility will be sized to handle the current design peak flows for the North (separate system) and South (combined system) influent flow streams. The design flows are shown in Table 2-1. Table 2-1: Headworks Design Flows Flow North Flow South Flow *Includes Cowan and Shelby Pump Station Flows. Flow Sources Brown s Creek* and 28 th Avenue Pump Stations Central Pump Station Peak Design Flow Rate (MGD) During dry weather conditions, the North and South flows can be combined upstream of the headworks and screened and degritted together. Dry weather flow discharging from the headworks can be diverted to the North primary settling tanks. During wet weather, the North and South flow streams will be segregated and treated separately through the headworks. Treated North flow will discharge from the headworks to the North primary settling tanks, and treated South flow will discharge from the headworks to the South primary settling tanks with a portion of the flow potentially being diverted to the Excess Flow Treatment Unit (EFTU). Based on historical plant data for the North and South flow contingents, the total average dry weather flow into the new Headworks Facility is expected to be approximately 125 MGD. The design criteria will therefore be: Average Dry Weather Flow (ADWF): 125 MGD Peak Sustained Flow North: 200 MGD Peak Sustained Flow South: 240 MGD 2.2 Process / Operations Rapid, Heavy Storm Flows There is limited data available to determine the effects of heavy storm flows with respect to solids loading. CWWTP staff have provided anecdotal information regarding high peaking factors (PFs) and quickly rising flows observed at the existing east/west coarse screens at the existing South Grit Facility. Generally, flows can increase from average to peak in the matter of one to two hours, depending on the intensity, duration, and geographic distribution of the storm. Overflows of the existing upstream screen channels have been observed during storm events, especially when the existing screens cannot keep up C-3

9 with the heavy load of debris or when a screen is disabled by an obstruction. These events have occurred within minutes of the first high flows arriving at the plant First Flush Conditions and Heavy Grit Loads Due to the rapid increase in flow during wet weather, the CWWTP commonly experiences a high debris and grit load at the beginning of a storm event. This first flush condition results from high flows suspending larger and heavier debris (e.g. bricks, rocks) that have collected in the sewer system during dry weather and have been subsequently scoured during a storm event. This debris and grit ends up at the CWWTP, and those items that make it through the Brown s Creek, 28 th Avenue, and CPS have resulted in the need for significant tank and channel cleaning and maintenance issues in plant process areas. At times, plant staff have observed significant loads of stone and other debris. During conversations with staff, examples were provided of significant No. 57 stone at the existing screens; this loading was suspected to have been caused by upstream construction in the sewershed that was then transported to the WWTP through the incoming sewers. The May 2011 Grit Removal Assessment Technical Memorandum (Gresham Smith and Partners) includes data for grit characterization at the South plant conducted over two consecutive wet weather days (April 12-13, 2011). Testing showed high concentrations of influent grit (approximately 94 lb/mgal and 26 lb/mgal during the second day). It is suspected that the first day was first flush conditions Seasonal Leaf Loads A common challenge during the fall season is leaves which rapidly blind influent screens, overwhelm the screenings conveyor system, and cause maintenance issues downstream (see Figure 2-1). Leaf loading is predominant in the South flow from the combined system. Equipment selections and recommendations will aim to address high leaf loads and reduce rapid screen blinding potential. Screens will also be capable of multiple operational speeds, with a faster cleaning mode for high differential levels typically associated with wet weather and high loading conditions. C-4

10 Figure 2-1: Leaves Causing Overflow at Existing West Screen Compactor in South Headworks Large Debris The Brown s Creek and 28 th Avenue pump stations include 1-inch coarse bar screens to remove large debris from the wastewater prior to it entering the CWWTP. The CPS pumps, however, can pass up to a 9-inch diameter solid. Large debris, such as bricks, car batteries, two liter bottles, long pieces of pipe, etc., have been reported to be present in the CPS discharge and have caused problems in downstream processes. The recommended screens will remove this large debris at the front of the plant and will keep it from causing greater damage downstream. The existing CPS features a coarse grit well upstream of the pumps designed to capture these large debris for removal by a clamshell, which is operated from a manned bridge crane (see Figure 2-2). Historically, the depth of the coarse grit well (nearly 100 ft) made operation of the clamshell difficult, and the frequency of cleaning of the coarse grit well was historically low. The buildup of grit during these periods allowed for the passage of larger solids and grit through the CPS pumps to the rest of the treatment process. C-5

