The City of Baltimore Back River Wastewater. No more backups Reducing SSOs with headworks improvements in Baltimore

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1 No more backups Reducing SSOs with headworks improvements in Baltimore Brian Balchunas, Meredith Welle, Jason Kerns, Gurminder Singh, Misrak Shiferaw, Ben Asavakarin, Thomas Demlow, and Kara Hurtig The City of Baltimore Back River Wastewater Treatment Plant is improving its headworks to eliminate hydraulic restrictions at the facility. By doing so, it will reduce sanitary sewer overflows (SSOs) in the collection system. A large part of these improvements, which include the construction of new fine-screen grit and equalization facilities, is a new 2.3 million-m 3 /d (600-mgd) influent pumping station that will provide a hydraulic break between the sewer outfall and headworks facility. The problem The Back River Wastewater Treatment Plant serves approximately 1.3 million residents in a 363-km 2 (140-mi 2 ) area comprising Baltimore City and Baltimore County. Currently, the facility treats 680,000 m 3 /d (180 mgd) of wastewater and can handle peak flows of more than 1.5 million m 3 /d (400 mgd). Wastewater enters the Back River facility through two large conduits: a m (11-12-ft) outfall sewer and m (10-11-ft) outfall relief sewer. Influent directly enters the facility headworks, which consist of fine screens and grit removal, before entering primary sedimentation tanks. Because the grit tank effluent weirs are several feet higher in elevation than the outfall sewer inverts, a backwater effect is created for several miles into the collection system. This backwater effect reduces sewer capacity for several miles, which has led to SSOs. In addition, the backwater effect has contributed to reduced flow velocity in the collection system, which has resulted in significant grit deposition. 36 WE&T l FEBRUARY 2016 l

2 The project team built a 1:8 scale physical model of the influent junction chamber, influent conduit, coarse screen facilities, wet wells, and all eight influent pump suction lines. Brian Balchunas In 2002, the City of Baltimore entered into a wet weather consent decree with the United States and the State of Maryland that requires the city to convey and treat modelpredicted future wet weather flows. Although the facility s flow restrictions are not part of the consent decree, relieving this hydraulic restriction could eliminate 80% of SSOs. By creating a hydraulic break between the collection system and Back River facility, the city s hydraulic modeling team predicts that influent flows will increase to 2.3 million m 3 /d (600 mgd) during wet weather. Design solution New headworks facilities will be located in a greenfield area of the 1.9 km 2 (466-ac) site (see Figure 1, p. 38). To remove the hydraulic restrictions, flow will be intercepted from the outfall sewers with a new influent junction chamber (IJC), creating a free-fall from the outfall sewers to downstream facilities, including the influent pumping station. Flow from the IJC will be conveyed through a coarse-screening facility to protect the influent pumping station. Because wet weather flows into the influent pumping station could exceed the capacity of downstream treatment processes, new equalization facilities also are part of the project, including a 11,000-m 3 (3 million-gal) rectangular equalization tank, a 1.5 million-m 3 /d (400-mgd) equalization pumping station, and two above-grade circular equalization tanks with a total storage capacity of 125,000 m 3 (33 million gal). Influent pump station The new influent pump station, which will be operating full time, will be the key to lowering the upstream hydraulic grade line, thereby decoupling the outfall sewers and the collection system from the facility. The operating level of the influent pumping station was set to maintain a free-fall from the existing influent conduits under all conditions. Flow from the new IJC and outfall sewers will be conveyed to the new influent pump station via a rectangular influent conduit. Several design criteria (see table, below) were used to perform hydraulic analyses and select equipment and pumps. Influent pump station design flows Condition Flow (m 3 /d [mgd]) Minimum hourly 3.5 [80] Average daily 5.9 [135] Maximum hourly 2,300,000 [600] Maximum design 2,850,000 [752] l FEBRUARY 2016 l WE&T 37

