Sewerage Agency of Southern Marin HEADWORKS IMPROVEMENTS PROJECT PREDESIGN REPORT

Transcription

1 Sewerage Agency of Southern Marin HEADWORKS IMPROVEMENTS PROJECT PREDESIGN REPORT ADMINISTRATIVE DRAFT February YGNACIO VALLEY ROAD SUITE 300 WALNUT CREEK, CALIFORNIA (925) FAX (925) pw://carollo/documents/client/ca/sasm/9291b10/deliverables/ PDR.docx

2 SEWERAGE AGENCY OF SOUTHERN MARIN HEADWORKS IMPROVEMENTS PROJECT DRAFT PREDESIGN REPORT TABLE OF CONTENTS Page No. Predesign Report PURPOSE SUMMARY OF FINDINGS AND RECOMMENDATIONS BACKGROUND HEADWORKS CAPACITY FLOW MEASUREMENT Parshall Flume Improvement Alternatives Recommendation SCREENINGS REMOVAL Bar Screen Opening Size Clear Screen Velocity Screenings Volume Screenings Technology Alternatives Screenings Washing and Dewatering Screenings Conveyance Alternatives Recommendation GRIT REMOVAL SYSTEM CFD Analysis Potential Improvements Recommendation NFPA 820 IMPROVEMENTS ODOR CONTROL SYSTEM New Chemical Scrubber New Packaged Biofilter New Wood Chip Bed Biofilter Improvements to the existing system Recommendation MISCELLANEOUS IMPROVEMENTS CONSTRUCTION COST ESTIMATES Cost Estimate Assumptions Cost Estimates February 2014 i

3 Predesign Report HEADWORKS IMPROVEMENTS PROJECT 1.0 PURPOSE The purpose of this predesign report (PDR) is to establish preliminary design criteria, identify alternatives, and develop conceptual layouts for improvements to the existing headworks at the Sewerage Agency of Southern Marin (SASM) Wastewater Treatment Plant (WWTP). 2.0 SUMMARY OF FINDINGS AND RECOMMENDATIONS The key findings and recommendations of this PDR are summarized below: The Headworks facilities, including the parshall flumes, screens, screening handling equipment, vortex grit basin, and grit classifier, are in fair to poor condition and are underperforming. o o o o Due to poor upstream hydraulics, the parshall flumes do not accurately measure the influent flowrate during wet weather conditions. The mechanical bar screens are obsolete and are inefficient at removing both fine and large plastics, rags, and debris from the influent flow stream. The vortex grit basin is a non-standard design and does not efficiently remove grit at low or high influent flowrates. The grit classifier and grit piping are in poor condition and need to be replaced. The underperforming Headworks facilities negatively impact the WWTP during both dry and wet weather: o o During dry weather conditions, plastics, rags, and other debris pass through the mechanical bar screens and are removed by the downstream processes. The debris ultimately collects in the anaerobic digesters, which reduces the capacity of the digesters. In addition, the rags have the potential to clog critical WWTP pumps. During wet weather events, a large amount of grit is flushed from the collection system. Because of the issues with the vortex grit basin, a significant portion of the grit passes by the vortex grit basin. The grit is then deposited in the primary clarifier sludge hoppers, which can clog the primary sludge pumps. As a result, the primary clarifiers have to be taken out of service and drained in order for the grit to be removed. Replacement of the existing screens, screenings conveyance, and grit classifier equipment is recommended. Two options were found feasible, depending on available funding: DRAFT February

4 o Option 1 Headworks Rehabilitation: The existing Headworks would be rehabilitated by installing new screens, screenings washer compactors, a belt conveyor, and a new grit classifier. The new screens should have a bar spacing of 3/8-inch and should be a chain driven multi-rake type. In addition, the following improvements would be made: The accuracy of the parshall flume would be improved by relocating the existing bubbler to the other side of the flume. To improve grit removal performance, a baffle would be installed to prevent short circuiting, which reduces grit removal performance. Various improvements would be made to the Headworks Building to comply with the National Fire Protection Association s Fire Protection in Wastewater Treatment Collection Facilities (NFPA 820) guidelines. The existing odor control system would be maintained in its present state, with some modifications, until the vessel is past the end of its useful life. At that time, a biofilter, such as the Siemens ZB-8025 system, would be installed. The estimated construction cost of retrofitting the existing headworks, including structural modifications and wet well rehabilitation, is approximately $2,458,000. This option would provide for an improved Headworks but would not solve all of the issues with the existing headworks o Option 2 New Headworks and Influent Pump Station: A completely new headworks and influent pump station would be constructed, including screening, influent pumping, and vortex grit removal. The construction cost of a new headworks, influent pump station, and vortex grit basin is approximately $9,500,000. The new headworks would be state of the art and would be perform more effectively and efficiently than a rehabilitated headworks. Because of the relatively high construction costs of both Headworks improvement options, SASM to postpone a final decision on the path forward until after the ongoing Wastewater Treatment Plant Master Plan (MP) is complete. The Master Plan will provide a comprehensive capital improvements plan (CIP) that will allow SASM to prioritize necessary projects and match projects to available funding. In the meantime, a list of short-term projects that can be implemented to prolong the life of the existing Headworks until the MP is completed. These improvements are necessary to keep the Headworks operational and reliable over the next few years. The recommended improvements include the following: o New belt for belt conveyor ($15K-$20K). DRAFT February