11 Figure 2-2. Existing CPS Coarse Grit Well Clamshell Operations staff have begun regular and frequent clam-shelling of the coarse grit wells at the CPS and have noted a reduction in debris and grit in the existing coarse screen channels and aerated grit tanks. The recommended new headworks facility assumes the existing CPS coarse grit well will continue to be cleaned on a regular basis to minimize pass through of large debris. 2.3 Operational Flexibility The ability for MWS operational staff to efficiently operate the new Headworks Facilities based on operational conditions is a critical feature of the proposed improvements. Because the facilities are on the influent end of the CWWTP, the facility s ability to automatically adjust to different operational and influent conditions is important to prevent hydraulic backups and to maximize the volume of treated flow Automatic Flow Control Flow control automation will be integrated into the design where beneficial. Normal, dry weather flows will be routed through a limited number of screens and grit tanks to maintain high channel velocities and to minimize solids deposition in the facility. During wet weather events, additional screens and grit tanks will be brought online to accommodate the increased flow. The time lag between increased flow at one of the feeder pump stations and increased flow at the Headworks Facility may be very small. When a pump feeding the WWTP turns on, particularly at the CPS, a large slug of debris often is conveyed to the plant. To minimize the chance of blinding or damaging a screen from high flow or high loading when a pump turns on, all screens may be manually placed into service proactively at the onset of a wet weather event, as opposed to reactive attempts to place tanks or equipment in service based on observed flow increases. If screens have multiple speeds, the screens could be operated at high speed to limit the chances of blinding. Although operating at high speed would decrease the chances of blinding the screens, running C-6

12 the screens too often or too early can cause excessive wear on the screens. The Headworks Facility will otherwise automatically route flow and bring standby units in and out of service as flows increase and decrease based on operations-entered setpoints. For example, during wet weather events, influent gates will automatically open in front of standby units to ensure optimal treatment as flows increase. The control system will open the gates at operator-adjustable water level or influent flow setpoints, which have been calculated based on flow conditions that ensure optimal screening and grit removal Passive Hydraulic Bypass Two passive bypass channels will be provided in the new Headworks Facility. One bypass channel will serve the South side of the facility and one bypass channel shall serve the North side of the facility. Two bypass channels are required to maintain flow separation during wet weather should both channels be utilized at the same time. The bypass channels will run the length of the facility to bypass flow around the coarse screens, grit removal tanks, and fine screens. In addition to a passive overflow weir for each bypass channel upstream of the coarse screens, a slide gate will also provide an alternate means to divert flow into each bypass channel. The common fine screen influent channels will also be connected to the bypass channels via downward opening weirs Discharge to Flow Equalization Flow diversion to EQ/EFTU will occur downstream of the Headworks Facility. The flow diversion will only occur for the South (combined system) flows. All North flow will be conveyed to the North Primary Settling Tanks for treatment through the plant Ability to Transfer Flow Between North and South Areas of Plant The Headworks Facility will include provisions to comingle the North (separate system) and South (combined system) flows upstream of the coarse screens during dry weather and discharging the comingled flow to the North primary settling tanks. Similarly, during wet weather, the facility will be capable of maintaining flow stream separation during wet weather conditions with the screened and degritted North (separate system) flow being discharged to the North primary settling tanks and the screened and degritted South (combined) flow being discharged to the South primary settling tanks or to EQ/EFTU. 3. Existing Conditions & Constraints 3.1 Existing South Headworks Facilities The existing east and west screens and associated compactors have less than the required peak flow design capacity of 240 MGD, and they do not have adequate bypassing capability to convey the peak flow to downstream treatment units. The existing aerated grit chambers could pass 240 MGD hydraulically, but would provide very low grit removal during high flows (see TM 3B). C-7

13 3.1.1 East and West Screens The existing east and west screens were installed in the South Grit Facility in Photos are shown in Figure 3-1 and a plan drawing is shown in Figure 3-2. Each unit is a chain and multi-rake unit with 1-inch openings, and it has a design capacity of 80 MGD. Figure 3-1: East (left photo) and West (right photo) Screens at South Grit Facility Figure 3-2. Plan Drawing of East and West Screens The east and west screens and compactors have difficulty keeping up with seasonal leaf loads, especially at the entrance to the compactor unit, where bridging occurs due to the leaves. Other debris also causes C-8

