Pump power. Regional wastewater utility accomplishes several firsts to overcome pump station replacement challenges

Transcription

1 Pump power Regional wastewater utility accomplishes several firsts to overcome pump station replacement challenges Jessica M. W. Hou Nestled in a quiet residential neighborhood in Norfolk, Va., the Norchester Pump Station was constructed in the 1940s as one of the earliest facilities built by the Hampton Roads Sanitation District (HRSD). As the area s population grew, so too did the wastewater flows handled by this wet well/dry well facility. The pump station was built in a fairly remote area, but over the years a densely developed residential neighborhood grew around it. The pump station receives flow from 23 upstream pump stations that serve a mixture of moderately to densely populated residential developments, as well as commercial, institutional, and industrial facilities. Today, average flow is 9.8 ML/d (2.6 mgd); the projected wet weather peak flow is 40.9 ML/d (10.8 mgd), making it the third largest pump station in the HRSD system. The region s rather flat topography and relatively high water table cause runoff to enter the aged sewer system through cracks and joints during wet weather events, resulting in significant infiltration/ inflow. This, combined with extreme microbial corrosion of the Norchester Pump Station s concrete, had taken its toll on the system and further repairs to the existing wet well were not recommended. Simply put, a new pump station was urgently needed. HRSD tapped national wastewater engineering leader Gannett Fleming to design a station that would effectively manage Influenced by the architectural styling of surrounding homes, the Norchester Pump Station is a model for best practices in pump station aesthetics as it blends in with the community. Gannett Fleming JANUARY 2018 l WE&T

2 average day and wet weather conditions. This design presented many challenges, such as the application of new techniques, control of significant odors, a complicated bypass plan, vibration monitoring, and consideration of the unique needs and architecture of the surrounding community. Meeting the standard The typical options that wastewater engineers consider when replacing a pump in diverse dry and wet weather flow conditions include designing extra storage in the wet well or selecting larger pumps. However, larger pumps often run infrequently and at lower efficiencies when managing a wide range of flows. Further, strict adherence to the Hydraulic Institute (HI) Standards the global authority on pumps and pump stations must be balanced with other design drivers like availability of space, cost limitations, and operational philosophies. Since the Norchester Pump Station would be the first new pump station in decades for this portion of the service area, HRSD chose to follow the latest HI standards and directed the designer to comply with as many recommendations as possible, considering the site limitations. The application of new techniques During the design phase of the pump station replacement, HRSD already was constructing several self-cleaning trench-type wet wells in its treatment facilities. HRSD determined that the Norchester Pump Station would be the first application of this in its collection system. Self-cleaning, trench-type wet wells are designed for efficiency, ease of operation, and reduced maintenance. They enable easy wet well cleaning an extremely important benefit in the Hampton Roads region, where grit accumulation resulting from storms can be problematic. HI standards primarily recommend a self-cleaning, trench-type wet well to create optimum hydraulic conditions for the suction side of the pumps. This arrangement supports multiple normal pumping cycles, including maintaining a consistent level in the wet well or a fill-and-draw operation. The cleaning cycle sets this apart and simplifies the process for operators as it results in a significant reduction of manual or vacuum-cleaning of the wet well. In the cleaning cycle, the water level is drawn down by the furthest downstream of the four pumps, which is recessed below the others into a sump, until flow guided down an ogee weir creates a hydraulic jump. The hydraulic jump then moves along the bottom of the wet well past the other three pump suctions, scouring settled solids and incorporating floatables until they are taken up into the last pump for complete removal from the wet well. Maintaining the ideal hydraulic design in the wet well trench requires attention to HI pump intake design features intended to eliminate or minimize accumulations of solids and reduce maintenance. The key principle is to minimize horizontal surfaces anywhere but directly within the influence of the pump inlets to enable all solids to be directed to a location where they can be removed by the pumps. Confining the suctions in the narrow trench of the self-cleaning trench-type wet well accomplishes this goal. Compliance with HI standards regarding pump piping required spacing of suction pipes in the wet well, suction piping designed to create a uniform velocity profile to the pump inlet connection, proper structural restraint of the pump, and properly supported valves and fittings. A balanced approach was required to maintain a maximum width of 12 m (39 ft) on the 20-m-wide (62-ft-wide) site while also ensuring ease of maintenance in the dry well. The final design included two 75-hp, dry-pit submersible pumps sized to handle average flow conditions. These were supplemented with two 200- hp dry-pit submersible pumps, sized to accommodate wet weather conditions. The two smaller pumps operating in tandem with one of the larger pumps would efficiently manage the design peak flow of 40.9 ML/d (10.8 mgd). Another innovative design application included a permanent diesel bypass pump located just outside the station. Traditional design standards call for the use of generators in lieu of permanent The Norchester Pump Station boasts a beautifully designed façade that fully conceals the sewer infrastructure and provides aesthetic continuity with the rest of the neighborhood. Combined with the station s self-cleaning, trenchtype wet well, pumps that can manage various flows, and advanced odor-control technology, the project shines a spotlight on best practices for the seamless blending of critical sanitary infrastructure in a residential community. Gannett Fleming WE&T l JANUARY 2018

