In July, a four-alarm fire ravaged the engine room of the North River Wastewater Treatment Plant along the Hudson River, sending raw sewage into the Hudson, East and Harlem Rivers. It took the Department of Environmental Protection three days to stop the sewage from pouring into the waterways—but beaches were closed and other water activities were discouraged for weeks after while the cleanup continued. Now it seems the DEP has found the root cause of the damage: a loose nut.

According to the DEP's official report on the incident, the whole thing was caused by a single improperly tightened nut connecting a fuel injector to its pump which was tightened either too much or too little during routine maintenance in June. Once the fitting leaked, “a spray of fuel oil hitting the engine’s extremely hot turbocharger” may have started a small fire which then spread into the bigger fire.

The report emphasized that the equipment’s maintenance manual does not specify to what degree the nut should be tightened, so workers do not appear to have been at fault for the fire. North River processes an average of 120 million gallons of wastewater daily; if you want a good primer on how the city's wastewater systems work in general, Jim Dwyer's Times column from the summer is a good way to get you started.