14 equipment failure at these screens somewhat frequently, particularly because the CPS pumps are able to pass relatively large solids. Because there is no capability to bypass these screens, a failure or excessive blinding can quickly cause a hydraulic bottleneck in the system and a significant operational concern. These screens will be decommissioned and flow routed around the existing South Headworks Facility after the new merged Headworks Facility is commissioned South Aerated Grit Chambers There are three aerated grit chambers immediately downstream of the east and west screens in the existing South Grit Facility. The existing chambers were retrofitted in place of old gravity grit tanks and sludge storage tanks as the focus of a 1998 project. Plan drawings from this project are shown in Figure 3-3. Figure 3-3. Plan Drawing of Existing Aerated Grit Chambers Aerated grit chambers typically have lower removal performance than modern grit technologies, particularly at peak flows. These grit tanks will be decommissioned and flow routed around the South Grit Facilities at the completion of this project Abandoned Climber Screens Three climber screens with 1-inch openings (see photo in Figure 3-4 and the plan drawing in Figure 3-5) were installed in 1998 in a separate building downstream of the grit tanks. These screens have been abandoned in place for several years and will remain out of service. C-9

15 Figure 3-4. Photo of Abandoned Climber Screens at South Grit Facility Figure 3-5. Plan Drawing of Abandoned Climber Screens at the South Grit Facility C-10

16 3.2 Existing North Grit Facility The current North Grit Facility receives flow from the Browns Creek Pump Station and the 28 th Ave. Pump Station. Unlike the Central Pump Station which serves the combined sanitary sewer system, both Browns Creek and 28 th Avenue pump stations serve separate sanitary sewer systems. The current operational procedure is to send a majority of the dry weather flow from the South system to the North Grit Facility for treatment in addition to the other flows Screening There is no screening equipment in the North Grit Facility. All screening is currently performed upstream of the facility at collection system pump stations. Screens installed at the pump stations have coarse openings of 1-inch or greater Grit Removal The current grit removal system at the facility consists of a group of four aerated grit tanks. According to the O&M manuals, each tank has a maximum treatment capacity of 62 MGD. Settled grit from the tanks is directed into a grit hopper, and then the grit is pumped through a recessed impeller centrifugal pump to one of the six cyclone separators. The three pairs of cyclones each discharge into one of three grit classifiers that will discharge the grit onto conveyors to be disposed into a dump truck. The existing grit removal system should remove grit at a percent removal efficiency based on its design. However, the recent GS&P CWWTP Grit and Primary Clarifier Improvements evaluation showed that the North grit removal system had a negative grit capture rate. Figure 3-6 shows a plan drawing of the North Grit Facility. The existing North Grit Facility will be decommissioned after the new Headworks is constructed. C-11

17 Figure 3-6. Plan Drawing of the North Grit Facility 3.3 New Headworks Facility Site Selection The new Headworks Facility will be of a significant size, and, therefore, there are limited locations on the Central WWTP site where the facility will fit without significant modification to existing facilities or interference with future planned facilities. The Headworks Facility will occupy an area of approximately 40,000 square feet, and the processes will be supplied by and discharge to multiple 60-inch diameter or larger pipes. The CPS is located at the southern end of the WWTP, and the Browns Creek force mains also enters the site at the southern property line. Flow leaving the new headworks will be directed to the North PST influent channel and to the South PST influent pipe. The rd Avenue building (System Services building), located northwest of the CPS, is planned to be abandoned in the near future. Given the size of the proposed Headworks Facility, and the location and direction of the supply and discharge pipes, the C-12

18 most appropriate location for the new Headworks Facility is at the rd Avenue site. The proposed location also sits at a higher grade than the rest of the WWTP which benefits its design so that the treated effluent can flow by gravity to the downstream treatment processes. Figure 3-7 shows the approximate layout for the proposed new Headworks Facility in relation to the surrounding structures and site features. Figure 3-7. Site Layout of Proposed Headworks Facility 3.4 Facility Hydraulic Constraints The proposed Headworks Facility will need to accommodate numerous hydraulic constraints to accommodate. On the upstream side of the facility, wastewater is pumped to the WWTP; therefore, the upstream water surface elevation (WSE) of the headworks directly impacts the total dynamic head of the supply pumps. Higher WSE will result in reduced influent pumping capacity. On the downstream side of the facility, headworks effluent WSE needs to be high enough to convey flow by gravity to both the North and South PSTs. Preliminarily, the maximum upstream water surface elevation target has been determined to be elevation This elevation has preliminarily been determined to not have a detrimental impact on the hydraulic capacity of the Browns Creek pump station or the CPS. A draft hydraulic analysis has identified a maximum water surface elevation of approximately at the PST interconnect channel. The North Headworks effluent is proposed to connect directly to the PST interconnect channel and the South Headworks effluent is proposed to connect to piping at the South PST influent. The conveyance from the headworks to the interconnect channel and South PST influent pipe will be designed with a headloss which will bridge the new Headworks Facility effluent WSE to the downstream WSEs. C-13