5 o o o o Rebuild both of the existing bar screens (performed by SASM maintenance staff). Replace grit classifier piping ($30K-$50K). Design and install new vortex grit basin baffle ($30K-$50K). Parshall flume instrumentation improvements ($5K-$10K). 3.0 BACKGROUND The Sewerage Agency of Southern Marin s (SASM) Headworks and Solids Handling Building was constructed in 1981 as part of the Wastewater Treatment Plant Improvements Project at the Agency s Wastewater Treatment Plant located in Mill Valley, California. The Headworks and Solids Handling Building includes an influent junction box, two influent channels with parshall flumes, two mechanical bar screens, one bypass bar rack, a single vortex grit chamber with associated grit pumps, two wet wells, a dry pit pump room with seven influent pumps, and a screenings room with a grit cyclone and washer and belt conveyor with bin for screenings and grit removal. The building also contains a sample room, chemical room, and electrical room. An isometric of the existing Headworks and Solids Handling Building is shown in Figure 1. After 32 years of service, the Headworks and Solids Handling Building requires improvements to improve the operation and reliability of the WWTP. These improvements include new mechanical bar screens, new screenings handling equipment, modifications to the grit removal and handling system, and several other miscellaneous improvements such as ventilation and electrical improvements to update the Headworks and Solids Building to current National Fire Protection Association (NFPA) 820 recommendations. DRAFT February

6 Figure 1 Isometric Diagram of Existing Headworks and Solids Handling Building 4.0 HEADWORKS CAPACITY The headworks and influent pump station have a peak design capacity of 32.7 mgd. This peak hour design flowrate is similar to the peak hour flowrate generated from the collection system modeling during the preparation of SASM s 2010 Sewer Spill Reduction Action Plan (SSRAP). The SSRAP determined that for a wet weather event with a 20-year recurrence interval, the peak hour flowrate is 32.5 mgd. A summary of the design flowrates is shown in Table 1. To the maximum extent possible, the improvements will be designed to meet the current minimum flow conditions as well as the average dry weather, maximum day, and peak hour flow conditions. Table 1 Influent Flow Summary Headworks Improvement Project Sewerage Authority of Southern Marin Condition Minimum Hour (mgd) Average Dry Weather (mgd) Maximum Day (mgd) Peak Hour (mgd) Design SSRAP DRAFT February

7 There have been some events where the flowrate has exceeded 32.7 mgd. However, the flow chart from one of these events indicated that the peak flow rate was not sustained for a long period of time. The higher flow, up to 38.5 mgd, was likely due to water level conditions in either the influent pump station wet well or from a blockage in the bar screens. A momentary reduction in pump flowrate or a screen blockage could artificially raise the water level in the parshall flumes, causing the flow transmitters to register a high influent flow rate than the actual flowrate. 5.0 FLOW MEASUREMENT The influent flowrate to the WWTP is measured by two parshall flumes. The smaller 12-inch flume is used during dry weather flows. Once influent flowrates exceed 8 mgd, the larger 36-inch flume is used in conjunction with the smaller flume. In general, the flumes perform adequately, except that the larger flume does not accurately measure flowrates when the influent flowrate is between 8 and 25 mgd. During this flow range, the flowrate through the larger parshall flume reads approximately 4 mgd low. To accurately measure flow, the water entering the flume must be sub-critical, well distributed across the channel, and relatively tranquil. This is not the case with the existing larger parshall flume. The reason that the larger parshall flume does not accurately measure the flowrate during low flow periods is due to a trough, or wake, in the water surface that is caused by turbulence upstream of the flume. The trough results in an artificially low water level at the location of the bubble level measurement device. 5.1 Parshall Flume Improvement Alternatives To increase the accuracy of the influent flow measurement, several alternatives were developed Provide a New Influent Collection Vault A new influent collection vault would be located upstream of the existing influent vault. The purpose of the vault would be to improve the inlet conditions to the flume by reducing turbulence upstream of the flumes. However, the vault would not mitigate the turbulence caused by the isolation gates upstream of the flumes. In addition, the vault would require that the existing odor control system be relocated. The estimated construction cost of this alternative is approximately $100, Relocate the Bubbler Level Device The bubbler level device would be relocated to the other side of the flume. The new location would improve flume accuracy because it would not measure the water level in the trough. However, the water level in the flume would remain uneven across the width of the flume, and would still be inaccurate to some extent. If a new bubbler level transmitter is provided, the estimated construction cost of this alternative is approximately $5,000. Alternatively, an ultrasonic level transmitter could be provided for the same cost. DRAFT February

