Fracture Critical Sewershed

There was a four-alarm fire last week at one of my favorite examples of multifunctional infrastructure: NYC’s North River Wastewater Treatment Plant/Riverbank State Park. The fire damaged five engines that power the pumps that circulate the sewage, through the treatment process, so 120 million gallons of untreated sewage are being dumped into the Hudson and Bronx Rivers till the plant is back up and running. The plant serves a sewershed of 6,030 acres, home to almost 600,000 folks.

Because NYC has a Combined Sewer Overflow (CSO) system, the untreated sewage was discharged from 56 outfalls. If NYC had a more modern seperate storm sewer and sanitary sewer system, the untreated discharge would have been concentrated just at the plant creating a more severe local impact. This is one of those rare cases where having an obsolete CSO infrastructure may be reducing the environmental impacts by increasing the dilution of the sewage.

BTW: the treatment plant plinth was designed by Phillip Johnson in the 1960s!

map of outfalls and closed beaches impacted by the fire

The wastewater treatment process

Several stories underground, wastewater flows into the North River plant from an 11-mile-long intercepting sewer that extends along Manhattan’s west side. Upon entering the plant, the wastewater first passes through upright bars that remove large items, including rags, sticks, newspapers, cans and other debris. The trash is automatically scraped from the bars and later transported to a landfill. Five main sewage pumps lift the wastewater to the surface level primary settling tanks. The flow of the water is slowed, allowing the heavier solids to settle on the bottom and the lighter materials to float. Oil and grease are skimmed from the top of the tanks and the heavy solids, called “primary sludge,” are scraped off the bottom for further processing.

The partially-treated wastewater then flows to the secondary treatment system. Secondary treatment is called the “activated sludge process,” because air and “seed” sludge from the plant treatment process are added to the wastewater to break it down further. Air pumped into five, 30-foot-deep aeration tanks stimulates the growth of oxygen-using bacteria and other tiny organisms that consume most of the remaining organic materials that pollute the water.

The aerated wastewater then flows to 16 final settling tanks, where heavy particles and other solids again settle to the bottom. Some of this sludge is recirculated back to the aeration tanks as “seed” to stimulate the treatment process. The remaining solids are removed and join the primary sludge for further processing in sludge-handling facilities.

To destroy disease-causing organisms, the wastewater is disinfected with sodium hypochlorite, the same chemical found in common household chlorine bleach. The treated wastewater, called effluent, is then released into the Hudson River.

A few more pics of the treatment plant and park:

Bridgeandtunnelclub [above] has one of the best collection of pics of the park and plinth.

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