19 4. Headworks Unit Processes and Equipment This section focuses on preferred / selected equipment types for major headworks unit processes. Significant discussions were held with the project team to identify the most viable unit process equipment. Further detail is provided in meeting summaries. Figure 4-1 shows the overall layout of the proposed Headworks Facility. Figure 4-1: Proposed Headworks Facility The process train of the proposed facility was selected to allow for each unit process to operate to its maximum efficiency and extend the useful life of the equipment. Coarse screens with ½ spacing are proposed upstream of the grit tanks to prevent large items from entering the tanks which can damage the equipment or cause plugging to occur. Fine screens with ¼ spacing are proposed downstream of the grit tanks to minimize equipment wear from the grit in the wastewater and to minimize plugging due to grit slugs Screening Equipment The Headworks Facility will employ two levels of screening. Coarse screens will precede the grit removal tanks and fine screens will follow grit removal. C-14

20 4.1.2 Coarse Screens Chain and multi-rake bar screens are recommended for the coarse screens. The equipment design consists of three basic components: a drive head, raking device, and bar screen. The rakes are mounted to chains or links and move the debris up the screen and discharge the screenings at the operating level on the back side, then return on the front side to the bottom of the screen, staying entirely on the influent side of the bars. Two types of multi-rake screens, conventional and catenary, were evaluated for further consideration. Conventional multi-rake bar screens include multiple rakes attached to two roller chains, each with a submerged lower bearing or slide to guide the chain and rakes into the bar rack. The lower sprocket rides on a journal bearing that the manufacturers claim is no-maintenance (see Figure 4-2). An alternate design using a fixed curved plate is available in lieu of the conventional sprocket. The plate guides the chain around the lower hub to engage the bar rack just as the sprocket. Conventional multi-rake screens are manufactured by Headworks, Vulcan, Huber, and others. The advantage of the lower sprocket is that it provides more rigidity in the rakes, increasing their pressure on the front side of the screen and providing additional force to carry screened solids up to the discharge point. This additional pressure improves the overall capture efficiency of the screens. The lower bearing material varies across manufacturers, but generally consists of a stainless or hardened steel stub shaft with a low friction, high wear resistant material for the contact surfaces. The bearing is lubricated by the oils present in the wastewater. The MWS staff has had issues with jamming of the lower sprocket on the existing headworks bar screens at CWWTP. Each of the above named manufacturers also offers an alternate lower guide instead of sprocket with bearing. The guide is generally a fixed disc which the chain follows while using the chain rollers as the contact point. Manufacturers have numerous installations that have been operating for over ten years without operational issues related to the lower sprocket. Figure 4-2: Conventional Multi-Rake Bar Screen Lower Sprocket (courtesy Huber) Flex-link multi-rake screens, a type of catenary screen, are similar to conventional chain and multi-rake screens but have no fixed lower bearing or sprocket at the bottom of the screen (i.e. in the flow path). The chain is comprised of solid links which articulate in one direction only. Flex-link multi-rake screens are currently manufactured by several manufacturers. However, Duperon has the largest installation list and longest history with flex-link type screens. The screens are designed to resist jamming due to large C-15