8 5.1.3 Provide a Channel Flow Measurement Device. Install a level and velocity measurement device upstream of the parshall flume. This type of device (e.g. ISCO Laserflow) uses a level instrument to calculate the area of the flowpath and a velocity measurement device (laser or Doppler) to measure the velocity of the water in the flow path at several different locations. This type of device would minimize the error caused by the poor inlet conditions but would not be as precise as a well designed parshall flume installation. The estimated construction cost of this alternative is approximately $15, Replace Parshall Flumes with Magnetic Flowmeters in the Influent Pump Station Another alternative is to provide magnetic flow meters on the three pump station discharge pipelines and on the grit pump in the influent pump station room. The totalized flow from the three flowmeters would approximate the influent flowrate to the WWTP. The Regional Water Quality Control Board (RWQCB) would have to approve this change to the flow measurement location. The approximate construction cost for the magnetic flow meters is approximately $75, Recommendation Relocating the existing bubbler is the least cost alternative and should improve the accuracy of the large parshall flume. This alternative should be implemented as a first step; if the new bubbler location does not improve the accuracy of the meter, then another alternative, such as the channel flowmeter or the magnetic flow meter option, should be considered for implementation. 6.0 SCREENINGS REMOVAL Screenings removal at the front end of a wastewater treatment plant greatly improves reliability and reduces maintenance of downstream facilities. Effective screenings removal helps prevent clogging of pumps and pipelines, prevents buildup of rags on mechanical equipment such as mixers and sludge collection mechanisms, reduces accumulation of materials in downstream channels, and eliminates many of the existing maintenance activities downstream of the headworks. Both of the existing mechanically cleaned bar screens are front discharging bar screens with 3/4- inch bar spacing. The screens do a poor job of removing debris from the raw wastewater. After 30 years of service, they are also well past their useful lives and are due to be replaced. The bar screens are also unique in that they are front discharging screens, while most modern screens discharge the screenings to the rear of the screens. In the following section, screenings removal design criteria and technologies are discussed. In addition, recommendations and alternatives to improve the existing screenings facilities are presented. DRAFT February

9 6.1 Bar Screen Opening Size Selecting an opening size is important in establishing the design criteria for the screenings facilities. Screen opening size impacts screenings removal efficiency, dictates the size of screens, and affects plant hydraulics. In addition, screenings handling equipment needs to be sized to match the anticipated removal efficiency of the screen. Smaller opening sizes (or closer bar spacing) will remove more solids and provide greater protection for downstream equipment. Potential reductions in maintenance include reduced ragging of downstream pumps including influent pumps and primary sludge pumps. Removing inert material at the headworks reduces the buildup of this material in the trickling filter media and the digesters as well as reducing the frequency of digester cleaning. The removal of plastic material from the solids stream will also improve the quality of biosolids for reuse. The existing bar screens have a 3/4-inch spacing. The current industry trend is to install screens with smaller openings or spacing between bars in order to capture greater quantities of solids at the headworks and reduce downstream maintenance costs. Our experience indicates that the two most popular bar screen opening sizes used are 3/8 inch and 1/4 inch. A 3/8-inch bar spacing effectively removes most plastics and rags that can cause plugging problems but does not capture as much debris as a bar screen equipped 1/4-inch bar spacing. While a 1/4-inch bar spacing removes more debris, the small bar spacing results in more headloss across the stream. In addition, the debris captured by a screen with 1/4-inch bar spacing often contains organic matter which can cause odor issues. 6.2 Clear Screen Velocity The clear screen velocity (velocity through openings or between bars) is the primary criteria used to determine the number of screens required. Recommended values typically range between 2 and 5 feet per second (fps). For new screening facilities, we generally recommended that a clear screen velocity of 2 to 3 fps at average flows and 4 fps at peak hour flows be used (for multi-rake screens). These velocities, which are measured on the backside of the screens, provide efficient removal of screenings and limit the amount of solids that settle out in the screen channels. Because the new screens are fitting in existing channels, the screen widths are fixed. In addition, the downstream water elevations are also fixed by the influent pump station wet well water levels and the weir on the downstream side of the vortex grit basin. Therefore, the only variable that can be changed to comply with the recommended clear screen velocities is the bar spacing. A hydraulic analysis was performed to determine the clear screen velocities at the design flowrates for the existing screens as well as new screens with 1/4-inch and 3/8-inch bar spacing. The clear screen velocities for each condition are shown in Table 2. The table shows that the clear screen velocity at 32.7 mgd is 6.6 fps, which is well above recommended values. A 3/8-inch bar spacing has a clear screen velocity of 5.4 fps at 32.7 mgd. DRAFT February

10 This velocity is marginal but would occur rarely. The headloss across the screen at this flowrate is approximately 4.1 inches which is within the design requirements for the existing headworks. Therefore, a bar screen with a 3/8-inch bar spacing is recommended. It is important to note that a clear screen velocity in the range of 4-5 fps is only recommended for a multi-rake style screen that is continuously cleaned. Even with a multi-rake screen, at a flowrate above 21 mgd (~ 4 fps), screening performance will decrease. For a single rake type of screen, with a relatively long cleaning cycle, the recommended clear screen velocity is 3 fps. Table 2 Condition Clear Screen Velocity in Feet per Second Headworks Improvement Project Sewerage Authority of Southern Marin Average Dry Weather Flow (1) 2 mgd Maximum Day Flow (2) 24.7 mgd Peak Hour Flow (2) 32.7 mgd Existing Screens New Screens with 3/8-inch spacing New Screens with 1/4-inch spacing Notes: 1. With one screen in operation. 2. With both screens in operation. 6.3 Screenings Volume Because the new bar screens with smaller bar spacing will remove more screenings than the existing screens, it is important to estimate the anticipated screenings volume to size the new conveyor and washer compactors. The existing screening removal volumes based on 3-years of data provided by SASM are presented in Table 3. Because the grit and screenings are comingled in a common dumpster, the screening removal volumes were adjusted based on an assumed grit removal rate. The data shows that, on average, approximately 6.2 cubic feet is removed each day. The maximum month average daily removal rate is approximately 9.3 cubic feet per day. When compared to industry guidelines from the Water Environment Federation s (WEF) Manual of Practice (MOP), the screenings volume produced from SASM is on the lower end of the range. Table 4 shows screening volumes for a range of wastewater treatment plants with 3/4-inch, 3/8- inch, and 1/4-inch screens. As shown in the table, at the lower limit for a plant with a 3/4-inch screen, 2.6 cubic feet of screenings per million gallons (cf/mg) is produced. This is similar to the DRAFT February

