| Aeration tanks and operations building at Field's Point WWTF. |
An additional challenge for the design team was to develop a construction phasing plan that would keep the wastewater treatment plant operating within required treatment limits throughout the four-year modernization and construction effort. The clear and comprehensive construction phasing plan put in place prior to construction included approaches to minimize operating disruptions and optimize construction schedules.
Three specific examples of phased construction - aeration tank conversion, screening and caustic building construction, and turbo blower conversion - exhibit how the project was phased for efficiency.
In the case of the aeration tanks, permit limits required that the plant's 10 aeration tanks be converted to the IFAS process a few at a time. However, for efficient and cost-effective conversion, it was important that the contractor could move directly from one aeration tank to the next with little to no downtime. This required coordination between the owner, engineer, general contractor, and subcontractors to ensure that tank conversions went smoothly and without risking process upset or permit violations.
The 10 biological process reactors were converted from activated sludge to the IFAS process over a period of two years. In late 2012, all 10 basins at Field's Point WWTF had been converted to the IFAS process, started up and were operational. By September 2012, the total nitrogen concentration dipped to below 5 mg/L with no optimization of internal recycle rates and no alkalinity feed or carbon dosing.
The second example of phased efficiency was realized in the construction of the screening and caustic building. Due to site constraints, the structure was constructed directly above two live 60-inch diameter PCCP gravity pipelines connecting primary treatment to the screw pump lift station. To maintain flows and avoid costly bypass pumping, the building was designed with three distinct foundations and a series of temporary bulkheads that could be staged in such a way as to always have one of the 60-inch lines online. Once the new screening process was installed, the remaining 60-inch line was demolished and the remainder of the building was constructed. While a bypass pump system was installed as a standby system, it was never required to operate other than to exercise the equipment.
The third example of phased efficiency occurred during the installation of the turbo blower. The new IFAS process requires approximately three times the aeration capacity as the former process. Through the selection of innovative turbo blower technology, a total of nine new turbo blowers were installed in the footprint that was formerly occupied by the intake filter boxes for five centrifugal blowers.
Due to the small footprint and high efficiency of the turbo blowers, it was possible to provide additional aeration capacity without a building expansion. Also, a new electrical room and storage area was able to occupy the space formerly taken up by the centrifugal blowers. Finally, the flexibility of the technology allowed for installation in phases, while keeping full aeration capacity to the plant during construction.
Kleinfelder, as the engineering and construction team lead, is finalizing the IFAS process optimization with anticipated completion of the project by the end of winter 2014.
Ultimately, designing the buildings for phased construction eliminated the need to use bypass pumping, which saved the client $1.5 million and resulted in completion of the project one year ahead of schedule. This project exhibits how advanced planning can facilitate efficient construction, while limiting process interruptions and saving cost.
About the Authors: Edward V. DiSalvio, P.E., is a project manager and principal structural engineer with Kleinfelder. He can be reached at (617) 498-4609 or [email protected]. David Peterson, P.E., is a project professional with Kleinfelder. He can be reached at [email protected].