21 debris. Instead of a fixed rotation point at the bottom, the rakes (see Figure 4-3) are designed to pull debris up the bars under the power of their own weight. The links to which the rakes are attached allow the rakes to move away from the rack and work around large debris to pick it up and prevent equipment stoppage. Rake travelling downward to base of screen Rake engaged on screen bars Links act as lever arms to pull rakes over large objects Figure 4-3: Duperon FlexLink Elements The new Headworks Facility coarse screens will be designed around the Duperon flex-link, multi-rake screen. The layout will also be able to accommodate conventional chain and multi-rake screens Coarse Screen Design The grit removal process requires the influent wastewater to be screened to 1/2-inch openings or less. Since half of the facility will receive flow from the combined system, 1/2-inch opening bar screens were selected for the coarse screening application. Screen channel design is often a balancing act between channel velocity (to prevent deposition) and screen slot velocity (to minimize screenings passage). As channel deposition can lead to damaging material slugs being conveyed into equipment during increasing flow periods, channel velocities should be maintained above 2 feet per second, if possible, for all flow scenarios. The chain and multi-rake screens will be rated for 60 MGD each with a 6.0 ft channel width. Water depth in the channel will be determined based on optimizing velocity under various flow conditions. Design headloss will be 6 inches or less at 30% blinding. A total of 12-inch headloss will be provided for in the hydraulic profile to provide an operating margin during heavy leaf loading scenarios. Typically the screens are setup to operate off a 6-inch differential head, set to run at slow speed, and a 12-inch differential, set to run at a high speed. A standby screen is not proposed. A bypass channel will serve as the backup means to pass excess flow around the coarse screens. Table 4-1 shows the coarse screen design criteria. C-16

22 Table 4-1: Coarse Screen Design Criteria Item Amount Unit No. Screens North 4 Each No. Screens South 4 Each Capacity 60 MGD Drive Motor 1 hp Type Chain & Multirake Size ½ Inch Channel Width 6 feet Installation Angle 65 Degrees Minimum Downstream Water Depth At Peak Flow Maximum Headloss At Peak Flow & Blinding 7.0 feet 6 inches Blinding At Peak Flow 30 percent Solids Load 400 CFH Each Fine Screens Fine screens will be installed downstream of the grit removal tanks. Fine screen technology ranges from small opening bar racks, to perforated plate screens, as well as center flow and front flow screens. Bar racks with ¼ and smaller opening widths are considered fine screens. Chain and multi-rake fine bar screens are the same as the chain and multi-rake coarse screens discussed above, except that the bar spacing is smaller. The screens can be manufactured with lower sprocket and bearing or lower chain guide, or can be of the catenary type. Whereas bar screens only screen in one dimension, width, perforated plate screens provide screening in two dimensions, width and height, offering significantly improved capture for the equivalent size screen. Screen design includes multiple perforated plates attached to a set of roller chains. The plates fit tight to each other, and stationary components of the screen eliminate openings larger than the perforations through which solids can pass. See Figure 4-4 for an example perforated plate screen. Coarse screens operate with a fixed screen field. However, perforated plates operate with a moving screen field. The chain continually moves the perforated plates up out of the channel and over a top bearing where spray bars and a brush clean the captured screenings from the plates. The clean plates continue back down into the channel and around the lower bearing to repeat the process. Similar to the chain and multi-rake coarse screens, a lower bearing and sprocket or guide are offered. Unlike the coarse screens, the perforated plate screen design places the lower bearing and sprocket downstream of the screen elements where they are much less susceptible to catching debris. Manufacturers being considered include Enviro-Care, Huber, and Andritz. C-17

23 Figure 4-4: Perforated Plate Screen (courtesy Enviro-Care) Perforated plate screens are either front flow type or center flow type. Front flow screens are installed in screen channels with the screening face of the screen perpendicular to the flow of wastewater in the channel, as shown in Figure 4-4. The wastewater essentially travels linearly with the channel as it passes through the screen. The center flow screens are arranged with the screening face of the panels parallel to the flow of water in the channel. The wastewater flows into the center of the screen and then once in the screen, flows laterally through the perforated plates, and then turns back in line with the channel downstream of the screen; flow makes two 90 degree bends to get through a center flow screen. Figure 4-5 shows typical center flow screens. Figure 4-5: Center Flow Screens (courtesy of Hydro-Dyne) The new Headworks Facility fine screens will be designed around front flow perforated plate type screens Fine Screen Design 1/4-inch perforated plate front flow screens have been selected for the fine screen application. Similar to the coarse screens, the fine screen design depends on target channel velocity, perforation velocity, and headloss. Water depth will be determined during detailed design. C-18

