The Source Civil Engineering Magazine Preliminary treatment facility will boost Philadelphia’s wet weather treatment capability

Preliminary treatment facility will boost Philadelphia’s wet weather treatment capability

By Jay Landers

The Philadelphia Water Department expects to begin construction later this year on a 300 mgd preliminary treatment facility that will greatly increase the capacity of the city’s existing Northeast Water Pollution Control Plant. By enabling the Northeast WPCP to treat significantly more flows during wet weather events, the new preliminary treatment facility will help the PWD achieve its ambitious goals for reducing combined sewer overflows and improving water quality in local waterways.

On Jan. 25, the PWD announced that it had received a $100 million low-interest loan for the project from the Pennsylvania Infrastructure Investment Authority. Also known as PENNVEST, the authority provides funding for drinking water, wastewater, and stormwater projects in the state.

Greening the city, cleaning the waters

In 2011, the PWD initiated the Green City, Clean Waters program, a 25-year effort that has as its chief goal the elimination of approximately 8 billion gal. of sewer overflows annually by 2036. To achieve this goal, the Green City, Clean Waters program calls for a wide variety of infrastructure improvements, says Melanie Garrow, P.E., the director of the PWD’s Office of Watersheds.

Such improvements include upgrading the city’s wastewater treatment plants, lining interceptor sewers, and implementing green infrastructure to reduce stormwater runoff, Garrow says. Besides reducing CSOs, the improvements conducted as part of the Green City, Clean Waters program are intended to reduce levels of fecal coliform and total suspended solids and improve dissolved oxygen levels in local waterways, she says.

A second headworks

The planned preliminary treatment facility “is a major component of the overall program for Philadelphia,” says Michael Olivier, P.E., a vice president for Whitman, Requardt and Associates LLP, which designed the project. Essentially a second headworks for the Northeast plant, the preliminary treatment facility will receive the flow from one of the three major interceptor sewers that convey wastewater to the WPCP. “We’re taking that flow and diverting it to this new facility,” Olivier says. “The discharge from this facility will connect to the existing primary sedimentation tanks” at the Northeast WPCP.

The preliminary treatment facility will include mechanically cleaned bar screens and hydraulically induced vortex grit tanks, Olivier says. Ancillary facilities will include pumping and conveyance systems to send the screened material and the collected grit to dumpsters for disposal. A two-stage odor-control system will treat odorous air from the enclosed facility. The first stage of the odor-control system will comprise a biotrickling scrubber, while the second stage will consist of an activated carbon system for polishing.

cross section of a building showing above and below ground spaces
The preliminary treatment facility will include mechanically cleaned bar screens and hydraulically induced vortex grit tanks along with pumping and conveyance systems to send the screened material and the collected grit to dumpsters for disposal. (Courtesy of Whitman, Requardt and Associates LLP)

The Northeast WPCP has a permitted capacity of 210 mgd during dry weather and 435 mgd during wet weather. Currently, the existing preliminary treatment stage amounts to a “bottleneck” that limits the plant’s overall capacity, says Brian Sparks, a design unit project manager for the PWD. By alleviating this bottleneck, the new preliminary treatment facility will facilitate an increase in the wet weather treatment capacity of the Northeast WPCP by 215 mgd.

During significant wet weather events, the Northeast plant is permitted to discharge a certain amount of flow that has undergone primary treatment and disinfection but has bypassed secondary treatment. The PWD already has constructed a secondary treatment bypass system, Sparks says. Once the new preliminary treatment facility is in place, the Northeast WPCP will have a total peak instantaneous flow capacity of 650 mgd, he notes.

Further CSO reductions

Increased treatment capacity at the Northeast WPCP will lead to improved water quality and environmental conditions, Garrow says. “The project should have a direct impact” in terms of decreasing the volume of CSOs entering local waterways, Garrow says. “That obviously helps with the water quality of our waterways,” she notes.

Before the start of the Green City, Clean Waters program, Philadelphia discharged about 13.1 billion gal. of CSOs in a typical year, based on modeling conducted by the PWD, Garrow says. In just the first five years of the program, the department reduced CSOs by 2 billion gal. “Two of the 8 billion gallons (of CSO reductions) we need by the end of the program, we’ve achieved already,” she notes.

The PWD has until the end of the year to complete its modeling and determine the extent to which it has reduced CSOs further during the past five years. “We are still undertaking the modeling right now for year 10,” Garrow says. “But the modeling will obviously show that we’ve continued to reduce the amount of overflows that we’ve seen during the last five years.”

Moving to construction

Design work on the preliminary treatment facility began in 2011 but was delayed by a “number of hurdles,” Olivier says. In particular, a portion of the new facility is to be located adjacent to the existing Northeast WPCP, and properties needed to be acquired to enable soil borings to be taken to finalize the structural design. For its part, Whitman, Requardt and Associates is finalizing the design now, Olivier says.

Construction bids for the project will be solicited from contractors “sometime this summer,” Olivier says. Estimated to last four years, construction is scheduled to be completed in late 2025 or early 2026, he says.

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