11 average screenings volume production at SASM s WWTP, which is 3.1 cf/mg. Accordingly, it can be expected that in the future, when finer screens are installed, that screenings will remain at the lower end of the typical range of screenings volume production Table 3 Screenings Volumes Based on SASM Data ( ) Headworks Improvement Project Sewerage Authority of Southern Marin Average Monthly Screenings Weight of Combined Screenings and Grit (tons/month) Weight of Screenings (tons/month) (1) Volume (2) (cf/mo) Volume (cf/day) Volume (3) (cf/mg) Minimum Average Maximum Notes: (1) Assumes that 1.0 cf/mgd of screenings is removed per day at an average flowrate of 2 mgd with a grit density of 100 lb/cf. This is equivalent to 3 tons of grit removed per month. (2) Based on screenings density of 45 lb/cf (3) Based on influent flowrate of 2 mgd. Table 4 Monthly Screenings Typical Screenings Volume from Wastewater Treatment Plants Headworks Improvement Project Sewerage Authority of Southern Marin 1/4 inch screen (6 3/4 inch screen 3/8 inch screen mm) cf/mg cf/mg cf/mg Lower Limit Average Upper Limit Note: (1) Screenings volume based on Figures 11.1 and 11.2 of WEF MOP. Because the screenings volume production rates are average rates, a peaking factor is normally applied to accommodate peak hour or first flush screenings volumes. Per the MOP, the peaking factor can vary between 4 and 15. Table 5 shows the screenings volume for each of the three screen opening sizes with a range peaking factors applied. DRAFT February

12 Table 5 Required Washer/Compactor Capacity with Several Peaking Factors Applied Headworks Improvement Project Sewerage Authority of Southern Marin 3/4 inch screen (cf/hr) 3/8 inch screen (cf/hr) 1/4 inch screen (cf/hr) Monthly Screenings PF = 4 PF = 10 PF = 15 PF = 4 PF = 10 PF = 15 PF = 4 PF = 10 PF = 15 Lower Limit Average Upper Limit Note: PF = Peaking Factor Recommendation It is recommended that the screenings handling equipment be designed for a screenings production rate of approximately 60 cf/hr. This assumes that screens with 3/8-inch bar spacing will be installed, a lower limit screenings production rate, and a peaking factor of 15. This size would also be adequate for screens with 3/8-inch bar spacing, an average screenings production rate, and a peaking factor of 10. Vulcan manufactures the EWP washer compactor that has a rated capacity of 99 and 159 cf/hr for the EWP 250 and EWP 300, respectively. Huber manufactures the WAP washer compactor which has a rated capacity of 70 and 140 cf/hr for the WAP 2 and the WAP 4, respectively. Both of the lower capacity units would be appropriate for this project. 6.4 Screenings Technology Alternatives To select the best available technology for the proposed Headworks and Solids Handling Building, a number of screening technologies including perforated plate screens, band screens, step screens and screens that utilize bar racks were considered. Based on this initial pre-screening, it was decided that perforated plate screens, band screens and step screens would not be considered further for a number of reasons including: All three types of screens feature lighter duty construction than screens that utilize bar racks. All three types of screens have more submerged moving parts and consequently increased O&M requirements. DRAFT February