24 The perforated plate screens will be rated for 60 MGD each with a 6.0 ft channel width. Water depth in the channel will be determined based on optimizing velocity under various flow conditions. Design headloss will be 18 inches or less at 30 percent blinding. A total of 24-inch headloss will be provided for in the hydraulic profile to provide an operating margin during heavy leaf loading scenarios. A standby screen will not be provided. A bypass channel will serve as the backup means to pass excess flow around the fine screens. The fine screen design criteria are shown in Table 4-2. Table 4-2: Fine Screen Design Criteria Item Amount Unit No. Screens North 4 Each No. Screens South 4 Each Capacity 60 MGD Drive Motor 3 hp Brush Motor 3 hp Type Perforated Plate Size ¼ Inch Channel Width 6 feet Installation Angle 75 Degrees Minimum Downstream Water Depth At Peak Flow Maximum Headloss At Peak Flow & Blinding 7.5 feet 18 inches Blinding At Peak Flow 30 percent Solids Load 37.5 CFH Each Wash Water 40 gpm Fine Screenings Washing/Compacting Fine screenings will be discharged from the screens into washer/compactors. Each screen will have a dedicated washer/compactor which will discharge to a belt conveyor to transport the washed/compacted screenings to the dumpster room. Washer/compactor will be equipped with a receiving hopper and dewatering zone, wash zone, and press zone. Internal sprayers will wash organics from the screenings as they pass through the unit. The press zone will compress and dewater the screenings. Washed/compacted screenings will be discharged through an inclined, increasing diameter discharge chute onto the belt conveyor. Figure 4-5 shows a typical screenings washer/compactor and Table 4-3 includes the washer/compactor design criteria. C-19

25 Table 4-3: Washer/Compactor Design Criteria Item Amount Unit Washer/Compactor No. 8 Each Capacity 75 CFH Motor 10 hp Minimum Discharged Solids Dryness Minimum Volume Reduction 40 percent 60 percent Minimum Weight Reduction 60 percent Wash Water 20 gpm Screening Bypass Two bypass channels will be provided in the headworks. One channel will be dedicated to the North flow side of the facility and the other channel will be dedicated to the South flow side of the facility. The two channels will maintain flow stream separation should both bypass channels be used at the same time. A fixed weir upstream of the coarse screens will allow for excess flow to enter the bypass channel if the water level upstream of the coarse screens gets too high. A high water level could result from one or more screens being out of service during a wet weather event or severe screen blinding conditions. A slide gate will also be provided from the area upstream of the coarse screens into the bypass channels to allow for an alternate means to divert flow around the facility. An overflow weir will also be provided from the area upstream of the fine screens into the bypass channel. Rather than being fixed, the fine screen bypass weir will be a downward opening weir gate. Due to the tight hydraulic profile and the low differential head from the area upstream of fine screens to the bypass channel, the downward opening weir will allow for additional flow to be bypassed compared to a fixed weir Screenings Conveyance and Handling Several options for conveyance and compaction/dewatering of screenings were considered for the new headworks. Since approximately half the flow in the new headworks is combined flow, large debris is anticipated at the coarse screens. Large debris often causes problems with compaction equipment and can lead to frequent jams/shutdowns of this equipment. Therefore, it is recommended that screenings be conveyed directly to the dumpsters, with provisions for drainage from the dumpsters to meet the required paint filter test for landfill. A belt conveyor is recommended for transport of material because other conveyance methods, including sluicing channels and screw conveyors, are susceptible to failure or clogging with large volumes of leaves, coarse grit, and trash. Belt conveyors for transporting loose material typically are constructed of a smooth troughed belt with side skirts, a flat belt with corrugated vertical side walls, or a corrugated belt. Corrugated belts are C-20

26 superior to troughed belts when the conveyor needs to be installed on a steep incline as the material does not roll back. Corrugated belts, however, can require significant housekeeping due to the screenings hanging up or sticking in between the belt or sidewall corrugations and falling off on the underside of the conveyor. Troughed belt conveyors have been used successfully on numerous screenings applications including low slope installations. Figure 4-6 is an example troughed belt conveyor and Figure 4-7 shows a cleated belt conveyor. Equipment options will be further evaluated and selected during detailed design. Figure 4-6: Troughed Belt Conveyor (courtesy of Custom Conveyor Corporation) Figure 4-7: Cleated Belt Conveyor (courtesy of Serpentix) Conveyance of screenings is a critical link in effective screening and debris removal. If only one conveyor were installed and it happened to fail, multiple screens would have to be shut down. To ensure continuous screening removal, it is essential that conveyance has redundancy. The coarse screen system will include redundant belt conveyors. Half of the screens will be dedicated to one conveyor, and the other half will be dedicated to the second conveyor. Based on historical flow data, influent flow has been below 200 MGD approximately 95 percent of the time. Although the dry flow rate will stay at the same levels, the peak flow rates could vary once CPS is expanded from 160 MGD to 240 MGD. With only half of the facility in operation during dry weather periods, or periods when flow is less than 200 MGD, the conveyor redundancy is 100 percent. Diversion gates will also be included to allow for screenings to be diverted into an adjacent small receptacle, as shown in Figure 4-8. The small dumpster option also provides the advantage of clearing away large and rapidly discharged volumes of screenings during storm events. C-21