3 Despite significant complexities, including the site s limited size, extreme weather, and overwhelming odors, the project team developed solutions that combine both function and form. Gannett Fleming bypass pumps; however, the permanent bypass pump provides the automated ability to respond to high wet well levels. Planning informed by record rainfall During the project s flow monitoring phase sensors were positioned in manholes in the street. Also during this period, a nor easter battered the area and produced record storm tides and rainfall. The street in front of the Norchester Pump Station was hit especially hard. It had some of the highest levels of flooding in the area. This caused significant anomalies in the data collected. Normally, the area sees a wide range of dry and wet weather flows. The collection system in Hampton Roads is separated, and does not handle combined sanitary and stormwater during dry weather. However, during wet weather, stormwater from cracked pipes, manhole covers, and gutter connections runs into the sewer system. This affects flows significantly. Such added volume makes it difficult for a single pump to efficiently manage the wide range of flows. After careful consideration, Gannett Fleming advanced a plan to accommodate an average daily flow of 9.8 ML/d (2.6 mgd) with a projected wet weather peak flow of 40.9 ML/d (10.8 mgd). However, the smallest pump will function efficiently at flows as low as 5.7 ML/d (1.5 mgd). Extreme odor leads to specialized control system The original Norchester Pump Station drew neighbor complaints about odors. Odor sampling during design prompted strict odor control measures. Gas detectors inside the wet well at the existing pump station recorded hydrogen sulfide (H2S) concentrations for 7 days. Detailed analysis indicated that the wet well had an average H2S concentration of 114 ppm and a peak concentration of 312 ppm. This is an extremely high concentration compared to other areas within the interceptor sewer system. The good news is that the existing closed wet well environment represented conditions that would not be encountered with the new self-cleaning, trench-type wet well design. Unlike a typical wet well, a trench-type wet well does not store solids-bearing liquid. It is designed to have continuous flow with either constant or variable speed pumps. Variable speed pumping enables the pumps discharge to equal inflow and can be used to create a water level in the wet well equivalent to the water level in the upstream conduit. Therefore, it was more reasonable to conduct odor load sampling at manholes just upstream of the existing pump station. Gannett Fleming and HRSD then deployed gas detectors in the JANUARY 2018 l WE&T

4 An advanced odor control system with a biological trickling filter and a secondary carbon scrubber prevents hydrogen sulfide and other odors from affecting neighbors. This system uses biological media to treat the sewer gases, followed by carbon to adsorb any remaining odors. Gannett Fleming immediate upstream manholes. The data showed an average H2S concentration of 64 ppm. To combat even these moderate odors, the team conducted a detailed review of the chemical, biological, and carbon-based technologies. The design included an advanced odor control system with a biological trickling filter and a secondary carbon scrubber. This system uses biological media to treat the sewer gases from the wet well. It then employs carbon to adsorb any remaining odors. In addition, these units do not use or produce any type of hazardous chemical. Biotrickling filters require low maintenance and the media change-out typically occurs at 10-year intervals. Biotrickling filters do require a potable water source to maintain the biological population. Low-pressure drops allow these units to maintain an even flow distribution throughout the media, which minimizes power costs. The vertical orientation of the vessel results in a smaller footprint, which was key for the Norchester station s small site. While such high-strength odor control measures are used in treatment facilities, they had not been applied to collection system pump stations in the region until now. The biotrickling filter installed at the Norchester Pump Station has displayed an ability to handle H2S loading with greater than 99% removal efficiency. 25-step temporary bypass plan keeps Norchester running during construction The old Norchester Pump Station had to be taken offline to build the new one. The area had no viable relocation sites. To prevent collapse during initial construction, the area above the wet well was locked out of all construction activities until it was time to demolish the underground wet well structure. However, the major challenge of managing the wastewater flow remained. To that end, the design included a significant temporary bypass plan. Essentially, a temporary pump station was constructed in the street. Since there were multiple City of Norfolk and HRSD gravity influent lines as well as influent force mains from another pump station, the sequencing of incremental transfer of flow from the existing station to the temporary wet well, as well as the transfer of the force main flows was detailed to maintain service and avoid overflows. The bypass sequencing operation required the contractor to complete 25 steps to maintain flow continuously. The temporary bypass system included a precast concrete vault that served as a wet well with multiple influent lines and three temporary bypass pumps hard-piped into the force main. The bypass system included piping and valves ranging from 25 to 61 cm (10 to 24 in.) in diameter, emergency discharge connections, WE&T l JANUARY 2018