13 Further evaluation of three types of screens that utilize bar racks is provided below: 1) climber screens, 2) chain driven multi-rake screens, and 3) link driven multi-rake screens Alternative 1 - Climber Screens Figure 2 shows a typical climber bar screen. Climber screens have a reciprocating rake mechanism that maintains all drive components out of the flow stream under normal operating conditions. When a cleaning cycle is initiated, the cogwheels, housed in the frame of the bar screen, move down stationary pin racks, also housed in the frame. The drive assembly descends from its stopped position with the rake arm extended. When the cogwheels reach the bottom, they rotate around the bottom of the pin rack, engaging the rake teeth with the bar rack. As the cogwheels walk back up the pin racks, the screenings are carried out of the wastewater flow and are discharged into screenings conveyance equipment or bins located behind the bar screen. The involute gear and pin rack system is a proven and reliable system with many successful installations. The overall height of a climber-type bar screen is determined by both the channel depth and the height of the discharge point above the operating floor. This can result in screens that are tall and more difficult to maintain. Some options to minimize the height of climber screens are to specify motors or motor housings that allow the drive to be submerged and to minimize the height of the discharge point. The minimum recommended bar spacing for climber screens is 3/8 inch. At spacing below this dimension, the rake may experience difficulty engaging the bar rack and the rake teeth may be damaged Alternative 2 - Chain Driven Multi-Rake Screens Another screen technology suitable for replacement of the existing screens are heavy duty chain driven multi-rake screens shown in Figure 3. These screens are chain-and-sprocket-type bar screens with multiple rake bars mounted onto chains on both sides of a self-contained frame. The two manufacturers Carollo would specify use a design with a lower sprocket assembly located in a recess at the bottom of the frame. The bearings for the lower sprockets are made of self-lubricating polyethylene material with a ceramic collar bonded onto the sprocket stub shaft. The all stainless steel chains are roller-type, water lubricated, and designed for continuous submerged duty. The screens are configured so the rakes clean and return in front of the bar rack to prevent carry over of material to the downstream channel. A two-speed drive can be provided so that raking speed can be increased to accommodate high flows or high volumes of screenings in the influent flow stream. The features of this type of screen make it capable of accommodating smaller bar rack spacing (down to 1/4 inch) than climber screens Alternative 3 - Link Driven Multi-Rake Screens Link driven multi-rake screens considered for this project are manufactured by a single manufacturer, Duperon. Figure 4 shows a typical Duperon FlexRake screen. Similar to chain driven DRAFT February

14 multi-rake screens, this type of screen is also a front raked, front return type screen with multiple rake bars mounted onto chains on both sides of the channel. However, there are no lower sprockets and the parallel chains serve as their own frame. The chains are constructed from 316 stainless steel castings that form a bar-like chain that bends in only one direction, providing both flexibility and rigidity Screenings Technology Alternative Comparison Table 6 summarizes the key advantages and disadvantages associated with each screenings technology alternative. Replacing the existing screens with chain driven multi-rake screens is recommended, primarily for the following reasons: Multiple rakes and adjustable rake speed allow the rakes to respond more rapidly to high screening loading events than a climber screen. The height of the climber screen may not be compatible with the existing structure. The link driven multi-rake screen has limited operating experience in wastewater applications and can only be provided by a single manufacturer. DRAFT February

15 End of Travel Limit Switch Tie Member Side Frame Reverse Motion Limit Switch Pin Rack V Power Cable Support Wiper Assembly Junction Box for Incoming Service Cable Guide Base Frame Apron with Discharge Chute 1 1/2 Grout (By Others) 45 A C Bottom Pin LC Drive Motor Dead Plate T W Flow Rake Arm Floor Opening Bar Rack Anchors Lf Figure 2 TYPICAL CLIMBER BAR SCREEN sle309f ai

16 Drive Upper Sprocket Discharge Chute Dead Plate SST Chain Rakes Bar Rack 75 Lower Sprocket Source: Headworks, Inc. Figure 3 TYPICAL CHAIN DRIVEN MULTI-RAKE SCREEN sle309f ai

17 Bar Screen No Lower Sprocket, Bearings or Shaft Figure 4 DUPERON FLEX-RAKE SCREEN sle309f ai

18 6.4.5 Other Retrofit Considerations Because the screens must fit within the existing structure, several other key considerations need to be incorporated into the screen replacement project. The existing skylights in the screenings room do not line up with the screens below. New skylights are required to allow the new screens to be installed. The structure is built around vertically mounted bar screens. Chain driven multi-rake screens can be installed in a vertical position but remove more screenings when installed at a slight angle. Structural retrofit options are described in later sections. The concrete channels may need be modified to accommodate the new screens. The concrete in the wet wells and channels is corroded and needs to be repaired and coated. 6.5 Screenings Washing and Dewatering In addition to rags and inert material, at the recommended bar spacing of 3/8 inch the screens will remove fecal and other organic matter resulting in increased quantities of odorous material that needs to be handled. Although SASM does not currently wash the screenings, by washing screenings the organic matter can be separated from the inert screenings before dewatering and disposal. Lowering the organic content will reduce odor generation, improve dewatering and reduce the weight, volume, and cost of materials hauled Dewatering Requirements Most landfills require a minimum solids content of 50 percent for material to be landfilled directly other than sludge. However, the dewatering technology available for wastewater screenings has not reliably proven an ability to meet the 50 percent solids requirement and the requirement is not always enforced by landfills. Collection contractors and landfills usually accept material as long as no noticeable free water is present and the material passes a filter paper test. In this case, the screenings, if washed, will have less water and organic matter than the existing screenings so disposal should not be an issue. DRAFT February