27 Figure 4-8: Screenings Diverter to Dumpsters The belt conveyors will discharge screenings into either a 20 cubic yard (CY) dumpster or a dump truck parked in a loading bay. The dumpster would be a dewatering type in order to allow drainage of excess water from the coarse screenings prior removal and transport to final disposal. 4.2 Grit Removal Equipment Grit loading on the treatment facilities is significant and deteriorates process efficiency as grit accumulates in tanks and channels. Additional time and expense is also spent cleaning the grit out of the treatment plant. The new Headworks will incorporate stacked tray grit removal systems to improve grit capture and separation from the influent wastewater Grit Separation Stacked tray grit removal equipment uses a series of circular stacked trays within the square tank with conical bottom to assist with settling of finer grit particles. These units have shown superior performance in removal of fine grit, even at peak flows. To evaluate reliability, Hazen and Sawyer performed a detailed potential failure analysis on stacked tray systems. Some of the major considerations for installation and successful operation during wet weather are shown in Figure 4-9. C-22

28 Figure 4-9: Example Stacked Tray Grit Removal System (Courtesy Hydro International) The number of stacked tray units for varying performance (measured by minimum particle size target, or cut point ) was calculated to meet the peak flow design criteria of firm capacity at 200 MGD for the North flow and 240 MGD for the South flow, as shown in Table 4-5 and 4-5. Table 4-4: Quantity of 12-ft Diameter Stacked Tray Units to Treat 200 MGD* No. of Trays/Unit No. of Units for Particle Size Cut Point (95% Removal at 200 MGD) 106 micron 150 micron 212 micron * 200 MGD firm capacity (quantities include one standby) C-23

29 Table 4-5: Quantity of 12-ft Diameter Stacked Tray Units to Treat 240 MGD* No. of Trays/Unit No. of Units for Particle Size Cut Point (95% Removal at 240 MGD) 106 micron 150 micron 212 micron * 240 MGD firm capacity (quantities include one standby) There is a large increase in the number of units required to meet the 106 micron cut point versus 150 micron. The number of units required is directly related to the estimated construction cost of the facility. Because of the significant increase in number of units required when comparing 106 micron and 150 micron particle size cut points, the 150 micron cut point design standard at peak flow is recommended for implementation. Metro will achieve optimal return for this investment, achieving removal and lower size cutpoints during normal flow and maintaining grit removal of 150 micron particles during infrequent peak flow events. The new Headworks Facility will maintain flow segregation between the North and South flows during wet weather events. The grit tanks do not have any moving parts, thus minimizing failure components, so it has been decided to not include standby tanks. Each side of the facility will incorporate space for at least one future tank, however, if additional grit removal capacity is needed. These extra spaces will include only the concrete required for the HeadCells. The grit handling equipment would be installed in the future if additional capacity is needed. The number of tanks in the current design will therefore be six (+ 2 future) for the North flow, and seven (+ 1 future) for the South flow. Table 4-6 includes the design criteria for the grit removal tanks. Table 4-6: Grit Removal Design Criteria Item Amount Unit Grit Removal Tank No. North 6 Each Grit Removal Tank No. South 7 Each Capacity 34.3 MGD/each Type Stacked Tray Diameter 12 feet No. Trays 12 each Target Particle Size 150 um Target Particle Removal Efficiency 95 percent Grit Pump Capacity 550 gpm Grit Pump Motor 25 hp Scour Water 20 gpm C-24