3 Figure 1. New facilities at the Back River headworks Suction piping for the influent pumps will enter the basement level from the wet wells, and influent pumps will be located on the pump level. The discharge piping will discharge to the new fine screen facility. An intermediate bearing platform between the pump floor and the first floor will allow staff to access each pump intermediate bearing for maintenance. The influent pump motors will be located on the first floor. Floor openings strategically located on the first floor will allow equipment removal on the lower levels of the facility using an 18.1-Mg (20-ton) bridge crane. A pump station layout is presented in Figure 3 (p. 39). The minimum and average flows were based on plant historical data. The maximum hourly flow criterion was derived using model predictions of storm events. The influent pump station will house new coarse screens, influent pumps, bypass to the new equalization basins, a new headworks operations and controls center, office space, locker rooms, and training facilities. Coarse screens. Flow to the influent pump station will first pass through four 760,000-m 3 /d (200-mgd) mechanical coarse screens on the south side of the station. Each coarse screen will have a dedicated screening channel. A winch pulley system and dumpster rails will remove and replace dumpsters that collect coarse screenings. The coarse screens are arranged in a staggered manner to facilitate the removal and replacement of full/empty dumpsters. Overflow weirs. Overflow weirs, located at the entrance of the coarse screen influent channels and along the west wall of one wet well, will divert flows to the equalization facilities in case of coarse-screen blockage or pump station failure. These weirs can convey up to 1.5 million m 3 /d (400 mgd) to the equalization facilities and subsequently downstream to existing primary clarifiers. Wet wells. Flow from the coarse screening channels then will enter two self-cleaning trench wet wells. Each wet well will house the suction for four 380,000-m 3 /d (100-mgd) influent pumps. The wet wells are approximately 3.4 m (11 ft) wide, 28.7 m (94 ft) long, and 15.2 m (50 ft) deep at the deepest point. Pumps. The facility was designed in compliance with 2012 Hydraulic Institute pump intake design standards. Centrifugal pumps with a vertical arrangement were selected for this pumping application. There will be a total of eight influent pumps, four dedicated to each wet well. The influent pump station facility has three main levels: basement level, pump level, and the first floor. A 3-D view of the influent pump station first floor plan is provided in Figure 2 (right). Pump operation Each influent pump will be provided with a variable-frequency drive (VFD) to handle influent flow fluctuations. The pump VFDs, electrohydraulic actuated plug valves on the pump discharge, motor actuated valves on the pump suction, and motor actuated slide gates that isolate the two wet wells will be controlled automatically with the distributed control system (DCS). Four main modes of operation will be used for the system: normal operation, wet-well highlevel operation, wet-well cleanout cycle, and manual operation (for emergencies). Operators will be able to select the mode of operation at the DCS. The DCS also will control the minimum and maximum speed setpoints based on the best efficiency point for the selected pumps. Two types of level elements (radar and ultrasonic) located in each wet well will monitor the liquid level constantly. The DCS will be programmed with level setpoints that will control pump operation and trigger level alarms. The system will maintain a constant wet well level setpoint by automatically sequencing the number of pumps operating and the speed at which the pumps operate. Influent flow will serve as a backup method for controlling pump operation. During average flows (300,000 to 680,000 m 3 /d, or 80 to 180 mgd), Figure 2. Influent pump station 3-D view 38 WE&T l FEBRUARY 2016 l