19 Table 6 Comparison of Screening Technology Alternatives Headworks Improvement Project Sewerage Authority of Southern Marin Alternative Advantages Disadvantages Alternative 1 - Climber Screens Alternative 2 - Chain Driven Multi-Rake Screens Alternative 3 - Link Driven Multi-Rake Screens No submerged moving parts. Proven and reliable technology. Maintenance can be performed above deck level so channel entry is not necessary. Multiple rakes and continuous operation facilitate frequent cleaning passes and rapid screenings removal. Can reverse directions several times to clear obstructions. Reduced height of equipment above discharge point compared to climber screens. Two-speed drive improves removal rates at peak flows. No frame, underwater sprockets, bearings or shafts. Multiple rakes and continuous operation facilitate frequent cleaning passes and rapid screenings removal. Reduced height of equipment above discharge point compared to climber screens. Maintenance can be performed from deck level by pulling chains out of channel. Height of equipment above discharge point makes maintenance and installation more difficult. Permanently submerged moving parts (chains and sprockets). Maintenance of chain and bottom sprockets require channel access. Fewer number of installations compared to other screen technologies. Sole source procurement is required Washing and Dewatering Technologies Screenings washing and dewatering can be accomplished in a single piece of mechanical equipment called a washer/compactor. Washer/compactors are available from several manufacturers and two basic types of units were evaluated; washer/compactors with water sprays and washer/compactors with agitators. Figure 5 shows examples of both types of equipment. Dewatering of washed screenings in both types of units is accomplished with a screw compactor. A typical unit consists of a motor driven shafted or shaftless helical screw auger, a loading hopper, and discharge tube. Turning the auger pushes the wet screenings material from the loading hopper into the discharge tube, which is tapered for a short length. The reduction in the discharge tube diameter causes a squeezing action that removes the free water from the screenings material. The DRAFT February

20 water drains out of the unit through a perforated screen in the bottom and is returned to the flow stream for further treatment. Washer/compactors that use water sprays typically apply high pressure utility water to the sides of the compactor loading hopper and to the washing zone of the compaction unit. Some units also spray water from the center shaft of the auger. Water sprays have limited success in breaking up organic matter unless the washer unit also conducts batch washing process where the auger cycles backward and forward a number of times to break the organic material apart. This type of equipment is capable of producing dewatered screenings material with a solids content of 30 to 40 percent. Washer/compactors with in-tank agitators use mechanical agitation and a large volume of water to wash screenings. The loading hopper is first flooded with water in a batch process. The agitator breaks up the organic matter so that it can be carried out of the machine with the wash water through a fine perforated screen. The cleaned screenings material is then dewatered and discharged. Some of the more successful units are capable of reducing the weight and the volume of the raw screening by 50 to 80 percent and achieve solids content in excess of 40 percent. Carollo recommends the use of a washer/compactor with water sprays that can operate in a batch washing mode. Although washer/compactors with in-tank agitators can produce cleaner screenings materials, these units also have a higher capital cost, require more complicated control systems, and require a higher degree of operations and maintenance attention. DRAFT February

21 Drive Assembly Discharge Tube Screw Washwater (Typical) Drain Washer Compactor with Water Sprays Source: Vulcan Washer Compactor with Agitator Source: Hycor Figure 5 TYPICAL WASHER/COMPACTORS usd710f ai

22 6.6 Screenings Conveyance Conveyance technologies that can be used for screenings conveyance include belt conveyors, hydraulic sluiceways and shaftless screw conveyors. The existing belt conveyor has functioned well for SASM. However, belt conveyors can be fairly messy and require more O&M attention than screw conveyors. Screenings material placed on conveyor belt can be difficult to contain as screenings will spill over the sides of the conveyor. A hydraulic sluiceway for screenings conveyance is also not recommended due the amount of flushing water required. Sluiceways typically require 300 to 400 gpm amount of clean, chlorinated flush water to convey screenings properly. Although this flow would be used in a screening washing process, it would then need to be returned to the influent flow stream and re-routed through the plant process at a cost. By comparison, a typical washer/compactor fed by a screw or belt conveyor would consume approximately 10 to 20 percent of the volume needed in a sluiceway application. Shaftless screw conveyors are a simple, proven and reliable technology for screenings conveyance. The equipment consists of a hardened steel spiral installed in a stainless steel U- trough and driven by a motor and gearbox. The spiral rests on a replaceable ultra-high molecular weight polyethylene liner in the bottom of the U-trough. The screw conveyor is provided with removable stainless steel covers that completely enclose the screenings, containing the odors and preventing material from spilling out of the conveyor. Carollo recommends the use of either shaftless screw or belt conveyors for screenings conveyance, although shaftless screw conveyors have a smaller footprint and may be better suited for the tight screenings room. 6.7 Alternatives Five alternatives to replace and improve the existing screenings system. In addition, a sixth alternative, full replacement of the existing headworks was also developed for comparison purposes. The alternatives include: Alternative 1: New screens, new screenings washer compactors, new grit classifier, and rolling disposal bins. Alternative 2: New screens, shaftless screw conveyors, and combined dumpster for biosolids, screenings, and grit located outdoors under a canopy. Alternative 3: New screens, new screenings washer compactors, new belt conveyor, and new grit washer compactor located in the belt filter press room. Alternative 4: New screens, new screenings washer compactors, new belt conveyor, and new grit washer compactor located in screen room. DRAFT - February

