Building and maintaining anaerobic digesters and sludge storage tanks in Brooklyn proves daunting
Executing projects in remote locations often presents challenges; however, building storage and processing units in areas that are densely populated can also place considerable demands on a construction team.
One project that faced significant tight construction site challenges was the erection of egg-shaped digesters (ESD) in the heart of one of the most densely populated areas in the world—Brooklyn, N.Y.
CB&I was awarded a contract to engineer, procure, fabricate and construct eight 3-million gal ESDs and two 2.4-million gal sludge storage tanks providing 11,500 cubic meters of total capacity for the Newtown Creek Waste Water Treatment Plant (WWTP). The scope of this project also included internal piping, sludge removal boxes, walkway supports, and the supply and installation of the draft tube and mechanical mixers.
The project is part of an overall upgrade of the Newtown Creek WWTP solids handling facility, which treats wastewater from parts of Brooklyn and Manhattan. “When the Newtown Creek upgrade is completed, all of New York City’s 14 wastewater treatment facilities will be in compliance with federal mandates,” said a former New York City Department of Environmental Protection commissioner at the commencement of the project. “It is a far reaching project that will contribute to the continuing improvement of the city’s harbor environments and water quality. In addition, this facility has been honored by awards for its commitment to the community through its aesthetic design.”
Field construction on the egg-shaped digesters began in July 2003. From the beginning, the team sought creative and flexible solutions for the challenges presented by the tight construction site and congested work spaces.
“One of the biggest challenges to make this a successful project was to overcome the problem of an extremely tight site,” said CB&I’s Operations Manager Mike Santoro. “We had no lay down area, and many other contractors were working around us.”
Some of the solutions found include the incorporation of recent advances in ESD technology, the use of innovative procurement methodologies, and implementation of unique and flexible construction approaches.
The ESD anaerobic system may be one of the most efficient, cost-effective digestion systems for sludge treatment, according to the manufacturer. Its double-curvature shape, small top liquid surface area and liquid mixing system help reduce scum, grit build-ups and dead zones within the vessel to maximize solids stabilization and minimize solids accumulation. This shape enhances performance and provides efficient process control so that the vessel does not have to be taken out of service for cleaning.
Recent developments in the design of the ESD resulted in the simplification of its mechanical systems, improving the life cycle benefits of these systems. Steel is the predominant material used in erecting ESD vessels in North America today, which proved to be beneficial for the Brooklyn project because it allowed for an accelerated construction schedule. The versatility afforded by a welded steel design became especially effective because the project scope called for the construction of eight vessels within the confines of the existing wastewater treatment plant.
One unique feature of the egg-shaped digesters built for the Newtown Creek project is that all the piping is located inside the vessels. Most digesters have piping on the exterior of the vessels to provide accessibility for maintenance purposes.
However, the use of internal piping on the egg-shaped digesters proved to be particularly beneficial for the Newtown Creek project as the extensive insulation system on the vessels would have made external piping less aesthetically pleasing. The internal piping in the two sludge storage tanks also had a unique design feature because the piping traveled through the center of the containers and required an elaborate pipe support system, according to Santoro.
The internal piping braces are cantilevered from the inside surface of the tank and have crossbeams that give it the illusion of a bridge. Nicknamed the “Brooklyn Bridge” by the field crew for its size and appearance, it provides the necessary support to allow for the piping inside the vessels, again contributing to the aesthetic considerations of the overall project design.
The procurement element of this project presented unique challenges, including the challenge of finding a cost-effective way to transport material to the job site, the need to find sufficient space to store materials once they arrived, and the need to have all the material on hand in time for installation but not ahead of time, due to the space limitations on site. An innovative procurement strategy was developed that effectively solved all three of these challenges at the same time. Fabrication for the project took place in Houston and because the cost of transporting the fabricated pieces by truck from Houston was not economically feasible, it was shipped by rail.