30 4.2.2 Grit Washing and Classifying Several well established alternatives are available for grit washer/classifier systems, all of which generally use a cyclonic flow pattern and screw conveyors for transport of the grit to a dumpster. Hydro International offers the SlurryCup washer/classifier system (see Figure 4-10), which is often coupled with their stacked tray grit removal systems. Wemco offers a washer/classifier system (see Figure 4-11) that can be coupled with any grit removal equipment. Figure 4-10: Hydro International Washer/Classifier Systems Figure 4-11: Wemco Washer/Classifier System After evaluating grit capture efficiency, equipment size, washwater needs, and operation and maintenance requirements of the grit handling options, it was decided to design around cyclones and classifiers. C-25

31 4.3 Odor Control Because of the proximity of residential properties and other establishments near the CWWTP, odor control for exhaust foul air from the Headworks will be necessary. Odor control for the Headworks has been separated into three areas: Foul air under channel and equipment covers Coarse Screen Building Exhaust Fine Screen Building Exhaust Note that the air supply for the Coarse and Fine Screen Buildings will be provided as part of the HVAC system and is covered in Section 4.4. The fans for the Coarse Screen Building will be slightly oversized to match the size of the fans on the Fine Screen Building and simplify maintenance. Ventilation rates for the spaces described above are shown in Table 4-7. Space Table 4-7: Odor Control Ventilation Rates Design Criteria air changes per hour Required Ventilation Rate, cfm Proposed Fan Size cfm Area Under Covers 6 35,000 35,000 Coarse Screen Building 6 42,000 44,000 Fine Screen Building 6 44,000 44,000 General odor control design criteria shall be as follows in Table 4-8. Table 4-8: Odor Control Design Criteria Item Amount Unit Process Space Air Exchange 6 ACH Cover Leakage 0.5 CFM/SF Building Air Exchange Differential Maximum Ductwork Velocity 10 Percent* 2,500 fpm *Exhaust air flowrate shall be 10% higher than supply air flowrate to maintain negative pressure inside of the building Foul Air Under Covers MWS contracted with others to perform a plant-wide evaluation of odor control needs, including the new headworks and the other planned improvements. That evaluation determined that the foul air from under the covers at the Headworks can be treated by the existing Liquid Side Biofilter, and the design of the odor control systems for the Headworks is based on that assumption. Foul air from under the covers will C-26

32 be collected and transported via overhead duct to the existing Liquid Side Biofilter. The existing Liquid Side Biofilter fans will provide ventilation for the headspace under the covers Coarse and Fine Screen Buildings The recommended odor control facilities for the buildings will include ventilation for both code requirements and personnel health and safety for each facility area. As noted above, air supply for the two buildings will be provided by the HVAC system and is described in Section 4.4. In the Coarse Screen Building, the odor sources are the screenings on the open conveyors and the screening and grit dumpsters. In the Fine Screen Building, the odor sources are the washed screenings on the open conveyors and the screening and grit dumpsters. Because the odor concentrations in the buildings are expected to be low, odor control by dilution will be provided for these spaces. Odor control fans will discharge to stacks and odor levels will be controlled through dilution and dispersion. Three odor control alternatives were considered for the new headworks room spaces. Option 1: Install odor control by dilution for the coarse screen room, coarse screen dumpster room, fine screen room, and fine screen dumpster room. Option 2: Install odor control by dilution for the coarse screen room and coarse screen dumpster room. Provide HVAC in the fine screen room and fine screen dumpster room with ductwork optimized for odor control so that future odor control could be provided by changing out the fans and adding a stack. Option 3: Provide odor control by dilution for the coarse screen dumpster room. Provide HVAC in the coarse screen room, fine screen room, and fine screen dumpster room with ductwork optimized for odor control so that future odor control could be provided by changing out the fans and adding a stack. The room space odor control system will be designed around Option 2. Two FRP fans will be provided for each building, one duty and one spare. The fans will be located in sound attenuating enclosures on top of the bypass channels in the area between the two buildings. Coordination of final odor control system requirements and associated connections will be continued throughout detailed design. 4.4 Mechanical / HVAC System Selection Basis The mechanical system selected for the Headworks project was determined based on the ability to comply with air change requirements and compliance with Class 1 Division 1 requirements. In order to comply with the 6 ACH requirement for fresh air supply throughout the project, 100% outdoor air heating and ventilating units were selected. The Class 1 Division 1 explosive environment requirements apply to the areas inside the fine and coarse screen buildings, including the Grit Handling and Dumpster rooms. The North grit and South grit pump rooms are unclassified according to NFPA 820. C-27