5 To understand the level of the H2S concentration at the existing pump station, Gannett Fleming and HRSD deployed gas detectors inside the wet well to record H2S levels. After discerning that peak concentrations of 312 ppm of H2S were exacerbated by the closed wet well environment, sampling was performed upstream and yielded the above results, more indicative of the design criteria for the future odor control system, with averages of 64 ppm. tapping sleeves and valves, air release valves, and a 61-cm (24- in.) linestop. Creating the system required demolishing pipes and manholes abandoned at different points in the sequence, installing a polymer manhole and a 3-m 2-m (10-ft 6-ft) wet well as well as three short-term gravity line bypasses on adjacent streets. All short-term and long-term bypasses had to be tested for 24 to 48 hours depending on the criticality of the bypass. All bypass systems were equipped with supervisory control and data acquisition (SCADA) systems with alarms that communicated to the contractor, the bypass pump supplier, and HRSD to enable for rapid response. Vibration monitoring assesses threat of collapse and neighborhood disruption The nature of the project, including the danger of wet well collapse and the proximity to neighbors, demanded vibration monitoring during construction. A detailed preconstruction survey of buildings assessed nearby homes and a vibration monitoring plan helped minimize damage. Equipment, such as vibration monitors and survey control points, were used to inform the contractor s choice and sequence of multiple installation techniques to reduce vibration. When the project called for vibratory sheet piling, the team monitored subsurface vibrations to minimize the effect on the residences. This meant stopping piling to allow vibrations to dissipate and resettle. Vibration monitoring during construction provided the team with real-time data on the effect of construction on the site s subsurface. This enabled up-to-the-minute decisions to be made about sheet pile installation. Lastly, the sheet pile was left in place to avoid the extreme vibration that occurs when sheeting is removed. Engaged resident engineer establishes positive stakeholder relationships The project site was extremely small and tight: just 19 m (62 ft) wide with neighbors within 2 m (7 ft). The pump station s location in the heart of a residential community created unique issues related to disturbance, site access, and the availability of storage for construction materials. Given these circumstances, special emphasis was placed on public involvement with community residents during design and construction. Public meetings, in-person visits to homes, and participation in community days and parades were part of a proactive approach to ensure effective and comprehensive outreach. Site access issues were mitigated by a long-term closure of the portion of the road that is directly in front of the pump station. Collaboration with the City of Norfolk s Right-of-Way Division yielded a rare approval to close Norchester Avenue for the full 2.5-year construction duration. Careful consideration was given JANUARY 2018 l WE&T

6 A rear view of Norchester Pump Station s diesel bypass pump, wet well, and odor control system. Gannett Fleming to homeowners, emergency services, and city services access to the dwellings. Multiple stakeholders were involved in the Norchester Pump Station initiative, including HRSD, the City of Norfolk, nearby Norfolk State University, the civic league, and residents. This created numerous challenges with respect to competing interests, schedule, and cost. The new pump station also blends into the neighborhood architecturally. HRSD has created a set of guidelines for its service area to ensure that pump stations match their surroundings and do not stand out to neighbors. (Read more about this program in the article, Curb appeal on p. 38.) HRSD and Gannett Fleming listened to each stakeholder s opinions, needs, concerns, and priorities, and high levels of importance were placed on communication, relationship-building, flexibility, and collaboration. Gannett Fleming provided a resident engineer to engage in regular contact with neighbors and stakeholders affected by noise, vibration, traffic disruption, and construction. The trust that was developed between the resident engineer and the facility s neighbors was invaluable to the successful execution of the project. Applications for the future The new pump station can manage extreme wet weather events and minimizes the likelihood of a sanitary sewer overflow (SSO). Because this pump station reduced the risk for SSOs, the Chesapeake Bay, the largest estuary in the U.S. and one of the world s most prized ecosystems, is safer. Designed according to HI s exacting standards, the new $6.8 million Norchester Pump Station with self-cleaning, trench-type wet well brings efficiency, ease of operation and maintenance, and cost savings. This benchmark project is applicable to wastewater utilities that face regulatory requirements for SSO reduction and infiltration/inflow reduction, severe odor control issues, and construction needs in fully developed residential communities. The innovations implemented during the Norchester Pump Station replacement can be replicated at similar sites, including new architectural guidelines for pump stations to blend with their communities. The design elements, modifications, and lessons learned during this project hold relevance for design engineers, pump station operators, and construction managers in the wastewater collection systems field. Jessica M. W. Hou serves as vice president of Gannett Fleming (Camp Hill, Pa.) and office principal for the firm s Virginia Beach, Newport News, and Richmond, Va., offices. Electronic permissions to Gannett Fleming from Water Environment & Technology January 2018 Water Environment Federation Jessica M. W. Hou, PE, BCEE, ENV SP Gannett Fleming Vice President jhou@gfnet.com or