To make this possible, an arrangement was made with a storage yard/trucking company in Harrison, N.J., which was located alongside a railway. Once the fabricated pieces were completed in the Houston shops, they were shipped by rail to the adjacent storage yard, where they were unloaded and stored. Because space was so limited at the site, this also solved the second problem for the construction crew—the problem of finding enough space to store materials until they were needed.
Because the site offered no lay down area for materials, the crew had to coordinate logistics for just-in-time deliveries. To prevent the possibility of running out of material, shipments were scheduled in advance to ensure a steady supply of equipment and supplies in the yard without overcrowding the allotted work area. Each morning, the project team contacted the trucking company with a list of what was needed the following day, according to Santoro.
The trucks would be loaded during the afternoon and the material shipped at night. By traveling at night, the trucks were able to avoid any disruption of New York City traffic while moving over bridges and through tunnels, complying with permit requirements to be off the roads by 6 a.m. The materials were then unloaded at the site and often erected straight from the truck. This procedure was repeated day after day and proved to be an efficient way of solving some of the most significant challenges presented by this project.
The most prominent of the construction challenges was also a direct result of the space limitations at the job site: the team had no lay down area to assemble materials for installation. To manage this issue, a systematic approach was developed.
Because a total of eight digesters were being constructed, a procedure was established to assemble two digesters at a time. For digesters one and two, the areas inside the ring wall foundations of digesters three and four were used to assemble the materials. When the first two digesters were complete, digesters three and four were assembled using the areas inside the ring wall foundations of digesters five and six.
This assembly process worked well until the time came to assemble the last two digesters, digesters seven and eight. At this point, there was no lay down area available; a creative solution was urgently needed. To solve the problem, the crew devised a way to accomplish the ground assembly for the final two digesters—not on the ground, but 32 ft in the air. To accomplish this, the floors in the digesters were poured, and an assembly area was established on the top deck between the sludge tanks and the two digesters at the 32-ft elevation.
Although the space was very tight, there was just enough room to assemble the plates and perform the necessary welding. The crew needed every square foot of the available space to turn pieces over to finish the assembly. Another construction challenge the digester crew faced was the number of contractors working on other projects at the same tight site. Safety was paramount in the thoughts of all the contractors, and the construction activity had to be planned with the awareness that many contractors would be working in the same area at the same time.
For example, the welding procedures chosen minimized the need for scaffolding, which helped protect all workers from the various risks associated with scaffolds in such a congested work area. The welding work was primarily done using gas shielded flux cored arc welding (FCAW) and wire feeders. FCAW can be used on a wide range of steels and can also be used in all positions with a flat and smooth bead appearance. The air seams on the upper double-curved vessel rings were welded using an automated, curved track welder. Because the weld operator rode inside a cage alongside the welding head, the need for erecting scaffolding outside the vessel was eliminated. The only scaffolding used was at the round seams and were made from the steel wind girder sections, which were already attached to the digesters.
The welding group improved their efficiency by using automatic girth welders (AGW), a technique CB&I developed. AGW achieves high-quality welds and is significantly faster than manual welding.
For welds located in the extremely small area located between the top of the digester skirt and the vessel shell, welders used two mini AGWs, which were modified to run on top of the vessel without a special track. These machines were also customized to run twin wire, which saved time and labor on the project. Twin wire systems are arranged in a tandem arrangement—the lead wire produces deep penetration while the trailing wire spreads the weld pool, which is ideal for sealing the joint. This system minimizes the risk of fusion defects and porosity in the welded area, reducing the need for restoring defective welds.
Improving water runoff in Lower Manhattan
Even with all the advanced planning and scheduling, the project team had to remain flexible to accommodate the other activity in the area.
The crews on the Newtown Creek job site had to work harmoniously to establish safe working procedures for everyone in the area.
As a result, the Newtown Creek WWTP solids handling facility, the largest of 14 water treatment plants in the greater New York metropolitan area, will help provide New York City with clean waterways.
When the upgrade of the treatment facility is completed in 2007, Newtown Creek WWTP will process wastewater, producing clean water that meets all federal regulations and aids in the effort to treat wastewater from the commercial and residential areas in parts of Brooklyn and Manhattan.