4 This physical model represents the downstream view of the coarse screen channel. Screens were arranged in a staggered manner to facilitate efficient cleaning and replacement. Brian Balchunas two to three pumps normally will be in operation. During wet weather, the DCS will place more pumps in service as needed automatically. The second wet well will be placed in service when the primary wet well can no longer maintain the set level. When a wet well is taken out of service, an operator will oversee the wet well cleanout cycle. The cleanout pump, the last pump in either wet well, will run at 100% speed until the suction pressure drops below an operator-adjustable level, indicating that the wet well has been pumped dry. At that point, the cleaning cycle will end, and the wet well influent slide gate will close fully and the wet well returns to normal operation. Flow from the coarse screen channels to the two trench-type wet wells and into the pump suction piping was evaluated by assessing free-surface and subsurface vortices, flow pre-swirl, flow-patterns, flow distribution, and the time-averaged velocity fluctuation at the pump impellers. Nonuniform flow phenomena can lead to fluctuating loads on the pump impeller, vibration, accelerated bearing wear, cavitation damage, and a reduction in pump efficiency. Similarly, vortices can influence pump operation and life due to vibration, increased bearing wear, and potentially fatigue failure of the pump components. Strong surface vortices are capable of entraining air, which could cause a reduction in pump capacity and potentially the loss of prime. The model study determined that flow plunging from the existing conduits into the IJC resulted in significant air entrainment and nonuniform flow velocities exiting the junction chamber into the coarse-screen channel. However, the length of the influent channel and coarse-screen channel allowed the entrained air to vent prior to the wet well entrance. All flow through the coarse-screening channels was directed downstream with no areas of stagnation where solids could deposit. Testing on the original design showed that flow velocities approaching Screens No. 2 and 3 (the middle Figure 3. Influent pump station layout Modeling and results Both computational fluid dynamic (CFD) modeling and physical modeling were completed during design. CFD modeling was used to assess the flow through the IJC, coarse screen facility, and wet wells. CFD modeling identified flow imbalances and flow patterns that could influence both coarse screen and the influent pump performance. A 1:8 scale physical model was created of the IJC, influent conduit, coarse screen facilities, wet wells, and all eight influent pump suction lines. The objective of the physical model was to determine if the proposed pump station design could provide acceptable flow to the pumps in accordance with Hydraulic Institute standards (ANSI/HI ) and, if required, to develop and test modifications to the pump station layout to produce acceptable flow. The physical model also assessed the flow approaching the coarse screens and confirmed the cleanout cycle performance for each wet well. Note: A full-sized PDF of this figure can be viewed on WE&T s online version at l FEBRUARY 2016 l WE&T 39

5 The physical model helped to confirm that the design could provide acceptable flow to the pumps. It also enabled development and testing of modifications to the pumping station layout to produce acceptable flow. Brian Balchunas channels) were less uniform than those in the outer channels. Individual point velocities varied by much as 35%. Within the wet wells, the initial design was subject to unacceptable, well-organized (Type 2) subsurface vortices forming from the wet well floor and end walls and entering the last pump in each wet well. In addition, the flow pre-swirl angle and velocity fluctuations measured at the plane of the pump suction flange were outside the specified criteria. The initial design was modified to produce a more uniform distribution of flow to the coarse screens and wet wells and to improve the performance of the intake structure. Modifications to the initial design included the following: Revising an impact baffle beam to improve the flow distribution approaching the coarse screens and improve the flow approaching the wet wells. Adjusting the alignment of the outside walls at the entrance to the wet well influent channels for more uniform approach flow. Installing transition fillets upstream of the wet wells to minimize flow separation and spray at the top of the ramp during the cleaning cycle. Installing a flow splitter below the last pump in each wet well to eliminate the formation of floor vortices. Installing an end wall fillet in both wet wells to eliminate the formation of subsurface vortices. Adding three antirotation baffles along the end wall to reduce flow pre-swirl at the last pump in each wet well. With these modifications, the surface and subsurface vortices, swirl angle of the flow approaching the impeller, and velocity distribution and velocity fluctuations at the pump impeller location were all within the criteria established by the Hydraulic Institute 2012 standard. Therefore, the design is considered compliant with the standard. The flow distribution to the coarse screens with the modified impact baffle installed was also re-evaluated and found to be acceptable. Project status The city currently is finalizing procurement recommendations for the project. The project will be advertised in early 2016 with substantial completion by the end of Brian Balchunas is a vice president at the Calverton, Md., office, Meredith Welle is a project manager in the Pittsburgh office, and Jason Kerns is an associate vice president at the Newport News, Va., office of HDR Engineering (Omaha, Neb.). Gurminder Singh is an engineering supervisor and Misrak Shiferaw is a project manager at the City of Baltimore, Md. Ben Asavakarin is a vice president at Johnson, Mirmiran & Thompson Inc. (Sparks, Md.). Thomas Demlow is a principal at the Seattle office and Kara Hurtig is an associate at the North Vancouver, B.C., office of Northwest Hydraulic Consultants (Edmonton, Alberta). 40 WE&T l FEBRUARY 2016 l