23 Alternative 5: New screens, new screenings washer compactors, new belt conveyor, and new grit classifier. The location of the screens and flumes would be swapped (relative to their existing locations) Alternative 6: A new headworks facility located adjacent to the west side of the existing primary clarifiers Alternative 1 This alternative includes replacement with new screens, as recommended previously, new screenings washer compactors, and screenings disposal with a dedicated rolling bin to capture the washed and compacted screenings. A new grit classifier would also be installed in the screen room. A rolling bin would be provided to capture the washed grit as well. A motorized pallet jack would be used to move the rolling bins outside. Operations staff would use a forklift to dump the bins into a larger dumpster outside at regular intervals. Figures 6 and 7 show visual depictions of Alternative 1. One advantage of this alternative is each screen will have a dedicated screenings water compactor, which will provide redundancy and simplify the control of the screen and screenings water compactor. The screen and washer compactors will be almost fully enclosed, which will minimize the potential for messes. Another advantage is that screenings conveyance, via a belt conveyor or shaftless screw conveyor, is not required. The primary disadvantage of this alternative is that it requires additional attention and handling from operations staff. Because the bins are smaller than the existing dumpster, the bins will have to be emptied more often than the existing dumpster. Another major disadvantage is that access to the screens and washer compactors is restricted and maintenance will be difficult. A third disadvantage of this alternative is that a third dumpster must be located outside. However, this disadvantage can be mitigated by using the automatic bagging system on the screenings washer compactor to reduce odors Alternative 1A This subalternative is the same as Alternative 1 except the wall behind the screens is relocated to the south approximately 5 feet. This provides additional room for maintenance access to the screens and the screenings washer compactor. This additional benefit is not without cost. Major structural retrofit work will be required, including installation of a new beam under the new wall. In addition,two new columns and a beam will be required to frame the new opening in the existing wall. The cost of the additional work is approximately $250,000, including structural design and construction costs. Figure 8 shows a visual depiction of this subalternative. Another option is to remove the portion of the wall behind the screens and connect the Screen and Solids rooms. This would still require complex structural work but the cost would be approximately 50% less. Under this scenario, all of the equipment in the solids room, including the belt filter press, would have to be brought into compliance with NFPA 820. DRAFT - February

24 Figure 6 Alternative 1 Conceptual Plan DRAFT - February

25 Figure 7 Alternative 1 Section View Figure 8 Alternative 1A Partial Plan Alternative 2 This alternative includes installing a new shaftless screw conveyor to convey the screenings from the new screens to a dumpster located under a canopy to the east of the building. The canopy would shelter the dumpster from the elements and could be outfitted with solar panels to offset the WWTP s electricity usage. The grit classifier would also be located under the canopy. Because the dumpster location would block access to the belt filter press room, the existing shaftless screw conveyor would be replaced with a longer conveyor that would convey the dewatered biosolids to the dumpster located under the canopy. In order to provide access to haul the dumpster off-site, the landscaped area to the southeast of the Headworks building would have to be paved over. Figures 9 and 10 show visual depictions of Alternative 2. The advantages of this alternative are that only one disposal bin is required, more room is available in the screenings room for maintenance activities, and the project would generate renewable energy from the solar panels on the canopy. There are several disadvantages to this alternative. Most importantly, the screenings, grit, and biosolids would be combined in one dumpster. This means that the biosolids could no longer be DRAFT - February

26 used for land application. Also, the dumpster would be located outdoors, which may lead to increased odor complaints. Figure 9 Alternative 2 Conceptual Plan Figure 10 Alternative 2 Section View DRAFT - February

27 6.7.3 Alternative 3 This alternative is similar to Alternative 1A except a belt conveyor will be used to convey the washed and compacted screenings to a dumpster inside the room, similar to the existing setup. The new grit classifier would also be installed in the biosolids room to provide additional space in the screen room. Figure 11 shows a visual depiction of Alternative 3. One advantage of this alternative is each screen will have a dedicated screenings water compactor, which will provide redundancy and simplify the control of the screen and screenings water compactor. The screen and washer compactors will be almost fully enclosed, which will minimize the potential for messes. The primary disadvantage of this alterantive is that the grit would be discharged to one of the biosolids bins. If this bin is also used for biosolids disposal, those biosolids could not be land applied or used as daily cover at a landfill. Figure 11 Alternative 3 Conceptual Plan DRAFT - February

28 6.7.4 Alternative 4 This alternative is identical to Alternative 3 except the new grit classifier would be installed in the screen room, similar to the existing installation. Figure 12 shows a visual depiction of Alternative 4. This alternative has all the advantages of Alternative 3 and keeps the biosolids isolated from the screenings and grit. It also includes new washer compactors that will provide cleaner screenings and provides room to maintain the new screens and screenings washer compactors. The primary disadvantage of this alternative is that installing the grit classifier in the screen room results in limited space to work on and around the new grit classifier and belt conveyor. Figure 12 Alternative 4 Conceptual Plan Alternative 5 Alternative 5 was developed based on input from SASM Staff. This alternative is based on the concept of switching the positions of the bar screens and the parshall flumes. The concrete channels housing the flumes and screens would be completely rebuilt. The screens would discharge to the under-utilized chemical room on the ground floor. Washer compactors would transfer the screenings from this room to a belt conveyor in the existing screenings room. This alternative is shown in Figure 13. DRAFT - February

