Tuesday is the grand opening of a long-awaited $46 million development project on Manayunk's Venice Island. The island, located between the Manayunk Canal and the Schuylkill River, just off Main Street, is home to a new 250-seat theater, outdoor amphitheater, a park, and basketball courts.

All of those amenities exist above ground because of a less flashy project underneath: a massive stormwater overflow-storage tank the city needed to meet federal Clean Water Act requirements.

The tank is a colossal room of concrete, 25 feet deep and the size of a football field, with concrete pillars running its length.

The storage basin can hold about 4 million gallons of diverted stormwater and sewage during intense rainstorms.

A high-water line at the top of the staircase leading down into the tank is marked with the date April 30, 2014, when the basin filled and the Schuylkill overflowed its banks after a storm.

"This entire tank capacity has been full at one point," said Jim Giffear, construction project engineer for the Venice Island project, who said the tank has been operational for around a year and has been used only a handful of times.

In the past, the mixture of sewage and rainwater flowing down from Roxborough, Andorra and Manayunk that now goes into the tank would have exceeded the capacity of the city's combined sewer system and overflowed into the Schuylkill River, about a half mile upstream from the Queen Lane Water Treatment Plant's drinking water intake.

"In lieu of that, we're trying to store excess stormwater, wastewater, until the storm passes and there's capacity again in the sewer, and we pump it back to it," Giffear said, for treatment.

The pump station housed in a building above the tank is the only part of the project above ground.

"(The tank) protects the health of our waterways," Giffear said, by preventing contaminants transported by stormwater from flowing directly in the river.

This is the city's first underground sewage and stormwater storage tank, according to Philadelphia Water Department spokeswoman Joanne Dahme.

She said other large cities — including New York, Baltimore and Detroit — have underground storage tanks that collect and store combined sewage.

Venice Island also has a number of "green infrastructure" solutions including rain gardens, green roofs, porous paving, tree trenches and a cistern to capture rainwater before it enters the sewer system.