29 Figure 13 Section View of Alternative 5. This option is an improvement in that the screenings facility becomes less space constrained upstairs. In addition, at peak flows, the hydraulics through the flumes will be improved. Another advantage is that a wider screen can be provided in the high flow channel (5 wide vs 3 wide). This will improve the screenings removal efficiency. Implementing this option is structurally feasible but would most likely require that the concrete in this area be completely removed and rebuilt. This is feasible but would require a full bypass of the headworks during construction and would likely require extensive groundwater dewatering. The main disadvantage of this alternative is that positioning the flumes downstream of the screens will likely create flow pulses through the flumes as the screens go through their cleaning cycles. It is likely that this will create a NPDES discharge permit compliance issue. In addition, when both screens are in operation during low flow periods, the influent flow to the small parshall flume will have poor approach hydraulics. Therefore, this configuration may result in decreased flow measurement accuracy. DRAFT - February

30 While the advantages are significant, the disadvantages and potential to make the flow measurement issues worse cannot be ignored. Therefore, this alternative is not recommended Alternative 6 Alternative 6 is comprised of a completely new screening, influent pump station, and grit removal facility. The dimensions of the new facility are approximately 120 ft by 35 ft. The facility would be located on the west side of the existing primary clarifiers (as shown in Figure 14). The existing shop to the southeast of the primary clarifiers would be demolished to make room for the new facilities. The shop could be relocated to the existing Headworks building after the new Headworks facility is online. Raw sewage from the two existing influent sewer lines would be intercepted by a new junction box and diverted to the new headworks through a 48-inch diameter pipe. Flow would then pass through channels fitted with the screening equipment. The screening equipment would include two 4-ft wide, chain driven multi-rake screens with bypass channel equipped with a manually cleaned bar rack. A shaftless screw conveyor and a screenings washer compactor would be provided to clean and convey the screenings. The bar screen channels would discharge into a common channel and then into two, parallel, trench type, self cleaning wet wells containing submersible pumps. The submersible pumps would be located at the bottom of a narrow trench between vertical sidewalls. The sidewalls above the trench would be steeply sloped to minimize or eliminate the accumulation of solids. The wet wells would also be equipped with motorized isolation gates to both isolate the wet well and control the inlet flow to the wet well during periodic cleaning cycles. The submersible pumps in each wet well would discharge into parallel discharge headers. The flow would be recombined and directed through a magnetic meter and into the grit basin influent chamber. The grit facility would be located immediately downstream of the influent pump station. Screened sewage from the influent pump station would be directed into an inlet riser box. Flow would then pass through an inlet channel and into the vortex grit basin for grit separation. Degritted flow would pass out of the basin, into an outlet drop box and then on to the primary clarifiers. Settled grit would be pumped from the grit collection hopper to the cyclone/classifier units to be dewatered. DRAFT - February

31 Figure 14 Potential Location of New Headworks Facility The key advantage of Alternative 6 is that a completely new screening, pump station, and grit facility is provided that will be designed to modern industry standards. The performance and longevity of the new headworks would exceed that of the retrofitted headworks. The main disadvantage of this alternative is the construction cost, which would be approximately $9,500, Recommendation If funding is not available for a new headworks (Alternative 6), Alternative 4 is recommended as it has the most advantages and the least disadvantages of the five alternatives. However, replacing the existing screening and grit equipment essentially in-kind will leave SASM with a headworks facility that does not have the capacity to operate optimally at peak flow rates. While the new equipment and improvements will improve the performance of the facility, the improvements will be incremental and will not provide a modern, state-of-the-art headworks facility (as Alternative 6 would provide). While replacing the screenings equipment and making incremental improvements will be beneficial in the short term, it may be best to delay a final decision on the path forward with the headworks improvements project until after the Wastewater Treatment Plant Master Plan (Master Plan) is completed. This will allow the headworks project to be integrated into the overall master plan for the facility and will ensure that SASM s funds are allocated to the highest priority projects. DRAFT - February

32 7.0 GRIT REMOVAL SYSTEM The existing Pista-type vortex grit removal system is located between the screens and the influent pump station wet well. The screened wastewater enters a the vortex grit basin, a flat bottomed cylindrically shaped basin, turns 270 degrees, passes over a weir before entering the influent pumps station wet well. Inside the vortex grit basin, grit settles out into a chamber recessed in the center of the basin. Organics are kept in suspension by a motorized paddle that rotates just above the bottom of the cylindrically shaped basin. Grit is pumped out of the bottom of the recessed chamber by one of two grit pumps. The grit slurry is pumped to a grit cyclone and then is cleaned further in a grit classifer. The cyclone and classifier are located in the screen room. Due to the quantities of grit that accumulate in the downstream processes, including the digesters, the grit removal system is believed to be underperforming. This is consistent with the design of vortex grit basin, which does not comply with inlet and outlet design guidelines for Pista grit systems. The diameter of the grit basin does not appear well suited for either the average dry weather flow of 2.0 mgd or the peak wet weather flow of 32.7 mgd. The existing 14-ft diameter vortex grit basin is best suited for flows between 4 and 20 mgd. 7.1 CFD Analysis In an effort to improve the performance of the existing grit basin, a computational fluid dynamic analysis of the vortex basin was performed at the peak hour wet weather flowrate. The peak hour wet weather flowrate was selected because it is likely that a large portion of the grit enters the WWTP from the collection system during the first flush at the beginning of each wet weather season DRAFT - February